Research Note pubs.acs.org/IECR
Highly Efficient Photocatalytic Hydrogen Production over PdS@CdS+ZnS(en)0.5 Photocatalyst under Visible Light Irradiation Hideyuki Katsumata,*,† Hideki Ando,† Tohru Suzuki,‡ and Satoshi Kaneco† †
Department of Chemistry for Materials, Graduate School of Engineering, Mie University, Tsu, Mie 514-8507, Japan Mie Global Environment Center for Education & Research, Mie University, Tsu, Mie 514-8507, Japan
‡
S Supporting Information *
ABSTRACT: A new photocatalyst, CdS+ZnS(en)0.5 (Cd/Zn = 6:4), was prepared by a simple mechanical mixing method and characterized by various techniques including X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and UV−vis diffuse reflectance spectroscopy. The photocatalytic activity of CdS+ZnS(en)0.5 was evaluated for hydrogen production from an aqueous solution under visible light. PdS-loaded CdS+ZnS(en)0.5 exhibited an efficient H2 production rate of 2600 μmol h−1 g−1, which was higher than that observed with PdS-loaded CdS and Cd0.6Zn0.4S solid solution catalysts. The apparent quantum yield at 405 nm was 23%. Furthermore, PdS@CdS+ZnS(en)0.5 showed good stability for prolonged H2 production reactions. It was proposed that enhancement of hole and electron transfer by PdS and ZnS(en)0.5 contributed to the high activity of this photocatalyst for H2 production.
1. INTRODUCTION As a potential solution to the global energy crisis and environmental pollution, the application of hydrogen energy has attracted great attention. Extensive work has been devoted to the efficient production of hydrogen at low cost. Because of the advantages of photocatalytic H2 production, such as its low pollution levels and reduced energy consumption, researchers have made enormous efforts toward improving the efficiency of H2 production over various photocatalysts.1 Recently, considerable research has been focused on the development of visiblelight-responsive photocatalysts to take advantage of solar light resources more effectively, because visible light constitutes a larger proportion of the solar spectrum than UV light.2,3 Metal chalcogenides, because of appropriate band gap widths and band edge positions, have elicited increasing attention in photocatalytic H2 production.4−6 CdS was the first photocatalyst to be employed in hydrogen production by irradiation of its aqueous solutions with visible light using sulfide ions as electron donors. CdS is characterized by a band gap of 2.4 eV, and the positions of its valence and conduction bands are suitable for the photocatalytic decomposition of water.7−10 However, problems such as the rapid recombination of the photogenerated charge carriers and photocorrosion under visible-light irradiation still exist and prohibit the wider application of CdS. A possible way to enhance the photocatalytic activity of CdS is to develop composite materials based on CdS and broader-band semiconductors such as ZnS. ZnS and CdS have similar crystal structures, so they fairly readily form Cd x Zn 1−x S solid solutions. 11,12 The well-known CdxZn1−xS solid solution photocatalyst has been studied extensively due to its controllable band structure and excellent performance in photocatalytic H2 production under visible-light irradiation.13−18 However, it would be highly desirable to develop more efficient CdS-based visible-light-induced photocatalysts from the viewpoint of effectively utilizing solar light for © 2015 American Chemical Society
H2 production, because the bandgaps of CdxZn1−xS solid solutions are generally larger than that of CdS. ZnS is a very interesting semiconductor because it is resistant to photocorrosion and the position of its conduction band is favorably negative.19,20 Improved photophysical properties have been reported in ZnS materials which have been surfacemodified with ethylenediamine (en) to form the organic− inorganic hybrid ZnS(en)0.5. The high activity of this hybrid semiconductor has been shown in the photodegradation of dyes.21 Hernández-Gordillo et al. have additionally demonstrated the photocatalytic activity properties of the ZnS(en)0.5 hybrid in Cr(VI) reduction and H2 production reactions.22,23 The improved photoreductive properties were attributed to the presence of en in the ZnS structural layer of the hybrid.22,23 Therefore, ZnS(en)0.5 can be expected to be a very promising photocatalyst for H2 production in aqueous solution. However, the ZnS(en)0.5 photocatalyst is responsive only in the UV region, which largely limits its application as a photocatalyst. To overcome this disadvantage, ZnS(en)0.5−CdS heterojunction semiconductor was synthesized by a precipitation method, and it was shown to improve the photocatalytic activity in Cr(VI) reduction.24 In this study, CdS and ZnS(en)0.5 photocatalysts were synthesized by the solvothermal treatment of the respective metal cation−diethyldithiocarbamate (DDTC) complex in the presence of en. The photocatalysts were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), UV−visible diffuse reflectance spectroscopy (DRS), and X-ray photoelectron spectroscopy (XPS). Then, CdS +ZnS(en)0.5 was prepared by a simple mechanical mixing method. The photocatalytic activity of PdS (cocatalyst)-loaded Received: Revised: Accepted: Published: 3532
February 2, 2015 March 26, 2015 March 30, 2015 March 30, 2015 DOI: 10.1021/acs.iecr.5b00451 Ind. Eng. Chem. Res. 2015, 54, 3532−3535
Research Note
Industrial & Engineering Chemistry Research
morphology, because the en ligand promotes the growth of CdS along the c-axis direction.25,29,30 The SEM image of the mechanically mixed CdS and ZnS(en)0.5 composite showed a mixture of the plate-like and nanorod morphologies. Figure 3 displays the UV−vis DRS spectra of the assynthesized samples. Pure CdS has a broad absorption in the
CdS+ZnS(en)0.5 (PdS@CdS+ZnS(en)0.5) for H2 production from water is reported for the first time here. We demonstrate that PdS and ZnS(en)0.5 loading can greatly enhance the photocatalytic activity of CdS, and the photocatalyst prepared by this simple method is highly efficient for H2 production from water under visible-light irradiation.
2. RESULTS AND DISCUSSION CdS and ZnS(en)0.5 samples were synthesized from Cd(DDTC)2 and Zn(DDTC)2 precursors by the solvothermal method in the presence of en.25 The XRD patterns of CdS, ZnS(en)0.5, and their mixture (CdS+ZnS(en)0.5) are shown in Figure 1. The XRD pattern of CdS could be indexed as the
Figure 3. DRS of CdS, ZnS(en)0.5, and CdS+ZnS(en)0.5.
visible region with an absorption edge at about 520 nm, whereas pure ZnS(en)0.5 has an absorption in the UV region only. The bandgaps of CdS and ZnS(en)0.5 were estimated to be 2.38 and 4.58 eV, respectively, which are consistent with the values reported in the literature.22−25,29,30 As clearly seen in Figure 3, the mechanically mixed sample CdS+Zn(en)0.5 exhibited two distinctive absorption edges. The edge around 520 nm is the same as that of CdS, whereas the edge in the UV region is almost the same as that of ZnS(en)0.5. Further, in the visible and UV regions below 490 nm, the CdS+Zn(en)0.5 sample displayed stronger absorption than pure CdS, although the absorption of pure ZnS(en)0.5 was still the weakest. A similar phenomenon has been reported, indicating that there should be an interaction between CdS and ZnS(en)0.5 that enhances photoabsorption serving as one of the indicators of the photocatalytic activity.31 However, this phenomenon could not be observed in the case of the mixture of CdS and ZnS (Supporting Information, Figure S1d). Figure 4a shows the photocatalytic H2 production activities of CdS, ZnS(en)0.5, their mixture, and related photocatalysts (Cd0.6Zn0.4S solid solution, ZnS, and CdS+ZnS) in aqueous solutions containing 0.35 M Na2S and 0.25 M Na2SO3 in the presence or absence of PdS (1 wt %) as a cocatalyst under visible-light irradiation (λ = 405 nm). Pure CdS (i.e., in the absence of PdS) afforded a very low photocatalytic H2 production rate of 22 μmol h−1 g−1. Similarly, pure ZnS(en)0.5 exhibited negligible visible-light H2 production activity because its bandgap is too large to absorb visible light (Figure 3). In contrast, considerable visible-light photocatalytic H2 production activity was recorded for the PdS@CdS sample, at a rate of 1600 μmol h−1 g−1, because the loading of PdS onto CdS can greatly promote hole transfer processes under visible light. In other words, PdS acts as an oxidation cocatalyst.32,33 The XPS analysis clearly showed that PdS was loaded on the surface of the photocatalysts during visible light irradiation (Supporting Information, Figure S2). After the mechanical mixing with ZnS(en)0.5, the photocatalytic activity of PdS@CdS+ZnS(en)0.5 remarkably increased to 2600 μmol h−1 g−1, with 23% apparent quantum efficiency at 405 nm, although the activities of PdS@ ZnS(en)0.5 and CdS+ZnS(en)0.5 were still low. In addition, the
Figure 1. XRD patterns of CdS, ZnS(en)0.5, and their mixture, CdS +ZnS(en)0.5.
hexagonal structure (JCPDS no. 89-2944) with good crystallinity. The three diffraction peaks at 24.8, 26.5, and 28.2° could be assigned to the (100), (002), and (101) planes, respectively. The XRD pattern of ZnS(en)0.5 was in agreement with those reported in previous studies,26−28 which revealed that ZnS(en)0.5 is a layered complex with orthorhombic structure consisting of ZnS layers connected to each other through the bonding of the nitrogen atoms in the en molecules along the a-axis. For the CdS+ZnS(en)0.5 sample, the XRD pattern showed the presence of both compounds. Figure 2 shows the SEM images of the as-synthesized samples. SEM observation of ZnS(en)0.5 clearly shows regular plate-like shapes with micrometer widths and thicknesses of about 200 nm. In contrast, the CdS sample exhibited nanorod
Figure 2. SEM images of (a) CdS, (b) ZnS(en)0.5, (c) CdS +ZnS(en)0.5, and (d) magnification of image c. 3533
DOI: 10.1021/acs.iecr.5b00451 Ind. Eng. Chem. Res. 2015, 54, 3532−3535
Research Note
Industrial & Engineering Chemistry Research
modified Au electrode in the presence of catechol.35 The ZnS(en)0.5 particles strongly promoted electron transfer between catechol and the Au electrode. Therefore, the ZnS(en)0.5 should enhance the electron transfer between PdS and the sacrificial reagents SO32−/S2− in this study. In combination, then, PdS and ZnS(en)0.5 would be able to promote the hole and electron transfer processes, respectively. Consequently, charge separation would be accelerated on CdS, leading to the higher activity of the photocatalyst as well as the enhancement of photoabsorption in the visible region (Figure 3). However, further studies are needed to clarify the role of ZnS(en)0.5 and the mechanism for photocatalytic reactions over PdS@CdS+ZnS(en)0.5 in greater detail. Figure 4b shows the time course of H2 production obtained over PdS@CdS+ZnS(en)0.5 under visible-light irradiation for 35 h. The production of H2 steadily increased with prolonged irradiation. A total of 91 mmol g−1 H2 gas, corresponding to 2.0 L g−1, was produced over PdS@CdS+ZnS(en)0.5. No obvious deactivation of the photocatalyst was observed, suggesting its good stability toward photocatalytic H2 production under visible light. Furthermore, no changes in the photocatalyst sample were observed after the reaction in its XRD pattern, SEM image, or XPS spectra (Supporting Information, Figures S2 and S4). On the other hand, the photocatalytic H2 production over PdS@CdS decreased from 11000 μmol g−1 (first cycle) to 8000 μmol g−1 (fifth cycle) after 7 h of visible light irradiation, indicating a loss of about 27% (Supporting Information, Figure S6). Therefore, the mixing of ZnS(en)0.5 not only enhanced the photocatalytic activity but also improved the stability of the photocatalyst.
Figure 4. (a) Photocatalytic H2 production over various photocatalysts under visible-light irradiation (λ = 405 nm). Blue and red represent reactions without and with PdS (1 wt %), respectively. (b) Time courses of H2 production over PdS@CdS+ZnS(en)0.5 under visible light irradiation (λ = 405 nm).
3. CONCLUSIONS In summary, a CdS+ZnS(en)0.5 (Cd/Zn = 6:4) photocatalyst was fabricated via a simple physical mixing method. High photocatalytic activity was obtained for PdS@CdS+ZnS(en)0.5, offering a H2 production rate of 2600 μmol h−1 g−1 with 23% apparent quantum efficiency at 405 nm, which was about 1.6 and 1.2 times higher than those of PdS@CdS and the PdS@ Cd0.6Zn0.4S solid solution, respectively. Furthermore, PdS@ CdS+ZnS(en)0.5 showed good stability for prolonged H2 production reactions. The enhanced activity was due to the promotion of hole and electron transfer processes by PdS and ZnS(en) 0.5, respectively, leading to an efficient charge separation of the photogenerated electron−hole pairs from CdS. This present study provides new insights toward enhancing the photocatalytic H2 production activity of CdS by mixing with the ZnS(en)0.5 photocatalyst.
apparent quantum efficiencies of PdS@CdS+ZnS(en)0.5 were 21%, 16%, and 1.4% for 420, 450, and 500 nm, respectively. It should be noted that the H2 production rate from PdS@CdS +ZnS(en)0.5 was higher than that from the
[email protected] solid solution (2100 μmol h−1 g−1), which presented the highest H2 production activity in a series of PdS@CdxZn1−xS solid solutions, as reported in our previous study.34 The solid solution employed the same molar ratio of Cd to Zn as that in the mechanical mixture of CdS+ZnS(en)0.5. Furthermore, enhancement of the photocatalytic H2 production could not be obtained over PdS@CdS+ZnS (Cd/Zn = 6:4), which produced H2 at a rate of 1400 μmol h−1 g−1. PdS@CdS+ZnS was characterized by XRD and XPS analyses (Supporting Information, Figure S3). The XRD pattern showed the coexistence of CdS and ZnS, while there was no peak attributable to PdS, similar to the case of PdS@CdS+ZnS(en)0.5 (Supporting Information, Figure S4a). In addition, there were no apparent differences in the XPS spectra of PdS@CdS +ZnS(en)0.5 (Figure S2) and PdS@CdS+ZnS. The PdS amounts on CdS+ZnS, as revealed by the XPS results, were almost identical to those on CdS+ZnS(en)0.5, indicating that ZnS(en)0.5 plays an important role in effective photocatalytic H2 production. We consider the above experimental phenomena to be strongly related to the presence of ZnS(en)0.5, and the photocatalytic mechanism for the PdS@CdS+ZnS(en)0.5 sample is tentatively proposed and schematically illustrated in Supporting Information, Figure S5. Ni et al. revealed the electrochemical properties of ZnS(en)0.5 using a ZnS(en)0.5-
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ASSOCIATED CONTENT
* Supporting Information S
Experimental details and Figures S1−S6. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel: +81-59231-9425. Fax: +81-59231-9425. Notes
The authors declare no competing financial interest. 3534
DOI: 10.1021/acs.iecr.5b00451 Ind. Eng. Chem. Res. 2015, 54, 3532−3535
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Industrial & Engineering Chemistry Research
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ACKNOWLEDGMENTS This work was partly supported by a Grant-in-Aid for Scientific Research (C) (No. 24510095) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan, A-STEP (No. AS232Z00622C) from Japan Science and Technology Agency, and the Okasan-Kato Foundation.
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