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Multinary I‑III-VI2 and I2‑II-IV-VI4 Semiconductor Nanostructures for Photocatalytic Applications Michelle D. Regulacio* and Ming-Yong Han* Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), 2 Fusionopolis Way, Innovis, 138634, Singapore CONSPECTUS: Semiconductor nanostructures that can effectively serve as light-responsive photocatalysts have been of considerable interest over the past decade. This is because their use in light-induced photocatalysis can potentially address some of the most serious environmental and energyrelated concerns facing the world today. One important application is photocatalytic hydrogen production from water under solar radiation. It is regarded as a clean and sustainable approach to hydrogen fuel generation because it makes use of renewable resources (i.e., sunlight and water), does not involve fossil fuel consumption, and does not result in environmental pollution or greenhouse gas emission. Another notable application is the photocatalytic degradation of nonbiodegradable dyes, which offers an effective way of ridding industrial wastewater of toxic organic pollutants prior to its release into the environment. Metal oxide semiconductors (e.g., TiO2) are the most widely studied class of semiconductor photocatalysts. Their nanostructured forms have been reported to efficiently generate hydrogen from water and effectively degrade organic dyes under ultraviolet-light irradiation. However, the wide band gap characteristic of most metal oxides precludes absorption of light in the visible region, which makes up a considerable portion of the solar radiation spectrum. Meanwhile, nanostructures of cadmium chalcogenide semiconductors (e.g., CdS), with their relatively narrow band gap that can be easily adjusted through size control and alloying, have displayed immense potential as visible-light-responsive photocatalysts, but the intrinsic toxicity of cadmium poses potential risks to human health and the environment. In developing new nanostructured semiconductors for light-driven photocatalysis, it is important to choose a semiconducting material that has a high absorption coefficient over a wide spectral range and is safe for use in real-world settings. Among the most promising candidates are the multinary chalcogenide semiconductors (MCSs), which include the ternary I-III-VI2 semiconductors (e.g., AgGaS2, CuInS2, and CuInSe2) and the quaternary I2-II-IV-VI4 semiconductors (e.g., Cu2ZnGeS4, Cu2ZnSnS4, and Ag2ZnSnS4). These inorganic compounds consist of environmentally benign elemental components, exhibit excellent light-harvesting properties, and possess band gap energies that are well-suited for solar photon absorption. Moreover, the band structures of these materials can be conveniently modified through alloying to boost their ability to harvest visible photons. In this Account, we provide a summary of recent research on the use of ternary I-III-VI2 and quaternary I2-II-IV-VI4 semiconductor nanostructures for light-induced photocatalytic applications, with focus on hydrogen production and organic dye degradation. We include a review of the solution-based methods that have been employed to prepare multinary chalcogenide semiconductor nanostructures of varying compositions, sizes, shapes, and crystal structures, which are factors that are known to have significant influence on the photocatalytic activity of semiconductor photocatalysts. The enhancement of photocatalytic performance through creation of hybrid nanoscale architectures is also presented. Lastly, views on the current challenges and future directions are discussed in the concluding section. lies.1,2 These low-toxicity semiconductors possess band structures that can be conveniently adjusted through alloying to improve their properties. When studied at the nanoscale, they have been found to exhibit tunable emission properties, which have allowed their use in optoelectronics and biological applications. For example, quantum dots (QDs) based on CuInS2 have been incorporated as optically active materials for lighting devices due to their bright emission and good thermal stability.3 Meanwhile, highly emissive AgInS2-based QDs have been found useful in biological labeling applications.4 Aside

1. INTRODUCTION The quest for environmentally benign semiconductor materials with technologically applicable properties is actively pursued in many laboratories worldwide. Among the most widely investigated semiconductor materials in recent years are the multinary chalcogenide semiconductors (MCSs). These are inorganic compounds that consist of more than two component elements and whose anion(s) belong to the chalcogen family. Although oxygen is also a member of the chalcogen group, oxides are typically distinguished from chalcogenides due to their different chemical behavior. Many of the technologically promising MCSs belong to the ternary IIII-VI2 and the quaternary I2-II-IV-VI4 semiconductor fami© XXXX American Chemical Society

Received: December 10, 2015

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Accounts of Chemical Research from their exceptional emission properties, I-III-VI2 and I2-IIIV-VI4 semiconductors are also known for their remarkable solar-harvesting properties that can be utilized in photovoltaic applications. High solar conversion efficiencies have already been recorded for solar cell devices that make use of CuInxGa1−xSe2 and Cu2ZnSnS4 as absorber materials.1,2,5 The excellent light-harvesting properties that render these MCSs desirable in the field of photovoltaics can also be exploited in the area of photocatalysis. However, while their use as active components for solar cells has been heavily explored and well documented in several publications, their potential utility as light-responsive photocatalysts has only recently started to attract attention. Light-induced photocatalysis undoubtedly holds tremendous potential in providing possible solutions to the growing number of environmental and energy-related problems. An important photocatalytic reaction is the solar-powered photocatalytic cleavage of water, which can potentially provide a clean and sustainable approach to hydrogen fuel generation. Figure 1

In this Account, we provide a summary of recent research on nanostructured MCSs as potential visible-light-driven photocatalysts for hydrogen production and dye degradation. Because multinary tellurides have yet to be explored as photocatalytic materials, we limit our discussion to sulfides and selenides, particularly those that belong to the I-III-VI2 (where I = Cu, Ag; III = Ga, In; VI = S, Se) and I2-II-IV-VI4 (where I = Cu, Ag; II = Zn; IV = Ge, Sn; VI = S, Se) semiconductor families. As with other semiconductor photocatalysts, the photocatalytic performance of MCSs can be considerably enhanced through approaches that can modify the semiconductor band structure (e.g., alloying, doping) and promote efficient charge separation (e.g., cocatalyst loading, nanostructuring).7 In the case of nanostructuring, the size and morphology of the nanostructures are important factors that can be controlled to achieve fast charge transfer and efficient charge separation for the enhancement of photocatalytic activity. In addition, nanostructuring improves the photocatalytic performance by providing a larger surface area, thereby increasing the number of available surface active sites at which photocatalytic reactions can occur. A review of the solution-based approaches that have been successfully employed to synthesize nanoscopic multinary I-III-VI2 and I2-II-IV-VI4 semiconductors is included in this Account. The construction of hybrid nanostructures for enhanced photocatalytic activity is also discussed.

2. CONTROLLED SYNTHESIS OF NANOSTRUCTURED MULTINARY CHALCOGENIDE SEMICONDUCTORS Multinary I-III-VI2 and I2-II-IV-VI4 semiconductors can be readily prepared in the bulk form through solid-state methods that require extremely high temperature conditions. Meanwhile, their nanoscale structures are typically synthesized at relatively lower temperatures using solution-based approaches, such as the solvothermal/hydrothermal method and the colloidal chemical synthetic strategy. Significant advances in the solution-based synthesis of inorganic nanostructures have enabled the precise control of size, shape, composition, and crystal structure, which are crucial factors that can influence the photocatalytic activity of semiconductors. In this section, we give a summary of reports on how these factors have been tuned for I-III-VI2 and I2-II-IV-VI4 semiconductor nanostructures. This involves changing certain experimental parameters, including the reaction temperature, reaction time, precursor ratio, and nature of solvent/surfactant, among others. The creation of hybrid nanostructures is also presented.

Figure 1. Schematic illustration of solar-light-driven hydrogen production from water using a semiconductor photocatalyst. Electrons and holes are generated when a semiconductor with suitable band structure is irradiated with solar light. Water molecules are reduced by the photogenerated electrons to generate hydrogen while sacrificial reducing agents (Red) are oxidized by the photogenerated holes to their oxidized forms (Ox).

shows a schematic illustration of the role of a semiconductor photocatalyst in the solar-driven hydrogen production from water. Metal oxide semiconductors are the most often studied materials in photocatalysis owing to their high photocatalytic activity, nontoxicity, and chemical stability.6,7 A well-known metal oxide photocatalyst is TiO2, which has demonstrated usefulness in many photocatalytic applications including hydrogen generation, water treatment, and air purification.8 However, TiO2 and most oxide-based photocatalysts are widegap semiconductors that are only active under ultraviolet light, which constitutes only a small portion of the solar radiation spectrum. Because a large fraction of solar photons are visible photons, it is highly desirable for a photocatalyst to be responsive to visible light irradiation. Binary cadmium chalcogenide semiconductors (e.g., CdS) and their alloys are among the most popular visible-light-responsive photocatalysts but their practical use is limited by the intrinsic toxicity of cadmium.9

2.1. Composition-Tunable Synthesis

By tuning the semiconductor composition via alloying, the band structure of MCSs can be easily tailored to improve their photocatalytic properties. For example, it has been found that ZnS−CuInS2 (ZCIS), a solid solution that is created by alloying ternary CuInS2 with binary ZnS, exhibits much better photocatalytic performance for hydrogen production than its parent semiconductors.10,11 Figure 2a shows a schematic representation of the band structure diagram of ZnS, CuInS2, and alloyed ZCIS. ZnS has a band gap that is too wide for visible-light utilization while CuInS2 has a conduction band minimum that is not high enough for reduction of H2O to H2. These drawbacks have been eliminated by modifying the band structure through alloying. In our colloidal synthesis of alloyed ZCIS nanorods, we were able to obtain a series of samples with different ZnS/CuInS2 ratios by mixing the appropriate ratio of Zn, Cu, and In precursors for each sample.12 The compositionB

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induced change in the semiconductor band gap was monitored through absorption spectroscopy. It is worth noting that in our study, quantum confinement effect does not contribute to the observed change in band gap due to the much larger dimensions of our ZCIS nanorods relative to the bulk Bohr exciton radius. The absorption spectra of the as-prepared ZCIS nanorods with varying ZnS/CuInS2 ratios are shown in Figure 2b. The observed blue-shifting of the absorption band edge with increasing ZnS/CuInS2 ratio is consistent with the expected widening of the band gap as the alloy composition becomes more enriched in ZnS. The relationship between the band gap and alloy composition of ZCIS was found to be nonlinear, with a bowing parameter that is highly sensitive to alloy composition in the ZnS-rich region but not in the CuInS2rich region. Alloying of CuInS2 and CuGaS2 to form CuInxGa1−xS2 can also lead to enhanced photocatalytic performance. In preparing CuInxGa1−xS2 nanocrystals (NCs) in solution, different alloy compositions can be obtained by simply controlling the In/Ga precursor ratio as demonstrated in the work of Cabot et al.13 Different from the case of alloyed ZCIS, a linear relationship exists between the band gap and alloy composition of CuInxGa1−xS2. The absorption band edge systematically shifts to shorter wavelengths with increasing Ga content due to the wider band gap of CuGaS2 compared with CuInS2. 2.2. Size-Controlled Synthesis

Size is an important factor that can be adjusted to modify the photocatalytic properties of a semiconductor.7,14 Altering the size results in a change in surface area, which is a measure of the number of available surface active sites for photocatalytic reactions. In addition, size has been known to affect the dynamics of electron−hole recombination. In general, a decrease in size is expected to lead to enhanced photocatalytic performance due to the increased number of surface active sites and the reduction of bulk charge recombination. However,

Figure 2. (a) Schematic diagram of the band structures of ZnS, alloyed ZnS−CuInS2 (ZCIS), and CuInS2. (b) Absorption spectra of alloyed ZCIS with varying ZnS/CuInS2 ratios. Reproduced with permission from ref 12. Copyright 2012 Wiley-VCH.

Figure 3. TEM images of elongated nanostructures of (a) CuGaS2, (b) CuInS2, (c) Cu2ZnGeS4, (d) ZnS−CuInS2, and (e) Cu2ZnSnS4. Panel b reproduced with permission from ref 12. Copyright 2012 Wiley-VCH. Panel c reproduced with permission from ref 21. Copyright 2015 The Royal Society of Chemistry. C

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variety of crystal phases that can be classified into two types: (1) zinc-blende-type (ZB-type) and (2) wurtzite-type (WZtype).1 The ZB-type phases exhibit crystal structures that can be derived from the cubic ZB unit cell. Examples of ZB-related structures are the cation-ordered tetragonal structures (e.g., chalcopyrite, kesterite), and the cation-disordered cubic structure. Meanwhile, the WZ-type phases possess structures that are derivatives of the hexagonal WZ crystal lattice. These include the cation-ordered orthorhombic structures (e.g., WZchalcopyrite, WZ-stannite), and the cation-disordered hexagonal structure. In most cases, the ZB-type phases are thermodynamically more stable than their WZ-type counterparts. In order to obtain phase-pure products, it is necessary to know the experimental conditions that can selectively promote the formation of one crystal phase over another. In colloidal chemical synthesis, the nature of coordinating solvent is known to strongly influence the crystal phase of the resulting nanocrystals. Our study of the phase-controlled synthesis of nanostructured MCSs revealed that long-chain alkanethiols (e.g., dodecanethiol) play a crucial role in inducing the formation of the metastable WZ-type phases of multinary sulfide semiconductors, including CuInS2, CuGaS2, AgGaS2, and CZTS.12,20−22,24 Alkanethiols can serve not only as a surfactant that can passivate the nanocrystal surface but also as an additional sulfur source because they can coordinate strongly to the metal ions to form metal thiolates that can readily decompose to their corresponding sulfides upon heating.1 The ability of alkanethiols to affect the reactivity of the metal precursors can in turn influence the nanocrystal growth kinetics in a manner that is favorable to the stabilization of the metastable WZ-type phases. Phase-selective synthesis can also be achieved by varying the reaction temperature. In a study by Vittal et al. on colloidal CuInS2 NCs, they have noted that the maximum temperature at which phase-pure metastable WZtype CuInS2 NCs are obtained is 250 °C.25 When the reaction temperature is increased beyond this temperature, the WZ-type phase gradually transforms into the ZB-type phase. Aside from those mentioned above, other factors that have been observed to affect the nanocrystal phase of MCSs are the reaction time and the cation and anion precursors.1,18,21,26

when nanostructures are too small, there is considerable widening of the band gap due to strong quantum confinement, and this diminishes their visible light response. Also, surface charge recombination becomes significant in very small nanostructures and this can offset the advantages of having large surface area. In order to find the optimum size for maximum photocatalytic performance, it is first necessary to be able to prepare the nanostructured form of the photocatalyst in different sizes. Nanostructures of varying sizes can be prepared by simply changing the growth time during synthesis. This has been demonstrated in the controlled synthesis of CuInS2 and CuInSe2 QDs, where it was observed that prolonging the reaction time under identical synthetic conditions results in larger QDs.15,16 Meanwhile, Xie and co-workers have produced ZCIS NCs with size that grows larger with increasing reaction temperature.17 Different nanostructure sizes can also be obtained through postsynthesis size-sorting processes. As an example, Tuan et al. were able to obtain varying sizes of CuGaS2 NCs through size-selective precipitation.18 2.3. Shape-Controlled Synthesis

The ability to control the nanostructure shape allows for the creation of novel nanoscopic architectures with enhanced capabilities for functional applications. One-dimensional nanostructures (e.g., nanorods) have particularly shown enormous potential as light-responsive photocatalysts.19 Their elongated morphology is believed to be advantageous in efficiently promoting the separation of the photogenerated electrons and holes, which consequently leads to improved photocatalytic behavior. Using a facile colloidal synthetic technique, our group has successfully prepared nanostructured copper-based MCSs with elongated shapes, such as tadpoleshaped CuGaS2, bullet-shaped CuInS2, worm-like Cu2ZnGeS4, and rod-like ZCIS and Cu2ZnSnS4 (CZTS).12,20−22 Representative transmission electron microscopy (TEM) images are displayed in Figure 3. Our strategy involves the thermolysis of metal dithiocarbamate precursors in the presence of long-chain alkanethiols. An investigation of the growth mechanism of our CZTS and ZCIS nanorods revealed that the formation process begins with the nucleation of spherical copper sulfide seeds, which serve as the starting point for the anisotropic onedimensional growth of the desired material.12,22 Three-dimensional hierarchical microarchitectures having nanometer-sized building blocks have also been found to be beneficial for the enhancement of photocatalytic activity because the nanoscale components can serve as active sites for photocatalytic reactions. Through hydrothermal synthesis, Chen et al. have prepared AgInxGa1−xS2 nanoporous microspheres having nanosheet components that are gathered on the surface.23 The presence of long-chain alcohols (e.g., heptanol) in the reaction system is believed to be crucial to the formation of these unique hierarchical microarchitectures. Guo et al. have also employed the hydrothermal technique to synthesize ZCIS microspheres that consist of nanosized crystallite units.11 They have noted that the use of CTAB as surfactant leads to architectures having larger surface area.

2.5. Synthesis of Hybrid Nanostructures

It is well-known that the photocatalytic potential of a semiconductor photocatalyst can be considerably improved through cocatalyst loading.7 A schematic depiction of solarlight-induced hydrogen production from water using a cocatalyst-loaded semiconductor photocatalyst is displayed in Figure 4a. Noble metals, such as Au, Pt, and Ru, are among the most often used cocatalysts. When a noble metal cocatalyst is attached onto the surface of the semiconductor photocatalyst, an efficient electron−hole separation is realized as the photogenerated electrons tend to migrate to the noble metal due to its lower Fermi energy level. The efficient separation of photogenerated charges significantly reduces the occurrence of both bulk and surface charge recombination, and this leads to enhanced photocatalytic efficiency. In cases where a large amount/size of Au cocatalyst is used, the surface plasmon resonance effect of Au can also contribute to the improvement of photocatalytic activity. The cocatalysts are traditionally attached onto the photocatalyst surface through the in situ photodeposition technique. At the nanoscale, the creation of photocatalyst−cocatalyst

2.4. Phase-Selective Synthesis

For certain materials that exist in more than one crystallographic structure, the crystal phase is another factor that can influence their photocatalytic behavior. For example, in the case of TiO2, the anatase phase has been reported to be more photocatalytically active than the rutile phase.7 Multinary I-IIIVI2 and I2-II-IV-VI4 semiconductors are known to exist in a D

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3. PHOTOCATALYTIC APPLICATIONS OF NANOSTRUCTURED MULTINARY CHALCOGENIDE SEMICONDUCTORS 3.1. Hydrogen Generation

Solar-driven photocatalytic water splitting offers a clean and sustainable way of generating hydrogen fuel, a zero-emission energy carrier that can be used to power vehicles and electric devices. In photocatalytic overall water splitting, water (H2O) molecules are decomposed into molecular hydrogen (H2) and oxygen (O2) through redox reactions that proceed on the surface of the irradiated photocatalyst. The photogenerated electrons reduce H2O to H2 while the photogenerated holes oxidize H2O to O2. When evaluating the photocatalytic activity of a material for H2 production, it is typical to make use of only the reduction half-reaction of water splitting. In this case, the reduction of H2O to H2 is accompanied by oxidation of sacrificial reducing agents as depicted in the schematic illustration in Figure 1. Mixtures of sulfide (S2−) and sulfite (SO32−) ions are usually employed as sacrificial reducing agents, which minimize electron−hole recombination by scavenging the photogenerated holes. These ions also serve to prevent photocorrosion particularly when metal chalcogenide photocatalysts are used because their presence in the reaction solution protects the chalcogenide anions of the semiconductor from oxidation by the holes. During the photocatalytic reaction, S2− is oxidized by the holes to S and S22−, which are then removed in the reaction system by reacting with SO32− to form colorless S2O32−.27 Removal of S and S22− is necessary because they can compete with the photocatalyst for photon absorption. Our group has investigated the photocatalytic potential of CuInS2 and ZCIS (ZnS/CuInS2 ratio = 10:1) nanorods for H2 production from an aqueous solution containing S2− and SO32− as sacrificial reducing agents.27 While the CuInS2 nanorods showed almost zero activity, the alloyed ZCIS nanorods displayed significant activity even without cocatalysts present (Figure 5a). This means that the modified band structure that resulted from alloying CuInS2 with ZnS has a conduction band level that is high enough to allow reduction of H2O to H2. The photocatalytic behavior was further improved with the incorporation of cocatalysts as evidenced by the increase in H2 production rate when Pt and Pd4S are attached onto the ZCIS nanorod surface. We find that Pd4S, a chemically stable conducting compound, can be suitably used as an alternative to metals for a more stable photocatalytic activity under sulfidizing conditions. Meanwhile, Guo et al. have studied the effect of different alloy compositions on the photocatalytic activity of ZCIS microspheres.11 The highest activity was observed for Zn1.6Cu0.2In0.2S2 (ZnS/CuInS2 ratio of 8:1), with an apparent quantum yield of 15.45% at 420 nm when Ru-loaded, suggesting that this alloy composition has the optimum band structure. However, it should be noted that the nanocrystal units that make up the microspheres vary in size for the different ZCIS samples and the size variation also contributes to the difference in photocatalytic activities. The Zn1.6Cu0.2In0.2S2 microspheres possess the smallest crystallite size among the samples studied, and this has possibly contributed to their superior photocatalytic performance. In a similar study, Qian et al. compared the photocatalytic behavior of similar-sized (∼10 nm) ZCIS nanocrystalline samples having different alloy compositions. 32 Their best-performing sample is also Zn1.6Cu0.2In0.2S2, with a H2 production rate of 984 μmol g−1 h−1 (λ > 420 nm) without cocatalyst loading. Surface

Figure 4. (a) Schematic illustration of solar-powered hydrogen production from water using a cocatalyst-loaded semiconductor photocatalyst. (b, c) TEM and HRTEM (inset) images of hybrid nanostructures of (b) ZCIS−Pt and (c) CZTS−Pt. Panel b adapted with permission from ref 27. Copyright 2015 Wiley-VCH. Panel c adapted with permission from ref 28. Copyright 2014 American Chemical Society.

system can also be achieved by forming hybrid (or heterostructured) nanocrystals (HNCs). Oftentimes, the synthesis is carried out by mixing preformed nanostructures of the semiconductor photocatalyst with the cocatalyst precursor in suitable solvents, followed by heating. For semiconductor nanorods, the cocatalyst preferentially attaches onto the curved tips or ends, which are areas with a higher degree of reactivity. However, surface defects on the sidewalls can provide additional high-energy sites for the nucleation and growth of the cocatalyst, and this can lead to multiple attachments. Figure 4b displays the TEM image of the Pt-tipped ZCIS nanorods that our group has colloidally prepared.27 Meanwhile, Cabot et al. have decorated the surface of their preformed quasi-spherical CZTS NCs with Au and Pt to form HNCs of CZTS−Au and CZTS−Pt (Figure 4c), respectively.28 Vela et al. have shown that Au attachment onto the surface of CZTS NCs can be done through either thermal or photochemical deposition methods.29 On the other hand, Wong and co-workers have prepared HNCs of CZTS and Au with a core−shell structure, where preformed Au nanoparticles are coated with CZTS shell.30 Hybrid architectures that make use of MCSs as sensitizers for photocatalysis have also been fabricated. In this case, nanostructured MCSs are attached onto the surface of a wide-gap semiconductor photocatalyst. For instance, using an electrophoresis method, preformed CuInSe2 NCs have been immobilized onto the top surface of TiO2 nanotube arrays to form a heterojunction photocatalyst.31 Because the CuInSe2 NCs are visible-light-active, they are able to strongly absorb visible photons upon irradiation and transfer the photogenerated electrons into the conduction band of the TiO2 semiconductor. This allows for a more efficient solar-light absorption and electron−hole separation. E

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has a higher charge separation rate than the other alloyed ZCIS samples. Thus, aside from its optimum band structure, its excellent charge separation ability renders Zn1.6Cu0.2In0.2S2 with exceptional photocatalytic behavior. More recently, Torimoto et al. probed the effect of size and composition on the photocatalytic performance of alloyed ZnS−AgInS2 NCs. Through control of both size and alloy composition, the band structure was conveniently modified, and the highest activity was observed for ZnAg0.5In0.5S2 NCs (ZnS/AgInS2 ratio of 2:1) with size of about 4−5 nm and an optimal band gap of 2.3−2.4 eV.33 The influence of alloy composition on the photocatalytic behavior has also been investigated for CuInxGa1−xS2 nanorods (Figure 5b)13 and for AgInxGa1−xS2 nanosheet-laden microarchitectures.23 The highest H2 evolution rate was observed for CuIn0.3Ga0.7S2 and AgIn0.1Ga0.9S2 whereas the lowest was seen for CuInS2 and AgInS2. In both cases, substitution of In with Ga raises the bottom of the conduction band providing a larger driving force for photogenerated electrons to reduce H2O to H2. However, when the Ga concentration is very high, the band gap becomes too wide, and this translates to lower photon absorption and reduced photogenerated charge carrier density. Thus, there is an optimum concentration of Ga required to be substituted at the In site that leads to maximum H2 yield. CZTS, on its own, does not perform well as a photocatalyst, but its activity can be markedly improved when loaded with cocatalysts, which can effectively promote the separation of charge carriers. As an example, the CZTS−Au and CZTS−Pt HNCs prepared by Cabot et al. exhibited H2 production rates that are 6−8 times higher than that of the nonloaded CZTS NCs.28 In their CZTS−Au system, no Au-related plasmon resonance effect was observed due to the very small size (2 nm) of their Au nanoparticles. On the other hand, plasmon resonance effect contributed to the enhancement of photocatalytic efficiency seen for Au-core−CZTS-shell nanostructures, where the Au core size is 15 nm.30 The phase-dependent photocatalytic activity of CZTS has been examined by Wu et al. using Pt-loaded CZTS nanostructures.34 The ZB-type kesterite phase was found to produce H2 at a higher rate than the WZtype phase, and this is attributed to the more dispersive energy bands (leads to higher charge mobility) and the less stable surface atomic configuration (better adsorption) exhibited by the kesterite phase. However, since the kesterite-phase sample

Figure 5. (a) Photocatalytic H2 production under visible-light illumination by CIS nanorods, alloyed ZCIS nanorods, ZCIS−Pt HNCs, and ZCIS−Pd4S HNCs. Adapted with permission from ref 27. Copyright 2015 Wiley-VCH. (b) Photocatalytic H2 evolution rate under full-arc-light irradiation of alloyed CuInxGa1−xS2 nanorods. Reproduced with permission from ref 13. Copyright 2014 The Royal Society of Chemistry.

photovoltage spectroscopy (SPS) showed that the strongest SPS response is exhibited by Zn1.6Cu0.2In0.2S2, implying that it

Figure 6. Visible-light-induced photocatalytic degradation of rhodamine B using (a) ZCIS nanorods and (b) ZnS-coated ZCIS QDs. Panel a reproduced with permission from ref 12. Copyright 2012 Wiley VCH. Panel b reproduced with permission from ref 35. Copyright 2011 American Chemical Society. F

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activity of TiO2 nanotubes for the degradation of methyl orange.31 Meanwhile, both CuInS2 and CuInSe2 NCs have been shown effective in sensitizing ZnO microspheres for RhB degradation.40

was obtained through postannealing of the WZ-phase, it has better crystallinity as a result of the thermal annealing process, and this cannot be ruled out as a possible contributing factor. Synthesized samples of nanostructured MCSs normally have a large variety of structural defects that can influence their charge transport properties, which in turn determine their photocatalytic performance. High crystallinity, which can be achieved through thermal treatment, is desirable because it translates to having fewer defects that can act as carrier recombination centers.

4. CONCLUSION AND PERSPECTIVES This Account provides a summary of recent research on the controlled synthesis and photocatalytic properties of colloidal nanostructures of multinary I-III-VI2 and I2-II-IV-VI4 semiconductors, including their alloys and the hybrid structures they form with other materials. Although significant progress has been made in the study of the photocatalytic potential of nanostructured CZTS, ZCIS, CuInxGa1−xS2, and AgInxGa1−xS2, there are many other equally interesting related semiconductors that merit further investigation. One notable example is Ag2ZnSnS4. In a study by Kudo et al. on the photocatalytic potential of quaternary sulfides for hydrogen generation, Ag2ZnSnS4 displayed activity far superior to that of CZTS.41 Their Ru-loaded micrometer-sized Ag2ZnSnS4 exhibited a quantum efficiency of ∼3% at 500 nm. Meanwhile, Chai and co-workers have reported that with Pt-loading, their postannealed Ag2ZnSnS4 particles, with diameters of 100−200 nm, display a fairly good quantum efficiency of 15.2% at 420 nm.42 Smaller particle dimensions are expected to result in higher efficiencies. Torimoto et al. have employed the colloidal approach to prepare nanometer-sized Ag2ZnSnS4 but obtained mixed-phase products.43 The synthesis of smaller-sized Ag2ZnSnS4 nanostructures that are monodisperse and phasepure remains a big challenge, and this has hampered further study of the photocatalytic behavior of Ag2ZnSnS4 at the nanoscale. Theoretical calculations can aid in identifying other photocatalyst candidates. For instance, a theoretical study based on first-principles calculations predicted that AgGaSe2 and the alloys it forms with CuGaSe2 and AgAlSe2 have huge potential in photocatalytic water splitting, but this has yet to be proven experimentally.44 In preparing nanostructured semiconductors for photocatalysis, it is important to know the key factors that can be suitably tuned to produce highly efficient photocatalysts, and these include the chemical composition, size, shape or morphology, crystal phase, and crystallinity. In studying the composition dependence of photocatalytic activity of MCSs, focus is often placed on alloyed materials, where the electronic band structure is modified by altering the ratio of the alloy constituents (e.g., ZnS/CuInS2 ratio in ZCIS). Controlling the metal ratio in unalloyed MCSs (e.g., Cu/In ratio in CuInS2) could also be an effective way to tune the band structure toward improved photocatalytic performance as the band structure of these materials strongly correlates with the metal stoichiometry. A recent study by Zhong et al. revealed that their nonstoichiometric CuInS2 nanocrystalline sample with Cu/In ratio of 2.9:1 possesses a band structure that is potentially suitable for photocatalytic water splitting.45 Further work in this area is anticipated in the coming years. Although it is well-known that MCSs are polymorphic, reports on the phase dependence of their photocatalytic behavior have been very few. This may be due to the difficulty in preparing samples having different phases but with all other factors being similar. For example, in phase-selective syntheses of nanostructured CZTS that have been thus far reported, the as-prepared samples not only differ in phase but also vary in size, shape, and crystallinity. This makes it difficult to probe the effect of the variation in phase because there are other existing

3.2. Dye Degradation

Light-induced photocatalytic degradation of industrial dyes into smaller and less harmful compounds offers an effective means for removal of toxic dye pollutants from industrial effluents prior to its release into the environment. Rhodamine B (RhB), a nonbiodegradable organic dye that is widely utilized in the textile and printing industries, is typically used in photocatalytic dye degradation studies. The degradation of RhB can be easily monitored using absorption spectroscopy. Figure 6a shows the temporal evolution of the absorption spectrum of RhB in an aqueous dispersion containing ZCIS nanorods under visiblelight-irradiation.12 The absorption bands, which are attributed to the conjugated chromophore of the RhB molecule, weaken in intensity upon illumination, and this is indicative of the decrease in RhB concentration as the chromophore is destroyed due to photocatalytic degradation. Only 9% of RhB remained after 2 h of illumination. Interestingly, when ZnScoated ZCIS QDs were used as the photocatalyst, the disappearance of the characteristic RhB absorption bands is accompanied by concomitant blue-shifts of maximum absorption (Figure 6b),35 suggesting that the degradation involves the formation of N-deethylated intermediates prior to the destruction of the chromophore. Although the mechanism of photocatalytic RhB degradation differs depending on several factors, the end products are always colorless compounds. CZTS, CuGaS2, and Cu2ZnGeS4 nanostructures have also been shown capable of photocatalytically degrading RhB, albeit rather slowly.20,21,28 Complete degradation was not achieved even after several hours of irradiation. As expected, the heterostructures of CZTS with Au and Pt outperformed the nonloaded CZTS NCs.28 Photocatalytic degradation of RhB has also been demonstrated for nanostructured Ag-based compounds, such as AgGaS2, AgInS2, and the alloys ZnS− AgInS2 and AgInxGa1−xS2.24,36,37 For AgInxGa1−xS2, the composition dependence of photocatalytic activity has been examined, and it was found that AgIn0.3Ga0.7S2 displays the best photocatalytic performance.37 However, it is worth noting that the solvothermally prepared samples that have been used in the study not only differ in alloy composition but also vary in size, morphology, and crystal phase, which are factors that can have profound influence on the photocatalytic behavior. In a paper by Wang et al. on the photocatalytic activity of CuInS2 mesoporous nanospheres for RhB degradation, it was reported that the ZB-derived chalcopyrite phase performed better than the WZ-type phase.38 Meanwhile, Liang et al. have used methylene blue in investigating the morphology-dependent photocatalytic activity of nanostructured CZTS and noted that the rose-shaped microarchitectures outperformed the sphereshaped nanostructures due to the larger surface area provided by their nanosheet petals.39 When attached onto the surface of TiO2 nanotubes, CuInSe2 NCs behaved as sensitizers and were able to enhance the G

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(3) Liu, S.; Su, X. The Synthesis and Application of I−III−VI Type Quantum Dots. RSC Adv. 2014, 4, 43415−43428. (4) Regulacio, M. D.; Win, K. Y.; Lo, S. L.; Zhang, S.-Y.; Zhang, X.; Wang, S.; Han, M.-Y.; Zheng, Y. Aqueous Synthesis of Highly Luminescent AgInS2−ZnS Quantum Dots and their Biological Applications. Nanoscale 2013, 5, 2322−2327. (5) Wada, T.; Nakamura, S.; Maeda, T. Ternary and Multinary Cuchalcogenide Photovoltaic Materials from CuInSe2 to Cu2ZnSnS4 and other Compounds. Prog. Photovoltaics 2012, 20, 520−525. (6) Djurisic, A. B.; Leung, Y. H.; Ng, A. M. C. Strategies for Improving the Efficiency of Semiconductor Metal Oxide Photocatalysis. Mater. Horiz. 2014, 1, 400−410. (7) Chen, X.; Shen, S.; Guo, L.; Mao, S. S. Semiconductor-based Photocatalytic Hydrogen Generation. Chem. Rev. 2010, 110, 6503− 6570. (8) Fujishima, A.; Zhang, X.; Tryk, D. A. TiO2 Photocatalysis and Related Surface Phenomena. Surf. Sci. Rep. 2008, 63, 515−582. (9) Zhang, K.; Guo, L. Metal Sulphide Semiconductors for Photocatalytic Hydrogen Production. Catal. Sci. Technol. 2013, 3, 1672−1690. (10) Tsuji, I.; Kato, H.; Kobayashi, H.; Kudo, A. Photocatalytic H2 Evolution under Visible-Light Irradiation over Band-StructureControlled (CuIn)xZn2(1−x)S2 Solid Solutions. J. Phys. Chem. B 2005, 109, 7323−7329. (11) Zhang, X.; Du, Y.; Zhou, Z.; Guo, L. A Simplified Method for Synthesis of Band-Structure-Controlled (CuIn)xZn2(1−x)S2 Solid Solution Photocatalysts with High Activity of Photocatalytic H2 Evolution under Visible-Light Irradiation. Int. J. Hydrogen Energy 2010, 35, 3313−3321. (12) Ye, C.; Regulacio, M. D.; Lim, S. H.; Xu, Q.-H.; Han, M.-Y. Alloyed (ZnS)x(CuInS2)1−x Semiconductor Nanorods: Synthesis, Band Gap Tuning and Photocatalytic Properties. Chem. - Eur. J. 2012, 18, 11258−11263. (13) Yu, X.; An, X.; Shavel, A.; Ibáñez, M.; Cabot, A. The effect of the Ga Content on the Photocatalytic Hydrogen Evolution of CuInxGa1−xS2 Nanocrystals. J. Mater. Chem. A 2014, 2, 12317−12322. (14) Osterloh, F. E. Inorganic Nanostructures for Photoelectrochemical and Photocatalytic Water Splitting. Chem. Soc. Rev. 2013, 42, 2294−2320. (15) Nose, K.; Omata, T.; Otsuka-Yao-Matsuo, S. Colloidal Synthesis of Ternary Copper Indium Diselenide Quantum Dots and Their Optical Properties. J. Phys. Chem. C 2009, 113, 3455−3460. (16) Zhong, H.; Zhou, Y.; Ye, M.; He, Y.; Ye, J.; He, C.; Yang, C.; Li, Y. Controlled Synthesis and Optical Properties of Colloidal Ternary Chalcogenide CuInS2 Nanocrystals. Chem. Mater. 2008, 20, 6434− 6443. (17) Zhang, J.; Xie, R.; Yang, W. A Simple Route for Highly Luminescent Quaternary Cu-Zn-In-S Nanocrystal Emitters. Chem. Mater. 2011, 23, 3357−3361. (18) Chang, S.-H.; Chiu, B.-C.; Gao, T.-L.; Jheng, S.-L.; Tuan, H.-Y. Selective Synthesis of Copper Gallium Sulfide (CuGaS2) Nanostructures of Different Sizes, Crystal Phases, and Morphologies. CrystEngComm 2014, 16, 3323−3330. (19) Weng, B.; Liu, S.; Tang, Z.; Xu, Y. One-Dimensional Nanostructure based Materials for Versatile Photocatalytic Applications. RSC Adv. 2014, 4, 12685−12700. (20) Regulacio, M. D.; Ye, C.; Lim, S. H.; Zheng, Y.; Xu, Q.-H.; Han, M.-Y. Facile Noninjection Synthesis and Photocatalytic Properties of Wurtzite-Phase CuGaS2 Nanocrystals with Elongated Morphologies. CrystEngComm 2013, 15, 5214−5217. (21) Fan, C.-M.; Regulacio, M. D.; Ye, C.; Lim, S. H.; Lua, S. K.; Xu, Q.-H.; Dong, Z.; Xu, A.-W.; Han, M.-Y. Colloidal Nanocrystals of Orthorhombic Cu2ZnGeS4: Phase-Controlled Synthesis, Formation Mechanism and Photocatalytic Behavior. Nanoscale 2015, 7, 3247− 3253. (22) Regulacio, M. D.; Ye, C.; Lim, S. H.; Bosman, M.; Ye, E.; Chen, S.; Xu, Q.-H.; Han, M.-Y. Colloidal Nanocrystals of Wurtzite-Type Cu2ZnSnS4: Facile Noninjection Synthesis and Formation Mechanism. Chem. - Eur. J. 2012, 18, 3127−3131.

factors that contribute to the observed difference in activity. The crystal phase effect on the photocatalytic activity of these nanostructured semiconductors deserves to be given more attention in future studies, especially because many of the metastable phases of the MCSs are accessible only at the nanoscale. Another topic that warrants further research is the control of crystallinity in achieving better photocatalytic performance because this remains largely unexplored for nanostructured MCSs. Sample crystallinity can be easily improved by postannealing processes, but careful control of thermal treatment conditions is necessary because sintering and phase transformation can also occur at elevated temperatures. Moreover, a deeper understanding of the defect properties of these compounds would be useful when preparing high-quality samples that are suitable for photocatalysis.46 Hole-induced oxidative photocorrosion is a common problem faced by most metal chalcogenide photocatalysts.47 In photocatalytic hydrogen generation, this is typically suppressed with the use of sacrificial reducing agents that consume the photogenerated holes. However, these hole scavengers are irreversibly oxidized during the photocatalytic reaction, and thus a fresh supply is always needed to sustain good photocatalytic performance. An alternative solution is to integrate these multinary chalcogenides with a protective material that will lead to a composite photocatalyst that is resistant to photocorrosion, but this would entail additional manufacturing steps and cost. Further research should be directed toward finding other means of addressing the stability issues that result from photocorrosion. Also, challenges associated with the translation of laboratory-scale experiments into industrial-scale processes need to be overcome before commercial utilization becomes possible.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Michelle D. Regulacio received her B.S. in Chemistry from the University of the Philippines Los Baños and her Ph.D. in Chemistry from Georgetown University. She was a postdoctoral research associate at Princeton University before joining the Institute of Materials Research and Engineering (IMRE) as a research scientist. Her research focuses on the synthesis, properties, and applications of inorganic nanostructured materials. Ming-Yong Han obtained his B.S. and Ph.D. degrees in Chemistry from Jilin University. He was with IBM and Indiana University before his current appointment as senior scientist in IMRE. His research addresses problems at the interfaces of nanoscience, nanotechnology, and biotechnology.

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REFERENCES

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