- Email: [email protected]
Fabrication of CdS nanowires decorated with TiO2 nanoparticles for photocatalytic hydrogen production under visible light irradiation
international journal of hydrogen energy 33 (2008) 5975–5980
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Fabrication of CdS nanowires decorated with TiO2 nanoparticles for photocatalytic hydrogen production under visible light irradiation Jum Suk Janga, Hyun Gyu Kimb, Upendra A. Joshia, Ji Wook Janga, Jae Sung Leea,* a
Eco-friendly Catalysis and Energy Laboratory (NRL), Department of Chemical Engineering, Pohang University of Science and Technology, San 31, Hyoja-dong, Pohang 790-784, Republic of Korea b Busan Center, Korea Basic Science Institute (KBSI), Busan 609-735, Republic of Korea
article info
abstract
Article history:
A CdS/TiO2 composite photocatalyst consisting of one-dimensional CdS nanowire (NW)
Received 28 May 2008
with a high crystallinity decorated with nanosized TiO2 particles (NP) was fabricated by
Received in revised form
solvothermal method and sol–gel synthesis. The new configuration photocatalyst exhibi-
26 July 2008
ted higher rate of hydrogen production than that of single CdS NW under visible light
Accepted 26 July 2008
irradiation (l 420 nm) from water containing sulfide and sulfite ions as hole scavengers.
Available online 9 September 2008
Physicochemical properties of CdS NW/TiO2 NP composite photocatalysts were investigated together with the effect of the mole ratio of TiO2 and CdS for photocatalytic hydrogen
Keywords:
production from water under visible light irradiation.
CdS nanowires
ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights
TiO2 nanoparticles
reserved.
Composite H2 evolution Photocatalysis
1.
Introduction
Composite photocatalysts have been recognized as an attractive configuration because of their potential applications in solar cells, water splitting, organic decomposition and electrochromic devices [1–9]. In particular, CdS has ideal band gap energy (2.4 eV) and band positions that can drive both oxidation and reduction of water under visible light irradiation. CdS has been combined with other materials such as ZnO, TiO2 and LaMnO3 or intercalated into layered compounds [10–16] in attempt to improve the photo-efficiency and photostability. Among several configurations, a composite system consisting of CdS nanoparticles and TiO2 nanotubes has been reported in order to maximize the interfacial areas and
physical or chemical contact between the two compounds [17–23]. Li et al. [17] reported TiO2 nanotube-supported CdS and ZnS heterostructure synthesized via a simple wet chemical process and investigated the photocatalytic performance for decomposition of methyl orange. Kim et al. [18] synthesized a new composite material consisting of CdS nanoparticles and TiO2 nanotube and employed a bi-functional organic molecule to bind CdS strongly to TiO2 surface. Chen et al. [21] synthesized CdS nanoparticle-modified TiO2 nanotube-array photoelectrodes via anodizaiton and electrodeposition method. Kim et al. [18] fabricated ordered TiO2 based composite (TiO2–CdS) nanotube array using a layer-by-layer (LBL) assembly process. To the best of our knowledge, there was no report on the fabrication of CdS/TiO2 composite based on CdS nanowire and
* Corresponding author. Tel.: þ82 54 279 2266; fax: þ82 54 279 5528. E-mail address: [email protected] (J.S. Lee). 0360-3199/$ – see front matter ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.07.105
5976
international journal of hydrogen energy 33 (2008) 5975–5980
TiO2 nanoparticles, nor its photocatalytic activity for hydrogen production under visible light. Recently, we have found that the fabrication method of these composite photocatalysts is critical for their performance [5]. We also synthesized quasi-one-dimensional CdS nanowires via a solvothermal route in ethylenediamine as single solvent and evaluated their photoelectrochemical property as well as photoactivity for hydrogen production under visible light irradiation (l 420 nm) [24]. CdS nanowires with high crystallinity and large surface area are a good candidate as an active material for composite configuration. In this work, we fabricate a composite photocatalyst consisting of CdS nanowire (NW) with a high crystallinity decorated with TiO2 nanoparticles (NP), and evaluate its photoactivity for hydrogen production from aqueous solution containing hole scavengers such as sulfide and sulfite under visible light irradiation (l 420 nm).
2.
Experimental
2.1.
Materials preparation
In a typical procedure, 16.2 mmol of Cd(NO3)2$4H2O and 48.6 mmol NH2CSNH2 was added into a Teflon-lined stainless steel autoclave which had been filled with ethylenediamine to 80% of its capacity. The autoclave was maintained at 160 C for 48 h for solvothermal reaction and allowed to cool to room temperature. The colored precipitate was filtered and washed with absolute ethanol and deionized water to remove the residue of organic solvent [24]. To fabricate the CdS NW/TiO2 NP composite photocatalyst, the obtained 0.5 g CdS NW was stirred in 50 ml isopropyl alcohol and 0.25 g titanium isopropoxide (in a mole ratio of CdS to TiO2 from 0 to 4) and 0.623 g H2O were added drop-bydrop [5,15]. The prepared composite sample was solvothermally treated in an autoclave at 423 K for 24 h to improve the contact between two components and to increase the crystallinity of TiO2. All procedures for catalyst preparation are schematically shown in Fig. 1A.
2.2.
Fig. 1 – (A) A schematic procedure for fabrication of CdS NW/TiO2 NP composite photocatalyst. (B) X-ray diffraction patterns of the samples with different molar ratios of TiO2 in CdS NW/TiO2 NP composite photocatalysts; ([TiO2]/ [TiO2] D [CdS]) (a) 0 (CdS only), (b) 0.20, (c) 0.33, (d) 0.67. All samples were solvothermally treated at 423 K for 24 h in Teflon-lined stainless steel autoclave.
2.3.
Photocatalytic reactions
The photocatalytic reactions were carried out at room temperature under atmospheric pressure in a closed reactor system using an Hg-arc lamp (450 W) equipped with a UV cutoff filter (l 420 nm). Before reaction, 1 wt% of Pt was deposited on photocatalysts by photodeposition method under visible light (l 420 nm). The rate of H2 evolution was determined in an aqueous solution (100 ml) containing 0.1 g catalyst and 0.1 M Na2S þ 0.02 M Na2SO3. The evolved amounts of H2 were analyzed by gas chromatography (TCD, molecular ˚ column and Ar carrier). sieve 5-A
Characterizations
The crystalline phases of the products were determined by powder X-ray diffraction (XRD) on a diffractometer (Mac Science Co., M18XHF) with monochromatic Cu Ka radiation at 40 kV and 200 mA. The optical property was analyzed by UV-Visible diffuse reflectance spectrometer (Shimadzu, UV 2401). The morphology of photocatalysts was investigated by field emission scanning electron microscopy (SEM, Hitachi, S4200) and transmission electron microscope (JEOL JEM 2010F, Field Emission Electron Microscope) operated at 200 kV. The chemical states of sulfur in the samples were determined from X-ray photoelectron spectroscopy measurements (XPS VG Scientific, ESCALAB 220iXL) using Mg Ka radiation (1253.6 eV). The binding energy calibration was performed using C1s peak in the background as the reference energy (284.6 eV). The BET surface area was evaluated by N2 adsorption in a constant volume adsorption apparatus (Micrometrics, ASAP 2012).
3.
Results and discussion
The optimized preparation condition and characterization procedure of CdS nanowires (NW) have been described elsewhere [24]. The CdS NW as a visible light absorber component in composite photocatalyst was solvothermally synthesized at 160 C for 48 h. Fig. 1B shows X-ray diffraction patterns of the samples prepared with different molar concentrations of TiO2 in CdS NW/TiO2 NP composite photocatalysts. When the mole ratio of TiO2 NP to CdS NW was increased, (101) plane of TiO2 anatase phase became clearer around 2q ¼ 25.3 in XRD pattern as noted by a vertical line. The CdS NW showed welldeveloped hexagonal phase both in single and composite photocatalysts and TiO2 was in the anatase phase in the composite photocatalyst. The crystallinity of CdS in CdS NW/ TiO2 NP composite photocatalyst was as good as the single
international journal of hydrogen energy 33 (2008) 5975–5980
Fig. 2 – UV-VIS diffuse reflectance spectra of the samples with different molar ratios of TiO2 in CdS NW/TiO2 NP composite photocatalysts; ([TiO2]/[TiO2] D [CdS]) (a) 0 (CdS only), (b) 0.20, (c) 0.33, (d) 0.67. The edge position of TiO2 is indicated. All samples were solvothermally treated at 423 K for 24 h in Teflon-lined stainless steel autoclave.
CdS NW, considering its concentration (30 wt%) in the composite. The diffuse reflection spectra for these photocatalysts are shown in Fig. 2. The CdS NW photocatalyst showed a sharp
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edge at 520 nm. The spectra of CdS NW/TiO2 NP composite photocatalysts showed a combination of the two spectra coming from CdS NW and TiO2 NP. But, when the mole ratio of TiO2 to CdS is 0.33 or more, the spectra contributed by TiO2 NP in composite was clearly appeared as shown in Fig. 2(c, d). These results are well in accordance with those of XRD patterns. The morphology of CdS NW/TiO2 NP composite photocatalysts was observed by SEM and TEM images as shown in Fig. 3. CdS NW sample solvothermally synthesized at 160 C for 48 h showed the morphology of nanowires with a length of ca. 2–3 mm and a diameter of ca. 50 nm in Fig. 3(a). The dimensions were highly uniform among individual nanowires and the cross-section of nanowire was in a hexagonal shape (not shown here) [24]. For CdS NW/TiO2 NP composite, CdS NW of ca. 2–3 mm was decorated with TiO2 nanoparticles of ca. 10 nm. This clearly defined geometry of nanoparticles in contact with a nanowire is schematically shown in insert of Fig. 3(b). Note that TiO2 particles would form random multiple particle layers on CdS nanowire surface as discussed in our previous works [5,15]. The surface sulfur species was analyzed by XPS before photoreaction as shown in Fig. 4. The S2p peaks indicated that CdS NW and CdS NW/TiO2 NP composite contained SO2 4 as an impurity as well as sulfide (S2) species on its surface. Generally, CdS can be oxidized to CdO and CdSO4 as impurity phases and they can decrease the photoactivity. Especially, CdO inhibits H2 production because the conduction band position of CdO is more positive than the redox potential of Hþ/H2. CdSO4 also decreases visible light absorption of CdS [25]. Also, this surface oxidation may be responsible for the induction period as discussed below. In the previous work, we investigated the location of Pt cocatalysts by TEM images obtained after Pt deposition on
Fig. 3 – SEM images of the samples with different molar ratios of TiO2 in CdS NW/TiO2 NP composite photocatalyst; ([TiO2]/ [TiO2] D [CdS]) (a) 0 (CdS only), (b) 0.20, (c) 0.33, (d) 0.67. All samples were solvothermally treated at 423 K for 24 h in Teflonlined stainless steel autoclave.
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international journal of hydrogen energy 33 (2008) 5975–5980
Fig. 4 – XPS data of S2p of CdS NW and CdS NW/TiO2 NP samples with [TiO2]/[TiO2] D [CdS] ratio of; (a) 0 (CdS only) and (b) 0.2.
bulk CdS-nano TiO2 composite [9]. From TEM images and EDAX analysis, it was confirmed that Pt cocatalysts were randomly located on both nanosized TiO2 and bulk CdS particles. In the present work, we also investigated where the Pt cocatalysts were located. Thus, TEM images were obtained after Pt deposition on CdS NW/TiO2 NP composite. As shown in TEM images in Fig. 5, Pt cocatalysts are randomly located on both nanosized TiO2 (a) as well as CdS nanowires (b). Fig. 6A shows the effect of the mole ratio of [TiO2]/ [TiO2] þ [CdS] for hydrogen production from an aqueous
solution containing 0.1 M Na2S and 0.02 M Na2SO3 as sacrificial reagents under visible light irradiation (l 420 nm). The surface area of CdS NW/TiO2 NP composite linearly increases with increase of the concentration of TiO2. But, the molar concentration of TiO2 in CdS NW/TiO2 NP that showed the highest activity for H2 evolution was determined to be 0.2. Note that this molar ratio for activity has no correlation with the surface area of the composite as shown in Fig. 6A. Thus, an optimum coverage of TiO2 NP decorating CdS NW is needed for effective and fast charge separation of photogenerated electrons and holes and thus to increase the photocatalytic activity. But too high a coverage would hinder the contact of electrolyte and CdS surface responsible for hole scavenging. The time-dependence of H2 production was also measured for [TiO2]/[TiO2] þ [CdS] of (a) 0.0 (CdS only) and (b) 0.2 samples under visible light irradiation with cut-off filter (l 420 nm) as shown in Fig. 6B. The photocatalytic activity of hydrogen evolution was linearly increased with the reaction time for 3 h except for a brief induction period. As discussed above, the oxidation of surface CdS species to CdO or CdSO4 may be responsible for this induction period [5]. In any case, the photocatalyst remains stable under the present reaction conditions. In our previous works [5,15], it was reported that CdS(bulk)/ TiO2 composite photocatalyst showed a high photocatalytic activity for hydrogen production from electrolyte solution containing sulfide and sulfite as sacrificial reagents under visible right irradiation (l 420 nm). The activity was much higher than that of single CdS photocatalyst having a high crystallinity, and those of composite photocatalysts of different configurations, i.e. nano-CdS/bulk-TiO2 or nanoTiO2/nano-CdS. Although CdS NW/TiO2 NP composite did not show a dramatic improvement of photocatalytic activity, the configuration of CdS nanowire decorated with TiO2 nanoparticle led to increase in photocatalytic activity from that of CdS nanowire alone. However, CdS NW/TiO2 NP showed considerably lower activity compared with those for CdS(bulk)/TiO2 NP composite photocatalysts [5,15]. One
Fig. 5 – TEM images that show that location of Pt loaded on TiO2 NP (a) and CdS NW (b).
international journal of hydrogen energy 33 (2008) 5975–5980
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Fig. 6 – (A) Evolution rates of H2 (bars) and specific BET surface areas (curve) for photocatalysts with different [TiO2]/ [TiO2] D [CdS] ratios: (a) 0 (CdS only), (b) 0.20, (c) 0.33, (d) 0.67. (B) Time curve of photocatalytic hydrogen production for [TiO2]/ [TiO2] D [CdS] ratios of (a) 0 and (b) 0.20. All samples were solvothermally treated at 423 K for 24 h in Teflon-lined stainless steel autoclave. Catalysis: 0.1 g loaded 1 wt% Pt. Electrolyte solution: 0.1 M Na2S D 0.02 M Na2SO3. Light Source: 450-W Hg lamp with a cut-off filter (l ‡ 420 nm).
reason for the lower activity for hydrogen production could be lower crystallinity of CdS NW. Although the CdS NW showed a relatively good crystallinity as a nanoscale material as shown in Fig. 1, it was still much less crystalline than bulk CdS prepared at high temperatures. The crystallinity of CdS was found to be the most important variable for high activity [5]. Indeed, it was found that CdS nanowire or composite calcined at 400 C after hydrothermal treatment showed higher rate of hydrogen production than that of uncalcined samples. The high activity of the composite photocatalyst is considered to be due to a fast charge separation. Thus upon initial light absorption, difference in the positions of conduction bands drives photoelectrons generated in CdS NW to surrounding TiO2 nanoparticles as shown in a schematic
model in Fig. 7. The holes remain in CdS NW and react with hole scavengers of sulfide and sulfite ions. In order for this system to function efficiently, quality of individual components as well as the close contact between two compounds should be important.
4.
Conclusions
The CdS NW/TiO2 NP composite photocatalyst based on CdS nanowires with high crystallinity decorated with TiO2 nanoparticles has been successfully fabricated via solvothermal and sol–gel method to develop a highly active photocatalyst for hydrogen production from water containing sulfide and sulfite ions as hole scavengers under visible light irradiation. The molar concentration of TiO2 in CdS NW/TiO2 NP composite photocatalyst that showed the highest activity for H2 evolution was determined to be 0.2. This configuration of photocatalyst results in an efficient charge separation, caused by fast diffusion of photoelectrons generated from CdS NW toward surrounding TiO2 NP, leading to high photocatalytic activity of hydrogen production.
Acknowledgment This work has been supported by Hydrogen Energy Center (a Frontier Research Program of KOSEF) and BK-21 program. Fig. 7 – A configuration model consisting of CdS NW with high crystallinity decorated with nanosized TiO2 NPs. The possible role of TiO2 NP is to provide sites for collecting the photoelectrons generated from CdS NW, enabling thereby an efficient electron-hole separation as depicted. A TEM image of the interface between CdS NW and TiO2 NP is also shown.
references
[1] Brown P, Kamat PV. Quantum dot solar cells. Electrophoretic deposition of CdSe–C60 composite films and capture of photogenerated electrons with nC60 cluster shell. J Am Chem Soc 2008;130:8890–1.
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[2] Ranjbar M, Mahdavi SM, Zad A Iraji. Pulsed laser deposition of W–V–O composite films: preparation, characterization and gasochromic studies. Sol Energy Mater Sol Cells 2008;92: 878–83. [3] Wang D, Zou Z, Ye J. Photocatalytic water splitting with the Cr-Doped Ba2In2O5/In2O3 composite oxide semiconductors. Chem Mater 2005;17:3255–61. [4] Kim HG, Borse PH, Choi W, Lee JS. Photocatalytic nanodiodes for visible-light photocatalysis. Angew Chem Int Ed 2005;44: 4585–9. [5] Jang JS, Li W, Oh SH, Lee JS. Fabrication of CdS/TiO2 nanobulk composite photocatalysts for hydrogen production from aqueous H2S solution under visible light. Chem Phys Lett 2006;425:278–82. [6] Kida T, Guan G, Yamada N, Ma T, Kimura K, Yoshida A. Hydrogen production from sewage sludge solubilized in hotcompressed water using photocatalyst under light irradiation. Int J Hydrogen Energy 2004;29:269–74. [7] Leshkies KS, Duvakar R, Basu J, Pommer EE, Boercker JE, Carter CB, et al. Photosensitization of ZnO nanowires with CdSe quantum dots for photovoltaic devices. Nano Lett 2007; 7:1793–8. [8] Kim HG, Jeong ED, Borse PH, Jeon S, Yong K, Lee JS, et al. Photocatalytic ohmic layered nanocomposite for efficient utilization of visible light photons. Appl Phys Lett 2006;89: 64103. 1–3. [9] Jang JS, Kim HG, Borse PH, Lee JS. Simultaneous hydrogen production and decomposition of H2S dissolved in alkaline water over CdS/TiO2 composite photocatalysts under visible light irradiation. Int J Hydrogen Energy 2007;32:4786–91. [10] Arriaga LG, Fernandez AM. Determination of flat band potential and photocurrent response in (Cd, Zn)S used in photoelectrolysis process. Int J Hydrogen Energy 2002;27: 27–31. [11] Spanhel L, Weller H, Henglein A. Photochemistry of semiconductor colloids. 22. Electron injection from illuminated CdS into attached TiO2 and ZnO particles. J Am Chem Soc 1987;109:6632–5. [12] Kid T, Guan G, Yoshida A. LaMnO3/CdS nanocomposite: a new photocatalyst for hydrogen production from water under visible light irradiation. Chem Phys Lett 2003;371: 563–7. [13] Shangguan W, Yoshida A. Photocatalytic hydrogen evolution from water on nanocomposites incorporating cadmium sulfide into the interlayer. J Phys Chem B 2002;106:12227–30.
[14] Jang JS, Choi SH, Park H, Choi W, Lee JS. A composite photocatalyst of CdS nanoparticles deposited on TiO2 nanosheets. J Nanosci Nanotechnol 2006;6:3642–6. [15] Jang JS, Ji SM, Bae SW, Son HS, Lee JS. Optimization of CdS/ TiO2 nano-bulk composite photocatalysts for hydrogen production from Na2S/Na2SO3 aqueous electrolyte solution under visible light (l 420 nm). J Photochem Photobiol A Chem 2007;188:112–9. [16] So WW, Kim KJ, Moon SJ. Photo-production of hydrogen over the CdS–TiO2 nano-composite particulate films treated with TiCl4. Int J Hydrogen Energy 2004;29:229–34. [17] Li H, Zhu B, Feng Y, Wang S, Zhang S, Huang W. Synthesis, characterization of TiO2 nanotubes-supported MS (TiO2NTs@MS, M ¼ Cd, Zn) and their photocatalytic activity. J Solid State Chem 2007;180:2136–42. [18] Kim JC, Choi J, Lee YB, Hong JH, Lee JI, Yang JW, et al. Enhanced photocatalytic activity in composites of TiO2 nanotubes and CdS nanoparticles. Chem Commun 2006: 5024–6. [19] Guo YG, Hu JS, Liang HP, Wan LJ, Bai CL. TiO2 based composite nanotube arrays prepared via layer-by-layer assembly. Adv Funct Mater 2005;15:196–202. [20] Kukovecz A, Hodos M, Konya Z, Kiricsi I. Complex-assisted one-step synthesis of ion-exchangeable titanate nanotubes decorated with CdS nanoparticles. Chem Phys Lett 2005;411: 445–9. [21] Chen S, Paulose M, Ruan C, Mor GK, Varghese OK, Kouzoudis D, et al. Electrochemically synthesized CdS nanoparticle-modified TiO2 nanotube-array photoelectrodes: preparation, characterization, and application to photoelectrochemical cells. J Photochem Photobiol A Chem 2006;177:177–84. [22] Hodos M, Horvath E, Haspel H, Kukovecz A, Konya Z, Kiricsi I. Photosensitization of ion-exchangeable titanate nanotubes by CdS nanoparticles. Chem Phys Lett 2004;399:512–5. [23] Hsu MC, Leu IC, Sun YM, Hon MH. Fabrication of CdS@TiO2 coaxial composite nanocables arrays by liquid-phase deposition. J Cryst Growth 2005;285:642–8. [24] Jang JS, Joshi UA, Lee JS. Solvothermal synthesis of CdS nanowires for photocatalytic hydrogen and electricity production. J Phys Chem C 2007;111:13280–7. [25] Fujii H, Ohtaki M, Eguchi M, Arai H. Preparation and photocatalytic activities of a semiconductor composite of CdS embedded in a TiO2 gel as a stable oxide semiconducting matrix. J Mol Catal A Chem 1998;129:61–8.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Fabrication of CdS nanowires decorated with TiO2 nanoparticles for photocatalytic hydrogen production under visible light irradiation Jum Suk Janga, Hyun Gyu Kimb, Upendra A. Joshia, Ji Wook Janga, Jae Sung Leea,* a
Eco-friendly Catalysis and Energy Laboratory (NRL), Department of Chemical Engineering, Pohang University of Science and Technology, San 31, Hyoja-dong, Pohang 790-784, Republic of Korea b Busan Center, Korea Basic Science Institute (KBSI), Busan 609-735, Republic of Korea
article info
abstract
Article history:
A CdS/TiO2 composite photocatalyst consisting of one-dimensional CdS nanowire (NW)
Received 28 May 2008
with a high crystallinity decorated with nanosized TiO2 particles (NP) was fabricated by
Received in revised form
solvothermal method and sol–gel synthesis. The new configuration photocatalyst exhibi-
26 July 2008
ted higher rate of hydrogen production than that of single CdS NW under visible light
Accepted 26 July 2008
irradiation (l 420 nm) from water containing sulfide and sulfite ions as hole scavengers.
Available online 9 September 2008
Physicochemical properties of CdS NW/TiO2 NP composite photocatalysts were investigated together with the effect of the mole ratio of TiO2 and CdS for photocatalytic hydrogen
Keywords:
production from water under visible light irradiation.
CdS nanowires
ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights
TiO2 nanoparticles
reserved.
Composite H2 evolution Photocatalysis
1.
Introduction
Composite photocatalysts have been recognized as an attractive configuration because of their potential applications in solar cells, water splitting, organic decomposition and electrochromic devices [1–9]. In particular, CdS has ideal band gap energy (2.4 eV) and band positions that can drive both oxidation and reduction of water under visible light irradiation. CdS has been combined with other materials such as ZnO, TiO2 and LaMnO3 or intercalated into layered compounds [10–16] in attempt to improve the photo-efficiency and photostability. Among several configurations, a composite system consisting of CdS nanoparticles and TiO2 nanotubes has been reported in order to maximize the interfacial areas and
physical or chemical contact between the two compounds [17–23]. Li et al. [17] reported TiO2 nanotube-supported CdS and ZnS heterostructure synthesized via a simple wet chemical process and investigated the photocatalytic performance for decomposition of methyl orange. Kim et al. [18] synthesized a new composite material consisting of CdS nanoparticles and TiO2 nanotube and employed a bi-functional organic molecule to bind CdS strongly to TiO2 surface. Chen et al. [21] synthesized CdS nanoparticle-modified TiO2 nanotube-array photoelectrodes via anodizaiton and electrodeposition method. Kim et al. [18] fabricated ordered TiO2 based composite (TiO2–CdS) nanotube array using a layer-by-layer (LBL) assembly process. To the best of our knowledge, there was no report on the fabrication of CdS/TiO2 composite based on CdS nanowire and
* Corresponding author. Tel.: þ82 54 279 2266; fax: þ82 54 279 5528. E-mail address: [email protected] (J.S. Lee). 0360-3199/$ – see front matter ª 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2008.07.105
5976
international journal of hydrogen energy 33 (2008) 5975–5980
TiO2 nanoparticles, nor its photocatalytic activity for hydrogen production under visible light. Recently, we have found that the fabrication method of these composite photocatalysts is critical for their performance [5]. We also synthesized quasi-one-dimensional CdS nanowires via a solvothermal route in ethylenediamine as single solvent and evaluated their photoelectrochemical property as well as photoactivity for hydrogen production under visible light irradiation (l 420 nm) [24]. CdS nanowires with high crystallinity and large surface area are a good candidate as an active material for composite configuration. In this work, we fabricate a composite photocatalyst consisting of CdS nanowire (NW) with a high crystallinity decorated with TiO2 nanoparticles (NP), and evaluate its photoactivity for hydrogen production from aqueous solution containing hole scavengers such as sulfide and sulfite under visible light irradiation (l 420 nm).
2.
Experimental
2.1.
Materials preparation
In a typical procedure, 16.2 mmol of Cd(NO3)2$4H2O and 48.6 mmol NH2CSNH2 was added into a Teflon-lined stainless steel autoclave which had been filled with ethylenediamine to 80% of its capacity. The autoclave was maintained at 160 C for 48 h for solvothermal reaction and allowed to cool to room temperature. The colored precipitate was filtered and washed with absolute ethanol and deionized water to remove the residue of organic solvent [24]. To fabricate the CdS NW/TiO2 NP composite photocatalyst, the obtained 0.5 g CdS NW was stirred in 50 ml isopropyl alcohol and 0.25 g titanium isopropoxide (in a mole ratio of CdS to TiO2 from 0 to 4) and 0.623 g H2O were added drop-bydrop [5,15]. The prepared composite sample was solvothermally treated in an autoclave at 423 K for 24 h to improve the contact between two components and to increase the crystallinity of TiO2. All procedures for catalyst preparation are schematically shown in Fig. 1A.
2.2.
Fig. 1 – (A) A schematic procedure for fabrication of CdS NW/TiO2 NP composite photocatalyst. (B) X-ray diffraction patterns of the samples with different molar ratios of TiO2 in CdS NW/TiO2 NP composite photocatalysts; ([TiO2]/ [TiO2] D [CdS]) (a) 0 (CdS only), (b) 0.20, (c) 0.33, (d) 0.67. All samples were solvothermally treated at 423 K for 24 h in Teflon-lined stainless steel autoclave.
2.3.
Photocatalytic reactions
The photocatalytic reactions were carried out at room temperature under atmospheric pressure in a closed reactor system using an Hg-arc lamp (450 W) equipped with a UV cutoff filter (l 420 nm). Before reaction, 1 wt% of Pt was deposited on photocatalysts by photodeposition method under visible light (l 420 nm). The rate of H2 evolution was determined in an aqueous solution (100 ml) containing 0.1 g catalyst and 0.1 M Na2S þ 0.02 M Na2SO3. The evolved amounts of H2 were analyzed by gas chromatography (TCD, molecular ˚ column and Ar carrier). sieve 5-A
Characterizations
The crystalline phases of the products were determined by powder X-ray diffraction (XRD) on a diffractometer (Mac Science Co., M18XHF) with monochromatic Cu Ka radiation at 40 kV and 200 mA. The optical property was analyzed by UV-Visible diffuse reflectance spectrometer (Shimadzu, UV 2401). The morphology of photocatalysts was investigated by field emission scanning electron microscopy (SEM, Hitachi, S4200) and transmission electron microscope (JEOL JEM 2010F, Field Emission Electron Microscope) operated at 200 kV. The chemical states of sulfur in the samples were determined from X-ray photoelectron spectroscopy measurements (XPS VG Scientific, ESCALAB 220iXL) using Mg Ka radiation (1253.6 eV). The binding energy calibration was performed using C1s peak in the background as the reference energy (284.6 eV). The BET surface area was evaluated by N2 adsorption in a constant volume adsorption apparatus (Micrometrics, ASAP 2012).
3.
Results and discussion
The optimized preparation condition and characterization procedure of CdS nanowires (NW) have been described elsewhere [24]. The CdS NW as a visible light absorber component in composite photocatalyst was solvothermally synthesized at 160 C for 48 h. Fig. 1B shows X-ray diffraction patterns of the samples prepared with different molar concentrations of TiO2 in CdS NW/TiO2 NP composite photocatalysts. When the mole ratio of TiO2 NP to CdS NW was increased, (101) plane of TiO2 anatase phase became clearer around 2q ¼ 25.3 in XRD pattern as noted by a vertical line. The CdS NW showed welldeveloped hexagonal phase both in single and composite photocatalysts and TiO2 was in the anatase phase in the composite photocatalyst. The crystallinity of CdS in CdS NW/ TiO2 NP composite photocatalyst was as good as the single
international journal of hydrogen energy 33 (2008) 5975–5980
Fig. 2 – UV-VIS diffuse reflectance spectra of the samples with different molar ratios of TiO2 in CdS NW/TiO2 NP composite photocatalysts; ([TiO2]/[TiO2] D [CdS]) (a) 0 (CdS only), (b) 0.20, (c) 0.33, (d) 0.67. The edge position of TiO2 is indicated. All samples were solvothermally treated at 423 K for 24 h in Teflon-lined stainless steel autoclave.
CdS NW, considering its concentration (30 wt%) in the composite. The diffuse reflection spectra for these photocatalysts are shown in Fig. 2. The CdS NW photocatalyst showed a sharp
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edge at 520 nm. The spectra of CdS NW/TiO2 NP composite photocatalysts showed a combination of the two spectra coming from CdS NW and TiO2 NP. But, when the mole ratio of TiO2 to CdS is 0.33 or more, the spectra contributed by TiO2 NP in composite was clearly appeared as shown in Fig. 2(c, d). These results are well in accordance with those of XRD patterns. The morphology of CdS NW/TiO2 NP composite photocatalysts was observed by SEM and TEM images as shown in Fig. 3. CdS NW sample solvothermally synthesized at 160 C for 48 h showed the morphology of nanowires with a length of ca. 2–3 mm and a diameter of ca. 50 nm in Fig. 3(a). The dimensions were highly uniform among individual nanowires and the cross-section of nanowire was in a hexagonal shape (not shown here) [24]. For CdS NW/TiO2 NP composite, CdS NW of ca. 2–3 mm was decorated with TiO2 nanoparticles of ca. 10 nm. This clearly defined geometry of nanoparticles in contact with a nanowire is schematically shown in insert of Fig. 3(b). Note that TiO2 particles would form random multiple particle layers on CdS nanowire surface as discussed in our previous works [5,15]. The surface sulfur species was analyzed by XPS before photoreaction as shown in Fig. 4. The S2p peaks indicated that CdS NW and CdS NW/TiO2 NP composite contained SO2 4 as an impurity as well as sulfide (S2) species on its surface. Generally, CdS can be oxidized to CdO and CdSO4 as impurity phases and they can decrease the photoactivity. Especially, CdO inhibits H2 production because the conduction band position of CdO is more positive than the redox potential of Hþ/H2. CdSO4 also decreases visible light absorption of CdS [25]. Also, this surface oxidation may be responsible for the induction period as discussed below. In the previous work, we investigated the location of Pt cocatalysts by TEM images obtained after Pt deposition on
Fig. 3 – SEM images of the samples with different molar ratios of TiO2 in CdS NW/TiO2 NP composite photocatalyst; ([TiO2]/ [TiO2] D [CdS]) (a) 0 (CdS only), (b) 0.20, (c) 0.33, (d) 0.67. All samples were solvothermally treated at 423 K for 24 h in Teflonlined stainless steel autoclave.
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Fig. 4 – XPS data of S2p of CdS NW and CdS NW/TiO2 NP samples with [TiO2]/[TiO2] D [CdS] ratio of; (a) 0 (CdS only) and (b) 0.2.
bulk CdS-nano TiO2 composite [9]. From TEM images and EDAX analysis, it was confirmed that Pt cocatalysts were randomly located on both nanosized TiO2 and bulk CdS particles. In the present work, we also investigated where the Pt cocatalysts were located. Thus, TEM images were obtained after Pt deposition on CdS NW/TiO2 NP composite. As shown in TEM images in Fig. 5, Pt cocatalysts are randomly located on both nanosized TiO2 (a) as well as CdS nanowires (b). Fig. 6A shows the effect of the mole ratio of [TiO2]/ [TiO2] þ [CdS] for hydrogen production from an aqueous
solution containing 0.1 M Na2S and 0.02 M Na2SO3 as sacrificial reagents under visible light irradiation (l 420 nm). The surface area of CdS NW/TiO2 NP composite linearly increases with increase of the concentration of TiO2. But, the molar concentration of TiO2 in CdS NW/TiO2 NP that showed the highest activity for H2 evolution was determined to be 0.2. Note that this molar ratio for activity has no correlation with the surface area of the composite as shown in Fig. 6A. Thus, an optimum coverage of TiO2 NP decorating CdS NW is needed for effective and fast charge separation of photogenerated electrons and holes and thus to increase the photocatalytic activity. But too high a coverage would hinder the contact of electrolyte and CdS surface responsible for hole scavenging. The time-dependence of H2 production was also measured for [TiO2]/[TiO2] þ [CdS] of (a) 0.0 (CdS only) and (b) 0.2 samples under visible light irradiation with cut-off filter (l 420 nm) as shown in Fig. 6B. The photocatalytic activity of hydrogen evolution was linearly increased with the reaction time for 3 h except for a brief induction period. As discussed above, the oxidation of surface CdS species to CdO or CdSO4 may be responsible for this induction period [5]. In any case, the photocatalyst remains stable under the present reaction conditions. In our previous works [5,15], it was reported that CdS(bulk)/ TiO2 composite photocatalyst showed a high photocatalytic activity for hydrogen production from electrolyte solution containing sulfide and sulfite as sacrificial reagents under visible right irradiation (l 420 nm). The activity was much higher than that of single CdS photocatalyst having a high crystallinity, and those of composite photocatalysts of different configurations, i.e. nano-CdS/bulk-TiO2 or nanoTiO2/nano-CdS. Although CdS NW/TiO2 NP composite did not show a dramatic improvement of photocatalytic activity, the configuration of CdS nanowire decorated with TiO2 nanoparticle led to increase in photocatalytic activity from that of CdS nanowire alone. However, CdS NW/TiO2 NP showed considerably lower activity compared with those for CdS(bulk)/TiO2 NP composite photocatalysts [5,15]. One
Fig. 5 – TEM images that show that location of Pt loaded on TiO2 NP (a) and CdS NW (b).
international journal of hydrogen energy 33 (2008) 5975–5980
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Fig. 6 – (A) Evolution rates of H2 (bars) and specific BET surface areas (curve) for photocatalysts with different [TiO2]/ [TiO2] D [CdS] ratios: (a) 0 (CdS only), (b) 0.20, (c) 0.33, (d) 0.67. (B) Time curve of photocatalytic hydrogen production for [TiO2]/ [TiO2] D [CdS] ratios of (a) 0 and (b) 0.20. All samples were solvothermally treated at 423 K for 24 h in Teflon-lined stainless steel autoclave. Catalysis: 0.1 g loaded 1 wt% Pt. Electrolyte solution: 0.1 M Na2S D 0.02 M Na2SO3. Light Source: 450-W Hg lamp with a cut-off filter (l ‡ 420 nm).
reason for the lower activity for hydrogen production could be lower crystallinity of CdS NW. Although the CdS NW showed a relatively good crystallinity as a nanoscale material as shown in Fig. 1, it was still much less crystalline than bulk CdS prepared at high temperatures. The crystallinity of CdS was found to be the most important variable for high activity [5]. Indeed, it was found that CdS nanowire or composite calcined at 400 C after hydrothermal treatment showed higher rate of hydrogen production than that of uncalcined samples. The high activity of the composite photocatalyst is considered to be due to a fast charge separation. Thus upon initial light absorption, difference in the positions of conduction bands drives photoelectrons generated in CdS NW to surrounding TiO2 nanoparticles as shown in a schematic
model in Fig. 7. The holes remain in CdS NW and react with hole scavengers of sulfide and sulfite ions. In order for this system to function efficiently, quality of individual components as well as the close contact between two compounds should be important.
4.
Conclusions
The CdS NW/TiO2 NP composite photocatalyst based on CdS nanowires with high crystallinity decorated with TiO2 nanoparticles has been successfully fabricated via solvothermal and sol–gel method to develop a highly active photocatalyst for hydrogen production from water containing sulfide and sulfite ions as hole scavengers under visible light irradiation. The molar concentration of TiO2 in CdS NW/TiO2 NP composite photocatalyst that showed the highest activity for H2 evolution was determined to be 0.2. This configuration of photocatalyst results in an efficient charge separation, caused by fast diffusion of photoelectrons generated from CdS NW toward surrounding TiO2 NP, leading to high photocatalytic activity of hydrogen production.
Acknowledgment This work has been supported by Hydrogen Energy Center (a Frontier Research Program of KOSEF) and BK-21 program. Fig. 7 – A configuration model consisting of CdS NW with high crystallinity decorated with nanosized TiO2 NPs. The possible role of TiO2 NP is to provide sites for collecting the photoelectrons generated from CdS NW, enabling thereby an efficient electron-hole separation as depicted. A TEM image of the interface between CdS NW and TiO2 NP is also shown.
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