DOI: 10.1002/cssc.201500067
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Water-Soluble MoS3 Nanoparticles for Photocatalytic H2 Evolution Wei Zhang,[a, c] Tianhua Zhou,[a, b] Jianwei Zheng,[d] Jindui Hong,[a] Yunxiang Pan,*[a] and Rong Xu*[a, b, e] Polyvinylpyrrolidone (PVP)-modified MoS3 nanoparticles with unusual water solubility up to 1.0 mg mL 1 were synthesized through a facile hydrothermal method in the presence of thioacetic acid. The amorphous nanoparticles wrapped by PVP have sizes of around 2.5 nm, which represent the smallest MoS3 clusters reported. The photocatalytic performance of the MoS3 nanoparticles was evaluated under visible light for H2 evolution using xanthene dyes as photosensitizers. The quantum efficiency of the optimized system for H2 evolution under
green light irradiation (520 nm) is up to 36.2 %, which is comparable with those of other excellent photocatalytic systems involving earth-abundant catalysts. The excellent photocatalytic activity can be attributed to its good dispersion in water, amorphous nature and limited layers with abundant surface active sites, and possibly higher conduction band potential for proton reduction and larger indirect band gap for a longer lifetime of the excited electrons.
Introduction Development of active, stable, and low-cost catalysts for efficient photocatalytic H2 evolution has received increasing research interest.[1] As inspired by nature, homogeneous molecular systems employing photosensitizers utilizing green light have been widely investigated.[2] Among them, both noble metal[2a, b] and earth abundant elements such as Ni, Co, and Fe[2c–e, 3] have been involved in the photosensitizers or the catalytic centers. In many such systems, the use of organic solvents is necessary to achieve a good interaction between photosensitizer and catalyst in order to facilitate the charge transfer.[2c, d, 3] Methods for synthesis of water-soluble inorganic nanoparticles have been well established for II-VI chalcogenides, such as CdS, CdSe, CdTe quantum dots.[4] These quantum dots in their dispersed form have been employed as low-
cost and stable photosensitizers for photocatalytic H2 evolution despite the toxic nature of cadmium.[5] On the other hand, it is highly desirable to design active inorganic catalyst nanoparticles that are dispersible in aqueous solutions. To date, limited research work has been done in this aspect. Molybdenum sulfides have been experimentally and theoretically shown as potential alternatives to Pt as a hydrogen evolution catalyst in both electrocatalytic and photocatalytic water splitting systems.[6] Bulk MoS2, which naturally occurs as molybdenite, has a layered hexagonally packed structure consisting of S-Mo-S sheets held together in stacks by van der Waals interactions. The edges of the S-Mo-S sites are the active H2 evolution sites, while the basal plane is inactive.[6b, d] As hydrogen evolution catalysts, MoSx units are inspired from the nitrogenase enzymes for ammonia fixation.[7] The enzyme molecule is usually large and the number of Mo-S active centers inside is limited. It is expected that the optimal form of MoSx catalyst should be a molecular cluster with high density of Mo-S active centers. In this regard, the Mo3S44 + incomplete cubane species could be an ideal homogeneous H2 evolution catalyst. Although the cubane has been demonstrated to be active for H2 evolution, the problem lies in its instability during photocatalytic reactions as Mo4 + can be reduced irreversibly.[6b, 8] It was estimated that the intrinsic activity of Pt is around 102– 105 times greater than that of MoS2 edge.[9] As a result, recent studies on MoS2 in photocatalytic H2 evolution have been aimed at synthesizing MoS2 of both limited layers and lateral sizes to enhance the exposure of the edge sites.[6d, 10] For instance, MoS2 of a few layers prepared from thermal decomposition of (NH4)2MoS4 is reported to be efficient co-catalyst on CdS surface for photocatalytic H2 evolution from lactic acid solution.[11] Colloidal MoS2 by PVP-assisted synthesis is able to catalyze the H2 evolution reaction in the presence of Ru-and Ir-
[a] W. Zhang, T. Zhou, J. Hong, Y. Pan, Prof. R. Xu School of Chemical & Biomedical Engineering Nanyang Technological University 62 Nanyang Drive, 637459 (Singapore); E-mail: [email protected] [email protected] [b] T. Zhou, Prof. R. Xu SinBeRISE CREATE, National Research Foundation CREATE Tower, 1 Create Way, 138602 (Singapore) [c] W. Zhang School of Chemistry & Chemical Engineering Shaanxi Normal University Xi’an 710119 (China) [d] J. Zheng Institute of High Performance Computing 1 Fusionopolis Way, #16-16 Connexis North, 138632 (Singapore) [e] Prof. R. Xu C4T CREATE, National Research Foundation CREATE Tower, 1 Create Way, 138602 (Singapore) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201500067.
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Full Papers complex photosensitizers.[12] Exfoliated MoS2 nanosheets can Table 1. Weight percentages of the elements in MoS3 samples prepared enhance the activity for H2 evolution when linked to CdSe with or without PVP. quantum dots as reported by Osterloh’s group.[13] MoS2 nanoSamples N [%] C [%] S [%] H [%] Mo [%][a] S:Mo[b] particles are loaded on reduced graphene oxide for promoted charge transfer towards H2 formation sensitized by TiO2[14] or PVP-free MoS3 0.88 0.15 49.68 1.32 47.97 3.1 PVP-MoS3 5.04 23.06 30.30 4.11 31.72 2.9 Eosin Y.[10] Amorphous MoS2 formed through in situ photoreduction of (NH4)2MoS4 in dye-triethanolamine solution is re[a] Calculated from the balance after deducting the percentages of other elements by CHNS analysis. The percentage of oxygen in PVP-modified ported to be effective for H2 production by Wang and co-work[15] sample was estimated based on that of nitrogen. [b] Atomic ratio. ers. As an analogue to amorphous MoS2, MoS3 displays the same oxidation state for molybdenum but a different oxidation state atomic ratio of both samples is close to 3:1, which suggests for sulfur ligands. Both bridging S-S2 and S2 anions exist in the formation of MoS3. MoS3 at a theoretical ratio of 1:1.[16] MoS3 has been studied as Figure 1 A shows the XRD pattern of the as-synthesized amorphous MoS3 nanoparticles. After fitting the broad peak by an electrode material for electrocatalytic H2 evolution.[6e–g, 17] MoS3 is usually synthesized in an amorphous form through the Gaussian curve, the peak position at 13.098 is found for PVPacidification process of MoS42 anion. The amorphous nature usually assures an unsaturated environment of Mo4 + cation. There is no detailed structural information for MoS3 as its crystalline form has not been successfully obtained yet. Amorphous MoS3 has been also applied as a co-catalyst loaded on CdS/ CdSe nanorods for photocatalytic H2 evolution.[6c] Other than this, the synthesis and application of MoS3 for photocatalytic Figure 1. (A) XRD patterns of (a) PVP-free MoS3, (b) PVP-MoS3 (prepared with 0.2 g of PVP) and (c) bulk MoS2 ; (B) FTIR spectra of (a) PVP-free MoS3, (b) PVP-MoS3 prepared with 0.1 g of PVP, (c) PVP-MoS3 and (d) PVP. Inset of H2 evolution in literature is limit- A: pictures of PVP-MoS dispersed in water and ethanol. 3 ed. The use of MoS3 as a catalyst for photocatalytic H2 evolution MoS3 which corresponds to a lattice spacing of 0.68 nm. A simand the effect of the materials properties on catalytic activities is still a largely unexplored field. ilar diffraction pattern was reported for amorphous MoS3 in Herein, we report the facile synthesis of PVP-modified MoS3 a previous report with a weak and broad diffraction peak nanoparticles with unusual water solubility of up to around 148.[18] The spacing is slightly larger than that of the 1 1.0 mg mL . The as-synthesized amorphous nanoparticles PVP-free MoS3 sample at 0.62 nm. The difference is probably wrapped by PVP have particle sizes of approximately 2.5 nm, due to the broadened asymmetric XRD peak in PVP-MoS3 nanoparticles caused by the quantum size effect. As the peak which represent the smallest MoS3 or MoSx clusters reported in literature. Under visible light, the MoS3 nanoparticles dispersed is quite broad, the estimated grain size from the Scherrer equation of below 2 nm for PVP-MoS3 is not so accurate. Nevin water are demonstrated to be an efficient catalyst for H2 evolution using the low-cost xanthene dyes as photosensitizertheless, this indicates that there are limited layers in the sizeconfined MoS3 nanoparticles, since the Mo–S and Mo–Mo disers. The quantum efficiency of the optimized system for H2 tances are reported to be 2.44 and 2.78 in bulk MoS3 materievolution under green light irradiation (520 nm) is up to 36.2 %, which is comparable with those of other excellent phoal.[6c] tocatalytic systems involving earth-abundant catalysts. Fourier transform infrared spectroscopy (FTIR) spectra of MoS3 samples and PVP are shown in Figure 1 B. The absorption peaks originated from the chemical groups of PVP including C N, C=O and C-H can be clearly observed in PVP-MoS3 samResults and Discussion ples prepared with different amounts of PVP.[19] PVP as a water soluble capping reagent can wrap the surface of the nanoparWater soluble MoS3 was prepared by the hydrothermal ticles and make them easily dispersible in polar solvents withmethod using (NH4)2MoS4 as the precursor and thioacetic acid out any post-synthesis treatment.[20] As shown in pictures of (TAA) as the acidification agent in the presence of PVP. Elemental analysis results indicate that the weight percentage of PVP Figure 1 A inset, the as-synthesized PVP-MoS3 nanoparticles can is around 37 % in molybdenum sulfide nanoparticles synthebe well dispersed in water and ethanol, forming brown-colored sized (Table 1). There are negligible amounts of carbon and nistable and transparent solutions with solubility up to about trogen in the sample synthesized without PVP. The Mo/S 1 mg mL 1.
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Full Papers vis diffuse reflectance spectroscopy (UV–vis DRS) of the solid are shown in Figure S2. Both spectra show the absorbance of visible light at wavelength up to 800 nm which is consistent with its brown color. The oxidation states of Mo and S were investigated by X-ray photoelectron spectroscopy (XPS) analysis and the results are shown in Figure 4. The Mo3d5/2 and Mo3d3/2 doublet binding energies in the spectrum of PVP-free MoS3 (Figure 4 A) are located at 229.0 and 232.2 eV, respectively, which can be assigned to the binding energies of Mo4 + in MoS3.[6e,g] The XPS result indicates that the reduction of Mo6 + to Mo4 + is effective by the introduction of protons from TAA during the hydrothermal acidification synthesis.[16] Binding energies of Mo3d5/2 and Mo3d3/2 in the spectrum of PVP-MoS3 (Figure 4 B) are slightly shifted to lower positions at 228.5 and 231.6 eV, respectively. The estimated S/Mo ratio on the surface of the samples based on XPS results is 4.0 and 3.5 for PVP-free MoS3 and PVP-MoS3, respectively, which could be attributed to the presence of surface elemental S.[17] The higher elemental S in the PVP-free MoS3 is consistent with the S 2 s peak areas for S0 as shown in blue line in Figure 5. In both samples, the broad peak of S2p signals is characteristic to amorphous MoS3.[6e,g] The spectra can be fitted by three groups of doublets. Besides the elemental S with a S2p doublets at higher binding energies (blue line in Figure 5), there are two different kinds of sulfur ligands (S22 and S2 ) in MoS3 at the ratio of 1:1. However, it is difficult to quantify the exact ratio of the two sulfur ligands by XPS signals as their binding energies are quite similar. The S2p signal doublet with higher binding energies can be attributed to the bridging S22 and/or apical S2 ligands in MoS3, while the lower binding energy doublet is due to the terminal S22 and/or terminal S2 ligands.[6e, 16] The relative intensity ratio of the doublets between the higher and lower binding energies is 2.1 in PVP-free MoS3, and this ratio is decreased to 1.8 for PVP-MoS3. This can be well explained by the higher ratio of terminal sulfur ligands in PVP-MoS3 as a result of its much smaller particle sizes. The photocatalytic performance of H2 evolution over PVPMoS3 was studied in aqueous solutions containing triethanolamine (TEOA) as an electron donor and erythrosine B (ErB) as
Figure 2. TEM and HRTEM images of (A–C) PVP-MoS3 at different magnifications and (D) PVP-free MoS3. Insert of B shows the particle size distribution histogram.
The TEM images of PVP-MoS3 shown in Figure 2 A and 2B indicate the formation of rather uniform and well dispersed nanoparticles of approximately 2.5 nm in size. The histogram inset in Figure 2 B shows the particle size distribution in a narrow range of 1–3.5 nm. No clear lattice can be observed in the high resolution TEM (HRTEM) image (Figure 2 C) due to the amorphous nature of the material. On the other hand, the PVP-free MoS3 sample consists of agglomerated nanoparticles of around 20–50 nm (Figure 2 D) and presents poor water solubility. Besides TEM, atomic force microscopy (AFM) technique was also applied to investigate the morphological feature of PVP-MoS3. Figure S1 shows that the height of nanoparticles is around 5–8 nm at the same magnitude of the particle size obtained from HRTEM results, suggesting that round-shaped MoS3 nanoparticles were formed. The Raman spectra of the MoS3 and bulk MoS2 samples are displayed in Figure 3. The spectra of MoS3 are consistent with the Raman spectrum of amorphous MoS3 reported in literature.[21] By comparing with the Raman spectrum of the commercial bulk MoS2, the absence of typical E1g and A1g peaks from the spectra of MoS3 samples further excludes the possibility of the formation of crystalline MoS2 in our samples.[22] The UV-vis absorption spectra of PVP-MoS3 Figure 3. Raman spectra of MoS3 samples and bulk MoS2 in wide and narrow ranges, (a) PVP-MoS3, (b) PVP-free dispersed in water and the UV– MoS3, and (c) bulk MoS2. ChemSusChem 0000, 00, 0 – 0
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Figure 4. XPS spectra of Mo 3d, S 2s and S 2p orbitals of (A) PVP-free MoS3, and (B) PVP-MoS3. Mo (green); S0 (blue); bridging S22 /apical S2 (red); terminal S22 /S2 (purple).
Figure 5. Optimization of H2 evolution amount from the photocatalytic system using PVP-MoS3 as the catalyst by varying (A) the pH of the reaction solution, (B) the amount of ErB, (C) the amount of the catalyst, and (D) the type of xanthene dye. Reaction conditions: 300 W Xenon lamp with a cut-off filter at 420 nm, 100 mL 15 v % TEOA solution, pH 8.5, 0.2 g ErB, 0.2 g PVP-MoS3 (unless otherwise stated).
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a photosensitizer. Control experiments have been conducted to show that all the above components and light are essential for H2 production. There was no H2 evolved when only PVP was used as the catalyst. The reaction conditions including the concentrations of the photosensitizer and catalyst, the pH, and the type of photosensitizer were optimized to gain highest H2 evolution amount in 24 h irradiation from our photoreactor. Figure 5 A shows that a slightly basic TEOA solution with a pH of 8.5 (adjusted by HNO3) is optimal for H2 evolution, which is similar to the results from our previous studies.[1e, f] As both higher proton concentration in the reaction solution and lower protonation level of the amine group of TEOA are favorable, a suitable pH value of the solution assures the sufficient amount of both electron acceptor (H + ) and electron donor (TEOA). Figure 5 B demonstrates that a higher concentration of the photosensitizer leads to greater amounts of H2 evolved in 24 h irradiation. However, within the first 5 h, the amount of H2 evolved is decreased with the amount of ErB increased from 200 to 300 mg. This is probably due to the self-sheltering effect of the dye molecules.[1f] With a longer irradiation time, photodecomposition of dye molecules takes place extensively[23] and thus a higher dye concentration provides sufficient photosensitizer to compensate its degradation. Figure 5 C shows the effect of catalyst concentration on H2 evolution. Below 100 mg, the amount of H2 evolved increases almost linearly with the concentration of the catalyst. Beyond that, there is no significant difference in the amount of H2 evolved as the concentration of dye should be the limiting factor under such a condition. Among the xan-
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Figure 6. Visible light driven (l > 420 nm) photocatalytic H2 evolution over MoS3 and commercial MoS2, (B) photocatalytic H2 evolution over PVP-MoS3 with long-pass cut-off filters cut on different wavelengths, and (C) QEs with ErB and PVP-MoS3 under photons with different wavelengths. Reaction conditions: 300 W Xenon lamp, 100 mL 15 v % TEOA solution, pH 8.5, 0.2 g ErB, 0.2 g catalyst.
thene dyes studied (Figure 5 D), ErB is the best photosensitizer for MoS3 due to its suitable visible-light absorption range and relatively good resistance to photodecomposition.[1e] Figure 6 A shows the photocatalytic performance of different molybdenum sulfides as H2 evolution catalysts under the optimized reaction conditions mentioned above. Compared to the bulk commercial MoS2 and PVP-free MoS3, PVP-MoS3 with smaller particle sizes and excellent water solubility exhibits much enhanced photocatalytic performance for H2 evolution. After 24 h irradiation, 10.8 mmol of H2 was evolved over PVPMoS3, which is around 45 % higher than that over PVP-free MoS3. Compared to the bulk MoS2, the amount of H2 generated over PVP-MoS3 in 24 h is 7.3 times higher. The activity of PVP-MoS3 was further evaluated under irradiation with photons of different wavelengths. As shown in Figure 6 B, the amount of H2 evolved in 48 h does not vary much when the cut-off filters at 420, 455, and 475 nm were used. Even when a 500 nm cut-off filter was used, the amount of H2 evolved is still over 10 mmol. Therefore, such a dye sensitized system employing well-dispersed MoS3 catalyst is able to utilize visible light of longer wavelengths. It is notable that the catalyst is still active with a moderate H2 evolution rate of 120 mmol h 1 even under the irradiation of green light (l > 520 nm) and can even produce H2 under irradiation longer than 550 nm. As our reaction was carried out at a relatively large scale of 100 mL, the performance of the photocatalytic system can be better measured by the quantum efficiency (QE) rather than the turnover number (TON) from the catalyst. The QEs measured in the visible light region (420–550 nm) at various wavelengths are displayed in Figure 6 C together with the UV–vis absorbance spectrum of ErB. Before the wavelength of maximum absorption of ErB at around 526 nm, the QE increases from 9.1 % to the highest value of 36.2 % from 420 to 520 nm. Such a high QE obtained at 520 nm green light is comparative with other excellent QE results for photocatalytic systems involving earth-abundant catalysts.[1e, 2a,d,e, 24] As a result, the MoS3 amorphous nanoparticles synthesized by a simple method in the present work ChemSusChem 0000, 00, 0 – 0
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may be potentially a good candidate of co-catalyst for H2 evolution reaction. Under the irradiation of photons with wavelength at 550 nm, the QE obtained is still as high as 17 %. The utilization of longer wavelength photons is essential to achieve the successful application of solar energy by water splitting.[25] Since there is no confirmed structural information about the amorphous MoS3, as an alternative, we studied the size effect on the electronic band structure of MoS2 to give us some hints on the particle size effect of MoS3. Based on our previous study on the alignment of energy states of ErB using vertical ionization potential (IP), the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels of the ErB dye are located at 5.85 and 3.42 eV versus vacuum, respectively.[1e, 26] The positions of conduction and valence band edges of MoS2 relative to vacuum were calculated following the method reported by Moses et al.[27] For bulk MoS2, the calculated positions of conduction band minimum (CBM) and valence band maximum (VBM) are 3.79 and 4.99 eV versus vacuum, respectively. On the other hand, MoS2 of a single layer has a CBM at 3.71 eV and a VBM at 5.61 eV. Herein we are able to draw the alignment of energy levels of the photosensitizer ErB and the H2 evolution catalyst MoS2 as shown in Figure 7. Bulk MoS2 has a 0.37 eV lower conduction band potential than the LUMO level of ErB and a much higher potential than the normal hydrogen reduction potential ( 4.44 eV for NHE vs vacuum). The combination of ErB and MoS2 is hereby demonstrated theoretically to be an effective system for photocatalytic H2 evolution and the photoexcited ErB dye is quenched by TEOA as discussed in our previous publication.[1e] Compared to the bulk MoS2 with an indirect band gap at 1.2 eV, MoS2 with limited layers has a direct band gap at around 1.9 eV.[28] Based on our calculation, monolayered MoS2 has a higher CBM than the bulk MoS2, which corresponds to a higher driving force for proton reduction. Further, in MoS2 monolayer, the lifetime of photogenerated electrons is increased dramatically due to strong excitonic binding energies associated with low relaxation rate arising from the broadened 5
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Full Papers Figure 9 shows the XPS results of the catalyst after reaction. In the spectrum of Mo 3d, the major doublet of Mo 3d5/2 and Mo 3d3/2 located at 229.0 and 232.1 eV is attributed to Mo4 + in MoS3. A smaller doublet with binding energies at 231.8 and 234.9 eV may be associated with molybdenum of higher oxidation states, or molybdenum coordinated with sulfate species. Consistently in the spectrum of S 2p, a small peak at 168.1 eV indicates that a small percentage of sulfur is possibly oxidized to sulfate anion[31] due to self-corrosion. Nevertheless, only about 7 % of sulfur is oxidized based on the relative peak areas. The other two doublets with the main peaks at 161.1 and 162.6 eV indicate the presence S22 and S2 ligands. The estimated ratio of S/Mo is 2.5 based on the XPS results. Compared to the value in the as-prepared PVP-MoS3, the S/Mo ratio is decreased. It has been reported that amorphous MoS3 serves as a pre-catalyst and it undergoes an activation process to form MoS2 or MoS2 + x catalysts under the electrochemical study for H2 evolution.[6e,g, 17] In the current study, a similar phenomenon may have occurred and MoS3 can be partially converted to amorphous MoS2 + x phase under the photocatalytic condition.
Figure 7. Photocatalytic H2 evolution mechanism using ErB as a photosensitizer, MoS2 monolayer as the catalyst and TEOA as the sacrificial reagent. Energy levels of bulk MoS2 are presented for comparison.
indirect band gap.[29] Likewise, amorphous MoS3 with reduced sizes of around 5 nm prepared in this work should also compare favorably with its bulk counterparts for improved H2 evolution activity from the electronic structure point of view. The water soluble PVP-MoS3 has further been shown as an efficient electrocatalyst for proton reduction to molecular hydrogen based on the cyclic voltammetric study shown in Figure 8 A. After adding trifluoroacetic acid to the solution, the current was greatly enhanced at the electrocatalytic proton reduction potential. Similar observations were reported by other groups for photocatalytic H2 evolution catalysts.[30] Meanwhile, the water soluble PVP-MoS3 is able to quench the photoluminescence of ErB efficiently as shown in Figure 8 B, indicating that transfer of photoexcited electrons from ErB to the catalyst occurs efficiently. After 48 h of light irradiation, the PVP-MoS3 catalyst Figure 8. (A)1 Cyclic voltammetry study of the PVP- MoS3 dissolved in water at 0.1 mg mL with the addition of different amount of trifluoroacetic acid, and (B) fluoreswas collected by adding 300 mL of acetone into the cence spectra of ErB-TEOA solutions (dilute 10 times from the photoreaction concentrareaction mixture and the solid sample was character- tion) excited at 490 nm with or without addition of PVP-MoS3 (inset: the weight ratio of ized by FTIR, XRD, and XPS techniques. The XRD pat- MoS3 :ErB). terns (Figure S3 A) indicate that the catalyst after reaction is still amorphous in nature and the same broad peak at around 148 is observed. It is noted that a small and broad peak appears at around 358, which could be due to the slight oxidation of Mo species. Based on the FTIR spectra of the sample before and after reaction (Figure S3 B), the fingerprints of PVP can still be found but at lower intensities in the spectrum of the catalyst after reaction. The dissociation or decomposition of PVP on the surface of PVP-MoS3 nanoparticles is expected to occur during photoreaction. Figure 9. XPS spectra of Mo 3d and S 2p for PVP-MoS3 after photoreaction under irradiation of 48 h.
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Full Papers Conclusions
Research Initiative for Sustainable Energy (SinBeRISE) and Cambridge Centre for Carbon Reduction in Chemical Technology (C4T) CREATE Programmes.
Water soluble PVP-modified MoS3 nanoparticles of approximately 2.5 nm have been successfully synthesized. The as-prepared MoS3 nanoparticles can be well dispersed in water for photocatalytic H2 evolution sensitized by the low-cost xanthene dyes in aqueous solutions. The optimized system exhibits a high QE of 36.2 % at 520 nm. The efficient catalytic activity of PVP-MoS3 nanoparticles can be attributed to their good dispersion in water for optimal interaction with the photosensitizer molecules, amorphous nature and limited layers in the catalyst particles with abundant surface active sites, and possibly a higher CBM potential for proton reduction and larger indirect band gap for a longer lifetime of the excited electrons.
Keywords: hydrogen evolution · molybdenum sulfide · photocatalyst · photosensitizer · water splitting
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Experimental Section Synthesis of water-soluble MoS3 nanoparticles In a typical synthesis of water-soluble MoS3 nanoparticles, 2.0 mmol of (NH4)2MoS4 and 0.2 g of PVP were dissolved in 75 mL of deionized water. 1.0 mL of TAA was added to the solution before it was transferred into a Teflon-lined autoclave with a capacity of 120 mL. The mixture was subjected to hydrothermal treatment at 200 8C for 12 h. After cooling to room temperature, the obtained brown color suspension was mixed with 300 mL of acetone and kept still over night for the precipitation of the solid particles. The dark brown colored solid was washed thoroughly with acetone and centrifuged several times before being dried at room temperature to obtain PVP-modified MoS3 nanoparticles with a yield of around 60 %.
Photocatalytic activity measurement Visible-light-driven H2 evolution reactions were conducted in a closed gas circulation and evacuation system fitted with a top window Pyrex cell. The photocatalytic H2 production reactions were performed in 100 mL of 15 vol % TEOA aqueous solutions with pH 8.5 (adjusted by concentrated HNO3), typically containing 0.2 g of Erythrosine B and 0.2 g of MoS3. The light source is a 300 W Xenon lamp equipped with long-pass cut-off filters (cut on 420, 455, 475, 500, and 520 nm). The reaction cell was kept at room temperature with jacketed cooling water. The H2 evolved was detected using an online gas chromatography. The apparent quantum efficiency (QE) was measured by equipping band pass interference filters (Newport, center wavelength at 420, 440, 460, 480, 500, 520, and 550 nm, band width 10 nm). The reaction solutions were irradiated under l > 420 nm for 1 h before switching to bandpass filters for QE tests and the H2 evolution measured in the subsequent 5 h was used for calculation of QE. The number of photons from irradiation was measured using a photodiode. Detailed experimental information can be referred in the Electronic Supporting Information.
Acknowledgements This work was supported by National Environment Agency, Singapore under the Environment Technology Research Programme (ETRP) through Project no.: ETRP 1002 103, and Singapore National Research Foundation (NRF) through the Singapore-Berkeley ChemSusChem 0000, 00, 0 – 0
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FULL PAPERS Green light for H2 evolution: MoS3 nanoparticles with unusually high water solubility were synthesized through a facile hydrothermal method. Their excellent quantum efficiency for H2 evolution under green light irradiation can be attributed to their good dispersion in water, amorphous nature, abundant surface active sites, higher conduction band potential for proton reduction, and larger indirect band gap for a longer lifetime of the excited electrons.
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Water-Soluble MoS3 Nanoparticles for Photocatalytic H2 Evolution Wei Zhang,[a, c] Tianhua Zhou,[a, b] Jianwei Zheng,[d] Jindui Hong,[a] Yunxiang Pan,*[a] and Rong Xu*[a, b, e] Polyvinylpyrrolidone (PVP)-modified MoS3 nanoparticles with unusual water solubility up to 1.0 mg mL 1 were synthesized through a facile hydrothermal method in the presence of thioacetic acid. The amorphous nanoparticles wrapped by PVP have sizes of around 2.5 nm, which represent the smallest MoS3 clusters reported. The photocatalytic performance of the MoS3 nanoparticles was evaluated under visible light for H2 evolution using xanthene dyes as photosensitizers. The quantum efficiency of the optimized system for H2 evolution under
green light irradiation (520 nm) is up to 36.2 %, which is comparable with those of other excellent photocatalytic systems involving earth-abundant catalysts. The excellent photocatalytic activity can be attributed to its good dispersion in water, amorphous nature and limited layers with abundant surface active sites, and possibly higher conduction band potential for proton reduction and larger indirect band gap for a longer lifetime of the excited electrons.
Introduction Development of active, stable, and low-cost catalysts for efficient photocatalytic H2 evolution has received increasing research interest.[1] As inspired by nature, homogeneous molecular systems employing photosensitizers utilizing green light have been widely investigated.[2] Among them, both noble metal[2a, b] and earth abundant elements such as Ni, Co, and Fe[2c–e, 3] have been involved in the photosensitizers or the catalytic centers. In many such systems, the use of organic solvents is necessary to achieve a good interaction between photosensitizer and catalyst in order to facilitate the charge transfer.[2c, d, 3] Methods for synthesis of water-soluble inorganic nanoparticles have been well established for II-VI chalcogenides, such as CdS, CdSe, CdTe quantum dots.[4] These quantum dots in their dispersed form have been employed as low-
cost and stable photosensitizers for photocatalytic H2 evolution despite the toxic nature of cadmium.[5] On the other hand, it is highly desirable to design active inorganic catalyst nanoparticles that are dispersible in aqueous solutions. To date, limited research work has been done in this aspect. Molybdenum sulfides have been experimentally and theoretically shown as potential alternatives to Pt as a hydrogen evolution catalyst in both electrocatalytic and photocatalytic water splitting systems.[6] Bulk MoS2, which naturally occurs as molybdenite, has a layered hexagonally packed structure consisting of S-Mo-S sheets held together in stacks by van der Waals interactions. The edges of the S-Mo-S sites are the active H2 evolution sites, while the basal plane is inactive.[6b, d] As hydrogen evolution catalysts, MoSx units are inspired from the nitrogenase enzymes for ammonia fixation.[7] The enzyme molecule is usually large and the number of Mo-S active centers inside is limited. It is expected that the optimal form of MoSx catalyst should be a molecular cluster with high density of Mo-S active centers. In this regard, the Mo3S44 + incomplete cubane species could be an ideal homogeneous H2 evolution catalyst. Although the cubane has been demonstrated to be active for H2 evolution, the problem lies in its instability during photocatalytic reactions as Mo4 + can be reduced irreversibly.[6b, 8] It was estimated that the intrinsic activity of Pt is around 102– 105 times greater than that of MoS2 edge.[9] As a result, recent studies on MoS2 in photocatalytic H2 evolution have been aimed at synthesizing MoS2 of both limited layers and lateral sizes to enhance the exposure of the edge sites.[6d, 10] For instance, MoS2 of a few layers prepared from thermal decomposition of (NH4)2MoS4 is reported to be efficient co-catalyst on CdS surface for photocatalytic H2 evolution from lactic acid solution.[11] Colloidal MoS2 by PVP-assisted synthesis is able to catalyze the H2 evolution reaction in the presence of Ru-and Ir-
[a] W. Zhang, T. Zhou, J. Hong, Y. Pan, Prof. R. Xu School of Chemical & Biomedical Engineering Nanyang Technological University 62 Nanyang Drive, 637459 (Singapore); E-mail: [email protected] [email protected] [b] T. Zhou, Prof. R. Xu SinBeRISE CREATE, National Research Foundation CREATE Tower, 1 Create Way, 138602 (Singapore) [c] W. Zhang School of Chemistry & Chemical Engineering Shaanxi Normal University Xi’an 710119 (China) [d] J. Zheng Institute of High Performance Computing 1 Fusionopolis Way, #16-16 Connexis North, 138632 (Singapore) [e] Prof. R. Xu C4T CREATE, National Research Foundation CREATE Tower, 1 Create Way, 138602 (Singapore) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201500067.
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Full Papers complex photosensitizers.[12] Exfoliated MoS2 nanosheets can Table 1. Weight percentages of the elements in MoS3 samples prepared enhance the activity for H2 evolution when linked to CdSe with or without PVP. quantum dots as reported by Osterloh’s group.[13] MoS2 nanoSamples N [%] C [%] S [%] H [%] Mo [%][a] S:Mo[b] particles are loaded on reduced graphene oxide for promoted charge transfer towards H2 formation sensitized by TiO2[14] or PVP-free MoS3 0.88 0.15 49.68 1.32 47.97 3.1 PVP-MoS3 5.04 23.06 30.30 4.11 31.72 2.9 Eosin Y.[10] Amorphous MoS2 formed through in situ photoreduction of (NH4)2MoS4 in dye-triethanolamine solution is re[a] Calculated from the balance after deducting the percentages of other elements by CHNS analysis. The percentage of oxygen in PVP-modified ported to be effective for H2 production by Wang and co-work[15] sample was estimated based on that of nitrogen. [b] Atomic ratio. ers. As an analogue to amorphous MoS2, MoS3 displays the same oxidation state for molybdenum but a different oxidation state atomic ratio of both samples is close to 3:1, which suggests for sulfur ligands. Both bridging S-S2 and S2 anions exist in the formation of MoS3. MoS3 at a theoretical ratio of 1:1.[16] MoS3 has been studied as Figure 1 A shows the XRD pattern of the as-synthesized amorphous MoS3 nanoparticles. After fitting the broad peak by an electrode material for electrocatalytic H2 evolution.[6e–g, 17] MoS3 is usually synthesized in an amorphous form through the Gaussian curve, the peak position at 13.098 is found for PVPacidification process of MoS42 anion. The amorphous nature usually assures an unsaturated environment of Mo4 + cation. There is no detailed structural information for MoS3 as its crystalline form has not been successfully obtained yet. Amorphous MoS3 has been also applied as a co-catalyst loaded on CdS/ CdSe nanorods for photocatalytic H2 evolution.[6c] Other than this, the synthesis and application of MoS3 for photocatalytic Figure 1. (A) XRD patterns of (a) PVP-free MoS3, (b) PVP-MoS3 (prepared with 0.2 g of PVP) and (c) bulk MoS2 ; (B) FTIR spectra of (a) PVP-free MoS3, (b) PVP-MoS3 prepared with 0.1 g of PVP, (c) PVP-MoS3 and (d) PVP. Inset of H2 evolution in literature is limit- A: pictures of PVP-MoS dispersed in water and ethanol. 3 ed. The use of MoS3 as a catalyst for photocatalytic H2 evolution MoS3 which corresponds to a lattice spacing of 0.68 nm. A simand the effect of the materials properties on catalytic activities is still a largely unexplored field. ilar diffraction pattern was reported for amorphous MoS3 in Herein, we report the facile synthesis of PVP-modified MoS3 a previous report with a weak and broad diffraction peak nanoparticles with unusual water solubility of up to around 148.[18] The spacing is slightly larger than that of the 1 1.0 mg mL . The as-synthesized amorphous nanoparticles PVP-free MoS3 sample at 0.62 nm. The difference is probably wrapped by PVP have particle sizes of approximately 2.5 nm, due to the broadened asymmetric XRD peak in PVP-MoS3 nanoparticles caused by the quantum size effect. As the peak which represent the smallest MoS3 or MoSx clusters reported in literature. Under visible light, the MoS3 nanoparticles dispersed is quite broad, the estimated grain size from the Scherrer equation of below 2 nm for PVP-MoS3 is not so accurate. Nevin water are demonstrated to be an efficient catalyst for H2 evolution using the low-cost xanthene dyes as photosensitizertheless, this indicates that there are limited layers in the sizeconfined MoS3 nanoparticles, since the Mo–S and Mo–Mo disers. The quantum efficiency of the optimized system for H2 tances are reported to be 2.44 and 2.78 in bulk MoS3 materievolution under green light irradiation (520 nm) is up to 36.2 %, which is comparable with those of other excellent phoal.[6c] tocatalytic systems involving earth-abundant catalysts. Fourier transform infrared spectroscopy (FTIR) spectra of MoS3 samples and PVP are shown in Figure 1 B. The absorption peaks originated from the chemical groups of PVP including C N, C=O and C-H can be clearly observed in PVP-MoS3 samResults and Discussion ples prepared with different amounts of PVP.[19] PVP as a water soluble capping reagent can wrap the surface of the nanoparWater soluble MoS3 was prepared by the hydrothermal ticles and make them easily dispersible in polar solvents withmethod using (NH4)2MoS4 as the precursor and thioacetic acid out any post-synthesis treatment.[20] As shown in pictures of (TAA) as the acidification agent in the presence of PVP. Elemental analysis results indicate that the weight percentage of PVP Figure 1 A inset, the as-synthesized PVP-MoS3 nanoparticles can is around 37 % in molybdenum sulfide nanoparticles synthebe well dispersed in water and ethanol, forming brown-colored sized (Table 1). There are negligible amounts of carbon and nistable and transparent solutions with solubility up to about trogen in the sample synthesized without PVP. The Mo/S 1 mg mL 1.
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Full Papers vis diffuse reflectance spectroscopy (UV–vis DRS) of the solid are shown in Figure S2. Both spectra show the absorbance of visible light at wavelength up to 800 nm which is consistent with its brown color. The oxidation states of Mo and S were investigated by X-ray photoelectron spectroscopy (XPS) analysis and the results are shown in Figure 4. The Mo3d5/2 and Mo3d3/2 doublet binding energies in the spectrum of PVP-free MoS3 (Figure 4 A) are located at 229.0 and 232.2 eV, respectively, which can be assigned to the binding energies of Mo4 + in MoS3.[6e,g] The XPS result indicates that the reduction of Mo6 + to Mo4 + is effective by the introduction of protons from TAA during the hydrothermal acidification synthesis.[16] Binding energies of Mo3d5/2 and Mo3d3/2 in the spectrum of PVP-MoS3 (Figure 4 B) are slightly shifted to lower positions at 228.5 and 231.6 eV, respectively. The estimated S/Mo ratio on the surface of the samples based on XPS results is 4.0 and 3.5 for PVP-free MoS3 and PVP-MoS3, respectively, which could be attributed to the presence of surface elemental S.[17] The higher elemental S in the PVP-free MoS3 is consistent with the S 2 s peak areas for S0 as shown in blue line in Figure 5. In both samples, the broad peak of S2p signals is characteristic to amorphous MoS3.[6e,g] The spectra can be fitted by three groups of doublets. Besides the elemental S with a S2p doublets at higher binding energies (blue line in Figure 5), there are two different kinds of sulfur ligands (S22 and S2 ) in MoS3 at the ratio of 1:1. However, it is difficult to quantify the exact ratio of the two sulfur ligands by XPS signals as their binding energies are quite similar. The S2p signal doublet with higher binding energies can be attributed to the bridging S22 and/or apical S2 ligands in MoS3, while the lower binding energy doublet is due to the terminal S22 and/or terminal S2 ligands.[6e, 16] The relative intensity ratio of the doublets between the higher and lower binding energies is 2.1 in PVP-free MoS3, and this ratio is decreased to 1.8 for PVP-MoS3. This can be well explained by the higher ratio of terminal sulfur ligands in PVP-MoS3 as a result of its much smaller particle sizes. The photocatalytic performance of H2 evolution over PVPMoS3 was studied in aqueous solutions containing triethanolamine (TEOA) as an electron donor and erythrosine B (ErB) as
Figure 2. TEM and HRTEM images of (A–C) PVP-MoS3 at different magnifications and (D) PVP-free MoS3. Insert of B shows the particle size distribution histogram.
The TEM images of PVP-MoS3 shown in Figure 2 A and 2B indicate the formation of rather uniform and well dispersed nanoparticles of approximately 2.5 nm in size. The histogram inset in Figure 2 B shows the particle size distribution in a narrow range of 1–3.5 nm. No clear lattice can be observed in the high resolution TEM (HRTEM) image (Figure 2 C) due to the amorphous nature of the material. On the other hand, the PVP-free MoS3 sample consists of agglomerated nanoparticles of around 20–50 nm (Figure 2 D) and presents poor water solubility. Besides TEM, atomic force microscopy (AFM) technique was also applied to investigate the morphological feature of PVP-MoS3. Figure S1 shows that the height of nanoparticles is around 5–8 nm at the same magnitude of the particle size obtained from HRTEM results, suggesting that round-shaped MoS3 nanoparticles were formed. The Raman spectra of the MoS3 and bulk MoS2 samples are displayed in Figure 3. The spectra of MoS3 are consistent with the Raman spectrum of amorphous MoS3 reported in literature.[21] By comparing with the Raman spectrum of the commercial bulk MoS2, the absence of typical E1g and A1g peaks from the spectra of MoS3 samples further excludes the possibility of the formation of crystalline MoS2 in our samples.[22] The UV-vis absorption spectra of PVP-MoS3 Figure 3. Raman spectra of MoS3 samples and bulk MoS2 in wide and narrow ranges, (a) PVP-MoS3, (b) PVP-free dispersed in water and the UV– MoS3, and (c) bulk MoS2. ChemSusChem 0000, 00, 0 – 0
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Figure 4. XPS spectra of Mo 3d, S 2s and S 2p orbitals of (A) PVP-free MoS3, and (B) PVP-MoS3. Mo (green); S0 (blue); bridging S22 /apical S2 (red); terminal S22 /S2 (purple).
Figure 5. Optimization of H2 evolution amount from the photocatalytic system using PVP-MoS3 as the catalyst by varying (A) the pH of the reaction solution, (B) the amount of ErB, (C) the amount of the catalyst, and (D) the type of xanthene dye. Reaction conditions: 300 W Xenon lamp with a cut-off filter at 420 nm, 100 mL 15 v % TEOA solution, pH 8.5, 0.2 g ErB, 0.2 g PVP-MoS3 (unless otherwise stated).
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a photosensitizer. Control experiments have been conducted to show that all the above components and light are essential for H2 production. There was no H2 evolved when only PVP was used as the catalyst. The reaction conditions including the concentrations of the photosensitizer and catalyst, the pH, and the type of photosensitizer were optimized to gain highest H2 evolution amount in 24 h irradiation from our photoreactor. Figure 5 A shows that a slightly basic TEOA solution with a pH of 8.5 (adjusted by HNO3) is optimal for H2 evolution, which is similar to the results from our previous studies.[1e, f] As both higher proton concentration in the reaction solution and lower protonation level of the amine group of TEOA are favorable, a suitable pH value of the solution assures the sufficient amount of both electron acceptor (H + ) and electron donor (TEOA). Figure 5 B demonstrates that a higher concentration of the photosensitizer leads to greater amounts of H2 evolved in 24 h irradiation. However, within the first 5 h, the amount of H2 evolved is decreased with the amount of ErB increased from 200 to 300 mg. This is probably due to the self-sheltering effect of the dye molecules.[1f] With a longer irradiation time, photodecomposition of dye molecules takes place extensively[23] and thus a higher dye concentration provides sufficient photosensitizer to compensate its degradation. Figure 5 C shows the effect of catalyst concentration on H2 evolution. Below 100 mg, the amount of H2 evolved increases almost linearly with the concentration of the catalyst. Beyond that, there is no significant difference in the amount of H2 evolved as the concentration of dye should be the limiting factor under such a condition. Among the xan-
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Figure 6. Visible light driven (l > 420 nm) photocatalytic H2 evolution over MoS3 and commercial MoS2, (B) photocatalytic H2 evolution over PVP-MoS3 with long-pass cut-off filters cut on different wavelengths, and (C) QEs with ErB and PVP-MoS3 under photons with different wavelengths. Reaction conditions: 300 W Xenon lamp, 100 mL 15 v % TEOA solution, pH 8.5, 0.2 g ErB, 0.2 g catalyst.
thene dyes studied (Figure 5 D), ErB is the best photosensitizer for MoS3 due to its suitable visible-light absorption range and relatively good resistance to photodecomposition.[1e] Figure 6 A shows the photocatalytic performance of different molybdenum sulfides as H2 evolution catalysts under the optimized reaction conditions mentioned above. Compared to the bulk commercial MoS2 and PVP-free MoS3, PVP-MoS3 with smaller particle sizes and excellent water solubility exhibits much enhanced photocatalytic performance for H2 evolution. After 24 h irradiation, 10.8 mmol of H2 was evolved over PVPMoS3, which is around 45 % higher than that over PVP-free MoS3. Compared to the bulk MoS2, the amount of H2 generated over PVP-MoS3 in 24 h is 7.3 times higher. The activity of PVP-MoS3 was further evaluated under irradiation with photons of different wavelengths. As shown in Figure 6 B, the amount of H2 evolved in 48 h does not vary much when the cut-off filters at 420, 455, and 475 nm were used. Even when a 500 nm cut-off filter was used, the amount of H2 evolved is still over 10 mmol. Therefore, such a dye sensitized system employing well-dispersed MoS3 catalyst is able to utilize visible light of longer wavelengths. It is notable that the catalyst is still active with a moderate H2 evolution rate of 120 mmol h 1 even under the irradiation of green light (l > 520 nm) and can even produce H2 under irradiation longer than 550 nm. As our reaction was carried out at a relatively large scale of 100 mL, the performance of the photocatalytic system can be better measured by the quantum efficiency (QE) rather than the turnover number (TON) from the catalyst. The QEs measured in the visible light region (420–550 nm) at various wavelengths are displayed in Figure 6 C together with the UV–vis absorbance spectrum of ErB. Before the wavelength of maximum absorption of ErB at around 526 nm, the QE increases from 9.1 % to the highest value of 36.2 % from 420 to 520 nm. Such a high QE obtained at 520 nm green light is comparative with other excellent QE results for photocatalytic systems involving earth-abundant catalysts.[1e, 2a,d,e, 24] As a result, the MoS3 amorphous nanoparticles synthesized by a simple method in the present work ChemSusChem 0000, 00, 0 – 0
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may be potentially a good candidate of co-catalyst for H2 evolution reaction. Under the irradiation of photons with wavelength at 550 nm, the QE obtained is still as high as 17 %. The utilization of longer wavelength photons is essential to achieve the successful application of solar energy by water splitting.[25] Since there is no confirmed structural information about the amorphous MoS3, as an alternative, we studied the size effect on the electronic band structure of MoS2 to give us some hints on the particle size effect of MoS3. Based on our previous study on the alignment of energy states of ErB using vertical ionization potential (IP), the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels of the ErB dye are located at 5.85 and 3.42 eV versus vacuum, respectively.[1e, 26] The positions of conduction and valence band edges of MoS2 relative to vacuum were calculated following the method reported by Moses et al.[27] For bulk MoS2, the calculated positions of conduction band minimum (CBM) and valence band maximum (VBM) are 3.79 and 4.99 eV versus vacuum, respectively. On the other hand, MoS2 of a single layer has a CBM at 3.71 eV and a VBM at 5.61 eV. Herein we are able to draw the alignment of energy levels of the photosensitizer ErB and the H2 evolution catalyst MoS2 as shown in Figure 7. Bulk MoS2 has a 0.37 eV lower conduction band potential than the LUMO level of ErB and a much higher potential than the normal hydrogen reduction potential ( 4.44 eV for NHE vs vacuum). The combination of ErB and MoS2 is hereby demonstrated theoretically to be an effective system for photocatalytic H2 evolution and the photoexcited ErB dye is quenched by TEOA as discussed in our previous publication.[1e] Compared to the bulk MoS2 with an indirect band gap at 1.2 eV, MoS2 with limited layers has a direct band gap at around 1.9 eV.[28] Based on our calculation, monolayered MoS2 has a higher CBM than the bulk MoS2, which corresponds to a higher driving force for proton reduction. Further, in MoS2 monolayer, the lifetime of photogenerated electrons is increased dramatically due to strong excitonic binding energies associated with low relaxation rate arising from the broadened 5
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Full Papers Figure 9 shows the XPS results of the catalyst after reaction. In the spectrum of Mo 3d, the major doublet of Mo 3d5/2 and Mo 3d3/2 located at 229.0 and 232.1 eV is attributed to Mo4 + in MoS3. A smaller doublet with binding energies at 231.8 and 234.9 eV may be associated with molybdenum of higher oxidation states, or molybdenum coordinated with sulfate species. Consistently in the spectrum of S 2p, a small peak at 168.1 eV indicates that a small percentage of sulfur is possibly oxidized to sulfate anion[31] due to self-corrosion. Nevertheless, only about 7 % of sulfur is oxidized based on the relative peak areas. The other two doublets with the main peaks at 161.1 and 162.6 eV indicate the presence S22 and S2 ligands. The estimated ratio of S/Mo is 2.5 based on the XPS results. Compared to the value in the as-prepared PVP-MoS3, the S/Mo ratio is decreased. It has been reported that amorphous MoS3 serves as a pre-catalyst and it undergoes an activation process to form MoS2 or MoS2 + x catalysts under the electrochemical study for H2 evolution.[6e,g, 17] In the current study, a similar phenomenon may have occurred and MoS3 can be partially converted to amorphous MoS2 + x phase under the photocatalytic condition.
Figure 7. Photocatalytic H2 evolution mechanism using ErB as a photosensitizer, MoS2 monolayer as the catalyst and TEOA as the sacrificial reagent. Energy levels of bulk MoS2 are presented for comparison.
indirect band gap.[29] Likewise, amorphous MoS3 with reduced sizes of around 5 nm prepared in this work should also compare favorably with its bulk counterparts for improved H2 evolution activity from the electronic structure point of view. The water soluble PVP-MoS3 has further been shown as an efficient electrocatalyst for proton reduction to molecular hydrogen based on the cyclic voltammetric study shown in Figure 8 A. After adding trifluoroacetic acid to the solution, the current was greatly enhanced at the electrocatalytic proton reduction potential. Similar observations were reported by other groups for photocatalytic H2 evolution catalysts.[30] Meanwhile, the water soluble PVP-MoS3 is able to quench the photoluminescence of ErB efficiently as shown in Figure 8 B, indicating that transfer of photoexcited electrons from ErB to the catalyst occurs efficiently. After 48 h of light irradiation, the PVP-MoS3 catalyst Figure 8. (A)1 Cyclic voltammetry study of the PVP- MoS3 dissolved in water at 0.1 mg mL with the addition of different amount of trifluoroacetic acid, and (B) fluoreswas collected by adding 300 mL of acetone into the cence spectra of ErB-TEOA solutions (dilute 10 times from the photoreaction concentrareaction mixture and the solid sample was character- tion) excited at 490 nm with or without addition of PVP-MoS3 (inset: the weight ratio of ized by FTIR, XRD, and XPS techniques. The XRD pat- MoS3 :ErB). terns (Figure S3 A) indicate that the catalyst after reaction is still amorphous in nature and the same broad peak at around 148 is observed. It is noted that a small and broad peak appears at around 358, which could be due to the slight oxidation of Mo species. Based on the FTIR spectra of the sample before and after reaction (Figure S3 B), the fingerprints of PVP can still be found but at lower intensities in the spectrum of the catalyst after reaction. The dissociation or decomposition of PVP on the surface of PVP-MoS3 nanoparticles is expected to occur during photoreaction. Figure 9. XPS spectra of Mo 3d and S 2p for PVP-MoS3 after photoreaction under irradiation of 48 h.
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Research Initiative for Sustainable Energy (SinBeRISE) and Cambridge Centre for Carbon Reduction in Chemical Technology (C4T) CREATE Programmes.
Water soluble PVP-modified MoS3 nanoparticles of approximately 2.5 nm have been successfully synthesized. The as-prepared MoS3 nanoparticles can be well dispersed in water for photocatalytic H2 evolution sensitized by the low-cost xanthene dyes in aqueous solutions. The optimized system exhibits a high QE of 36.2 % at 520 nm. The efficient catalytic activity of PVP-MoS3 nanoparticles can be attributed to their good dispersion in water for optimal interaction with the photosensitizer molecules, amorphous nature and limited layers in the catalyst particles with abundant surface active sites, and possibly a higher CBM potential for proton reduction and larger indirect band gap for a longer lifetime of the excited electrons.
Keywords: hydrogen evolution · molybdenum sulfide · photocatalyst · photosensitizer · water splitting
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Experimental Section Synthesis of water-soluble MoS3 nanoparticles In a typical synthesis of water-soluble MoS3 nanoparticles, 2.0 mmol of (NH4)2MoS4 and 0.2 g of PVP were dissolved in 75 mL of deionized water. 1.0 mL of TAA was added to the solution before it was transferred into a Teflon-lined autoclave with a capacity of 120 mL. The mixture was subjected to hydrothermal treatment at 200 8C for 12 h. After cooling to room temperature, the obtained brown color suspension was mixed with 300 mL of acetone and kept still over night for the precipitation of the solid particles. The dark brown colored solid was washed thoroughly with acetone and centrifuged several times before being dried at room temperature to obtain PVP-modified MoS3 nanoparticles with a yield of around 60 %.
Photocatalytic activity measurement Visible-light-driven H2 evolution reactions were conducted in a closed gas circulation and evacuation system fitted with a top window Pyrex cell. The photocatalytic H2 production reactions were performed in 100 mL of 15 vol % TEOA aqueous solutions with pH 8.5 (adjusted by concentrated HNO3), typically containing 0.2 g of Erythrosine B and 0.2 g of MoS3. The light source is a 300 W Xenon lamp equipped with long-pass cut-off filters (cut on 420, 455, 475, 500, and 520 nm). The reaction cell was kept at room temperature with jacketed cooling water. The H2 evolved was detected using an online gas chromatography. The apparent quantum efficiency (QE) was measured by equipping band pass interference filters (Newport, center wavelength at 420, 440, 460, 480, 500, 520, and 550 nm, band width 10 nm). The reaction solutions were irradiated under l > 420 nm for 1 h before switching to bandpass filters for QE tests and the H2 evolution measured in the subsequent 5 h was used for calculation of QE. The number of photons from irradiation was measured using a photodiode. Detailed experimental information can be referred in the Electronic Supporting Information.
Acknowledgements This work was supported by National Environment Agency, Singapore under the Environment Technology Research Programme (ETRP) through Project no.: ETRP 1002 103, and Singapore National Research Foundation (NRF) through the Singapore-Berkeley ChemSusChem 0000, 00, 0 – 0
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Received: January 14, 2015 Revised: February 28, 2015 Published online on && &&, 0000
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FULL PAPERS Green light for H2 evolution: MoS3 nanoparticles with unusually high water solubility were synthesized through a facile hydrothermal method. Their excellent quantum efficiency for H2 evolution under green light irradiation can be attributed to their good dispersion in water, amorphous nature, abundant surface active sites, higher conduction band potential for proton reduction, and larger indirect band gap for a longer lifetime of the excited electrons.
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W. Zhang, T. Zhou, J. Zheng, J. Hong, Y. Pan,* R. Xu* && – && Water-Soluble MoS3 Nanoparticles for Photocatalytic H2 Evolution
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