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Constructing Anatase TiO2 Nanosheets with Exposed (001) Facets/Layered MoS2 Two-Dimensional Nanojunction for Enhanced Solar Hydrogen Generation Yong-Jun Yuan, Zhi-Jun Ye, Hongwei Lu, Bin Hu, Yong-Hui Li, Daqin Chen, Jia-Song Zhong, Zhen-Tao Yu, and Zhigang Zou ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02036 • Publication Date (Web): 11 Dec 2015 Downloaded from http://pubs.acs.org on December 13, 2015

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ACS Catalysis

Constructing Anatase TiO2 Nanosheets with Exposed (001) Facets/Layered MoS2 TwoDimensional Nanojunction for Enhanced Solar Hydrogen Generation Yong-Jun Yuan,†,* Zhi-Jun Ye,† Hong-Wei Lu,† Bin Hu, Yong-Hui Li,‡ Da-Qing Chen,†,* Jia-Song Zhong,† Zhen-Tao Yu,‡,* Zhi-Gang Zou‡ †

College of Materials and Enviromental Engineering, Hangzhou Dianzi University, Hangzhou

310018, People’s Republic of China. ‡

Jiangsu Key Laboratory for Nano Technology, College of Engineering and Applied Science,

Nanjing University, Nanjing 210093, People’s Republic of China.

*Corresponding authors: E-mail addresses: [email protected], [email protected] or [email protected] Fax:+86-0571-86919105; Tel: +86-0571-87713538

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ABSTRACT. Cocatalyst has been extensively used to accelerate the rate of hydrogen evolution in semiconductor-based photocatalytic system, but the influence of interface state between semiconductor and cocatalyst has been rarely investigated. Here, we demonstrate a feasible strategy of two-dimensional (2D) nanojuction to enhance solar hydrogen generation of MoS2/TiO2 system. Loading of 2D MoS2 nanosheets on the surface of 2D anatase TiO2 nanosheets with exposed (001) facets greatly increases the interfacial contact. At an optimal ratio of 0.50 wt% MoS2, the 2D-2D MoS2/TiO2 photocatalyst shows the highest H2 evolution rate of 2145 µmol h-1 g-1, which is almost 36.4 times higher than that of pure TiO2 nanosheets. The apparent quantum yield of hydrogen evolution system reaches 6.4% at 360 nm. More importantly, the 2D-2D MoS2/TiO2 composite exhibits much higher photocatalytic activity than those of noble metals (such as Pt, Rh, Ru, Pd or Au) loaded-TiO2 photocatalysts. The decisive factors in improving the photocatalytic H2 production activity are the intimate and large contact interface between the light-harvesting semiconductor and cocatalyst. The effective charge transfer from TiO2 to MoS2 is demonstrated by the significant enhancement of photocurrent responses in 2D-2D MoS2/TiO2 composite electrodes. This work creates new opportunity for designing and constructing highly-efficient photocatalysts by interface engineering.

KEYWORDS. Two-dimensional nanojunction; interface engineering; solar hydrogen generation; anatase TiO2 nanosheets; layered MoS2.


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ACS Catalysis

1. INTRODUCTION Direct conversion of solar energy to hydrogen fuel from water via artificial photosynthesis is considered to be an effective strategy to mitigate the aggravating energy crisis and address the worsening environmental problems.1,2 In such a photosynthesis scheme, the reductive side of water splitting is the light-induced generation of hydrogen from aqueous protons. Catalytic systems that have been successfully constructed for photoinduced hydrogen evolution typically use semiconductor material as a photocatalyst to harvest solar light in conjunction with a noble metal cocatalyst.3-5 Recently, functional two-dimensional (2D) layered materials have attracted considerable interest in the field of solar hydrogen generation due to the unique properties attributed to their novel structural features, which is different from zero-dimensional (0D) and one-dimensional (1D) materials.6,7 For example, the 2D light-harvesting semiconductors are excellent cocatalyst supports, which provide larger specific surface areas for cocatalyst dispersion. One the other hand, the nano-sized 2D layered cocatalysts exhibit a great number of active sites for hydrogen evolution reaction, resulting in enhanced photocatalytic activities. Up to now, many 2D materials such as graphene,8,9 molybdenum disulfide,10,11 and graphitic carbon nitride (g-C3N4) etc,12,13 have been explored for photocatalytic hydrogen production. Recently, 2D anatase TiO2 nanosheets with exposed (001) facets is considered to be the most promising candidate for solar-to-H2 conversion because it is highly-efficient, low-cost, earth-abundant, stable, and nontoxic.14-16 Traditional methods for designing highly efficient solar hydrogen generation system based on anatase TiO2 nanosheets with exposed (001) facets mainly focus on the development of noble metal loaded-TiO2 nanosheets photocatalysts with a 0D-2D structure.17-19 Although the photogenerated electron-hole pairs in TiO2 nanosheets can be effectively segregated by loading noble metal cocatalysts, these 0D-2D composites usually


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exhibit relatively low photocatalytic activities because of the limited contact interface between the noble metals and TiO2 nanosheets. In addition, the expensive price of noble metals limits their widespread use. Hence, it is highly desirable to develop noble-metal-free cocatalysts modified TiO2 nanosheets with exposed (001) facets composite photocatalysts with intimate and large contact interface for robust and efficient solar hydrogen generation system. Molybdenum disulfide is an expeditiously rising star 2D layered material, which has sparked enormous scientific interest in developing novel MoS2-based materials for a cornucopia of potential applications. Very recently, MoS2 has been reported to be an excellent catalyst for both photocatalytic and electrocatalytic H2 evolution reaction due to the existence of abundant exposed edges, and the active sites were stemmed from the sulfur edges of MoS2 crystal layers.20-24 However, to the best of our knowledge, no prior work regarding the application of layered MoS2-loaded anatase TiO2 nanosheets with exposed (001) facets hybrid photocatalyst for solar hydrogen generation has been reported to data. More importantly, coupling different 2D materials of anatase TiO2 nanosheets and layered MoS2 would greatly enhance the photocatalytic activity of this 2D-2D photocatalyst due to the increased contact interface and charge transfer rate.25,26 Accordingly, it is expected that the combination of (001) facets exposed anatase TiO2 nanosheets and layered MoS2 is promising to obtain a highly-efficient, low-cost, and stable photocatalyst for solar-to-H2 conversion. In this work, we designed and synthesized functional 2D-2D MoS2/TiO2 composite photocatalyst by using 2D anatase TiO2 nanosheets with exposed (001) facets as the light-harvesting semiconductor and 2D layered MoS2 as the cocatalyst for solar H2 generation. Such a novel architecture provides intimate and large contact interface for fast interfacial charge separation. As a result, the 2D nanojunction photocatalyst is anticipated to exhibits superior photocatalytic activity for H2 evolution.


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ACS Catalysis

2 EXPERIMENT SECTION 2.1

Materials:

Thioacetamide

(CH3CSNH2),

sodium

molybdate

dehydrate

(Na2MoO4·2H2O), and 47% hydrofluoric acid aqueous solution (HF) were purchased from Sinopharm Chemical Reagent Co., Ltd. Tetrabutyl titanate (Ti(OBu)4, 99%) was obtained from Alfa Aesar China (Tianjin) Co., Ltd. Degussa P25 TiO2, composite of rutile and anatase, was purchased from Degussa Company, German. All reagents were used as received without further purification. 2.2 Synthesis of TiO2 Nanosheets: Anatase TiO2 nanosheets with exposed (001) facets were prepared by the hydrothermal method similar to that reported by Xie et al.27 In a typical process, 5 mL of Ti(OBu)4 and 0.8 mL of hydrofluoric acid solution were mixed in a dried Teflon-lined stainless steel autoclave with a capacity of 25 mL, and then kept in an electric oven at 200 oC for 24 h. Caution: Hydrofluoric acid is extemely corrosive and a contact poison, and it should be handled with extreme care! Hydrofluoric acid solution is stored in Teflon containers in use. After hydrothermal reaction, the autoclave was cooled naturally to room temperature, the produced off-white powder was separated by high-speed centrifugation, followed by washing with ethanol and distilled water for several times and drying at 80 oC for 12 h. 2.3 Synthesis of 2D-2D MoS2/TiO2 Photocatalysts: The 2D-2D MoS2/TiO2 composite photocatalysts were prepared by hydrothermal reaction of the above-prepared TiO2 nanosheets powders in an aqueous solution with sodium molybdate dehydrate and thioacetamide. The nominal weight ratios of MoS2 to TiO2 were 0.25, 0.50, 1.00, 1.50, and 2.00 wt%, and the obtained samples were labeled as 0.25, 0.50, 1.00, 1.50, and 2.00 wt% 2D-2D MoS2/TiO2 photocatalysts, respectively. In a typical synthesis, 200 mg of the above-prepared TiO2 nanosheet was dispersed in 18 mL of aqueous solution consisting of 30 mg sodium molybdate dehydrate


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and 60 mg thioacetamide to form a transparent solution. The mixed solution was then transferred into a 25 mL Teflon-lined stainless steel autoclave and then treated at 200 oC for 24 h in an electric oven. The gray product was collected via centrifugation, and washed thoroughly with ethanol before drying at 80 oC for 12 h to obtain a 1.00 wt% 2D-2D MoS2/TiO2 sample. The other 2D-2D MoS2/TiO2 photocatalysts with different loading amounts of MoS2 were synthesized by changing the amount of sodium molybdate dehydrate and thioacetamide under the same method. For comparison, MoS2 modified P25 (MoS2/P25) photocatalyst was prepared through the same procedures. All products were ground and heated at 400 oC for 2 h under nitrogen atmosphere. The MoS2/TiO2 photoelectrodes were prepared using electrophoretic deposition method. Typically, 5 mg iodine and 10 mg MoS2/TiO2 powder were dispersed in 25 mL of acetone. The electrophoretic deposition process was conducted between two parallel ITO electrodes with the distance of 1 cm under 15 V of bias for 5 min. Finally, the electrodes were heated at 400 °C for 30 min under nitrogen atmosphere. 2.4 Photocatalytic Hydrogen Production: Photocatalytic hydrogen evolution experiments were mainly carried out in a Pyrex glass cell with a top window connected to a gas-closed system. In a typical photocatalytic hydrogen production experiment, 100 mg H2-evolving photocatalyst powders were suspended by a magnetic stirrer in 100 ml aqoeous solution containing 10% methanol in volume. The reaction temperature of reactant solution was maintained at 293 K. The reactant solution was irradiated with light using a 300 W Xe-arc lamp after the reaction solution was evacuated several times to remove air completely. The amount of evolved H2 was determined by using a gas chromatograph (JieDao, GC1609, MS-5A column, TCD, Ar carrier). The different noble metals, including Pt, Pd, Ru, Rh and Au, were loaded on the surface of TiO2 nanosheets with exposed (001) facets by an in situ photodeposition method


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ACS Catalysis

using H2PtCl4, PdCl2, RuCl3, RhCl3 and HAuCl4 aqueous solutions, respectively. The apparent quantum yield (AQY) of H2 evolution system was measured by using 0.50 wt% MoS2/TiO2 as the photocatalyst and low power UV-LED (3 W, 360 nm) (Shenzhen Warner JPLED Co. Ltd. China) as light source. The AQY was calculated according to the following equations: nphotons =

Pλ hc

AQY[%] =

=

×t

number of reacted electrons number of incident photons

(1) × 100

2 × number of evolved H2 molecules number of incident photons

× 100

(2)

where P is the input optical power; λ (360 nm) is the wavelength of the monochromatic light; h is Planck’s constant; c is the speed of light; and t is the illumination time. 2.5 Characterization: Powder X-ray diffraction (XRD) patterns were taken with a Bruker D8 Advance X-ray diffractometer operated at a voltage of 40 kV and a current of 40 mA by using Cu Kα radiation (λ = 0.15406 nm). UV-visible diffuse reflectance spectra of samples were performed using a Varian cary 500 UV-vis spectrophotometer, in which BaSO4 was used as the background. The high resolution transmission electron microscopy (HR-TEM) images of TiO2 and 2D-2D MoS2/TiO2 photocatalysts were obtained using a JEOL JEM 2010 transmission electron microscope with an accelerating voltage of 200 kV. All TEM samples were obtained by depositing a drop of diluted suspensions in ethanol on a carbon-film-coated copper grid and naturally dried. The field emission scanning electron microscopic (FE-SEM) images of MoS2 and TiO2 were taken using a Carl Zeiss Gemini (vltra55) field emission scanning electron microscope. All SEM samples were obtained by depositing a drop of diluted suspensions in ethanol on a silicon chip. X-ray photoelectron spectroscopy (XPS) measurements were performed on a VG ESCALAB MKII XPS system with Al Kα X-ray source and a charge


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neutralizer. All the binding energies were calibrated using the C1s peak at 284.8 eV. Nitrogen adsorption-desorption isotherms were conducted at 77 K using a Quant achrome Autosorb instrument. Prior to the measurement, the sample was degassed at 473 K for approximately 3 h under high vacuum. Raman analysis of the samples was carried out on a J-Y T64000 Raman spectrometer with 514.5 nm wavelength incident laser light. The photocurrent curves were recorded on a CHI-B600 electrochemical analyzer by using the prepared MoS2/TiO2 films as the working electrodes, Pt wire as the counter electrode, and Ag/AgCl as the reference electrode, the light source was a 300 W Xe lamp. The photoluminescence (PL) spectra were measured on an Edinburgh FS5 spectrofluorometer. 3 Results and Discussion 3.1 Characterization of 2D-2D MoS2/TiO2 Nanocomposites. The MoS2/TiO2 nanocomposites were prepared by a two-step hydrothermal process, and the overall synthetic procedure is illustrated in Figure S1 (Supporting Information). In the first step, anatase TiO2 nanosheets with exposed (001) facets were prepared in a HF aqueous solution using Ti(OBu)4 as the precursor according to a previously reported method.27 Subsequent hydrothermal treatment of CH3CSNH2 and Na2MoO4·2H2O in an aqueous solution of the as-prepared TiO2 nanosheets at 200 oC for 24 h led to the formation of 2D-2D MoS2/TiO2 nanojunction photocatalysts. The morphology of pure TiO2 nanosheets and 2D-2D MoS2/TiO2 photocatalysts were investigated by scanning electron microscope (SEM) and transmission electron microscopy (TEM). As shown in Figure 1a and b, the SEM and TEM images of as-prepared (001) facets exposed anatase TiO2 nanosheets show that the product consists of well-defined sheet-shaped structures with a specific surface area of 105.6 m2 g-1 (Figure S2). The size distribution of TiO2 nanosheets derived from the TEM image were measured in the range of 80-140 nm in size as measured by the length. The


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ACS Catalysis

average size of TiO2 nanosheets from the TEM image shown in Figure 1c was calculated to be 120 nm in the side length, 100 nm in the side width and 12 nm in thickness. The high-resolution TEM (HRTEM) image in Figure 1d shows that the lattice spacing parallel to the top and bottom facets was 0.235 nm, corresponding to the (001) planes of anatase TiO2.18,27 In addition, as shown in Figure 1e, the HRTEM image recorded from a single nanosheet clearly shows the continuous (200) atomic planes of anatase TiO2 with a lattice spacing of 0.19 nm.28,29 On the basis of the above structural information, it can be concluded that the exposed top and bottom surfaces of TiO2 nanosheets were (001) facets. According to the dimensions of TiO2 nanosheets, the percentage of (001) facets in the as-prepared TiO2 nanosheets was calculated directly from this regular geometry to be approximately 84% (Figure S3).30 Figure 2 shows the XRD patterns of pure TiO2 nanosheets and 2D-2D MoS2/TiO2 photocatalysts. As for the pure TiO2 sample, all the diffraction peaks are indexed to the anatase phase (JCPDS card No. 21-1272).15,18 The diffraction peaks appeared at 2θ = 25.44o, 38.20o, 48.29o, 54.22o, 55.30o, 62.82o, 70.48o, 75.58o and 76.52o can be attributed to the (101), (004), (200), (105), (211), (204), (220), (215) and (301) lattice plane of the anatase TiO2, respectively.30 In addition, Raman spectroscopy was applied to further check the phase purity of anatase TiO2 nanosheets with exposed (001) facets. As shown in Figure S4, the pure TiO2 sample has four peaks appeared at 144, 394, 514 and 636 cm-1, which can be assigned to the Eg, B1g, A1g and Eg anatase tetragonal vibration modes of TiO2, respectively.31,32 The result indicates the presence of typical anatase TiO2 phase, which is consistent with the XRD result. The Eg, B1g and A1g peaks are correspond to the symmetric stretching vibration, symmetric bending vibration and antisymmetric bending vibration of O-Ti-O, respectively.33,34 It has been known that the higher the percentage of (001) facets are exposed, the intensity of the Eg peaks in the Raman spectra


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becomes decreased, but the intensity of the A1g and B1g peaks in the Raman spectra become increased. Therefore, the percentage of exposed (001) facets in anatase TiO2 nanosheets can be calculated from the peak intensity ratio of A1g and Eg peaks.35 In the present study, the percentage of exposed (001) facets of as-prepared anatase TiO2 nanosheets was calculated to be 83%, which is close to the measured percentage from the electron microscope. The pure MoS2 prepared by hydrothermal method was fully characterized by XRD, Raman and SEM. The XRD pattern of pure MoS2 nanosheets illustrated in Figure S5 shows three main diffraction peaks at values of 14.06 o, 33.27 o and 57.45 o, which can be assigned to the (002), (100) and (106) lattice plane of hexagonal phase MoS2 (JCPDS card No. 37-1491).11 The crystal structure of MoS2 was further confirmed by Raman spectrum. As shown in Figure S6 the pure MoS2 shows two strong Raman speaks at 387 and 412 cm-1, corresponding to high energy A1g 1 mode and low energy E2g mode, respectively. The Raman result confirms the formation of 2H-

MoS2 phase in nanosheets.36 The SEM image illustrated in Figure S7 reveals that the MoS2 exhibits a typical layered structure, and the specific 2D structure is important for constructing 2D nanjunction. After loading of MoS2 on the surface of TiO2 nanosheets, no characteristic diffraction peak of MoS2 was observed in the XRD patterns of as-prepared 2D-2D MoS2/TiO2 photocatalysts, probably due to the small amount of MoS2 component and its high dispersion on the surface of TiO2 nanosheets. However, the MoS2 component in 2D-2D MoS2/TiO2 photocatalysts can be confirmed by Raman, HRTEM and XPS analyses. As shown in Figure S8, 1 the characteristic peaks of MoS2 (A1g and E2g modes) were observed in the Raman spectra of 2.00

wt% MoS2/TiO2 photocatalyst, indicating the MoS2 was loaded on TiO2 surface successfully. As compared to pure TiO2, a red-shift of the Eg mode was observed in MoS2/TiO2 photocatalyst from 142 to 147 cm-1. Such a shift could suggest a strong intimate interaction existed between


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TiO2 and MoS2. A similar phenomenon was reported for MoS2/TiO2 composites in previous study.34 To determine the chemical states of as-prepared 2D-2D MoS2/TiO2 composites, XPS measurement was carried in the region of 0-800 eV. As shown in Figure S9, the wide-scan XPS spectra of 2.00 wt% 2D-2D MoS2/TiO2 photocatalyst shows the binding energy peaks at 163.1, 230.2, 458 and 530.6 eV, which can be attributed to the S 2p, Mo 3d, Ti 2p and O 1s peaks, respectively.37,38 As illustrated in Figure 3a and b, three peaks are observed at 464.6, 458.8 eV and 531.0 eV, which can be attributed to Ti 2p1/2, Ti 2p3/2, and O1s, respectively. Figure 3c shows the high-resolution XPS spectra of Mo 3d, which contains Mo 3d3/2 at 231.9 eV and Mo 3d5/2 at 228.8 eV. In addition, the doublet peaks for S 2p at 161.7 (S 2p3/2) and 162.8 eV (S 2p1/2) in Figure 3d indicate the formation of MoS2 in the 2D-2D MoS2/TiO2 photocatalyst. The XPS measurement confirms that the layered MoS2 was loaded on TiO2 surface successfully. To gain a clearer understanding of the interface between MoS2 and TiO2, the morphology of the MoS2/TiO2 nanostructures was vividly investigated by the TEM characterization. The typical TEM image of 2.00 wt% 2D-2D MoS2/TiO2 photocatalyst presented in Figure 4a shows that the composite exhibits a similar morphology with pure TiO2 sample. As shown in Figure 4b and c, the HRTEM image recorded from a single nanosheet clearly shows that the composite is made up of anatase TiO2 nanosheets with lattice fringes of 0.19 nm attributes to the (200) plane of anatase TiO2 and MoS2 nanosheets with interplanar spacing of 0.62 nm corresponds to the (002) plane of hexagonal MoS2.39,40 The lattice fringes of (200) plane of anatase TiO2 in the HRTEM images of 2D-2D MoS2/TiO2 photocatalyst are blurry as compared to that of pure TiO2 (Figure 1e), which could be mainly attributed to the interference effect of MoS2 loaded on TiO2 surface. The layer numbers of MoS2 was determined to be approximately 6-9 from the HRTEM images illustrated in Figure 4b and c. It is worth noting that the intimate 2D nanojunction


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between MoS2 and TiO2 is clearly discernable in the HRTEM images, indicating their prefect contact between each other. More importantly, the intimate 2D nanojunction favors the photogenerated charge carrier transfer from TiO2 to MoS2, which may be a key factor in determining the photocatalytic activities of these 2D-2D MoS2/TiO2 photocatalysts. Based on the XPS and TEM analysis, it is concluded that the layered MoS2 was coated on the surface of TiO2 nanosheets with a novel 2D-2D structure. 3.2 Photoelectrochemical Properties of 2D-2D MoS2/TiO2 Photocatalysts. Diffuse reflectance spectroscopy was used to measure the UV-vis absorbance of TiO2 alone and 2D-2D MoS2/TiO2 photocatalysts. As shown in Figure S10, the pure TiO2 nanosheets show an absorption edge at 388 nm, corresponding to a band gap of 3.20 eV. The absorbance spectra of MoS2/TiO2 composites show a combination of the spectral features of TiO2 and MoS2 alone, with TiO2 absorbing mainly UV light while MoS2 absorption extends into the visible range. It is clearly that the absorption intensity in visible light region increases with the increased amounts of MoS2, which is in good agreement with the color changing from white to gray (Figure S11). Previous studies have suggested that the interface between the light-harvesting semiconductor and cocatalyst is one of the key factors that can determine the over separation efficiency of photogenerated charge carriers.41,42 In the present study, the photoluminescence (PL) emission spectra were used to investigate the efficiency of the interfacial electron transfer from TiO2 to MoS2 since the PL emission results from the recombination of free charge carriers.18,43 Figure 5a presents the comparison of PL emission spectra of pure TiO2 and MoS2/TiO2 photocatalysts in ethanol solutions (0.1 mg ml-1) excited at 300 nm. A strong emission peak at approximately 368 nm could be assigned to the band gap transition of TiO2, which is significantly different from the published values in region of 385-395 nm obtained from


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ACS Catalysis

bulk TiO2 sample.19 The blue-shift of emission wavelength was caused by the reabsorption mechanism.44 That is, the emission peaks shift to the short-wavelength with the decreased concentration of TiO2 (Figure S12). Remarkably, the emission peak can be effectively quenched in intensity in the presence of MoS2 due to the efficient interfacial electron transfer from excited TiO2 to MoS2. That is to say, the MoS2 cocatalyst loaded on the surface of TiO2 can act as an electron sink, which suppresses the recombination of photogenerated electron-hole pairs. In order to further evaluate the efficiency of charge carrier trapping, migration, transfer and separation, the photocurrent responses of both bare TiO2 electrode and MoS2/TiO2 composite electrodes loaded with different amounts of MoS2 were tested. Figure 5b shows the periodic on/off photocurrent response of all samples when irradiated under a 300 W Xe lamp. The bare TiO2 electrode generates a relatively low short-circuit photocurrent density with a value of 4.1 µA cm-2. After loading the layered MoS2 on the surface of TiO2 nanosheets, which results in significantly increased photocurrent density. The 0.50 wt% MoS2/TiO2 electrode shows the highest photocurrent density of 54.3 µA cm-2, which is about 13 times higher than that of bare TiO2 electrode. This conclusively demonstrates that the intimate 2D nanojunction can significantly enhance the separation efficiency of photogenerated charge carriers in these 2D-2D MoS2/TiO2 photocatalysts. 3.3 Photocatalytic Activities of 2D-2D MoS2/TiO2 Photocatalysts. To investigate the photocatalytic activities of 2D-2D MoS2/TiO2 photocatalysts, we utilized them for photocatalytic hydrogen production from water in the presence of methanol as the sacrificial agent under a 300 W Xe lamp. Without light or photocatalyst, no H2 was observed which indicates that H2 evolution is a light catalyzed reaction. As shown in Figure 6a, the pure TiO2 sample was used as the control sample showed a very low photocatalytic activity with a H2 evolution rate of 61 µmol


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h-1 g-1, which can be assigned to the fast recombination of photogenerated charge carriers. Meanwhile, it has been known that the pure MoS2 was inactive for photocatalytic hydrogen evolution reaction even it has intensive absorption in the ultraviolet and visible regions.45 However, the H2 evolution rates were significantly increased when using the 2D-2D MoS2/TiO2 nanocomposites as photocatalysts, which indicates a synergistic interaction between the two components for the enhanced photocatalytic activities. The intimate 2D nanojunction between TiO2 to MoS2 provides an efficient transport channel for charge carrier transfer, accelerating the hydrogen evolution reaction. After only loading of 0.25 wt% MoS2 on TiO2 surface, the MoS2/TiO2 photocatalyst exhibits a H2 evolution rate of 568 µmol h-1 g-1, which is 9.3 times higher than that of pure TiO2 sample. Especially, the loading amount of MoS2 cocatalyst was found to dramatically influence the photocatalytic activities 2D-2D of MoS2/TiO2 photocatalysts. The H2 evolution rates increased remarkably and then decreased gradually, which is in accordance with previous literatures.46,47 The highest H2 evolution rate of 2145 µmol h-1 g-1 was obtained when using 0.50 wt% 2D-2D MoS2/TiO2 nanocomposite as the photocatalyst, which is 36.4 times higher than that of bare TiO2 sample. When the MoS2 loading amount exceeds 0.50 wt%, the H2 evolution rates decreased gradually with the increasing amounts of MoS2. The 1.00, 1.50, 2.00 wt% MoS2/TiO2 photocatalyst exhibits a H2 evolution rate of 1534, 877 and 546 µmol h-1 g-1, respectively. The decreased in photocatalytic hydrogen evolution activities of 2D-2D MoS2/TiO2 photocatalysts with relatively larger amounts of MoS2 is likely due to the shading effect, which can block the light absorption of the incident light by the TiO2 component.46,47 This hypothesis is supported by the absorption spectra of 2D-2D MoS2/TiO2 photocatalysts shown in Figure S10. Meanwhile, the apparent quantum yield (AQY) of hydrogen evolution system reaches 6.4% at 360 nm, which is higher than that of Pt-loaded TiO2 photocatalyst (360 nm,


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AQY = 5.0%) photocatalyst but lower than that of MoS2-graphene/TiO2 (365 nm, AQY = 9.7%).32,48 Previous studies have shown that the amounts of photocatalyst can effectively affect the H2 evolution rates. To gain more insight into the effect of photocatalyst amounts on photocatalytic activities, the photocatalytic performance of H2 evolution systems containing 25, 50, 75, 100, 125 and 150 mg MoS2/TiO2 composite were illustrated. As illustrated in Figure S13, the amounts of H2 increase from 219 to 746 µmol after 3h of irradiation when the amounts of MoS2/TiO2 composite photocatalyst increase from 25 to 150 mg. However, the H2 evolution rates decrease from 2941 to 1691µmol h-1 g-1, which can be assigned to the limiting concentration of sacrificial reagents. We noted that the 2D-2D MoS2/TiO2 composite photocatalyst shows a much higher photocatalytic activity than those previously reported systems, in which the MoS2 nanosheets were loaded on TiO2 nanoparticles (753 µmol h-1 g-1), TiO2 nanocrystals (2000 µmol h-1 g-1) or TiO2 nanobelt (1600 µmol h-1 g-1).11,26,34 The most substantial reason for this phenomenon could be attributed to the fact that the combination of different 2D layered materials can greatly increased contact surface and charge transfer rate, resulting in a significant improvement in the photocatalytic activity. It is well-known that noble metals such as Pt, Pd, Rh, Ru, and Au are highly-efficient cocatalysts for photocatalytic hydrogen production reaction because of their large work functions.49-52 For comparison, 0.50 wt% of these noble metals was, respectively, loaded on the surface of TiO2 nanosheets by an in situ photodeposition method under irradiation from the corresponding complexes of metals. As shown in Figure 6b, among these noble metals modified TiO2 photocatalysts, the Pt/TiO2 photocatalyst shows the highest H2 evolution rate of 1368 µmol h-1 g-1, while it is still lower than that of the 0.50 wt% 2D-2D MoS2/TiO2 photocatalyst. To gain more insight into the catalytic activity of MoS2 and Pt, the photocatalytic performance of Pt/TiO2


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photocatalysts loaded with various amounts of Pt were investigated. As shown in Figure 6c, the optimal loading amount of Pt was found to be 1.50 wt%, and the1.50 wt% Pt/TiO2 photocatalyst shows a H2 evolution rate of 2036 µmol h-1 g-1, which is lower than that of 0.50 wt% 2D-2D MoS2/TiO2 photocatalyst. The result suggests that the MoS2 nanosheets could act as a more efficient cocatalyst than Pt for photocatalytic H2 production. Similar phenomenon was observed in previous studies.6,41 Although Pt metal shows a higher catalytic activity than that of MoS2 for electrochemical H2 evolution reaction,21,53 the 2D-2D MoS2/TiO2 photocatalyst exhibits a higher H2 evolution rate than that of Pt/TiO2 photocatalyst in the present study. The reason is most probably due to the intimate 2D nanojunction between MoS2 and TiO2, which favors the interfacial electron transfer from photoexcited TiO2 to MoS2, resulting in an enhancement of the photocatalytic activity.54-57 To verify this hypothesis, the Pt-laoded TiO2 photocatalyst was characterized by TEM. Figure 7a shows that the Pt nanoparticles with particle sizes of approximately 3 nm were uniformly deposited on the surface of TiO2 nanosheets. The magnified HRTEM image in Figure 7a clearly shows fringes with lattice fringes of 0.23 nm, which can be assigned to the crystallographic planes of Pt (111).17 Moreover, the TEM analyses clearly reveal that the Pt-loaded TiO2 photocatalyst exhibits a typical 0D-2D structure, and the contact area is relatively small due to the point contact between two components (Figure 7b). However, in comparison with the 0D-2D Pt/TiO2 composite, the 2D-2D MoS2/TiO2 photocatalyst possesses much larger contact area between the MoS2 and TiO2 (Figure 7c), leading to more efficient interfacial charge carrier transfer. As a result, a much higher photocatalytic H2 evolution activity of the 2D-2D MoS2/TiO2 photocatalyst was observed. Based on the above results, a proposed mechanism for the enhanced photocatalytic H2 production activities of 2D-2D MoS2/TiO2 photocatalysts was illustrated in Figure 8. Upon the absorption light, the electrons in the valence


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band (VB) of TiO2 are excited to the conduction band to create electron-hole pairs. The photogenerated electron-hole can quickly recombine within the TiO2 nanosheets without any cocatalyst loading, resulting in a low H2 evolution activity of pure TiO2 sample. Due to the comparable energy difference between the CB of TiO2 (ECB(TiO2) = -0.51 V vs. NHE, pH 7)58 and MoS2 (ECB(MoS2) = -0.16 V vs. NHE, pH 7),59 there is a strong thermodynamic driving force for electron transfer from excited TiO2 to MoS2. More importantly, the intimate and large contact interface between TiO2 and MoS2 are the key factors in determining the superior H2 evolution activities of 2D-2D MoS2/TiO2 photocatalysts, which favor the charge transfer from TiO2 to MoS2 for H2 evolution, resulting in enhanced photocatalytic activities for hydrogen evolution (Figure S14). The effective charge transfer from excited TiO2 to MoS2 is demonstrated by the significant enhancement of photocurrent responses in 2D-2D MoS2/TiO2 composite electrodes (Figure 5b). Finally, the electron on the surface of MoS2 can react with the absorbed protons to evolve H2 efficiently, while the holes in the VB of TiO2 are consumed by methanol sacrificial reagent. As we all know, the commercially available TiO2 (Degussa P25) is a distinguished photocatalyst, which has been widely used for solar hydrogen evolution.17 For comparison, the MoS2/P25 composite photocatalyst was prepared by a hydrothermal method similar to that of 2D-2D MoS2/TiO2 composite. The TEM images of 2.00 wt% MoS2/P25 composite illustrated in Figure S15 clearly shows that the MoS2 was loaded around TiO2 nanoparticle. It is very obvious that the microstructure of MoS2/P25 is different from 2D-2D MoS2/TiO2 composite, which exhibits much larger contact surfaces between MoS2 and TiO2 (Figure 4b and c). The contact surfaces between MoS2 cocatalyst and TiO2 could play an important role in determining the photocatalytic activities photocatalysts. The photocatalytic H2 production activities of as


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prepared 2D TiO2 nanosheets, P25, 2D-2D MoS2/TiO2 and MoS2/P25 were shown in Figure 9a. The H2 evolution rate of 2D TiO2 nanosheets (61 µmol h-1 g-1) is significantly higher than that of P25 (42 µmol h-1 g-1) under the same conditions, which is consistent with previous studies.17 The result demonstrates the high catalytic activity of the (001) facets for 2D TiO2 nanosheets. This observation is consistent with previous studies. After loading 0.50 wt% MoS2 cocatalyst, the H2 evolution rates of 2D TiO2 and P25 were increased to 2146 and 975 µmol h-1 g-1, respectively. The H2 evolution activity of 2D-2D MoS2-TiO2 is 2.2 times higher than that of MoS2/P25, which could be assigned to both the higher reactivity of high-energy (001) facets and the larger contact interface between MoS2 and TiO2. Furthermore, in order to determine the reusability of 2D-2D MoS2/TiO2 photocatalysts, H2 production experiment was carried out for three cycles by using the 0.50 wt% 2D-2D MoS2/TiO2 nanocomposite as the photocatalyst. As shown in Figure 9b, no apparent loss of H2 evolution rate was observed, indicating that the composite is structurally stable during the solar hydrogen evolution reaction. It is worth noting that no obvious change in material structure was observed for the photocatalyst after 9 h of irradiation, which was confirmed by the XRD examination (Figure S16). 4 Conclusions In conclusion, we fabricated noble-metal-free MoS2-modified TiO2 photocatalyst with an in situ-formed 2D nanojuction interface for photocatalytic H2 production. The 2D-2D MoS2/TiO2 photocatalyst shows high photocatalytic activity with the maximum H2 evolution rate of 2145 µmol h-1 g-1 for the sample containing 0.5 wt% MoS2, which is about 36.4 times higher than that of pure TiO2 nanosheets. An excellent apparent quantum efficiency of 6.4% is attained at 360 nm. It is believed that the intimate and large contact interface between MoS2 and TiO2 can efficiently promote the photoinduced charge carriers separation, which plays a key factor in


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determining the high photocatalytic performance of MoS2/TiO2 photocatalyst. The present study demonstrates the importance of 2D nanojuction between light-harvesting semiconductor and cocatalyst in improving the photocatalytic H2 production efficiency, and the interface engineering is expected to be an excellent strategy for constructing highly-efficient solar hydrogen generation systems. Supporting Information Figure S1-S16 are available free of charge via the Internet at http://pubs.acs.org. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 51502068, 21271170 and 51572065), the Natural Science Foundation of Zhejiang Province (No. LQ16B030002), the Natural Science Foundation of Zhejiang Province for Distinguished Young Scholars (LR15E020001), the National Basic Research Program of China (No. 2013CB632404), and the 151 talent’s projects in the second level of Zhejiang Province. References (1) Lewis, N. S.; Nocera, D. G. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15729-15735. (2) Tong, H.; Ouyang, S. X.; Bi, Y. P.; Umezawa, N.; Oshikiri, M.; Ye, J. H. Adv. Mater. 2012, 24, 229-251. (3) Ma, Y.; Wang, X. L.; Jia, Y. S.; Chen, X. B.; Han, H. X.; Li, C. Chem. Rev. 2014, 114, 998710043. (4) Kudo, A.; Miseki, Y. Chem. Soc. Rev. 2009, 38, 253-278. (5) Yang, L.; Zhong, D,; Zhang, J. Y.; Yan, Z. P.; Ge, S. F.; Du, P. W.; Jiang, J. Sun, D.; Wu, X. J.; Fan, Z. Y.; Dayeh, S. A.; Xiang. B. ACS Nano, 2014, 8, 6979-6985.


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Figure 1. (a) SEM image of pure TiO2 nanosheets. (b and c) TEM images of pure TiO2 nanosheets, inset of (b) is the histograms of particle size distribution. (d) HRTEM image from the vertical nanosheets. (e) HRTEM image recorded from the single TiO2 nanosheets.


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Figure 2. XRD patterns of pure TiO2 and 2D-2D MoS2/TiO2 composites, TiO2 (1), 0.25 wt% (2), 0.50 wt% (3), 1.00 wt% (4), 1.50 wt% (5), 2.00 wt% (6) MoS2/TiO2.


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Figure 3. High resolution XPS spectra of Ti 2p (a) and O 1s (b), Mo 3d (c) and S 2p (d)..


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Figure 4. (a) TEM images of 2.00 wt% 2D-2D MoS2/TiO2 photocatalyst. (b and c) HRTEM images of 2.00 wt% 2D-2D MoS2/TiO2, inset of (b) is the schematic diagram of 2D-2D MoS2/TiO2 photocatalyst, note: the HRTEM images clearly show the intimate 2D nanojunction between TiO2 and MoS2.


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Figure 5. (a) PL spectra of pure TiO2 and MoS2/TiO2 nanocomposites in ethanol solution (0.1 mg ml-1). (b) Transient photocurrent responses of TiO2 and MoS2/TiO2 composite electrodes recorded in 0.5 M NaSO4 aqueous solution under with light-on and light-off cycles. TiO2 (1), 0.25 wt% (2), 0.50 wt% (3), 1.00 wt% (4), 1.50 wt% (5), 2.00 wt% (6) MoS2/TiO2.


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Figure 6. (a) The rate of H2 production on 2D-2D MoS2/TiO2 photocatalysts loaded with various amounts of MoS2 under irradiation from a 300 W Xe lamp in 100 mL of 10 vol % aqueous methanol solution. (b) The rate of H2 production on TiO2 loaded with 0.50 wt% of different cocatalysts under irradiation from a 300 W Xe lamp in 100 mL of 10 vol % aqueous methanol solution. (c) The rate of H2 production on Pt/TiO2 photocatalysts loaded with various amounts of Pt under irradiation from a 300 W Xe lamp in 100 mL of 10 vol % aqueous methanol solution.


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Figure 7. (a) TEM of Pt-loaded TiO2 nanosheets photocatalyst, the inset shows the magnified HRTEM image of the selected frame from (a). (b and c) Schematic diagram of 0D-2D Pt/TiO2 and 2D-2D MoS2/TiO2 photocatalysts, which clearly show the 2D-2D MoS2/TiO2 photocatalyst exhibits much larger contact interface between the light-harvesting semiconductor and cocatalyst than that of 0D-2D Pt/TiO2 photocatalyst.


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ACS Catalysis

Figure 8. Illustration and energy diagram of charge transfer and photocatalytic processes for efficient solar-to-H2 conversion system using the 2D-2D MoS2/TiO2 as the photocatalys.


ACS Catalysis

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Figure 9. (a) The rate of H2 production on TiO2 nanosheets, 2D-2D MoS2/TiO2, P25, MoS2/P25 under irradiation from a 300 W Xe lamp in 100 mL of 10 vol % aqueous methanol solution. (b) Cyclic H2 production on 0.50 wt% 2D-2D MoS2/TiO2 photocatalyst (100 mg) under irradiation from a 300 W Xe lamp in 100 mL of 10 vol % aqueous methanol solution.


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ACS Catalysis

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Constructing Anatase TiO2 Nanosheets with ... - ACS Publications

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