MoS2 Nanosheets As Efficient Photocatalysts for ... - ACS Publications

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In-plane heterojunctions enable multiphasic 2D MoS2 nanosheets as efficient photocatalysts for hydrogen evolution from water reduction Rui Peng, Liangbo Liang, Zachary D. Hood, Abdelaziz Boulesbaa, Alexander A. Puretzky, Anton Ievlev, Jérémy Come, Olga Ovchinnikova, Hui Wang, Cheng Ma, Miaofang Chi, Bobby G. Sumpter, and Zili Wu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02076 • Publication Date (Web): 30 Aug 2016 Downloaded from http://pubs.acs.org on September 3, 2016

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In-plane heterojunctions enable multiphasic 2D MoS2 nanosheets as efficient photocatalysts for hydrogen evolution from water reduction Rui Peng,a Liangbo Liang,a Zachary D. Hood,a,b Abdelaziz Boulesbaa,a Alexander Puretzky,a Anton Ievlev,a Jeremy Come,a Olga Ovchinnikova,a Hui Wang,a Cheng Ma,a Miaofang Chi,a Bobby Sumpter,a and Zili Wu*a a. Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37831. b. School of Chemistry & Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332.

Two-dimensional (2D) single-layer MoS2 nanosheets are demonstrated as efficient photocatalysts for hydrogen evolution reaction (HER) from water reduction thanks to specific inplane heterojunctions constructed in the MoS2 monolayer. These functional heterojunctions are formed among the different phases of chemically exfoliated MoS2 monolayers: semiconducting 2H, metallic 1T, and quasi-metallic 1T’ phases. The proportion of the three MoS2 phases can be systematically controlled via thermal annealing of the nanosheets. Interestingly, a volcano relationship is observed between the photocatalytic HER activity and the annealing temperature with an optimum activity obtained after annealing at 60ºC. First-principles calculations were integrated with experimental studies to shed light on the role of the multiphases of MoS2 and reveal that optimum photocatalytic HER activity results from the formation of the in-plane heterojunctions between 1T’-2H MoS2. Importantly, this facilitates not only balanced light absorption and charge generation by the 2H phase, efficient charge separation at the 1T’-2H 1

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interface, but also favorable HER over the basal sites of 1T’ MoS2. Our work manifests how the confluence of the optical, electronic and chemical properties of 2D MoS2 monolayers can be fully captured for efficient photocatalytic water reduction.

Keywords: MoS2, multiphases, photocatalytic, hydrogen evolution reaction, heterojunctions.

Introduction Recently, MoS2 has attracted a wealth of research interest as a promising electrocatalyst for hydrogen evolution reaction (HER)1-4 due to the favorable Gibbs free energy of hydrogen adsorption on the edge of MoS2.5 The active sites for HER were generally found at the edges of the MoS2 particles6 and thus efforts have been devoted to maximize the density of active edge sites.7 Lately, 2D single-layer MoS2 nanosheets have shown improved HER activity over traditional MoS2 nanoparticles, thanks to the possibility of phase tuning of MoS2 nanosheets. In addition to the common semiconducting 2H phase, chemically exfoliated MoS2 via lithium intercalation also exists in a mixture of meta-stable metallic 1T and quasi-metallic 1T’ phases (structure models shown in Figure S1 in supporting information).8-13 Note that here we denote 1T’ phase as quasi-metallic, since it possesses only a tiny band gap compared to the semiconducting 2H phase and it shares similar properties to metallic 1T phase.14-15 The well balanced hydrogen binding Gibbs free energy on the basal sites of the 1T’ MoS2 and its higher electric conductivity enables 2D MoS2 as excellent HER electrocatalysts, with potential to replace Pt catalysts.11,

16-19

Recently, the multiphasic 2D MoS2 nanosheets were also 2


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demonstrated as efficient co-catalysts for solar hydrogen production by using dye molecules or semiconductors as the photosensitizers.15, 20-22 Notably, to date, 2D MoS2 nanosheets have not been demonstrated as direct photocatalysts for HER from water reduction. Herein, we take advantages of the merits of the different phases of 2D MoS2 and show that multiphasic single layer MoS2 nanosheets can be constructed as efficient photocatalysts to directly carry out solar HER from water reduction. Combining various experimental characterizations with density functional theory (DFT) calculations, we reveal that an optimal HER activity can be reached through the construction of in-plane heterojunctions consisting of a balanced proportion of 2H and 1T’ MoS2. The 2H MoS2 portion, with the proper bandgap (~2 eV) and direct bandgap nature,18,

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is responsible for light absorption and charge carrier

generation, the heterojunctions between 1T’–2H MoS2 help to separate the photo-generated carriers while the 1T’ portion evolves H2 over both its basal and edge sites. This kind of phase engineering, also effective in oxide semiconductors,18,

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could be a powerful strategy for

developing efficient photocatalysts based on 2D transition metal dichalcogenides.13, 30 Experimental Synthesis Li intercalation of MoS2. Bulk MoS2 sample (ultrafine powder obtained from Graphene Supermarket) was dried in a vacuum oven at room temperature for 48 hours to remove adsorbed water molecules on the surface. Afterwards, the dried MoS2 sample was transferred to an Argonfilled glove box. Then bulk MoS2 was added to a Li superhydride solution (Sigma Aldrich) with the concentration of 10 mg/mL. The mixed solution was sealed in an air-tight round bottom flask. After that, the round-bottomed flask was discharged from the glove box and put in a 3


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sonication bath at room temperature. The mixed MoS2/Li superhydride solution was sonicated for 2h to allow thorough diffusion of the Li sources into the layers of pristine MoS2. Subsequently, the round-bottomed flask was placed into an oil bath and heated at 100ºC for 3 days. The sample was then washed with THF (Sigma Aldrich) at room temperature for four times to remove excessive Li precursor and organic residuals from the sample. Finally, the LiMoS2 sample was dried under Argon atmosphere at room temperature. Exfoliation of Li intercalated MoS2. In order to obtain the exfoliated ultrathin MoS2 nanosheets, the freshly prepared LixMoS2 sample was dispersed in D.I. water (18.2 MΩ/cm). Upon the addition of LixMoS2 to D.I. water, continuous bubbles can be observed, which is ascribed to the formation of hydrogen gas. The mixture was then subjected to an additional 1 h of sonication in an ice bath to maintain the temperature. After sonication, the mixed solution was centrifuged several times to remove the unexfoliated MoS2 while simultaneously washing out excess Li residuals from the sample. The exfoliated MoS2 was collected and stored at room temperature for further characterizations. MoS2 Adsorption onto Al2O3. The photocatalysts for solar hydrogen production were assembled by adsorbing exfoliated MoS2 nanosheets onto Al2O3 (Sigma Aldrich, surface area: 155 m2/g) support materials. 5 g of Al2O3 were added into 50 mL of exfoliated MoS2 aqueous solution. The mixed solution was stirred overnight at room temperature in a dark environment. The MoS2Al2O3 powder was obtained by centrifuging the mixed solution and subsequently drying the sample in a vacuum oven for 2 days at room temperature. The sample that was dried at room temperature is denoted as MoS2–Al2O3-R.T. Since MoS2 is the component that is accountable for photocatalytic performance in the mixed materials, it is critical to understand the loading of 4


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MoS2 on the Al2O3 in the final sample. Hence, the accurate MoS2 content on the Al2O3 supporting material was determined by ICP-MS analysis. The loading of MoS2 was found to be 0.33 wt.% in all composite materials. This sample was further divided into six batches and the remainder five parts were heated at 60ºC, 120ºC, 180ºC, 240ºC, and 300ºC under vacuum. These samples were labelled accordingly as MoS2-Al2O3-60ºC, MoS2-Al2O3-120ºC, MoS2-Al2O3-180ºC, MoS2Al2O3-240ºC, and MoS2-Al2O3-300ºC. Photocatalytic hydrogen evolution reaction In a typical photocatalytic experiment, 40 mg of the photocatalyst (MoS2-Al2O3) were dispersed in 40 mL of deionized water with 0.01 M lactic acid (Alfa Aesar) as electron donors in a quartz reactor. Prior to the photocatalytic reaction, the quartz cell was deaerated by continuous flushing with ultrahigh pure Argon gas for 30 minutes. The light input is provided by a 200-W Hg lamp equipped with a cut-off filter to allow the pass of visible light with wavelength higher than 400 nm. The suspension was stirred vigorously during the entire photocatalytic reaction and the temperature of the system was remained at 25ºC by the cooling water circulating around the entire quartz cell. The yielded hydrogen gas was determined and quantified by using gas chromatography compiled with hydrogen calibration plot, with model BUCK 910 (molecular sieve column, TCD detector, and Argon as carrier gas). Measurement of photoelectrochemical water splitting The experiment of photoelectrochemical (PEC) water splitting was carried out under Ar atmosphere by measuring the photocurrent correlated with the photocatalytic decomposition of water. To measure the photocurrent, an electrochemical workstation (BioLogic SP150) was used with a three-electrode configuration, i.e., the prepared MoS2/Nafion solution spin-coated on 5


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indium tin oxide (ITO) substrate, the Ag/AgCl electrode, and Pt as the working, reference and counter electrodes, respectively. This configuration of experimental setup is similar to the previous literature and our main goal is to investigate the photoinduced charge generation and separation efficiency in the different phase of MoS2 nanosheets.31 Since the purpose of this PEC investigation is not the HER, we choose Pt as the best cathode for HER so that the system is not limited by the reaction but rather more related to the efficiency of the charge generation and separation from the multiphasic MoS2 samples. The 200-W Hg lamp light source with a light filter (Long-wave pass cut at 400 nm) was used to provide the visible illumination on samples. The buffer solution uses 0.1 M KH2PO4 aqueous solution with pH = 7.0. No other sacrificial reagent was added to the PEC studies. All the measurements were carried out under ambient conditions.

Characterizations Powder X-ray diffraction (XRD) patterns of the freshly lithiated LiMoS2 and Al2O3 supported MoS2 were recorded from PANalytical X’Pert Pro Powder Diffractometer equipped with a Nifiltered Cu Kα radiation. Scanning electron microscopy (SEM) images of freshly lithiated LiMoS2 and exfoliated MoS2 nanosheets deposited on Si wafer were collected with a Zeiss Merlin SEM at 3kV. The transmission electron microscopy (TEM) images of exfoliated MoS2 nanosheets deposited on lacy carbon grid was performed on an aberration-corrected FEI Titan S 80-300 TEM/STEM operated at 300 kV with a Gatan charge coupled device (CCD) camera. 6


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Raman and photoluminescence spectra of exfoliated MoS2 aqueous sample and Al2O3 supported MoS2 nanosheets were performed on Acton Trivista 555 spectrometer (Princeton Instruments) with laser excitation at 532 nm. Diffuse reflectance spectroscopy (DRS) analysis over the MoS2-Al2O3 samples was carried out by Cary 5000 UV-visible spectrophotometer equipped with a praying mantis kit. Atomic force microscope (AFM) topography images of exfoliated MoS2 deposited on a mica chip were collected on a MFP-3D AFM (Asylum Research, Oxford Instrument Company, USA). Pt-Ir coated Nanosensors™ tips were used (PPP-EFM-50, k = 0.5-9.5 Nm-1). Conductive atomic force microscopy (AFM) measurements have been performed for exfoliated MoS2 nanosheets deposited on highly doped Si wafer using Asylum Research Cypher Scanning probe microscope (SPM) with Budget Sensor Multi-75E tips with Pt coating. These investigations allowed nanoscale mapping of the MoS2 nanosheets topography. For conductivity measurements electric biased was applied to the SPM tip and current was measured from conductive Si substrate through the ballast resistor of 50 MΩ. Conductivity measurements were performed simultaneously with scanning in contact AFM mode. X-ray photoelectron spectroscopy (XPS) measurements were completed on each powder sample (MoS2-Al2O3) with a Thermo Scientific K-Alpha spectrometer. All spectra were collected using an Al Kα microfused monochromatized source (1486.6 eV) with a step size of 0.1 eV over 50 scans. For all spectra, the spot size was 400 μm and the operating pressure was under 3.0 × 10-7 mbar.

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Femtosecond transient absorption measurements of the MoS2 nanosheets deposited on glass slide were carried out on a home-built pump-probe spectrometer (PPS). A full description of the PPS can be found elsewhere.32 Briefly, the PPS is based on a titanium sapphire (Ti:Sa) oscillator (Micra, Coherent) with its output seeded a Ti:Sa Coherent Legend amplifier (USP-HE) operating at 1 kHz repetition rate. The Legend amplifier provides pulses centered at 800 nm, with ~45 fs duration and 2.2 mJ energy per pulse. The output of the Legend amplifier was divided onto two portions: 90% was attenuated to 0.5 mJ and focused on a BBO crystal to generate 400 nm pump pulse. The second portion (10%) was used to generate the white light continuum (WLC) probe in a 2 mm thick sapphire window. The WLC, which covers the spectral range from 450 nm to 900 nm, was collimated after generation and focused onto the sample using high reflective parabolic mirrors to minimize temporal chirp. After that, the transmitted probe was focused onto a 100 micron core fiber coupled with a spectrometer linear CCD array (USB2000ES, Ocean Optics). The pump passes through a controllable stage-delay and was chopped at 500 Hz frequency to allow measurement of absorbance change in the transmitted probe between each two successive laser shots. At the sample, the pump and probe spot sizes were 100 and 50 micron, and the pump energy was ~ 4 μJ/cm2. Results and Discussions Construction of in-plane heterojunctions in monolayer MoS2 nanosheets

The

physico-chemical properties of the chemically exfoliated MoS2 nanosheets via lithium intercalation process were characterized by powder XRD, SEM, TEM, Raman spectroscopy, photoluminescence and AFM (Figures S2 through S7). All these characterization results indicate the successful synthesis of single layer MoS2 nanosheets. The as-synthesized 2D MoS2 8


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nanosheets are a mixture of 1T+1T’ (denoted as 1T-like for simplicity) and 2H phases.12, 15 It is noted that although the term of 2H is historically named for bulk MoS2, it is commonly adapted to refer the hexagonal phase of monolayer MoS2,14-15, 25, 33 while the term of 1H has been also used for monolayer MoS2.34 In this work, 2H will be used for consistency. The formation of the 1T-like phase is due to the lithium intercalation process and they can be converted to the thermodynamically more stable 2H phase via annealing.9, 12, 18 To tune the proportion of different phases of MoS2 via thermal annealing, the 2D MoS2 nanosheets (0.33 wt.%) were dispersed on high surface area Al2O3 (155 m2/g) to prevent restacking of the exfoliated MoS2 monolayers, designated as MoS2-Al2O3-x (x is the vacuum annealing temperature in ºC). XRD patterns (Figure S8) of the MoS2/Al2O3 materials treated at different temperatures exhibit peaks solely due to Al2O3, indicating no aggregation of the MoS2 nanosheets even after annealing at 300ºC. Raman spectra of these samples are shown in Figure 1a. The phase transition from 1T-like to 2H can be visualized from the weakening of the peak at 330 cm-1 due to 1T-like phase25 and the strengthening of those at 385 (E2g) and 405 cm-1 (A1g) due to 2H MoS2. It is interesting that although the transition from 1T to 1T’ MoS2 should happen during the heating process, there are no observable Raman bands that can be ascribed to 1T’ MoS2, possibly due to very weak Raman scattering of the 1T’ phase or its overlapping with the Raman signals from the 1T phase. The constant peak positions of the E2g and A1g modes of MoS2 throughout the annealing process further indicate the single-layer nature of the MoS2 nanosheets and no appreciable restacking of the sheets. The band gap of these annealed samples is estimated to be ~ 2.0 eV from the diffuse reflectance spectroscopy measurement (Figure S9), consistent with the reported value for a single layer MoS2.25 The exact amount of 1T-like and 2H components in the MoS2/Al2O3 samples can be quantified via X-ray photoelectron spectroscopy (XPS). Figure 1b depicts the Mo 9


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3d region from XPS spectra of the samples heated at various temperatures. The two main peaks at 232.9 and 229.2 eV can be assigned to Mo 3d3/2 and Mo 3d5/2 in 2H MoS2, in line with the Mo 3d peaks from the bulk MoS2 sample.7 Two extra peaks at 232.0 and 228.3 eV can be separated from the Mo 3d regions after a deconvolution of the peaks, due to the presence of 1T-like MoS2.25 The S 2p region of the XPS spectra (Figure 1c) also displays the similar phenomenon. The XPS spectra in both Mo 3d and S 2p regions show the evolution trend for the quantity of 2H and 1T-like MoS2 due to the thermal annealing. This trend is depicted in Figure 1d. Clearly, a continuous tuning of 1T-like to 2H MoS2 ratio can be realized by controlling the annealing temperature. Because the 2D MoS2 nanosheets are single-layer in nature, the presence of different phases implies the existence of in-plane heterojunctions. Taking advantage of the drastic differences in the electric conductivity of 2H and 1T-like phases of MoS2,11 a AFM conductivity measurement was used to map the geographical distribution of 2H and 1T-like phases in a single flake MoS2. Figure 1e depicts the AFM images of MoS2 samples annealed at various temperatures. In the as-prepared sample, it is interesting that the conductive 1T-like phase of MoS2 (blue areas) occupies the basal plane while the lessconductive 2H phase is formed predominately at the edge portion of the flake. Obviously a lot of interfaces are formed between the different MoS2 phases. Under mild heating (60ºC), no appreciable change in the ratio of the relative areas is seen in the mapping, consistent with the marginal change of the 1T-like/2H ratio from XPS analysis in Figure 1d. Further increase of annealing temperature initiates the shrinkage of conductive area towards the core of the flake and eventually the whole flake turns into less-conductive 2H phase after heated at 240ºC.

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Figure 1. a) Raman spectra of MoS2/Al2O3 samples annealed at various temperatures; b) Corresponding XPS spectra of Mo 3d, and c) S 2p core level regions; d) Proportion of 2H and 1T-like MoS2 in the annealed samples; e) AFM topography (top) and conductivity mapping (bottom) of a single layer MoS2 flake deposited on doped silicon annealed at different temperatures. The more negative (blue-coded areas) the voltage, the more conductive the phase (1T-like phase) is. Notice that the silicon substrate is highly conductive and thus acts as a blue background.

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Photocatalytic HER over 2D MoS2 Figure 2a shows the visible light photocatalytic hydrogen evolution rate of MoS2/Al2O3 samples annealed at different temperatures and the detailed values of the hydrogen evolution rate along with their apparent quantum yield (AQY) are listed in Table S1. Interestingly, a volcano relationship is observed between the annealing temperature and the HER rate. The MoS2/Al2O3 sample heated at 60ºC shows the best solar hydrogen generation rate (1.5 mmol/h/gMoS2), comparable to the activity of MoS2 as co-catalyst coupled with other semiconductor photosensitizers (~1 – 5 mmol/h/g).13 This volcano trend is drastically different from the monotonic trend observed from electrochemical HER over monolayer MoS2 where the activity tracks exclusively the quantity of 1T-like MoS2.13, 17 This clear contrast lies in the fact that the use of multiphasic single layer MoS2 as direct photocatalyst requires a balance between the 2H phase for photo-excited carrier generation and the 1T-like phase for H2 evolution. However, the optimum photocatalytic HER activity for MoS2-Al2O3– 60ºC seemingly contradicts with the fact that the ratio of 1T-like/2H varies marginally for samples annealed at R.T., 60ºC and 120ºC. This observation indicates that the ratio of 1T’/1T changes in the 1T-like region and 1T phase is mostly transformed into 1T’ phase upon the mild heating at 60ºC. This is supported by recent predictions18, 35 that 1T’ phase is more stable than 1T phase, and is an intermediate during the conversion of 1T to 2H MoS2, also supported by recent STEM observations12, 15, 33 on multiphasic MoS2 nanosheets. Although Raman, XPS and AFM conductivity mapping could not distinguish the 1T’ from 1T MoS2, recent experimental and theoretical works15,

18

showed distinct differences

between them specifically for HER. The basal plane of 1T phase is HER inactive while that of 1T’ phase is active for HER, as also corroborated by our DFT calculations in the following section. Since the total number of edge sites (active for HER) in the MoS2 nanoflakes is fairly 12


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constant during the annealing process, activation of the basal plane of the 1T’ phase for HER would greatly enhance the overall HER activity and thus accounts for the optimum activity for 60ºC annealed sample. The declining HER activity of the MoS2/Al2O3 samples at increasing annealing temperature is attributed to the decrease of the proportion of 1T’ MoS2, and thus decrease in the number of active sites for HER. The level off of the HER activity for samples annealed above 180 ºC is due to the dominance of 2H MoS2.

Figure 2. a) Visible light photocatalytic hydrogen evolution rate of all MoS2/Al2O3 samples from 0.1 M lactic acid aqueous solution; b) Recycling study over MoS2-Al2O3-60ºC sample from 0.1 M lactic acid aqueous solution. Three reaction cycles were tested over the MoS2-Al2O3–60ºC sample and each cycle lasted for 8 hours. Figure 2b shows that the photocatalytic activity of the MoS2-Al2O3–60ºC sample remains highly reproducible. This indicates that the heterojunctions built within the 2D MoS2 nanosheets are robust and stable on Al2O3 under our experimental conditions. The stability of the 1T’ MoS2 phase is consistent with the predicted15 considerable barrier (>0.7 eV) to bar its 13


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transformation into the most stable 2H phase at 60ºC annealing temperature. It was shown recently that covalent functionalization of the 1T-like phase MoS2 can greatly enhance its stability and thus may provide a route for further developement of robust multiphasic MoS2 photocatalysts.36-37 We devoted enormous effort to identify the presence of 1T’ phase of MoS2 in the nanosheets. However, it turned out that to visualize the 1T’ MoS2 at the scale of atomic level by using HR-TEM is an exceedingly challenging task for the chemically exfoliated MoS2 nanosheets with lateral size in the range 100 – 400 nm, except a limited number of prior work that has succeeded in observing the structure of 1T’ by HR-TEM12, 14-15 on MoS2 nanosheets synthesized with the similar exfoliation method. Nonetheless, our photocatalysis activity test (Figure 2), the photoelectrochemical study and the time-resolved spectroscopy results (Figure 3) coupled with the DFT modeling (Figure 4) all suggest the formation of 1T’ phase MoS2 during the mild annealing process (60 ºC), which leads to the maximum photocatalytic HER rate.

Role of the in-plane heterojunctions in 2D MoS2 Photoanodes composed by ITO substrate spin-coated with MoS2 nanosheets were prepared and annealed at various temperatures for photoelectrochemical (PEC) HER (Pt as the cathode) study. Linear sweep voltammograms of these samples were presented in Figure S10. The correlative ON/OFF switching photocurrent response plots of these samples are shown in Figure 3a. The photocurrent of the as-synthesized MoS2 is much lower than the sample annealed at 60ºC, suggesting that 1T’-2H heterojunctions in the 60ºC annealed sample are more efficient in facilitating charge separation than the 1T-2H ones present in the R.T. sample, likely due to the larger conduction band offset and stronger interfacial 14


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electrostatic field between 1T’ and 2H phase MoS2 (see Figure S11). The MoS2 sample prepared at 120ºC displays slightly lower PEC performance than the sample annealed at 60ºC, in contrast to the large difference in their photocatalytic HER activity (Figure 2a). This is because that although 1T’ portion is decreasing (thus decreased HER activity), more charge carriers can be generated from the sample annealed at 120ºC due to the formation of higher percentage of semiconducting 2H phase MoS2. Annealing of the sample at 240ºC further decreases the current, due to the annihilation of the 1T’-2H interfaces. Overall, the PEC result suggests that the presence of 1T’-2H heterojunctions serves as a vital factor for efficient charge carriers separation in 2D MoS2 nanosheets. Time-resolved spectroscopy measurements were carried out to track the destiny of the photoinduced electrons in 2D MoS2 nanosheets deposited on a glass substrate. Figure 3b plots the time-delay curves of MoS2 samples annealed at R.T., 60ºC, 120ºC, and 300ºC. The corresponding values of the dynamic parameters are listed in Table S2. These multi-exponential delay curves can be assigned to three distinct time constant (τ) values reflecting three steps during the exciton relaxation process in ultrathin MoS2.3, 38-39 The fast component (τ1), typically in time spam of 2-4 ps, can be ascribed to the exciton captured by the surface trap states of single layer MoS2 nanosheets. In the next step (τ2), electrons have the tendency to migrate to the 1Tlike phase of MoS2 which serves as electron acceptors. The variation of the content of 1T/1T’ phase MoS2 in each sample causes the disparity on τ2. The shortest τ2 can be perceived in the sample annealed at 60ºC (τ2=8.2 ps), indicating the most efficient electron transfer from the trap states to the 1T’ phase charge acceptor, in good agreement with the photocatalysis and PEC results. The final portion ultrafast dynamics (τ3) can be assigned to the interband recombination of photogenerated electron-hole pairs. Typically, this is the slowest part in the decay process that 15


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can last from hundreds to even thousands picoseconds. Hence, both the PEC and electron lifetime studies make a comprehensive case that the presence and balance of both 1T’ and semiconducting 2H phase MoS2 and the in-plane heterojunctions are critical for the desirable photo charge generation, migration and separation.

Figure 3. a) Normalized amperometric l-t cycles with potential at 0.8 V (vs. Ag/AgCl) with MoS2/ITO photoanodes treated at different temperatures; b) Transient absorption dynamics with 400 nm pump and a probe wavelength of 670 nm for MoS2 dispersed on glass slide prepared at different temperatures. To understand the role of the different phases of MoS2 in catalytic HER, we first calculated the Gibbs free energy of H adsorption (ΔG ) on isolated 2H, 1T and 1T’ phases (Figure S1). It is an indicator of HER activity, and the optimal value should be |Δ

| ≈ 0 so that

the hydrogen does not bind with the surface too weakly or too strongly.5 According to our calculations and prior works,15, 18 the basal planes of 2H and 1T phases are inactive as the Δ 16


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deviates from zero significantly. In contrast, for 1T’ phase, there are two types of S atoms due to the structural distortion, denoted as “S-low” and “S-high” based on the relative height. The “Slow” atoms are HER active as they satisfy the optimal condition |Δ

| ≈ 0, and hence the basal

plane of 1T’ phase, at least half of the surface corresponding to the “S-low” atoms, is active (more details in Figure S1). We then studied the ΔG on the different sites of heterostructured MoS2 planes. Figure 4a shows a theoretical model of monolayer MoS2 plane consisting of 2H and 1T’ phases. Similar model was also built for 1T-2H heterojunction (Figure S12). The ΔG was calculated for H adsorption on sites from 2H edge, 2H interior, 1T’-2H interface, 1T’ interior to 1T’ edge (from sites 1 to 20 as marked in Figure 4a). As summarized in Figure 4b, except the “S-low” atoms in the 1T’ basal plane (atoms numbered 15-18) and edge atoms (atoms numbered 1 and 20), other sites are not suitable for HER as their ΔG values are too far from zero, consistent with the results on isolated phases. Clearly, the “S-low” sites on the basal plane of 1T’ MoS2 can make it a favorable choice for HER as the density of the basal sites is much higher than the edge sites in a MoS2 flake. In contrast, in the 1T-2H heterojunction, only the edge sites are active for HER. Furthermore, the 1T’ phase is thermodynamically more stable than the 1T phase.12, 15, 18 Hence, this explains the optimum photocatalytic HER from the MoS2–Al2O3-60ºC sample in which 1T’ is the dominant phase for hydrogen production.

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a

b

2.0

2H 1T ' 1T

1.6

ΔGH (eV)

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1.2 0.8 0.4 0.0

1T ' basal + edge

2H edge 0

2

4

6

8

10

12

14

16

18

20

Position of S atom

Figure 4. a) Structure for a 1T’-2H heterojunction. Calculated (DFT) free energy of H adsorption (ΔG ) on different S atoms is shown at the bottom. The red numbers indicate favorable sites for HER. b) Comparison of calculated Δ

as a function of S atom positions in 1T’-2H and 1T-2H

heterojunctions. S atoms below the blue dash line (~0.3 eV) are HER active.

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A more indicative schematic that imitates the photogenerated charge carriers transportation at the heterojunctions in the multi-phasic MoS2–Al2O3-60ºC sample is depicted in Figure 5. This scheme illustartes the paths of photogenerated electron flowing from 2H phase MoS2 into the 1T’ MoS2 in the lactic acid aqeuous condition. Due to the presence of metalsemiconductor and solution interfaces, the Fermi level (Ef) of both MoS2 species are corresponded and normalized to the oxidation potoential of lactice acid in this plot.40,41 Briefly, 1T and 2H phase MoS2 alone hold similar values of work function (Φ) of ~ 4.2 eV42 while 1T’ MoS2 possesses higher work function than its 2H counterpart (Φ1T’-MoS2 > Φ2H-MoS2). Moreover, our theoretical work (as shown in Figure S11) also indicates, althrough only qualitatively, that the Fermi level of 1T’ MoS2 falls at more negative potential level compared to that of 2H and 1T MoS2.

Therefore, an upward band bending Schottky barrier will be formed at the n-type

seminconducting 2H and the quasi-metallic 1T’ MoS2 interfaces. In addition, the electron affinity of 2H MoS2 (χ2H-MoS ≈ 4.0 eV) is less but close to its work function.43-45 The height of the Schottky barrier at the 1T’/2H MoS2 interfaces can be determined by the equation: Φb = Φ1T’-MoS2 - χ2H-MoS2. With the fact of higher work function of 1T’ MoS2 than that of 1T MoS2, 1T’/2H heterojunction in MoS2 generates a higher Schottky barrier than does the 1T/2H MoS2 interface. This rationalizes the better solar hydrogen yield in the 60ºC annealed sample.44, 46

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Figure 5. Schematic of the formation of heterojunctions at the 2H and 1T’ phases MoS2 interfaces. The denotations in the scheme are described as: Φ1T’-MoS2, work function of 1T’ phase MoS2; Φ2H-MoS2, work function of 2H phase MoS2; χ2H-MoS2, electron affinity of 2H phase MoS2; Ef, Fermi level of the 2H and 1T’ phases MoS2 in contact; Evac, vacuum energy; Ec, energy of conduction band minimum of 2H phase MoS2; Ev, energy of valence band maximum of 2H phase MoS2; Φb, height of the Schottky barrier.

Conclusions We have demonstrated that single-layer multiphasic MoS2 nanosheets can be directly employed as efficient photocatalysts for solar hydrogen production. The presence of in-plane 20


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heterojunctions among the different MoS2 phases is a key to the photocatalytic hydrogen evolution rate. Therefore, an optimal 2D MoS2 photocatalyst can be constructed with wellbalanced composition of 2H and 1T’ phase MoS2 for water reduction to produce H2, owing to a confluence of the optical (2H phase), electronic (1T’-2H heterojunction) and chemical properties (1T’ phase) of the 2D MoS2 monolayers. Our finding represents a potentially general strategy for designing 2D transition metal dichalcogenides as efficient photocatalysts for solar-to-fuel conversion.

ASSOCIATED CONTENT Supporting Information Sample preparations, more experimental details, additional characterizations and calculations details are available in the supporting information. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author [email protected]. Notes The authors declare no competing financial interests. Notice: This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do

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so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doepublic-access-plan).

ACKNOWLEDGMENTS This research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. LL was supported by Eugene P. Wigner Fellowship at ORNL. ZDH acknowledges support from the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1148903 and the Georgia Tech-ORNL Fellowship.

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MoS2 Nanosheets As Efficient Photocatalysts for ... - ACS Publications

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