Interface engineering of heterojunction photocatalysts based on 1D nanomaterials

Abstract

Semiconductor photocatalysis is considered a promising approach to solve the current energy shortage and environmental pollution problems. Due to their unique structures and outstanding physicochemical properties, one-dimensional (1D) nanomaterials have attracted great attention in the field of photocatalysis. However, they still suffer from many obstacles, such as rapid recombination of photogenerated electron–hole pairs and low apparent quantum efficiency, which result in unsatisfactory photocatalytic performance in practical applications. To overcome these challenges, photocatalytic heterojunctions have been constructed using 1D nanomaterials as basic building blocks and demonstrated unique and improved properties. In this review, we systematically summarize several different types of 1D nanomaterial-based heterojunctions based on the transfer mechanisms of photogenerated electron–hole pairs, including type II heterojunction, p–n type heterojunction, Schottky junction, Z-type heterojunction, and S-scheme heterojunction. At the end, some challenges and future directions of 1D nanomaterial-based photocatalytic heterojunctions for practical applications are discussed.

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1 Introduction

Semiconductor photocatalysis is considered to be one of the most promising strategies in the fields of energy and environment in the 21st century.^1–5 Therefore, significant efforts have focused on developing semiconductor photocatalysts. Among them, one-dimensional (1D) nanomaterial photocatalysts, including nanorods, nanobelts, nanofibers, nanowires and nanotubes, are viewed as one of most prospective materials in the photocatalysis field.^6–9

Firstly, due to their large surface areas and the number of active sites on the catalytic surface, 1D nanomaterials exhibit excellent photocatalytic activity.^10–12 In general, larger surface areas can facilitate reactant absorption, and abundant active sites can promote charge separation and interfacial reaction.¹³ Furthermore, by tuning the length-to-diameter ratios of 1D nanomaterials, the photocatalysis and photoelectrochemical (PEC) performance can be controlled due to the change of the specific surface area, photo-absorption and light scattering.¹⁴ In addition, the channel structure of 1D nanomaterials serves as a charge transport pathway to accelerate the transport of electron–hole pairs.^15–17 Finally, due to the unique 1D structure, the 1D semiconductor photocatalyst is extremely suitable as a prominent basic unit to fabricate composite photocatalysts with a multilevel hierarchy structure.¹⁸ Compared to 1D nanomaterials, large-scale synthesis of 2D nanomaterials with well-controlled nanostructures is difficult and still challenging.^19,20 In particular, the ultrathin nanosheet structure (2D) has a tendency toward restacking, consequently weakening the light absorption and decreasing active sites.^21,22 What's more, although the ultrathin structure of 2D nanomaterials can reduce the recombination of carriers in bulk, the electrons and holes would still recombine on the surfaces due to insufficient active sites.²³ For 3D nanomaterials, three-dimensional (3D) nanostructures often have a complex morphology, resulting in multiple reaction pathways.²⁴ Therefore, various 1D photocatalysts, including nanowires, nanotubes, nanoribbons, nanorods, nanoarrays, nanonetworks, and nano-bundles, such as TiO₂ nanowires, g-C₃N₄ nanorods, ZnO nanofibers, Fe₃O₄ nanotubes, and WO₃ nanobelts, have been explored and widely applied in photocatalytic hydrogen production, CO₂ reduction, and pollutant degradation for the environment.^25–32

However, pure 1D nanomaterials are limited in practical applications of photocatalysis due to fast recombination of photogenerated charge carriers and poor selectivity for the reaction product. To tackle these difficulties, many strategies have been employed to modify 1D photocatalysts, including morphology control,³³ element doping,³⁴ defect engineering,³⁵ and heterojunction construction.³⁶ Among these methods, constructing heterojunctions is considered as a promising and effective method to improve the photocatalytic efficiency. In particular, considering the outstanding advantages such as unique structure, high surface area, large number of active sites and direct transport pathway for charge carriers, 1D semiconductor photocatalysts are considered as a prominent basic unit to combine with another semiconductor.^16,37–41 It is expected that the design and construction of 1D nanomaterial-based composite photocatalysts with different kinds of hierarchical structure such as core–shell, three-dimensional (3D) (2D/1D) and 0D/1D structure can effectively boost the separation and transfer of space charges and broaden the light absorption.^42–47 For example, compared with 1D nanomaterials, 3D structures from assembling two-dimensional (2D) nanosheets on 1D nanorods can improve the light absorption due to the increased optical path as well as additional light trapping through reduced reflection and multi-scattering.⁴⁸ Furthermore, many small heterojunctions and high surface area are beneficial for fast and long-distance electron transport and surface activity. These useful properties coming from the unique 3D structure (2D/1D) lead the 1D-based composite photocatalysts to excellent photocatalytic activity. Therefore, fabrication of composite photocatalysts using 1D nanomaterials as building blocks has stimulated widespread interest.^49–51

The purpose of constructing heterojunctions is to solve the key problem of high recombination rate of photogenerated electrons and holes. Due to the difference in preparation methods, band energies, morphologies and structures, heterojunction photocatalysts have different transfer mechanisms of photogenerated carriers. Consequently, heterojunctions are generally divided into several types, including type I heterojunction, type II heterojunction, p–n type heterojunction, Schottky junction, Z-type heterojunction, and S-type heterojunction.⁵² Thus, this review focuses on the types and structures of photocatalytic heterojunctions based on 1D nanomaterials and highlights the different transfer mechanisms of photogenerated electrons and holes. Finally, the challenges, opportunities and future directions of practical applications of these heterojunction photocatalysts are discussed.

2 The types of heterojunctions

The charge separation mechanisms of different 1D nanomaterial-based heterojunctions can be classified into the following:^53–55 (1) type I heterojunction, (2) type II heterojunction, (3) p–n type heterojunction, (4) Schottky junction, (5) Z-type heterojunction, and (6) S-type heterojunction. They are shown schematically in Fig. 1 with a focus on their energy levels and charge carriers following photoexcitation. The performance of various 1D nanomaterial-based heterojunction photocatalysts are summarized in Table 1. Due to the special band structure of type I heterojunction, there are few reports about it, and we will not discuss it further.

Fig. 1 Band structure of various types of heterojunctions in a photocatalytic hybrid nanocomposite: (a) type I heterojunction, (b) type II heterojunction, (c) p–n junction, (d) Schottky junction, (e) Z-scheme heterojunction, and (f) S-scheme heterojunction. A, D and E_f represent electron acceptor, electron donor and Fermi level, respectively. Adapted with permission from ref. 55 and 138. Copyright 2019 Elsevier.

Table 1 The performance of various 1D nanomaterial-based heterojunction photocatalysts

Heterojunction	Structure	Types	Light source	Application	Performance		Advantages	Disadvantages
					Yield (μmol h^−1 g^−1)	Efficiency (AQE)
g-C3N4/LaPO4 (ref. 67)	1D/1D	Type II	Xe lamp (300 W)	CO2 reduction	CO 14.4	—	Spatial separation of photo-generated electrons and holes	Weaker redox potential
CdS/g-C3N4 (ref. 69)	1D/1D		Xe lamp (350 W)	H2 evolution Pt co-catalyst (0.6 wt%)	4152 (λ ≥420 nm)	4.3% (λ = 420 nm)
CdS/MoS2 (ref. 144)	1D/2D		Xe lamp (300 W)	H2 evolution Pt co-catalyst (3 wt%)	6020 (λ ≥400 nm)	22.0% (λ = 475 nm)
CdS/ZnO (ref. 145)	0D/1D		Xe lamp (500 W)	H2 evolution	851 (λ ≥400 nm)	3% (λ = 420 nm)
g-C3N4/TiO2 (ref. 79)	0D/1D		Xe lamp (300 W)	H2 evolution	4.58 (μmol h^−1 cm^−2) (λ ≥400 nm)	—
TiO2/MoS2 (ref. 146)	1D/2D		Xe lamp (350 W)	CO2 reduction	CH4 2.86 CH3OH 2.55	0.16% (λ = 420 nm)
BiOBr/TiO2 (ref. 89)	1D/2D	p–n	Xe lamp (300 W)	H2 evolution	122 (>420 nm)	—	Internal electrostatic field induces rapid separation of photogenerated electrons and holes	Weaker redox potential
NiWO4/Zn0.7Cd0.3S (ref. 149)	1D/2D		LED lamp (50 W)	H2 evolution	15 950 (>420 nm)	—
WO3–CuO (ref. 147)	1D/2D		Xe lamp (300 W)	CO2 reduction	CH4 0.84	—
P3HT/ZnO (ref. 91)	1D/1D		Xe lamp (300 W)	Degradation	—	—
P3HT/TiO2 (ref. 94)	1D/1D		Xe lamp (70 W)	Photo-detectors	—	—
Fe–Ag/TiO2 (ref. 108)	0D/1D	Schottky junction	Xe lamp (300 W)	Photocatalytic antibacterial	—	—	Cocatalysts, as charge carrier traps, can accelerate photo-generated carrier spatial separation and expand light absorption	Weaker redox potential
AuPd/ZnO (ref. 109)	0D/1D		Xe lamp (300 W)	Water splitting activity	—	—
TiO2/GDY (ref. 118)	1D/2D		Xe lamp (300 W)	CO2 reduction	CO 50.53 CH4 2.8	0.2% (λ = 400 nm)
CdS/Ti3C2 (ref. 148)	1D/2D		Xe lamp (300 W)	H2 evolution	2407	35.6% (λ = 420 nm)
WO3/Au/In2S3 (ref. 113)	0D/1D	Z-scheme	Xe lamp (500 W)	CO2 reduction	CH4 0.42 (>420 nm)	—	Stronger redox potential; photo-generated carrier spatial separation	Difficult to characterize
g-C3N4/Pt/TiO2 (ref. 126)	1D/2D		Xe lamp (500 W)	H2 evolution	13.77 (>400 nm)	—
ZnIn2S4/TiO2 (ref. 130)	1D/2D		Xe lamp (300 W)	CO2 reduction	CH4 1.135 (>420 nm)	—
CuInS2/TiO2 (ref. 131)	1D/2D		Xe lamp (350 W)	CO2 reduction	CH4 2.5 CH3OH 0.86	—
MoS2/Ag2Mo2O7 (ref. 133)	1D/2D		Xe lamp (150 W)	Degradation	—	—
TiO2/CdS (ref. 140)	1D/2D	S-scheme	Xe lamp (350 W)	H2 evolution	2320	—	Stronger redox potential; high Fermi level; photo-generated carrier spatial separation	Difficult to characterize
g-C3N4/Ag/Ag3VO4 (ref. 142)	1D/2D		LED lamp (50 W)	Degradation	—	—

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2.1 Type II heterojunction

The type II photocatalytic system is the most common.^56–58 In type II heterojunctions, photogenerated electrons in the CB of semiconductor 1 will transfer to the CB of semiconductor 2 under light irradiation, and the photogenerated holes in the VB of semiconductor 2 will be transferred to the VB of semiconductor 1 (Fig. 1b).^54,59 Due to the internal electric field (IEF) formed at the interface between these two semiconductors, the photogenerated electron–hole pairs can be separated effectively, resulting in improved photocatalytic activity.^60,61

Taking advantage of the large specific area and unique structures of 1D nanomaterials, type II heterojunctions using 1D nanomaterials as substrates have been widely fabricated to broaden the light absorption of the photocatalyst and improve the separation efficiency of photogenerated electron–hole pairs.^62–64 In particular, type II heterojunction photocatalysts with core–shell structures are most favorable because they effectively avoid agglomeration and provide a direct pathway for the rapid transport of photogenerated charge carriers.^65,66 Furthermore, the large interfacial area between two semiconductors promotes rapid charge separation. For example, the g-C₃N₄/LaPO₄ core/shell heterojunction photocatalyst has been demonstrated to be highly efficient for CO₂ reduction.⁶⁷ The CB and VB potentials of g-C₃N₄ are −0.88 and 1.98 eV, while the CB and VB potentials of LaPO₄ are −0.74 and 3.11 eV, respectively (Fig. 2b). The more positive CB and VB levels of g-C₃N₄ than those of LaPO₄ suggested that a type II heterojunction is easily constructed for photoinduced charge carrier transfer and separation. The g-C₃N₄/LaPO₄ heterojunction photocatalyst exhibited excellent photocatalytic activity and stability ascribed to the well-matched band structure and extensive interface from close contact, which greatly enhances light sensitivity and effectively accelerates the charge carrier separation/transfer (Fig. 2a). Many other heterojunction photocatalysts with 1D core–shell structures such as TiO₂/C₃N₄,⁶⁸ C₃N₄/CdS,⁶⁹ Ni₃S₂/VO₂,⁷⁰ and ZnO/WS₂ (ref. 71) have also been reported to fit the type II photocatalytic system, which can greatly improve the separation and transfer efficiency of photogenerated pairs.

Fig. 2 Schematic illustration of the proposed mechanism in the nanocomposite. Adapted with permission from ref. 67. Copyright 2017 Elsevier.

Despite the many merits of type II core–shell structures, the light absorption of the 1D photocatalyst is weak because the core is almost completely covered by the shell layer. Therefore, type II heterojunctions with 3D (2D/1D) and 0D/1D structure are often utilized for photocatalysis.^72,73 For example, a 3D heterojunction composite of ZnO nanosheets coated onto TiO₂ nanowire arrays was designed as an efficient photoelectrode for photoelectrochemical (PEC) solar hydrogen production (Fig. 3).⁷⁴ The unique 3D structure (2D/1D) of the ZnO/TiO₂ heterojunction not only can greatly improve the light absorption due to the increased optical path by reducing reflection and multi-scattering but also can enable the electron–hole pairs to efficiently separate and enable charge carrier transport at the interface, resulting in significant enhancement of hydrogen generation in a neutral electrolyte solution. Meanwhile, the decoration of 1D nanomaterials with semiconductor quantum dots (0D/1D) is another method to fabricate type II heterojunctions. In particular, after decoration with small bandgap semiconductor quantum dots (QDs), the light absorption efficiency is enhanced. For example, TiO₂ nanowires decorated by CdSe QDs have demonstrated an enhanced photoactivity in the wavelength region from 350 to 550 nm.⁷⁵ ZnO nanowire arrays sensitized by CdTe QDs show significant photocurrent improvement.⁷⁶ Similar type II 0D/1D structure heterojunctions such as CdS/ZnO,⁷⁷ C₃N₄/ZnO,⁷⁸ and C₃N₄/TiO₂ (ref. 79) have been reported to improve the photocatalytic and PEC performance.

Fig. 3 (a) Scheme for the fabrication of 3D ZnO/TiO₂ 2D/1D nanosheet–nanowire array architectures. (b–d) SEM images of (b) pure 1D TiO₂ nanowire arrays, (c) ZnO seed nanoparticle-decorated 1D TiO₂ nanowire arrays. (c) SEM images of 3D ZnO/TiO₂ 2D/1D nanosheet–nanowire array architectures fabricated in 0.1 M zinc nitrate aqueous solution. Adapted with permission from ref. 74. Copyright 2019 Royal Society of Chemistry.

2.2 p–n type heterojunction

Although type II heterojunctions can in principle separate electron–hole pairs for 1D nanomaterial-based composites, they still cannot substantially address the issue of fast recombination of electron–hole pairs.^54,82 To further suppress electron–hole pair recombination, the concept of p–n junction is proposed to accelerate electron–hole pair migration through the synergy between the band orientation of the interface and the internal electric field (IEF).^79–87 In general, before light irradiation, electrons on the n-type semiconductor diffuse into the p-type semiconductor through the p–n junction interface. At the same time, holes in the p-type semiconductor tend to diffuse into the n-type semiconductor. The electron and hole diffusion will continue until the system reaches the Fermi level balance.⁸⁶ As a result, an IEF is formed at the p–n junction interface (Fig. 1c). Under the synergy of the IEF and light irradiation, electrons and holes will migrate in a directional way, which greatly inhibits the recombination of electrons and holes and further improves the photocatalytic performance.

Recently, a number of p–n heterojunction composite photocatalysts with hierarchical structures, such as core–shell, 3D (2D/1D), and 0D/1D structures, have been designed to improve the photocatalytic and photoelectrocatalytic activities.^87,88 For instance, a 3D (2D/1D) structure of p–n junction composite photocatalyst has been constructed by assembling p-type BiOBr nanosheets on the surface of n-type TiO₂ nanobelts. As shown in Fig. 4, the n-type TiO₂ and p-type BiOBr have different Fermi levels. After contact, the Fermi level difference between them leads to electron transfer from TiO₂ to BiOBr until the Femi levels of BiOBr and TiO₂ reach an equilibrium state. Consequently, an IEF is formed between n-TiO₂ and p-BiOBr. Driven by the IEF and light irradiation, the photogenerated carriers were effectively separated, resulting in enhanced photocatalytic activity.⁸⁹ A (0D/1D) structure of p–n junction photocatalyst constructed by assembling p-type NiS nanoparticles on the surface of n-type CdS nanorods was reported to show enhanced H₂-production activity (Fig. 5).⁹⁰ In addition, organic/inorganic hybrid p–n heterojunctions based on a combination of p-type donor organic semiconductors with n-type acceptor inorganic semiconductors have been widely used in photocatalysis. For example, Lee et al. constructed a poly (3-hexylthiophene) (P3HT) nanomesh/ZnO nanorod p–n junction that showed the best visible light photodegradation rate of 80% within 30 minutes. The excellent photocatalytic activity of the composite is attributed to the enhanced visible-light absorption and separation efficiency of charge carriers, boosting numerous redox sites by the self-assembled p-type P3HT nanofibers on the surface of ZnO nanorods. At the same time, the fabrication of unique p–n nanorod heterojunctions effectively solves the weak chemical durability of organic semiconductors.⁹¹ In addition, similar enhanced photocatalytic and photoelectric performances have been observed in other organic/inorganic hybrid p–n heterojunctions such as polyaniline (PANI)/TiO₂ nanofibers, polypyrrole (PPY)/TiO₂ nanotubes, and P3HT/TiO₂ nanofibers.^92–94 Thus, the construction of organic/inorganic hybrid composites is regarded as a most effective and common method to form p–n heterojunctions, which is also widely applied in other research fields such as solar cells.

Fig. 4 (a) The band energy of BiOBr and TiO₂ before contact and (b) the formation of the p–n junction and the charge transfer and separation process under visible light irradiation. Adapted with permission from ref. 79. Copyright 2016 Elsevier.

Fig. 5 (a) SEM image of 5 wt% NiS-loaded CdS, (b) HRTEM image of as-prepared 5 wt% NiS-loaded CdS, (c) schematic diagram of charge transfer and separation, (d) the p–n junction band structure and schematic illustration of the electron–hole separation process under visible-light irradiation. Adapted with permission from ref. 90. Copyright 2013 Royal Society of Chemistry.

2.3 Schottky junction

A Schottky junction is a heterojunction formed by combining semiconductor photocatalysts with appropriate cocatalysts such as Au, Ag, Pt, MoS₂, and graphene. When n-type or p-type semiconductors with cocatalysts form Schottky junctions, the cocatalysts, as charge carrier traps, can promote the rapid transfer of photogenerated electrons in the CB or photogenerated holes in the VB to the cocatalyst, achieving spatial separation of photogenerated electrons and holes (Fig. 1d).^91–100 In addition, due to the different Fermi levels of semiconductors, a Schottky barrier may be formed at the interface, which will effectively inhibit the backflow of electrons or holes from the cocatalyst to the semiconductor, forming a unidirectional flow channel from the photocatalyst to the cocatalyst (Fig. 6).⁹⁵

Fig. 6 Schematic band diagrams illustrating the charge transfer driven by the Schottky junction between metal and n-type (a) or p-type (b) semiconductor. Adapted with permission from ref. 95. Copyright 2015 Wiley.

Construction of a Schottky junction by choosing a suitable cocatalyst load onto the 1D nanomaterial is one of the effective methods to promote the separation of photogenerated carriers, accelerate the surface reaction rate, and improve the selectivity of products.⁹⁵ Thus, many kinds of cocatalysts have been used to modify 1D nanomaterials to improve the photocatalytic and photoelectric activities. Besides promoting the separation of photogenerated carriers, cocatalysts can also form reactive sites to accelerate the surface reaction rate.^101–103 Noble metals such as Au, Ag and Pt are the most common cocatalysts used. Metallic cocatalysts assembled on the surface of 1D nanomaterials can form Schottky junctions with photocatalysts, which can expand light absorption in addition to enhancing charge separation, resulting in the enhancement of the catalytic activity.¹⁰⁴ In addition, except for monometallic cocatalysts, bimetallic cocatalysts may have greater potential in catalytic applications because the activity, selectivity and light absorption of the photocatalyst can be drastically influenced by the presence of a second metal.^105,106 1D photocatalysts such as TiO₂, ZnO and Cd_0.5Zn_0.5S decorated by bimetallic cocatalysts including AuPd, FeAg or PtCo have shown higher H₂ production activity than their corresponding monometallic counterparts.^107–110 Yu et al.¹¹¹ designed a well-defined Au@Pt core–shell nanostructure as a cocatalyst to construct a TiO₂ nanotube-Au (core)–Pt (shell) system for photocatalytic hydrogen generation (Fig. 7). Au can induce localized surface plasmon resonance (SPR) under light irradiation and provide a beneficial electric field nearby to facilitate the separation of photoexcited carriers, while the Pt shell with porous architecture and a high surface area can offer more active sites for the hydrogen evolution rate (HER). This Au@Pt core–shell cocatalyst system combines the characteristics of the localized surface plasmon effect of gold and the excellent ability of proton reduction of platinum, which has a synergistic effect for photocatalytic hydrogen generation.^111–113

Fig. 7 (a) SEM images of TiO₂ nanotubes. (b) Elemental line-scan of Au@Pt nanostructure. (c) High-resolution TEM image and corresponding lattice fringe of Au (200) and Pt (111) planes. (d) Corresponding high-angle annular dark field (HAADF) micrograph and elemental mapping of Ti, Au, and Pt. adapted with permission from ref. 111. Copyright 2016 Royal Society of Chemistry.

Recently, the rational design of spatially separated dual cocatalysts on photocatalysts has attracted extensive attention since charge separation can be further promoted by loading reductive and oxidation cocatalysts simultaneously. Due to the larger surface area and long light irradiation pathway of 1D nanomaterials, the use of 1D photoelectrodes and photocatalysts decorated with spatially separated dual cocatalysts is recognized as an effective and promising method for improving the photocatalytic and photoelectric activities.¹¹⁵ Qin et al.¹¹⁴ prepared a Pt/TiO₂ nanotube/CoO_x composite photocatalyst with Pt nanoparticles on the inner surface of TiO₂ nanotubes and CoO_x nanoclusters on the outer surface (Fig. 8). The spatially separated Pt and CoO_x dual cocatalysts, acting as electron and hole collectors, respectively, have a synergetic effect on the separation of photogenerated carriers, achieving remarkably high photocatalytic efficiency. Similarly, the construction of photoelectrodes with spatially charge separated oxidation and reduction cocatalysts have been demonstrated to improve the photoelectrode for photoelectrochemical (PEC) conversion efficiency. For example, Wang¹¹⁶et al.¹¹⁶ employed a simple but effective “bottom-up” method, metallic replacement combined with hydrolytic reactions, to selectively grow FeOOH and Pt cocatalysts on hematite nanoflake photoanodes (FeOOH/α-Fe₂O₃ nanofiber/Pt). Pt nanoparticles function as an electron acceptor and transfer layer to facilitate water reduction, and FeOOH is regarded as an active site to promote interfacial hole transfer, This novel FeOOH/α-Fe₂O₃ nanofiber/Pt photoanode shows excellent performance for the PEC water splitting reaction. Likewise, in order to further improve the light absorption and reduce the charge recombination rate, Liu¹¹⁷et al.¹¹⁷ studied a quaternary system including ZnO nanorods, CdS nanoparticles, Au and FeOOH simultaneously. ZnO nanorods are firstly modified by CdS nanoparticles to broaden the light absorbance spectrum and reduce charge recombination. Then Au and FeOOH cocatalysts decorated on inner and outer surface sites of the photocatalysts can drive photogenerated electrons and holes to flow in opposite directions, further promoting charge separation and PEC performance.

Fig. 8 The proposed mechanism for photocatalytic hydrogen production mediated by CoO_x/TiO₂/Pt. (a) The reaction process. (b) Band structure of the photocatalysts. (c) Synthetic process for TiO₂/Pt and CoO_x/TiO₂/Pt photocatalysts by template-assisted ALD. Adapted with permission from ref. 114. Copyright 2017 Wiley.

Considering the high price of noble metals, new and efficient cocatalysts such as organic cocatalysts have been explored to boost the photocatalytic reduction activity. Xu et al.¹¹⁸ synthesized a new Schottky junction using graphdiyne (GDY) cocatalyst coupled TiO₂ nanofibers for enhancing photocatalytic CO₂ reduction efficiency (Fig. 9a and b). First-principle calculations and in situ X-ray photoelectron spectroscopy (XPS) measurements show that there may be electron transfer or a strong chemical interaction between TiO₂ and GDY, resulting in the formation of an IEF at the interfaces between GDY and TiO₂ (Fig. 9c). In addition, GDY has strong chemisorption to CO₂ molecules that can enhance CO₂ adsorption and activation as well as accelerate catalytic reactions over the TiO₂/GDY heterostructure. The formed IEF, in conjunction with the photothermal effect and strong chemisorption of GDY on CO₂ molecules, leads to significantly improved CO₂ photoreduction efficiency.

Fig. 9 (a and b) TEM and HRTEM images of 0.5 wt% graphdiyne (GDY) loaded TiO₂ nanofibers. (c) Schematic illustration of TiO₂/GDY heterojunction: internal electric field-induced charge transfer and separation under UV-visible light irradiation for CO₂ photoreduction. Adapted with permission from ref. 118. Copyright 2019 Wiley.

2.4 Z-scheme heterojunction

One issue with type II heterojunction, p–n heterojunction and Schottky junction is that the redox ability of photoexcited electrons and holes for each reaction is often weakened by the fact that the photogenerated electrons in the CB of semiconductor 1 could transfer to the CB of semiconductor 2 with relatively more positive band energy (Fig. 1e), and the photogenerated holes could transfer to the VB of the semiconductor with a more negative band energy due to the well-matched bandgap energy levels between these two semiconductors.^119,120 Inspired by the mechanism of natural photosynthesis (NPS) in green plants, the construction of an artificial Z-scheme system by two connected semiconductor photocatalysts might overcome this problem.¹²¹ Specifically, in a typical Z-scheme photocatalytic system, the photogenerated electrons in semiconductor 1 would transfer and recombine with the photogenerated holes in semiconductor 2, giving the photogenerated electrons a relatively higher position and the photogenerated holes a lower position to participate in the redox reactions.¹²²

Recently, the design and synthesis of Z-scheme 1D nanomaterial-based heterojunctions with hierarchical structures have been studied for photocatalytic hydrogen production, photocatalytic CO₂ reduction and PEC reactions.^54,121 Initially, Z-scheme photocatalytic systems consisting of two semiconductors and a solid-state electron mediator at the interface of two photocatalysts were developed for water splitting.^119,122 Noble metal particles (such as Au, Ag) and reduced graphene oxide (RGO) were explored as electron mediators to accelerate the transport of photogenerated electrons and holes.¹²⁰ For example, Z-scheme WO₃/Au/In₂S₃ nanowire arrays¹²³ were successfully constructed through a two-step vapor deposition process (Fig. 10a and b). A photo-assisted Kelvin probe force microscope (KPFM) technique was utilized to detect the interfacial charge transfer of Z-scheme WO₃/Au/In₂S₃ nanowires, showing that the vectorial hole transfer of In₂S₃ → Au → WO₃ occurs upon excitation of both WO₃ and In₂S₃. Au nanoparticles embedded between WO₃ and In₂S₃, working as charge mediators, can accelerate the recombination between the photogenerated electrons in the CB of WO₃ and the photogenerated holes in the VB of In₂S₃, leaving the photogenerated electrons in the CB of In₂S₃ and the photogenerated holes in the VB of WO₃ with a strong redox ability (Fig. 10c). Therefore, a strong charge carrier separation ability and redox ability of the unique Z-scheme photocatalytic system provide the WO₃/Au/In₂S₃ heterostructure with great photocatalytic activity toward CO₂ reduction (Fig. 10d). Similarly, other Z-scheme system-based 1D photocatalysts with noble-metal nanoparticles as electron mediators, such as TiO₂/Au/Cu₂O, CdS/Ag/TiO₂, g-C₃N₄/Pt/TiO₂, have also been reported to exhibit enhanced photocatalytic activities.^124–126

Fig. 10 (a and b) TEM and HRTEM images of WO₃/Au/In₂S₃. (c) Schematic illustration of the charge separation and transfer in the Z-scheme system of WO₃/Au/In₂S₃. (d) The interfacial electron transfer between WO₃ and In₂S₃. Adapted with permission from ref. 123. Copyright 2016 American Chemical Society.

In addition to noble metals, some low-cost nonmetal materials such as reduced graphene oxide (RGO) with excellent conductivity can also be explored as electron mediators in indirect Z-scheme systems.¹²⁰ For example, a Z-scheme (TNT–GR–CdS) system with graphene (GR) film as the electron mediator in contact with both TiO₂ nanotube (TNT) arrays and CdS QDs was constructed to be applied in waste water treatment.¹²⁷ Under ultraviolet visible (UV-vis) light irradiation, the photoexcited electrons in the CB of the TiO₂ nanotube are transferred to GR and then to the VB of CdS, subsequently recombining with the holes photogenerated in CdS. The remaining VB holes of the TNT with a strong oxidation power oxidize dyes or H₂O to produce hydroxyl radicals (˙OH). The charge transport of this Z-scheme system not only achieves efficient spatial separation of photogenerated carriers and keeps the photogenerated electron and hole with strong reduction and oxidation ability but also restricts the photocorrosion of CdS, leading to the high photocatalytic activity and photostability of the TNT–GR–CdS composite film (Fig. 11). Similarly, Zhou et al.¹²⁸ fabricated a Fe₂V₄O₁₃ nanoribbon/reduced graphene oxide (RGO)/CdS nanoparticle (Z-scheme system) composite photocatalyst for photoconversion of gaseous CO₂ to methane. The Z-scheme photocatalytic system leads to more efficient spatial separation of the electrons and holes and subsequent high photoconversion efficiency of CO₂.

Fig. 11 Design and architectures of (a) the original TiO₂ nanotube (TNT) electrode, (b) the TNT–CdS quantum dots (QDs), (c) the TNT–graphene (GR), (d) the TNT–GR–CdS, (e) the TNT–CdS–GR, and (f) the TNT–CdS–GR–CdS composite film electrodes. Adapted with permission from ref. 127. Copyright 2014 American Chemical Society.

Although Z-scheme systems with appropriate mediators have been well developed and applied to water splitting, CO₂ reduction and pollutant degradation, the preparation of ternary system composite photocatalysts consisting of two semiconductors and an electron mediator is often complex. In addition, due to the surface plasmon resonance (SPR) effect of noble metals, the electron mediator can absorb visible light, consequently affecting the photocatalytic activity. Therefore, the fabrication of direct Z-scheme system photocatalysts without redox mediators has aroused widespread interest in recent years. Like the type II heterojunction, direct Z-scheme 1D-based composite photocatalysts with core–shell, 3D (2D/1D) and 0D/1D structures have been designed and investigated to improve the photocatalytic activity.^120,129 In our previous work,¹³⁰ a Z-scheme 3D ZnIn₂S₄/TiO₂ composite photocatalyst was constructed by assembling 2D ZnIn₂S₄ nanosheets onto 1D TiO₂ nanobelts for photocatalytic CO₂ reduction (Fig. 12a and b). The unique 3D morphology not only provides a large surface area but also promotes the separation and transfer of photogenerated electrons and holes. This 3D ZnIn₂S₄/TiO₂ composite photocatalyst demonstrated excellent photocatalytic reduction activity, about 39 times higher than that of bare ZnIn₂S₄ nanosheets. The enhanced photocatalytic activity is ascribed to the unique 3D structure and effective separation of the charge carriers between ZnIn₂S₄ and TiO₂ in the Z-scheme system (Fig. 12c). In order to determine whether a ZnIn₂S₄/TiO₂ composite photocatalyst is a direct Z-scheme photocatalytic system rather than a type II heterojunction system, photoluminescence (PL) spectra and transient time-resolved PL decay measurements were employed. In addition, the band potentials of ZnIn₂S₄ and TiO₂ corresponding with reaction products can further confirm this proposal. Similarly, other Z-scheme 3D structure (2D/1D) composite photocatalysts, such as TiO₂/CuInS₂, 1D/2D carbon nanotubes (CNTs)/protonated carbon nitrides (p-C₃N₄), and MoS₂/Ag₂Mo₂O₇ have also been reported to exhibit enhanced photocatalytic activities.^131–133 Except for some Z-scheme 3D structure (2D/1D) heterojunctions, a number of composite photocatalysts with 0D/1D structure have been constructed as Z-scheme systems. For example, graphene quantum dot (GQD) decorated ZnO nanowires (0D GQD/1D ZnO)¹³⁴ are proposed to follow a direct Z-scheme system heterojunction, which showed a considerable improvement in the photocatalytic degradation of methylene blue with about 3-fold enhancement as compared to pure ZnO nanowires. Similar enhancement of photocatalytic and photoelectric activity has been observed in other 0D/1D direct Z-scheme heterojunctions such as CdS QDs/CeO₂ nanorods,¹³⁵ Co₂P nanoparticles/CdS nanorods,¹³⁶ and InVO₄ nanoparticles/β-AgVO₃ nanoribbons.¹³⁷ Based on the mechanisms of photogenerated charge transfer, Z-scheme systems have more advantages for photocatalysis than type II systems. Thus, a variety of heterojunctions has been constructed according to the Z-scheme concept. Many characteristic methods such as time-resolved photoluminescence (PL), Kelvin probe force microscopy (KPFM), photochemical tests, and trapping experiments have been used to demonstrate the transfer pathways of electrons and holes. However, there is no direct evidence for the actual transfer paths for photogenerated carriers. Thus, it is still a challenge to find a new characteristic method to directly confirm the Z-scheme system.

Fig. 12 (a and b) SEM images of TiO₂ nanobelts and ZnIn₂S₄/TiO₂. (c) Schematic diagrams for energy bands of TiO₂ nanobelts and ZnIn₂S₄ nanosheets and the transfer of photogenerated electrons from TiO₂ to ZnIn₂S₄ forming a Z-scheme system under UV-vis light irradiation. Adapted with permission from ref. 130. Copyright 2017 Elsevier.

2.5 S-scheme heterojunction

Recently, Yu et al. proposed a new step scheme (S-scheme) system heterojunction. Like the Z-scheme system, it can provide a stronger redox ability and promote separation and transfer of photoexcited carries.¹³⁸ Specifically, S-scheme heterojunctions consist of two n-type semiconductors 1 and 2 (Fig. 1f). Semiconductor 1 is an oxidized photocatalyst with a larger work function and a lower Fermi energy level, and semiconductor 2 is a reductive photocatalyst with a smaller work function and a higher Fermi energy level. The transfer pathway of photo-excited charge in the S-scheme heterojunctions is more like a “step” or “N”, which means the electrons from the CB of semiconductor 1 recombine with the holes in the VB of semiconductor 2, then the electrons and holes with stronger reduction and oxidation ability are separated spatially. Furthermore, due to the difference in the Fermi energy levels between the two semiconductors, the electrons of semiconductor 1 will continue to transfer to semiconductor 2 until the Fermi energy levels are at equilibrium when the heterojunction is formed. As a result, an IEF is formed at the interface between these two semiconductors, which is the main driving force for the photogenerated carrier migration.¹³⁹

1D nanomaterial-based S-scheme system heterojunctions have been designed and fabricated to accelerate the separation and transfer efficiency of photogenerated charges. For example, a TiO₂/CdS core–shell S-scheme heterojunction prepared by an in situ electrospinning method showed excellent H₂ generation activity, which is 35 times higher than that of pure 1D TiO₂ nanofibers (Fig. 13).¹⁴⁰ The enhanced photocatalytic activity can be attributed to the synergy between the core–shell structure and the rapid transfer of photogenerated carriers in the S-scheme photosystem. The S-scheme heterojunction was confirmed by XPS analysis and density functional theory (DFT) calculations. Another S-scheme heterojunction based on Bi₂O₃ nanoparticle modified TiO₂ nanotube (Bi₂O₃/TiO₂) was found to improve the photocatalytic degradation performance for phenol.¹⁴¹ In addition to binary composite photocatalysts, some 1D nanomaterial-based ternary heterojunctions with an S-scheme photosystem have also been reported to improve photocatalytic activity. For instance, Mei et al. constructed a porous g-C₃N₄/Ag/Ag₃VO₄ S-scheme by a chemical deposition method that showed the best photodegradation rate of 99.3% within 8 min.¹⁴² The Ag particles not only have a role of charge mediator to promote the recombination of the photogenerated electrons in the CB of Ag₃VO₄ with the photogenerated holes in the VB of Pg-C₃N₄ but also have an SPR effect to enhance visible light absorption (Fig. 14). A similar ternary S-scheme g-C₃N₄/Bi/BiVO₄ heterojunction was applied to improve CO₂ reduction.¹⁴³ In this work, the metal Bi has the same role as that of Ag particles in the above work. In fact, the S-scheme system heterojunction is a kind of Z-scheme system heterojunction. They have the same transfer mechanism of photogenerated carriers. The difference between these two semiconductors is that the S-scheme system heterojunction is composed of two n-type semiconductors with suitable Fermi energy levels. Thus, most research studies seldom distinguish S-scheme system heterojunctions from Z-system heterojunctions. Both systems can facilitate the separation efficiency of photogenerated carriers and improve the photocatalytic activity.

Fig. 13 FESEM images of (a) T, and (b) TC10. (c) STEM image of 10 wt% CdS-loaded TiO₂ nanofibers and the corresponding EDX elemental mappings of Ti, O, Cd, and S. (d) Schematic of the S-scheme pathway for charge carrier separation in TiO₂/CdS nanofibers. Adapted with permission from ref. 140. Copyright 2019 Wiley.

Fig. 14 The possible charge transfer diagram of Pg-C₃N₄/Ag₃VO₄ composites. Adapted with permission from ref. 142. Copyright 2019 Elsevier.

3 Summary and perspective

In summary, 1D nanomaterials have attracted considerable interest in photocatalysis and photoelectrocatalysis owing to their many advantages including large surface area, rapid and long-distance electron transport, and improved light absorption and scattering. In particular, due to their morphology, 1D nanomaterials have been widely used as excellent building blocks to construct heterojunctions for solving the bottleneck problem of fast recombination of photogenerated carriers. 1D-based heterostructured photocatalysts with various types of hierarchical structures have been designed and demonstrated to enhance the separation and transfer efficiency of photogenerated charge carriers, consequently improving the photocatalytic and photoelectrocatalytic activities. Due to the different preparation methods, band energies, morphologies, and structures, heterostructured photocatalysts exhibit different photogenerated charge transfer mechanisms and photocatalytic activities. We have discussed several mechanisms of photogenerated charge carriers in 1D nanomaterial-based heterojunctions, including type II heterojunction, p–n type heterojunction, Schottky junction, Z-type heterojunction and S-type heterojunction.

Despite great progress made to date, there are still many challenges remaining for photocatalytic and photoelectrocatalytic applications of 1D nanomaterials and improvements are needed. First, the apparent quantum efficiency and separation efficiency of photogenerated charges in photocatalysis are generally still low. To develop new 1D-based composite photocatalysts with different hierarchical structures and optimal control of composition at the atomic level is highly desired. For example, use of advanced technology and equipment such as atomic-layer deposition (ALD) method and microsynthesis should be exploited to achieve more accurate design and control. Second, most current characterization methods, such as electron paramagnetic resonance (EPR), reactive species scavenging, XPS characterization, and terephthalic acid-assisted PL spectroscopy, cannot provide direct evidence about the pathway of electron and hole transfer. It is of strong interest to develop more advanced characterization methods to directly probe the charge transfer pathways and gain deeper mechanistic insight. With rational structural design/control and more in-depth mechanistic probes, it is expected that 1D nanomaterials will play a major role in photocatalysis and photoelectrocatalysis in the future.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was partly supported by the National Natural Science Foundation of China (No. 21978276), the Fundamental Research Funds for the Central Universities (Grant No. 2652019157; 2652019158; 2652019159), and the Innovation Ability Improvement Project of Hebei Province, China (No. 20543601D).

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