One-dimensional nanostructure based materials for versatile photocatalytic applications

Abstract

One-dimensional (1D) nanostructures are believed to play a significant role on the horizon of material science, and are a promising class of ideal high performance candidates for energy storage and conversion owing to their unique optical, structural and electronic properties. In particular, 1D nanostructure-based photocatalysts have been attracting ever-growing research attention. In this review article, we mainly focus on systematically summarizing the applications of 1D-based nanocomposites in photocatalysis, including nonselective processes for the degradation of pollutants, direct solar energy conversion to storable fuels and selective transformations for organic synthesis. Particularly, we explore the new directions for boosting the photocatalytic performances of 1D nanostructures, including graphene-1D nanocomposites, surface modification, 1D core–shell nanostructures and different exposed facet effects. It is hoped that this article will promote the efficient harnessing and rational development of the outstanding structural and electronic properties of 1D nanostructures to design more efficient 1D-based photocatalysts towards numerous applications in the field of solar energy conversion.

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1. Introduction

Since the discovery of carbon nanotubes (CNTs) in the early 1990s,¹ there has been increased interest in one-dimensional (1D) nanostructures (nanowires, nanotubes, nanobelts and nanoribbons) due to their fascinating physical properties and potential technological applications.^2–11 Based on their unique geometric characteristics, 1D nanostructures are believed to play an important role as next-generation building blocks for electronic or photoelectronic devices, for chemical or biological sensors, and for energy harvesting, storage, and conversion.^3,12–16 Both descriptors are pertinent to the physical and technological significance of 1D nanostructures. Firstly, when the diameter puts the radial dimension of the nanowire at or below certain characteristic lengths, such as the Bohr radius, the wavelength of the light, phonon mean-free path, and others, quantum mechanical effects become important.^17,18 As a result, many physical properties of semiconductors are significantly altered by the surfaces of 1D nanostructures. Additionally, with a large surface-to-volume ratio and two-dimensional confinement, 1D nanostructures also show distinct chemical and structural behavior and greater chemical reactivity, which are different from those of their corresponding bulk materials. Secondly, the large aspect ratio of 1D nanostructures is also beneficial to their technological applications. The one unfettered dimension can directly transmit quantum particles, such as photons, phonons, and electrons. As a result, 1D nanostructures are believed to be ideal materials to manufacture advanced solid-state devices due to their control over various forms of energy transport. Moreover, lengths of 1D nanostructures are normally satisfactory to interface with top-down manufacturing processes, such as photolithography. Thus, 1D nanostructures may provide a convenient platform to researchers to study confined transport phenomena.¹⁹ At the beginning of this century, many impressive reviews have been published on topics related to 1D nanostructures, mainly concerning the synthesis and characterization of 1D nanostructures.^2,5,17,19,20 However, relatively few papers have focused on systematically summarizing the possible practical applications of 1D nanomaterials, which is of great importance to both scientific and industrial concerns. Thus, with the potential for solving the current drawbacks and problems related to energy and the environment, possible practical applications of 1D based nanomaterials arguably deserve more separate dedicated reviews. In recent years, the application of 1D nanostructures in photocatalysis, namely solar energy conversion, is becoming one of the most active research areas within the nanoscience community. However, up to now, very few reviews have systematically outlined the utilization of 1D based nanostructures in heterogeneous photocatalysis. This situation is a strong incentive for us to write this feature article, providing a review on this significant topic regarding 1D based photocatalysts for solar energy conversion.

Since the 21st century, aggravating energy and environmental problems such as fossil fuel depletion, global warming and pollution have walked into our lives and rung alarm bells for human society.^21,22 As one of candidates with most potential for resolving energy and environmental issues, heterogeneous photocatalysis has been developed extensively in the past 40 years.^23–29 In a broad sense, photocatalysis involves the use of light to promote chemical transformations on organic or inorganic substrates which are penetrable in the wavelength range applied.^30,31 Photocatalysts can be excited by absorbing radiation and thus induce energy- or electron-transfer reactions, thereby driving the chemical reactions.^26,29,32,33 However, the practical applications of this technique are still limited by the inability to utilize visible light efficiently, low quantum efficiency, or the possible photo-degradation of the photocatalyst. Thus, to develop photocatalysts with improved photoactivity and efficiency has remained an enduring theme both fundamentally and technologically in the photocatalysis area.

Recent years have witnessed an exponential growth in the synthesizing of 1D nanostructures and their potential applications in photocatalysis. As compared to the commonly used nanoparticles or bulk materials, 1D nanostructures can provide unique benefits as photocatalysts in view of the following features.^34–42 First, the 1D geometry facilitates fast and long-distance electron transport. Second, 1D nanostructures are expected to have a larger specific surface area and pore volume as compared with corresponding bulk materials. Third, based on the high length-to-diameter ratio for the 1D nanostructures, the light absorption and scattering are thought to be markedly enhanced. Therefore, it should be of significant interest to investigate the potential applications of 1D-based nanocomposites in the field of heterogeneous photocatalysis.

Most recently, multi-component 1D-based nanocomposites, which are expected to overcome the drawbacks of single phase nanostructures, such as the inability to utilize visible light efficiently, low quantum efficiency, or the possible photo-degradation of photocatalyst, are investigated in heterogeneous photocatalysis. The formation of p–n junction, Z-scheme and spatial structures, such as core–shell structures, nanotree structures etc., makes these multi-component 1D-based nanocomposites with unique properties for versatile photocatalytic applications, especially for solar energy conversion to storable fuels, selective transformations for organic synthesis and photocatalytic disinfection.^43–45 In this review article, we provide a mini-review with a focus on the promising utilization of 1D nanostructures for target applications in heterogeneous photocatalysis. To the best of our knowledge, such a systematically summarized report on 1D nanostructure based photocatalysis, including non-selective processes for environmental remediation, solar energy conversion to storable fuels, selective transformations for organic synthesis and photocatalytic disinfection has remained unavailable. Some typical examples have been chosen to be discussed in each section as shown below, without bias on research works from specific research groups. Particularly, we emphasize the new directions for boosting the photocatalytic performances of 1D nanostructures, including graphene-1D nanocomposites, surface modification, 1D core–shell nanostructures and different exposed facet effects. It is hoped that this review article will inspire ongoing efforts to rationally utilize the outstanding structural and electronic properties of 1D nanostructures to design more efficient 1D-based photocatalysts towards numerous applications in the conversion of solar energy.

2. Nonselective photocatalytic processes for the degradation of pollutants

Nonselective photocatalysis has been attracting research interest because of its great potential for environmental remediation, such as the removal of stubborn, noxious compounds or non-biodegradable molecules from the air, soil and water.^{25,26,46–49} The contaminant materials are mineralized to a large extent into stable inorganic compounds, such as water, carbon dioxide and salts. Therefore, nonselective photocatalysis can be utilized in environmental protection and amelioration because of its strong oxidation power to eliminate hazardous chemical compounds from the air, soil, and water.⁴⁹ On account of the superior electron conductivity and mobility, high adsorption capacity and high specific surface area of 1D nanostructures, 1D-based nanocomposites can serve as a new type of promising photocatalyst and have been utilized in the nonselective degradation of pollutants for environmental amelioration.^{32,33,45,50–197}

As one of the most studied semiconductors, 1D TiO₂ nanostructures have been synthesized for investigation for the nonselective degradation of pollutants.^{33,58,81,87,88,134,147–149,152,154,165,175–177,190,198} Fujishima and co-workers¹⁹⁷ have synthesized highly ordered TiO₂ nanotube arrays by electrochemical anodization for the photocatalytic degradation of gaseous acetaldehyde pollutants in air, as shown in Fig. 1. The length effect of the TiO₂ nanotube arrays for photoactivity has also been investigated. The results show that increasing the lengths of nanotubes arrays within a certain range could significantly improve the degradation rate of acetaldehyde molecules. This provides a speculative explanation for the length effect on the photoactivity of TiO₂ nanotube arrays. Moreover, by detecting the products of CO₂, the main mineralization of the acetaldehyde process has been demonstrated. Wu and coworkers¹¹⁹ have reported a mechanistic study on a novel nanobelt structure that overcomes the drawback of sphere-shaped nanoparticles. The single-crystalline anatase TiO₂ nanobelts exhibit enhanced photocatalytic activity for the degradation of MO over the nanosphere counterparts with an identical crystal phase and similar specific surface area (Fig. 2). The effects of the shape and the surface structure of the nanobelts are investigated to understand the underlying mechanism of the enhanced photoactivity of the anatase TiO₂ (101) nanobelts. The density functional theory (DFT) calculations show that the exposed (101) facet of the nanobelts yields an enhanced reactivity with molecular O₂, facilitating the generation of superoxide radicals. Moreover, the group has also demonstrated that the nanobelts exhibit a lower electron–hole recombination rate than the nanospheres due to the greater charge mobility in the nanobelts, fewer localized states near the band edges and in the bandgap and enhanced charge separation due to the trapping of photogenerated electrons by chemisorbed molecular O₂ on the (101) facet. The exposed specific crystal facets with special properties that are formed in the anisotropic growth of 1D nanostructures are considered to lead to an exciting direction for developing highly active new 1D-based photocatalysts.

Fig. 1 (A) Top view SEM image of the as-prepared TiO₂ nanotube arrays and (B) Plots of the decrease in acetaldehyde concentration vs. irradiation time during the photocatalytic degradation of acetaldehyde by TiO₂ nanotube arrays with lengths of 0.2, 8.4, and 17 μm. (Reprinted with permission from the American Chemical Society.)¹⁹⁷

Fig. 2 (A) Bright-field TEM image of a TiO₂ nanobelt; the inset is a selected area electron diffraction (SAED) pattern taken along the [100] direction of the nanobelt, (B) schematic illustrations of the nanobelt and the relationship between the incident beam and the nanobelt whilst taking images and diffraction patterns and (C) the degradation of MO in an aqueous solution by the TiO₂ nanobelts and nanospheres as a function of exposure time to UV irradiation. (Reprinted with permission from the American Chemical Society.)¹¹⁹

Other single phase semiconductors have also been reported for nonselective photocatalytic processes. Wang's group¹⁰¹ has reported the synthesis of ZnO nanotubes grown via a Ga-catalyzed vapor transport method at low temperature, as shown in Fig. 3A and B. They have proposed that the active growth surface is at the root of the nanotube, and the growth of nanotubes occurs by a vapor–liquid–solid (VLS) process. The photocatalytic degradation of an azobenzene-containing polymer PAZO solution and rhodamine B (RhB) on ZnO nanotubes has been studied, as shown in Fig. 3C and D, respectively. The photoactivity of the ZnO nanotubes is much higher than the ZnO thin films and ZnO nanowires. Although the effects of the electrical and optical properties on the enhanced photoactivity are ignored, this work provides a new synthesis method for ZnO nanotubes. Zhu's group¹⁶⁶ has reported the transformation of graphitic carbon nitride (g-C₃N₄) from nanoplates to nanorods by a simple reflux method (Fig. 4A and B). The photoactivity for the degradation of methylene blue (MB) (Fig. 4C) and the intensity of the photocurrent response of g-C₃N₄ nanorods under visible light are about 1.5 and 2.0 times higher than that of g-C₃N₄ nanoplates, respectively. The optical and electrical properties are investigated as a result of the enhanced photoactivity. Moreover, the photocatalytic mechanism is also studied. Combined with the electron spin resonance (ESR) results, superoxide radicals are still the main oxidative species for the g-C₃N₄ nanorod samples, as shown in Fig. 4D. Thus, the nanorod structures and high crystallinity of g-C₃N₄ may enhance the separation of electron–hole (e–h) pairs, accelerate the transfer of photo-induced charge carriers, and inhibit the recombination of the e–h pairs, resulting in the increase of the number of main superoxide (O₂˙⁻) and hydroxyl (˙OH) radicals participating in the photo-oxidation process. The morphology and crystallinity play an important role in enhancing the photocatalytic activity. In this regard, the 1D-based nanostructures are worthy of investigation in order to improve the photoactivity of a specific semiconductor. Besides, this metal-free material also shows economical potential for large scale synthesis.

Fig. 3 (A) Transmission electron microscopy (TEM) image of a ZnO nanotube; the upper inset displays the diffraction pattern of the nanotube; the lower inset is a high-resolution TEM image of the nanotube, (B) proposed growth mechanism of the ZnO nanotubes, (C) UV-vis absorption spectra of PAZO as a function of irradiation time with the presence of ZnO nanotubes and (D) plot of degradation ln(C/C₀) as a function of irradiation time by RhB. (Reprinted with permission from the American Chemical Society.)¹⁰¹

Fig. 4 (A) TEM image of g-C₃N₄ nanoplates, (B) TEM image of g-C₃N₄ nanorods, (C) apparent rate constants for MB photodegradation over g-C₃N₄nanoplates and g-C₃N₄ nanorods under visible light (λ > 420 nm, [MB] = 0.03 mM) and (D) ESR spectra of g-C₃N₄ nanoplates and g-C₃N₄ nanorods in water and DMSO solvents under visible light irradiation: (a) in water; (b) in DMSO solvent (DMPO as a radical trapper, ★ labels the carbon radicals, ◆labels the superoxide radicals). (Reprinted with permission from the American Chemical Society.)¹⁶⁶

However, the inability to utilize visible light efficiently, low quantum efficiency, or the possible photo-degradation of photocatalyst still limits the utilization of single phase 1D nanostructures. Recently, multi-component 1D-based nanocomposites have shown potential to overcome these drawbacks of single phase 1D nanostructures. With utilizing different polyelectrolytes for the adjusting of surface charge properties, Xiao has developed a facile and easily accessible layer-by-layer (LBL) self-assembly route to synthesize a series of 1D noble metal/semiconductor nanocomposites (Fig. 5A and B).^{182–185,199} The intimate interfacial contact between the metal nanoparticles (NPs) and semiconductor substrate arising from the pronounced electrostatic attractive interaction is afforded by the multilayering of polyelectrolytes. These integrated heterostructures show remarkably enhanced photoactivity and outstanding photo-stability for the photo-degradation of organic dyes (Fig. 5C). Photoelectrochemical exploitations have substantiated the role of the metal NPs acting as an “electron reservoir” in prolonging the lifetime of photogenerated electron–hole charge carriers (Fig. 5D). These works provide a new and facile strategy for designing highly ordered metal/1D semiconductor hybrid nanostructures based on the facile LBL self-assembly method.

Fig. 5 (A) SEM image of the titania nanotube arrays (TNTs) substrate, (B) top-view SEM image of the Pt/TNTs hybrid nanostructures prepared via the LBL self-assembly approach, (C) photocatalytic performances of the TNTs and M/TNTs (M = Au, Ag, Pt) heterostructures towards the degradation of organic dye pollutant (MO) under the irradiation of UV light (365 ± 15 nm) and (D) pictorial representation of the proposed mechanism for the photocatalytic degradation of dye pollutant over the M/TNTs (M = Au, Ag, Pt) heterostructures. (Reprinted with permission from the American Chemical Society.)¹⁸⁵

Except for the 1D semiconductor-based nanocomposites, 1D carbon nanotube (CNT)-based nanocomposites and 1D metal-based nanocomposites are also beneficial for photocatalysis processes. Zeng's group⁹⁶ has prepared mesocrystals of anatase TiO₂ onto multiwalled carbon nanotubes (CNTs) by a one-pot chemical approach, as displayed in Fig. 6. By investigating the formation mechanism, mesoporous products are simply resulted from a re-crystallization event.²⁰⁰ Under their mild reaction conditions, part of the gaseous HF might be trapped/adsorbed on growing TiO₂ crystallites. This gas–solid interface may prevent the crystallites from further growth, as the supply of TiF₄ from liquid is terminated, noting that new growth can always take place at any available liquid–solid interface, resulting in a well connected mesocrystalline structure. However, other less plausible mechanisms cannot be entirely ruled out.^201,202 The observation of a high photoactivity for MO degradation can be attributed to the large specific area of the TiO₂ phase, through-porosity, the overall oriented attachment among TiO₂ nanocrystallites and surface defect sites and conductive CNTs support which may serve as electron reservoirs to suppress the recombination of electron–hole pairs. Although CNTs themselves show no photoactivity, their unique optical and electrical properties make them excellent substrates for supporting other semiconductors as efficient 1D photocatalysts. Ye's group⁴⁵ has also developed a facile and effective approach for preparing core–shell Ag/AgCl hetero-nanowires with uniform structures by an in situ oxidation reaction at room temperature (Fig. 7B). The formation mechanism considers that the particular stabilization and structure induction of PVP may promote the epitaxial growth of AgCl crystals on the surface of Ag nanowires and cause the formation of the resultant Ag/AgCl nanowires with a uniform and perfect core–shell structure. The Ag/AgCl core–shell hetero-nanowires with a uniform and perfect structure are the result of the Ostwald ripening process (Fig. 7A). In addition, by studying the nanostructures of Ag/AgCl synthesized with other metal ions, this suggests that the redox potential of both the silver species and metal ions play important roles in determining the morphology and structures of silver/silver halide nanostructures by varying the oxidation rate between Ag crystals and metal ions. From the photoactivity test of the decomposition of methyl orange (MO) dye under visible light irradiation as shown in Fig. 7C,²⁰³ it is notable that the photocatalytic properties of these 1D core–shell nanostructures are tunable by changing the inner core and outer shell composition. Combining the unique optical and electrical properties of 1D nanostructures with protecting and coupling advantages of core–shell structures, 1D core–shell nanostructures for multiple photocatalysis reactions might lead to an intriguing research current for 1D-based nanostructure photocatalysts.

Fig. 6 Scanning electron microscopy (SEM) images of mesoporous TiO₂/CNTs nanocomposites: (A) panoramic views at low magnification and (B) a single strand of TiO₂/CNTs at higher magnification. (Reprinted with permission from the American Chemical Society.)⁹⁶

Fig. 7 (A) SEM images of Ag/AgCl core–shell nanowires during the growth process at different reaction times from 0 min to 40 min, (B) schematic illustration of the in situ oxidation process for Ag/AgCl core–shell nanowires and (C) photocatalytic activities of Ag/AgCl core–shell nanowires for the decomposition of MO dye under visible light irradiation. (Reprinted with permission from the Royal Society of Chemistry.)⁴⁵

3. Solar energy conversion to storable fuels

As the energy issue becomes increasingly serious worldwide, the development of low-cost and efficient light-harvesting materials has attracted increasing research interest for their great potential to produce renewable energy sources.²² As a potential solution to the global energy crisis and environmental pollution, the application of hydrogen energy has attracted great attention.²⁰⁴ At present, hydrogen is mainly produced from fossil fuels or high-energy consumption processes, which are not environmentally friendly and economic.²⁰⁵ Therefore, photocatalytic H₂-production using solar energy, which is a promising approach for clean and low-cost production of hydrogen,^27,206 has received more and more attention.²⁰⁷ On the other hand, the growing concerns about the emissions of CO₂, which is a major anthropogenic greenhouse gas, have driven research activities for CO₂ capture, storage and utilization. Particularly, the catalytic conversions of CO₂ to fuels and chemicals have attracted much attention in recent years. Thus far, some 1D nanostructures-based nanocomposites have been utilized for solar energy conversion to storable fuels.^{43,44,208–264}

Yang's group⁴⁴ has chosen earth-abundant and stable semiconductors, silicon (Si) and titanium dioxide (TiO₂), as the hydrogen-generating photocathode and oxygen-generating photoanode, respectively, for direct solar water splitting. Owing to the differences in catalytic and electrical transport properties of the two materials, the formation of a nanoscale tree-like light-harvesting architecture displays much higher photoactivity than the configuration where a TiO₂ thin-film is partially deposited onto Si nanowire, suggesting that the structural features for the nanoscale-tree architecture are essential due to the vastly different optical and electrical properties of the two materials. Moreover, the photocatalytic “Z-scheme”^265,266 mechanism of the nanocomposites is also investigated. Under solar illumination, due to the band-bending at the semiconductor–electrolyte interfaces,²⁶⁷ the photogenerated electrons in the Si nanowires migrate to the surface and reduce protons to generate H₂; meanwhile the photogenerated holes in the TiO₂ nanowires oxidize water to evolve O₂ (Fig. 8). The holes from Si and electrons from TiO₂ recombine at the ohmic contact, completing the relay of the “Z-scheme”,^265,266 similar to that of natural photosynthesis. At the same time the spatially separated photo-electrodes with a local ohmic contact help to segregate the products to mitigate back- reactions.²⁶⁸ This nanoscale tree-like design is thought to be in principle applicable for other “Z-scheme” materials in solar-to-fuel conversion. The example of combining Si and TiO₂ here demonstrates an important approach to enhance the charge separation/transfer, and increase the photocatalytic performances of 1D based nanostructures. Additionally, this creation of well-defined complex architectures based on 1D nanostructures might also provide opportunities for spatial charge transfer arising from their 2D or 3D structural organization in a controlled manner.

Fig. 8 Schematics of the asymmetric nanoscale tree-like heterostructures used for solar-driven water splitting. (A) Structural schematic of the nanotree heterostructure and (B) energy band diagram of the nanotree heterostructure for solar-driven water splitting. (Reprinted with permission from the American Chemical Society.)⁴⁴

Yu and co-workers⁴³ have reported a NiS–CdS system with a p–n junction that shows enhanced visible-light photocatalytic H₂-production activity, which is even higher than that of the optimized Pt–CdS nanorods. A possible photoactivity enhancement mechanism is proposed and discussed, as shown in Fig. 9A and B. The formation of p–n junctions leads to an inner electric field because of the different Fermi levels of p-type NiS and n-type CdS.^{134,269–271} This internal field leads to the excitation of photogenerated holes at the valence band (VB) of CdS towards the VB of NiS, and acceleration of the photogenerated electrons from the conduction band (CB) of NiS into the CB of CdS. Thus, the photogenerated electron–hole pairs are effectively separated by the p–n junction and a high concentration of electrons is obtained in the CB of CdS, while a high concentration of holes is obtained in the VB of NiS, which leads to the effectively reduction of H₂O (or H⁺) to produce H₂ (Fig. 9C and D). This work provides new insight into the design and fabrication of new photocatalysts combining p–n junctions with 1D nanostructures for enhancing H₂-production photoactivity.

Fig. 9 (A) Schematic diagram of charge transfer and separation, (B) the p–n junction band structure and schematic illumination of the electron–hole separation process under visible-light irradiation, (C) comparison of the visible-light photocatalytic H₂-production activity of Ni0, Ni0.5, Ni1, Ni3, Ni5, Ni10, 1 wt% Pt–CdS, NiS and Ni5-NPs samples under visible light and (D) Time course of photocatalytic H₂-production over Ni5 and 1 wt% Pt–CdS samples. (Reprinted with permission from the Royal Society of Chemistry.)⁴³

It is known that the well-defined morphology and components synergy of hybrid nanocatalysts are beneficial for the photo-induced charge carriers separation and transfer processes. Therefore, by controlling and modifying the structural morphology and composition of 1D nanostructures it is possible to adjust the charge carriers separation and transfer kinetics, thereby improving the photocatalytic efficiency for targeting reactions. For example, Song et al. have investigated geometrical (single- or double-tipped) and compositional (Pt or Au) variations of active metal components on the charge carriers transfer over CdSe nanorods under visible light irradiation.²⁷² It is considered that the tip regions are commonly regarded as highly active areas, and the sacrificial reagents, SO₃²⁻ and S²⁻ ions, are likely to receive the holes through these sites.²⁷³ The single Pt-tipped structure has the tip of a CdSe nanorod directly open to the reagent, where the hole transfer favorably occurs. However, in the double Pt-tipped structure, both tips are already occupied by the Pt dots. Therefore, the holes must be transferred to the reagents through the less active walls having no defects with strong coordination of the surfactants, which leads to a decreased performance on hydrogen generation of the double-tipped structure compared to that of the single-tipped structure (Fig. 10). This work encourages us to pay attention to the rational surface engineering of 1D based nanocomposites towards tuning photogenerated charge carriers separation and transfer and thus achieving improved photocatalytic performance.

Fig. 10 (A) Schematic representation of photocatalytic H₂ generation and (B) (a) time course of H₂ evolution by 1 (◆), 2 (▲), 3 (■), and 4 (●); 4.0 mg catalysts based on the CdSe amount in a 0.35 M Na₂SO₃/0.25 M Na₂S aqueous solution; λ ≥ 420 nm. (Reprinted with permission from the American Chemical Society.)²⁷²

In addition to the examples using 1D nanostructures as photocatalysts for H₂-production as mentioned above, Zou et al.^242,258,259 have synthesized a series of 1D Zn₂GeO₄ nanostructures for the photo-reduction of CO₂ into CO and CH₄ (Fig. 11). The mechanism of the photo-reduction of CO₂ into CO and CH₄ has also been investigated. The photogenerated holes in the valence band oxidize water to generate hydrogen ions via the half-reaction H₂O → ¹/₂O₂ + 2H⁺ + 2e⁻ (E_redox = 0.82 V vs. NHE), and the photogenerated electrons in the conduction band reduce CO₂ to CH₄ via the reaction of CO₂ + 8e⁻ + 8H⁺ → CH₄ + 2H₂O (E_redox = −0.24 V vs. NHE). Investigation of the electronic structure of a Zn₂GeO₄ nanoribbon using density functional theory has demonstrated that the photogenerated electrons and holes in the irradiated Zn₂GeO₄ can react with adsorbed CO₂ and H₂O to produce CH₄, as described in the following equation: CO₂ + H₂O → CH₄ + O₂. Similarly, the mechanism of photo-reduction of CO₂ into CO can be described as follows: under UV-vis light irradiation, the photogenerated holes on the valence band top (potential: +3.8 V vs. NHE) of Zn₂GeO₄ can lead to the oxidation of water to produce hydrogen ions via H₂O → ¹/₂O₂ + 2H⁺ + 2e⁻ (E_redox = 0.82 V vs. NHE), and the photogenerated electron on the conduction band bottom (potential: −0.7 V vs. NHE) of Zn₂GeO₄ can drive the reduction of CO₂ into CO via CO₂ + 2 e⁻ + 2H⁺ → CO + H₂O (E_redox = −0.53 V vs. NHE). Investigation into the fundamental mechanism would remarkably promote the further understanding and design of 1D photocatalysts for the photo-reduction of CO₂ into solar fuels.

Fig. 11 (A) A TEM image of the Zn₂GeO₄ nanoribbons, (B) Structural model of a nanoribbon, (C) TEM image of a Zn₂GeO₄ nanorod, (D) crystal structure model of the nanorod, (E) CH₄ generation over (a) the SSR sample, (b) nanoribbons, (c) 1 wt% Pt-loaded nanoribbons, (d) 1 wt% RuO₂-loaded nanoribbons, and (e) 1 wt%RuO₂ + 1 wt% Pt-coloaded nanoribbons as a function of light irradiation time and (F) CH₄ and CO generation over Zn₂ GeO₄ prepared by the solution phase route at 40 (a) and 100 °C (b) and solid state reaction at 1300 °C (c) as a function of UV-vis light irradiation time. (Reprinted with permission from the American Chemical Society and Royal Society of Chemistry.)^242,258,259

Additionally, some other 1D-based nanostructures have also been synthesized for the photo-reduction of CO₂ into solar fuels.^{255–257,260–264} An interesting work from Xue's group has reported that 1D TiO₂ nanofibers co-decorated with Au and Pt nanoparticles can lead to remarkably enhanced photoactivities for CO₂ reduction.^255a This great enhancement is attributed to the synergy of the electron-sink function of Pt and surface plasmon resonance (SPR) of Au nanoparticles, which significantly improves the charge separation of photoexcited TiO₂. This work demonstrates that through the rational design of composite nanostructures and coupling with the SPR effect, one can harvest visible light and improve the photoactivities of semiconductors in the whole solar spectrum for the photo-reduction of CO₂ to solar fuels.

Besides, similarly to all photocatalytic processes, other groups have also reported that the shape, defect, crystal phase or co-catalyst can significantly influence the photoactivity of nanocrystal TiO₂.^255b,c This useful information can be utilized to enhance the photocatalytic performance of 1D semiconductor-based nanostructures for CO₂ photo-reduction, e.g., improving the lifetime and transfer of photogenerated charge carriers, the adsorption and activation capacity of CO₂ over the photocatalyst surface.

4. Photocatalytic selective organic transformations

Wastewater decontamination, solar energy conversion, and photoelectrochemical processes are the main research targets in the field of photocatalysis.^{26,29,274,275} However, the possibility that this technique could also provide an alternative to more conventional organic synthetic pathways has attracted growing interest.^28,276–280 Significant examples of photocatalytic processes employed for synthetic purposes are selective oxidation and reduction processes, isomerization reactions, C–H bond activations, and C–C and C–N bond-forming reactions. It is now recognized that heterogeneous photocatalysis holds great potential for organic synthesis due to its possibility of avoiding environmentally detrimental heavy metal catalysts, strong chemical oxidants or reducing agents, such as Cr^IV, MnO₄⁻, ClO⁻, Cl₂, H₂ and CO, as well as harsh reaction conditions, such as high temperature and high pressure.^22,281 Hitherto, most reports on 1D nanostructures often focus on their applications in nonselective processes for environmental protection, whereas the utilization for selective organic transformations is relatively scarce.

Our group has carried out a series of relevant studies in this respect.^282–286 The semiconductors ZnO, TiO₂ and CdS are chosen to prepare the 1D nanostructures for the aerobic selective oxidation of alcohols to aldehydes, an important synthesis reaction of industrial importance,^287–291 under ambient conditions. As shown in Fig. 12, ZnO nanorods with a small-diameter (20–30 nm) size distribution exhibit a significantly enhanced photoactivity and selectivity compared to its precursor, commercial ZnO powder. Detailed characterizations show that the 1D morphology of ZnO nanorods favors the anti-recombination of photogenerated charge carriers and the selective favorable adsorption of reactant alcohols over product aldehydes, which are the primary reasons leading to the enhanced photoactivity and, particularly, high selectivity of 1D ZnO nanorods towards the oxidation of alcohols to their corresponding aldehydes. This work provides a generic example exemplifying the basic mantra of chemistry, “structure-dictates-function”, i.e., the 1D structure-induced highly selective photocatalyst of ZnO nanorods compared to non-1D commercial ZnO.

Fig. 12 (A) TEM image of the as-prepared ZnO nanorods, (B) HRTEM image of an as-prepared ZnO nanorod, (C) time-online profile of the photocatalytic conversion of benzyl alcohol to benzaldehyde and (D) selectivity of benzyl alcohol to benzaldehyde over the as-prepared ZnO nanorods and commercial ZnO powder under the irradiation of UV light. (Reprinted with permission from the Royal Society of Chemistry.)²⁸⁴

The optical and electrical properties of titanate nanotubes (TNT) can be tuned by doping various metal ions (Cu²⁺, Co²⁺, Ni²⁺, Fe²⁺, and Mn²⁺) via an ion-exchange method in an aqueous phase (Fig. 13).²⁸⁸ This incorporation of metal ions into the matrix of TNT is able to extend its light absorption to the visible-light region, thus making TNT have both enhanced UV-light and visible-light photoactivity as compared to the un-doped TNT, which is primarily attributed to the prolonged lifetime of the photogenerated electron–hole pairs. Our research work not only demonstrates the tunable optical property of TNT by doping metal ions, but also opens up promising prospects of 1D nanotubular TNT or TNT-based materials as visible-light-driven photocatalysts in the area of selective transformation using molecular oxygen as a benign oxidant under ambient conditions. In addition, to develop visible-light driven photocatalysts for efficiently utilizing solar energy, we also prepared the 1D CdS@TiO₂ core–shell nanocomposites (CSNs) in a subsequent work (Fig. 14).²⁸⁷ As compared to bare CdS nanowires (NWs), an obvious enhancement of both conversion and yield for the selective oxidation of alcohols to aldehydes is achieved over 1D CdS@TiO₂ CSNs (Table 1). By carrying out a series of control experiments, we have found the dual role of the TiO₂ shell. On one hand, the TiO₂ shell improves the lifetime of photogenerated charge carriers of 1D CdS@TiO₂ CSNs resulting from the transfer of photo-excited electrons from the CdS core. On the other hand, it can also block the photo-excited holes from the CdS core moving to the surface of the TiO₂ shell. Our studies might promote further interest in understanding the photocatalytic mechanism of the selective organic transformation over 1D core–shell photocatalysts.

Fig. 13 (A) UV-vis diffuse reflectance spectra (DRS) of undoped TNT and various metal-ion-doped TNT and (B) a tentative proposed mechanism for the photocatalytic selective oxidation of alcohols to corresponding aldehydes over the metal-ion-doped TNT photocatalyst. (Reprinted with permission from the American Chemical Society.)²⁸³

Fig. 14 (A) A typical SEM image of the as-prepared 1D CdS@TiO₂ CSNs; the inset is the corresponding schematic model and (B) illustration of the proposed reaction mechanism for the selective oxidation of alcohols to corresponding aldehydes over the 1D CdS@TiO₂ CSNs under visible-light irradiation. (Reprinted with permission from the American Chemical Society.)²⁸²

Table 1 Photocatalytic selective oxidation of a series of alcohols into corresponding aldehydes over CdS NWs and 1D CdS@TiO₂ CSNs under visible light (λ = 520 ± 15 nm) irradiation for 8 h

Entry	Substrate	Product	Conversion (%)		Yield (%)		Selectivity (%)
			CdS	CdS@TiO2	CdS	CdS@TiO2	CdS	CdS@TiO2
1			13	34	12	33	97	97
2			7	41	4	30	60	74
3			21	39	8	25	36	65
4			7	30	5	21	73	70
5			4	38	3	33	88	87
6			6	37	5	36	97	96

---

The 1D nanostructures have also been demonstrated to have the potential for photocatalytic selective organic reduction reactions. Choosing CdS nanowires as a substrate, by coupling with graphene and Au nanoparticles via a simple and efficient electrostatic self-assembly method, CdS nanowires-reduced graphene oxide nanocomposites (CdS NWs–RGO NCs) and CdS nanowire–Au nanocomposites (CdS NW–Au NCs) are successfully synthesized, respectively (Fig. 15 and Fig. 16).^285,286 Both of the two 1D-based nanocomposites show enhanced photoactivity towards the selective reduction of nitro organics, as compared to CdS NWs. The doping of graphene and Au nanoparticles can tune the optical and electrical properties of 1D-based nanocomposites, thereby leading to the increased photoactivity of CdS NWs. Moreover, the surface plasmon resonance (SPR) effect of Au NPs is proven to not play an efficient role in the reduction reaction. Although the carbon materials and metal nanoparticles generally show no photoactivity for these organic transformation reactions, the unique optical and electrical properties of these materials make them efficient modifiers for 1D photocatalysts.

Fig. 15 (A) Typical TEM image of the as-prepared samples of CdSNWs–RGO NCs and (B) the photocatalytic performance of the as-prepared CdS NWs and CdS NWs–RGO NCs for 4-nitroaniline reduction under visible light irradiation (λ > 420 nm) with the addition of ammonium formate as a quencher for photogenerated holes and N₂ purging at room temperature in the aqueous phase. (Reprinted with permission from the American Chemical Society.)²⁸⁵

Fig. 16 (A) Schematic flowchart for the synthesis of CdS NW–Au NCs by using an electrostatic self-assembly method at room temperature and (B) a comparison of the rate constant over CdS NWs and CdS NW–Au NCs with different weight addition ratios of Au NPs for the selective reduction of 4-nitro-aniline. (Reprinted with permission from the Royal Society of Chemistry.)²⁸⁶

5. Photocatalytic disinfection

Recent years have witnessed an increasing interest of the use of 1D nanostructures for photocatalytic disinfection.^292–295 Bacteria and viruses can be destroyed by a number of different techniques, including heat, UV radiation, antibiotics and chemical oxidation. However, for bacteria, antibiotics show a selective but slow bactericidal rate. Chemical oxidation although effective in inactivating bacteria and most viruses, generates unwanted disinfection byproducts.⁴⁸ Thus, disinfection by solar light, which is clean and effective, is becoming increasingly important. For example, Sun and co-workers²⁹² have investigated the antibacterial activity of TiO₂ nanorod spheres with or without solar light irradiation, as shown in Fig. 17. The effect of different calcination temperatures on the antibacterial activity is also studied. There is a direct relationship between the antibacterial activity and the sharpness of the TiO₂ nanorods which is based on the calcination temperature, thus leading to the highest antibacterial activity of TiO₂-500 without light irradiation. Additionally, the increase in calcination temperature promotes crystallisation and leads to the enhanced light absorption capability, which makes TiO₂-500 exhibit the highest antibacterial photoactivity under solar light irradiation. The proposed shape-determined mechanism suggests the promising potential of 1D nanostructures towards photocatalytic disinfection.

Fig. 17 Antibacterial capabilities of the obtained TiO₂ nanorod spheres against E. coli: (A) without and (B) with solar light irradiation. (Reprinted with permission from John Wiley & Sons, Inc.)²⁹²

6. Summary and outlook

In this review, we have summarized the recent progress on the utilization of 1D nanostructures in heterogeneous photocatalysis. The 1D nanostructures with new physicochemical properties, such as a high length-to-diameter ratio and efficient electron mobility, show very promising potential in the diverse field of photocatalysis.

The promising opportunities offered by 1D nanostructures are outstanding. However, some existing challenges merit attention in order to achieve more efficient 1D nanostructure based photocatalysts. For example, single phase 1D semiconductors still show a high recombination rate of photogenerated charge carriers. Photo-induced charge carriers separation represents an essential step in the process of solar energy conversion through photocatalytic reactions. The ongoing search for a system exhibiting efficient charge-separating capabilities is of key importance. Some reports have shown the selective growth of metal tips onto 1D nanostructures with enhanced photoactivity.^{225,229,272,296,297} In such structures, holes are three-dimensionally confined to the 1D semiconductors, whereas the delocalized electrons are transferred to the metal tip, leading to a long-lived charge separated state. Additionally, the integration of the two light-absorbing semiconductor domains also exhibits a prolonged lifetime of charge carriers due to the formation of a “Z-scheme”.^44,266 Thus, research works on developing the rational surface modification or engineering strategy towards facilitating photogenerated charge carriers separation and transfer are significant to improve the 1D nanostructure based photocatalysts more efficiently.

Electronic conductivity is another important effect for photocatalytic performance. For example, anatase TiO₂ has a larger band gap than that of rutile TiO₂; however, anatase TiO₂ shows better photoactivity than rutile TiO₂. This can be attributed to the low electron mobility in rutile TiO₂, which can be an obstacle to reach the electrical contact. To further increase the conductivity of 1D-based nanostructures, a new class of the nano-hetero-structures of graphene-1D nanocomposites has been recently synthesized.^{85,179,285,298–301} This class shows enhanced photoactivity due to the excellent electron conductivity of graphene. In addition, for single wide-band-gap semiconductors, the overall photocatalytic efficiency is limited by insufficient light absorption. Doping graphene into 1D semiconductors could also enhance light absorption as graphene has a high optical transparency. The synthesis of graphene-1D nanocomposites and graphene-1D-based multi-component composites is of particular interest in photocatalysis.

As important as component modification, structural modification is another effective way to improve the photocatalytic performance of the 1D-based nanocomposites. As the surface properties of nanostructures are especially important to the overall charge collection efficiency, since they can influence the recombination velocity and the chemical reaction dynamics, one method to decrease surface recombination velocity is the surface coating. Recently, some 1D core–shell nanocomposites have been reported for photocatalysis.^{45,96,141,158,175,282} Coating another component (such as noble metal nanoparticles, semiconductors and carbon materials) onto 1D nanostructure cores can efficiently prolong the lifetime and transfer the photogenerated charge carriers. On the other hand, the outer shell could also protect the inner core component from photo-corrosion or dissolution. Besides, compared to the common complex composites, 1D core–shell nanostructures provide a homogeneous environment for reactions to proceed.²⁸¹ Thus, designing various 1D core–shell nanostructures for multiple photocatalysis reactions might lead to an intriguing current for 1D-based nanostructure photocatalysts. Additionally, the creation of well-defined complex architectures based on 1D nanostructures, such as 1D nanostructured arrays and 1D-based nanotree-like nanocomposites, might also provide opportunities for the spatial charge transfer arising from their 2D or 3D structural organization.

Stimulated by the strong influence of the characteristics of the crystal facets, such as atomic arrangement, electronic structure, and defects, on the activity of photocatalysts, great interest has emerged in tuning the crystal facets of photocatalysts to optimize solar-driven photocatalytic reactivity.^302,303 Recently, some reports have shown the enhanced photoactivity of the samples with exposed specific crystal facets as compared to the common nanoparticles.^{119,304−307} The exposed specific crystal facets can lead to some special properties, such as the enhanced adsorption capability of O₂ or reactants and effective separation of the photo-induced electron–hole pairs.^119,308 The anisotropic growth of 1D structures is often driven by using capping ligands that can bind selectively onto a particular facet of the seed particles. Thus, facet engineering fully exposed with reactive facets in 1D nanostructures is an exciting direction for developing highly active new 1D-based photocatalysts.

1D-based nanostructures hold great potential to address various environmental and energy-related issues. In spite of the remarkably rapid progress, the ability of 1D-based nanostructures in this field has yet to be exploited fully. The research field of 1D-based photocatalysts is still very young. Opportunities and challenges exist together, which await us for the development of highly efficient 1D-based multifunctional photocatalytic systems. With the reasonable design and full exploration of 1D structural and electronic potential, the applications of 1D-based composite photocatalysts for the conversion of solar energy would be significantly enriched in a more rational way. It is hoped that this review will provide useful information in the utilization of 1D nanostructures for versatile photocatalytic applications, and inspire growing efforts in the area of 1D nanostructure based photocatalysis.

Acknowledgements

The support by the National Natural Science Foundation of China (NSFC) (20903023, 21173045), the Award Program for the Minjiang Scholar Professorship, the Natural Science Foundation (NSF) of Fujian Province for Distinguished Young Investigator Grant (2012J06003), Program for Changjiang Scholars and Innovative Research Team in Universities (PCSIRT0818), Program for Returned High-Level Overseas Chinese Scholars of Fujian province, and the Project Sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry, is gratefully acknowledged.

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