Hybrid materials based on conjugated polymers and inorganic semiconductors as photocatalysts: from environmental to energy applications

Marta Liras *, Mariam Barawi and Víctor A. de la Peña O’Shea *
Photoactivated Processes Unit, IMDEA Energía, Ramón de la Sagra, 3, 28935, Móstoles, Madrid, Spain. E-mail: marta.liras@imdea.org; Victor.delapenya@imdea.org

Received 26th May 2019

First published on 14th October 2019

Photocatalysts provide a sustainable way to remove pollutants or store energy in the form of solar fuels by processes such as water splitting and CO2 photoreduction (artificial photosynthesis). Research in this topic is an expansive field evidenced by the large number of contributions published in the past few years. Hybrid photocatalysts based on inorganic semiconductors (ISs) and conjugated polymers (CPs) have emerged as novel promising photoactive materials. In addition to the well-known behaviour of ISs in photocatalytic processes, CPs have emerged as an interesting alternative to improve the photocatalytic efficiency due to the possibility of controlling their optoelectronic, textural and morphological properties at the molecular level. Thus, the synergy between ISs and CPs leads to more efficient photocatalysts with enhanced light absorption in the overall solar spectrum, improved photocharge generation and transport, higher stability to photo-corrosion and higher surface areas. Here, we present an overview of the advances in the development of hybrid IS–CP photocatalysts for pollutant degradation and energy conversion through water splitting, CO2 reduction and/or N2 fixation using photo- and photo(electro)catalytic processes.

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Marta Liras

Dr Marta Liras is a Senior Assistant Researcher at the Photoactivated Processes Unit from IMDEA Energy since 2017. She received her PhD in organic chemistry in 2003 from Universidad Complutense de Madrid (UCM) for her work at the Organic Chemistry Institute (IQOG-CSIC) and Science and Technology of Polymers Institute (ICTP-CSIC) about solid state laser dyes. In 2008 she joined the J.C. Scaiano research group at the University of Ottawa as a postdoctoral researcher. She was awarded with Juan de la Cierva (2004), Junta Ampliación de Estudios (2013) driving nanostructures functionalization with smart polymers projects, and Ramon y Cajal grants (2016). Her interests are focused on polymer designs for energy applications.

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Mariam Barawi

Dr Mariam Barawi is a Juan de la Cierva postdoctoral researcher at IMDEA Energy. She received her PhD from Universidad Autónoma de Madrid in 2015 which based on the investigation of photoelectrochemical cells for energy conversion. In this period, she did a research stage under a COST action at the Institut de Chimie et des Matériaux, Paris (France). After that, she moved to the Italian Institute of Technology for almost two years where she developed smart window devices based on colloidal metal oxide-based nanostructures. Since January 2017 she has been in charge of the photoelectrochemical solar fuel production research line at the Photoactivated Process Unit of IMDEA Energy.

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Víctor A. de la Peña O’Shea

Dr Víctor A. de la Peña O’Shea received his BSc and MSc from the Universidad Autonoma de Madrid (2000) and his PhD (2003) from the Institute of Catalysis and Petrochemistry (CSIC). In 2008 he joined IMDEA Energy as a Ramón y Cajal researcher, where since 2015 he has been the Head of the Photoactivated Processes Unit. His research is focused on catalysis for energy production/storage and environmental technologies. He coordinates several projects on Solar Fuels and CO2 Conversion including an ERC-CoG. He is the Chair of CO2 uses in the Spanish technological platform of CO2 (PTECO2) and the Coordinator of Spanish solar-fuels network (FOTOFUEL). In 2018, he received the Comunidad de Madrid prize for the best ERC project.

1. Introduction

Photocatalysis plays a critical role in the development of emerging environmental and energy conversion and storage technologies such as water treatment and the production of solar fuels by artificial photosynthesis, as H2 production and CO2 reduction.1 The interest about this technology lies in the use of sunlight as a renewable energy source. This strategy meets perfectly with the green chemistry concept and with the imperative challenges regarding clean energy and decarbonised society proposed for the near future.2Fig. 1 depicts the increase in the number of published papers in this field with more than 12[thin space (1/6-em)]500 references (by WOS) in the last 10 years. Also, the number of citations of these references has exponentially increased in the last decade. As an example, in 2018 alone these works were cited more than 95[thin space (1/6-em)]000 times. Despite the fact that published papers on organic pollutant removal are significantly higher, H2 photoproduction from water and CO2 photoreduction received greater attention. Thus, the average number of citations per paper is 34 and 33 for H2 production and CO2 photoreduction, respectively, versus 24 in the case of environmental remediation.
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Fig. 1 Number of original research articles and reviews published in the last decade concerning photocatalysts for organic pollutant removal (blue bars), H2 production from water (red bars) and CO2 photoreduction (green bars). The number of citations of these works during the same period of time have been included (coloured lines). The data come from the ISI web of Knowledge database. Search: Photocat* AND (organic pollutant), (H2 and water) or (CO2 photored*), respectively. Note that contributions related to oxidation processes by Fenton reaction have not been taken into consideration.4 So, in all cases “Fenton” keyword has been removed including NOT (Fenton) in the search.

The key pillar of this success is that among the several available strategies to produce green fuels and chemicals, photocatalytic, electrocatalytic or photoelectrocatalytic approaches are particularly attractive since the required energy input can be potentially supplied by renewable sources. While electrochemical technologies are closer to scale up to the industrial level, its development is highly dependent on the cost of electricity.3 In addition, this technology requires two steps: the first one related to the electricity production, and second one, the conversion of that electricity into chemical energy. This leads to higher efficiency losses. On the other hand, photo(electro)catalytic processes achieve a direct conversion of solar to chemical energy, mimicking the natural photosynthesis, and hold an advantage of mobility afforded by being independent of an electricity source, relying purely on solar radiation to produce fuels.5 However, photocatalytic efficiencies must be improved to go forward and develop industrial processes. The bottlenecks that must be improved for the successful scale up in these technologies are manly focused on the limited solar light spectrum absorption, the high rates of charge recombination, the low electronic transport and low photo-stability of the current photocatalysts.6 In this sense, the recent and stimulating paper published by Kim et al.7 explore the latest advances implemented by the scientific community in the development of new materials and reactors that avoid these blocks and allow the scale up of these technologies.

Advances in photocatalytic materials have focused on the use of inorganic semiconductors (ISs) such as metal oxides and chalcogenides.8–12 Currently, the most used semiconductor is TiO2 (being 25% of the contributions in the photocatalyst field), which constitutes the benchmark photocatalyst in photo-oxidation of organic compounds for water and air purification and solar energy conversion. However, the main drawback of TiO2 is that due to its high band gap energy (around 3.2 eV) it absorbs only ultraviolet (UV) radiation and also presents a fast electron–hole recombination rate.8 Different strategies have been developed to modulate the light response of ISs and improve the charge separation and transport such as: (i) band-gap engineering approaches;13 (ii) use of co-catalysts;14,15 (iii) addition of plasmonic nanoparticles (Au, Pt, Cu);16 (iv) design of multijunction materials based on two or more ISs10,17,18 or (v) design of heterogeneous catalysts with controlled nanostructures.10,19

A new strategy to improve the performance of ISs is to develop organic–inorganic hybrid (OIH) materials.20 The first and most studied OIH materials are based on ISs and organic molecules. These organic molecules can be organic dyes able to sensitize ISs,21 or molecular catalysts capable of acting as photocatalysts.22 The second kind of IOH material is based on the use of organic macromolecules or polymers. Sometimes, in this configuration, the polymer does not act as a photoactive material but acts just as a protective layer, and even endows the hybrid with its own properties (i.e. hydrophobicity, hydrophilicity, thermoresponse, etc.).20,23–25 In this case, the polymer improves the IS performance but could not be considered as part of the photocatalytic system.

Other times, the polymer presents photocatalytic behaviour and also conductive properties as it happens with conjugated polymers (CPs). CPs are characterized by a backbone chain of alternating double and single bonds. Their overlapping π-orbitals create a system of delocalised π-electrons responsible for their photo and electronic properties. Semiconductor polymers have attracted attention for many optoelectronic applications such as in light-emitting diodes, field-effect transistors, and photovoltaic solar cells.26–28 This is due to their interesting properties such as: tailor made synthesis, light absorption and emission tunability covering the whole solar spectrum, hole injection and electron-blocking abilities and, sometimes, processability and stretchability. Although the scientific community showed interest at first in their use as photocatalysts, this interest declined over time due to their low photostability compared with the ISs. However, in the past few years there has been a renaissance in the development of novel conjugated polymers (CPs) as photocatalysts due to the upgrading of their optoelectronic, textural, morphological and photostability properties which has led to an improvement in their efficiencies as photocatalysts.29,30

The synergistic effect of semiconductor nanocrystals and CPs for energy applications in photovoltaics and photocatalysts was revised by Y.-W. Su et al. in 2014.31 Recently, two revision works related to the use of graphitic carbon nitride, g-C3N4, a 2D conductive polymer, have appeared highlighting the promising potential of hybrid materials based on these materials in the photocatalytic field.32,33 The paper presented here timely focuses on the use of organic–inorganic hybrid materials based on organic conductive polymers (CPs) and inorganic semiconductors (ICs) as photocatalysts for environmental and energy applications which is an expanding and promising topic of research that has not been reviewed yet (Fig. 2). The revision will be of valuable interest to those scientific communities related to chemistry and materials science, showing the latest advances in the development of novel hybrid multifunctional materials based on CPs and ISs. Furthermore, it will be interesting for the chemical engineering community, in particular for those related to the catalysis field showing the latest advances in photocatalysis and photoelectrochemical processes for energy production and storage as well as environmental technologies. Along the present revision, we will see how the combination of ISs with CPs leads to a remarkable improvement of the photo-efficiencies when compared with the bare materials. This synergistic effect will be analysed from different points of view such as hybrid morphology, charge transport, charge recombination and so on.

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Fig. 2 General scheme of the review topic. Hybrid organic–inorganic materials based on inorganic semiconductors (IS) and conductive polymers used for pollutant removal or artificial photosynthesis including hydrogen production from H2O and CO2 photoreduction by both photocatalytic and photoelectrocatalytic processes.

Fig. 3 depicts the molecular structures of some of the most studied linear CPs for photocatalysts as part of an OIH system. One of the main advantages of CPs versus inorganic semiconductors is that they can be tailor made thanks to the development of organic synthesis tools. This led to the control of their morphological, structural, textural, superficial and more importantly optoelectronic properties such as light harvesting (including UV, visible or NIR), electronic structure, conductivity type (n or p), photocurrent, and charge separation and transport. However, these linear molecular structures present, as a main disadvantage, low stability under irradiation in the presence of oxidants, and hence are less studied than ISs as photocatalytic systems.34

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Fig. 3 Molecular structure of some linear conjugated polymers (CPs) used as part of OIH photocatalytic systems. Abbreviations are as follows: poly(dopamine), PDA; poly(aniline), PANI; camphorsulfonic acid doped poly(aniline), CSA-PANI; poly(thiophene), PT; poly(3-hexilthiophene), P3HT; poly(p-phenylenevinylenes), PPVs; poly(3,4-ethelenedioxythiophene), PEDOT; poly(pyrrole), PPy; derivatives of poly(isoprene), CDPIP; poly(diketopyrrolopyrrole-carbazole), DPP-Car; poly(bithiophene-co-fluorene), BThF; poly(vinyl alcohol), PVA; degraded poly(vinyl alcohol), PVAD and poly(benzothiadiazole), B-BT-1,4-E.

The development of organic photovoltaics (OPV) has been the main promotor of the development of novel synthetic approaches to achieve more active photo-conductive polymers, and therefore of their expansion.35 The advantage of OPV versus other photovoltaic (PV) applications is the access to thin-film systems in a roll-to-roll process at room temperature with extremely thin absorbers (around 100 nm), mostly consisting of carbon and polymers.36 The most developed polymers has been linear polymers which present in their structure lateral hydrocarbon chains that facilitate their dissolution in organic solvents. This characteristic makes them stretchable and easily processable to fabricate thin films. In addition, the presence of these hanging chains avoids π–π stacking interactions from the aromatic groups present in the main backbone that can act as crystallinity points in the processed film. However, as explained before, these CPs are prone to degradation under irradiation due to these lateral chains. In particular, the carbon atom next to the conjugated polymer backbone (αC) appears to be vulnerable to radical formation (e.g. poly(hexylthiophene) (P3HT), see structure in Fig. 3).37 Another negative point is the presence of exocyclic double bonds along the backbone because they can be photo-oxidized in the presence of oxygen causing chain scission (e.g. poly(p-phenylenevinylenes) (PPVs) see structure in Fig. 3).38 To overcome this problem, Manceu et al. established a series of rules to attain more photostable conductive polymers based on their structure–activity studies.39 Among them, the use of aromatic polycyclic units and the cleavage of the side chain generally lead to good photochemical stability. For this reason, polymers completely based on aromatic polycyclic units, without side chains in the network, are highly photostable polymer conductors, although expensive in terms of their inherent processability.

One of the most interesting examples of the use of aromatic polycyclic units is the work published by Wang et al. which proposes graphitic carbon nitride (g-C3N4) (see structure in Fig. 4) as a stable photocatalyst for the production of hydrogen from water.40 Since the publication of this work, the number of contributions based on this photocatalytic material has increased considerably.33,41–44 g-C3N4 is a conjugated polymeric system constructed with s-triazine or tri-s-triazine units interconnected by tertiary amines forming interconnected honeycomb atomic layers, thanks to van der Waals forces. So, essentially, g-C3N4 is a conductive polymer based on aromatic polycyclic units without side chains in the network which meets perfectly the photostability criteria commented previously. More recently, new classes of conjugated polymeric networks such as conjugated porous polymers (CPPs) and covalent organic frameworks (COFs) have emerged as novel photocatalytic systems (Fig. 4), mainly for pollutant removal and hydrogen production.45–54

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Fig. 4 Molecular structure of some conjugated porous polymers (CPPs) used as part of organic–inorganic hybrid (OIH) photocatalytic systems. Abbreviations are as follows: graphitic carbon nitride, g-C3N4; covalent organic polymer number 64, COP-64; poly(catechol), aza-fused conjugated microporous polymer, aza-CMP; conjugated microporous poly(benzothiadiazole), CMPBTT; triptycene-based covalent polymer, TCP and conjugated polymer number 1, CP1.

This revision elucidates how hybrid photocatalysts based on both ISs and CPs have emerged as novel and promising photocatalysts for environmental remediation and solar fuel production processes. Initially, linear CPs and hybrid thereof were developed following the ground-breaking discovery of g-C3N4 networks solely or as a part of organic–inorganic hybrids. However, examples of hybrid materials based on ISs and CPs networks are scarce. Due to the wide variety of exhibited structures, the simplicity of synthesis of these polymer networks and their high photostability versus linear polymers we predict that they will open new horizons in the development of highly efficient photocatalysts.

1.1. Photocatalysis for hybrid materials

For a better comprehension of this review paper, a series of fundamental concepts in photocatalysis will be described.55 Essentially, a photocatalyst is a semiconductor with adequate band gap energy to absorb light from the solar spectrum, resulting in the excitation of electrons from the valence band (VB) to the conduction band (CB). This leads to the formation of an exciton or an electron–hole pair. These electrons and holes are transferred to the photocatalyst surface where they react with the absorbed molecules (water, CO2, pollutants, etc.) causing their reduction and oxidation. It is important to mention that band energy levels (CB and VB) should have an adequate position with respect to redox pairs of the target reactions, i.e., the valence band edge must be at more positive potential than the oxidation redox potential, while the conduction band edge must be more negative than the reduction redox potential. So, photogenerated electrons have enough energy to carry out the reduction process and holes can accept electrons and accomplish the oxidation reaction. Fig. 5a depicts the energy edge relative to vacuum and a normal hydrogen electrode (NHE) for the most used inorganic and conductive polymer semiconductors in combination with the redox potentials of several pairs involved in the water splitting and CO2 reduction reactions.
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Fig. 5 Schematic illustration of the energy level diagram of some conjugated polymers (CPs) and inorganic semiconductors (ISs) as well as five types of charge-transfer mechanism in hybrid semiconductor materials: (a) valence band (VB) edge position (red squares), conduction band (CB) edge position (blue squares) and band gap for several CPs and ISs. Redox potentials versus vacuum and NHE of CO2 reduction and water splitting at pH = 0 are included; (b) sensitization mechanism; (c) type I heterojunction mechanism; (d) type II heterojunction mechanism; (d) type II heterojunction mechanism; (e) Z-scheme heterojunction mechanism; (f) Z-scheme mechanism through a conductive intermediate. Note that in (b–f) CP is represented as yellow and IS as blue.

Based on the literature, light absorption and charge transfer processes associated with hybrid organic–inorganic heterojunctions may proceed following five possible mechanisms: (A) sensitization (Fig. 5b); (B) type I heterojunction (Fig. 5c); (C) type II heterojunction (also named p–n junction or double transfer structure) (Fig. 5d) and (D) Z-scheme (Fig. 5e and f).10,56,57

In the sensitization mechanism, conductive polymers mainly act as visible sensitizers (i.e. PANI) generating photoelectrons from their valence band to the conduction band (CB), which can be transferred to the CB of the inorganic material (Fig. 5b).

Type I implies that both semiconductors absorb light and that one of them has a broad band edge where the band gap of the other one is localized. So, both electrons and holes recombine from the broader semiconductor to the shorter semiconductor CB and VB, respectively (Fig. 5c). So, the electrons recombine from the CB of the broader semiconductor to CB the shorter one, while hole does the same way from their respective VB.

However, in type II, their relative band gap edge position allows that photogenerated electrons and holes are correspondingly transferred towards both semiconductors (Fig. 5d). In both type I and type II heterojunctions, electrons move towards lower energy potentials where the reduction occurs, meanwhile holes move to higher energy potentials where the oxidation occurs.

On the other hand, the Z scheme mechanism is inspired by the Photosystem I and Photosystem II processes of natural photosynthesis.58 The photogenerated electrons in the CB of one of the semiconductors are transferred to the VB of the other one (Fig. 5e). Alternatively, both semiconductors can be connected in series with reversible redox couple shuttles (electron donor/acceptor pairs) or a conductive medium (Fig. 5f). In general, in the Z-scheme, the relative band energy edge positions of both semiconductors are the same as in the type II heterojunction mechanism. As we will see vide infra, to clarify the electron flow pathways, one of the most reliable methods employed in the literature is to trace back the localization of photodeposited metals (such as Pt), monitoring their location by transmission electron microscopy (TEM). If the metal nanoparticles are deposited on the semiconductor surface with the lower band edge the mechanism is type II and if they are localized in the semiconductor with the upper band edge the mechanism is the Z-scheme. In other cases, the redox potential of the intermediate products is the parameter that decides between both mechanisms as we will see vide infra.

Photo-induced electron transfer from conjugated polymers to TiO2 was firstly reported in 1999 by van Hal et al.59 They used poly(p-phenylenevinylene)s (PPVs) and poly(thiophenes) as electron donors and nanocrystalline TiO2 as the acceptor material. The photo-driven electron transfer was studied using near-steady-state photoinduced absorption (PIA) spectroscopy and electron spin resonance (ESR) spectroscopy.

Both PIA and ESR spectroscopies showed that the recombination of charge was characterized by a distribution of lifetimes at cryogenic temperatures ranging from milliseconds to tenths of seconds. A decade later, Tachibana et al.60 observed by transient absorption spectroscopy (TAS) an increment of the photo-induced charge separated state lifetime by a factor of 50–70 after electro-polymerising a thin layer of poly(bithiophene-co-fluorene) over TiO2 (Fig. 6).

Herein, we are going to present our point of view about the state of the art of the use of hybrid materials based on conjugated polymers and inorganic semiconductors as photocatalyst as well as future research lines. Up to now, their use has been fundamentally related to the pollutant removal process. However, new cutting edge contributions for solar fuel production have been recently described.

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Fig. 6 (A) Molecular structure of the bithiophene-fluorene-bithiophene derivatives (F1BT2R, R = H or Br). (B) Conducting wire formation on the metal oxide (TiO2 or Al2O3) surface by voltage application. (C) Charge recombination kinetics of F1BT2H/TiO2 (curve B) and P-F1BT2/TiO2 (curve A), monitored at 600 nm and 800 nm with excitation wavelengths at 450 nm and 480 nm, respectively. Both monitored by transient absorption spectroscopy. Solid red lines indicate fits to a stretched exponential function, DOD(t) = DOD(0)[thin space (1/6-em)]exp(−t/t)a. Reproduced from ref. 60 with permission from The Royal Society of Chemistry.

2. Environmental applications

Pollutant removal is the most studied process in photocatalysis and this is also the case for hybrid photocatalysts based on inorganic semiconductors and conjugated polymers. To do that, different kinds of dye molecules such as Rhodamine B (RhB), Methylene Blue (MB), Methyl Orange (MO), Orange G (OG), and so on are employed as benchmark pollutants (see molecular structures in Fig. 7). These molecules present high extinction coefficients, and good solubility in water, making it possible to monitor their photodegradation by conventional spectroscopic measurements. In addition, other chemical species such as phenols, ciprofoxacin (CIP), tetracycline hydrochloride (TC), naproxene (NP) or even Cr(VI) and NOx have been used as harmful or toxic pollutant models for photoactivity tests (see molecular structures in Fig. 7).
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Fig. 7 Molecular structure of most common dyes and drugs used as photocatalyst testers.

Table 1 collects a series of representative examples of the use of OIHs based on CPs and ISs for photocatalytic pollutant removal. All reported hybrids show an increase in photocatalytic activity compared with single pristine materials. Without doubt, the most popular inorganic semiconductor is TiO2 (ca. 60% of revised contributions) followed by ZnO and CdS. On the other hand, the most studied organic polymer is g-C3N4 (ca. 30% of revised contributions).33

Table 1 Some hybrid photocatalytic systems based on CPs and ISs used for pollutant removal
IS CP Pollutant Light source Charge transfer mechanisma Result Morphology Ref. (year)
a Here we include the mechanism name as appearing in the original scientific paper. Note that type II and p–n junction refer to the same mechanism. b TNF means TiO2 nanofibers. c TNT means TiO2 nanotube. d m-TiO2 means TiO2 treated with 3-thiophenecarboxylic acid. e N-TiO2 means nitrogen doped TiO2 nanofibers. f 2,3-Dichlorophenol; 4-CP = 4-chlorophenol; RB 198 = reactive blue 198; RB 5 = reactive black 5 and RY 145 = reactive yellow 145; CIP = ciprofloxacin; TC = tetracycline hydrochloride; IBF = ibuprofen; NPX = naproxen. g LMCT means ligand to metal charge transfer complex.
TiO2 Ppy RhB Xe lamp (>400 nm) Sensitization Ppy–TiO2-24 showed 4.5 times higher photoactivity than TiO2 Composite 61 (2014)
TiO2/AgAgCl PPy RhB Xe lamp (>400 nm) Sensitization TiO2/Ag–AgCl@PPy showed 4.9 fold higher activity than TiO2 nanofibers Polymeric coating 62 (2016)
TNFb/Ag PPy Acetone (g) Hg lamp (>400 nm) Sensitization 1%-Ppy–Ag–TiO2 sample provided the optimum photocatalytic activity Polymeric coating 63 (2013)
TiO2 PANI MB, RhB Visible light (>450 nm) Vis: sensitization PANI-modified TiO2 photocatalysts with remarkably enhanced activity Polymeric coating 64 (2008)
UV light (λ = 254 nm). UV: p–n junction
TiO2 PANI RhB, MB, phenol UV light TiO2/PANI 20% of TiO2 displayed the highest photocatalytic activity Polymer coating 65 (2016)
TNTc CSA-PANI Xe lamp (420 nm) p–n junction 100CSA-PANI/TNTs 2.4 fold more active than TNT Polymer coating 66 (2015)
TNT PTs Phenole Na lamp, (>400 nm) Sensitization Enhanced photoactivity Polymer coating 67 (2009)
TiO2 P3HT OG UV light p–n junction Enhanced photoactivity Composite 50–50 68 (2007)
TiO2 P3HT MO Iodine tungsten lamp (400 nm) Sensitization Enhanced photocatalytic activity Coating polymer 69 (2009)
TiO2 P3HT MO UV and visible light Sensitization Enhanced photoactivity Polymer coating 70 (2010)
TiO2 P3HT MB Xe lamp (>400 nm) Sensitization Enhanced photoactivity 71 (2010)
TiO2 P3HT MO Visible light Sensitization It detected oxidation of the side chain of P3HT (beneficial for photocatalytic activity) and oxidation of the backbone of P3HT which decreased the photocatalytic activity Coating polymer 72 (2012)
TiO2 P3HT MO UV and visible light Sensitization SAM-containing TiO2–P3HT composite exhibited superior photocatalytic activity P3HT polymerized as coating 73 (2013)
TiO2 P3HT/graphene RhB Tungsten-halogen lamp (>420 nm) Highest performance was provided by composite samples containing graphene of medium size (150 m2 g−1). Composite 74 (2019)
TiO2 CDPIP MO Fluorescent lamps (400 nm) Sensitization TiO2/CDPIP showed much more enhanced photoactivity if it was iodine doped Polymer coating 75 (2012)
TiO2 Polycatechol Cr(VI) Visible light (>400 nm) LMCTg The hybrid material showed 5- and 10-fold enhanced activity for photocatalytic Cr(VI) reduction compared with pure TiO2. Polymer coating 76 (2017)
TiO2 PDA RhB Xe lamp (>420 nm) Sensitization TiO2@PDA 1 nm coating gave the best result versus thicker ones Polymeric coating 77 (2016)
TiO2 PVA and PVAD MO and phenols Halogen lamp (>450 nm) Sensitization Enhanced photoactivity Coating 78 (2014)
TiO2 doped with Mo PVAD MB 365 lamp (8W) Mo doping enhanced photoactivity Composite thin film 79 (2019)
TiO2 DPP-Car MO Visible light Sensitization TiO2–DPP-Car showed enhanced photoactivity when the carbazole protecting group was removed by hydrolysis Polymer coating 80 (2018)
TiO2 B-BT-1,4-E CIP Xe-lamp (≥420 nm) Sensitization CIP degradation with B-BT-1,4-E/TiO2 as the photocatalyst was increased 7.3 times compared with CMPBTT/TiO281 Polymer as coating 82 (2018)
N-TiO2 g-C3N4 RhB Xe lamp p–n junction 100% degradation in 120 min Polymer as coating 83 (2015)
TiO2 B doped g-C3N4 Acetaldehyde Xe-lamp (≥420 nm) Sensitization High visible-light activity compared with the undoped CN Composite 84 (2016)
TiO2 CMPBTT CIP Xe-lamp (≥420 nm) Sensitization BBT/TiO2 heterojunction exhibited enhanced photocatalytic activity (20.4 times higher activity for ciprofloxacin degradation) compared with BBT alone. Polymer as coating 81 (2017)
Bi2SnTIO7 PANI MB Xe lamp (400–800 nm) MB acts as the sensitizer of the hybrid Novel polyaniline/Bi2SnTiO7 possessed higher catalytic activity compared with Bi2InTaO7 or pure TiO2 or N-doped TiO2 Polymer coating 85 (2012)
CdS PANI MB Visible (≥450 nm) p–n junction Enhanced visible photocatalytic activity, PANI-showed anti photo-corrosion properties Polymer coating 86 (2010)
CdS P3HT MO Fluorescent lamps (400 nm) p–n junction Photocatalytic enhancement due to suppressed photo-corrosion effect. Polymer coating 87 (2014)
CdS g-C3N4 MO 300 W Xe lamp The injection of electrons from excited g-C3N4 to CdS increased the stability of the hole–electron pair and that ultimately enhanced the photocatalytic activity CdS decorating C3N4 88 (2018)
ZnO PANI MO, 4-CP UV lamps 365 nm p–n junction The interface hybrid structure can boost the photocatalytic activity Polymer coating 89 (2014)
ZnO PT MO 9 W LED lamp (white light, 400–700 nm) and solar light Sensitization The hybrid with high PT content (>20%) showed an increase in the photocatalytic activity Polymer coating 90 (2014)
ZnO PEDOT MB UV (λ = 254 nm) Sensitization Enhanced photocatalytic efficiency with respect to bare materials. Composite 59 (1999)
ZnO g-C3N4 MB Fluorescent lamp Z-scheme ZnO/g-C3N4 (98.83%) composite exhibited higher decolourisation efficiency as compared with pure g-C3N4 (35.21%). Polymer coating 91 (2018)
BiO-Br g-C3N4 RB 198, RB 5, RY 145 Visible and solar Z-scheme 30% pGCN–BiOBr resulted in the highest photocatalytic activity towards the degradation of all the three dyes in the presence of UV, visible and solar irradiation. Composite 92 (2017)
BiVO4 g-C3N4 MB 410 nm LED light Z-scheme 4.6 and 7.2 times higher than that of BiVO4 (0.00938 min−1) and g-C3N4 Composite 93 (2018)
β-Bi2O3 g-C3N4 TC Xe lamp (>420 nm) Z-scheme 5 wt% g-C3N4 loaded core/shell sample (5%CN@BO) showed optimum photocatalytic efficiency, with a rate constant (k = 0.0311 min−1) Polymer coating 94 (2018)
β-Bi2O3 g-C3N4 RhB Xe lamp (>420 nm) Z-scheme 70 wt% g-C3N4/Bi2O2CO3 exhibited the highest activity for RhB degradation, and the apparent reaction rate Composite 95 (2018)
ω-Bi2O3 g-C3N4 Phenol Xe lamp (>400 nm) Z-scheme Bi2O3/g g-C3N4 composite exhibited higher photocatalytic activity than pure Bi2O3 and g-C3N4 Composite synthesis of BiO3 over polymer 96 (2018)
BiOBr/OG g-C3N4 RhB, TC Xe lamp (>420 nm) Z-scheme The total organic carbon (TOC) removal ratios of RhB and TC over 10% BiOBr/RGO/g-C3N4 were 88% and 59%, respectively Composite 97 (2018)
BiOBr g-C3N4 NO Tungsten halogen lamp (>420 nm) Type II BiOBr/g-C3N4 is showed a rate 1.5–2.0 higher than bare materials Composite 98 (2014)
BiWO6 g-C3N4 IBF g-CN/m-BWO (1[thin space (1/6-em)]:[thin space (1/6-em)]4, UTCB-25) reached almost 96.1% removal efficiency of IBF within 1 h, which was about 2.7 times as that of pure m-BWO Composite 99 (2017)
SnCoS4 mpg-C3N4 RHb, MB Xe lamp (>420 nm) Type II Photocatalytic performance of MCN/SnCoS4-50 was much higher than that of pristine materials SnCoS4 decorating g-C3N4 100 (2017)
In2S3 g-C3N4 RhB, MO Tungsten lamp (>420 nm) Type II The In2S3/g-C3N4 nanocomposite showed 8.5 times higher photocurrent density than the single-phase g-C3N4 under visible light In2S3 decorating g-C3N4 101 (2017)
SDAg–CQDs/UCN Ultrathin g-C3N4 (UCN) NPX Xe lamp (>420 nm and 290 nm) and monochromatic 365 nm (3 W) and 740 nm (9 W) Hybrid analyzed as a whole The content of 1.0 wt% of CQDs and 3.0 wt% of Ag resulted in a 10-fold higher reaction rate than that of UCN under visible light irradiation CQDs decorating g-C3N4 sheets 102 (2018)
Ag2CO3 PANI MB Tungsten filament lamp (λ > 400 nm) p–n heterojuction The optimum photocatalytic activity of Ag2CO3–PANI with 50 wt% PANI for the degradation of MB was almost 86%, much higher than that of the pure Ag2CO3 and PANI Composite 103 (2018)
SrO Amorphous C3N4 NOx Tungsten halogen lamp Increased photocatalytic activity SrO cluster decorating the C3N4 surface 104 (2017)
SrSO4 g-C3N4 NO 30 W LED lamp The hybrid showed 3.8-fold higher activity than bare g-C3N4 SrO4 decorating the C3N4 surface 105 (2019)

An important parameter in the design of hybrid materials is their distribution and morphology (Fig. 8). Polymeric capping over the inorganic semiconductor is the most frequent distribution (Fig. 8a, see Table 1). In these cases, the polymers are generally made by in situ polymerization over the ISs, using precursors that allow a good control of the polymeric capping thickness. An alternative method is IS impregnation from a polymer solution. In other cases, hybrid systems are composite materials formed by incorporating an inorganic semiconductor into the polymer or vice versa (Fig. 8b). Also, if the ISs are nanoparticulates, the ISs can be found decorating the polymer surface (Fig. 8c) or even inside the polymer pores. From the synthetic point of view these materials can be achieved by other alternative ways: (i) physical mixture of both semiconductors; (ii) synthesis of the polymer in the presence of ISs or vice versa and; (iii) growth of both materials simultaneously. In the first situation (Fig. 8a), the thickness of the polymer capping seems to play an important role in the photocatalytic activity as it was reported by Mao et al. where a very thin layer of polydopamine (PDA) over TiO2 nanoparticles (ca. 1 nm) was found to be critical for the degradation of RhB under visible light (Fig. 9).77 Although the light absorption ability increases with the PDA thickness, the photocatalytic activity shows reverse capability which means the thicker the PDA layer, the worse the photocatalytic activity. The organic–inorganic interface nature is one of the corner stones of the charge transfer mechanism and therefore, of their photocatalytic activity. Thus, the presence of electron donor elements (such as sulphur or nitrogen) in the backbone of the polymer favours the inorganic particle coating. The S-atoms present in poly(thiophenes) (i.e. PT and P3HT) and N-atoms present in polyaniline (PANI), polypyrrole (PPy) or g-C3N4 have lone electron pairs that can be donated to the inorganic surface. In fact, it has been recently reported that the use of diketopyrrolopyrrole-carbazole-based conjugated polymers (DPP-Car) as capping films and the consequent removal of amine protecting groups increase the interface area causing an increase in photocatalytic activity (Fig. 10).80 This enhancement of interfacial interactions was confirmed by FT-IR, UV/Vis DRS, fluorescence and XPS analysis, and showed the following results: (i) an increased absorption in the 800–1000 nm range, (ii) the generation of Ti3+ and Ti5+ species and (iii) efficient charge transport between DPP-Car and TiO2. Furthermore, this work also revealed that as polymeric molecular weight increase the photocatalytic activity is higher.

image file: c9cs00377k-f8.tif
Fig. 8 Overview of the different morphology scenarios of IS–polymer hybrid materials. The hybrid morphology can be varying depending on the synthetic condition. (a) Polymer can act as a coating over the IS surface by two different synthetic routes such as impregnation of IS with a polymer solution or direct thermal/photochemical/electrochemical polymerization from monomer precursors in the presence of IS. The TEM image shows a 0.71 nm PANI coating of TiO2 nanoparticles (NPs).64 (b) The mixture of both IS and conductive polymer leads to a nanocomposite. The TEM image shows a hierarchical composite based on β-Bi2O3/g-C3N4 synthesized by a combined hydrothermal-calcination approach.95 (c) Also, the IS NPs can decorate a polymer surface, and the HRTEM image shows mpg-C3N4@SnCoS4 prepared by the in situ hydrothermal method indicating a homogeneous dispersion of SnCoS4 nanoparticles on the ordered mesoporous structure of mpg-C3N4.100 The TEM image of panel (a) is used with permission from ref. 64, American Chemical Society, copyright 2008. The TEM image of panel (b) is used with permission from ref. 95, Elsevier, copyright 2018. The TEM image of panel (c) is adapted with permission from ref. 100, Elsevier, copyright 2017.

image file: c9cs00377k-f9.tif
Fig. 9 (a) Photograph image of temporal evolution of RhB solution over TiO2@PDA with a 1 nm PDA shell photocatalyst under visible light irradiation. (b) UV-vis adsorption spectra showing photodegradation of RhB using TiO2@PDA with a 1 nm PDA shell photocatalyst. (c) Evaluation of RhB concentration versus reaction time of different TiO2 photocatalysts. (d) Recycling performance of TiO2@PDA with 1 nm coatings for RhB photo-degradation in 8 cycles. Reproduced from ref. 77 with permission from The Royal Society of Chemistry. Copyright 2016.

image file: c9cs00377k-f10.tif
Fig. 10 The proposed mechanism diagram of enhanced photocatalytic efficiency for the DPP-Car@TiO2 hybrid material. Reproduced from ref. 80 with permission from Elsevier publishing group. Copyright 2018.

Another strategy to improve the photo-induced charge transfer processes and therefore the photocatalytic activity is to force the contact between organic and inorganic semiconductors by means of the formation of covalent bonds. These more effective interactions are favoured by the functionalization of polymers with organic groups (i.e. hydroxyl, carboxylic, etc.) that interact with the hydroxyl groups of metal oxide semiconductor surfaces. An example of this effect is the use of polymers based on catechol moieties,76 or polymers based on dopamine units77 (see polymer structure in Fig. 2) over TiO2 to photodegrade Cr(IV) and Rhodamine B (RhB), respectively. Related to that, Lei et al. reported a poly(vinyl alcohol) (PVA) based hybrid, exhibiting pendant hydroxyl groups capable of coating the TiO2 surface, active towards MO and phenol photodegradation.78 Note that PVA is not a conjugated polymer, but once PVA coated TiO2 the hybrid sample was post-treated this make unsaturated bonds into the PVA backbone. Thus, the new coating polymer, degraded PVA (PVAD), showed conductive properties (see both molecular structures in Fig. 2). Another example of this interfacial engineering to covalently bond the polymer to the inorganic surface was published by Huang et al.73 Prior to 3-hexylthiophene (P3HT precursor) polymerization, TiO2 was treated with thiophene carboxylic acid to form a self-assembling monolayer (SAM). These carboxylic groups interact with the P3HT polymer achieving an increase of the MO removal photocatalytic activity. The SAM decreased the barrier for charge transfer between the TiO2 nanoparticles and the P3HT coating, leading to an improved MO removal photocatalytic activity under visible light when compared with a hybrid based on the direct coating with P3HT.

As was previously commented, the vast majority of examples involve the use of TiO2 as an IS. Also, these examples imply the use of visible light to drive the pollutant photodegradation process where TiO2 is unable to absorb energy (see Table 1 for light source details). Thus, from the charge transfer mechanistic point of view the conjugated polymer acts as a sensitizer in most cases (Fig. 5b). The same situation applies to hybrids containing ZnO, which is also unable to absorb visible light by itself. Thus, a hybrid based on ZnO and PT for the degradation of MO under visible light90 and a hybrid based on ZnO and PEDOT for MB degradation59 show a sensitization charge transfer mechanism. However, ZnO shows a type II mechanism under UV-light exposure when a hybrid with PANI is prepared for MO photocatalytic degradation.89

A special case regarding the charge transfer process is reported by Karthik et al. for a poly(catechol)@TiO2 hybrid used as a photocatalyst for H2 production from water and the removal of Cr(VI).76 Here, poly(catechol) was attached onto the TiO2 surface via photopolymerization using a catechol monomer. This poly(catechol)@TiO2 based hybrid shows a type-II following a ligand to metal charge transfer (LMCT) mechanism, where photoexcited electrons in the HOMO level of catechol are directly injected into the conduction band of TiO2. Furthermore, during LMCT, holes are also promoted to the valence band of TiO2.

The quantum dots (such as CdS) are completely different, because their band gap is shorter than TiO2 and ZnO allowing it to absorb visible light (Fig. 5d). Its band edge positions versus the polymers employed up to now (PANI and P3HT) favour the heterojunction mechanism (Fig. 5b).86,87

On the other hand, when g-C3N4 is part of the hybrid material, the Z-scheme charge transfer mechanism is the most commonly assumed mechanism.91–97 This has been proposed due to the relative band positions of the g-C3N4 compared with the studied inorganic semiconductors as well as by the analysis of the intermediate species. An example is described by Y. Bao et al. using a β-Bi2O3@g-C3N4 hybrid photocatalyst on the degradation of RhB and TC in aqueous solution under visible light irradiation (>420 nm) (Fig. 11).97 In this case, the authors discard the type II mechanism due to the inability to form an oxygen radical anion O2˙ (identified by ESR spectra) since the valence band of BIOBr is below the O2/O2˙ = 0.33 eV redox potential. Thus, the reduction pathway should involve VB holes of g-C3N4 and the oxidation pathway should have CB electrons on BiOBr following the subsequent Z-scheme mechanism.

image file: c9cs00377k-f11.tif
Fig. 11 Two models of charge separation proposed for BiOBr/RGO/g-C3N4 composites under visible irradiation: traditional heterojunction-type (a) and Z-scheme type (b). Reproduced from ref. 97 with permission from Elsevier publishing group. Copyright 2018.

As in the case of previous examples, the most common method to analyse the charge transfer mechanism is based on the evaluation of band gap energy and edge energy of each one of the components of the hybrid system. Another established alternative is to determine the feasible charge transfer mechanism considering the hybrid as a single entity instead as a mixture of two individual materials. Thus, Wang et al. reported a novel ternary photocatalyst composed of single atom-dispersed silver (SDAg) and carbon quantum dots (CQDs), co-loaded with ultrathin g-C3N4 (UCN) (SDAg–CQDs/UCN). This hybrid system exhibited a significant enhancement of photoresponse, a broad absorption spectrum (UV, visible, and near-infrared light) and improved photocatalytic activity for NPX degradation (see molecular structure in Fig. 7).102 The authors established an effective energy band alignment in the hybrid materials, CQDs/UCN and SDAg–CQDs/UCN (Fig. 12), using a combination of electrochemical impedance spectroscopy (EIS) and valence band X-ray photoelectron spectroscopy (VB-XPS) to determine the energy of Fermi level (EF) and its relative difference with the VB energy edge, respectively. Both CB and EF of the hybrid down-shifted to a more positive potential until an equilibrium was achieved, which resulted in a less energetic migration path for the photogenerated electrons (Fig. 12 middle and right). Therefore, the electronic structure of SDAg–CQDs/UCN was expected to facilitate the transport of photogenerated carriers, and thus improve the photocatalytic performance.

image file: c9cs00377k-f12.tif
Fig. 12 Schematic band structure evolution of ultrathin g-C3N4 (UCN) (left), a hybrid based on carbon quantum dots (CQDs), loaded with UCN (CQDs/UCN) (middle), and a hybrid co-loaded with single atom-dispersed silver (SDAg) (SDAg–CQDs/UCN) (right). Reproduced from ref. 102 with permission from Elsevier publishing group. Copyright 2018.

Almost all the hybrid photocatalytic systems used for pollutant removal reported show good recyclability parameters (at least 3-recycling path). However, taking into account the oxidative nature of the inorganic semiconductors (i.e., TiO2, ZnO) one of the long-term drawbacks is the low photostability of some polymers (mainly linear). This is highlighted by Xu et al., who studied the influence of the oxidation degree of poly(3-hexylthiophene) (P3HT) under visible light irradiation on the photocatalytic activity of hybrids based on TiO2.72 They observed that although P3HT is oxidized partially, the P3HT@TiO2 hybrid still presents some photocatalytic activity. While the oxidation of the side chain of P3HT was beneficial and increased the photocatalytic activity of the P3HT@TiO2 hybrid, the corrosion of the P3HT backbone decreased the photocatalytic activity. Although the polymer photodegradation seems not to have a real impact or even can be beneficial in the photocatalytic pollutant removal processes, we must not forget that this issue could have a significant impact on other applications like H2 production or CO2 photoreduction (vide infra), where polymer photodegradation products could be wrongly assigned as hydrogen or CO2 photoreduction products, respectively.

An important issue in this section dedicated to pollutant removal is the photocatalytic elimination of NOx. Nitrogen(II) oxide and nitrogen(IV) oxides (NOx) are very harmful and poisonous gases emitted primarily from combustion processes. NOx pollution is one of the most important environmental and health problems in industrialized and high traffic areas. In fact, many governments set up very strict laws to eliminate NOx. Among the possible atmospheric NOx removal strategies, photocatalytic technology is a cost-effective and environmentally friendly way that uses solar energy as an energy source.106 Over the past few years, a lot of ISs (mainly TiO2) have been used as photocatalysts for this purpose.107 Furthermore, conjugated polymers have been receiving attention to be used for this application, and several examples can be found generally based on g-C3N4 as photocatalysts.108,109 However, just a few examples with hybrid or heterojunctions composed of an inorganic semiconductor and a polymer have been found (Table 1). For instance, BiOBr/C3N4 nanojunctions show high photoactivity for the removal of NO in air under visible light irradiation, which can be ascribed to the highly efficient separation of photo-induced charges at the interface of nanojunctions.98 Also, celestine mineral (SrSO4) particle modified g-C3N4 exhibits highly enhanced photocatalytic activity, under visible light irradiation.105 The introduction of an appropriate amount of celestine can improve the surface area (SBET) which allows the catalytic material to adsorb and transfer more NO reactant, providing more active sites for the photocatalytic reaction. The combined effect of the amorphous carbon nitride structure and SrO clusters causes outstanding photocatalytic NO removal efficiency.104

Considering all this information, an important conclusion can be reached: in every pollutant removal study, OIHs show an increase in their photocatalytic activity versus the respective bare materials.

3. Solar fuel production by artificial photosynthesis

3.1. Photocatalytic H2 production

In the past few years, the application of hybrid materials for H2 photocatalytic production from H2O in the presence or absence of an electron scavenger has strongly emerged (eqn (1)–(3)). Without doubt, it is a green and friendly route to store and convert energy into a fuel such as H2. However, beyond examples of particulate systems based on g-C3N4 and some inorganic semiconductors,58 a review on hybrid heterogeneous catalysts formed by conductive polymers and their inorganic counterparts has not yet been reported.
2H2O → O2 + 4H+ + 4e, E0 = 1.23 V vs. NHE(1)
4H+ + 4e → 2H2, E0 = 0 V vs. NHE(2)
2H2O → O2 + 2H2, ΔG0 = +237.2 kJ mol−1(3)
For an efficient overall water splitting reaction, photocatalyst performance is strongly determined by the band structure, i.e. appropriate conduction band (CB) and valence band (VB) energy regarding redox reactions (see Fig. 5a), as well as the charge separation capabilities. Thus, in terms of energy, the overall water splitting is an uphill process, which requires the photocatalyst to have a wide enough band gap (>1.23 eV) (see eqn (1) and (2)) to overcome the Gibbs free energy (see eqn (3)). Indeed, a larger band gap (>1.6 eV) is actually needed due to the additional overpotential associated with the electron transfer and gas evolution steps.58,110

In addition, the photocatalyst should maintain good optical absorption capacity in a wide range of wavelengths for efficient solar spectrum utilization. Obviously, these counteractive requirements are very difficult to find in just one semiconductor. Therefore, it is necessary to explore a new type of photocatalytic system that can be combined with the classical ones in order to satisfy all the aforementioned requirements. OIH photocatalysts open the door to these new configurations.

Finally, water splitting is kinetically restricted by the oxygen evolution reaction (OER) (eqn (1)). The formation of molecular oxygen (O2) is a kinetically slow process which involves complex multielectron transfers and requires a large overpotential. Developing viable and efficient catalysts for the O2 evolution reaction (OER) has proven to be rather difficult, although investigations into the photocatalytic OER process could be potentially useful for H2 production and CO2 fixation.123 In the literature it is possible to find just a few examples related to conductive polymer–inorganic hybrid systems as photocatalysts where the OER is reported. These examples imply the use of an inorganic catalyst based on cobalt.120,121,124 For instance, graphitic carbon nitride, g-C3N4, hybrids exhibit higher O2 evolution rates when loaded with cobalt-oxide-phosphate (CoPi) or CoO.120,121 Furthermore, Wang et al. have reported a hybrid based on low band gap conjugated microporous polymer nanosheets (aza-CMP) using Co(OH)2 as a cocatalyst (3 wt%) showing the highest O2 evolution rates under visible light reported.125,126

An alternative to improve the hydrogen evolution reaction (HER) rates is the use of a sacrificial agent (hole scavengers such as methanol, sodium sulphite or amines, among others) for the oxidation reaction instead of oxygen oxidation. Some relevant examples of this process are summarized in Table 2 and some of the polymeric structures employed are depicted in Fig. 3 and 4. Note that there are a lot of examples using graphitic carbon nitride (g-C3N4) based hybrids used for hydrogen generation, also compiled in a recent review,41 but examples with hybrids based on other types of polymeric networks are scarce being an attractive and promising alternative. Also it is an important highlight that there are no examples of hybrids based on the most popular linear polymers used in pollutant removal (i.e. P3HT PANI, PPy, etc.), maybe due to the recent development of CPPs for this application.

Table 2 Some relevant examples of hybrid materials used in photocatalytic hydrogen production from water
IS CP Cocatalyst Reaction conditions Morphology Light source Charge transfer mechanisma Formation rate (units) Cycles (time per cycle (h)) AQYb (%), (λ (nm)) Ref.
a Here we include the mechanism name as appearing in the original scientific paper. Note that type II and p–n junction refer to the same mechanism. b AQY, apparent quantum yield AQY (%) = (2 × number of evolved H2 molecules)/(number of incident photons) × 100. c LMCT means ligand to metal charge transfer complex. d Pt is pre-loaded on TiO2 by photoreduction. e Pt loaded on hybrid photocatalyst. f TiO2 nanosheet. g Less than 3 wt% of polymer loading TiO2. h N-TiO2 means N doped TiO2 nanofibers. i In this work long term experiments were carried out.
TiO2 Polycatechol 5 vol% TEOA aq. Polymer coating Solar light LMCTc 10.925 mmol g−1 h−1 5 (2) N/A 76 (2017)
TiO2 B-BT-1,4-E 0.03 wt% residual Pd TEOA aq. Polymer as coating Xe-lamp (λ ≥ 420 nm) Sensitization 7.333 mmol g−1 h−1 5 (5) 1.91 (420) 82 (2018)
TiO2 B-BT-1,4-E 1 wt% Au 10 vol% TEOA Polymer flakes decorated with TiO2 nanocrystals Xe-lamp (λ ≥ 420 nm) Sensitization 26.640 mmol g−1 h−1 6 (3)i 7.8 (420) 111 (2018)
TiO2-TiH2black TiO2 B-BT-1,4-E residual Pd 10 vol% TEOA Polymer as coating Xe-lamp (λ ≥ 420 nm) Type II heterojunction 15.604 mmol g−1 h−1 6 (4)i 3.36 (420) 112 (2019)
TiO2 BFB or BFBA 0.4 wt% residual Pd TEOA aq. Polymer as coating Xe-lamp (λ ≥ 420 nm) Sensitization 3.670 mmol g−1 h−1 5 (5) 1.6 (420) 113 (2019)
7.333 mmol g−1 h−1 2.46 (420)
TiO2 CMPBBT 0.5 wt% Ptd TEOA Polymer as coating Xe-lamp (λ ≥ 420 nm) Sensitization 5.933 mmol g−1 h−1 N/A N/A 81 (2017)
TiO2f COP64 3 wt% Pte 10 vol% MeOH aq. Compositeg Xe-lamp without a UV-cut off filter p–n junction 15.020 mmol g−1 h−1 5 (3) N/A 114 (2017)
TiO2 g-C3N4 Certain amount of Pt 10 vol% TEOA aq. rotating frame Composite Xe-lamp without a UV-cut-off filter Z-scheme 13.8 mmol h−1 m−2 N/A N/A 115 (2018)
N-TiO2h g-C3N4 Pt (amount no provided) 20 vol% MeOH aq. Polymer as coating Simulated solar light p–n junction 8.931 mmol g−1 h−1 N/A N/A 83 (2015)
TiO2 B-Doped g-C3N4 20 vol% MeOH aq. TiO2 synthesized over polymer Xe-lamp (λ ≥ 420 nm) Sensitization 0.150 mmol g−1 h−1 N/A 3.08 (420) 84 (2016)
Black-TiO2 g-C3N4 20 vol% MeOH aq. Composite Simulated solar light Type II 0.558 mmol g−1 h−1 5 (5 h) N/A 116 (2017)
Cd0.5Zn0.5S TCP Na2S/Na2SO3 (0.35 M) aq. Composite Cd0.5Zn0.5S decorating polymer Xe-lamp (λ ≥ 420 nm) n–n junction 50.670 mmol g−1 h−1 5 (3 h) N/A 117 (2018)
CdS B-BT-1,4-E 1 wt% of Pt Na2S/Na2SO3 aq. Polymerized over IC Xe-lamp (λ ≥ 420 nm) Z scheme 100.200 mmol g−1 h−1 7.5 (420) 118 (2018)
ZnO/ZnS g-C3N4 0.25 M Na2S/0.25 M Na2SO3 aq. Composite Simulated solar light Double Z-scheme 0.301 mmol g−1 h−1 N/A N/A 115 (2018)
α-Fe2O3 g-C3N4 3 wt% of Pt 10 vol% TEOA aq. Polymer as coating Xe lamp (λ > 400 nm) Z scheme 31.400 mmol g−1 h−1 2 (5) 44.3 (420) 119 (2017)
CoO g-C3N4 3 wt% of Pt 10 vol%TEOA aq. Composite CoO loaded over g-C3N4 Xe lamp (λ > 400 nm) p–n junction 0.651 mmol g−1 h−1 3 (5) N/A 120 (2017)
CoPi g-C3N4 Ag (NO3) No sacrificial agent Co and Ag loaded over polymer Visible light Sensitization 0.626 mmol g−1 h−1 N/A N/A 121 (2013)
WO3 g-C3N4 1 wt% of Pt 10 vol%TEOA aq. sol 2D g-C3N4 layers stand vertically on the flat facets of WO3 nanocuboids Xe lamp Z-scheme 3.120 mmol g−1 h−1 4 (3) N/A 122 (2017)

As happens in the case of pollutant remediation, TiO2 is the most used inorganic semiconductor. In addition, it should be mentioned that, for solar fuel production, noble metal nanoparticles are usually employed as reduction cocatalysts. These cocatalysts can act as electron scavengers (enhancing electron–hole separation) and also extend the light absorption of the system to the visible region due to the localized surface plasmon resonance present on it, improving the photocatalytic results.14,15,127 The most suitable cocatalyst for hydrogen evolution is Pt, which exhibits a high work function, low Fermi level and strong electron extracting capacity. In addition, it is easy to load it by photodeposition from a molecular precursor. The comparison between published results is not easy because in most of the cases a lack of information of the utilized operation conditions is observed. Thus, H2 generation is reported as accumulative production without indication of the reaction time or illumination parameters (such as wavelength and irradiance). To facilitate this comparison we have transformed the literature data (when it is possible) using the units mmol of H2 per hour and grams of catalyst (see Table 2). Herein, we summarized the most relevant results for H2 production reported in the literature.

Yang et al. described a hybrid system based on CMP constituted by pyrene moieties (COP-64, Fig. 4), in contact with TiO2 nanosheets (Fig. 8b) and with 3% Pt as a cocatalyst, which exhibited the highest hydrogen generation (ca. 15.020 mmol g−1 h−1).114 Hou et al. reported hydrogen production (ca. 5.933 mmol g−1 h−1) using a system based on CMP containing benzothiadiazole building blocks (CMPBBT, Fig. 4), where the polymeric material was polymerized over TiO2 forming a crosslinked coating (Fig. 8a); furthermore 0.5 wt% Pt was used as a co-catalyst.81 The same group reported a hybrid photocatalyst (TiO2@B-BT-1,4-E) based on TiO2 capped by a linear polymer, formed by benzothiadiazole monomers (Fig. 3) and analogous to CMPBBT and 0.03 wt% Pd, which exhibited an H2 production of 7.333 mmol g−1 h−1.82 The increase in photocatalytic activity is ascribed to the close interface contact achieved between TiO2 and the linear polymer compared with the crosslinked polymer coating. Another interesting example using the (B-BT-1,4-E) polymer was performed by Xiao et al.111 that evaluated the effect of Au nanoparticles as a cocatalyst. In this work the (B-BT-1,4-E)–Au–TiO2 hybrid photocatalyst attained a H2 production of 26.640 mmol g−1 h−1 (AQY of 7.8% at 420 nm) which is ∼1.9 and ∼68 times faster than that of (B-BT-1,4-E)–TiO2 and Au–TiO2, respectively.

However, due to the high cost of noble metals it is of vital importance to achieve metal free photocatalyst systems. Several examples are published without cocatalysts such as the work of Karthik et al. using TiO2–polycatechol hybrid systems with a H2 production of 10.925 mmol g−1 h−1.76

Also, as was commented previously in the environmental applications, light driven hydrogen production can be boosted by increasing the interfacial charge transfer by covalent bonds between the CP and the IS. This is the case described by Chen et al. where a donor–acceptor conjugated polymer composed of alternative units of benzene and 2-fluorobenzene (BFB) is functionalized with benzoic acid as an end-chain monomer (BFBA).113 Thus, the hybrid BFBA–TiO2 shows a H2 production of 7.333 mmol g−1 h−1 (AQY = 2.46 at 420 nm) versus 3.670 mmol g−1 h−1 (AQY = 1.6 at 420 nm) recorded by TiO2–BFB.

On the other hand, a stimulating example was described by Zhang et al. using B-BT-1,4-E and black titania obtaining a H2 evolution of 15.604 mmol g−1 h−1 (AQY = 3.36 at 420 nm). This production is ∼30% higher than that of the same polymer combined with TiO2 nanoparticles (B-BT-1,4-E–TiO2).112 This enhancement was attributed to the formation of sub band gap states in black titania. These intermediate states are responsible for the narrow band-gap and visible light absorption for black-TiO2, acting as electron traps and accelerating the charge transfer from B-BT-1,4-E to TiO2.

Although scarce, other examples different from TiO2 are also published. She et al. reported a hybrid photocatalyst based on α-Fe2O3 and g-C3N4 with 3 wt% Pt as a cocatalyst whose H2 production was 31.400 mmol g−1 h−1 (AQY = 44.3% at 420 nm).119 This high production was attributed to the morphology of the hybrid photocatalyst which played an important role in the photocatalytic activity as is highlighted by recent studies focused on the use of g-C3N4 with atomistic thickness.128,129 In this case the presence of α-Fe2O3 nanoparticles decorating the g-C3N4 nanosheets (Fig. 8c) promote the g-C3N4 exfoliation, leading to higher activity 2D hybrid photocatalysts (Fig. 13).

image file: c9cs00377k-f13.tif
Fig. 13 (a) Scheme of the proposed synthetic route to produce the α-Fe2O3/2D g-C3N4 hybrids. The presence of iron oxide is essential to form 2D structures. (b) AFM of pure 2D g-C3N4, obtained after etching away α-Fe2O3 using HCl. Scale bar: 1 μm. (c) Photocatalytic H2 evolution over carbon-nitride-based hybrid samples under visible light irradiation, using Pt (3%) as a co-catalyst. (d) Turnover frequency of different materials. Reproduced and adapted from ref. 119 with permission from Wiley. Copyright 2017.

Q. Liang et al. reported elevated H2 production (ca. 50.670 mmol h−1 g−1) using a highly stable hybrid photocatalyst based on the synergic interaction between a triptycene-based covalent polymer (TCP) (Fig. 4) and a Cd0.5Zn0.5S (CZS) quantum dot as an inorganic semiconductor.117 This high photocatalytic H2 production rate and superior photostability were achieved without the need for noble metals. The enhanced H2 evolution over novel TCP–CZS heterojunction composites was ascribed to the inherent structural properties (free volume and 3D structure of the triptycene), porous morphology, high surface area (SBET = 1729 m2 g−1) and good dispersion in the reactive media of TCP nanoparticles. Due to the highly effective interfacial interaction between the TCP and CZS, the photoexcited electrons in the LUMO of the TCP are transferred to the CB of CZS, and the holes in the VB of CZS are rapidly injected into the HOMO of the TCP, facilitating efficient separation and migration of photogenerated electron–hole pairs (Fig. 14). In addition, the CB of CZS (−0.65 eV vs. NHE) is more negative than the redox potential of H+/H2, resulting in the fast and efficient electron transfer to generate H2.

image file: c9cs00377k-f14.tif
Fig. 14 (a) Valence band XPS spectra of the TCP and Cd0.5Zn0.5S (CZS); (b) the electronic band structure of the TCP and Cd0.5Zn0.5S (left) and schematic illustration of photocatalytic H2 production over the TCP–CZS heterojunction under visible light irradiation (right). Reproduced from ref. 117 with permission from The Royal Society of Chemistry. Copyright 2018.

To date, the highest recorded H2 production using a hybrid photocatalyst (ca. 100.200 mmol h−1 g−1) has been achieved by Zhang et al. who described a system composed of conjugated polybenzothiadiazole (B-BT-1,4-E, Fig. 3) flakes and CdS nanorods (BE-CdS), in the presence of 1 wt% Pt as a cocatalyst.118 The results in the absence of the Pt cocatalyst exhibited a H2 production rate of 40.000 mmol h−1 g−1 (AQY = 7.5% at 420 nm) which is 8.3 and 23.3 times higher than that presented for CdS and B-BT-1,4-E respectively.

The authors propose a Z-scheme electron transfer mechanism with a rapid charge transfer of photogenerated electron from the CdS to the polymer. This mechanism is evidenced by the detection of Pt particles, deposited through an in situ photodeposition over the B-BT-1,4-E, indicating that the reduction steps takes place on the polymer surface.

As a summary, photocatalytic hydrogen production from water by the use of hybrid materials based on conjugated polymers and an inorganic semiconductor is an interesting and continuously expanding research area. The emergence of alternative polymeric networks to g-C3N4 such as conjugated porous polymers opens the door to a new study field.

3.2. Photoreduction of CO2

The mitigation of CO2 anthropogenic emission is one of the most significant challenges of this century as is illustrated by the Paris Agreement (COP21) and ratified in subsequent COPs. According to the International Energy Agency, the full implementation of climate pledges to hold the increase in the global average temperature at 1.5 °C with respect to preindustrial values needs a collective effort. The actions proposed are mainly focused on CO2 capture and valorisation. To reach this ambitus objective, one of the most desirable and challenging processes is CO2 photoreduction to produce fuels or value-added chemicals.130,131

However, CO2 is a very stable molecule with a high C[double bond, length as m-dash]O bond dissociation energy (750 kJ mol−1) and therefore, this energy needs to come from renewable sources in order to make CO2 reduction a sustainable process. The direct photoreduction of the CO2 molecule in the gas phase is really unfavourable from the thermodynamic point of view due to the high negative redox potential of CO2/CO2˙ (−1.90 V vs. NHE, at pH 7.00) (see eqn (4)). Nevertheless, proton-assisted CO2 photoreduction is easier because of the relatively lower potential (vs. NHE, at pH 7.00), as shown in eqn (5)–(10) and Fig. 5d. The CO2 mechanism, which is still a matter of discussion, is quite complex and leads to a great variety of photoproducts from CO, CH4 and low chain hydrocarbons to oxygenated species (such as formic acid formaldehyde, methanol and ethanol) (eqn (5) and (10)).132

CO2 + e → CO2˙E0 = −1.90 V(4)
CO2 + 2H+ + 2e → CO + H2O E0 = −0.53 V(5)
CO2 + 2H+ + 2e → HCOOH E0 = −0.61 V(6)
CO2 + 4H+ + 4e → HCHO + H2E0 = −0.48 V(7)
CO2 + 6H+ + 6e → CH3OH E0 = −0.38 V(8)
CO2 + 8H+ + 8e → CH4 + 2H2O E0 = −0.24 V(9)
2CO2 + 12H+ + 12e → C2H5OH + 3H2E0 = −0.33 V(10)
Besides these thermodynamic requirements, there is an activation barrier in the charge transfer process due to interface interactions between reagents and semiconductors. The most used semiconductor for CO2 photoreduction is TiO2 (more than 50% of published studies)8,56,131,133–137 and examples using hybrid photocatalysts are still scarce in the literature (Table 3).

Table 3 Some relevant examples of hybrid materials used in CO2 photoreduction
IS Polymer Cocatalyst Reactor Preparation details Morphology Light source Charge transfer Rate (units) AQY (%), λ (nm) Ref. (year)
a In this case the CO2 was generated in situ by reaction of NaHCO3 and H2SO4. b In order to generate Al–O bridges.
N-TiO2 g-C3N4 No metal cocatalyst Gas–solid From thermal treatment of mixed urea and Ti(OH)4 (7[thin space (1/6-em)]:[thin space (1/6-em)]3) wt/wt Composite Xe lamp (UV-Vis) Type II heterojunction CO: 11.91 μmol g−1 h−1 N/A 143 (2014)
4 times higher than that of P25
TiO2 (P25) PANI 0.2 wt% Pt Gas–solid In situ oxidative polymerization of PANI Polymer coating Xe lamp (320–780 nm) N/A CO: 0 μmol g−1 h−1 N/A 138 (2015)
CH4: 50 μmol g−1 h−1
H2: 320 μmol g−1 h−1
3.3 (CH4) and 2.8 (H2) times higher than those of Pt–TiO2
TiO2 B-Doped g-C3N4 No metal cocatalyst Gas–liquid TiO2 synthesized by sol–gel over polymer Composite Xe lamp (>420 nm) Sensitization CH4: 131.25 μmol g−1 h−1 1.68, (420 nm) 84 (2016)
Bi2WO6 PANI No metal cocatalyst Gas–liquid In situ chemically oxidative polymerization method N/A Xe lamp (>420 nm) Type II heterojunction CH3OH: 56.5 μmol g−1 h−1 0.0066 (475 nm) 139 (2015)
CH3CH2OH: 20.5 μmol g−1 h−1
Bi2WO6 PPY N/A Xe lamp (>420 nm) Type II heterojunction CH3OH: 30.4 μmol g−1 h−1 0.0053 (475 nm)
CH3CH2OH: 13.7 μmol g−1 h−1
Bi2WO6 PT Polymer coating Xe lamp (>420 nm) Type II heterojunction CH3OH: 44.3 μmol g−1 h−1 0.0086 (475 nm)
CH3CH2OH: 15.5 μmol g−1 h−1
2.8 times higher than that of Bi2WO6
Bi2WO6 g-C3N4 No metal cocatatalyst Gas–solid Bi2WO6 growth on g-C3N4 Composite Xe lamp (>420 nm) Z-scheme CO: 51.8 μmol g−1 h−1 N/A 144 (2015)
22 and 6.4 times that over pure g-C3N4 and Bi2WO6, respectively
BiOI g-C3N4 No metal cocatalyst Gas–solid 7.4-BiOI/g-C3N4 (means 7.4 wt% of BIOI) Composite BiOI growth on g-C3N4 Xe lamp (>420 nm) Z-scheme CO: 3.44 μmol g−1 h−1 N/A 145 (2016)
CH4: 0.164 μmol g−1 h−1
H2: 0.37 μmol g−1 h−1
O2: 1.89 μmol g−1 h−1
Bi2O2CO3/CoFe2O4 g-C3N4 No metal cocatalyst Gas–solid Synthesis of ISs over g-C3N4 Composite Xe lamp (>400 nm) Type II heterojunction CH4: 119 μmol g−1, N/A 146 (2018)
CO: ∼131 μmol g−1
O2: ∼242 μmol g−1
ZnO g-C3N4 No metal cocatalyst Gas–liquid ZnO deposited on g-C3N4 Composite Xe lamp (>420 nm) Type II heterojunction CO: 30 μmol g−1 h−1 N/A 147 (2015)
CH3OH: 10 μmol g−1 h−1
CH4: 4 μmol g−1 h−1
4.9 and 8.2 times higher than those of g-C3N4 and P25, respectively
WO3 g-C3N4 Ag or Au 0.5 wt% Gas–liquid 50% of each compound in a planetary mill Composite LED lamp (435 nm) Z-scheme CH3OH: 104.16 μmol g−1 h−1 N/A 148 (2014)
In2O3 g-C3N4 0.5 wt% Pt Gas–solid In situ growth of In2O3 nanocrystals onto the sheet-like g-C3N4 surface (best result 10 wt%) Composite In2O3 decorating g-C3N4 Xe lamp (UV-Vis) Type II heterojunction CH4: 159.2 ppm (20 mg catalyst, 4 h irradiation) N/A 149 (2014)
CeO2 g-C3N4 No metal cocatalyst Gas–solid 3% CeO2/g-C3N4 Composite Xe lamp (UV-Vis) Type II heterojunction CO: 118 μmol g−1 h−1 N/A 150 (2016)
CH4: 13.88 μmol g−1 h−1
Recycling experiment: reactivation by heating
SnO2−x g-C3N4 No metal cocatalyst Gas–liquid Calcination of g-C3N4 and Sn6O4(OH)4. Obtaining 42.2 wt% SnO2 Composite Xe lamp (UV-Vis) Z-scheme CO2 conversion: 22.7 μmol g−1 h−1 N/A 151 (2015)
CO: 17 μmol g−1 h−1
CH3OH: 5 μmol g−1 h−1
CH4: 2 μmol g−1 h−1
4.3 and 5 times higher than those of g-C3N4 and P25, respectively.
Sb doped SnO2 g-C3N4 No metal cocatalyst Gas–liquid Mixture of bare materials Sb doped SnO2 decorating g-C3N4 Xe lamp (>420 nm) Type II heterojunction CO: 4.49 μmol g−1 h−1 N/A 152 (2018)
CH4: 0.60 μmol g−1 h−1
21.38 times higher than that of g-C3N4 and 12.83 times higher than that of an inorganic material
NaNbO3 g-C3N4 Pt 0.5 wt% Gas–solid g-C3N4 polymerized over NaNbO3 nanowires Polymer coating Xe lamp (>420 nm) Type II heterojunction CH4: 6.4 μmol g−1 h−1 N/A 153 (2014)
8-Fold higher activity than that of naked g-C3N4.
Ag3PO4 g-C3N4 No metal cocatalyst Gas–liquid Ag3PO4 (10–50%) loading g-C3N4 Composite Simulated sunlight Z-scheme CO2 conversion: 57.5 μmol g−1 h−1 N/A 154 (2014)
CO: 45 μmol g−1 h−1
CH3OH: ∼9 μmol g−1 h−1
α-Fe2O3 g-C3N4 No cocatalyst Gas–solida Impregnation–hydrothermal method Composite Visible light Z-scheme CO: 27.2 μmol g−1 h−1 N/A 155 (2018)
2.2 times higher than g-C3N4
α-Fe2O3 g-C3N4 AlCl3b Gas–liquid Mixture of bare materials Composite Xe lamp (no filter) Z-scheme CO: ∼24 μmol g−1 h−1 N/A 156 (2018)

It should be kept in mind that the use of conjugated polymers as single photoactive materials for CO2 photoreduction is relatively new. This is mainly due to the low photostability of polymers as previously commented in this review. This instability could cause an overestimation in the quantification of obtained products due to the generation of polymer photodegradation products. For this reason, only verified photostable polymers are good candidates for this application.

Despite these remarks, some linear conjugated polymers such as PANI, PPy and PT have been reported as part of hybrid systems with inorganic semiconductors. Liu et al. reported a combination of polyaniline with TiO2 showing a significant enhancement of the photocatalytic reduction of CO2 with H2O. The reasons behind this enhancement were related to the increased CO2 chemisorption at the hybrid with respect to the parent materials and the facilitated separation of photogenerated electron–hole pairs.138 As compared to those for TiO2, the 0.85% PANI–TiO2 hybrid photocatalyst shows rates of CO, CH4 and H2 formation 2.8, 3.8 and 2.7 times higher, respectively. Since PANI by itself showed low activity, this result suggests a significant synergistic effect between TiO2 and PANI for the reduction of CO2 in the presence of H2O. Furthermore, the loading of these composite with noble metal Pt nanoparticles, a Pt–0.85% PANI–TiO2 photocatalyst, led to H2 (∼320 μmol g−1 h−1) and CH4 (∼50 μmol g−1 h−1) production, which were 3.3 and 2.8 times higher than those achieved over the Pt–TiO2 catalyst. Note that in this case CO was not detected. This work constitutes, as far as we know, the first example in the literature of a linear conjugated polymer as a part of an organic–inorganic hybrid system for CO2 photoreduction. Moreover, high PANI stability, as a part of the hybrid even in the presence of Pt, was confirmed by FTIR analysis before and after 10 cycles.

In a later study, Dai et al. use a photocatalyst based on Bi2WO6 hierarchical hollow microspheres (HHMS) modified with different conducting polymers (poly(aniline), poly(pyrrole), and poly(thiophene)) by ‘in situ’ oxidative polymerization deposition.139 They found that the introduction of conducting polymers avoid the recombination of photogenerated electron–hole pairs, enhancing the photocatalytic activity of inorganic semiconductors. Among all photocatalysts, poly(thiophene) modified Bi2WO6 (PT/Bi2WO6) exhibits the best optoelectronic properties and photocatalytic performance, due to the narrow band gap and good charge mobility of polythiophene leading to the production of methanol and ethanol (56.5 and 20.5 μmol gcat−1 respectively) as main products after 4 h of reaction. The total yield of products was 2.8 times higher than when using bare Bi2WO6. Also in this case, PT was found to be stable after 5 recycling cycles by FTIR analysis.

Once again, g-C3N4 is the most studied conjugated polymer as part of an hybrid system for CO2 photoreduction mainly due to its robust networked structure and ability for CO2 activation.33,43,140 Modification of g-C3N4 with Ru140 or Co141 complexes or even Mg142 allows extension of its use under visible light. Other alternatives to improve the optical absorption and to create a surface molecular heterojunction able to promote charge separation is the use of barbituric acid (CNU-BAX).157

One of the most recent contributions described a hierarchical direct Z-scheme system consisting of urchin-like hematite and carbon nitride (α-Fe2O3/g-C3N4).155 This hybrid enhances the photocatalytic activity of the CO2 reduction, yielding a CO evolution rate of 27.2 μmol g−1 h−1 which is >2.6 times higher than that produced by bare g-C3N4 (10.3 μmol g−1 h−1). This is a remarkable result obtained without the use of metal cocatalysts or sacrificial agents. The suggested Z scheme mechanism (see Fig. 5c) is supported by the examination of the band structure and ESR results (Fig. 15). In general, the α-Fe2O3/g-C3N4 hybrid photocatalyst is described as a traditional double-transfer structure (see Fig. 5b). Due to the matched band structure, an inner electrical field is established from g-C3N4 to α-Fe2O3. When exposed to visible light, the excited electrons are transferred from the CB of g-C3N4 to the CB of α-Fe2O3, while the photoinduced holes can be transferred from the VB of α-Fe2O3 to VB of g-C3N4. Considering the fact that as the CB potential of α-Fe2O3 (0.28 V vs. NHE) is more positive than the standard potential of O2/˙O2 (−0.33 V vs. NHE), ˙O2 can’t be thermodynamically generated via this pathway (Fig. 15C). Similarly, the VB potential of g-C3N4 (1.57 V vs. NHE) is more negative than those of H2O/˙OH (1.99 V vs. NHE) and OH/˙OH (2.34 V vs. NHE)), and thus, the production of ˙OH via this pathway is also forbidden. In addition, the internal static electric fields of α-Fe2O3 promote the movement of the excited electrons in the CB of α-Fe2O3 towards the interface to combine with the photoinduced holes in the VB of g-C3N4, preserving the strong reducing power of the electrons in the CB of g-C3N4 and the oxidizing power of the holes in the VB of α-Fe2O3. Thus, an increased density of ˙O2 and ˙OH signals could be detected by ESR analysis.

image file: c9cs00377k-f15.tif
Fig. 15 (A and B) DMPO spin-trapping ESR spectra recorded for ˙OH (A) and ˙O2 (B) under visible light for g-C3N4, α-Fe2O3, and α-Fe2O3/g-C3N4, and (C) band energy alignment and (D) Z-scheme photocatalytic system. Reproduced from ref. 155 with permission from Wiley. Copyright 2018.

Previously to these investigations, Wang et al. published a study with a similar hybrid system based on α-Fe2O3 and g-C3N4 which produces CO (∼15.8 μmol g−1 h−1), CH4 (∼3.1 μmol g−1 h−1) and O2 (∼18.5 μmol g−1 h−1).156 The differences in the photocatalytic production could be ascribed to the absence of a hierarchical structure in the α-Fe2O3 material. The same study described a pre-treatment of α-Fe2O3 with an AlCl3 solution to enhance α-Fe2O3/polymer interactions in the interface by the formation of Al–O bridges. The obtained sample shows CO production (∼24 μmol g−1 h−1), which is 1.5 times higher than the one obtained for the untreated material, and at the same level as the result obtained by Jiang et al.155

Other polymeric networks as alternative photoactive materials to g-C3N4 have been proposed recently. Some of them have been proven alone without an inorganic semiconductor counterpart. Thus, Lin et al.158 described a covalent organic framework (COF), comprising cobalt porphyrins, for photocatalytic CO2 reduction in water. This material exhibits a 26-fold improvement in activity compared with the molecular cobalt complex, with no degradation over 24 hours. In the same line, Shen et al. have proposed several conjugated porous polymers based on a pyrene core with different aromatic substituents159 (see the molecular structure of one of them, PC1, in Fig. 4). The catalytic system involving a task-specific ionic liquid, [P4444] [p-2-O], was developed for direct CO2 capture from air and its further photoreduction to CO under visible light irradiation, affording a CO production rate of 47.37 μmol g−1 h−1 with a selectivity of 98.3%. Also, Yang et al. described three triazine-based conjugated microporous polymers to capture, activate and reduce CO2 to CO upon irradiation with visible light.160 Due to their similarities with g-C3N4, robustness and photoactivity, together with the great diversity of molecular structures accessible by organic synthesis, it may be expected that in the near future, hybrids based on this kind of network polymer would be used for this application, as happens for the hydrogen production process.

Note that, to the best of our knowledge, all the examples of OIH used for CO2 photoreduction found in the literature use linear conjugated polymers and that beyond g-C3N4 no examples of the use of conjugated porous polymers (CPPs) as part of hybrid materials can be found.

3.3. Photoelectrochemical processes

The basic principle behind photoelectrochemical (PEC) cells is the use of solar energy to produce water splitting or reduce CO2 by illuminating one or two semiconductor materials, which are immersed in an electrolyte containing a redox couple. The principal difference from a photocatalyst is that the reduction and oxidation reactions occur in different places (semi-reactions at the anode and cathode), Fig. 16. This division creates an electrical field which contributes to the charge separation in the photoelectrode limiting electron–hole recombination. Ideally, the light captured by the photoelectrodes provides energy to drive both semi-reactions but sometimes an additional external voltage is applied to facilitate the reaction. Nevertheless, this applied voltage is always smaller than that needed to drive the reactions under pure electrochemical conditions. There are different possible configurations for PEC cells, such as the use of a single photoelectrode and a metal counter electrode or both photoactive electrodes (tandem cell). Although many materials have been tested to date (mainly inorganic semiconductors), the conversion efficiencies using a single material remain relatively poor so far.161 From all the different strategies that have been used to improve the performance of PEC cells until now, the use of an organic semiconductor polymer to build hybrid photoelectrodes has only been discreetly addressed yet, as can be seen in Table 4.
image file: c9cs00377k-f16.tif
Fig. 16 Schematic illustration of the electronic band structure of materials assembled in hybrid photoelectrodes and the photogenerated charge transfer mechanism for hydrogen generation. (a) Photocathode. The IS or CP (showed as green) absorbs light and generates charge separation. The energy band edge positions of the rest of materials involved allow both hole and electron transport to the support or reaction media, respectively. The layer disposition allows the HER (reduction pathway). (b) Photoanode. In this case hole transport to the media is able to produce water oxidation thankfully to the band position of the ISs and CPs layers. Note that redox levels for both hydrogen evolution reaction (HER, red dashed line) and oxygen evolution reaction (OER, blue dashed line) are also shown.
Table 4 Hybrid electrodes in photoelectrochemical processes
Inorganic semiconductor Polymer Cocatalyst Type of photoelectrode Contra-electrode Light source Max photocurrents density/cm2 H2 production rate Ref. (year)
a NTiO2 = TiO2 nanotubes. b NRGO nitrogen doped graphene. c The authors included evolved H2 quantified by gas chromatography but used it to calculate the faradaic efficiency; they not present experimental H2 production in the manuscript. d The authors show a singular injection chromatograph in ESI but they do not show a quantified flow data number. e The authors present only the linear sweep voltammetry photocurrent not stationary ones. f The authors present photocatalytically evolved H2 but not in PEC experiments.
TiO2 PEDOT:PSS Photoanode and photocathode Pt 500 W Xe lamp 0.5 μA at 0.001 V 162 (2006)
TiOx X-PEDOT:PSS, P3HT:PCBM Pt layer Photocathode Graphite bar 300 W Xe lamp 0.4 mA at 0.15 V vs. RHE 1.5 μmol h−1 cm−2 at 0 V vs. NHE 163 (2015)
CuI, TiO2 P3HT:PCBM Pt Photocathode Pt 300 W Xe lamp 8 mA at 0 V vs. RHE 164 (2016)
CuI, TiO2 P3HT:PCBM Pt/MoS3 Photocathode Pt 300 W Xe lamp 6 mA At −0.2 V vs. RHE 165 (2017)
MoS3/TiO2 PEDOT:PSS/P3HT:PCBM Photocathode Glassy carbon plate 300 W Xe lamp 0.15 mA at 0.16 V vs. RHE 166 (2013)
NTiO2a g-C3N4 Photoanode Pt mesh 5 W LED 65 μA cm−2 at 0.5 V vs. Ag/AgCl 167 (2015)
NTiO2a g-C3N4 (QDs) Photoanode Pt Xe lamp 1.3 mA at 0.3 V vs. Ag/AgCl 22.0 μmol h−1 cm−2 at 0.3 V vs. Ag/AgCl 168 (2016)
NTiO2a g-C3N4 (QDs) Photoanode Pt Simulated solar light 3.40 mA cm−2 at 0 V vs. Ag/AgCl 169 (2016)
NTiO2a g-C3N4 Photoanode Pt 300 W Xe arc lamp 0.85 mA cm−2 at 0.7 V vs. Ag/AgCl 170 (2018)
NRGO/MoS2b g-C3N4 Photoanode Pt foil 150 W Xe lamp 28 μA cm−2 at 0.8 V vs. Ag/AgCl 171 (2013)
V2O5 g-C3N4 Photoanode Pt wire 100 W Xe 0.9 μA cm−2 (not found bias potential information) 172 (2015)
WO3 g-C3N4 Photoanode Pt wire 300 W Xe lamp 0.73 mA cm−2 at 1.23 V vs. RHE 173 (2016)
BiPO4 g-C3N4 Au Photoanode Pt wire 300 W Xe lamp 150 μA cm2 at 1.1 V vs. Ag/AgCl 174 (2016)
NiTiO3 g-C3N4 Photoanode Pt wire 30 W visible LED * 175 (2017)
ZnGa1.9Al0.1O4 g-C3N4 Photoanode Pt foil 150 W Xe lamp * 176 (2018)
In2S3 g-C3N4 Pt 200 W tungsten lamp 5 μA cm−2 at 1.5 V vs. Ag/AgCl 101 (2017)
TiO2 + QD:CdS, CdSe PEDOT Pt Photoanode Pt 150 W Xe lamp 15 mA at 0 V vs. Ag/AgCl 177 (2015)
ZnO PANI Photoanode Pt UV-source: 100 mW cm−2 0.8 mA at 1 V vs. Ag/AgCl 178 (2016)
CdSe P3HT Photocathode Pt coil AM 1.5G (100 mW cm−2) −1.24 mA at 0 V vs. RHE 40 μL cm−2 at 0 V RHE 179 (2015)
MoO3 P3HT:PCBM Pt Photocathode Pt 450 W Xe lamp (A.M 1.5) 4 mA at −0.3 V vs. RHE 180 (2016)
MoS4 Poly(pyrrole)- Ru(L)32+ Photocathode Pt 150 W Xe lamp 12 μA at −0.3 V Ag/AgCl 181 (2015)
Ti–MoS3 PEDOT:PSS/P3HT:PCBM Photocathode Glassy carbon 200 W Hg–Xe lamp 7.6 mA at −0 V vs. RHE 182 (2015)
CuBi2O4 Polythiophene (PTh) Photocathode Pt 300 W Xe lamp 0.41 mA at 0.3 V vs. RHE 183 (2018)

In some works, the term organic–inorganic hybrid photoelectrochemical cell is used to refer to, for example, an inorganic photoanode spatially separated from an organic photocathode, or vice versa.184 This review is focused only on hybrid materials/composites that constitute a full photoelectrode (i.e. the inorganic and organic materials are together).

The first reported work in photoelectrochemistry using a hybrid system containing a conjugated organic polymer as a part of a photoelectrode was reported by Sakai et al. in 2006.162 In this work, the authors assembled a thin film of TiO2 bulk anatase on poly(styrenesulfonate)/poly(2,3-dihydrothieno(3,4-b)-1,4-dioxin) (PEDOT-PSS). The deposition of the PEDOT-PSS/TiO2 bilayer was repeated 10 times to synthesize a multilayer assembly: substrate/PEI/(PEDOT-PSS/TiO2)10 (note that PEI means poly(ethylenimide), a nonconjugated polymer used as a binder between the substrate and active layers). They found a reversible change in the photocurrent signal under alternate irradiation of UV and visible light. This difference in current can be attributed to the conductivity type change (n for TiO2 and p for PEDOT-PSS). This work was focused on the photoinduced tuning of the electrical conductivity but did not explore the behaviour of the material as a photoelectrode in an energy conversion PEC cell. Since this initial work most of the assemblies of ISs with CPs as electrodes in PEC cells are based on the combination of conventional conjugated polymers such as PEDOT, P3HT, PT and PANI (see molecular structures in Fig. 3) with metal oxides and/or chalcogenides with examples based on g-C3N4 (Table 4). In fact, hybrids based on g-C3N4 used as photoelectrodes have been revised recently by Safaei et al.57 Although there are already examples of the use of conjugated porous polymers in photoelectrochemical cells,185 we could not find any hybrid photoelectrode based on them.

The highest photocurrents recorded by using an hybrid photoelectrode were reported by Miyauchi et al. in 2015.177 In this work, the authors prepared a hybrid photoelectrode composed of a mesoporous TiO2 film, a CdS and CdSe quantum dot sensitized thin film and a PEDOT coating. The PEDOT coating prevented the inherent photocorrosion of chalcogenide quantum dots and at the same time reduced recombination rates and increased electron lifetime values. They found an outstanding performance under visible-light irradiation in the absence of an applied bias potential. Under this condition, the hybrid photoelectrode generated H2 at a rate of 370 μmol cm−2 h−1 using a hole scavenger as the electrolyte (solution containing 0.24 M Na2S and 0.34 M Na2SO3). Furthermore, a quantum efficiency of 6.9% was achieved using a PEDOT-coated electrode. This electrode also showed superior photo-stability compared with the uncoated one. An improvement in the charge transfer, photocurrents and stability was obtained by electro-polymerizing the PEDOT polymer over the TiO2/QD photoelectrode surface.

Other interesting results published by Fumagalli et al. reported a hybrid organic–inorganic photocathode comprising a combination of a FTO transparent conductive oxide, a hole blocking α-MoO3 layer, a P3HT:PCBM bulk heterojunction, an electron selective TiO2 layer and a final Pt electrocatalyst layer.

In this system, the P3HT:PCBM bulk heterojunction (BJH) (being PCBM phenyl-C61-butyric acid methyl ester, a well-known fullerene additive used on PV cells) acts as the photo-active and charge generating layer.180

A similar approach has also been employed by Rojas et al. with the same polymeric BJH, but using a cuprous iodide (CuI) solution-based layer as a hole selective contact.165 In this work they used a protective amorphous TiO2 film layer, as well as a Pt thin film to catalyse the H2 evolution reaction (FTO/CuI/P3HT/PCBM/TiO2/Pt) reproducing a similar photocathode reported in previous works from the same lab.164,180 The hybrid organic/inorganic architecture was designed to have a favourable energetic alignment, matching the electrochemical potential needed for efficient HER, and also to provide a potential gradient to separate electrons and holes and avoid their recombination (Fig. 16a).

On the other hand, Bourgeteau et al. presented a work with the same organic bulk heterojunction architecture (P3HT:PCBM).182 They added another conductor polymer, PEDOT:PSS, as a hole transporting layer and MoO3 as the principal inorganic semiconductor. These studies were based on their previous works,166 but in this case they incorporated an interfacial layer to improve the charge transfer between P3HT:PCBM and MoS3. For this new goal the authors described two strategies: (1) a metallic material used to improve the electronic collection and electronic transfer to the catalyst and (2) a nanocarbon layer used as a fully organic interfacial layer. They found that the C60 layer slightly improved the photocurrents. However, the best performance was found upon using the metallic Al/Ti interfacial layer, which led to an increase of the photocurrent by up to 10 mA cm−2 at −0.5 V vs. reversible hydrogen electrode (RHE). Besides, it showed a 0.6 V anodic shift of the H2 evolution reaction onset potential, a value close to the open-circuit potential of the P3HT:PCBM solar cell.

It can also be said that the presence of noble metals in the PEC cells based on organic–inorganic hybrid can boost hydrogen production. A hybrid formed by TiO2/Au/PT shows an optimal photoelectric conversion (0.11%, at 0.22 V vs. Ag/AgCl) and PEC hydrogen production rates 3 times higher than those for the TiO2/PT hybrids (2.929 mmol h−1 m−2, at 50 mW cm−2 and 0.4 V vs. Ag/AgCl).186

In a recent work, Xu et al. used polystyrene spheres as a matrix to obtain a porous hierarchal CuBi2O4 nanostructure that increases the light trapping ability of this semiconductor. The hybrid photoelectrode was composed also of polythiophene obtained by photopolymerization which led to a modest improvement of the photoelectrochemical properties of bare CuBi2O4.183

In short, although the use of hybrids photocatalysts based on ISs and CPs has experienced a limited growth they have demonstrated to be used as efficient photocatalysts in PEC H2 production. In our opinion, they have potential properties to emerge also as alternatives to other materials in photoelectro-reduction of CO2. Bare ISs and CPs and inorganic–inorganic heterojunctions are abundant in solar fuel production.187 The optoelectronic properties of CPs, presenting often a p-type conductivity, open the door to their use as efficient and stable photocathodes, this being one of the main challenges in PEC cells. The development of novel conjugated organic polymers is a useful strategy to enhance the photoelectrochemical performance of current inorganic semiconductors, leading to a remarkable improvement in their light absorption, electronic photogeneration and transport properties.

3.4. Photoproduction of ammonia

Ammonia (NH3) is the second most produced industrial chemical and its production means 2% of the global enegy188 and is mainly used as a fertilizer to sustain agricultural activities. In addition, it has been proposed as an energy carrier and is expected to play a crucial and sustainable role in the new horizons of energy scenarios.189 The Haber–Bosch process is the main route for industrial ammonia synthesis by using N2 and H2 (eqn (11)) at 150–200 atm and 400–500 °C, with evident energy consumption and CO2 emissions. Therefore more sustainable alternatives are necessary to produce NH3.
N2(g) + H2(g) → 2NH3(g), ΔH298K = −92.2 kJ mol−1(11)
One of the most interesting strategies is the photocatalytic N2 fixation190 in which fertilisers and fuels can be produced locally, lowering the costs and risks associated with transportation of large volumes. Combined with CO2, the photochemical production of N-based chemicals as well as N-rich polymers and materials will be made possible. In 2018 X. Chen et al. reviewed the state-of-the-art and future prospects of photocatalytic ammonia production. They have concluded that solar energy conversion for N2 fixation will be enriched to begin a revolution of renewable energy for practical benefits and future commercialization.191 Also, X. Xue et al. reached at similar conclusions: they connected the development of this technology with the reduction of our reliance on fossil fuels and the mitigation of its impact on the climate change.192 Hybrid materials based on conjugated polymers and inorganic semiconductors are a very interesting alternative to conventional photocatalysts leading to a higher NH3 production in near future. However, this is a novel topic and only a few contributions have been published (Table 5).
Table 5 Some relevant examples of hybrid materials used on photochemical N2 fixation to NH3
IS Polymer Scavenger Reaction conditions Preparation details Morphology Light source Charge transfer NH3 rate (units) AQY (%), λ (nm) Ref. (year)
a Amorphous type TiOwx anodic oxide films were formed in a phosphoric acid solution (0.85 wt%) at 20 °C. The typical potential applied to the titanium plate was 10 V versus the counter Pt plate at which a brownish yellow oxide layer, TiOwx, was formed on the titanium plate. b 3,4-Dihydroxybenzaldehyde-functionalized Ga2O3.
TiO2 P3MeT Gas ± solid reactor Both polymer and IS layers were prepared electrochemically Composite Pseudosolar lamp (1200 lx) 8.3 μmol h−1 m2 of TiOx N/A 195 (2000), 196 (2001)
TiOw[thin space (1/6-em)]xa P3MeT Gas ± solid reactor Both polymer and IS layers were prepared electrochemically Composite White light (260 W m2) 9.5 μmol h−1 m2 of TiOx N/A 198 (2008)
TiO2 PF and PCBz Gas ± solid reactor Both polymer and IS layers were prepared electrochemically Composite White light (260 W m2) 19 μmol h−1 m2 of TiOx N/A 197 (2004)
TiO2 C/g-C3N4 Methanol (20 v%) Gas ± liquid reactor Calcination of a mixture of Ti3C2 (MXene) and melamine IS decorating g-C3N4 300 W Xe lamp (λ > 420 nm) Type II 250.6 μmol g−1 h−1 0.14% (420 nm) 203 (2018)
ZnMoCdS g-C3N4 Ethanol (0.789 g L−1) Gas ± liquid reactor Hydrothermal synthesis of IS over g-C3N4 IS decorating g-C3N4 250 W high-pressure Na lamp Type II 18.8 μmol L−1 h−1 gcat−1 N/A 199 (2016)
ZnSnCdS g-C3N4 Ethanol (0.789 g L−1) Gas ± liquid reactor Hydrothermal method IS decorating g-C3N4 250 W high-pressure Na lamp Type II 0.44 mmol L−1 h−1 gcat−1 N/A 200 (2016)
Ga2O3-DBDb g-C3N4 Methanol (0.8 mol L−1) Gas ± liquid reactor Post-grafting strategy via Schiff base chemistry Covalently bound 500 W Xe lamp Z-scheme 5.6 mmol L−1 h−1 gcat−1 N/A 202 (2017)
MgAlFeO g-C3N4 Ethanol (0.789 g L−1) Gas ± liquid reactor g-C3N4 synthesized in situ Composite 250 W high-pressure Na lamp Z-scheme 0.53 mmol L−1 h−1 gcat−1 N/A 201 (2017)
Fe2O3 g-C3N4 Ethanol (20 v%) Gas ± liquid reactor Calcination of Fe(NO3)3·9H2O and prepared g-C3N4 IS decorating g-C3N4 300 W high pressure Xe lamp Z-scheme 2.81 mmol L−1 h−1 gcat−1 N/A 204 (2019)
CdS C3N4 nanosheets Ethanol (0.789 g L−1) Gas ± liquid reactor CdS prepared over C3N4 IS decorating g-C3N4 500 W Xe lamp Type II 1.93 mmol L−1 h−1 g−1 N/A 205 (2019)

The first work describing photocatalytic N2 reduction to ammonia was published by Schrauzer and Guth in 1977193 using TiO2 as a photocatalyst in presence of N2 and H2O leading to the formation of O2 and NH3 (eqn (12)). Note that other hydrogenation reactions related to the photoreduction of N2 to NH3 could occur (eqn (13)–(17)).

image file: c9cs00377k-t1.tif(12)
N2 + e → N2E0 = −4.16 V(13)
N2 + H+ + e → N2E0 = −3.2 V(14)
N2 + 4H+ + 4e → N2H2E0 = −1.1 V(15)
N2 + 6H+ + 6e → NH3E0 = 0.55 V(16)
N2 + 8H+ + 8e → N E0 = −0.38 V(17)
From then to now, as in the production of other solar fuels described in this review, the photocatalysts used for ammonia production have been mainly based on metal oxides and chalcogenides.194 In fact, the first work related to the use of hybrid based on conjugated polymers and ISs did not appear until 2000 by Hoshino and coworkers.195 In this letter, they described a system composed of TiO2 and poly(3-methylthiophene) (P3MeT), both of which were prepared electrochemically in the form of layers. This catalyst was able to produce NH4+ClO4 needles by N2 and H2O flow under pseudo-solar irradiation. Later, the same authors determined the reaction yield as 8.3 μmol h−1 m2 of TiO2.196 Following the same philosophy other polymers such as polyfuran (PF) and polycarbazole (PCz) were tested as counterparts with layered TiO2.197 Thus, the PCz/TiO2 hybrid system showed 1.6-fold higher production of NH3 than P3MeT/TiO2 under the same reaction conditions. Note that to complete the study this research group explored different ways of polymer electrochemical synthesis and TiO2 phases, preparing several P3MeT/TiO2 catalysts, concluding that the amorphous type TiO2 was more reactive than the rutile or anatase phase.198

Other recent examples found in the literature consist of the combination of g-C3N4 with metal oxides or chalcogenides.199–205 Noteworthily, in all cases, the setup of the reaction involves the irradiation of the hybrid system dispersion in aqueous solution in the presence of methanol and ethanol as a scavenger under N2 flow. Thus, the scavenger is easily oxidized than water and the product is dissolved in the reaction media facilitating its detection by colorimetric tests. Both type II and Z-Scheme charge transfer mechanism can be found depending on the hybrid system (see Table 5). Note that the Z-scheme (Fig. 17) achieves higher overall N2 photoreduction (2.81 mmol L−1 h−1 gcat−1) than type II.

image file: c9cs00377k-f17.tif
Fig. 17 Schematic illustration of photocatalytic N2 reduction to NH3 over a g-C3N4@Fe2O3 hybrid system. Reproduced from ref. 204 with permission of Elsevier. Copyright 2019.

We would like to highlight the strategy employed by Cao et al. to covalently bind both semiconductors.202 They functionalized Ga2O3 with 3,4-dihydroxybenzaldehyde (DBD) taking advantage of the IS's superficial hydroxylic groups. Then, g-C3N4 is grown from urea over the Ga2O3-DBD nanoparticles, tethering both semiconductors by Schiff base chemistry. NH3 production in these studies is 5.6 mmol L−1 h−1 gcat−1.

4. Conclusions and perspectives

The development of highly active photocatalytic materials for environmental and energy application is the key stone in the expansion of innovative and environmentally friendly technologies that ensure the foundation of sustainable models based on a circular economy. Over the past few years, different strategies have been used to develop photoactive materials optimizing the photons and electrons leading to improved catalytic activity such as: (i) band-gap engineering approaches; (ii) use of co-catalysts; (iii) use of nanocrystals or molecules as sensitizers; (iv) design of heterojunctions, including both organic and/or inorganic semiconductors. Among all of these possibilities, conjugated organic polymers have emerged as an innovative type of photocatalyst. These materials possess interesting properties such as: tailor made synthesis, light absorption and emission tunability and, sometimes, processability and stretchability. They are used as photocatalysts themselves or in combination with other materials as dye sensitizers and protective layers or forming hybrid photoactive heterojunctions. This review summarized the recent developments of photocatalytic hybrid heterojunctions formed by conjugated organic polymers and inorganic semiconductors with environmental and energy applications.

For a better comprehension of this revision a description of the fundamentals on photocatalysts and photoelectrochemistry related to hybrid systems has been described. As a first point to keep in mind, a knowledge of the relative energy band edge positions of both conductive polymers and inorganic semiconductors is mandatory in order to determine whether the photochemical process of interest is thermodynamically possible. Also, it is necessary to know the possible charge transfer mechanism between both semiconductors (i.e. sensitization, type II heterojunction or Z-scheme) to explain the photocatalytic behavior. Both parameters allow the design of improved hybrid photocatalytic systems adequate for each application. Other factors such as hybrid morphology, nanoparticle size, coating size and covalent bonds between different materials should also be taken into consideration. In terms of morphology, several scenarios such as polymeric coatings, composite mixtures and inorganic nanocrystal decorating bulk polymers can be found. In fact, with the same couple of semiconductors the photocatalytic results could be changed because of morphological issues. Also, the interface between both semiconductors and so, the photocatalytic activity can be improved by covalent bonds between them.

Although the use of conjugated polymers as a counterpart of an organic inorganic hybrid (OIH) photocatalyst is an old concept, the scientific community was losing interest mainly due to the problems related to the low photostability under oxidizing conditions. However, in the past few years, and thanks to advances in novel and efficient synthetic approaches, we have experienced a renaissance in the development of conductive polymers and hybrid heterojunctions mainly due to their application in other fields such as photovoltaics or optoelectronics. This evolution has led to the progress of robust and photostable polymer families with high activity in photocatalytic reactions. Regarding photostable conductive polymers, conjugated porous polymers (CPPs) (including carbon nitride) or 2D–3D polymeric structures have arisen as an attractive option to produce hybrid materials that can be used as efficient photocatalysts and photoelectrodes. These polymers, and also the hybrid materials, are derived from their interesting optoelectronic, textural, morphological and superficial properties that are crucial for the development of industrial processes. In fact, beyond carbon nitride, over the last five years, CPPs and covalent organic frameworks (COFs) have appeared as attractive alternative photocatalysts in hydrogen production or water splitting processes.53 Meanwhile, hybrids based on them used as photocatalysts in hydrogen production are an approximation used only since two years ago.

In the case of inorganic semiconductors the most used photocatalyst is TiO2, followed to a lesser extent by other materials (ZnO, CdS, Bi2WO6, BiVO4, among others). From the polymer side, most part of the papers are focused on g-C3N4 followed by others such as poly(thiophenes) (i.e. PT and P3HT) polyaniline (PANI), polypyrrole (PPy), among others.

The most common environmental application of these OIHs is in pollutant remediation via photocatalytic oxidation of emerging pollutants (i.e. drugs, aromatics, Cr(VI) or NOx) or dyes. After this bibliographic revision the most important conclusion is that in every pollutant removal studied case, OIHs show an increase in their photocatalytic activity versus the respective bare materials. In most of these reactions, the polymer acts as a sensitizer, although other mechanisms such as type II heterojunction or Z-scheme have been described. However, the biggest drawback of these technologies is the low photostability of some polymers due to the high oxidant atmosphere achieved under the operation conditions.

Solar fuel production by artificial photosynthesis has led to new breakthroughs in the renewable energy conversion area. Although in smaller proportion than in pollutant remediation, the use of hybrid materials for solar fuel production has emerged in recent years. Although most of the articles related to this area were focused on H2 photoproduction, in the past few years new photoactive materials have been developed also for the reduction of CO2, and NH3 production. As in the case of environmental applications, the most used inorganic photocatalyst is TiO2 and from the organic counterpart it is g-C3N4. Another important issue is that in most of the examples the use of a metal co-catalyst is required to improve the oxidation (complexes or oxides based on Co) and reduction (metallic nanoparticles of Pt, Ag, Au). Until now, the highest H2 production (ca. 100.200 mmol h−1 g−1) has been achieved by Zhang et al, using a hybrid photocatalyst composed of a conjugated polybenzothiadiazole and CdS in the presence of 1 wt% Pt as a cocatalyst. The same materials in the absence of Pt displayed a H2 production rate of 40.00 mmol h−1 g−1. These values are higher than those obtained with bare CdS or Pt/CdS. The authors propose a Z-scheme charge transfer mechanism to explain this enhancement in the H2 evolution.

CO2 photoreduction is a very complex multi-electronic reaction and the use of conjugated polymers even as single photoactive materials is relatively new. One of the possible bottlenecks is their photodegradation which could cause an overestimation in the product quantification. For this reason, only confirmed photostable polymers such as g-C3N4 are good candidates for this application. Despite these remarks, some linear conjugated polymers such as PANI, PPy and PT have been successfully used as photocatalysts. A comparison of the achieved results between different research groups is difficult due to the non-standardized process. However, after the topic revision an important conclusion can be reached: in all cases, the highest production of solar fuels from CO2 photoreduction was achieved by hybrid materials based on CPs and ISs compared with the bare materials.

As the most relevant example of the use of this kind of hybrid we would like to mention the results achieved by Liu et al. who reported a combination of PANI with TiO2 and Pt as a cocatalyst leading to H2 (∼320 μmol g−1 h−1) and CH4 (∼50 μmol g−1 h−1) production without CO detection.143 Also, Li et al. reported production of CO (∼51.8 μmol g−1 h−1) using a hybrid of Bi2WO6 grown on g-C3N4 in the absence of a metal cocatalyst.144

Thus, another important conclusion can be mentioned which is the fact that the selectivity to different possible products changes with the nature of the photocatalyst system.

On the other hand, designs of photoelectrochemical cells (PEC) based on hybrid systems, where one of the semiconductors is a conjugated polymer, have been discreetly addressed so far, not finding any conjugated porous polymer for this application. As far as we know, the highest photocurrent recorded in a PEC hybrid system was using a mesoporous TiO2 film, sensitized with CdS and CdSe quantum dots covered with a PEDOT film.177 PEDOT inhibited the photo-corrosion of the quantum dots and at the same time reduced recombination rates and increased electron lifetime values. We found just a few examples in which the authors also showed the HER quantification, as it is very difficult to make a comparison between them because of the differences in terms of reaction conditions (bias potential, irradiation, etc.). Moreover, these results still present modest efficiencies, and we have not yet found any example for CO2 reduction. This scenario ratify that this is still an open possibility with great potential ahead where there are still great improvements. In fact, the optoelectronic properties of conjugated polymers, presenting often a p-type conductivity, open the door to their use as photocathodes, this being one of the main challenges in PEC cells.

One of the main limitations for commercialization of conjugated porous polymers beyond g-C3N4 materials in PEC systems is the lack of their processability. Those materials are usually composed of large particles making thin film fabrication necessary for multilayer PECs systems very difficult. In fact, the possibility of using other synthetic tools to develop thin films such as nanostructuring or electropolymerization is currently being investigated.206

Another emerging topic is the fixation of N2 through its photoreduction to produce NH3. In the last decade a few examples using the OIH system can be found in the literature. Also in this case it is evident that the use of this kind of material as a photocatalyst leads to better performance than bare semiconductors. The production of NH3 has economic and environmental importance and we suppose that the number of contributions related to this topic will increase in the near future.

We believe that future developments of hybrid photocatalysts based on organic conjugated polymers and inorganic semiconductors have huge potential to provide highly efficient photoactive materials with improved optoelectronic and photocatalytic properties. These hybrid systems could have a significant impact not only on environmental processes but also on solar fuel production by artificial photosynthesis, leading the research advances in this critical topic in the fight against climate change.

Conflicts of interest

There are no conflicts to declare.


Financial support from MINECO-AEI/FEDER, UE, through the RaPHUEL project (ENE2016-79608-C2-1-R) is gratefully acknowledged. Also, the authors thank the regional government of “Comunidad de Madrid” and European Structural Funds for their financial support to the FotoArt-CM project (S2018/NMT-4367). M. L. thanks the Spanish MINECO-AEI/ESF, UE, the “Ramón y Cajal” grand (RyC-2015-18677). M. B. thanks the Spanish MINECO-AEI, the “Juan de la Cierva” grand (FJCI-2016-30567). This work has also received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (HyMAP project, grant agreement No. 648319). The results reflect only the authors’ view and the Agency is not responsible for any use that may be made of the information provided.

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