Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Controlled Synthesis of CuS/TiO2 Heterostructured Nanocomposites for Enhanced Photocatalytic Hydrogen Generation through Water Splitting Moumita Chandra, Kousik Bhunia, and Debabrata Pradhan* Materials Science Centre, Indian Institute of Technology, Kharagpur, 721302 West Bengal, India S Supporting Information *
ABSTRACT: Photocatalytic hydrogen (H2) generation through water splitting has attracted substantial attention as a clean and renewable energy generation process that has enormous potential in converting solar-to-chemical energy using suitable photocatalysts. The major bottleneck in the development of semiconductor-based photocatalysts lies in poor light absorption and fast recombination of photogenerated electron−hole pairs. Herein we report the synthesis of CuS/TiO2 heterostructured nanocomposites with varied TiO2 contents via simple hydrothermal and solution-based process. The morphology, crystal structure, composition, and optical properties of the as-synthesized CuS/TiO2 hybrids are evaluated in detail. Controlling the CuS/TiO2 ratio to an optimum value leads to the highest photocatalytic H2 production rate of 1262 μmol h−1 g−1, which is 9.7 and 9.3 times higher than that of pristine TiO2 nanospindles and CuS nanoflakes under irradiation, respectively. The enhancement in the H2 evolution rate is attributed to increased light absorption and efficient charge separation with an optimum CuS coverage on TiO2. The photoluminescence and photoelectrochemical measurements further confirm the efficient separation of charge carriers in the CuS/TiO2 hybrid. The mechanism and synergistic role of CuS and TiO2 semiconductors for enhanced photoactivity is further delineated. thus exposed facets of TiO2,10 producing mesoporous structures with enhanced surface area,11 surface modification,12,13 band gap engineering by doping with a transition metal14 or nonmetal,15 and forming heterostructures with other semiconductors.16 Although plasmonic coupling with noble metals such as Au, Pt, and Pd increases the light harvesting ability through surface plasmon resonance and also improves the charge carrier separation,17,18 the cost and scarcity of noble metals have been the limiting factors for their wide-scale use. The morphology effect was demonstrated by Zhang et al. with four-truncated bipyramid TiO2 single-crystal morphology, which shows excellent hydrogen generation activity from ethanol−water solution.10 Daghrir et al. reviewed the performance of modified TiO2 for the enhanced photocatalytic activity.13 Hou et al. reported the synthesis of a ternary heterostructure of TiO2 for enhanced visible light-driven H2 generation.19 Xie et al. reported a comprehensive overview on graphene-based materials for hydrogen generation from lightdriven water splitting.20 Similarly, although the band gap of TiO2 can be reduced by metal/nonmetal doping to increase the solar light absorption, it also creates new recombination centers and lowers the efficiency by alternative recombination of the charge carriers.21 Thus, integration of TiO2 with another lowcost narrow band gap semiconductor such as metal
1. INTRODUCTION Rapid depletion of fossil fuels and increasing energy demand are becoming major concerns of the present world. The development of alternate clean and renewable energy sources is thus an area that needs significant attention from the scientific community.1 Solar light-assisted water splitting is a promising strategy for solar fuel production, named “artificial photosynthesis”.2 Hydrogen (H2) generation through solar water splitting is an attractive approach since water and solar energy are not only the most abundant on the earth but also environmentally friendly and economically viable. A pioneering report by Fujishima and Honda on photoelectrochemical (PEC) water splitting on TiO2 photoelectrode in 1972 has attracted wide and persistent research interest for the development of efficient and stable photocatalysts.3 Numerous efforts have been made to develop efficient photocatalysts for water splitting in the past decades.4−6 As a classical photocatalyst, TiO2 has been extensively researched for photocatalytic H2 production because of its low cost, high abundance, high chemical stability, nontoxicity, and suitable reduction potential for proton reduction.7,8 However, the efficiency of TiO2 is mainly restricted by means of its wide band gap (Eg = 3.2 eV for anatase TiO2) and rapid recombination of the charge carriers. Thus, efforts have been directed to design and sensitize the TiO2 to improve its photocatalytic performance. The major strategies employed to enhance the photocatalytic activity of TiO2 include plasmonic coupling,9 tuning the morphology and © XXXX American Chemical Society
Received: January 31, 2018
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DOI: 10.1021/acs.inorgchem.8b00283 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
The precipitate formed was washed several times with ethanol and water and dried at 60 °C for 4 h. 2.2.3. Synthesis of CuS/TiO2 Nanocomposites. In a typical synthesis procedure, the as-synthesized TiO2 NSPs powder of chosen quantity was added to a mixed solvent of 10 mL of water and 30 mL of EG and sonicated for 30 min to form a homogeneous dispersion. Then 341 mg of CuCl2·2H2O (2 mmol) and 496 mg of Na2S2O3·5H2O (2 mmol) were added to the preceding solvent. The resulting solution was stirred until a yellowish green solution was obtained. The final solution was transferred to a 100 mL round-bottom flask placed in an oil bath held at 70 °C and heated for 4 h with constant stirring. After 4 h, the flask was taken out of the oil bath and cooled naturally at room temperature. The precipitate formed was washed several times with ethanol and water. Finally, the product was dried at 60 °C for 4 h. In order to investigate the effect of different ratio of CuS and TiO2 in the nanocomposite on photocatalytic water splitting, a series of samples was prepared by adding different quantities of TiO2 powder, i.e., 800 (10 mmol), 480 (6 mmol), 400 (5 mmol), and 320 mg (4 mmol), while keeping the CuS precursor concentration (2 mmol) fixed. The resulting CuS/TiO2 nanocomposites were designated as per the mole ratio of CuS to TiO2 as CTr, where r is the mole ratio (r = 0.2, 0.3, 0.4, 0.5). 2.3. Material Characterization. The surface morphology and microstructure of the products were examined by a MERLIN (Zeiss) field emission scanning electron microscope (FESEM) and a TECNAI G2 (FEI) transmission electron microscope (TEM) operated at 200 kV, respectively. The crystallographic structure of the products was obtained by a PANalytical high-resolution X-ray diffractometer (HRXRD) operated at 40 kV and 30 mA using Cu Kα X-rays. The Raman studies were performed with a Horiba Jobin Yvon T64000 Raman spectrometer using an argon−krypton mixed-ion gas laser as the excitation source (Spectra-Physics, USA). The UV−vis diffuse reflectance spectroscopy (UV−vis DRS) and photoluminescence (PL) study were carried out with a Cary 5000 UV−vis spectrophotometer (Agilent Tech.) and a LS 55 spectrophotometer (PerkinElmer), respectively. The surface composition of the samples was studied by X-ray photoelectron spectroscopy (XPS) with a PHI5000 Versa Probe II XPS Microprobe with a monochromatic Al Kα source (1486.6 eV). The effective Brunauer−Emmett−Teller (BET) surface area was measured with an iQ 2 Autosorb (Quantachrome, USA) ChemBET analyzer. 2.4. Photocatalytic H2 Production. The photocatalytic H2 production experiment was carried out in a 300 mL three-necked round-bottom flask sealed with a rubber septum. A 300 W xenon arc lamp (ISS instrument, USA) was used as a light source to initiate the photolysis reaction, positioned 20 cm away from the reactor. In a typical experiment, 50 mg of the catalyst was dispersed in 100 mL of aqueous solution containing 0.35 M Na2S and 0.25 M Na2SO3 (catalyst to solvent ratio of 0.5 g/L), which acts as a sacrificial reagent to capture photogenerated holes during the photocatalysis. The system was bubbled with nitrogen gas for 30 min prior to light irradiation to remove the dissolved oxygen and ensure an anaerobic condition. The H2 generation activity was also studied under visible light irradiation by placing a 420 nm cutoff filter on the light path. The evolved hydrogen gas was quantitatively measured using a gas chromatograph (GC) (Agilent Technologies 7890B GC system). 2.5. Photoelectrochemical (PEC) Study. PEC measurement was carried out with a CHI 760D electrochemical workstation (CH Instruments, Inc., USA) using a three-electrode configuration in 0.5 M Na2SO4 electrolyte and a 300 W xenon arc lamp as irradiation source. The catalyst-coated indium−tin−oxide (ITO) glass substrate, Pt wire, and saturated calomel electrode (SCE) were used as working, counter, and reference electrode, respectively. The working electrode was prepared by coating a slurry of the as-synthesized catalyst on a 1 cm2 area of ITO-glass substrate and drying in a vacuum oven at 100 °C for 1 h. The slurry was prepared by ultrasonic dispersion of 5 mg of the assynthesized CuS/TiO2 nanocomposite or pristine TiO2 in 1 mL of isopropanol with 20 μL of Nafion solution (Aldrich, 5% Nafion) as a binder. The linear sweep voltammogram, transient photocurrent (i−t) response, electrochemical impedance spectroscopic (EIS) analysis, and
chalcogenide which would enhance the light absorption phenomenon and promote charge carrier separation is appealing.22−24 In this regard, among the metal chalcogenides, in recent years, significant performance has been demonstrated by incorporation of CdS, which has been proved to be an efficient cocatalyst for TiO2 for H2 production through water splitting.25 However, CdS is toxic and an environmental pollutant. It is thus desirable to explore other semiconductors which not only sensitize TiO2 but also are environmentally friendly. CuS has recently been proved to be an alternative cocatalyst for H2 generation,26 which is nonhazardous, cheap, and abundant. Zhang et al. reported that CuS loading significantly enhances the performance of ZnS for photocatalytic H2 generation.27 Recently, a CuS/TiO2 system has been considered to be an effective photocatalyst for pollutant degradation.28 A CuS/TiO2 heterojunction forms a type-II band gap configuration that facilitates separation of photoinduced charge carriers. In type-II configuration, the conduction band (CB) and valence band (VB) positions are staggered between two semiconductors.29 Upon excitation, photogenerated electrons in CuS move to the CB of TiO2 as the CB band energy of TiO2 is lower than that of CuS and photogenerated holes of TiO2 move to the VB of CuS, thus facilitating charge carriers separation. Herein, we report the synthesis of CuS/TiO2 heterostructure nanocomposites with varying percentage of TiO2 and their improved photocatalytic and PEC activity toward H 2 generation. The H2 production activity of the nanocomposites is also compared by irradiating both the ultraviolet and the visible (UV−vis) and only visible light. At an optimal ratio of CuS to TiO2, the H2 generation activity was found to be the highest because of the synergistic effect of both materials that contribute toward maximum light absorption and charge carrier separation. A plausible mechanism on the photocatalytic H2 evolution from the as-prepared heterostructure nanocomposite is discussed in detail.
2. MATERIAL AND METHODS 2.1. Chemicals. Titanium tetraisopropoxide (TTIP) (97%) from Sigma-Aldrich, glacial acetic acid (AcOH) (99−100%), copper chloride dihydrate (CuCl2·2H2O), sodium thiosulfate pentahydrate (Na2S2O3·5H2O), and ethylene glycol (EG) were purchased from Merck, India. All of the above reagents were analytical grade and used without any further purification. 2.2. Synthesis. 2.2.1. Synthesis of TiO2 Nanospindles (NSPs). TiO2 NSPs was synthesized using the hydrothermal method. In a typical synthesis, 1 mL (3.3 mmol) of TTIP was added dropwise to a beaker containing 20 mL of AcOH and stirred for 20 min. The resulting milky white solution was transferred to a 50 mL Teflon-lined stainless steel autoclave and heated at 180 °C for 20 h in a muffle furnace. Then the autoclave was cooled to room temperature under ambient condition, and the ensuing white precipitate was collected by centrifuging. The precipitate was washed with distilled water and ethanol several times and dried at 60 °C for 24 h. The dried product was finally calcined at 400 °C under air for 2 h. 2.2.2. Synthesis of CuS Nanoflakes (NFs). CuS NFs was synthesized using a solution-based method reported earlier.30 Typically, 10 mL of water and 30 mL of EG were mixed and stirred for 5 min. Then 341 mg of CuCl2·2H2O (2 mmol) and 496 mg of Na2S2O3·5H2O (2 mmol) were added to the preceding solution. The resulting solution was stirred until it turned yellowish green, then transferred to a 100 mL round-bottom flask, placed in an oil bath held at 70 °C, and heated for 4 h with constant stirring. Then the flask was taken out of the oil bath and cooled naturally at room temperature. B
DOI: 10.1021/acs.inorgchem.8b00283 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Mott−Schottky experiments were carried out with the same electrode configuration in 0.5 M Na2SO4. The EIS was carried out in the frequency domain from 1 Hz to 100 kHz and ac voltage amplitude of 5 mV.
3. RESULTS AND DISCUSSION 3.1. Morphology and Microstructure. The surface morphology of the as-synthesized catalysts was studied using a FESEM. Figure 1a shows a representative FESEM image of Figure 2. (a, c, e) TEM and (b, d, f) HRTEM images of (a, b) TiO2 NSPs, (c, d) CuS NFs, and (e, f) CT0.4 nanocomposite. (Inset in a) Magnified single NSP. (Insets in b and d) SAED patterns of the corresponding samples.
CuS NFs shows a randomly scattered spot SAED pattern (inset in Figure 2d), which confirms the crystalline nature of the irregularly arranged CuS NFs. Figure 2e and 2f shows the TEM and HRTEM images of the CT0.4 nanocomposite. The uniform distribution of CuS NFs and TiO2 NSPs in the nanocomposite (CT0.4) is evident from the TEM image (Figure 2e). Furthermore, HRTEM (Figure 2f) also shows the interface of TiO2 and CuS, indicating close contact with a lattice spacing of 0.35 and 0.301 nm corresponding to the 101 (TiO2) and 102 (CuS) crystal planes, respectively. More importantly, the interface of TiO2 and CuS appears to be well bonded as apparent in the HRTEM image (Figure 2f). 3.2. Structural Property. The powder X-ray diffraction (PXRD) technique was used to investigate the phase and crystal structure of the as-synthesized materials. Figure 3
Figure 1. FESEM images of (a) TiO2 NSPs, (b) CuS NFs, and (c) CT0.4 nanocomposite, and (d) EDX spectrum of CT0.4 nanocomposite. (Inset in a) Magnified image of the corresponding sample.
TiO2 nanoparticles in the shape of nanospindles (NSPs) obtained hydrothermally using TTIP and AcOH as precursors at 180 °C for 20 h. These NSPs were found to be uniform in shape and size of 250−300 nm length and 100−150 nm width. The inset in Figure 1a shows a magnified image of TiO2 NSPs, which appeared to be composed of finer nanoparticles. On the other hand, CuS was synthesized by the precipitation method in the shape of nanoflakes (NFs) as shown in Figure 1b. The length/width of these nanoflakes was measured to be CT0.2 > CT0.5 > CT0.3 > CT0.4 nanocomposites, which is well matched with the photocatalytic H2 generation activity and is just opposite of the photocatalytic H2 generation rate. The PL spectra (Figure S4, SI) were also collected near the band edge of CuS at an excitation wavelength of 500 nm for all of the composites and CuS NFs. PL spectra show an intense peak near 566 nm corresponding to the band gap of CuS. For all of the
Table 2. Calculated CB and VB Positions of TiO2 NSPs and CuS NFs sample
electronegativity
band gap
CB
VB
TiO2 NSPs CuS NFs
5.81 eV 4.85 eV
3.15 eV 2.16 eV
−0.27 −0.73
2.88 1.43
supporting the type-II band gap configuration.49 Therefore, electrons move from the CB of CuS to the CB of TiO2 and holes move from the VB of TiO2 to the VB of CuS as the VB edge of TiO2 is greater (more positive) than that of CuS. This resulted in the separation of electrons and holes through the CuS/TiO2 interface as there was an intimate contact between TiO2 NSPs and CuS NFs (as confirmed from the HRTEM image, Figure 2f). Electrons can thus efficiently reduce protons to H2 at the surface of TiO2 NSPs, and photogenerated holes get scavenged by sulfide−sulfite solution at the CuS surface, reducing the chances of recombination with electrons. On the basis of the above results, a plausible mechanism for photocatalyic H2 generation can be written in the following equations CuS + hν → CuS(e− + h+) −
(9) −
CuS(e ) + TiO2 → TiO2 (e ) −
2e (TiO2 ) + H 2O → H 2 + 2OH
(10) −
(11)
2h+(CuS) + SO32 − + 2OH− → SO4 2 − + H 2O
(12)
2S2 − + 2h+ → S2 2 −
(13)
S2 − + SO32 − → S2 O32 −
(14)
The photogenerated electrons on the CB of TiO2 (eq 11) reduce water molecules and produce H2. Meanwhile, holes react with SO32− and S2−, producing SO42− and S22−, respectively (eqs 12 and 13). Combination of S2− and SO32− into colorless S2O32− (eq 14) does not cause any effect on the photocatalyst for light absorption. The above proposed mechanism is shown schematically in Figure 8b. To demonstrate the stability and recyclability, CT 0.4 nanocomposite was used for H2 production for several cycles (for 35 h) as shown in Figure 9. After each cycle of 5 h, the reaction vessel was purged by N2 and the experiment was performed under the same conditions. No significant decrease in H2 generation activity was observed for up to three cycles.
Figure 9. Stability test of CT0.4 nanocomposite for H2 generation under irradiation of a 300 W Xe arc lamp (UV−vis) for seven cycles. Each cycle was run for 5 h. After the fifth cycle, fresh sacrificial solution was used. G
DOI: 10.1021/acs.inorgchem.8b00283 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 10. (a) FESEM image, (b) EDX spectrum, and (c) XRD pattern of CT0.4 after 7 cycles of photocatalytic H2 generation.
Figure 11. Photoluminescence spectra of TiO2 NSPs and CuS/TiO2 nanocomposites.
synthesized composites, PL emission intensity decreases due to a lower charge recombination rate and the order of emission intensity follows CuS NFs > CT0.2 > CT0.5 > CT0.3 > CT0.4. The lowest PL intensity for CT0.4 nanocomposite suggests efficient separation, lower recombination, and migration of charge carriers through the formation of type-II band structure by TiO2 NSPs and CuS NFs, which in turn contributes to the enhanced photoactivity. 3.11. Photocurrent Measurement. To further confirm the efficient charge carrier separation in CuS/TiO2 nanocomposite, an electrochemical study was undertaken through linear sweep voltammogram (LSV), chronoamperometry, and electrochemical impedance spectroscopy (EIS) under illumination in aqueous 0.5 M Na2SO4 as electrolyte. The linear sweep voltammogram with CT0.4 electrode shows a much higher photocurrent than that of pristine TiO2 NSPs (Figure S5, SI) under illumination. This confirmed an enhanced charge separation in the nanocomposite. Electrochemical transient photocurrent responses were further recorded at 0.6 V versus SCE with CT0.4 nanocomposite as anode for several on−off cycle of irradiation and compared with pristine TiO2 NSPs (Figure 12a). The CT0.4 nanocomposite delivers 3.6 times higher photocurrent than that of pristine TiO2 NSPs, confirming the higher light absorption and better charge separation in the former. In addition, the photocurrent response was found to be reproducible on irradiation, suggesting the high stability of the catalyst. To get an idea on superior charge separation and transport in the composites under irradiation, an EIS study was performed. Figure 12b shows the Nyquist plots of CT0.4 nanocomposite and pristine TiO2 NSPs in the dark and under irradiation. The diameter of the semicircle arc represents the charge transfer resistance (Rct) of the respective electrode. An equivalent circuit diagram as shown in the inset of Figure 12b consists of solution resistance (Rs), Rct, constant phase element (CPE), and Warburg impedance (Zw). A smaller Rct value in the Nyquist plot for CT0.4 (4.4 × 104 Ω) as compared to pristine TiO2 NSPs (6.6 ×
Figure 12. Photoelectrochemical properties of pristine TiO2 NSPs and CT0.4 nanocomposite under dark and illumination. (a) Transient photocurrent response (i−t) at 0.6 V versus SCE with light chopping and (b) Nyquist plots. (Inset) Equivalent circuit diagram. Mott− Schottky plots of pristine (c) TiO2 NSPs and (d) CuS NFs.
104 Ω) under dark condition suggests better charge transport property of the composite at the electrode/electrolyte interface. Under light irradiation, a significant decrease in the Rct value was observed and also lower for CT0.4 (1441 Ω) than that of pristine TiO2 NSPs (5860 Ω). Thus, the charge transport property follows the order CT0.4 nanocomposite (light) > TiO2 NSPs (light) > CT0.4 nanocomposite (dark) > TiO2 NSPs (dark). The higher light absorption, lower band gap, efficient charge separation, and superior charge transport properties of semiconductor heterostructure catalyst play important roles in its enhanced photocatalytic and photoelectrocatalytic H2 generation activity than that of their individual counterparts. To further confirm the major charge carriers in the assynthesized pristine TiO2 NSPs and CuS NFs, the Mott− Schottky measurements were performed in aqueous 0.5 M Na2SO4 as electrolyte. Figure 12c and 12d shows the Mott− Schottky plots of pristine TiO2 NSPs and CuS NFs, which show n-type and p-type behavior, respectively, in accord with the earlier reports.14,52
4. CONCLUSIONS In conclusion, a series of CuS/TiO2 heterostructured nanocomposites is synthesized and their photocatalytic H 2 generation activity is demonstrated in the present work. The growth of CuS NFs on TiO2 NSPs and formation of CuS/TiO2 H
DOI: 10.1021/acs.inorgchem.8b00283 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
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heterostructure effectively facilitates the separation of photoinduced charge carrier through the heterostructure interface and enhances the photocatalytic activity of TiO2 NSPs. An optimal mole ratio of CuS to TiO2 at 0.4 delivers the highest H2 production rate of 1262 μmol h−1 g−1, which is 9.7 times higher than that of pristine TiO2 NSPs. Further PL and photocurrent measurement study establishes the efficient separation, transportation, and reduced recombination of the charge carriers. This work demonstrates a highly promising strategy to fabricate semiconductor heterostructures for enhanced photoactivity for H2 generation instead of using expensive noble metal as cocatalyst.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00283. Tauc band gap, photocatalytic hydrogen generation activity, and N2 adsorption−desorption isotherm plots, PL spectra of CuS NFs and all synthesized composites near band age of CuS, LSV plots for TiO2 and CT0.4under illumination (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Debabrata Pradhan: 0000-0003-3968-9610 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Science and Engineering Research Board (SERB), New Delhi, India, through the research grant EMR/2017/000697. The authors acknowledge the DST-FIST funded facility at the Department of Physics for XPS measurement.
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REFERENCES
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DOI: 10.1021/acs.inorgchem.8b00283 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
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DOI: 10.1021/acs.inorgchem.8b00283 Inorg. Chem. XXXX, XXX, XXX−XXX
