Two-Dimensional Photocatalysts: Properties ...

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Energy and Environment Focus Vol. 3, pp. 330–338, 2014 (www.aspbs.com/efocus)

Two-Dimensional Photocatalysts: Properties, Synthesis, and Applications Dongman Hou1 , Weijia Zhou2, ∗ , Xiaoyan Liu3 , Kai Zhou2 , Guoqiang Li1, ∗ , and Shaowei Chen2, 4 1

School of Materials Science and Engineering, South China University of Technology, Guangzhou, 510641, China New Energy Research Institute, School of Environment and Energy, South China University of Technology, Guangzhou Higher Education Mega Center, Guangzhou, 510006, China 3 Guangdong General Research Institute of Industrial Technology, Guangzhou, 510650, China 4 Department of Chemistry and Biochemistry, University of California, Santa Cruz, California 95064, United States 2

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ABSTRACT Recent advances in the properties, synthesis, and applications of two-dimensional photocatalysts are reviewed. The properties of two-dimensional photocatalysts, such as adsorption, ion-exchange and energy bandgap, are described. The structures, morphologies and corresponding synthesis process are critically discussed. Examples of applications of two-dimensional nanostructures in photocatalytic hydrogen production and decomposing organic pollutants are presented. KEYWORDS: Two-Dimensional, Photocatalysis, Heterostructure, Photocatalytic Water Splitting.

CONTENTS

drawbacks in practical application. After the discovery and Delivered by Publishing Technology to: Guest User

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330 of 05:44:31 graphene nanosheets,6–8 the research on IP: 166.111.120.71 On: Mon,applications 12 Jan 2015 2. Structures and Properties of Two-Dimensional Nanomaterials . 332 two dimensional (2D) nanomaterials have experienced a Copyright: American Scientific Publishers 2.1. Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332 rapid growth. The novel electrical, optical, mechanical, 2.2. Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332 and chemical properties make them attractive from a tech3. Synthesis of Two-Dimensional Nanomaterials . . . . . . . . . . . 333 nological standpoint, which is particularly important for 3.1. Type I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333 applications of photocatalytic hydrogen production9 10 and 3.2. Type II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334 3.3. Type III . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334 decomposing hazardous organic pollutants.11 12 Recently, 3.4. Type IV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334 various morphological 2D nanomaterials have been pre3.5. Heterostructure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334 pared by hydrothermal treatment, oil phase reaction or 4. Applications of Two-Dimensional Nanomaterials . . . . . . . . . 335 exfoliation. In this review, we classify the reported 2D lay4.1. Photocatalytic Splitting of Water . . . . . . . . . . . . . . . . 335 ered materials into four main types, including type (I) the 4.2. Photocatalytic Decomposing Organic Pollutants . . . . . . . 336 5. Conclusions and Prospect . . . . . . . . . . . . . . . . . . . . . . . . 336 transition metal disulfides with sandwiched structure, such Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 337 as MoS2 ,13 Sb2 Te3 ,14 GeSe,15 WS2 :16 type (II) the monReferences and Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . 337

1. INTRODUCTION Titanium dioxide has been intensively investigated as photocatalysts since Fujishima and Honda discovered the photocatalytic splitting of water by TiO2 in 1972.1 Up to now, a lot of research results about TiO2 photocatalysts have been reported,2 such as synthesis of lowdimensional structures,3 doping treatment,4 and growth of heterostructures.5 However, the low photocatalytic efficiency and the lack of visible light response due to large band gap of TiO2 photocatalysts remain the main ∗

Authors to whom correspondence should be addressed. Emails: [email protected], [email protected] Received: 14 January 2014 Accepted: 24 March 2014

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oclinic structure composed of octahedron layers and the free cations between layers, such as K2 Ti3 O7 ,17 MnO2 ,18 Bi2 WO6 ;19 type (III) nanoplates synthesized by crystal control growth, such as TiO2 ,20 Cu2 ZnSnS4 ,21 PbS;22 and type (IV) graphene-like nanosheets, such as C3 N4 .23 Due to the excellent structure characteristics, two-dimensional photocatalysts with high specific surface area and fewlayer structure can effectively suppress recombination of photo-electron and photo-hole, which enhance the photocatalytic activity. Especially, transition metal disulfides nanosheets with narrow energy gap have high visible light or near-infrared photocatalytic activity. In recent years, breakthroughs have continually been made in the preparation, modification, and applications of two-dimensional nanomaterials. Several excellent reviews of the subject exist in the literature. Nevertheless, we believe that a professional review fasten on 2326-3040/2014/3/330/009

doi:10.1166/eef.2014.1120

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Two-Dimensional Photocatalysts: Properties, Synthesis, and Applications

Dongman Hou received B.S. degree from the School of Chemistry and Chemical, South China University of Technology in 2013. She is pursuing her M.D. degree under the supervision of Professor Guoqiang Li in the State Key Laboratory of Luminescent Materials and Devices, South China University of Technology. Her research interests include the synthesis and applications of hybrid nanostructures based on two-dimensional nanomaterials.

Weijia Zhou received his B.S. degree from Shandong Institute of Light Industry in 2006, and completed his Ph.D. with Professor Hong Liu and Professor Jiyang Wang at Shandong University in 2012. He was doing research at Nanyang Technological University from 2011 to 2012. Now, he is a lecturer in New Energy Research Institute, School of Environment and Energy, South China University of Technology, China. His research interests are related to designs and synthesis of devices and nanomaterials for energy conversion and storage, including photo and electro-catalytic water splitting and supercapacitor.

Kai Zhou is currently pursuing a Ph.D. in the School of Environment and Energy at the South China University of Technology, China. His research is focused on the designs and synthesis of nanomaterials for energy conversion and storage.

Guoqiang Li received his Ph.D. degree of materials science at Northwestern Polytechnical University, Xi’an, China, in 2004. Afterwards, he joined GE Global Research as an R&D scientist, and then carried out two postdoctoral research experiences in the University of Tokyo (2005–2007) under the JSPS fellowship, and University of Oxford (2007–2010) under the Royal Society International Incoming Fellowship. He has been a full professor at South China University of Technology, China since 2010. Prof. Li is a standing committee member for Chinese Materials Association-UK (CMA-UK), and the co-founder and chair in materials science of Oxford Forum of Science and Technology (OXFOST). He has received a few awards and honors, including National Award for Technological Invention of China, National Excellent Ph.D. Thesis of China, Guangdong Provincial Outstanding Youth in Science and Technology, etc. He has published over 70 peer-reviewed articles and patented over 40 techniques. He is also the author of 1 monograph and 2 book chapters. Energy Environ. Focus, 3, 330–338, 2014

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Xiaoyan Liu received her Ph.D. with Professor Xiaopeng Hao and Xiangang Xu at Shandong University in 2013. Now she is working in Guangdong Research Institute of Semiconductor Lighting Industrial Technology, Guangdong General Research Institute of Industrial Technology, China. Her major research interest is preparation and optimization of LEDs, Delivered by Publishing Technology to: Guest User including epitaxy technique, chip manufacturing preparation of nano materials on chip IP: 166.111.120.71 On: Mon, 12 Jan 2015 and 05:44:31 surface to enhance the performance of LEDs. Copyright: American Scientific Publishers


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Shaowei Chen finished his undergraduate studies in China in 1991 with a B.Sc. degree in Chemistry from the University of Science and Technology of China, and then went to Cornell University receiving his M.Sc. and Ph.D. degrees in 1993 and 1996, respectively. Following a postdoctoral appointment in the University of North Carolina at Chapel Hill, he started his independent career in Southern Illinois University in 1998. In summer 2004, he moved to the University of California at Santa Cruz and is currently a Professor of Chemistry. He is also an adjunct professor at South China University of Technology. His research interest is primarily in the electron transfer chemistry of nanoparticle materials.

attracted extraordinary attention due largely to the unique physical and chemical properties associated with the 2D structural confinement. MoS2 is the typical type I two-dimensional nanomaterials, composed of Mo atoms between two layers of hexagonally close-packed sulfur atoms, which can be exfoliated to single- or few-layer nanosheets.13 The simulated diagram of the molecular structure is shown in Figure 1.24 Second, layered structures of type II have been studied for a long time as intercalation materials before the extensive research of 2D nanomaterials. The simulated diagrams of the layered structure of titanate and titanoniobate are shown in Figure 2,25 26 Fig. 1. Atomic ball model showing a hypothetical, bulk-truncated MoS2 respectively. The stacked polyanion nanosheet is comhexagon exposing the two types of low-index edges, the edges and Mo Technology Delivered bySPublishing Guest User polyhedron, such as TiO6 octaposed of to: crystal package edges. Mo atoms are blue, S atoms areIP: yellow. Reprinted with permis166.111.120.71 On: Mon,hedra. 12 Jan 05:44:31 As2015 for the synthesis of type III 2D nanomaterials, sion from [24], J. Lauritsen, et al., J. Catal. 221, 510 (2004).American © (2004), Scientific Publishers Copyright: there is no requirement for crystal structure. Tetragonal Elsevier. structure (such as anatase),27 orthorhombic structure (such as brookite)28 and hexagonal structure (such as ZnO)29 two-dimensional photocatalysts would further promote the can be all synthesized into two-dimensional morphology. corresponding research. In this review, two-dimensional For example, uniform anatase TiO2 single crystals with photocatalysts are the focus. Some recent progress on high percentage (47%) of {001} facets were synthesized structure, properties, preparation, and applications of twousing hydrofluoric acid as a morphology controlling agent dimensional photocatalysts are reviewed. (Fig. 3).27 The single-molecule imaging and kinetic analysis of the fluorescence from the products show that reac2. STRUCTURES AND PROPERTIES OF tion sites for the effective reduction of the probe molecules TWO-DIMENSIONAL NANOMATERIALS are preferentially located on the {101} facets of the crystal 2.1. Structures rather than the {001} facets with a higher surface energy.30 According to the types, two-dimensional photocatalysts At last, following the discovery of graphene, great attenpossess the different crystal structures. Over the past tion has been paid to analogs of graphene, such as C3 N4 decade, two-dimensional transition metal disulfides have possessing the graphene-like structure (Fig. 4).31

Fig. 2. Schematic structures of (a) titanate25 and (b) titanoniobate. Reprinted with permission from [25, 26], N. Miyamoto, et al., J. Mater. Chem. 14, 165 (2004), T. Shibata, et al., Energy. Environ. Sci. 4, 535 (2011). © (2004, 2011), Royal Society of Chemistry.

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2.2. Properties 2.2.1. Adsorption Properties The 2D nanomaterials with high surface-to-volume ratio have a large surface area, which facilitates reaction/interaction between the photocatalysts and organic dye molecules. The theoretical surface area of individual isolated graphene sheets is about 2630 m2 /g. The high BET value of C3 N4 was reported as ∼ 122 m2 /g.32 Zhou et al. found that the MoS2 nanosheets have a strong adsorption ability towards the Rhodamine B, with the adsorption value of 97.05 mg/g.10 Sun et al. reported that the enhanced adsorption ability of the Bi2 WO6 ultrathin Energy Environ. Focus, 3, 330–338, 2014

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(a)

(c)


(b)

(d)

Fig. 3. Slab models of anatase TiO2 (001) and (101) surfaces. Ti and O atoms are represented by grey and red spheres. (a), (b) Unrelaxed, clean (001) and (101) surfaces. (c), (d) Unrelaxed (001) and (101) surfaces surrounded by adsorbate X atoms. Reprinted with permission from [27], H. G. Yang, et al., Nature 453, 638 (2008). © (2008), Nature Publishing Group.

Samples

TiO2

MoS2

WO3

SnSe

InSe

Bandgap (eV, indirect/direct) Bulk Nanosheets Reference

3.2 3.84 [34]

1.2 1.8 [35]

2.68 2.88 [36]

0.90/1.30 0.86/1.10 [37]

1.26 1.54 [38]

in Table I. As with aqueous colloids of 2D titanate nanosheets, the bandgap has been estimated to be 3.84 eV, much higher than the value of 3.2 eV for bulk TiO2 .34 Zhou et al. reported that the dramatically enhanced photocurrent response was deemed to the change from indirect to direct band structure when bulk MoS2 and WS2 were exfoliated into monolayer or few-layer nanosheets.13 Bulk MoS2 is a semiconductor with an indirect band gap of 1.2 eV, whereas single-layer MoS2 is a direct band gap semiconductor with a band gap of 1.8 eV. Such an indirectto-direct band gap transition leads to a strong photoluminescence (PL) effect in monolayer MoS2 .35

3. SYNTHESIS OF TWO-DIMENSIONAL NANOMATERIALS

3.1. Type I Guest Over theto:past fewUser years, many methods have been 2.2.2. Ion-Exchange PropertiesDelivered by Publishing Technology IP: 166.111.120.71 On: Mon,developed 12 Jan 2015 05:44:31 for the successful fabrication of 2D nanoIn the layered structure, protons occupy the American cavities Scientific Publishers Copyright: materials. According to the types of 2D structures, the between the layers of the octahedra, which results in preparation methods were summarized. As for the transieffective ion-exchange properties. In the layered structure + + + tion metal disulfides with sandwiched structure (type I), of titanate, protons (K , Na and H ) occupy the cavthere is the weak Van der Waals force among layities between the layers of the TiO6 octahedra, which ers, which can be easily exfoliated by weak external reveals high protonic conductivity properties along interforce. The effective exfoliation methods include mechanilayer direction. Britvin et al. reported LHT-9, a laycal exfoliation,39 solvent exfoliation,13 40 and electrochemered hydrazinium titanate with an interlayer spacing of ical lithium intercalation.16 Zeng et al.16 reported that the ∼ 0.9 nm, was synthesized by one-step mild fluoride route high-yield, single-layer MoS2 nanosheets can be obtained involving hydrazine-induced hydrolysis of hexafluoroti33 by a controllable electrochemical lithiation process, which tanic acid under near-ambient conditions. The layered was shown in Figure 5. This method is suitable for the titanate has the good ion-exchange properties and the large preparation of many layered bulk materials, such as MoS2 , surface area. WS2 , TiS2 , TaS2 , ZrS2 or graphite. However, the morphol2.2.3. Energy Band Structure ogy of nanosheets obtained by exfoliation methods from bulk layered nanostructures is random. To facilitate highThe layer numbers of nanosheets can affect the energy quality nanosheets with regular shapes, uniform lateral band structure of photocatalysts, which is summarized dimensions, and tunable thicknesses, synthetic methods is critically important. Vaughn et al. described one pathway that generates colloidal nanosheets of SnSe with uniform lateral dimensions and tunable thicknesses.41 Freestanding a uniform square-like SnSe nanosheets sol was synthesized by slowly heating a one-pot reaction mixture of SnCl2 , oleylamine, trioctylphosphine selenide, and hexamethyldisilazane to 240 C. They found that the nanosheets first Fig. 4. Scheme of (a) s-triazine and (b) tri-s-triazine based connec“grow out” laterally via coalescence of individual nanopartion in g-C3 N4 . Blue and gray spheres represent nitrogen and carbon ticle building blocks to yield a single-crystal nanosheet atoms, respectively. Reprinted with permission from [31], Y. Zheng, template and then “grow up” vertically in a pseudo layeret al., Energy. Environ. Sci. 5, 6717 (2012). © (2012), Royal Society of by-layer fashion. Chemistry. Energy Environ. Focus, 3, 330–338, 2014

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nanosheets is not only due to its larger surface area (39.2 m2 /g), which is about 5 times that of the nanodisk sample (7.8 m2 /g), but also related to the particular terminated crystal facets.19

Table I. The energy bandgap of bulk and nanosheet photocatalysts.


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Fig. 5. Electrochemical lithiation process for the fabrication of 2D nanosheets from the layered bulk material. Reprinted with permission from [16], Z. Y. Zeng, et al., Angew. Chem. Int. Ed. 50, 11093 (2011). © (2011), John Wiley and Sons.

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an aqueous solution of propylamine. In additions, it was commonly accepted that Na2 Ti3 O7 nanosheets can be synthesized under alkali hydrothermal reaction (lower than 120 C).44 The lamellar titanate product possesses a twodimensional structure with dangling bonds of edges. In all, a number of layered compounds can be exfoliated by organic molecule intercalation, but this method is timeconsuming, also causing structural deformation and reaggregation after removal of the ions. Based on the problems, Kai et al. reported that the first single-step approach to directly access the MnO2 monosheets comprised of edges hared MnO6 octahedra by the chemical oxidation of Mn2+ ions in the presence of tetramethylammonium cations in an aqueous solution at room temperature.18 Also, the process makes it possible to fabricate high-purity organicinorganic layered hybrids and LB films composed of the MnO2 monosheets.

3.2. Type II Different from the sandwiched structure with Van der 3.3. Type III Waals force, the titanate and niobate with monoclinic Controlling anisotropy is a key concept in the generastructure can’t be exfoliated by mechanical exfoliation, tion of complex functionality in advanced materials. For solvent exfoliation, or electrochemical lithium intercalatype III, the nanosheets, such as TiO2 , were synthesized by tion. Because of the strong force between layers, the laycrystal control growth. The synthesis process of type III ered titanate or niobate were usually exfoliated by organic is from bottom to top. Gordon et al.45 reported TiO2 intercalation agents, such as propylamine (Fig. 6)42 and nanosheets with size of 10–100 nm were synthesized using tetrabutylammonium (TBA+) cations.43 Miyamoto et al. the precursor TiF4 inUser the presence of OLAM and 1-ODOL, Delivered by Publishing Technology to: Guest reported K4 Nb6 O17 possesses two layer environments IP:inter 166.111.120.71 On: Mon,as12shown Jan 2015 05:44:31 in Figure 7. The precursor TiF4 enables the (inter layer I and II) with differentCopyright: reactivities. Potas- Scientific Publishers American exposure of the {001} facet of anatase TiO2 through in sium ions in the inter layer I can be easily exchanged, situ release of HF. Jiang et al.46 reported BiOCl singlewhile those in the inter layer II are relatively difficult crystalline nanosheets with exposed {001} and {010} facets to be replaced.42 Exfoliated K4 Nb6 O17 bilayer nanosheets were selectively synthesized by reacting Bi(NO3 )3 · 5H2 O in extraordinarily large size (ca. 100 mm) were prepared and KCl in distilled water via a facile hydrothermal route by the direct reaction of K4 Nb6 O17 · 3H2 O crystals with at 160 C. The facet control was realized by adjusting pH of the solution.

3.4. Type IV The C3 N4 nanosheets were usually synthesized by carbonization of a variety of carbon compounds.31 47 Yang et al. reported that g-C3 N4 nanosheets have been successfully extracted via a simple and cost-effective liquid exfoliation method of bulk g-C3 N4 powder. Bulk g-C3 N4 was suspended in various solvents such as IPA, N -methyl-pyrrolidone (NMP), water, ethanol, and acetone to create separated yellow dispersions, and then sonicated at room temperature. g-C3 N4 nanosheets were gradually exfoliated from bulk ones with the increase of sonication time.

Fig. 6. Schematic image of exfoliation of K4 Nb6 O17 . Reprinted with permission from [42], N. Miyamoto, et al., Chem. Commun. 2378 (2002). © (2002), Royal Society of Chemistry.

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3.5. Heterostructure The thin two-dimensional nanomaterials have high specific surface area, but it is easy to be reaggregated due to high surface energy. Growth of nanosheets on the surface of substrates is one good method to solve this problem. Zhou et al. reported few-layer MoS2 nanosheets can be grown on the surface of TiO2 nanobelts as shown in Figure 8.10 TiO2 Energy Environ. Focus, 3, 330–338, 2014

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Fig. 7. TEM images of TiO2 nanosheets synthesized using the precursor TiF4 (a), (d), a mixed precursor of TiF4 and TiCl4 (b), (e), and TiCl4 (c), (f). Those depicted in (a)–(c) and (d)–(f) are synthesized in the presence of OLAM and 1-ODOL, respectively. Reprinted with permission from [45], T. R. Gordon, et al., J. Am. Chem. Soc. 134, 6751 (2012). © (2012), American Chemical Society.

substoichiometric MoO3 core provides a high aspect ratio foundation and enables facile charge transport, while the conformal MoS2 shell provides excellent catalytic activity and protection against corrosion in strong acids.48 Kong presented a synthesis process to grow MoS2 and MoSe2 thin films with vertically aligned layers, maximally exposing the edges on the molybdenum metal surface, which possesses to: theGuest high User hydrogen evolution reaction (HER) Delivered by Publishing Technology 49 IP: 166.111.120.71 On: Mon,activity. 12 Jan 2015 05:44:31 Copyright: American Scientific Publishers

4. APPLICATIONS OF TWO-DIMENSIONAL NANOMATERIALS

Fig. 8. (a), (b) Low-magnification TEM images of TiO2 @MoS2 heterostructures (50 wt% of MoS2 . Inset in (b): the corresponding SAED pattern. (c), (d) HRTEM images of the designated square parts in Figure 3(b). (e) EDS mapping results from TiO2 @MoS2 heterostructure (50 wt% of MoS2 ). Reprinted with permission from [10], W. J. Zhou, et al., Small 9, 140 (2013). © (2013), John Wiley and Sons. Energy Environ. Focus, 3, 330–338, 2014

4.1. Photocatalytic Splitting of Water During the past few decades, extensive efforts have been made to convert solar radiation to a renewable H2 energy with high calorific value from water. New initiatives were required to develop highly efficient light energy-harvesting materials, which has the suitable energy band structure, high specific surface area and low recombination rate of photo-electron and hole pairs. The two-dimensional structures meet the all above requirements. Comparing the band edge positions with the redox potentials of water shows that single or few layers chalcogenides are potential photocatalysts for water splitting. Moreover, the band edge positions and optical absorption of the chalcogenides nanosheets can be tuned by biaxial strain to increase the conversion efficiency of solar energy.50 Metal dichalcogenides usually have the narrow band gap, such as MoS2 , which can overlap fairly well with the solar spectrum. Meng et al reported that p-type MoS2 nanoplatelets with size of 5–20 nm were deposited on then-type nitrogendoped reduced graphene oxide (rGO) nanosheets to form multiple nanoscale p–n junctions in each rGO nanosheet,51 which shows significant photocatalytic activity toward the HER in the wavelength range from the ultraviolet light through the near-infrared light (Fig. 9). The enhanced 335

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nanobelts@MoS2 nanosheets core–shell structure have high photocatalytic activity for photocatalytic hydrogen production and photocatalytic decomposition of organic dyes. Chen et al. synthesized vertically oriented core– shell nanowires with substoichiometric MoO3 cores of ∼ 20–50 nm and conformal MoS2 shells of ∼ 2–5 nm. The


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Fig. 9. Hydrogen generated by the MoS2 , the MoS2 /rGO, and the p-MoS2 /n-rGO photocatalysts. Reprinted with permission from [51], F. K. Meng, et al., J. Am. Chem. Soc. 135, 10286 (2013). © (2012), American Chemical Society.

photoelectrochemical activity is due to the charge generation and suppresses the charge recombination.9 Semiconductors with the wide bandgap, such as K4 Nb6 O17 , adjust the light absorption by doping or sensitization treatment.52–55 At the same time, the thin nanosheets photocatalysts have wider band gap than that of bulk ones. Fig. 10. (a) Comparison of the photocatalytic degradation rates of MO for TiO2 nanosheets on carbon fibers (triangles), TiO2 nanosheets on a Maeda et al.54 reported that potassium hexaniobate nanoFTO substrate and pure carbon fiber (squares). (b) Schematic Publishing Technology to: (circles), Guest User scrolls formed by exfoliation ofDelivered lamellar Kby Nb O were 4 6 17 representation of the05:44:31 same amount of TiO2 nanosheets grown on FTOIP: 166.111.120.71 On: Mon, 12 Jan 2015 studied as visible-light-driven H2 photocatalysts using glass and CF substrates with same surface area. Reprinted with permisCopyright: American Scientific Publishers tris(2,2 -bipyridyl)ruthenium(II) chloride (Ru(bpy)2+ sion from [58], W. X. Guo, et al., Adv. Mater. 24, 4761 (2012). © (2012), 3 ) as a sensitizer and ethylenediaminetetraacetic acid (EDTA) as John Wiley and Sons. an electron donor. The rate of visible light H2 production for photocatalytic decomposing organic pollutants. Guo in the nanoscroll-based system is 10 times higher than that et al.58 reported single-crystalline TiO2 nanosheets, mainly of similarly sensitized K4 Nb6 O17 . dominated by (001) facets grow on carbon fibers by a The growth of heterostructures is one of the important facile solvothermal synthetic route, which as shown in approaches for the development of advanced photocataFigure 10. In comparison with TiO2 nanosheets grown 56 lysts. Zhang et al. reported a novel visible-light-driven on a planar substrate, the TiO2 nanosheets/carbon fibers CuS/ZnS porous nanosheet photocatalysts prepared by a hybrid structure exhibited 3.38-fold improved photocatsimple hydrothermal and cation exchange reaction. Even alytic degradation of methyl orange (MO) and showed without a Pt co-catalyst, the as-prepared CuS/ZnS porous excellent stability under ultraviolet–visible light irradiananosheets reach a high H2 -production rate of 4147 mol 59 reported that the BiOCl single-crystalline tion. Jiang et al. −1 −1 h g at CuS loading content of 2 mol% and an apparnanosheets with exposed {001} facets exhibited higher ent quantum efficiency of 20% at 420 nm. At last, carbon activity for direct semiconductor photo-excitation pollunitride semiconductors, such as C3 N4 nanosheets, are also tant degradation under UV light, but the counterpart with important photocatalysts for the efficient and sustained use exposed {010} facets possessed superior activity for indi47 57 of solar radiation for hydrogen photosynthesis. rect dye photosensitization degradation under visible light. 4.2. Photocatalytic Decomposing Organic Pollutants Upon absorption of photons with energy larger (lower wavelengths of light) than the band gap of photocatalyst, electrons are excited from the valence band to the conduction band, creating electron–hole pairs. These charge carriers migrate to the surface and react with the chemicals adsorbed on the surface to decompose them. So, the adsorption capacity of organic pollutants, the ability of light capture and photo-induced generation performance of electron–hole pairs are important parameters 336

5. CONCLUSIONS AND PROSPECT Over recent decades, the tremendous effort invested in two-dimensional nanomaterials due to their specific structure with unique physical and chemical properties that provide promising photocatalytic applications. In this tutorial review, we have discussed the most recent progress in properties, synthesis, and applications of twodimensional photocatalysts. Various methods for the synthesis of two-dimensional photocatalysts were reviewed, Energy Environ. Focus, 3, 330–338, 2014

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