Self-assembled aligned Cu doped ZnO nanoparticles for photocatalytic hydrogen production under visible light irradiation

Materials Chemistry and Physics 102 (2007) 98–104

Self-assembled aligned Cu doped ZnO nanoparticles for photocatalytic hydrogen production under visible light irradiation K.G. Kanade a,b , B.B. Kale c , Jin-Ook Baeg a,∗ , Sang Mi Lee a , Chul Wee Lee a , Sang-Jin Moon a , Hyunju Chang a a

Korea Research Institute of Chemical Technology, Yuseong, Daejeon 305-600, Republic of Korea b Department of Chemistry, Mahatma Phule College, Pimpri, Pune 411017, India c Center for Materials for Electronics Technology (C-MET), Panchawati, Off Pashan Road, Pune 411008, India Received 6 January 2006; received in revised form 7 August 2006; accepted 13 November 2006

Abstract We report here the synthesis of self-assembled aligned hexagonal prismatic Cu doped ZnO nanoparticles in aqueous and organic medium. The average particle size was found to be in the range of 40–85 nm. Structural study revealed the existence of ZnO phase with wurtzite structure. The copper was present as oxide, situated at the core of prismatic form of wurtzite ZnO nanoparticle with clear edges and faces. The maximum hydrogen production rate achieved was 1932 ␮mol h−1 over the as synthesized Cu doped ZnO suggesting as an active photocatalyst for hydrogen sulfide decomposition under visible light irradiation. The effect of copper concentration as a dopant on the photocatalytic activity of Cu-ZnO was studied. The photocatalytic activity of Cu-ZnO synthesized in organic media was observed to be higher as compared to aqueous medium. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanoparticle; Zinc oxide; Copper doped; Photocatalyst; Hydrogen

1. Introduction Nanostructured ZnO is especially important in Hi-tech applications owing to its unique chemical and physical properties [1–4]. The size and shape of nanostructured ZnO is mainly controlled by the understanding of its nucleation and crystal growth process. Among the various nanosize metal oxides, ZnO is a promising semiconductor material, due to its environmental stability and low cost as compared to other binary nanosize metal oxides. It has been reported that in some of the applications (e.g., catalysts) the particle morphology plays a vital role in the reactivity of the ZnO surface [5–7]. In this regard, extensive work has been expended to tune the morphology of ZnO nanostructure for specific applications [8–15]. Recently, the compounds like ZnO, CuO and SrCO3 have shown the significance of morphology vis-a-vis applications [16–18]. Photocatalytic production of hydrogen from water, H2 S and organic wastes using semiconductors is one of the potential strategies for converting the sunlight energy into chemical

∗

Corresponding author. Tel.: +82 42 860 7560; fax: +82 42 860 7582. E-mail address: [email protected] (J.-O. Baeg).

0254-0584/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2006.11.012

energy. Among the various methods of solar energy conversion, much attention has been paid to photocatalytic decomposition of H2 S splitting for its potential in obtaining clean and high energy containing H2 from abundant H2 S. Every year, millions of tonnes of H2 S is produced in petroleum refinery plants and is expected to increase considerably in the future [19,20]. The direct thermal decomposition of H2 S for the production of hydrogen and elemental sulfur is energy intensive and economically unviable. Therefore, currently there has been immense emphasis on the development of visible light photocatalyst for the production of hydrogen. Beydoun et al. [21] reviewed nanocrystalline photocatalysts and described the effect of nanoparticle on photocatalytic activity. Recently, CdS–TiO2 nanocomposite film [22] has been used for the photodecomposition of H2 S. It is also noticed that the photodecomposition of H2 S by sulfide semiconductor photocatalysts, is not consistent due to photocorrosion of the photocatalyst. Therefore, there is a need to find stable, economical and efficient new metal oxide photocatalyst for hydrogen production. Most of the studies have focused on large band gap semiconductor oxides, such as TiO2 and ZnO, whose photoexcitation by UV light provides electron–hole pairs that are able to initiate

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the production of hydroxyl radicals in water. ZnO has nearly the same band gap and electron affinity as TiO2 , making it a likely candidate as semiconductor material for dye-sensitized solar cells (DSSC). It is also perceived that photocatalysis with nanostructured ZnO can actually become a versatile alternative to TiO2 [23]. ZnO nanoparticle DSSC’s have shown high efficiencies next to TiO2 [24,25]. The oxidative reforming of methanol with ZnO based catalysts has been extensively studied [26–28]. However, the effect of dopants on the photocatalytic activity of semiconducting oxide photocatalyst is scantily studied. In this paper, we report a new approach for the synthesis of lattice doped copper in the nanocrystalline wurtzite ZnO in different solvents and their use as a photocatalyst for the decomposition of H2 S to generate hydrogen. The effect of Cu doping and particle morphology on photocatalytic characteristics of the ZnO nanoparticles has been described. 2. Experimental 2.1. Materials The raw materials, viz. zinc acetate [Zn(CH3 COO)2 ·2H2 O]; oxalic acid [H2 C2 O4 ·2H2 O], copper chloride (CuCl2 ), methanol and ethylene glycol (EG), all from Aldrich were used as received.

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3. Results and discussion 3.1. XRD analysis The XRD patterns of copper doped with different atomic wt% of Cu is shown in Fig. 1 (a) 0.0%, (b) 0.1%, (c) 0.5%, (d) 1.0% and (e) 1.5% of aqueous mediated ZnO. All the d-values of XRD peaks showed the presence of a hexagonal (wurtzite) phase of ZnO, which is in good agreement with the reported values [30]. The intensity of peaks is not significantly changed with the increasing concentration of Cu in ZnO, which indicates that the crystallinity of the particles is retained. No signals of metallic Cu and its oxides were detected in XRD. This suggests that Cu/CuO has been doped into the ZnO lattice [31]. When organic solvents were used for the synthesis of these photocatalysts there was slight decrease in peak intensities in XRD pattern as compared with samples prepared in water. Since organic mediated Cu-ZnO did not show any extra features in XRD pattern, only X-ray diffraction patterns of aqueous mediated Cu-ZnO are furnished here. The average crystallite size calculated from FWHM using Scherer’s formula for (1 0 0), (1 0 1) and (1 1 0) plane was found to be 35 nm for the aqueous mediated Cu-ZnO and 30 nm for the organic mediated Cu-ZnO. The solvent with higher

2.2. Preparation of undoped and doped ZnO The zinc oxalate and co-precipitated copper-zinc oxalate were prepared by following the procedure as reported in our earlier communication [29]. Copper chloride was used to dope copper (Cu = 0.1, 0.5, 1.0 and 1.5 wt%) in ZnO. Zinc oxalates/Cu-zinc oxalates were synthesized using water as a medium and then decomposed at 450 ◦ C under aerobic conditions to obtain ZnO/Cu-ZnO. The same procedure was followed using methanol and ethylene glycol as solvents as a reaction medium. The ZnO prepared in water, methanol and EG medium is termed as water mediated, methanol mediated and ethylene glycol mediated ZnO, respectively. The decomposition of intermediate complex was studied using thermogravimetric analyzer (TGA–DTA, Metler-Toledo Star System).

2.3. Characterization of sample powder The X-ray diffractograms (XRD) of powdered samples was recorded with a Model Rigaku-D/MaX-2200 V X-ray Diffractometer with Cu K␣ radiation with Ni filter to study the crystal phase formed by catalyst. Inductively coupled plasma optical emission spectrophotometer (ICP-OES) was used to determine the percentage of copper in the samples. The surface morphology and particle size were determined using a field emission scanning electron microscope (FESEM Model JEOL-JSM6700F). The absorbance of the undoped and doped photocatalyst was measured by using UV–vis spectrophotometer (SHIMADZU UV-2450 at diffuse reflectance mode).

2.4. Measurement of photocatalytic activity For the photocatalytic measurements, the photocatalyst was introduced into cylindrical Pyrex photochemical reactor with quartz window and thermostat water jacket. A Xe-lamp light source (Oriel) of intensity 450 W was directly used for the undoped ZnO sample while a cut-off filter (>420 nm) was used for Cu-ZnO samples. Each experiment was conducted by using 0.5 g of catalyst in 250 ml of KOH solution (0.5M) with H2 S flow, 2.5 ml min−1 . The vigorously stirred suspension was purged with argon for 1 h followed by bubbling of hydrogen sulfide (H2 S) for about 1 h at 25 ◦ C. The excess hydrogen sulfide was trapped in NaOH solution. The amount of evolved hydrogen was measured using graduated gas burette and gas chromatograph (Model Shimadzu GC-14B, ˚ column, TCD, Ar gas carrier). MS-5 A

Fig. 1. X-ray diffractograms corresponding to copper doped: (a) 0.0%, (b) 0.1%, (c) 0.5%, (d) 1.0% and (e) 1.5%, zinc oxide prepared in water.

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Table 1 Elemental analysis of Cu (%) in Cu-ZnO samples Number

1 2 3 4

Sample

Cu (0.1%)-ZnO Cu (0.5%)-ZnO Cu (1.0%)-ZnO Cu (1.5%)-ZnO

Copper (%) Calculated

Observed

0.1 0.5 1.0 1.5

0.085 0.46 0.94 1.44

dielectric constant (water) hinders the growth of crystallites by forming agglomerates, resulted in the formation of bigger particle sizes as observed in FESEM. On the other hand, organic solvents with lower dielectric constant significantly reduced the agglomerations and thus induced better crystallinity. 3.2. Compositional analysis by ICP-OES The elemental copper in ZnO was analyzed using ICP-OES and the data are presented in Table 1. It is revealed from ICPOES analytical data that the copper content in water mediated Cu-ZnO and organic mediated Cu-ZnO is in good agreement with the calculated values. 3.3. Particle size and morphology by FESEM The scanning electron micrographs (SEM) of undoped ZnO powder prepared in water, methanol and ethylene glycol are shown in Fig. 2a–c, respectively. SEM micrographs evidently show the microstructural homogeneities and remarkably different morphologies for undoped ZnO prepared in different solvents. An aqueous mediated ZnO displayed rectangular chunk shape morphology (Fig. 2a), puffy or cotton-like morphology was observed in methanol mediated sample (Fig. 2b), whereas nearly spherical crystallites were seen in ethylene glycol mediated ZnO (Fig. 2c). The morphological behavior of an undoped nanosized ZnO prepared in different solvents is already explained in our earlier communication [29]. Fig. 3a–c represents the FESEM images of aqueous, methanol and ethylene glycol mediated Cu (1%) doped ZnO, respectively. Fig. 4a–c represents the FESEM images of aqueous, methanol and ethylene glycol mediated Cu (1.5%) doped ZnO, respectively. Interestingly, self-aligned prismatic nanoparticles were observed in aqueous and organic mediated ZnO. In the case of aqueous mediated ZnO, the particles are agglomerated and non-uniform in size and shape. Methanol and ethylene glycol mediated ZnO shows well-aligned prismatic nanoparticles with average particle size of 40–55 nm. The shape and size of the particles were virtually uniform. In the case of water mediated samples more compact agglomerates of self-aligned prismatic nanoparticles of 50–85 nm were observed. The shape and size of these particles were not uniform. Furthermore, in the case of ethylene glycol mediated samples a well-defined hexagonal prismatic morphology with active facets was observed, which is regarded as a general morphology of one-dimensional (1D) ZnO nanostructures [32]. Slightly different morphology with virtually uniform particle size obtained in organic medium

Fig. 2. SEM micrographs of nano ZnO (undoped) powder prepared in: (a) water, (b) methanol and (c) ethylene glycol.

reveals that organic solvents are playing key role in controlling the nucleation and crystal orientation [33,34]. 3.4. Electronic band structure by DRS The diffuse reflectance spectra of copper doped (0.0, 1.0 and 1.5%) ZnO powder prepared in aqueous, methanol and ethylene glycol solvents is shown in Fig. 5A–C. The DRS of undoped nanosized ZnO powder synthesized in aqueous and organic solvents (Fig. 5A) showed absorption edge cut off at 370–380 nm (band gap: 3.35–3.26 eV). The copper doped samples showed

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Fig. 3. FESEM micrographs of 1.0% copper doped ZnO powder prepared in: (a) water, (b) methanol and (c) ethylene glycol.

Fig. 4. FESEM micrographs of 1.5% copper doped ZnO powder prepared in: (a) water, (b) methanol and (c) ethylene glycol.

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Fig. 5. UV–DRS spectrum of copper doped ZnO powders with Cu atomic wt%: (A) 0.0, (B) 1.0 and (C) 1.5 prepared in water, methanol and ethylene glycol.

broad adsorption edge cut off (Fig. 5B and C) in the visible region (>400 nm). The absorption edge has been shifted to visible region due to copper doping, which implies that copper has gone into lattice of ZnO. In view of this, we have used as prepared copper doped (1.0 and 1.5%) ZnO powders for the photocatalytic study under visible light irradiation. 3.5. Photocatalytic activities of copper doped ZnO catalysts Photocatalytic activity of undoped and Cu doped ZnO photocatalyst prepared in water, methanol and ethylene glycol was investigated. The photocatalytic activity for photodecomposition of H2 S into hydrogen was studied using undoped and Cu doped ZnO under UV light and visible light irradiation, respectively. The photocatalytic activity of water, methanol and ethylene glycol mediated ZnO (undoped and Cu doped) are summarized in Table 2. The photocatalytic activity of undoped ZnO under visible and UV light irradiation were found to be lower than the Cu doped ZnO, irrespective of the solvents used for their synthesis. The photocatalytic activity of organic mediated ZnO (undoped) for H2 S splitting was found to be higher as compared to aqueous mediated ZnO. Similar trend was observed in the photocatalytic study of Cu doped ZnO. From the FESEM micrographs, it is quite clear that the organic mediated ZnO has higher degree of crystallanity as compared to that of water mediated samples. At the same time, the nanoparticles of organic mediated Cu doped ZnO are self-aligned and well crystallined. It is reported that the photocatalytic activity is dependent on crystallinity rather than the surface area or particle size [35]. Interestingly, higher photocatalytic activity

was observed for higher concentration of Cu in Cu doped ZnO samples. From the UV–DRS of ZnO samples it is evident that band gap has narrowed with copper doping, largely due to the Cu 3d levels [36]. This suggests that the Cu 3d level forms a new energy level below the conduction band of ZnO. This is consistent with the UV–DRS spectra, which showed a broad absorption edge (>400 nm). The narrower band gap suggests easier excitation for an electron from the valence band to the conduction band in the oxide semiconductor. This results in higher photocatalytic activity for Cu doped ZnO. Table 2 Photocatalytic evolution of H2 (␮mol h−1 ) by the decomposition of H2 S using Cu-ZnO prepared in different solvents Serial number

Catalyst

H2 (␮mol h−1 )

1 2 3 4 5 6 7 8 9 10 11 12

Undoped ZnO-W Undoped ZnO-W Cu (1.0%)-ZnO-W Cu (1.5%)-ZnO-W Undoped ZnO-M Undoped ZnO-M Cu (1.0%)-ZnO-M Cu (1.5%)-ZnO-M Undoped ZnO-EG Undoped ZnO-EG Cu (1.0%)-ZnO-EG Cu (1.5%)-ZnO-EG

785.3a 682.6b 1472.3b 1606.1b 1128.7a 785.2b 1739.9b 1753.4b 1383.0a 861.0b 1708.7b 1931.8b

W, water; M, methanol; EG, ethylene glycol solvents used for synthesis. a UV light source. b Visible light source.

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In this system copper doped zinc oxide acts as a photocathode and adsorption of light promote electrons in the conductance band (CB) of semiconductor where the potential (−0.88 V) is sufficient to liberate hydrogen. At the same time, holes in the valence band (VB) move in to the bulk to facilitate the oxidation process, where they are transferred to the reduced species S2− , which acts as a hole-scavenger and prevents the photocorrosion of the photocatalyst. Based on the observations of our experiments, we propose the following reaction mechanism for photocatalytic decomposition of H2 S. hv

Photocatalyst−→eCB − + hVB +

(1)

hVB + + H2 S → 2H+ + S

(2)

H2 S + 2OH− ⇔ S2− + 2H2 O

(3a)

or H2 S + OH− ⇔ HS− + H2 O −

+

−

2HS + 2hVB → S2 + H

(3b)

+

(4)

2S2− + hVB + → S2 −

(5)

HS− + OH− ⇔ S◦ + 2e− + H2 O

(6)

S2− + 2hVB + → 2S

(7)

2H+ + 2eCB − → H2

(8)

hv,Photocatalyst

Net reaction : H2 S

−→

H2 + S

(9)

It is known from the literature that, in alkaline medium, the photocatalytic decomposition of aqueous sulfide leads to the formation of disulfide ions (Eq. (5)) which is why in our case also, no sulfur precipitation was observed [37–39]. Thus in the present work, only the formation of disulfides and polysulfides were observed when the photocatalytic reaction was carried out in alkaline medium (pH 12–13). According to the reaction (3b), hydrosulfide ion was produced by the dissociation of H2 S in alkaline medium. The photogenerated holes on the surface of the photocatalyst oxidize hydrosulfide ion to give disulfide ion (Eq. (4)). Finally, the hydrogen is formed by the reduction of protons by the photogenerated electrons (Eq. (8)). Eq. (9) illustrate the overall process corresponds to the cleavage of H2 S by two photons of light which require the Gibbs energy of 7.892 kcal mol−1 [40]. 4. Conclusion Self-assembled aligned Cu doped ZnO nanoparticles were obtained in aqueous and organic reaction medium. Well-aligned nanocrystalline hexagonal prismatic particle morphology was observed for the Cu doped ZnO. The average particle size was found to be in the range of 40–55 nm for organic mediated Cu-ZnO and 50–85 nm for aqueous mediated ZnO. Structural studies revealed the existence of ZnO as wurtzite phase. The copper was found to be in the form of oxide, situated at the core of prismatic form of ZnO nanoparticle with clear edges and faces. Organic mediated Cu-ZnO showed higher photocatalytic

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activity under visible light irradiation due to well crystallined self-aligned particles. We believe that our synthetic strategy for doping of transition metal ions in semiconductor nanostructures would be useful for the development of visible light photocatalysts and photovoltaic devices. Further work in this direction is under progress in our laboratory. Acknowledgements This research (paper) was performed for the Hydrogen Energy R&D Center, one of the 21st Century Frontier R&D Program, funded by the Ministry of Science and Technology of Korea. One of the authors (K.G. Kanade) gratefully acknowledge to Dr. Arun Andhale, Principal of Mahatma Phule College, Pimpri, Pune 411017, India. We also thank to Dr. K. Gurunathan and Mr. J.Y. Park for many stimulating discussions. References [1] C.M. Lieber, Solid State Commun. 107 (1998) 607. [2] J. Hu, T.W. Odom, C.M. Lieber, Acc. Chem. Res. 32 (1999) 435. [3] J. Goldberger, R.R. He, Y.F. Zhang, S.W. Lee, H.Q. Yan, H.J. Choi, P.D. Yang, Nature 422 (2003) 599. [4] H.Q. Yan, R.R. He, J. Pham, P.D. Yang, Adv. Mater. 15 (2003) 402. [5] M. Bowker, H. Houghton, K.C. Waugh, T. Giddings, M. Green, J. Catal. 84 (1983) 252. [6] V. Bolis, B. Fubini, E. Giamello, A. Reller, J. Chem. Soc., Faraday Trans. 185 (1989) 855. [7] D. Li, H. Haneda, Chemosphere 51 (2003) 129. [8] M.H. Huang, Y.Y. Wu, H. Feick, N. Tran, E. Weber, P.D. Yang, Adv. Mater. 13 (2001) 113. [9] W.L. Hughes, Z.L. Wang, Appl. Phys. Lett. 82 (2003) 2886. [10] M.S. Arnold, P. Avouris, Z.W. Pan, Z. Wang, J. Phys. Chem. B 107 (2003) 659. [11] X.D. Wang, P.X. Gao, J. Li, C.J. Summers, Z.L. Wang, Adv. Mater. 14 (2002) 1732. [12] P.X. Gao, Z.L. Wang, J. Phys. Chem. B 106 (2002) 12653. [13] H. Gleiter, Acta Mater. 48 (2000) 1. [14] W.I. Park, D.H. Kim, S.W. Jung, G.C. Yi, Appl. Phys. Lett. 80 (2002) 4232. [15] Y.J. Zhang, N.L. Wang, S.P. Gao, R.R. He, S. Miao, J. Liu, J. Zhu, X. Zhang, Chem. Mater. 14 (2002) 3564. [16] B. Liu, H.C. Zeng, J. Am. Chem. Soc. 126 (2004) 16744. [17] J.L. Yang, S.J. An, W. Park, G. Yi, W. Choi, Adv. Mater. 16 (2004) 1661. [18] D. Rautaray, S.R. Sainkar, M. Sastri, Langmuir 19 (2003) 888. [19] B. Bandapani, N.J.C. Packham, J.O. Bockris, in: V.T. Veziroglu (Ed.), Hydrogen Energy Progressed, Pergamon, Oxford, 1984, p. 283. [20] S.V. Tambwekar, M. Subrahmanyam, Int. J. Hydrogen Energy 22 (1997) 956. [21] D. Beydoun, R. Amal, G. Low, S. Mcevoy, J. Nanoparticle Res. 1 (1999) 439. [22] W. So, K. Kim, S. Moon, Int. J. Hydrogen Energy 29 (2004) 229. [23] R. Comparelli, E. Fanizza, M.L. Curri, P.D. Cozzoli, G. Mascolo, A. Agostiano, Appl. Catal. B: Env. 60 (2005) 1. [24] K. Keis, E. Magnusson, H. Lindstrom, S.E. Lindquist, A. Hagfeldt, Sol. Energy Mater. Sol. Cells 73 (2002) 51. [25] Z.S. Wang, C.H. Huang, Y.Y. Huang, Y.J. Hou, P.H. Xie, B.W. Zhang, H.M. Cheng, Chem. Mater. 13 (2001) 678. [26] S. Velu, K. Suzuki, M. Okazaki, M.P. Kapoor, T. Osaki, F. Ohashi, J. Catal. 194 (2000) 373. [27] S. Velu, K. Suzuki, M. Okazaki, M.P. Kapoor, F. Ohashi, T. Osaki, Appl. Catal. A: Gen. 213 (2001) 47. [28] S. Murcia-Mascar´os, R.M. Navarro, L. G´omez-sainero, U. Costantino, M. Nocchetti, J.L.G. Fierro, J. Catal. 198 (2001) 338.

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