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Jun 12, 2017 - ABSTRACT: With an increase in global warming due to an increase in CO2 emission, there have been immense efforts to convert CO2 into ei...
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Titania Cowrapped α‑Sulfur Composite as a Visible Light Active Photocatalyst for Hydrogen Evolution Using in Situ Methanol from CO2 as a Sacrificial Agent Rajkumar Yadav and Anil Kumar Sinha* Conversions and Catalysis Division CSIR-Indian Institute of Petroleum (IIP) Mohkampur, Dehradun 248005, India Academy of Scientific and Innovative Research (AcSIR), Chennai, Tamil Nadu 600113, India S Supporting Information *

ABSTRACT: With an increase in global warming due to an increase in CO2 emission, there have been immense efforts to convert CO2 into either renewable fuels or useful chemicals. At the same time, because hydrogen is a clean energy source, there has been much interest in the hydrogen evolution reaction (HER). In this paper, we report a titania cowrapped α-sulfur composite material that under a light-emitting diode light source showed the highest level of hydrogen evolution reported for any elemental material. In situ methanol from CO2 photoreduction conversion was used as a sacrificial agent for the hydrogen evolution reaction. The effect of in situ methanol on hydrogen production was around 165 times stronger than the effect of ex situ methanol as a sacrificial agent. A plausible mechanism was proposed to explain the higher level of hydrogen production with in situ methanol as a sacrificial agent. The S8−TiO2−Au photocatalysts were thoroughly characterized by powder X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectrometry, X-ray photoelectron spectroscopy, ultraviolet−visible diffuse reflection spectroscopy, Fourier transform infrared spectroscopy (FT-IR), and N2 sorption studies. Photocatalytic products were analyzed using a gas analyzer (gas chromatograph with a thermal conductivity detector and a flame ionization detector). In situ, in operando FT-IR spectroscopy established the formation of formaldehyde and methanol as intermediate products. Density functional theory calculations showed the creation of additional density of states in the band gap of the photocatalyst, as well as a shift in the conduction band to the energy potential for hydrogen formation. KEYWORDS: Photocatalysis, Hydrogen generation, CO2 reduction, Water splitting, α-Sulfur, Titania

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semiconductors, α-sulfur and red phosphorus, have attracted much attention in photocatalytic reactions because of their visible light absorption ability, low cost, and abundance.15,16 Recently, Ansari and Cho reported titania modified with red phosphorus that showed a strong visible light response in photodegradation of Rhodamine B dye as well as in photoelectrochemical applications.17 There has not been much examination of the photocatalytic reduction of CO2 and photocatalytic hydrogen evolution from water using such elements. Even design, preparation, and modification of such materials have not been explored much. Much attention has been focused on the sulfided metal oxides, but using elemental sulfur for photocatalytic CO2 reduction and hydrogen evolution has not been explored, to the best of our knowledge. Recently, Liu et al. reported α-sulfur that showed photoelectrochemical water splitting and photodegradation of Rhodamine B (RhB) but suffered from low photocatalytic activity, poor hydrophilicity, and fast recombination of photoexcited e−−h+ pairs.16 Wang et al. reported graphene and g−C3N4 nanosheet

INTRODUCTION There has been great interest in developing alternative renewable energy resources because of the shortage as well as restrictions on the use of fossil fuels. There is also a continuous need to have more energy resources with the increase in demand.1−8 With the sun as the primary energy source, providing continuous energy to the earth, there is much interest in harvesting this energy by developing photocatalysts that are active in the visible region.3,4 Among various possibilities, one alternative could be a photocatalyst that can absorb sunlight in the visible range, and the excited electrons and generated holes can be used simultaneously for the evolution of hydrogen from water and for the conversion of CO2 into CH3OH, HCHO, HCOOH, etc.9−11 Such a photocatalytic conversion would help not only in controlling global warming but also in producing useful chemicals.10,11 An ideal photocatalyst should absorb in the visible region, should be inexpensive, and should be environmentally friendly, which can be achieved using abundant materials and one way to use elemental materials. Several elemental materials such as silicon, selenium, red phosphorus, and sulfur (a new class of photocatalysts for solar energy conversion) have shown photocatalytic activities in the decomposition of dyes and in hydrogen generation from water splitting.12−15 Elemental © 2017 American Chemical Society

Received: April 2, 2017 Revised: April 27, 2017 Published: June 12, 2017 6736

DOI: 10.1021/acssuschemeng.7b00996 ACS Sustainable Chem. Eng. 2017, 5, 6736−6745

Research Article

ACS Sustainable Chemistry & Engineering cowrapped elemental α-sulfur photocatalysts that showed bacterial inactivation under visible light.18 Considering hydrogen as an ideal clean energy, various efforts are being made to develop photocatalysts for water splitting that are active not only in UV light but also under visible light illumination. Theoretically, all the photocatalysts that have a suitable band gap for water splitting should show the hydrogen evolution reaction (HER). TiO2 has been widely used as a photocatalyst for hydrogen evolution reaction from water due to its strong catalytic reaction, high chemical stability, and long lifetime for generated electron−hole recombination, whereas elemental sulfur has not been explored for the hydrogen evolution reaction. Because of fast electron−hole recombination for such photocatalysts, it is difficult to achieve significant hydrogen evolution from water splitting. Combining two semiconductors together or adding electron donors (sacrificial reagents or hole scavengers) can increase the photocatalytic hydrogen evolution efficiency by preventing electron−hole recombination. As these electron donors are consumed continuously because of reaction with generated valence band (VB) holes, there is need for the continuous addition of an electron donor to achieve photocatalytic hydrogen evolution for a long run time. Cheap, simple, and commercially available light-emitting diode (LED) light sources were preferred for photocatalytic reactions, which reduces not only the amount of heat generated, thereby avoiding the need for additional cooling of the reaction mixture, but also the amount of electric power consumed compared to those of traditional light sources. Therefore, a white LED light source setup was used in this work for photocatalytic hydrogen production. In this paper, we report a simple, inexpensive, tunable synthesis of a composite α-sulfur cowrapped with a titania semiconductor to minimize the electron−hole recombination issue of elemental sulfur. This composite photocatalyst loaded with Au nanoparticles produced methanol from photocatalytic CO2 reduction. The produced methanol was used as an in situ sacrificial agent to react irreversibly with the photogenerated valence band holes, producing hydrogen evolution from water splitting. Continuous production of methanol permitted sacrificial hydrogen generation for a long run.

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Synthesis of a Au−Sulfur−TiO2 Powder Photocatalyst. The Au (1.2 wt %)−S8−TiO2 powder photocatalyst was synthesized by the sol immobilization method as described in a previous report.19 First, 0.02 g of HAuCl4·3H2O was dissolved in 10 mL of water while the mixture was being continuously stirred for 20 min, and 1.2 mL of a 1 wt % PVA aqueous solution was added while the mixture was being continuously stirred for 20 min. To this was slowly added 2.0 mL of a freshly prepared 0.1 M NaBH4 aqueous solution, which resulted in the immediate formation of a brown dark sol. After 0.5 h, S8−TiO2 powder (1.0 g) was added to the mixture, and the resulting solution was further stirred for 12 h at room temperature. The resulting precipitate was then washed with 200 mL of distilled water and dried at 120 °C for 10 h. Photocatalytic Activity Study. Photocatalytic hydrogen evolution experiments were executed in a designed lab-made photocatalytic reactor (Figure S1), which consisted of a visible light illuminating system (six 7 W LED bulbs as a light source) and a 50 mL roundbottom glass vessel made of borosilicate glass. The wavelength range of commercially available LEDs with single-output power is 360−950 nm, which is mainly in the visible light range. The measured intensity at the vessel was calculated to be 100 W/m2 using a solar power meter (TM-207). In this typical experimental setup, a round-bottom flask containing 0.1 g of the sample was suspended in 20 mL of HPLC grade water using a magnetic stirrer. The vessel was septum (silicon rubber) sealed and first degassed by N2 purging for 10 min. After that, the resulting suspended solution was saturated with CO2 for the next 30 min and then sealed for the first run. The vessel was then illuminated with the visible light illuminating system providing uniform illumination. After a fixed run time, liquid product samples were collected from the vessel using a syringe, and the catalyst was then removed using a syringe filter (2 nm PTFE, 13 mm diameter). We did not observe sample heating above 40 °C during the reaction. The sample was cooled intermittently, during regular sample analysis time. The reaction products adsorbed over the catalyst were analyzed by Fourier transform infrared (FT-IR) spectroscopy. Qualitative and quantitative analysies of final liquid products after different run times were performed using a gas chromatography-flame ionization detector (GC-FID, injector temperature of 120 °C and FID temperature of 275 °C, column flow of 0.8 mL/min, starting oven temperature of 40 °C) equipped with a 30 m long DBwax w/Integra-Guard column. A calibration curve was prepared for quantification and for confirmation of the linear response of the GC-FID system (Figure S2). The quantitative analysis of the final product that resulted from photocatalytic reaction was performed by injecting 1.0 μL of the sample for GC measurements and then integrating the respective peak areas of characteristic product peaks in the chromatogram. Evolved gaseous products were collected after a fixed run time, using an airtight gas syringe, and immediately injected into a gas analyzer (Agilent 7890) and analyzed by a thermal conductivity detector. Controlled experiments were performed to eliminate the surrounding interference and to ensure that hydrogen evolution and methanol production were caused by water splitting and photoreduction of CO2, respectively. One blank test was done in the absence of the photocatalyst. Another blank test was performed in the dark, in the presence of the photocatalyst, under identical experimental conditions. Yet another blank test was performed in the presence of the photocatalyst using N2 instead of CO2 (Table 2). Recycling studies for the catalyst were performed after removing the adsorbed species on the catalyst surface (predominantly methoxides) by treatment with acidified dilute hydrogen peroxide and drying. Electrochemical Measurements. The working electrode was prepared by mixing 70 wt % active material [S8−TiO2 (40)−Au], 20 wt % conducting agent (activated carbon), 10 wt % binder [polyvinylidene difluoride (PVDF), Aldrich], and a paste prepared in N-methyl-2-pyrrolidone (NMP) solvent. The resulting paste was pressed onto aluminum foil and dried at 60 °C. A 2 M KOH aqueous solution was used as an electrolyte. All measurements were taken on a Princeton Applied Research workstation with cyclic voltammetry and energy functions using a three-electrode system with Pt as the counter

EXPERIMENTAL SECTION

Materials and Reagents. Elemental sulfur (Merck), polyvinyl alcohol [PVA (Loba Chemie)], gold(III) chloride trihydrate [HAuCl4· 3H2O (Sigma-Aldrich)], ethylene glycol (S D Fine Chemical Ltd.), NaBH4 (Sigma) were used. Titanium(IV) tert-butadioxide [Ti{OCH(CH3)2}4, 95%, Alfa Aesar] was used as a source of titanium. Highpurity chemicals were used as received for synthesis. High-performance liquid chromatography (HPLC) grade water was used for experiments and synthesis. Synthesis of the Sulfur−TiO2 Composite. In a typical optimized experimental procedure, 0.7 g of S8 was added to a 100 mL solution of ethylene glycol and water (80:20) and the mixture stirred for 2 h at 125 °C. To this was added dropwise under continuous stirring 1 mL of titanium tert-butyloxide (TBT). After the addition of TBT, 5 mL of HPLC grade water was added at an interval of 30 min thrice and left for 3 h for vigorous stirring. After that, 1 mL of the NH3 solution (25% in water) was added to the resulting solution. The temperature was then increased to 130 °C, and the suspension was heated for 12 h while being continuously stirred in a closed vessel. The mixture was filtered under vacuum, washed with water several times, and dried at 120 °C for 10 h. The resulting powder was named the S8−TiO2 composite. 6737

DOI: 10.1021/acssuschemeng.7b00996 ACS Sustainable Chem. Eng. 2017, 5, 6736−6745

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most common among these forms at STP.15,19 Powder XRD patterns of S8−TiO2 composites (Figure 1) show a series of

electrode and standard calomel electrode (SCE) as the reference electrode. Characterization. Powder X-ray diffraction (XRD) patterns of the samples were acquired using a Bruker D8 advance diffractometer with Cu Kα radiation (40 kV, 40 mA) operating in reflection mode. All samples were scanned with 2θ scan range of 10−60. Raman spectra were recorded on a STR 500 Airix instrument with He−Ne 532 nm laser excitation in the range of 50−2000 cm−1. The morphological and structural observations were investigated using transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM), which were performed on a field emission gun transmission electron microscope (JEOL-TEM-2010), operating at 200 kV. Pretreatment of samples for the TEM measurements was performed by depositing an ethanol-dispersed sample on amorphous carbon-coated grids. The surface morphology of samples was determined using scanning electron microscopy (SEM), and elemental analysis using SEM with EDX was performed on a field emission scanning electron microscope (Quanta 200F). Textural properties of the samples were investigated from the N2 adsorption/desorption isotherms to calculate the specific surface area by the Brunauer− Emmett−Teller (BET) method, and the average pore size and pore volume were determined by the Barrett−Joyner−Halenda (BJH) method, recorded on a BELSORP max instrument at 77 K. Prior to measurements, the samples were degassed and dried under a vacuum system at 125 °C for 2−3 h as a pretreatment. Optical absorption properties of the samples were determined by UV−visible diffuse reflectance recorded on a PerkinElmer model Lambda 19 instrument for wavelengths ranging from 200 to 800 nm. An in situ FTIR spectroscopy technique was performed for the photocatalytic CO2 reduction reaction (water + CO2 system + 1.2 wt % Au−S8−TiO2 catalyst) under visible light irradiation. A mid-IR 187 fiber-optic system (SYS-IRX Reaction View-X, Remspec Corp.) was used, and the obtained data were further analyzed using the GRAMS/AI & GRAMS 3D (version 9.1) part of the GRAMS Spectroscopy software suite that was provided by Thermo Fisher Scientific Inc. A background for the water + CO2 atmosphere + photocatalyst reaction (without any irradiation source and after saturation with CO2 for 30 min) was taken and subtracted from all the spectra obtained during analysis. Spectra were recorded continuously for 16 h with 37 scans in 1 min at 4 cm−1 resolution. The sulfur content in composites was measured by using a PerkinElmer thermogravimetric analyzer (TGA) with a temperature ramp of 10 °C/min in an air atmosphere, and elemental analysis was performed using a CHNS Elementar analyzer. X-ray photoelectron spectroscopy (XPS) experiments were performed on an ESCA+ instrument from Omicron nanotechnology, Oxford instrument, with monochromatic Al Kα radiation (1486.7 eV) as the X-ray source.

Figure 1. XRD pattern of TiO2, S8−TiO2 (40), and S8.

diffraction peaks at 2θ values of 15.48°, 23.13°, 25.85°, 27.70°, 31.40°, 34.21°, 34.04°, 42.79°, 47.75°, and 51.25°, which correspond to (113), (222), (040), (313), (044), (400), (422), (319), (515), and (226), respectively, and can be readily indexed to the pure phase of elemental α-sulfur (JCPDS Card No. 78-1889). The appearance of characteristic peaks of orthorhombic α-sulfur in the composite showed the existence of elemental sulfur in the composite. The crystallinity of αsulfur was reduced after it had been wrapped with TiO2 as observed by the decreased intensity of diffraction peaks in the as-synthesized samples. There were no additional peaks due to titania in the PXRD pattern for the composite material, suggesting that the titania shell was amorphous and highly dispersed over the S8 surface. The weight ratio of α-sulfur was evaluated by elemental analysis and TGA (Figure 2). In TGA of the samples, a weight loss from room temperature to 200 °C was due to evaporation of adsorbed water. Further weight loss from 200 to 300 °C was

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RESULTS AND DISCUSSION Synthesis of the Photocatalyst. α-Sulfur powder was heated above its melting temperature (115 °C) to 125 °C to melt it in the solvent mixture (ethylene glycol and water). To obtain a homogeneous titania layer on elemental sulfur, the titania precursor was first gradually adsorbed on the dispersed elemental sulfur solution. Water was added in a controlled manner to the mixture of the titania precursor and α-sulfur melt solution (2:1 Ti:S), and then NH3 solution was added gradually to hydrolyze the titania precursor wrapping the elemental sulfur, thereby creating stable, homogeneous titania layers around elemental sulfur. In addition, the resulting composite was dried at 120 °C to retain the sulfur in its elemental form inside titania layers. Addition of excess α-sulfur during synthesis resulted in precipitation of unwrapped α-sulfur agglomerates, and the wrapped α-sulfur content was 34 wt %. Characterization of the Photocatalyst. S8 is the most stable allotrope among its more than 30 allotropes at STP. In solid form, it can crystallize into orthorhomic (α-sulfur), monoclinic (β-sulfur), and γ-sulfur forms, where α-sulfur is

Figure 2. Thermogravimetric analysis of the S8−TiO2 (40)−Au photocatalyst. 6738

DOI: 10.1021/acssuschemeng.7b00996 ACS Sustainable Chem. Eng. 2017, 5, 6736−6745

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ACS Sustainable Chemistry & Engineering ascribed to evaporation of α-sulfur. The weight loss of S8−TiO2 (40)−Au used around 450 °C was observed due to removal of polymeric sulfur and adsorbed methoxy species.20 The percentage decrease due to sublimation of α-sulfur was found to be 40%. Elemental analysis showed the amount of α-sulfur is 42%, which remained intact after gold deposition. From TGA and elemental analysis, it was confirmed that the Au/α-sulfur− TiO2 composite contained approximately 40−42 wt % α-S8. The melting point of α-sulfur was 115 °C,15 whereas the composite showed a melting point of 280 °C, which signifies that as-synthesized composite α-sulfur became more stable after being cowrapped with titania. The surface area decreased by 20% (18.5 m 2 /g) for the Au-deposited α-sulfur−TiO 2 composite compared to that of the α-sulfur−TiO2 composite (23.4 m2/g). The decrease in the surface area was probably due to covering of the pores in the composite by Au nanoparticles. Nitrogen sorption analysis showed a type III isotherm, indicating weak interaction between the adsorbent and adsorbate, with the wide H3 hysteresis loop indicating aggregates (loose assemblages) of platelike particles forming slitlike pores (Figure 3). Panels a and b of Figure 4 show SEM

Figure 4. (a) SEM image of S8−TiO2. (b) SEM image of S8−TiO2 (40)−Au. (c) EDX spot scanning of S8−TiO2 (40)−Au. (d−f) Elemental mapping of Ti, O, and S, respectively.

peak for S compared to that for Ti could be due to wrapping of S by titania. Au (4f7/2) binding energies were observed at 84− 86 eV, indicating multiple oxidation states of Au (0, I, and III). Low-magnification TEM images (Figure 5c) show that Au Figure 3. Isotherm and hysteresis loop of the S8−TiO2 (40)−Au photocatalyst.

images of the α-sulfur−TiO2 composite and the Au-deposited α-sulfur−TiO2 composite, respectively. The average size of the as-synthesized particles was found to be 6−10 μm, which is much smaller than the commercial α-sulfur (25−30 μm).15 There is an indication of α-sulfur covered with a deposition of a less dense titania layer. Titania particles are less dense than αsulfur is (Figure S14). There was no major change in the morphology after deposition of Au nanoparticles, indicating that the composite was intact during deposition. To verify the composition at the surface of the composite material, we performed EDX spot scanning and elemental mapping (Figure 4c−f). Elemental mapping showed highly dense Ti and O elements and a less dense S element, indicating most of the sulfur was covered with a titania layer (Figure 4d−f). This was also observed in EDX spot elemental analysis where the Ti:S ratio was 6.4 (Figure 4c), whereas elemental analysis and TGA showed a Ti:S ratio of around 0.5 indicating most of the sulfur is wrapped inside the titania layer (Figure 4c). XPS analysis showed a Ti 2p3/2 peak at 458.5 eV along with a Ti 2p1/2 peak at 465 eV that is ascribed to TiO2. A weak S 2p3/2 peak at 164.5 eV due to elemental sulfur was observed21 (Figure S3). A weak

Figure 5. (a) HRTEM images of Au nanoparticles deposited on S8− TiO2 (40). (b) HRTEM image of the TiO2 layer. (c) TEM image of deposited Au nanoparticles. (d) SAED of the S8−TiO2 (40)−Au composite. 6739

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absorption in the visible range as compared to those of α-sulfur and P25 titania. A Tauc plot revealed that the band gap of the composite material is at 2.54, which is very suitable for photocatalytic reactions (Figure 8). Figure 9 compares UV−vis

nanoparticles (black spots) were homogeneously well dispersed on the α-sulfur−TiO2 composite support. High-magnification TEM images (Figure 5a) show that the deposited Au nanoparticles were spherical in nature. Lattice-resolved TEM images showed well-resolved lattice fringes that correspond to Au (111) (d = 0.23 nm) and titania (211) (d = 0.15 nm) (Figure 5a,b). Selected area electron diffraction (SAED) showed spots assigned to crystalline titania and Au planes (Figure 5d). The Raman spectrum (Figure 6) of the Au-

Figure 8. Tauc plot of the S8−TiO2 (40)−Au photocatalyst.

Figure 6. Raman spectra of the S8−TiO2 (40)−Au photocatalyst.

deposited composite exhibited characteristic peaks located below 500 cm−1 corresponding to the bending and stretching mode of α-sulfur.22 Peaks corresponding to titania were not detectable in the composite as the thickness of the titania layer was small. Figure 7 compares UV−vis absorption spectra for α-sulfur, TiO2, and Au/α-sulfur−TiO2 samples. Absorption spectra of

Figure 9. UV−vis pattern of the S8−TiO2−Au photocatalyst with different sulfur contents.

absorption spectra of samples with different amounts of wrapped α-sulfur. The Au (1.2 wt %)−α-sulfur−TiO2 nanocomposite with 40 wt % α-sulfur absorbed maximally in the visible region compared to samples with 20 and 34 wt % αsulfur (Table 1). The wrinkle in the UV−vis spectrum of the TiO2−S8 composite could be an artifact from experiment. To understand the electronic properties of the composite, we conducted density functional theory (DFT) calculations. The calculated band structure of α-S with a direct band gap of 2.10 eV at the Γ point is 0.69 eV smaller than the experimental value (2.79 eV) as a result of well-known band gap underestimation

Figure 7. UV−vis pattern of the S8−TiO2 (40)−Au photocatalyst.

different S8−TiO2 (40)−Au composites with 1 wt % Au loading showed two sets of peaks: a band gap transition band of TiO2 and S8 with a maximum in the UV region and a surface plasmon resonance band (∼540 nm) in the visible region due to Au nanoparticles, as clearly observed in the TiO2−Au spectrum. It is noticeable that the level of absorption increased in the visible region for gold-deposited composite material. Thus, the assynthesized composite material showed an increased level of

Table 1. CHNS Analysis of S8−TiO2−Au Photocatalysts

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sample

%N

%C

%H

%S

S8−TiO2 (20)−Au S8−TiO2 (40)−Au S8−TiO2 (30)−Au

0.02 0 0.02

4.724 3.903 5.847

1.693 1.532 1.979

20.51 42.00 34.00

DOI: 10.1021/acssuschemeng.7b00996 ACS Sustainable Chem. Eng. 2017, 5, 6736−6745

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ACS Sustainable Chemistry & Engineering within the framework of standard DFT (Figure S6).15 The conduction band lies along the Fermi level. Both conduction and valence bands consist of major sulfur p orbitals, s orbitals, and minor sulfur d orbitals that are moderately dispersed along all of the high-symmetry directions in the Brillouin zone (Figure S7). The valence band maximum (VBM) of α-S was determined to be 1.63 eV, which is 0.37 eV smaller than that of anatase TiO2, suggesting that α-S has a VBM that is higher than that of anatase TiO2, which can help in absorption of visible light as well as in reduction of charge recombination (Figures S6 and S8). Highly separated holes (due to reduction of charge recombination) can be effectively consumed by the sacrifical agent methanol. The photocatalytic efficiency of titania photocatalysts in water splitting reactions increases with the use of methanol as a sacrificial agent, due to consumption of generated valence band holes by methanol.23 The O−Ti−O molecule was adsorbed onto the S8 (111) plane most favored by α-sulfur as it is regarded as an elongated fcc lattice with a c axis approximately twice as long as the a and b axes (Figure S5).24 The adsorbate molecule created additional density of states in the band gap of α-sulfur that can facilitate faster electronic excitation in visible light, as well as a longer lifetime of excited electrons by reducing the level of recombination of generated electron−hole pairs (Figure 10). Deposition of the

absence of light (Figure 11d). Such observations can open ways to a new class of transparent battery materials with a reduced discharge rate and charging/discharging potentials under visible light. Cyclic voltammetry under visible light showed an enhanced current in a positive applied potential, suggesting that the photocatalyst is sensitive to visible light in a positive applied potential (Figure 11c). Visible light-irradiated photocatalytic hydrogen generation was performed on the synthesized α-sulfur−TiO2 catalyst in water, and Au was loaded as a co-catalyst to boost hydrogen production. Control experiments showed that no hydrogen was generated in the absence of the photocatalyst, whereas little hydrogen was observed in the absence of visible light, which could be due to background visible light (Table 2). This confirms that hydrogen production was due to the photocatalyst and visible light irradiation. The photocatalyst showed hydrogen generation at a rate of 57.01 μmol g−1 h−1 for a 25 h run time under a CO2 atmosphere, whereas α-sulfur and TiO2 alone showed hydrogen production rates of 0.44 and 0.92 μmol g−1 h−1, respectively, under the same experimental conditions (Figure 12 and Table 2). Higher photocatalytic hydrogen production was attributed to in situ methanol generation due to photocatalytic CO2 reduction under visible light irradiation. To verify this, we performed the same experiment in a N2 atmosphere, in the absence of CO2, where the rate of photocatalytic hydrogen generation was only 3.4 μmol g−1 h−1 (Table 2). The in situ-generated methanol in the presence of CO2 was continuously being consumed. The rate of hydrogen production increased with an increase in the rate of in situ methanol generation and decreased with a decrease in the rate of in situ methanol generation (Figure 13). This indicates that the rate of hydrogen production was dependent on the rate of in situ methanol generation. To determine whether in situ methanol was acting as a sacrificial agent, the same experiment was performed with externally added methanol under a N2 atmosphere. The ex situ methanol as a sacrificial agent was also effective in photocatalytic hydrogen generation with a rate of 34.07 μmol g−1 h−1, but in situ methanol was around 165 times more effective as a sacrificial agent than ex situ methanol was (Figure 14). Vigorus stirring of the reaction mixture did not produce any change in methanol production rates. The photocatalytic efficiency was increased in the case of in situ methanol because of the higher probability of the sacrificial agent (methanol) generated on the catalyst surface finding the reaction sites, unlike in the case of ex situ methanol where it has to diffuse across the solvent molecules to reach and be adsorbed at the reaction sites. To check that hydrogen is not being evolved from lowtemperature precurser impurities, the photocatalyst was dried at 220 °C. The obtained photocatalyst showed a hydrogen production rate of 90.59 μmol g−1 h−1 vs a rate of 115.89 μmol g−1 h−1 for the photocatalyst dried at 120 °C with a 10 h run time (Figure 15). The decrease in hydrogen production rate could be attributed to a change in the elemental sulfur form that was observed as a change in the color of the photocatalyst after drying at 220 °C (Figure S10). The activity of the photocatalyst decreased after each cycle because of the formation of polymeric sulfur, which was observed as a weight loss at 410 °C during TGA of the used photocatalyst after the third run (Figures 2 and 16) and deposition of methoxy species on the surface of the used photocatalyst as observed in FT-IR spectra (Figure S12). Such a change in the form of sulfur from elemental to polymeric

Figure 10. Total density of states (DOS) of the O−Ti−O molecule adsorbed on α-sulfur.

titania layer on α-sulfur caused the conduction band shift from 0.0 eV (α-sulfur) to −0.5 eV, which is very suitable for hydrogen generation potential (−0.41 eV) for a one-step photoexcitation system (Figure 10).25 The valence band of O− Ti−O-adsorbed α-sulfur shifted from 3.0 eV (α-sulfur) to 2.0 eV. The overall band gap from DOS was observed to be 2.5 eV, which matches experimental values well. To evaluate efficient charge separation under visible light on the photocatalyst, we determined the transient photocurrent in the presence and absence of light. In the presence of visible light, the current increased from 59 nA to 61 μA in the absence of visible light (Figure 11a,b). Thus, the photocatalyst increased the current density in the presence of visible light (Figure 11c). The photocatalyst reduced the discharge rate in the presence of visible light. The charging voltage and discharging voltage were reduced to 0.1 and −0.45 V, respectively, in the presence of light compared to 0.25 and −1.15 V, respectively, in the 6741

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Figure 11. Electrochemical profile of the S8−TiO2 (40)−Au photocatalyst (a) in absence of visible light and (b) in the presence of visible light. (c) Cyclic voltammetry in the absence and presence of visible light. (d) Charge−discharge profile in the absence and presence of visible light.

Table 2. Controlled Photocatalytic Hydrogen Evolution and Methanol Production Experiments entry

catalyst

reaction precursor

mg of catalyst

visible light illumination

T (h)

1 2 3 4 5 6 7

Au−S8−TiO2 Au−S8−TiO2 Au−S8−TiO2 none S8 TiO2 S8−TiO2

CO2 N2 CO2 CO2 CO2 CO2 CO2

100 100 100 none 100 100 100

yes yes no yes yes yes yes

25 25 25 25 25 25 25

a

H2 production (μmol g−1 h 57.01 3.41 4.84 NDa 0.44 0.92 0.44

−1

)

methanol production (μL/g) 74.3 NDa NAb NDa NDa NAb NAb

Not detected. bNot analyzed.

Figure 12. Photocatalytic in situ sacrificial hydrogen production of the S8−TiO2 (40)−Au photocatalyst (1.2 wt %) in a CO2 atmosphere.

Figure 13. Hydrogen evolution and methanol production rate under a CO2 atmosphere for the S8−TiO2 (40)−Au photocatalyst.

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affected the band structure of the composite, resulting in an inability to absorb the visible light and therefore reducing the photocatalytic efficiency after each run. The activity of the photocatalyst decreased after the first cycle because of the formation of polymeric sulfur, which could be ascribed to agglomeration of sulfur. This could be prevented by using less sulfur in the composite. The photocatalyst with a lower S content [S8−TiO2 (30)−Au], for which the formation of polymeric sulfur was inhibited, due to better wrapping of S8 by titania, was also used for cycling experiments. The photocatalyst showed repeatable stable activity after a regeneration and recycling study, proving the stability of the catalyst after light irradiation (Figure S13a,b). The recycled catalyst had a hydrogen production rate slightly higher than that of the fresh catalyst, which may be attributed to rearrangement of Au nanoparticles during catalyst regeneration, as also reported previously (TEM image).26,27 In situ reaction monitoring using infrared spectroscopy revealed the formation of methanol in the reaction mixture (Figure 17). The peak that corresponds to methanol was

Figure 14. Comparison of hydrogen production with the S8−TiO2 (40)−Au photocatalyst (1.2 wt %) for in situ methanol and externally added methanol.

Figure 15. Photocatalytic hydrogen production of the S8−TiO2 (40)− Au photocatalyst dried at 120 and 220 °C.

Figure 17. In situ FT-IR profile of the production of methanol from photocatalytic CO2 reduction for the S8−TiO2 (40)−Au photocatalyst: (a) O−H stretching mode of CH3OH and (b) H−C (sp3) mode of CH3OH.

assigned from the spectra of pure methanol dissolved in water at a wavenumber of 3300 cm−1 (Figure S11). In the in situ FTIR spectra, the peak at 3300 cm−1 was assigned to -OH stretching bending of methanol, the peak at 2900 cm−1 was assigned to sp3 C−H stretching bending of methanol, and the weak band at ∼2800 cm−1 could be attributed to formaldehyde (Figure 17). We also observed peaks around 3300 cm−1 that could be attributed to other chemical forms of reduced CO2 or products of methanol after reaction with generated holes. The production of methanol occurred in periodic intervals because of its role as a sacrificial agent, which confirmed the result of GC-FID for methanol production that changed with time intervals due to the consumption as a sacrificial agent in the hydrogen production reaction.

Figure 16. Cyclability of photocatalytic hydrogen production of the S8−TiO2 (40)−Au photocatalyst (1.2 wt %) in a CO2 atmosphere.

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ACS Sustainable Chemistry & Engineering

photocatalytically reduced to methanol in the reaction. In situ, in operando FT-IR studies showed spectra with weak bands at ∼2800 cm−1 that could be attributed to formaldehyde. This indicates that formaldehyde could be an intermediate in the photocatalytic reduction of CO2 to methanol. DFT calculations showed the creation of additional density of states in the band gap of titania, resulting in the absorption of visible light. A gold (Au) co-catalyst allowed faster electron utilization resulting in hydrogen evolution.28 Concluding Remarks. The remarkably high rate of photocatalytic hydrogen production of the gold-deposited composite can be attributed to the shift of the conduction band to an energy potential needed for hydrogen production as well as creation of additional density of states between the valence band and conduction band. Further in situ methanol production allowed better utilization of methanol as a sacrificial agent than of ex situ methanol. Continuous generation of in situ methanol allowed its continuous utilization as a sacrificial agent, resulting in continuous hydrogen production. The photocatalyst with a higher S8 content (40 wt %) was not active in multiple cycles because of a change in the form of elemental sulfur into polymeric sulfur, but the photocatalyst with a reduced S8 content (30 wt %) was active in multiple cycles, after regeneration, as the elemental form of sulfur was retained because of the better wrapping by the titania layer, thus proving the stability of the catalyst after light irradiation.

Figure 18 and Scheme 1 explain the mechanism for hydrogen production using in situ methanol as a sacrificial agent. Upon

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Figure 18. Proposed mechanism for photocatalytic hydrogen production for the S8−TiO2 (40)−Au photocatalyst using methanol generated in situ as a sacrificial agent.

ASSOCIATED CONTENT

S Supporting Information *

Scheme 1. Proposed Reaction Mechanism for Hydrogen Production for the Au/S8−TiO2 Photocatalyst

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00996. DFT calculation method, lamp setup, calibration curve, XPS spectra, DFT molecule structures, band structures and density of states, FT-IR spectra, hydrogen evolution spectra, and SEM images (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. irradiation with visible light on the designed photocatalyst, electrons become excited to the conduction band by generating holes in the valence band because the valence of α-sulfur is higher than that of titania. The generated electron−hole pairs are stabilized with a longer lifetime due to the created additional density of states in the band gap. The generated holes are consumed by water molecules, which produces H+. The generated H+ atoms are then converted into hydrogen via the consumption of conduction band electrons through gold nanoparticles. Atmospheric CO2 is converted into methanol via the consumption of generated H+ and conduction band electron. Thus, in situ-generated methanol consumes omitted holes, resulting in a higher rate of hydrogen production. The FT-IR spectrum of as-prepared S8−TiO2 (40)−Au and S8−TiO2 (40)−Au−used photocatalysts (after reaction) was measured as shown in Figure S12. The characteristic peaks at approximately 570, 1640, and 3400 cm−1 can be ascribed to TiO2. Figure S12 shows that after the first cycle, the exposed catalyst surface is saturated with methoxy species due to dissociative methanol adsorption as indicated by bands at 1100 and 1070 cm−1 corresponding to type I and II methoxy species, respectively.23 This indicates that the CO2 molecule was

ORCID

Anil Kumar Sinha: 0000-0003-2844-7368 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.Y. acknowledges CSIR, New Delhi, for a junior research fellowship. REFERENCES

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