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ZnO from aqueous Na2S + Na2SO3 solution
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Photocatalytic hydrogen production with CuS/ZnO from aqueous Na2S D Na2SO3 solution Paramasivan Gomathisankar a,*, Katsumasa Hachisuka a, Hideyuki Katsumata a, Tohru Suzuki b, Kunihiro Funasaka c, Satoshi Kaneco a,b,** a
Department of Chemistry for Materials, Graduate School of Engineering, Mie University, Tsu, Mie 514-8507, Japan Environmental Preservation Center, Mie University, Tsu, Mie 514-8507, Japan c Department of Urban Environment, Osaka City Institute of Public Health and Environmental Sciences, Osaka 543-0026, Japan b
article info
abstract
Article history:
The photocatalytic hydrogen production in the sacrificial S2eSO2 anions was investi3
Received 22 January 2013
gated with ZnO in the addition of metal sulfides containing Ag2S, CuS, Fe2S3, and NiS. In the
Received in revised form
absence of metal sulfides, the photocatalytic H2 evolution using ZnO was observed with
17 April 2013
255 mmol g1. The CuS amount and the concentrations of S2 and SO2 3 ions were opti-
Accepted 23 April 2013
mized. It was found that ternary component semiconductor CuS/ZnS/ZnO was formed
Available online xxx
during the photocatalytic hydrogen production in the aqueous Na2S þ Na2SO3 solution. The photocatalytic hydrogen evolution with CuS/ZnS/ZnO in the 0.4 M Na2Se0.4 M Na2SO3
Keywords:
solution was more than about 8.5 times better compared with those obtained with only
Photocatalytic hydrogen generation
ZnO. The CuS clusters on the surface of ZnS/ZnO seem to play an important role on the
Metal sulfide
separation for electronehole pair and the enhancement of H2 production. Nano-sized ZnS/
ZnS/ZnO
ZnO photocatalytic hydrogen technology has great potential for low-cost, environmentally
CuS
friendly solar-hydrogen production to support the future hydrogen economy.
Hetero junction semiconductor
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Recently, zinc oxide (ZnO) has been widely used as the photocatalyst, owing to its high activity, low cost and environmentally friendly feature [1]. The band gap of ZnO is almost the same as that of TiO2, and their conduction band (CB) and valence band (VB) edges are very close to those of TiO2 [1]. However, there are only several reports concerning the photocatalytic H2 production with ZnO, because the system needs the suitable sacrificial agents in order to enhance the consumption of the photogenerated holes for minimizing the photocorrosion of ZnO and the electronehole pair recombination [2e5].
Since the extraction products of fossil energy resources containing sulfides and sulfites, which is now produced in large quantities, are undesirable and polluting byproduct in hydrogenation and flue-gas desulfurization processes at chemical plants, the photocatalytic reaction of hydrogen formation over sulfides from aqueous solution containing S2 and SO2 3 may become one of the promising methods for the H2 energy [6e13]. The metal/semiconductor and semiconductor/semiconductor composite materials are used to modify band gap position, facilitate charge rectification, and improve carrier separation for photocatalytic hydrogen generation [6e13].
* Corresponding author. Tel./fax: þ81 59 231 9427. ** Corresponding author. Department of Chemistry for Materials, Graduate School of Engineering, Mie University, Tsu, Mie 5148507, Japan. E-mail addresses: [email protected] (P. Gomathisankar), [email protected] (S. Kaneco). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.04.131
Please cite this article in press as: Gomathisankar P, et al., Photocatalytic hydrogen production with CuS/ZnO from aqueous Na2S þ Na2SO3 solution, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.04.131
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Experimental
2.1.
Chemicals and materials
Photocatalyst ZnO was purchased from SigmaeAldrich (BET specific surface area 15e25 m2/g, mean particle size 50e70 nm). A standard stock solution of metal ion (1 mg mL1) was prepared by the dissolution of AgNO3, CuCl2, Fe(NO3)3 and Ni(NO3)2. Sodium sulphide nonahydrate (98%) and sodium sulphite (97%) were obtained from Wako Pure Chemical Industries, Ltd. and were used as received. Laboratory pure water was obtained from an ultrapure water system (Advantec MFS Inc., Tokyo, Japan) resulting in a resistivity >18 MU cm.
2.2.
Photocatalytic hydrogen production
The pyrex column vessel reactor (inner volume: 55.6 mL) was used for the photocatalytic production of hydrogen from aqueous Na2SeNa2SO3 solution. The spout of vessel was tightly closed with septum and aluminum sealing. The pyrex glass cuts off all wavelength below 300 nm. Typically, 20 mg of the ZnO photocatalysts were added to 30 mL of aqueous Na2SeNa2SO3 solution in the photoreactor. A xenon lamp (500 W, UXL‒500D‒O) was applied as light source, which was positioned on the side of photoreactor. The light intensity was measured by a UV radio meter with a sensor of 320e410 nm
3.
Result and discussions
3.1. Effect of metal sulfides on the photocatalytic H2 evolution The effect of metal sulfides on the photocatalytic hydrogen production using ZnO was investigated in the aqueous Na2S þ Na2SO3 solution (Fig. 1). The suspension concentration of metal sulfide was 6.7 ppm as metal in the Na2SeNa2SO3 solution. The photocatalyst of ZnO in the absence of metal sulfides exhibited a small amount of H2 evolution (255 mmol g1). However, the hydrogen production with CuS/
900
-1
2.
wavelengths (UVRe400, Iuchi Co., Osaka, Japan), and the value was 1.0 mW/cm2. The ZnO photocatalysts were continuously dispersed in the aqueous Na2SeNa2SO3 solution by a magnetic stirrer during the irradiation. The temperature of the suspension in the photoreactor was kept constant at 50 C by the hot stirrer. Metal ion solution was added into the aqueous Na2SeNa2SO3 solution immediately before the irradiation. The formations of metal sulfides were confirmed by color changes of the reaction solutions. The irradiation time was 3 h. The hydrogen product from the reaction solution was analyzed by gas chromatography (GL Sciences, GC‒3200) with thermal conductivity detector (TCD). The stainless column (4 m long, 2.17 mm i.d.) packed with Molecular Sieve 5A was used for the separation. The carrier gas was high purity argon gas. The temperature conditions of GC were 50 C for injection, column and detector. After the illumination, ZnO powders were separated from the solution through the 0.45 mm Advantec membrane filter. The concentrations of metal ion in the filtrate were evaluated by flame atomic absorption spectroscopy (Thermo Fisher Scientific K.K., Solaar S2 GFS97).
H2 Production, mol g
Upon optical excitation, photogenerated electrons accumulate at the lower-lying conduction band of one of the two semiconductors while holes accumulate at the valence band of the other semiconductor particle [14]. Arai et al. [15] have reported the Cu-doped ZnS hollow particle with high activity for hydrogen generation from alkaline sulfide solution. Sang et al. [10] have described the enhanced photocatalytic H2 production from glycerol solution over ZnO/ZnS core/shell nanorods prepared by a low temperature route. The selfassembled aligned Cu doped ZnO nanoparticles for photocatalytic hydrogen production have been presented by Kanade et al. [2]. Liu et al. [16] have described the Cu-doping ZnO/ZnS nanorods for the photoanode to enhance photocurrent and conversion efficiency. The vertically oriented CuO/ ZnO nanorod arrays from plasma-assisted synthesis to photocatalytic H2 production have been described by Simon et al. [5]. Andronic et al. [17] have presented the photochemical synthesis of copper sulphide/titanium oxide photocatalyst. Zhang et al. [18] have reported the doped solid solution, (Zn0.95Cu0.05)1xCdxS Nanocrystals, with high activity for H2 evolution from aqueous solutions. The photocatalytic H2 evolution systems containing ternary Cr2O3eSiCeTiO2 semiconductors have been described by Zhang et al. [19]. However, there are few reports on the in-situ metal sulfide preparation and their application into the photocatalytic hydrogen production using ZnO with high efficiency. Herein, we reported the highly efficient and stable CuS/ ZnO photocatalyst for the hydrogen production from aqueous S2 þ SO2 3 solution. The experimental parameters, such as the amount of CuS and the concentration of S2 and SO2 3 ions, were optimized.
600
300
0
ZnO
CuS
Ag2S
NiS
Fe2S3
Fig. 1 e Effect of metal sulfide on the photocatalaytic hydrogen production with ZnO from Na2S D Na2SO3 solution. ZnO, 20 mg; Metal sulfide suspension concentration, 6.7 ppm (as metal); Reaction time, 3 h; Reaction temperature, 50 C; [Na2S] [ 0.1 M; [Na2SO3] [ 0.1 M.
Please cite this article in press as: Gomathisankar P, et al., Photocatalytic hydrogen production with CuS/ZnO from aqueous Na2S þ Na2SO3 solution, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.04.131
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3.2. Effect of CuS amount on the photocatalytic H2 production The effect of CuS amount on the photocatalytic hydrogen production with ZnO was studied in the aqueous Na2SeNa2SO3 solution, as shown in Fig. 2. The photocatalytic activity increased with increasing the CuS suspension concentration up to 30 ppm. The maximum photocatalytic H2 evolution was 820 mmol g1. The 30 ppm of CuS corresponded to 4.3 wt% for ZnO. Above the value of CuS concentration, the hydrogen evolution decreased gradually. The possible reason could be attributable to (1) light filtration by the excess CuS, (2) aggregation of CuS particles, (3) dramatic change in surface modification
-1
2000
H2 Production, µmol g
ZnO (810 mmol g1) was approximately 3 times larger relative to that obtained with only ZnO. During the photoprocess, the surface of ZnO undergoes dissolution in alkaline sulfide solution and Zn2þ ion reacts with S2 ion to form ZnS on the surface of ZnO particles [15]. When Ag2S, Fe2S3 and NiS were formed in the Na2SeNa2SO3 solution, the photocatalytic activity was even lower compared with that of ZnO in this work. The NiS cocatalyst in Na2SeNa2SO3 solution enhanced the H2 evolution with CuGa3S5 [20]. Under the Sm2Ti2S2O5 and CdS photocatalysts, the NiS cocatalyst did enhance the activity of Sm2Ti2S2O5, and there was little enhancement of CdS [20]. Therefore, Tabata et al. [20] have concluded that the effect of the NiS cocatalyst on H2 evolution depends on which photocatalyst is employed. Pb-, or Cu-doped ZnS showed better ability to photoreductively defluorinate hexafluorobenzene under visible light irradiation and its defluorination mechanism was different from that on Ni-doped ZnS [21]. The particular combination of support photocatalyst and cocatalyst relationship in terms of electron transfer may play an important role in the photocatalytic hydrogen evolution [20]. Since it was found that CuS was very effective for the photocatalytic H2 production with ZnO, the amount of CuS was optimized for the photocatalytic H2 evolution using ZnO.
1500
1000
500
0 0
0.2
0.4
0.6
0.8
1.0
[Na2S], M Fig. 3 e Effect of Na2S concentration on the photocatalaytic hydrogen production with CuS/ZnO from Na2S D Na2SO3 solution. ZnO, 20 mg; CuS suspension, 30 ppm (4.3 wt% CuS/ZnO); Reaction time, 3 h; Reaction temperature, 50 C; [Na2SO3] [ 0.1 M.
of ZnO, and (4) deterioration of the charge mobility between CuS and ZnO [20,22]. After 3 h of illumination, the concentration of Cu in the filtrate was evaluated by flame atomic absorption spectroscopy. Since the concentration of Cu in the filtrate solution was very low, it seems that the complete formation and precipitation of CuS occurred in the solution.
3.3. Effect of Na2S concentrations on the photocatalytic H2 production First, the effect of Na2S concentration on the photocatalytic hydrogen production using ZnO was investigated with the 2500
H2 Production, mol g
-1
H2 Production, mol g
-1
900
600
300
2000
1500
1000
500
0 0
0 0
20
40
60
80
100
[CuS], ppm Fig. 2 e Effect of CuS amount on the photocatalaytic hydrogen production with ZnO from Na2S D Na2SO3 solution. ZnO, 20 mg; Reaction time, 3 h; Reaction temperature, 50 C; [Na2S] [ 0.1 M; [Na2SO3] [ 0.1 M.
0.1
0.2
0.3
0.4
0.5
0.6
[Na2SO3], M Fig. 4 e Effect of Na2SO3 concentration on the photocatalaytic hydrogen production with CuS/ZnO from Na2S D Na2SO3 solution. ZnO, 20 mg; CuS suspension, 30 ppm (4.3 wt%CuS/ZnO); Reaction time, 3 h; Reaction temperature, 50 C; [Na2S] [ 0.4 M.
Please cite this article in press as: Gomathisankar P, et al., Photocatalytic hydrogen production with CuS/ZnO from aqueous Na2S þ Na2SO3 solution, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.04.131
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aqueous Na2SeNa2SO3 solution. The results are illustrated in Fig. 3. An experiment with only Na2SO3 solution confirmed that hydrogen could not be produced. The photocatalytic hydrogen production increased with an increase in the Na2S concentration. There existed an optical concentration of 0.4 M Na2S for the maximal hydrogen evolution rate. BaO et al. [7] have reported the optimal concentration of 0.25 M for Na2S and 0.35 M for Na2SO3 in the photocatalytic hydrogen production with CdS. Generally, concentrated sacrificial reagents could be expected for the better diffusion of the reacting species to the surface of photocatalysts. The largest hydrogen evolution rate can not be attained in the presence of either diluted or concentrated sacrificial reagents. The present results were almost the same as those observed by BaO et al. [7].
hydroxyl radical and Hþ ions. Na2S þ Na2SO3 solution act as hole scavenger. The surface of ZnO undergoes dissolution in alkaline sulfide solution for the formation of ZnS on the surface of ZnO particles [15]. For the X ray diffraction analysis, the peak pattern of ZnO drastically changed after 3 h photoirradiation in the Na2S þ Na2SO3 solution, and the peaks were perfectly matched with ZnS crystal structure (although the data is not shown). Maybe, three different routes for scavenging the hole are possible (Eqs. 3, 4 and 6). The production of ions, which act as an optical filter and compete with S2 2 reduction of protons, is efficiently suppressed by mixing SO2 3 ions [7]. SO2 3 ions yields mainly thiosulfate ions. Finally, the formation of ZnS on the surface of ZnO appears to occur in the aqueous S2 þ SO2 3 solutions.
3.4. Effect of Na2SO3 concentrations on the photocatalytic H2 production
ZnO þ hn / ZnO (eCB þ hVBþ)
(1)
Next, the influence of Na2SO3 concentrations on the photocatalytic hydrogen evolution with CuS/ZnO was examined in the aqueous Na2SeNa2SO3 solution (Fig. 4). The hydrogen amount of 1310 mmol g1 was generated from the 0.4 M Na2S solution. The results may indicate that S2 anion was more effective for scavenging the hole compared with SO2 3 . The curve for the photocatalytic H2 production rose gradually with increasing the Na2SO3 concentration up to 0.4 M, and then the H2 production turned to decrease. The best photocatalytic hydrogen production with 2180 mmol g1 was observed in the aqueous 0.4 M Na2SO3 þ 0.4 M Na2S solution.
2H2O þ 2eCB / H2 þ 2OH
(2)
SO3 2 þ H2O þ 2hVB þ / SO4 2 þ 2Hþ
(3)
2S2 þ 2hVB þ / S2 2
(4)
S2 2 þ SO3 2 / S2O3 2 þ S2
(5)
SO3 2 þ S2 þ 2hVB þ / S2O3 2
(6)
S2O3 2 þ Hþ / HSO3 þ S
(7)
S þ 2e / S2
(8)
Zn2þ þ S2 / ZnS
(9)
3.5.
Reaction mechanism
In order to better understand the photocatalytic process of hydrogen formation from an aqueous Na2SeNa2SO3 solution, a possible mechanism based on the literature is shown in Figs. 5 and 6 [7e10,15,17]. The formed electrons and holes participate in redox processes at the semiconductor/solution interface. In photoirradiation process, light is bombarded on the surface of the ZnO, and holes and electrons are generated. At the same time, hole reacts with water molecule to produce
ZnS -2
H2O
CB
e e
ZnO CB
+
H /H2 0.0 V H2O
h
1
e e
0
1
3
h
CuS
2-
S2 , S2O3
+
h h+ 2-
VB
+
h h
+
ZnO
2-
S , SO3 2-
2
2-
S2 , S2O3
2-
3
h
+
T
2-
+
H /H2 0.0 V
+
h
+
O2/H2O 1.23 V
Cu2S IFCT
2-
S , SO3
H2
h
H2O
e e
O2/H2O 1.23 V 2
e e ZnS
-1 H2
0
ENHE, eV
ZnO ZnO CuS
e e
ENHE, eV
-1
ZnS
Cu2S
ZnO
ZnS H2
-2
+
h VB
2-
S , SO3 2-
+
2-
S2 , S2O3
h h
+
2-
4
Fig. 5 e Proposed mechanism for hetero junction ZnS/ZnO semiconductor.
Fig. 6 e Schematic diagram for the photocatalytic H2 generation with CuS/ZnS/ZnO.
Please cite this article in press as: Gomathisankar P, et al., Photocatalytic hydrogen production with CuS/ZnO from aqueous Na2S þ Na2SO3 solution, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.04.131
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Zn2þ þ S þ 2e / ZnS
(10)
CuS/ZnS/ZnO þ hn / CuS (eCB ) þ ZnS/ZnO (hVB þ)
(11)
2CuS þ 2e / Cu2S þ S2
(12)
Cu2S þ 2Hþ þ S2 / 2CuS þ H2
(13)
The p-type and n-type combined semiconductor (ZnS/ZnO) is formed by the photodeposition of ZnS on the surface of ZnO in the Na2S þ Na2SO3 solution (Fig. 5). The conduction band (CB) of ZnS lies on a more negative potential than that of ZnO, whereas the valence band (VB) of ZnO is more positive than that of ZnS. The photogenerated electrons from the conduction band of ZnS nanoparticles can transfer to that of the ZnO and holes on the valence band of ZnO can move to that of the ZnS. The combination of CuS with ZnS/ZnO will create pepen type hetero junction semiconductor (CuS/ZnS/ZnO) [10]. Since the band gap energy for CuS is near 0 eV [23], it almost works as the conductor. The conduction band of CuS is below the energy level of the Hþ/H2 redox reaction (Fig. 6). Therefore, CuS is not able to work as the center for H2 production. However, Cu2þ/Cuþ redox couple is thermodynamically possible. CuS may be interacted with ZnS/ZnO. Under the light irradiation, the electrons are photoexcited from the valence band of ZnS and ZnO directly to CuS by photoinduced interfacial charge transfer (IFCT). Then, CuS would be partially reduced to Cu2S [9,17]. Hence, the CuS/Cu2S clusters could work as an electron sink and cocatalyst to promote the separation and transfer of photogenerated electrons from valence band of ZnS to CuS/Cu2S cluster, where Hþ is reduced to hydrogen molecules. The potential of CuS/Cu2S is approximately 0.5 V vs. SHE, pH ¼ 0, which is more negative than the potential of Hþ/H2 and favors the reduction of Hþ, thus enhancing the photocatalytic hydrogen production activity [9]. Consequently, the recombination of eehþ can be delayed by the movement of the photogenerated electron to the CuS/Cu2S clusters.
4.
Conclusions
In summary, an in-situ photopreparation of ternary component semiconductor, CuS/ZnS/ZnO, was applied into the photocatalytic H2 evolution system containing the S2 þ SO2 3 solution. In the absent of CuS, the hydrogen production of 255 mmol g1 was observed on ZnO in S2 þ SO2 3 solution under the irradiation of 3 h. The maximum photocatalytic activity for H2 evolution on CuS/ZnS/ZnO was about 8.5 times better compared with those obtained with only ZnO. The developed system may provide a strategy for the design of stable and inexpensive technologies for highly efficient H2 production.
Acknowledgments The present research was partly supported by Grant-in-Aid for Scientific Research (C) 24510096 from the Ministry of
5
Education, Culture, Sports, Science, and Technology of Japan. Support was provided to P. G. as a Postdoctoral Fellowship for Foreign Research for the Public Foundation of Chubu Science and Technology Center. All experiments were conducted at Mie University. Any opinions, findings, conclusions, or recommendations expressed in this paper are those of the authors and do not necessarily reflect the view of the supporting organizations.
references
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[14] Karunakaran C, Dhanalakshmi R, Gomathisankar P. Phenolphotodegradation on ZrO2. Enhancement by semiconductors. Spectrochimica Acta Part A 2012;92:201e6. [15] Arai T, Senda S, Sato Y, Takahashi H, Shinoda K, Jeyadevan B, et al. Cu-doped ZnS hollow particle with high activity for hydrogen generation from alkaline sulfide solution under visible light. Chem Mater 2008;20:1997e2000. [16] Liu C, Liu Z, Li J, Li Y, Han J, Wang Y, et al. Cu-doping ZnO/ ZnS nanorods serve as the photoanode to enhance photocurrent and conversion efficiency. Microelectron Eng 2013;103:12e6. [17] Andronic L, Isac L, Duta A. Photochemical synthesis of copper sulphide/titanium oxide photocatalyst. J Photochem Photobiol A Chem 2011;221:30e7. [18] Zhang W, Zhong Z, Wang Y, Xu R. Doped solid solution: (Zn0.95Cu0.05)1xCdxS nanocrystals with high activity for H2 evolution from aqueous solutions under visible light. J Phys Chem C 2008;112:17635e42.
[19] Zhang Y, Xu Y, Li T, Wang Y. Preparation of ternary Cr2O3eSiCeTiO2 composites for the photocatalytic production of hydrogen. Particuology 2012;10:46e50. [20] Tabata M, Maeda K, Ishihara T, Minegishi T, Takata T, Domen K. Photocatalytic hydrogen evolution from water using copper gallium sulfide under visible-light irradiation. J Phys Chem C 2010;114:11215e20. [21] Kohtani S, Ohama Y, Ohno Y, Tsuji I, Kudo A, Nakagaki R. Photoreductive defluorination of hexafluorobenzene on metal-doped ZnS photocatalysts under visible light irradiation. Chem Lett 2005;34:1056e7. [22] Zhang HJ, Chen G, He X, Xu J. Synthesis and visible light photocatalysis water splitting property of Cu-doped NaInS2 synthesized by a hydrothermal method. Mater Res Bull 2012;47:4483e6. [23] Xu Y, Schoonen MAA. The absolute energy positions of conduction and valence bands of selected semiconducting minerals. Am Min 2000;85:543e6.
Please cite this article in press as: Gomathisankar P, et al., Photocatalytic hydrogen production with CuS/ZnO from aqueous Na2S þ Na2SO3 solution, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.04.131
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Photocatalytic hydrogen production with CuS/ZnO from aqueous Na2S D Na2SO3 solution Paramasivan Gomathisankar a,*, Katsumasa Hachisuka a, Hideyuki Katsumata a, Tohru Suzuki b, Kunihiro Funasaka c, Satoshi Kaneco a,b,** a
Department of Chemistry for Materials, Graduate School of Engineering, Mie University, Tsu, Mie 514-8507, Japan Environmental Preservation Center, Mie University, Tsu, Mie 514-8507, Japan c Department of Urban Environment, Osaka City Institute of Public Health and Environmental Sciences, Osaka 543-0026, Japan b
article info
abstract
Article history:
The photocatalytic hydrogen production in the sacrificial S2eSO2 anions was investi3
Received 22 January 2013
gated with ZnO in the addition of metal sulfides containing Ag2S, CuS, Fe2S3, and NiS. In the
Received in revised form
absence of metal sulfides, the photocatalytic H2 evolution using ZnO was observed with
17 April 2013
255 mmol g1. The CuS amount and the concentrations of S2 and SO2 3 ions were opti-
Accepted 23 April 2013
mized. It was found that ternary component semiconductor CuS/ZnS/ZnO was formed
Available online xxx
during the photocatalytic hydrogen production in the aqueous Na2S þ Na2SO3 solution. The photocatalytic hydrogen evolution with CuS/ZnS/ZnO in the 0.4 M Na2Se0.4 M Na2SO3
Keywords:
solution was more than about 8.5 times better compared with those obtained with only
Photocatalytic hydrogen generation
ZnO. The CuS clusters on the surface of ZnS/ZnO seem to play an important role on the
Metal sulfide
separation for electronehole pair and the enhancement of H2 production. Nano-sized ZnS/
ZnS/ZnO
ZnO photocatalytic hydrogen technology has great potential for low-cost, environmentally
CuS
friendly solar-hydrogen production to support the future hydrogen economy.
Hetero junction semiconductor
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Recently, zinc oxide (ZnO) has been widely used as the photocatalyst, owing to its high activity, low cost and environmentally friendly feature [1]. The band gap of ZnO is almost the same as that of TiO2, and their conduction band (CB) and valence band (VB) edges are very close to those of TiO2 [1]. However, there are only several reports concerning the photocatalytic H2 production with ZnO, because the system needs the suitable sacrificial agents in order to enhance the consumption of the photogenerated holes for minimizing the photocorrosion of ZnO and the electronehole pair recombination [2e5].
Since the extraction products of fossil energy resources containing sulfides and sulfites, which is now produced in large quantities, are undesirable and polluting byproduct in hydrogenation and flue-gas desulfurization processes at chemical plants, the photocatalytic reaction of hydrogen formation over sulfides from aqueous solution containing S2 and SO2 3 may become one of the promising methods for the H2 energy [6e13]. The metal/semiconductor and semiconductor/semiconductor composite materials are used to modify band gap position, facilitate charge rectification, and improve carrier separation for photocatalytic hydrogen generation [6e13].
* Corresponding author. Tel./fax: þ81 59 231 9427. ** Corresponding author. Department of Chemistry for Materials, Graduate School of Engineering, Mie University, Tsu, Mie 5148507, Japan. E-mail addresses: [email protected] (P. Gomathisankar), [email protected] (S. Kaneco). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.04.131
Please cite this article in press as: Gomathisankar P, et al., Photocatalytic hydrogen production with CuS/ZnO from aqueous Na2S þ Na2SO3 solution, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.04.131
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Experimental
2.1.
Chemicals and materials
Photocatalyst ZnO was purchased from SigmaeAldrich (BET specific surface area 15e25 m2/g, mean particle size 50e70 nm). A standard stock solution of metal ion (1 mg mL1) was prepared by the dissolution of AgNO3, CuCl2, Fe(NO3)3 and Ni(NO3)2. Sodium sulphide nonahydrate (98%) and sodium sulphite (97%) were obtained from Wako Pure Chemical Industries, Ltd. and were used as received. Laboratory pure water was obtained from an ultrapure water system (Advantec MFS Inc., Tokyo, Japan) resulting in a resistivity >18 MU cm.
2.2.
Photocatalytic hydrogen production
The pyrex column vessel reactor (inner volume: 55.6 mL) was used for the photocatalytic production of hydrogen from aqueous Na2SeNa2SO3 solution. The spout of vessel was tightly closed with septum and aluminum sealing. The pyrex glass cuts off all wavelength below 300 nm. Typically, 20 mg of the ZnO photocatalysts were added to 30 mL of aqueous Na2SeNa2SO3 solution in the photoreactor. A xenon lamp (500 W, UXL‒500D‒O) was applied as light source, which was positioned on the side of photoreactor. The light intensity was measured by a UV radio meter with a sensor of 320e410 nm
3.
Result and discussions
3.1. Effect of metal sulfides on the photocatalytic H2 evolution The effect of metal sulfides on the photocatalytic hydrogen production using ZnO was investigated in the aqueous Na2S þ Na2SO3 solution (Fig. 1). The suspension concentration of metal sulfide was 6.7 ppm as metal in the Na2SeNa2SO3 solution. The photocatalyst of ZnO in the absence of metal sulfides exhibited a small amount of H2 evolution (255 mmol g1). However, the hydrogen production with CuS/
900
-1
2.
wavelengths (UVRe400, Iuchi Co., Osaka, Japan), and the value was 1.0 mW/cm2. The ZnO photocatalysts were continuously dispersed in the aqueous Na2SeNa2SO3 solution by a magnetic stirrer during the irradiation. The temperature of the suspension in the photoreactor was kept constant at 50 C by the hot stirrer. Metal ion solution was added into the aqueous Na2SeNa2SO3 solution immediately before the irradiation. The formations of metal sulfides were confirmed by color changes of the reaction solutions. The irradiation time was 3 h. The hydrogen product from the reaction solution was analyzed by gas chromatography (GL Sciences, GC‒3200) with thermal conductivity detector (TCD). The stainless column (4 m long, 2.17 mm i.d.) packed with Molecular Sieve 5A was used for the separation. The carrier gas was high purity argon gas. The temperature conditions of GC were 50 C for injection, column and detector. After the illumination, ZnO powders were separated from the solution through the 0.45 mm Advantec membrane filter. The concentrations of metal ion in the filtrate were evaluated by flame atomic absorption spectroscopy (Thermo Fisher Scientific K.K., Solaar S2 GFS97).
H2 Production, mol g
Upon optical excitation, photogenerated electrons accumulate at the lower-lying conduction band of one of the two semiconductors while holes accumulate at the valence band of the other semiconductor particle [14]. Arai et al. [15] have reported the Cu-doped ZnS hollow particle with high activity for hydrogen generation from alkaline sulfide solution. Sang et al. [10] have described the enhanced photocatalytic H2 production from glycerol solution over ZnO/ZnS core/shell nanorods prepared by a low temperature route. The selfassembled aligned Cu doped ZnO nanoparticles for photocatalytic hydrogen production have been presented by Kanade et al. [2]. Liu et al. [16] have described the Cu-doping ZnO/ZnS nanorods for the photoanode to enhance photocurrent and conversion efficiency. The vertically oriented CuO/ ZnO nanorod arrays from plasma-assisted synthesis to photocatalytic H2 production have been described by Simon et al. [5]. Andronic et al. [17] have presented the photochemical synthesis of copper sulphide/titanium oxide photocatalyst. Zhang et al. [18] have reported the doped solid solution, (Zn0.95Cu0.05)1xCdxS Nanocrystals, with high activity for H2 evolution from aqueous solutions. The photocatalytic H2 evolution systems containing ternary Cr2O3eSiCeTiO2 semiconductors have been described by Zhang et al. [19]. However, there are few reports on the in-situ metal sulfide preparation and their application into the photocatalytic hydrogen production using ZnO with high efficiency. Herein, we reported the highly efficient and stable CuS/ ZnO photocatalyst for the hydrogen production from aqueous S2 þ SO2 3 solution. The experimental parameters, such as the amount of CuS and the concentration of S2 and SO2 3 ions, were optimized.
600
300
0
ZnO
CuS
Ag2S
NiS
Fe2S3
Fig. 1 e Effect of metal sulfide on the photocatalaytic hydrogen production with ZnO from Na2S D Na2SO3 solution. ZnO, 20 mg; Metal sulfide suspension concentration, 6.7 ppm (as metal); Reaction time, 3 h; Reaction temperature, 50 C; [Na2S] [ 0.1 M; [Na2SO3] [ 0.1 M.
Please cite this article in press as: Gomathisankar P, et al., Photocatalytic hydrogen production with CuS/ZnO from aqueous Na2S þ Na2SO3 solution, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.04.131
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3.2. Effect of CuS amount on the photocatalytic H2 production The effect of CuS amount on the photocatalytic hydrogen production with ZnO was studied in the aqueous Na2SeNa2SO3 solution, as shown in Fig. 2. The photocatalytic activity increased with increasing the CuS suspension concentration up to 30 ppm. The maximum photocatalytic H2 evolution was 820 mmol g1. The 30 ppm of CuS corresponded to 4.3 wt% for ZnO. Above the value of CuS concentration, the hydrogen evolution decreased gradually. The possible reason could be attributable to (1) light filtration by the excess CuS, (2) aggregation of CuS particles, (3) dramatic change in surface modification
-1
2000
H2 Production, µmol g
ZnO (810 mmol g1) was approximately 3 times larger relative to that obtained with only ZnO. During the photoprocess, the surface of ZnO undergoes dissolution in alkaline sulfide solution and Zn2þ ion reacts with S2 ion to form ZnS on the surface of ZnO particles [15]. When Ag2S, Fe2S3 and NiS were formed in the Na2SeNa2SO3 solution, the photocatalytic activity was even lower compared with that of ZnO in this work. The NiS cocatalyst in Na2SeNa2SO3 solution enhanced the H2 evolution with CuGa3S5 [20]. Under the Sm2Ti2S2O5 and CdS photocatalysts, the NiS cocatalyst did enhance the activity of Sm2Ti2S2O5, and there was little enhancement of CdS [20]. Therefore, Tabata et al. [20] have concluded that the effect of the NiS cocatalyst on H2 evolution depends on which photocatalyst is employed. Pb-, or Cu-doped ZnS showed better ability to photoreductively defluorinate hexafluorobenzene under visible light irradiation and its defluorination mechanism was different from that on Ni-doped ZnS [21]. The particular combination of support photocatalyst and cocatalyst relationship in terms of electron transfer may play an important role in the photocatalytic hydrogen evolution [20]. Since it was found that CuS was very effective for the photocatalytic H2 production with ZnO, the amount of CuS was optimized for the photocatalytic H2 evolution using ZnO.
1500
1000
500
0 0
0.2
0.4
0.6
0.8
1.0
[Na2S], M Fig. 3 e Effect of Na2S concentration on the photocatalaytic hydrogen production with CuS/ZnO from Na2S D Na2SO3 solution. ZnO, 20 mg; CuS suspension, 30 ppm (4.3 wt% CuS/ZnO); Reaction time, 3 h; Reaction temperature, 50 C; [Na2SO3] [ 0.1 M.
of ZnO, and (4) deterioration of the charge mobility between CuS and ZnO [20,22]. After 3 h of illumination, the concentration of Cu in the filtrate was evaluated by flame atomic absorption spectroscopy. Since the concentration of Cu in the filtrate solution was very low, it seems that the complete formation and precipitation of CuS occurred in the solution.
3.3. Effect of Na2S concentrations on the photocatalytic H2 production First, the effect of Na2S concentration on the photocatalytic hydrogen production using ZnO was investigated with the 2500
H2 Production, mol g
-1
H2 Production, mol g
-1
900
600
300
2000
1500
1000
500
0 0
0 0
20
40
60
80
100
[CuS], ppm Fig. 2 e Effect of CuS amount on the photocatalaytic hydrogen production with ZnO from Na2S D Na2SO3 solution. ZnO, 20 mg; Reaction time, 3 h; Reaction temperature, 50 C; [Na2S] [ 0.1 M; [Na2SO3] [ 0.1 M.
0.1
0.2
0.3
0.4
0.5
0.6
[Na2SO3], M Fig. 4 e Effect of Na2SO3 concentration on the photocatalaytic hydrogen production with CuS/ZnO from Na2S D Na2SO3 solution. ZnO, 20 mg; CuS suspension, 30 ppm (4.3 wt%CuS/ZnO); Reaction time, 3 h; Reaction temperature, 50 C; [Na2S] [ 0.4 M.
Please cite this article in press as: Gomathisankar P, et al., Photocatalytic hydrogen production with CuS/ZnO from aqueous Na2S þ Na2SO3 solution, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.04.131
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aqueous Na2SeNa2SO3 solution. The results are illustrated in Fig. 3. An experiment with only Na2SO3 solution confirmed that hydrogen could not be produced. The photocatalytic hydrogen production increased with an increase in the Na2S concentration. There existed an optical concentration of 0.4 M Na2S for the maximal hydrogen evolution rate. BaO et al. [7] have reported the optimal concentration of 0.25 M for Na2S and 0.35 M for Na2SO3 in the photocatalytic hydrogen production with CdS. Generally, concentrated sacrificial reagents could be expected for the better diffusion of the reacting species to the surface of photocatalysts. The largest hydrogen evolution rate can not be attained in the presence of either diluted or concentrated sacrificial reagents. The present results were almost the same as those observed by BaO et al. [7].
hydroxyl radical and Hþ ions. Na2S þ Na2SO3 solution act as hole scavenger. The surface of ZnO undergoes dissolution in alkaline sulfide solution for the formation of ZnS on the surface of ZnO particles [15]. For the X ray diffraction analysis, the peak pattern of ZnO drastically changed after 3 h photoirradiation in the Na2S þ Na2SO3 solution, and the peaks were perfectly matched with ZnS crystal structure (although the data is not shown). Maybe, three different routes for scavenging the hole are possible (Eqs. 3, 4 and 6). The production of ions, which act as an optical filter and compete with S2 2 reduction of protons, is efficiently suppressed by mixing SO2 3 ions [7]. SO2 3 ions yields mainly thiosulfate ions. Finally, the formation of ZnS on the surface of ZnO appears to occur in the aqueous S2 þ SO2 3 solutions.
3.4. Effect of Na2SO3 concentrations on the photocatalytic H2 production
ZnO þ hn / ZnO (eCB þ hVBþ)
(1)
Next, the influence of Na2SO3 concentrations on the photocatalytic hydrogen evolution with CuS/ZnO was examined in the aqueous Na2SeNa2SO3 solution (Fig. 4). The hydrogen amount of 1310 mmol g1 was generated from the 0.4 M Na2S solution. The results may indicate that S2 anion was more effective for scavenging the hole compared with SO2 3 . The curve for the photocatalytic H2 production rose gradually with increasing the Na2SO3 concentration up to 0.4 M, and then the H2 production turned to decrease. The best photocatalytic hydrogen production with 2180 mmol g1 was observed in the aqueous 0.4 M Na2SO3 þ 0.4 M Na2S solution.
2H2O þ 2eCB / H2 þ 2OH
(2)
SO3 2 þ H2O þ 2hVB þ / SO4 2 þ 2Hþ
(3)
2S2 þ 2hVB þ / S2 2
(4)
S2 2 þ SO3 2 / S2O3 2 þ S2
(5)
SO3 2 þ S2 þ 2hVB þ / S2O3 2
(6)
S2O3 2 þ Hþ / HSO3 þ S
(7)
S þ 2e / S2
(8)
Zn2þ þ S2 / ZnS
(9)
3.5.
Reaction mechanism
In order to better understand the photocatalytic process of hydrogen formation from an aqueous Na2SeNa2SO3 solution, a possible mechanism based on the literature is shown in Figs. 5 and 6 [7e10,15,17]. The formed electrons and holes participate in redox processes at the semiconductor/solution interface. In photoirradiation process, light is bombarded on the surface of the ZnO, and holes and electrons are generated. At the same time, hole reacts with water molecule to produce
ZnS -2
H2O
CB
e e
ZnO CB
+
H /H2 0.0 V H2O
h
1
e e
0
1
3
h
CuS
2-
S2 , S2O3
+
h h+ 2-
VB
+
h h
+
ZnO
2-
S , SO3 2-
2
2-
S2 , S2O3
2-
3
h
+
T
2-
+
H /H2 0.0 V
+
h
+
O2/H2O 1.23 V
Cu2S IFCT
2-
S , SO3
H2
h
H2O
e e
O2/H2O 1.23 V 2
e e ZnS
-1 H2
0
ENHE, eV
ZnO ZnO CuS
e e
ENHE, eV
-1
ZnS
Cu2S
ZnO
ZnS H2
-2
+
h VB
2-
S , SO3 2-
+
2-
S2 , S2O3
h h
+
2-
4
Fig. 5 e Proposed mechanism for hetero junction ZnS/ZnO semiconductor.
Fig. 6 e Schematic diagram for the photocatalytic H2 generation with CuS/ZnS/ZnO.
Please cite this article in press as: Gomathisankar P, et al., Photocatalytic hydrogen production with CuS/ZnO from aqueous Na2S þ Na2SO3 solution, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.04.131
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Zn2þ þ S þ 2e / ZnS
(10)
CuS/ZnS/ZnO þ hn / CuS (eCB ) þ ZnS/ZnO (hVB þ)
(11)
2CuS þ 2e / Cu2S þ S2
(12)
Cu2S þ 2Hþ þ S2 / 2CuS þ H2
(13)
The p-type and n-type combined semiconductor (ZnS/ZnO) is formed by the photodeposition of ZnS on the surface of ZnO in the Na2S þ Na2SO3 solution (Fig. 5). The conduction band (CB) of ZnS lies on a more negative potential than that of ZnO, whereas the valence band (VB) of ZnO is more positive than that of ZnS. The photogenerated electrons from the conduction band of ZnS nanoparticles can transfer to that of the ZnO and holes on the valence band of ZnO can move to that of the ZnS. The combination of CuS with ZnS/ZnO will create pepen type hetero junction semiconductor (CuS/ZnS/ZnO) [10]. Since the band gap energy for CuS is near 0 eV [23], it almost works as the conductor. The conduction band of CuS is below the energy level of the Hþ/H2 redox reaction (Fig. 6). Therefore, CuS is not able to work as the center for H2 production. However, Cu2þ/Cuþ redox couple is thermodynamically possible. CuS may be interacted with ZnS/ZnO. Under the light irradiation, the electrons are photoexcited from the valence band of ZnS and ZnO directly to CuS by photoinduced interfacial charge transfer (IFCT). Then, CuS would be partially reduced to Cu2S [9,17]. Hence, the CuS/Cu2S clusters could work as an electron sink and cocatalyst to promote the separation and transfer of photogenerated electrons from valence band of ZnS to CuS/Cu2S cluster, where Hþ is reduced to hydrogen molecules. The potential of CuS/Cu2S is approximately 0.5 V vs. SHE, pH ¼ 0, which is more negative than the potential of Hþ/H2 and favors the reduction of Hþ, thus enhancing the photocatalytic hydrogen production activity [9]. Consequently, the recombination of eehþ can be delayed by the movement of the photogenerated electron to the CuS/Cu2S clusters.
4.
Conclusions
In summary, an in-situ photopreparation of ternary component semiconductor, CuS/ZnS/ZnO, was applied into the photocatalytic H2 evolution system containing the S2 þ SO2 3 solution. In the absent of CuS, the hydrogen production of 255 mmol g1 was observed on ZnO in S2 þ SO2 3 solution under the irradiation of 3 h. The maximum photocatalytic activity for H2 evolution on CuS/ZnS/ZnO was about 8.5 times better compared with those obtained with only ZnO. The developed system may provide a strategy for the design of stable and inexpensive technologies for highly efficient H2 production.
Acknowledgments The present research was partly supported by Grant-in-Aid for Scientific Research (C) 24510096 from the Ministry of
5
Education, Culture, Sports, Science, and Technology of Japan. Support was provided to P. G. as a Postdoctoral Fellowship for Foreign Research for the Public Foundation of Chubu Science and Technology Center. All experiments were conducted at Mie University. Any opinions, findings, conclusions, or recommendations expressed in this paper are those of the authors and do not necessarily reflect the view of the supporting organizations.
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