Chem. Mater. 2008, 20, 1997–2000
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Cu-Doped ZnS Hollow Particle with High Activity for Hydrogen Generation from Alkaline Sulfide Solution under Visible Light Takeo Arai,† Shin-ichiro Senda,‡ Yoshinori Sato,‡ Hideyuki Takahashi,‡ Kozo Shinoda,§ Balachandran Jeyadevan,‡ and Kazuyuki Tohji* Institute of Fluid Science, Tohoku UniVersity, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan; Graduate School of EnVironmental Studies, Tohoku UniVersity, 6-6-20 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan; and Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan ReceiVed July 10, 2007. ReVised Manuscript ReceiVed December 17, 2007
Visible light sensitive and highly active Cu-doped ZnS hollow photocatalyst particles “Cu-ZnS-shell” were successfully developed. These particles could generate H2 through photocatalytic decomposition of HS- ion in Na2S solution. The photoactivity under xenon lamp irradiation was 6 and 130 times higher than that of copper-free “ZnS-shell” and coprecipitated ZnS particles, respectively. “Cu-ZnS-shell” particles were prepared by doping the “ZnS-shell” particles synthesized using zinc oxide as precursor with Cu, utilizing the difference in ionization tendency between zinc and copper. Though the photoactivity of “ZnS-shell” was higher than that of coprecipitated ZnS, their photoactivity under visible light conditions was low. However, the “Cu-ZnS-shell” was active to light of wavelengths higher than 440 nm and is preferred over CdS, which requires an expensive support metal catalyst such as platinum to decompose HS- ion.
1. Introduction Hydrogen sulfide is a harmful gas generated in oil refinery, sewage treatment plant, volcano, and so on. Generally, hydrogen sulfide is treated using the oxidation process and converted into water and sulfur or sulfate ion. However, it will be environmentally beneficial if hydrogen sulfide is decomposed to produce hydrogen, which is a source of clean energy. In this study, we focused our attention on the photocatalytic decomposition of hydrogen sulfide using sunlight to generate hydrogen. Decomposition of hydrogen sulfide (HS-) ion consumes energy lower than that is required to split water and could be realized with already available semiconductors. Sulfide semiconductors such as ZnS1 and CdS2–6 are reported as effective photocatalysts to decompose hydrogen sulfide ion. However, CdS could photosplit hydrogen sulfide efficiently under visible light irradiation only when their particle surface is deposited with expensive noble metals, such as platinum. Though ZnS can split hydrogen sulfide even without any metal particles as cocatalyst, the operating * To whom correspondence should be addressed: Ph +81-22-795-7390; Fax +81-22-795-7390; e-mail
[email protected]. † Institute of Fluid Science. ‡ Graduate School of Environmental Studies. § Institute of Multidisciplinary Research for Advanced Materials.
(1) Reber, J. F.; Meier, K. J. Phys. Chem. 1984, 88, 5903–5913. (2) Borgarello, E.; Kalyanasundaram, K.; Grätzel, M.; Pelizzetti, E. HelV. Chim. Acta 1982, 65, 243–248. (3) Boragarello, E.; Erbs, W.; Grätzel, M.; Pelizzetti, E. NouV. J. Chim. 1983, 7, 195–198. (4) Rufus, I. B.; Ramakrishnan, V.; Viswanathan, B.; Kuriacose, J. C. Langmuir 1990, 6, 565–567. (5) Rufus, I. B.; Viswanathan, B.; Ramakrishnan, V.; Kuriacose, J. C. J. Photochem. Photobiol. A 1995, 91, 63–66. (6) Bühler, N.; Meier, K.; Reber, J. F. J. Phys. Chem. 1984, 88, 3261– 3268.
range is limited only to wavelengths under ultraviolet irradiation. Therefore, it is necessary to design and synthesize semiconductor particles that could respond to visible light even in the absence of any expensive noble metal cocatalyst. It has been envisaged that ZnS particles doped with transition metals such as Cu, Ni, or Pb could show the necessary activity for H2 evolution under visible-light irradiation and does not require cocatalyst such as platinum.7–9 In this paper, we synthesized a unique shell structured ZnS particles using ZnO as precursor to improve the photocatalytic activity compared with that of ZnS particle prepared by ordinary coprecipitation method. Furthermore, we attempted the visible light photocatalytic activity of shell structured ZnS particles by doping their surfaces with copper using the difference in ionization tendencies between zinc and copper ions. 2. Experimental Section Sample Preparation. ZnO (99.999%, 200-600 nm) and CuCl (99.9%) of Kojundo Chemicals and Zn(NO3)2 · 6H2O, Na2S · 9H2O, NH3 solution (25%), HCl, and Na2SO3 of Wako Chemicals were used as received. The preparation method of shell structured ZnS is described as follows: 1.25 mol of ZnO was introduced into 500 mL of 0.5 M Na2S solution, and the suspension was sonicated and stirred for 48 h at room temperature to precipitate ZnS on ZnO. The particles were filtered and washed with deionized water and then dried in an oven at 60 °C for 12 h. The particles thus obtained by this sulfuration process were named as “ZnO/ZnS”. Then, 5 g of ZnO/ (7) Kudo, A.; Sekizawa, M. Cat. Lett. 1999, 58, 241–243. (8) Kudo, A.; Sekizawa, M. Chem. Commun. 2000, 1371–1372. (9) Tsuji, I.; Kudo, A. J. Photochem. Photobiol., A 2003, 156, 249–252.
10.1021/cm071803p CCC: $40.75 2008 American Chemical Society Published on Web 02/08/2008
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ZnS was dispersed in 100 mL of deionized water using sonication, and solution pH was maintained at 4 by adding HCl (pH 0.2). Since the pH of the solution shifted to higher values with the dissolution of ZnO, the point at which the pH remained constant was considered the end point of the reaction. Then the particles were filtered and dried in an oven at 60 °C for 12 h. The ZnS particles thus obtained were named “ZnS-shell”. For comparison, ZnS particle was also prepared by coprecipitation method as follows: 50 mL of 1 M Na2S solution was introduced into 50 mL of 0.2 M Zn(NO3)2 solution while stirring at 500 rpm. The particles precipitated in the solution were centrifuged, washed with deionized water, and dried in an oven at 60 °C for 12 h. To verify the visible light response, Cu ion was doped into ZnS-shell utilizing the difference in ionization tendencies between copper and zinc ions. First, 1 mmol of CuCl was dissolved in 3.8 mL of NH3 solution and diluted to 50 mL using deionized water. Then, 495.5 mg of ZnS-shell was dispersed in 50 mL of deionized water using sonication and stirred at 500 rpm. Then, ZnSshell samples doped with varying concentrations of copper were prepared by adding 2.5, 5, and 12.5 mL (ca. 1, 2, and 5 mol %) of Cu ion solutions into ZnS-shell/water suspension and stirred for 10 min, and the particles were filtered and dried in an oven at 60 °C for 12 h. Hereafter, these particles are referred to as Cx-Z-S (where x ) 1, 2, and 5). And also, CuxZn1-xS solid solution with similar copper concentration was prepared by the coprecipitation method as follows: First, 50 mL of 1 M Na2S solution was stirred at 500 rpm. Then, 50 mL of solution containing 0.2 M Zn(NO3)2 and 0.02 M Cu(NO3)2 was introduced into the Na2S solution and stirred for 24 h to obtain a yellow precipitate. Finally, CuxZn1-xS precipitate was filtered and dried in an oven at 60 °C for 12 h. Characterization. The crystal phases of the samples were identified by using an X-ray diffractometer (Rigaku, MultiFlex). The ratio between Zn and Cu was determined using an X-ray fluorescence element analyzer (HORIBA, MESA-500W). Ultraviolet–visible (UV–vis) reflection spectra of the samples were obtained using a UV–vis spectrometer (Hitachi, U3300) equipped with integrating sphere and quartz window. The surface area of the sample was estimated by the BET method using a volumetric sorption analyzer (Quantachrome, NOVA1200). The morphology of the particle was observed using a field emission type transmission electron microscope (Hitachi, HF-2000). The band structure and the partial density of states were estimated on the basis of density functional theory (DFT), using CASTEP package software (Accelrys Co.). Hydrogen generation reaction was performed in the Pyrex cell assembly shown in Figure 1. The cell was filled with Na2S suspension dispersing the photocatalyst. The bottom of the cell was irradiated using a 550 W xenon lamp (WACOM, KXL-552HPF) and a reflecting mirror (Sigma, TFAH100-15-0.3), as shown in Figure 1. The cell was air cooled using a fan to minimize the temperature rise of the suspension due to light irradiation. The average cell temperature was ca. 30 °C. The light wavelength dependence of activity was determined using a one-pot Pyrex glass cell connected to a closed gas-circulating system. The wavelength of 300 W xenon lamp (CERMAX, LX-300) was controlled by using several optical filters (HOYA, L-42, Y-44, Y-46, Y-48). The amount of hydrogen generated through photosplitting of HS- ions was determined by using a gas chromatograph (Shimadzu, GC-8A) with molecular sieve column. In each hydrogen generation reaction, 150 mL of 0.1 M Na2S-Na2SO3 mixed solution, which was prepared by dissolving 0.1 mol of Na2S and Na2SO3 in 1 L of deionized water, was used.
Figure 1. Sketch of reactor used in hydrogen generation experiment.
Figure 2. XRD patterns of ZnS-shell and ZnS synthesized by coprecipitation.
The absorbance of visible light by yellow S22- ion was minimized by reacting the same with SO32- ions to form colorless S2O32ion.
3. Results and Discussion XRD patterns of ZnS-shell and coprecipitated ZnS correspond to zinc-blende type ZnS (Figure 2). However, the diffraction peaks of ZnS-shell were shifted to higher angles than that of reference ZnS. This suggested the decrease in lattice spacing, and it was supposedly due to the replacement of some sulfide atoms in zinc sulfide by smaller radius oxygen atoms. Furthermore, the narrower peak width of ZnSshell compared to coprecipitated ZnS suggested larger crystallite size. The UV–vis reflection spectra of ZnS-shell, coprecipitated ZnS, and commercial ZnS (Kojundo Chemicals, 99.999%) are shown in Figure 3. ZnS-shell showed a specific absorption around 350 nm while the absorption edge of commercial ZnS was around 370 nm. It is supposed that the shift of absorption spectrum observed in ZnS-shell was caused by narrowing of the lattice or high crystallinity. Coprecipitated ZnS showed a broad absorption edge that
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Figure 3. UV–vis reflection spectra of ZnS-shell and ZnS by coprecipitation.
Figure 6. Hydrogen generation activity of ZnS-shell and coprecipitated ZnS.
Figure 4. TEM image of ZnS-shell (a) and coprecipitated ZnS (b).
Figure 5. Preparation scheme of ZnS-shell.
extended from ultraviolet to near-ultraviolet range, which was believed due to lattice defects. TEM images of ZnSshell and coprecipitated ZnS are shown in Figure 4. The presence of shell-like structure resulting from the dissolution of ZnO core was confirmed in ZnS-shell particles, while highly aggregated 5 nm diameter ZnS particles were present in the coprecipitaed sample. The schematic diagram illustrating the formation of the shell structured ZnS-shell is shown in Figure 5. The formation of the ZnS layer on the surface of ZnO particles is accomplished through the following mechanism. The surface of ZnO undergoes dissolution in alkaline sulfide solution, and Zn2+ ion reacts with S2- ion to form ZnS on the surface of ZnO particles, which is energetically favored. In other words, ZnO was consumed for the growth of ZnS layer. Then, the ZnO core is dissolved preferentially utilizing the differences in solubility of ZnO and ZnS phases in HCl to prepare shell structured ZnS-shell. The results of hydrogen generation experiment carried out using ZnS-shell and coprecipitated ZnS photocatalysts are shown in Figure 6. ZnS-shell could generate a larger amount of hydrogen than coprecipitated ZnS, in spite of ZnS-shell
surface area was being only 90 m2/g, against 230 m2/g for coprecipitated ZnS. The above results suggested that the enhancement in ZnS-shell activity was due to the crystal structure and morphology of particle rather than their specific surface area. Furthermore, it was confirmed that the enhancement in the activity was not a consequence of the expansion of operating light absorption range, which was limited to a maximum of 370 nm. This result suggested that the quantum efficiency under UV light including the transfer of photoexcited electrons and holes to the reaction medium may have been enhanced. We considered that the contact between particle and reaction solution was improved by the hollow structure of ZnS-shell. In the case of ordinary particle aggregates, even if the light is absorbed by the aggregates, unless the reaction solution is in contact with the particle, the photoexited electrons and holes cannot react and the absorbed light go wasted. On the other hand, in the case of ZnS-shell, the thin layer composed of nanoparticles forms the porous shell of the hollow structure. It is supposed that this porous structure facilitates the penetration of reaction solution into aggregate and eases the desorption of hydrogen from aggregate. Therefore, most of the particles could react with the solution and generate hydrogen. And we also believed that a charge gradient is maintained across the shell wall, which facilitates the separation of electron and hole and reduces the recombination probability, and enhanced the photocatalytic activity of ZnS-shell compared to coprecipitated ZnS. Though the photocatalytic efficiency of ZnS-shell was higher than the coprecipitated counterpart, it should be noted that the above particles are not capable of responding to visible light. Thus, an attempt was made to dope the surface of ZnS-shell particles with Cu to extend the operating range to visible light region. Varying concentrations of Cu ions were doped, and the results of XRD patterns of each sample showed no appreciable change in lattice constant. However, the change in UV–vis reflection spectra was observed as shown in Figure 7. The degree of absorbance varied with the Cu concentration in ZnS-shell. C1-Z-S and C2-Z-S
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Figure 7. UV–vis reflection spectra of Cu-doped ZnS-shell for various Cu concentrations.
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Figure 9. Wavelength-dependent photoactivity of C2-Z-S 250 mL of 0.1 M Na2S/Na2SO3 solution dispersing 200 mg of C2-Z-S was exposed to 300 W xenon lamp (CERMAX, LX-300). The amount of hydrogen generated in the reaction was determined by using a gas chromatograph (Shimadzu, GC-8A) with molecular sieve column.
the activity of C5-Z-S was higher than that of ZnS-shell but was half of C2-Z-S. It is believed that at high Cu concentrations large area of ZnS surface is converted to CuS, which has low activity and also prevents the light irradiation reaching active area within the particle. The wavelength dependence of photoactivity of C2-Z-S is shown in Figure 9. It was confirmed that hydrogen could be generated even with Y-46 filter (>440 nm). The above result indicates that the visible light response of ZnS-shell could be extended to higher wavelength region (>350 nm) by doping with Cu. It should be noted that the Cu-doped ZnS-shell particles are superior to CdS, which cannot photosplit hydrogen sulfide in the absence of cocatalyst such as platinum. 4. Summary Figure 8. Dependence of hydrogen generation activity on Cu concentrations in ZnS-shell; 150 mL of 0.1 M Na2S/Na2SO3 solution dispersing 100 mg of sample was exposed to 550 W xenon lamp (WACOM, KXL-552HPF). The amount of hydrogen generated in the reaction was measured using a cylinder.
showed yellowish color in comparison to the white ZnSshell. The reflection spectra of C1-Z-S and C2-Z-S indicated that the doped copper formed a new energy level in the band structure of ZnS. From DFT calculation, it was estimated that the donor level has partially filled 3d orbital of Cu2+ as reported by Kudo et al.9 Though the initial concentration of copper used in the synthesis of both C5-Z-S and CuxZn1-xS (ca. x ) 0.05) solid solution was the same, the color of the particles were gray and dark yellow, respectively. The gray color of C5-Z-S was believed due to incomplete doping of copper in ZnS-shell. It could be that some Cu ions preferentially formed CuS, which has lower solubility than ZnS. The high rate of light absorption in C5-Z-S at wavelengths over 400 nm indicates the presence of CuS. The results of hydrogen generation experiment of the Cu-doped ZnS-shell are shown in Figure 8. It was confirmed that the activity extremely improved by the addition of 1-2 mol % of Cu ion in ZnS-shell. Among the samples tested, the highest activity was recorded for C2-Z-S, which was 6 times higher than ZnS-shell and 130 times higher than coprecipitated ZnS under xenon lamp irradiation conditions. On the other hand,
ZnS particle with unique morphology named as “ZnS-shell” was prepared using ZnO particle as precursor. ZnS-shell had hollow shell structure with the shell consisting of particles with a smaller lattice constant than ZnS. This particle showed higher activity than the ZnS prepared by coprecipitation method in photosplitting hydrogen sulfide. Furthermore, visible light response of ZnS-shell particle was realized by doping the same with Cu ions. The photosplitting efficiency of 2 atom % of copper doped ZnS-shell was about 6 and 130 times higher than copper-free ZnS-shell and coprecipitated ZnS, respectively. The operating region of copper-doped particle was extended to wavelengths higher than 440 nm. Furthermore, copper-doped ZnS-shell is preferred over CdS, which require an expensive cocatalyst such as platinum to split hydrogen sulfide. The result of this study has demonstrated that the activity of a well-known material such as ZnS could be improved drastically by controlling the composition and morphology of the particle. Authors believe that the findings of the current study could pave the way for the development of new highly efficient photocatalysts. Acknowledgment. This study was supported by 21st century COE program “Flow Dynamics International Research Educational Base”. The authors also thank Dr. Kazuhiro Sayama for his assistance with the measurement of wavelength dependence of photocatalytic activity and for useful discussions. CM071803P
