Synergetic Effect of In and Ag Co-doped ZnS for Enhanced Photocatalytic Hydrogen Evolution under Visible Light Irradiation

Melody Kimi1,2,a*, L. Yuliati3,b, M. Shamsuddin1,c

1Department of Chemistry, Faculty of Science, Universiti Teknologi Malaysia, 81310 Johor Bahru,

Johor, Malaysia.

2Centre for Pre-University Studies, Universiti Malaysia Sarawak, 94300 Kota Samarahan,

Sarawak, Malaysia.

3Ibnu Sina Institute for Fundamental Science Studies, Universiti Teknologi Malaysia, 81310 Johor

Bahru, Johor, Malaysia.

akmelody@preuni.unimas.my, bleny@ibnusina.utm.my, cmustaffa@kimia.fs.utm.my

Keywords: hydrogen, visible light, ZnS, co-doped, photocatalyst

Abstract. In (Indium) and Ag (Argentum) co-doped ZnS photocatalysts were successfully prepared by hydrothermal method to extend the light absorption of ZnS to the visible light region. The concentration of In was constant while for Ag was varied to optimize the photocatalytic activity. The In and Ag co-doped ZnS (Zinc Sulfide) photocatalysts showed smaller band gap energy compared to single doped In(0.1)-ZnS and undoped ZnS. The photocatalytic activity of In and Ag co-doped ZnS photocatalysts was evaluated from the amount of hydrogen produced. The hydrogen evolution rate from aqueous solution containing Na2SO3 (Sodium Sulfite) and Na2S (Sodium Sulfide) as sacrificial reagent under visible light irradiation obtained from In and Ag co-doped ZnS is higher compared to the single doped In(0.1)-ZnS when optimum amount of Ag dopant was added. The highest photocatalytic activity is observed for In(0.1),Ag(0.01)-ZnS with hydrogen production rate of 26.82 µmol/h. The higher performance of this photocatalyst is ascribed to the extended visible light absorption, efficient charge separation as well as improved electron transfer associated with synergistic effect of appropriate amount of In and Ag co-doped ZnS.

Introduction

Hydrogen production under solar light irradiation with the assistance of semiconductor photocatalyst is one of the potential solutions to resolve the depletion of fossil fuels [1]. ZnS possesses high conduction band that is essential for photocatalytic hydrogen production [1] but ZnS is not active under visible light irradiation. Indium based sulfide material has gain interest due to the visible light active properties which can utilize most of the solar light energy. The In2S3 (Indium Sulfide) has showed good efficiency for hydrogen production under visible light irradiation [2]. Thus, indium can be doped into ZnS to improve the absorption properties. Ag dopant is frequently used as electron trap, enhancing electron-hole separation and promoting the interfacial electron transfer processes [3]. The addition of Ag increases the ability of a photocatalyst to absorb visible light, due to localized surface Plasmon resonance [4]. Studies on Ag doped ZnS for hydrogen production has also been done previously [5]. It would be beneficial if both In and Ag are doped into ZnS to improve the visible light absorption and also enhance the photocatalytic activity by inhibiting fast electron-hole recombination. In this study, In(0.1),Ag(x)-ZnS was successfully synthesized by hydrothermal method. The amount of Ag was varied to optimize the performance of the photocatalysts and systematically studied the physical-chemical properties and photocatalytic activities under visible light irradiation. The effects of co-doping on the photocatalytic activities were also discussed.

Advanced Materials Research Vol. 1024 (2014) pp 368-371Online available since 2014/Aug/28 at www.scientific.net© (2014) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.1024.368

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Experimental

Synthesis of Samples. Hydrothermal method was used to synthesize the powder samples of In(0.1)-ZnS and In(0.1),Ag(x)-ZnS (x = 0.01, 0.03, 0.05, x represents mol ratio) in a similar way to the previous literatures [6]. In a typical synthesis for In(0.1),Ag(0.01)-ZnS, 0.2 mmol of AgNO3 (Unilab, 99.9%), 2 mmol of In(NO3)2.xH2O (Aldrich, 98%), 18 mmol of Zn(CH3COO)2.2H2O (GCE chemicals, 98%) and 20 mmol of CH3CSNH2 (Merck, 99%) were dissolved in 50 mL of distilled water. The solution was added into a Teflon-lined autoclave that was sealed and heated in an oven at 433 K for 8 h. After natural cooling to room temperature, the precipitates were washed with distilled water for several times and dried in vacuum at room temperature.

Characterizations. Powder X-ray diffraction (XRD) patterns were obtained with an X-ray diffractometer, Bruker Advance D8 Siemens 5000 using Cu Kα radiation (λ = 0.15418 nm, 40 kV, 40 mA). The elemental analysis was completed on a Bruker S4 PIONEER X-ray fluorescence spectrum (XRF), using Ru (Ruthenium) target and 4 kW power. The morphologies and nanocrystal sizes of the samples were determined with field emission scanning electron microscopy (FESEM) using JEOL JSM 6701F with platinum coating (2 kV, 10 mA). The elemental ratio was measured using energy dispersive X-ray (EDX) spectroscopy utilizing an Oxford Instrument INCA X-sight analyzer equipped with a Sapphire super ultra window detector attached to the Zeiss Supra 35VP FE-SEM. The accelerating voltage of the electron gun was set to 15 kV. The diffuse reflectance UV–visible (DR UV–vis) spectra were recorded on a Perkin Elmer Ultraviolet–visible Spectrometer Lambda 900. BaSO4 (Barium sulfate) was used as the reference.

Photocatalytic Testings. Photocatalytic hydrogen evolution was performed in a closed-side irradiated-Pyrex cell equipped with an outer water cooling system to keep the temperature throughout the reaction at 25°C. In all experiments, the powder sample (0.2 g) was dispersed by magnetic stirring in an aqueous solution (190 mL) containing 0.25 M Na2SO3 and 0.35 M Na2S [6] as sacrificial agents. Nitrogen gas was purged through the reaction cell for 30 min before reaction to remove air. A 300 W Xe lamp was focused on the side window of the cell through a cut-off filter (λ ≥ 425 nm, TrusTech PLS-SXE 300).

Results and Discussion

Crystal Structure and Morphology. Figure 1 shows the XRD patterns of ZnS, In(0.1)-ZnS and In(0.1),Ag(x)-ZnS. Major diffraction peaks can be indexed to the (111), (200), (220) and (311) planes of cubic zinc blende phase (Joint Committee on Powder Diffraction Standards (JCPDS) 772100) [6]. No impurity peaks were observed. There were no obvious peak shifts when In or Ag was added, although the diffraction peaks of ZnS shifted slightly to the left with addition of In as the ionic radius of In3+ (0.80 Å) is larger than Zn2+ (0.72 Å). Further increase in Ag amount did not shift the diffraction peaks. This shows that Ag+ might not be incorporated into the ZnS lattice due to the larger ionic radius of Ag+(1.26 Å) but probably attached on the surface. The peak intensity decreased and became broader when Ag was added into In(0.1)-ZnS and it continued to decrease as the amount of Ag increased. This indicates that the addition of Ag resulted in smaller crystallite size. Ag probably inhibited the crystal growth. The particle morphologies of the prepared samples are shown by FESEM in Figure 2. ZnS have small nanospheres in the range of 20 to 50 nm, which agglomerated into undefined shape. For In(0.1)-ZnS and In(0.1),Ag(x)-ZnS, the large spheres with the range of 200 to 400 nm was observed as the result of agglomerated nanospheres with the range of 20 to 50 nm. The addition of Ag did not disrupt the morphology of the samples regardless of the amount. The EDX and XRF of In(0.1),Ag(0.01)-ZnS as representative sample confirmed the existence of In, Ag, Zn and S on the surface and bulk (not shown). The XRF result confirmed the amount of elements present in bulk is similar to the mol ratio of elements added during synthesis.

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Optical Properties. Figure 3 depicts the DR UV-visible spectra of ZnS, In(0.1)-ZnS and In(0.1),Ag(x)-ZnS samples. The addition of In dopant only improved the light absorption of ZnS slightly due to its low loading content. Ag added samples showed an improved absorption towards longer wavelength as the amount of Ag added increased. Since Ag+ ion ionic radius is much larger than both In3+ and Zn2+, the Ag would not only present as a dopant but also present on the surface. Therefore, the visible light region caused by Ag doping could be resulted from surface Plasmon absorption due to Ag0 particles [4]. Although surface plasmon absorptions are around 470 to 580 nm, the broadening of absorption to longer wavelength has been reported [7]. The broadening could be due to highly dispersed Ag which results in increased interaction between Ag and the host particles [7]. Improvement of visible light absorption in the Ag added samples led to a reduction of the band gap energy. The addition of Ag facilitated light harvest and charge carrier separation by acting as electron trap on the surface which was expected to improve the photocatalytic activity efficiencies.

Figure 1 XRD patterns of (a) ZnS, (b) In(0.1)-ZnS, (c) In(0.1),Ag(0.01)-ZnS

(d) In(0.1),Ag(0.03)-ZnS, and (e) In(0.1),Ag(0.05)-ZnS

10 20 30 40 50 60 70

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Figure 2 FESEM images of (a) ZnS, (b) In(0.1)-ZnS, and (c) In(0.1),Ag(0.01)-

ZnS

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Figure 4 Photocatalytic hydrogen evolution on In(0.1)-ZnS (●), In(0.1),Ag(0.01)-ZnS ( ), In(0.1),Ag(0.03)-ZnS (□), and In(0.1),Ag(0.05)-ZnS (○) under visible light irradiation.

Irradiation time (h)

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Figure 3 DR UV-vis spectra of (a) ZnS (b) In(0.1)-ZnS, (c) In(0.1),Ag(0.01)-ZnS (d) In(0.1),Ag(0.03)-ZnS, and (e) In(0.1),Ag(0.05)-ZnS

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370 Advancement of Materials and Nanotechnology III

Photocatalytic Activity. Figure 4 shows the time course of hydrogen production over In(0.1)-ZnS and In(0.1),Ag(x)-ZnS samples under visible light irradiation. The In(0.1),Ag(0.01)-ZnS and In(0.1),Ag(0.03)-ZnS showed much higher photocatalytic activity compared to In(0.1)-ZnS. The In(0.1),Ag(0.01)-ZnS hydrogen production rate was 26.82 µmol/h, which was 1.6 times higher than the single In(0.1)-ZnS. The method of co-doping efficiently enhanced the photocatalytic activity. However, when too much of Ag was added, as in In(0.1),Ag(0.05)-ZnS, the photocatalytic activity drastically decreased. It may be because Ag0 was oxidized to Ag+ acted as recombination center of the electron-hole pairs on the surface. Excessive Ag particles adsorbed on the surface might also block the light absorption [8]. As mentioned above, the optimum photocatalyst was In(0.1),Ag(0.01)-ZnS. Although the crystallinity decreased because smaller crystallite size was formed when Ag was added, the visible light absorption increased. The smaller crystallite size benefits from fast electrons transferring to the surface to participate in water reduction process. When the absorption shifts to longer wavelength, more photon energy could be utilized to generate electron-hole pairs. This result indicated that Ag on the surface improved the photocatalytic activity by increasing visible light absorption and successfully enhanced the charge separation by trapping electrons. Conclusions

In summary, In(0.1),Ag(0.01)-ZnS had the highest photocatalytic activity with hydrogen production rate of 26.82 µmol/h. The co-doping of Ag at optimum amount can effectively capture the photoinduced electrons for water reduction process and enhance ZnS response range to visible light region. The improved photocatalytic activity was also due to the small crystallite size of In(0.1),Ag(0.01)-ZnS. References

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