Synthesis and Characterization of Template-Free VS4 Nanostructured

Article pubs.acs.org/IECR

Synthesis and Characterization of Template-Free VS4 Nanostructured Materials with Potential Application in Photocatalysis Gregory Lui,† Gaopeng Jiang,† Aoshu Duan,‡ Josh Broughton,† Jason Zhang,‡ Michael W. Fowler,*,† and Aiping Yu*,† †

Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada Department of Chemical Engineering, University of Ottawa, Ottawa, Ontario, K1N 6N5, Canada

‡

S Supporting Information *

ABSTRACT: Since its discovery, little work has been done on the vanadium chalcogenide VS4. Recently, a facile method for synthesizing VS4 was discovered using a graphitic template. Here we show for the first time that template-free VS4 can be synthesized in a hydrothermal reaction by controlling key parameters of the reaction: mainly time, temperature, and pH. The phase and morphology of VS4 materials are tracked carefully using X-ray diffraction (XRD) and scanning electron microscopy (SEM) under each reaction condition. It is found that lower reaction temperatures and longer reaction times are sufficient to form VS4 crystals, while variations in pH do not appear to greatly affect VS4 crystallinity but rather surface area and morphology. By use of optimized reaction parameters, further characterization shows template-free VS4 to be comparable with VS4 templated with graphene oxide. Initial photocatalytic testing of these materials shows that VS4 has the potential to be used in photocatalysis.

1. INTRODUCTION Photocatalysis is an important area of research because of its promising future in various applications, including waste and pollution control, photovoltaics, water splitting, and various catalyzed reactions.1−4 Currently, titanium dioxide (TiO2) is most widely used as a photocatalytic material because of its large redox potential and chemical stability. Unfortunately, this large band gap makes TiO2 photoactive only in the ultraviolet range of the electromagnetic spectrum,5,6 as has also been shown in our previous work in this area.7 It is therefore pertinent to explore smaller band gap materials that are active in the visible spectrum and can act as sensitizers for larger band gap materials in photocatalysis.8,9 Chalcogenides are receiving great attention because of their strong electrical, optical, magnetic, sensing, and mechanical properties.10−12 These materials have been used in applications such as optics and sensors,13,14 Li-ion batteries,15,16 photovoltaics,17,18 fuel cells,19,20 LEDs,21,22 organocatalysis,23 and photocatalysis.24,25 VS4 is one such chalcogenide that has received relatively little attention from the scientific community. Naturally occurring VS4 (also called patronite mineral) was first discovered by Hewett and Bravo in 1906 and published by Hillebrand in 1907.26 Its crystal structure was then presented by Allmann and co-workers in 1964.27 VS4 is a monoclinicprismatic mineral (a = 6.78, b = 10.42, c = 12.11) with a linear chain structure (Figure 1). Since this time, little work has been done to synthetically produce such a material. VS4 was documented in the 1990s as being readily synthesized by heating V2S3 to 400 °C in the presence of excess sulfur.28 However, since then, this material and its synthesis method have not seen much attention in scientific research.29 This is likely due to the complex nature of the vanadium−sulfide system: both the need for precise control over the partial pressure of sulfur and the consequent difficulty of isolating © 2015 American Chemical Society

Figure 1. Structure of a linear chain of VS4 (side view and perspective view along the axial direction): gray atoms, V; yellow atoms, S.

specific vanadium sulfide phases.30,31 Because of vanadium’s oxophilic character, sulfidation of vanadium oxides is difficult.32 The synthesis of VS4 is often complex, involving the prior synthesis of V2S3 through either high temperature of sulfidation of V2O5 or sulfur replacement and reduction of orthovanadate ions.29,30,32,33 VS2, a more well-known vanadium sulfide phase, can also be synthesized via sulfidation of V2O5 at high temperatures but has also been shown to form in controlled hydrothermal reaction.34−36 Recently, Rout and co-workers were able to show that VS4 could be successfully synthesized in a hydrothermal reaction using graphitic templates such as graphene oxide (GO), carbon nanotubes (CNTs), and perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA).37 This evolved into further work demonstrating that VS4 could be used as an anode material Received: Revised: Accepted: Published: 2682

October 29, 2014 February 9, 2015 March 2, 2015 March 2, 2015 DOI: 10.1021/ie5042287 Ind. Eng. Chem. Res. 2015, 54, 2682−2689

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Industrial & Engineering Chemistry Research in Li-ion batteries.38 These results contrast with how VS2 is normally hydrothermally synthesized in the absence of such a template.36 It is still unclear how the graphitic structure aids in the nucleation and formation of VS4 crystals. Also to the best of our knowledge, there is no further literature concerning this specific reaction. In order to better understand how VS4 can be hydrothermally synthesized, the reaction was carried out in the absence of any graphitic template under varying reaction conditions to determine whether crystallization is still be possible. Herein we demonstrate that VS4 can indeed be synthesized in the absence of any graphitic template. By modification of the parameters of the reaction (time, temperature, pH), VS4 can be synthesized and its crystallinity can be tuned to resemble that of VS4 synthesized using a graphitic template. This removes the necessity of using a template or additive in the synthesis of VS4, increases the overall purity of the product, and allows for more flexibility in VS4 synthesis. More work is required in order to understand the nature of VS4 and its characterization; however, a reaction pathway can be suggested to explain this reaction. VS4 materials were evaluated based on the photocatalytic degradation of methylene blue (MB) under UV irradiation, showing the potential of VS4 for use in photocatalysis.

Table 1. List of Synthesized VS4 Samples with Their Corresponding Experimental Parameters: Reaction Time, Temperature, and pH sample name

reaction time (h)

reaction temp (°C)

reaction pH

VS4-A VS4-B VS4-C VS4-D VS4-E VS4-F VS4-G VS4-RGO

24 48 72 48 48 48 48 24

160 160 160 140 180 140 140 160

12 12 12 12 12 9 5 12

measured using a surface area and porosity analyzer (Micrometrics ASAP2020). Materials were evaluated based on the photocatalytic degradation of MB under UV light irradiation (500 W Hg lamp). 10 mg of photocatalyst was added to 40 mL of MB aqueous solution (0.0625 M) and stirred in the dark for 1 h to achieve adsorption−desorption equilibrium. This is a standard approach in photocatalytic experiments.41−43 During reaction, 2 mL solutions were drawn at 30, 60, 120, 150, and 180 min. Samples were centrifuged in order to remove the catalyst, and the concentration of MB was determined using a UV−vis photospectrometer (Fischer Scientific GENESYS 10S). The 664 nm characteristic peak of MB was used to determine its absorbance. The reaction rate was calculated using a first-order reaction equation, kt = −ln(C/C0), where k is the reaction rate (h−1), C is the measured dye concentration at a given time interval, and C0 is the measured dye concentration at absorption−desorption equilibrium.7

2. METHODS Materials. Sodium orthovanadate (Na3VO4, 99.98% trace metals basis), thioacetamide (CH3CSNH2, ACS reagent, ≥99.0%), methylene blue (C16H18N3SCl, MB) powder, and hydrochloric acid (ACS reagent, 37%) were purchased from Sigma-Aldrich. Graphite powder (natural, microcrystal grade) was purchased from Alfa Aesar. All chemicals were used as received. Graphitic oxide (GO) was synthesized from graphite powder using a modified Hummers method as described in previous work.39,40 VS4 was first synthesized according to literature.37,38 0.55 g of sodium orthovanadate (3 mmol) and 1.123 g of thioacetamide (15 mmol) were added to 40 mL of deionized water containing 60 mg of GO. After stirring for 10 min, the solution was placed in an autoclave and heated to 160 °C for 24 h. After the reaction was sufficiently cooled, the black product was washed with deionized water several times and dried in a freeze-dryer. This sample was denoted as VS4-RGO. This synthesis procedure was repeated in the absence of GO in order to determine whether nontemplated growth is possible. The purity of VS4 was optimized by first adjusting the length of the reaction (24, 48, and 72 h) and then the temperature of the reaction (140, 160, 180 °C). Lastly, the pH of the reaction was adjusted by adding HCl to the reaction mixture (pH = 5, 9, and 12). For the sake of simplicity, all samples were denoted as shown in Table 1. Characterization. The general morphology and composition of materials were determined using field-emission scanning electron microscopy (FESEM, Zeiss Leo 1530, 10 kV acceleration voltage) with energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD, Bruker AXS D8 Advance), transmission electron microscopy (TEM JEOL 2010F; 200 kV acceleration voltage), Raman spectroscopy (Renishaw Ramanscope; 633 nm, 7.5 mW laser), and Fourier transform infrared spectroscopy (FTIR, Thermo Nicolet Avatar 320 FTIR spectrophotomer). Absorbance spectra were obtained using a UV−vis−NIR spectrophotometer (Varian Cary 5000). Branauer−Emmett−Teller (BET) specific surface areas were

3. EXPERIMENTAL RESULTS Effect of Reaction Time on VS4 Formation. SEM images and XRD spectra corresponding to the temporal study of the VS4 reaction are shown in Figure 2. In terms of morphology, increasing the length of the reaction causes the formation of a nanorod structure. At 24 h, the rods have a diameter of ∼50 nm and a length up to 500 nm. At 48 h the rods have a more defined crystalline shape, with an average diameter of ∼200 nm and lengths up to several micrometers. However, by 72 h the distinct rodlike structure begins to disappear and the material becomes less uniform. In all samples, the rods tend to aggregate into larger particles. XRD spectra of these materials reveal that an increased reaction time also increases VS4 crystallinity. At 24 h, the two major peaks at 2θ = 15.9° (110) and 2θ = 17.0° (020) of monoclinic VS4 [JCPDS, 01-072-1294] do not fully appear. It is difficult to resolve all relevant peaks at this stage because of the low signal-to-noise ratio of the XRD spectra. It should be noted that the major peaks of VS2 are not present as one would expect from literature.36,37 At 48 h, the (110) and (020) peaks are now easily resolved. However, a peak at 21° begins emerging at this point that does not correspond to the VS4 phase or to the best of our knowledge any known vanadium sulfide. It is possible that this peak corresponds to a vanadium oxide phase such as V4O7 [JCPDS, 01-072-1718] or V2O5.44 By 72 h, these unknown peaks increase in intensity, while the crystallinity of VS4 remains relativity unchanged. The growth of this unknown peak may be the reason for the increase in nonuniformity of the sample at 72 h. By use of the Scherrer equation, crystallite sizes of products at 24, 48, and 72 h were estimated to be 29.7, 39.5, 2683

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Figure 2. VS4 temporal study: XRD spectra of VS4 materials with increasing reaction time, along with reference VS4 spectra [JCPDS, 01072-1294] (a) and SEM images of VS4 materials synthesized for 24 h (b), 48 h (c), and 72 h (d).

Figure 3. VS4 temperature study: XRD spectra at different reaction temperatures, along with reference VS4 spectra [JCPDS, 01-072-1294] (a) and SEM images of VS4 materials synthesized at 180 °C (b), 160 °C (c), and 140 °C (d). All reactions were performed for 48 h.

and 40.6 nm, respectively. This result is understandable, as crystallite size tends to increase with reaction time. This demonstrates immediately that it is possible to synthesize VS4 hydrothermally in the absence of a template or support. However, longer reaction times are required in order to fully form and improve crystallinity. Reaction times exceeding 48 h do not appear to significantly affect crystallinity. Unfortunately, in all cases minor impurities are introduced into the material that may correspond to oxides. This may be due to oxygen present in the system and may be solved by extensive purging with inert gas before reaction. Further work must be done in order to minimize these impurity peaks. Effect of Reaction Temperature on VS4 Formation. The temperature of the reaction was then adjusted to determine its effect on VS4 formation (Figure 3). Increasing the reaction temperature from 160 to 180 °C appears to

negatively affect the formation of VS4 nanorods. SEM images show that the nanorods have a diameter of ∼100 nm but are also less uniform. XRD of this material shows a complete disappearance of the major (110) and (020) peaks. The unknown peak at 21° increases in intensity, while a new peak at 23.2° also appears. Again, the identity of these peaks are unknown but bear some resemblance to vanadium oxide phases V4O7 and V6O13.44 Reducing the reaction temperature from 160 to 140 °C causes the VS4 to assemble into much smaller nanorods structures, approximately 200 nm long and 40 nm wide. It would therefore appear that a higher temperature promotes the formation of nanorods while also providing the necessary activation energy to oxidize vanadium to form other vanadium compounds. This demonstrates and confirms the sensitivity of the template-free VS4 reaction toward reaction conditions and 2684

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Industrial & Engineering Chemistry Research the difficulty of its synthesis in hydrothermal synthesis. It appears more beneficial to synthesize template-free VS4 at lower temperatures to avoid oxidation and the introduction of impurities. The average crystallite sizes of VS4 materials synthesized at 140, 160, and 180 °C were estimated to be 37.0, 39.5, and 39.8 nm, respectively. There is only a marginal increase in average size from 140 to 160 °C. At 180 °C vanadium undergoes a transition to an oxidized form of vanadium. This makes a direct comparison in terms of crystallite size more difficult. Effect of Reaction pH on VS4 Formation. Lastly, the pH of the reaction was adjusted (using HCl to adjust the pH to 5 and 9 from an original pH of 12) prior to hydrothermal reaction to determine the effect of pH on the formation of VS4 (Figure 4). Acidic conditions are known to encourage the formation of H2S. When the pH is lowered to 9, the VS4 product loses its rodlike shape and instead appears to consist only of nanoparticles. At pH = 5, VS4 preferentially forms into cubic nanoparticles less than 200 nm in size. From the XRD spectra, it can be seen that the crystallinity and purity of all three samples are somewhat similar. TEM was performed on VS4-D and confirmed the nanorod morphology of VS4 with a lattice spacing of 0.56 ± 0.01 nm corresponding to (110) plane (Figure 5b). The estimated crystallite size of VS4 materials at pH = 5, 9, and 12 are 26.1, 29.2, and 37.0 nm. It is possible that at higher pH values, a highly reducing environment drives the formation and precipitation of vanadium sulfides. This not only encourages the nucleation of more VS4 crystals but also produces a high concentration environment that drives the selfassembly of particles into nanorods which are seen with all VS4 products performed at pH = 12. As pH decreases, the reaction is less favorable and slower reaction kinetics and lower concentrations favor the formation of nanoparticles over nanorods. Comparison of Template-Free VS4 and VS4-RGO. As a reference material, VS4 was also synthesized using GO as demonstrated in literature. A direct comparison of XRD spectra shows that both the template-free VS4 and templated VS4 share all major VS4 peaks (Figure 5c). The SEM image of VS4-RGO (inset of Figure 5c) shows that VS4 particles are less aggregated when synthesized with GO. It is possible that GO provides nucleation sites that aid VS4 formation and reduces aggregation of particles, as is also posited in literature.37 Therefore, it can be stated that GO acts as a substrate for VS4 nucleation rather than a template that is necessary for the precipitation of VS4. The crystallite size of VS4 in this material was estimated to be 35.7 nm, which is similar in size to the template-free VS4 synthesized at the same pH (pH = 12). It should be noted that even with the current optimized reaction parameters, template-free VS4 still contains additional minor peaks that likely correspond to vanadium oxide phases. Regardless, these results show that template-free VS4 can indeed be synthesized successfully and that an alternative reaction pathway should be proposed to account for this. FTIR spectra of both VS4 and VS4-RGO materials were taken for comparison (Figure 6a) and to the best of our knowledge is the first instance of FTIR characterization for this compound in literature. In the VS4-G sample, the bands at 545 and 997 cm−1 bear a strong resemblance to the ν(V−S−V) doubly bonded S2− and doubly bridged S2− and the ν(VS) terminal S stretches, respectively.34 The slight blue shift in the bands may be due to minor oxidation of VS4, which would be in agreement with XRD results. These same peaks are also found in the

Figure 4. VS4 pH study: XRD spectra at VS4 materials synthesized at varying pH values, along with reference VS4 spectra [JCPDS, 01-0721294] (a) and SEM images of VS4 materials synthesized at pH = 12 (b), pH = 9 (c), and pH = 5 (d). All reactions were performed at 140 °C for 48 h.

spectra for VS4-RGO, along with additional peaks corresponding to the presence of skeletal vibrations (1578 cm−1), C−O−C (1224 cm−1), and C−O (1080 cm−1) stretching vibrations of RGO.45 It is difficult to resolve other vibrational modes of RGO because of its low loading on VS4. The Raman spectra of both VS4 and VS4-RGO materials were also obtained (Figure 6b). Similar to the FTIR data, both materials are near identical in their spectra. When compared to the RRUFF Raman database (ID R070737, 532 nm laser), all spectra appear almost identical. Known bands at 190 and 279 cm−1 correspond to the V−S stretching (A1) and bending (B1) modes, respectively.46 The absorbance spectra of VS4 and VS4-RGO was obtained using a UV−vis−NIR (Figure 7). Samples were dispersed in N,N-dimethylformamide, and absorbance was measured between 300 and 1700 nm. A Tauc plot was also generated 2685

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falls above that of VS4-RGO and the DFT calculated value at ∼1.13 eV. This small band gap may make VS4 suitable for various optical and electronic applications. Until recently, there has been little discussion on the formation mechanism of VS4. At the moment, there is no documented reaction pathway for the synthesis of monoclinic VS4 from sodium orthovanadate and thioacetamide. Feng et al. has proposed that VS2, a more well-known vanadium sulfide phase, can be synthesized through the reduction of V5+ in Na3VO4 to V4+ by HS−.36 However, the oxidation state of sulfur in VS4 is −1 as opposed to −2 in VS2. This is because VS4 consists of V4+ coordinated with two disulfur dianions (S22−) as opposed to S2− ions as in VS2.37 Therefore, the oxidation of HS− is insufficient to account for the reduction of V(V) to V(IV). There are two potential solutions to this issue. First, excess HS− provides a strong reducing environment that can lead to the formation of various intermediates. It is possible that the reduction of vanadium can lead to the formation of transient V2S3, which can further react with sulfur (another oxidized intermediate of HS−) to form VS4 (eqs S1 and S2 in Supporting Information). Second, the oxidation of HS− has been suggested to form S22− under both alkaline and acidic conditions (eqs S3−S8).49,50 Disulfide dianions can then react with vanadate to form VS4 (eq S9). The true and complete reaction pathway is still unknown and is likely much more complex. Further in-depth study of the reactants and products must be performed to provide greater insight into this reaction. Finally, the preliminary photocatalytic activity of VS4 materials was determined via the degradation of methylene blue under UV irradiation. This reaction follows a pseudo-firstorder reaction, kt = −ln(C/C0),7 as shown by the photocatalytic data (Figure 8). On the basis of a pseudo-first-order reaction, it was found that VS4-F had the highest degradation rate constant (k = 0.408 h−1), above that of both VS4-G (0.244 h−1) and VS4D (0.028 h−1). However, the VS4-RGO sample performed even better, achieving rate constant of 0.548 h−1. The higher performance of VS4-F may be due to the increased purity of the material compared to VS4-D and VS4-G. BET measurements also reveal a similar trend in specific surface area. VS4-D, VS4-G, and VS4-F have specific surface areas of 7.0 10.8 and 11.6 m2 g−1, respectively (VS4-RGO has a surface area of 12.6 m2 g−1). Although the surface area of these samples are relatively low,

Figure 5. Representative TEM image of VS4 material (VS4-D) (a) and HRTEM image of VS4 nanorod morphology showing lattice spacing of 0.56 ± 0.01 nm (b). XRD spectra comparing template-free VS4 and VS4 synthesized using a graphene oxide template (inset, SEM image of VS4-RGO) (c).

from these data in order to provide an estimate of the material’s band gap.7,47 The absorbance spectra of both VS4 materials show similarities to that found in literature;37 however, the presence of RGO in VS4-RGO presents background absorption well into the NIR region. This background absorption is commonly seen in materials containing RGO.48 The generated Tauc plot shows that the band gap of VS4-RGO is estimated to be ∼0.94 eV. This value is also similar to results found in literature and falls below the DFT-calculated value of 1.0 eV.37 Typically, DFT values underestimate the band gap; however, a reduced band gap can also be explained by the presence of RGO which is known to decrease the overall band gap of semiconductor materials.37 The estimated band gap of pure VS4

Figure 6. FTIR (a) and Raman (b) spectra comparing template-free VS4 and VS4 synthesized using a graphene oxide template. Raman spectra are compared against known RRUFF data (ID R070737). 2686

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Figure 7. UV−vis−NIR spectra comparing template-free VS4 and VS4 synthesized using a graphene oxide template (a) and the corresponding Tauc plot providing an estimate of optical band gap (b).

The goal of this paper is to show that VS4 can be synthesized without the presence of a graphitic template. Current obtained VS4 nanostructured materials still have relatively low reaction rates with minor instability under UV irradiation. The limited effectiveness of this material as a photocatalyst indicates that additional work is required in terms of structure modification and purity in order to improve the photocatalytic performance of VS4. Nevertheless, this work shows that VS4 has the potential to be used as a photocatalyst.

4. CONCLUSION It has been shown that VS4 can be synthesized in a hydrothermal reaction without the aid of a graphitic template. By modification of the reaction conditions, including time, temperature, and pH, the phase and purity of VS4 can be improved to the point at which its XRD spectra resemble that of VS4 synthesized using GO. It was found that increasing the reaction time is necessary for the formation of VS4, while decreasing the reaction temperature is necessary to prevent the oxidation of vanadium to form vanadium oxides. Changes in pH (ranging from 5 to 12) were found to not significantly change VS4 crystallinity. These findings appear to contrast with results found in current literature; however, further research and study are required to make a definitive conclusion regarding the nature of VS4 synthesis. The photocatalytic performance of VS4 materials at varying pH values and VS4-RGO was determined using the degradation of MB under UV irradiation. It was found that the VS4 synthesized at pH = 9 (VS4-F) had the highest photocatalytic activity among the VS4 materials. VS4-RGO provided an even higher photocatalytic activity than the other VS4 materials, and this is likely due to the high conductivity of surface area of RGO. SEM and XRD analyses of VS4 materials after UV irradiation show minimal change in VS4 crystallinity, implying the VS4 may be viable as a photocatalyst. The small band gap of VS4 (∼1.13 eV) implies that it may be useful in various electronic and optical applications. It is believed that these results can provide a new perspective concerning VS4 and its characteristics, as well as additional insight into how VS4 may be synthesized in a hydrothermal reaction. Additional study and characterization of VS4 are still necessary in order to further improve its crystallinity and to better understand the nature of its formation and potential applications.

Figure 8. Photocatalytic performance of VS4 materials. Photocatalytic activity was determined by the photodegradation of methylene blue under UV irradiation.

the trend in surface area is possibly one factor in the difference in performance. A larger active surface area allows more sites to be available for catalysis, thus increasing reaction rate. The improved performance of VS4-RGO is likely due to the welldocumented benefits of graphene in photocatalysis. Graphene is often used in photocatalytic applications in order to improve the conductivity and surface area of the primary catalyst and act as an electron acceptor and reduce electron−hole recombination.51 It is important to study the stability of VS4 during photocatalysis, since a catalyst should remain unchanged during reaction. SEM images and XRD spectra were obtained after UV irradiation (Figure S1). SEM shows that the nanorod morphology of VS4 tends to aggregate into larger particles approximately 2.5 μm in diameter. A comparison of XRD spectra with VS4 before UV irradiation shows a minimal change in the crystallinity of VS4. EDX spectra was also obtained before and after UV irradiation and show similar results (Figure S2). As-prepared VS4 shows a near stoichiometric V/S ratio corresponding to VS4. A slightly smaller amount of sulfur likely corresponds to oxide impurities in the sample. After UV irradiation, the ratio remains relatively the same. A small increase in sulfur content is likely due to the oxidation of sulfur in VS4 during irradiation. 2687

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(13) Sanghera, J. S.; Aggarwal, I. D. Active and passive chalcogenide glass optical fibers for IR applications: a review. J. Non-Cryst. Solids 1999, 256−257, 6. (14) Frumar, M.; Frumarova, B.; Nemec, P.; Wagner, T.; Jedelsky, J.; Hrdlicka, M. Thin chalcogenide films prepared by pulsed laser depositionnew amorphous materials applicable in optoelectronics and chemical sensors. J. Non-Cryst. Solids 2006, 352, 544. (15) Murphy, D. W.; Carides, J. N.; Di Salvo, F. J.; Cros, C.; Waszczak, J. V. Cathodes for nonaqueous lithium batteries based on VS2. Mater. Res. Bull. 1977, 12, 825. (16) Bhandavat, R.; David, L.; Singh, G. Synthesis of surfacefunctionalized WS2 nanosheets and performance as Li-ion battery anodes. J. Phys. Chem. Lett. 2012, 3, 1523. (17) Todorov, T. K.; Reuter, K. B.; Mitzi, D. B. High-efficiency solar cell with earth-abundant liquid-processed absorber. Adv. Mater. 2010, 22, E156. (18) Jeong, K. S.; Tang, J.; Liu, H.; Kim, J.; Schaefer, A. W.; Kemp, K.; Levina, L.; Wang, X.; Hoogland, S.; Debnath, R.; Brzozowski, L.; Sargent, E. H.; Asbury, J. B. Enhanced mobility-lifetime products in PbS colloidal quantum dot photovoltaics. ACS Nano 2011, 6, 89. (19) Liu, G.; Zhang, H.; Hu, J. Novel synthesis of a highly active carbon-supported Ru85Se15 chalcogenide catalyst for the oxygen reduction reaction. Electrochem. Commun. 2007, 9, 2643. (20) Bezerra, C. W. B.; Zhang, L.; Liu, H.; Lee, K.; Marques, A. L. B.; Marques, E. P.; Wang, H.; Zhang, J. A review of heat-treatment effects on activity and stability of PEM fuel cell catalysts for oxygen reduction reaction. J. Power Sources 2007, 173, 891. (21) Preier, H. Recent advances in lead-chalcogenide diode lasers. Appl. Phys. 1979, 20, 189. (22) Fu, H.; Tsang, S.-W. Infrared colloidal lead chalcogenide nanocrystals: synthesis, properties, and photovoltaic applications. Nanoscale 2012, 4, 2187. (23) McGarrigle, E. M.; Myers, E. L.; Illa, O.; Shaw, M. A.; Riches, S. L.; Aggarwal, V. K. Chalcogenides as organocatalysts. Chem. Rev. 2007, 107, 5841. (24) Kale, B. B.; Baeg, J. O.; Lee, S. M.; Chang, H.; Moon, S. J.; Lee, C. W. CdIn2S4 nanotubes and “marigold” nanostructures: a visiblelight photocatalyst. Adv. Funct. Mater. 2006, 16, 1349. (25) Zong, X.; Wu, G.; Yan, H.; Ma, G.; Shi, J.; Wen, F.; Wang, L.; Li, C. Photocatalytic H2 evolution on MoS2/CdS catalysts under visible light irradiation. J. Phys. Chem. C 2010, 114, 1963. (26) Hillebrand, W. F. The vanadium sulphide, patronite, and its mineral associates from Minasragra, Peru. J. Am. Chem. Soc. 1907, 29, 1019. (27) Allmann, R.; Baumann, I.; Kutoglu, A.; Rösch, H.; Hellner, E. Die kristallstruktur des patronits V(S2)2. Naturwissenschaften 1964, 51, 263. (28) Richards, R. L. Vanadium: Inorganic & Coordination Chemistry. In Encyclopedia of Inorganic Chemistry; John Wiley & Sons, Ltd: Brighton, U.K., 2006. (29) Naman, S. A. Photoproduction of hydrogen from hydrogen sulfide in vanadium sulfide colloidal suspensioneffect of temperature and pH. Int. J. Hydrogen Energy 1997, 22, 783. (30) Yokoyama, M.; Yoshimura, M.; Wakihara, M.; Somiya, S.; Taniguchi, M. Synthesis of vanadium sulfides under high pressure. J. Solid State Chem. 1985, 60, 182. (31) Taniguchi, M.; Wakihara, M.; Shirai, Y. Growth of single crystals of vanadium sulfides and their electrical conductivity. Z. Anorg. Allg. Chem. 1980, 461, 234. (32) Guillard, C.; Lacroix, M.; Vrinat, M.; Breysse, M.; Mocaer, B.; Grimblot, J.; des Courieres, T.; Faure, D. Preparation, characterization and catalytic properties of unsupported vanadium sulphides. Catal. Today 1990, 7, 587. (33) Janssens, J.-P.; Dick van Langeveld, A.; Moulijn, J. A. Characterisation of alumina- and silica-supported vanadium sulphide catalysts and their performance in hydrotreating reactions. Appl. Catal., A 1999, 179, 229. (34) Vadivel Murugan, A.; Quintin, M.; Delville, M.-H.; Campet, G.; Vijayamohanan, K. Entrapment of poly(3,4-ethylenedioxythiophene)

ASSOCIATED CONTENT

S Supporting Information *

XRD, SEM, and EDX analysis after photocatalytic reaction to show the stability of VS4 under UV irradiation; possible reaction pathways for the formation of VS4 from sodium orthovanadate and thioacetamide in aqueous solution, with two main pathways proposed: (1) the formation of reduced vanadium and oxidized sulfide intermediates in a highly reducing environment and (2) the formation of S22− via the formation of HS• radicals. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*M.W.F.: e-mail, [email protected]. *A.Y.: e-mail, [email protected]. Funding

This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Dr. Norman Zhou and Robert Liang for providing Raman characterization of samples. We also thank Dr. Michael Collins for providing UV−vis−NIR characterization.

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REFERENCES

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DOI: 10.1021/ie5042287 Ind. Eng. Chem. Res. 2015, 54, 2682−2689