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Enhanced photocatalytic hydrogen production over In-rich (Ag–In–Zn)S particles
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 3 ) 1 e9
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Enhanced photocatalytic hydrogen production over In-rich (AgeIneZn)S particles Po-Chang Lin b, Pei-Ying Wang b, Yuan-Yao Li b, Chi Chung Hua b,**, Tai-Chou Lee a,* a b
Department of Chemical and Materials Engineering, National Central University, 300 Jhongda Rd., Jhongli, Taoyuan 320, Taiwan Department of Chemical Engineering, National Chung Cheng University, 168 University Rd., Min-Hsiung, Chia-Yi 621, Taiwan
article info
abstract
Article history:
The ratio of ZnS to AgInS2 is usually adjusted to tune the band gaps of this quaternary (Ag
Received 14 January 2013
eIneZn)S semiconductor to increase photocatalytic activity. In this study, the [Zn]/[Ag]
Received in revised form
ratio was kept constant. The hydrogen production rate was enhanced by increasing the
11 April 2013
content of indium sulfide. Compared to the steady H2 evolution rate obtained with equal
Accepted 24 April 2013
moles of indium and silver ([In]/[Ag] ¼ 1, 0.64 L/m2 h), that obtained with In-rich photo-
Available online xxx
catalyst ([In]/[Ag] ¼ 2, 3.75 L/m2 h) is over 5.86 times higher. The number of nanostep structures, on which the Pt cocatalysts were loaded by photodeposition, increased with the
Keywords:
content of indium. The indium-rich samples did not induce phase separation between
Photocatalyst
AgxInxZnyS2xþy and AgIn5S8, instead forming a single-phase solid solution. Although the
Hydrogen production
photocatalytic activity decreased slightly for bare In-rich photocatalysts, Pt loading played
Water splitting
a critical role in the hydrogen production rate. This study demonstrates the significant
Solar energy
effect of In2S3 on this unique (AgeIneZn)S photocatalyst. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Due to deep concerns over global warming and increasing energy costs, an increasing number of researchers have focused on improving the efficiency of energy utilization and the development alternative energy sources. Semiconductor systems can convert solar energy into hydrogen, which is an energy carrier that produces no harmful emissions. This conversion has been considered an ideal solution for reducing the use of fossil fuels [1e6] and mitigating environmental problems [7e9]. Metal oxide photocatalysts, such as TiO2, SrTiO3, Ta2O5, LiTaO3, KTaO3, CaTa2O6, K4Nb6O7, AgTaO3, NaTaO3, and Ga2O3 [5,10e12], have been studied for decades for application in artificial photosynthesis, typically the solar splitting of water into hydrogen and oxygen. However, due to
their large band gap energies (3.6e4.7 eV), metal oxide photocatalysts are mainly limited to ultraviolet (UV) light activity [5,10,13], which account for only approximately 3e5% in the solar spectrum. Since visible light accounts for almost half of the photons, it is necessary to develop a visible-light-responsive photocatalyst [14,15]. Some metal sulfide and metal oxide materials can split water to generate hydrogen and oxygen under visible-light irradiation [16e20]. CdS is one of the photocatalysts for H2 production from water [21,22]. However, since CdS includes harmful cadmium, rigorous safety measures must be taken during the processing steps. The design of solid-solution particles and quantum dots with controlled electronic structures, including (AgeIneGa)S [23], (ZneIn)S [24], (AgeIneZn)S [25e32], (CueIneZn)S [33,34], (CueAgeIneZn)S [35], (CdeZn)S [36e41], and (ZneCueCd)S [42,43], has progressed
* Corresponding author. Tel.: þ886 3 4227151 34211; fax: þ886 3 4252296. ** Corresponding author. Tel.: þ886 5 2720411 33412; fax: þ886 5 2721206. E-mail addresses: [email protected] (C.C. Hua), [email protected] (T.-C. Lee). 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.2013.04.125
Please cite this article in press as: Lin P-C, et al., Enhanced photocatalytic hydrogen production over In-rich (AgeIneZn)S particles, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.04.125
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in recent years. A series of multi-component metal sulfide solid solutions that show apparent activity for hydrogen evolution from water containing sacrificial reagents, SO2 3 and S2 under visible-light irradiation have been reported [13,15,25,44]. AgInZn7S9 exhibits a high quantum yield (19.8% at 420 nm) of H2 production, with a reported steady evolution rate of 1.8 L/m2 h under simulated sunlight [25]. The photoelectrochemical properties of (AgeIneZn)S solid solutions were investigated by immobilizing the nanoparticles on ZnO nanorod electrodes [44]. The structure of (AgeIneZn)S was investigated and the binary AgInS2eZnS phase diagram was reported [45]. The single-phase regions are: AgInS2 (with less than 2 mol.% ZnS), sphalerite ZnS (with 97e100 mol.% ZnS), and a solid solution of ZnSeAgInS2 with a hexagonal structure (60e90 mol.% ZnS) [46,47]. ZnS, with a wide band gap energy of 3.5 eV, is a photocatalyst under UV irradiation. (AgeIn)S, including AgInS2 and AgIn5S8, is a narrow-band-gap semiconductor with a band gap energy of 1.87e2.03 eV [48]. Tsuji et al. synthesized a high-efficiency photocatalyst by making a solid solution of these two materials. A theoretical basis was provided [25,35]. The band gap of ZnSeAgInS2 solid solution can be tuned continuously from 1.8 to 2.4 eV by adjusting the ZnS content [25]. With this strategy, the H2 production efficiency can be maximized by optimizing the ratio of ZnS to AgInS2 [25,29]. However, the preparation of single-phase AgInS2 with high purity is difficult. The operating window is rather small; in theory, only equal moles of Ag2S and In2S3 will suffice, according to the phase diagram of Ag2S and In2S3 [49]. Based on our experience, high-temperature (>800 C) heat treatment is also required to prepare good-quality photocatalysts [29]. Even with an optimal ratio of ZnS to AgInS2, the routine fabrication of (AgeIneZn)S solid solution will be unstable and probably not even repeatable. Therefore, a robust synthetic protocol is needed for further industrial applications. In the present study, the [In]/[Ag] ratio is tuned and the phase type and activity of the obtained photocatalysts is discussed. By fixing the [Zn]/[Ag] ratio, changes in the band gap energy are negligible because there is only a minor variation of ZnS concentration ratios during the processing steps. However, the effect of indium on the activity of Pt-loaded photocatalyst is dramatic and independent of the effect of the band gap. In our wet chemistry process, metal sulfides were generated from reactions between metal nitrate and thioacetamide (TAA), the source of S2. The precipitates of metal sulfides can be collected, when concentration products exceed the solubility constant KSP. In order to keep the [Zn]/[Ag] ratio at 7, the optimal composition for hydrogen production [25,29], the reaction was divided into two steps. Ample powders of (AgeZn)S were synthesized in the first step. The powders were then used for preparing photocatalysts with various amounts of indium. Under suitable experimental conditions and heat treatment in a tube furnace with flowing ultra-pure nitrogen, (AgeIneZn)S solid solutions were obtained, as determined by X-ray diffraction (XRD). Scanning electron microscopy (SEM) images show nanostep structures on the surface of the particles. In the photochemical study, the hydrogen production was found to be a function of [In]/[Ag], with the optical band gap kept constant. The highest H2 evolution rate ([In]/[Ag] ¼ 2)
is 4.86 times higher than that of the original sample ([In]/ [Ag] ¼ 1) under 100-mW/cm2 Xe irradiation. The excess photocatalysts were loaded to make sure no light could escape from the rear of the illumination area. Evidence of the enhanced semiconductor-assisted photoreaction is provided and a possible mechanism is proposed.
2.
Experimental details
2.1.
Preparation of (AgeIneZn)S photocatalyst
A completely aqueous solution route without toxic H2S gas was adopted. The sulfur ions were provided by the decomposition of thioacetamide in an acidic environment [29]. The detailed experimental procedure was reported previously [29]. Briefly, aqueous solutions of 0.4 M indium nitrate (In(NO3)3$xH2O, Sigma Aldrich; 99.99%), 1.6 M zinc nitrate (Zn(NO3)2$6H2O, J. T. Baker; 98%), and 1 M thioacetamide (TAA, CH3CSNH2, S2 source, Merck; 99%) were prepared separately. The zinc nitrate solution (43.75 mL; 70 mmol) and silver nitrate (1.7 g; 10 mmol) (AgNO3, Sigma Aldrich; 99.9%) were added into a 250 mL round-bottom flask, fully dissolved and mixed well. The pH value of the solution was then adjusted to be around 1 by adding concentrated sulfuric acid (H2SO4, Merck; 95e97%). 100 mL of TAA (100 mmol) was poured into the flask, mixed well, and maintained at 80 C in a thermostat bath for 8 h. The precipitates were rinsed thoroughly with deionized (DI) water. They were then dried at 80 C in an oven for 12 h. In the second stage, 0.806 g (Ag:Zn ¼ 1:7) of the dried powders with a fixed [Zn]/[Ag] ratio were mixed with various amounts of 0.4 M indium nitrate solution (2.5, 3.125, 3.75, 4.375, 5, and 5.625 mL). The precursor solutions were diluted using 20 mL of DI water in a 100-mL round-bottom flask under rigorous magnetic stirring. The pH value of the suspension was adjusted to be around 2 by adding concentrated sulfuric acid. Finally, prescribed TAA solutions (2, 2.5, 3, 3.5, 4, and 4.5 mL) were added into the suspension. After a reaction at 80 C for 5 h, the precipitates were collected and rinsed thoroughly with DI water. They were then dried at 80 C in an oven for 12 h. The obtained powders were covered with a quartz plate and heat-treated at 850 C for 5 h in an ultra-pure N2 environment in a tube furnace. The molar ratios of the precursor solutions are given in Table 1.
2.2.
Characterization of samples
The crystal structures of the samples were determined using XRD (Bruker KAPPA APEX II). The XRD patterns were recorded in the 2q range of 20 e70 at a scanning rate of 4 /min. Fieldemission scanning electron microscopy (FE-SEM, Hitachi S4800-I) was employed to investigate the surface morphology of the powders. Transmission electron microscopic images were obtained using a JEOL JEM-2010 microscope operated at 200 kV. The absorption spectra (UVeVisible (UVeVis), Jasco V670) were measured with an integrating sphere in the wavelength range of 300e800 nm at room temperature. Surface areas were determined by the BET method (Micromeritics ASAP 2020). Photoluminescence spectra were recorded using a
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photoluminescence spectrometer (PerkinElmer LS-55). The samples were suspended in ethanol with a concentration of 1.25 mg/mL. The excitation wavelength was set at 350 nm. Inductively coupled plasma optical emission spectrometry (ICP-OES, Horiba Jobin Yvon JYULTIMA 2000, 120e180 nm) was used to analyze the composition of Zn, In, Ag, and Pt of selected samples. The standard solutions for calibration were prepared from indium nitrate (30e150 ppm), zinc nitrate (150e270 ppm), silver nitrate (20e100 ppm) and H2PtCl6$H2O (Alfa; 99.95%; 1e5 ppm). 0.1 g of each sample was weighted and soaked in aqua regia solution (20 mL) overnight. The transparent yellow-orange solution was then further diluted tenfold by adding distilled water. The concentrations of each element were in the range from 8 ppm to 400 ppm, which were suitable for ICP-OES measurement.
2.3.
Photocatalytic reaction
Photocatalytic reactions were conducted in a glass-made cell with a quartz side window with an illumination area of 23 cm2. 0.5 g of the annealed photocatalyst powders was dispersed in an aqueous solution that contained 220 mL sacrificial reagents (0.25 M K2SO3 and 0.35 M Na2S) with a magnetic stirrer under 300-W Xe lamp irradiation with an intensity of 100 mW/cm2. The water displacement method was employed to collect hydrogen gas. Bare photocatalysts and Pt-loaded photocatalysts were tested separately. First, the hydrogen production rate was measured using bare photocatalysts. The time was recorded after the first bubble appeared. The steady hydrogen evolution rate was averaged from a 5-h reaction. For the Pt-loaded photocatalyst, the aforementioned batch was used successively. After the addition of the desired weight percentage of H2PtCl6$H2O into the cell, the Pt photodeposition process was carried out under a light intensity of 400 mW/cm2 for 30 min. The corresponding hydrogen production was then recorded for the next 3 h. Throughout this paper, the average hydrogen evolution rate in units of L/m2 h is given. The compositions of the evolved gas were analyzed by gas chromatography with a thermal conductivity detector (GC-14B, Shimadzu; Molecular sieve 5 A columns; Ar carrier gas). Almost 99% of the gas evolved was hydrogen.
Table 1 e Molar ratios of metals in precursor solutions. Sample
A B C D E F B-2 E-1 E-2
Results and discussion
As mentioned in the Introduction, only the ZnSeAgInS2 binary phase diagram can be found in the literature [47]. Approximately 60e90 mol.% of ZnS can generate a single-phase AgInS2eZnS solid solution. The corresponding energy band gap can be varied from 1.8 to 2.4 eV. This unique optical property enables researchers to investigate photocatalytic hydrogen production as a function of the relative ZnS-toAgInS2 composition [13,24,25,29]. Although many factors affect photocatalytic activity, including band structure, crystal structure, and crystallinity, it appears that, in this (AgeIneZn) S system, the position of the absorption edge and energy band gap are decisive parameters [25,29]. Our previous study found that mixtures of AgInS2 and AgIn5S8 possess high photocatalytic activity [48], perhaps due to the excited charge carrier separation resulting from the band alignment. It is of
Ag
In
Zn
1 1 1 1 1 1 1 1 1
1 1.25 1.5 1.75 2 2.25 1.25 2 2
7 7 7 7 7 7 7 7 7
Pt loading (wt%) 3 3 3 3 3 3 4.5 1.5 4.5
fundamental interest to study other critical parameters that influence photocatalytic activity, specifically the effect of In content on this (AgeIneZn)S quaternary metal sulfide semiconductor. Note that AgInS2 and AgIn5S8 have similar energy band gaps. Typically, they are in the range of 1.8e2.0 eV [48]. Therefore, the band gap of the single phase of this (AgeIneZn) S quaternary compound semiconductor is primarily decided by the molar ratio of ZnS. To clarify the critical role of In content in the photocatalyst, the molar ratio of [Zn]/[Ag] should remain constant. According to the literature [25,29], a [Zn]/[Ag] ratio of 7 in the precursor solution leads to the highest photocatalytic activity. Therefore, only the results of In-rich samples with a [Zn]/[Ag] ratio of 7 are reported here. However, it is rather difficult to control the [Zn]/[Ag] ratio through one-pot chemistry using our wet process. Our synthetic strategy was to synthesize (AgeZn)S powders first. The powders served as the seeds for various In2S3 coatings. In this way, the energy band gap, or [Zn]/[Ag] ratio, of the photocatalysts can be easily controlled. Six different amounts of In2S3 were used to investigate the effect of In content. Table 1 shows the experiential conditions in the precursor solutions. With the [Zn]/[Ag] ratio kept constant (samples AeF), the [In]/ [Ag] concentration ratios were varied from 1.0 to 2.25 for samples A to F. All the samples were annealed in a tube furnace at 850 C for 5 h in an ultra-pure nitrogen environment.
3.1.
3.
Precursor solution (molar ratio)
X-ray diffraction patterns
Fig. 1 shows the diffraction patterns of all samples (A-F) before Pt loading. The diffraction peaks match the single phase of (AgeIneZn)S solid solutions with a hexagonal structure (wurtzite-type). Note that the peak intensities of sample B ([In]/[Ag] ¼ 1.25) are the strongest. The peak heights decreased with increasing [In]/[Ag] in samples C to F. These XRD patterns indicate that the crystals obtained were not mixtures of AgInZn7S9 and In2S3/AgIn5S8, but single-phase (AgeIneZn)S solid solutions. Moreover, the crystallinity of these solid solutions decreased with indium content. The cell parameter calculation was carried out by fitting the XRD patterns (Fig. 1) using the Powder Cell program. The P63mc space group for the wurtzite crystal structure was used. No peak shift due to the difference in ion radii was observed. The variation of cell parameters (a ¼ b ¼ 3.872e3.882 A; c ¼ 6.338e6.358 A) seemed to be within the experimental error. These data indicate that no significant distortion of the crystal was induced by the
Please cite this article in press as: Lin P-C, et al., Enhanced photocatalytic hydrogen production over In-rich (AgeIneZn)S particles, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.04.125
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and AgIn5S8 (spinel) can be observed. The peak intensities of AgIn5S8 increased at 550 C. With a further increase in the annealing temperature, all the peaks started to merge to those of (AgeIneZn)S solid solution with a wurtzite structure. Ternary phase diagrams of Ag2S, In2S3, and ZnS could not be found in the literature. Our results shed light on the manipulation of the compositions and crystal structures of this quaternary metal sulfide material. The proposed method can be used to tune the properties of this compound semiconductor for various optical and energy applications.
3.2.
Fig. 1 e Powder X-ray diffraction patterns of (AgeIneZn)S samples. All samples were annealed in a N2 environment at 850 C for 5 h.
formation of the solid solution. The atomic coordinates of ZnS and the AgeIneS system can be referenced to Yeh et al. [50] and Delgado et al. [51], respectively. To our best knowledge, however, the detailed crystal structures of ZnSeAgInS2 solid solutions and corresponding coordination number of each atom cannot be found in the literature. Tsuji et al. reported the band structure calculation using wurtzite ZnS and wurtzitelike AgInS2 [25]. In this case, all the ions, including Agþ, In3þ, and Zn2þ have a coordination number of 4. The corresponding ˚ , 0.76 A, and 0.74 A, radius of Agþ, In3þ, and Zn2þ are 1.14 A respectively [52]. In order to investigate the crystal structure evolution, i.e., the formation of the solid solution, various heat treatment conditions were used and ex-situ XRD experiments were performed. Fig. 2 shows XRD patterns of sample E annealed at various temperatures. After 5-h annealing at 450 C, mixed diffraction peaks of ZnS (sphalerite), AgInS2 (chalcopyrite),
Fig. 2 e Powder X-ray diffraction patterns of sample E annealed at various temperatures (450e650 C).
UVeVis spectra
The band gaps of silver indium sulfide are located in the range of 1.8e2.0 eV, for both AgInS2 and AgIn5S8. The [Zn]/[Ag] ratio can be used to tune the band gaps of the solid solutions of the ZnSeAgInS2 system. In this study, the absorption edge was kept in the same wavelength when [Zn]/[Ag] was fixed. Fig. 3 shows that the UVeVis spectra have sharp absorption edges in the visible-light region around 500 nm. The scattering due to the particulate samples was collected by the integrating sphere, resulting in spectra similar to those measured using the diffuse reflectance technique. This result indicates that the absorption edge did not vary significantly as a function of [In]/[Ag]. The second peak around 600 nm might be induced by the defects inside the materials. Since sulfur, zinc, and indium can be easily volatilized during the heat treatment process in N2 flow [25], vacancies in the local area generate various structures, changing the absorption. Several annealing processes (with various annealing durations and temperatures) were conducted, but this secondary absorption appeared random and was very difficult to remove. The dark spots can be easily distinguished from the yelloweorange colors of the samples (Sample B to Sample F) by using a regular optical microscope. Sample A, which exhibited a grayeblack color, had the weakest absorption at 500 nm and a loner tail in the near-infrared range, probably due to the defects. The defectinduced optical absorption in the visible light range was observed experimentally [28] and theoretically [53]. In summary, similar absorption edges indicate that samples B to F possess similar optical band gaps, and thus should have a
Fig. 3 e Absorption spectra of (AgeIneZn)S solid solutions.
Please cite this article in press as: Lin P-C, et al., Enhanced photocatalytic hydrogen production over In-rich (AgeIneZn)S particles, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.04.125
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similar driving force for hydrogen production from water splitting.
3.3.
Hydrogen production
Photocatalytic H2 evolution from an aqueous solution of 0.25 M K2SO3 and 0.35 M Na2S (220 mL) over the In-rich photocatalysts with and without 3 wt% Pt cocatalyst was investigated (samples AeF). The sacrificial reagents served as the electron donors to decrease the photocorrosion of sulfide catalysts and to enhance photocatalytic activity [29]. Fig. 4 shows the effect of indium content on the photocatalytic activity of unloaded and Pt-loaded AgeIneZneS solid solutions under the illumination of a 300-W Xe lamp with an intensity of 100 mW/cm2. Without the Pt cocatalyst, the hydrogen production rate was relatively low, on the order of 0.1 L/m2 h. The H2 evolution rate was highest for sample B (0.374 L/m2 h). The rate then decreased gradually to 0.169 L/m2 h for sample F. The decreasing H2 generation rate of the bare solid solutions (without Pt loading) can be explained by the lower crystallinity for higher-In-content samples. From the XRD patterns in Fig. 1, sample B has the most pronounced diffraction peaks, which implies the highest crystallinity. This high crystallinity (few grain boundaries) might decrease the probability of electronehole recombination. In contrast, the hydrogen production rate depends strongly on the [In]/[Ag] ratio after loading 3 wt% Pt. Sample B showed the highest photocatalytic activity without Pt loading (0.374 L/m2 h), but it was not the highest after Pt loading (1.74 L/m2 h). The highest H2 evolution over the photocatalysts with Pt loading was observed for sample E (3.75 L/m2 h). The UVeVis spectra indicate that the optical energy band gap of the In-rich solid solutions does not significantly vary. This enhancement was thus not related to the absorption edge. In order to understand the origin of this enhancement, the morphologies, nanostructures, and compositions of the Inrich photocatalysts were studied. Fig. 5 shows SEM images of all the unloaded samples. The photocatalysts are agglomerates with micrometer-scale diameters, as determined
Fig. 4 e Photocatalytic H2 evolution over (AgeIneZn)S photocatalysts in a 0.25 M K2SO3 and 0.35 M Na2S aqueous solution. Irradiation area: 23 cm2.
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from direct observation. It can be expected that the specific surface areas of the samples are low. However, careful observation of the images reveals nanostep structures in all samples. These nanostep structures were generated during the heat treatment process. The quantity of nanosteps increases with increasing indium content. This suggests that the amount of indium is the key to increasing the number of nanosteps in this quaternary photocatalyst. Samples E and F have the most nanostep structures. When H2PtCl6 was added, the Pt particles were rapidly photodeposited at the initial stage of the photocatalytic reaction. Fig. 6a shows a SEM image of the surface of the photocatalyst after Pt loading. It appears that the Pt was selectively deposited on the edges of the nanosteps. Fig. 6b shows a transmission electron microscopy (TEM) image, giving a clearer view of Pt deposition on the edges. Tsuji et al. demonstrated that the edges of nanostep structures facilitate electronehole charge separation [25]. The selective deposition of Pt particles also implies that photogenerated electrons migrate to the edge of the nanosteps, and these positions play a key role as reduction sites in the photocatalytic reaction [54]. Table 2 lists the atomic ratios of various metal components measured using ICP-OES, as well as the BET surface area data. The composition of the powders is a function of that of the precursor solution. The proposed two-step synthetic strategy was effective at keeping the [Zn]/[Ag] ratio constant. The standard deviation of [Zn]/[Ag] was lower than 5.1% for the various indium content levels. The BET surface area increased with increasing In content, perhaps due to more nanostep structures on the particle surface. The BET surface area varied from 0.83 to 3.82 m2/g, which is comparable to that of a similar system [13,25]. Table 2 also clearly shows that the Pt molar ratio increased with In content in the precursor solution. The same weight percentage of H2PtCl6 precursor was used to load the Pt cocatalyst onto the photocatalyst surfaces. The higher level of Pt content after loading suggests more active sites during photodeposition. Pt cocatalyst can trap electrons and provide active sites for hydrogen production from water under irradiation. Sample F had the most nanostep structures, which is consistent with the ICP-OES data (0.424 mol.% Pt and a BET surface area of 3.82 m2/g). However, sample E had the highest photocatalytic activity. This might be due to the competition among crystallinity, number of active sites, and specific surface area. To further verify the critical role of In content on Pt loading, two samples were selected to change the weight percentage of H2PtCl6 in the precursor solutions. Samples B and E had the highest hydrogen production rates of the bare and Pt-loaded photocatalysts, respectively. Since a lower Pt mol.% was observed for sample B (see Table 2), more H2PtCl6 (4.5 wt%, sample B-2) was added into the precursor. Less H2PtCl6 (1.5 wt %, sample E-1) was added into the precursor for sample E because more Pt was observed. The molar ratios of Pt after loading were 0.467 and 0.341 mol.% for samples B-2 and E-1, respectively. The amount of Pt was thus brought to similar levels for comparison. Sample B-2 had more Pt loading than that of sample E. However, the hydrogen production rate (1.565 L/m2 h) was lower than that of the latter (3.75 L/m2 h). The hydrogen production rate (3.043 L/m2 h) of less Pt-loaded sample E-1 was higher than that of sample B (1.74 L/m2 h). The
Please cite this article in press as: Lin P-C, et al., Enhanced photocatalytic hydrogen production over In-rich (AgeIneZn)S particles, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.04.125
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Fig. 5 e SEM images of bare (AgeIneZn)S solid solutions (no Pt loading) prepared with various [In]/[Ag] ratios.
hydrogen production rates obtained for various Pt loadings are plotted in Fig. 7. Furthermore, Sample E-2 (4.5 wt% Pt) shows almost the same photocatalytic activity (3.03 L/m2 h) as Sample E-1, but lower than Sample E. Considering the effective reaction surface area and the shielding for incident photons, this agrees with a study [25] that indicated that 3 wt% Pt loading is the optimal condition.
The luminescence properties of photocatalyst result from the electronehole recombination processes within the semiconductor [22]. Therefore, luminescence measurement is an efficient tool to detect the charge separation probability of our samples qualitatively. Fig. 8 shows the normalized photoluminescence spectra in two extreme cases, sample B and E. These spectra exhibit an emission peak at 500 nm,
Fig. 6 e (a) SEM and (b) TEM images of Pt-loaded sample E. Please cite this article in press as: Lin P-C, et al., Enhanced photocatalytic hydrogen production over In-rich (AgeIneZn)S particles, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.04.125
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Table 2 e Compositions and BET surface areas of In-rich photocatalysts. Atomic molar ratioa
Sample
A B C D E F B-2 E-1
Ag
In
Zn
Pt
1 1 1 1 1 1 1 1
1.05 1.34 1.51 1.82 2.1 2.25 1.32 2.09
7.23 7.32 7.27 7.43 7.39 6.88 7.35 7.42
0.0339 0.0369 0.0387 0.0419 0.044 0.043 0.0452 0.0358
Pt mol.%
BET (m2/g)
0.365 0.382 0.396 0.409 0.419 0.424 0.467 0.341
0.83 1.08 2.07 2.96 2.44 3.82 e e
a The atomic molar ratio of Ag:In:Zn was determined by ICP-OES.
Fig. 8 e Photoluminescence spectra of photocatalyst excited at 350 nm: (a) bare sample B, [In]/[Ag] [ 1.25, (b) Pt-loaded sample B, [In]/[Ag] [ 1.25, (c) bare sample E, [In]/[Ag] [ 2.00, (d) Pt-loaded sample E, [In]/[Ag] [ 2.00.
Fig. 7 e H2 evolution rate obtained for various Pt loading.
corresponding to the band-edge emission, which is consistent with the UVeVis spectra shown in Fig. 3. Curves a and c are measured from the bare samples B and E, respectively. Note that the normalized PL intensity of bare sample B is slightly lower than sample E, suggesting a lower charge recombination. This observation agrees with the hydrogen production rate shown in Fig. 4. After Pt cocatalyst loading, the PL intensities of both samples B and E decrease dramatically. This quenching effect produced by Pt loading is more pronounced for sample E, a 44.7% decrease in PL intensity. It is known that Pt particles are efficient traps for electrons [55]. This further demonstrates that Pt-loading treatment enhances the electronehole separation, especially for In-rich samples. Although the amount of Pt loading affects photocatalytic activity, the distribution of Pt cocatalyst is critical. The interaction between In and Pt contributes to the unique enhancement of hydrogen production using our In-rich photocatalysts. Nanostep structures were self-constructed during the heat treatment [25], especially for In-rich samples. Tsuji et al. proposed that this kind of solid solution consists of (Ag, In)-rich and Zn-rich planes in the wurtzite structure, and that (Ag, In)-rich planes generate nanostep structures. Our findings support their arguments. Tsuji et al.
also mentioned that excess indium is required in their recipe [25]; however, they did not discuss the effect of indium in detail. Furthermore, many factors affect the photocatalytic activity of particles, including specific surface area, crystal structure, crystallinity, and compositions of solid solutions [56]. These factors also change the charge transport properties, such as conductivity, transport mechanism, and mobility of the charge carriers. In this study, however, we first focus on the effect of In content and Pt loading on the photocatalytic hydrogen production. In fact, our results demonstrated that indium is a crucial factor for improving photocatalytic activity of the multi-component photocatalysts. In summary, the number of nanostep structures on the surface of this material depends on indium content. Although crystallinity decreases with increasing indium content, a suitable amount of Pt loading on the nanostep structures greatly improves the hydrogen production rate.
4.
Conclusion
An (AgeIneZn)S quaternary semiconductor was prepared using a two-step method in aqueous solutions to keep the band gap energy constant. After heat treatment at 850 C in a N2 environment, solid solutions with a wurtzite structure and various [In]/[Ag] ratios were obtained. The photocatalytic activity enhancement with increasing [In]/[Ag] ratio was mainly
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attributed to more nanostep structures and the dispersion of Pt cocatalysts. The highest evolution rate (3.75 L/m2 h), observed for sample E (Ag:In:Zn ¼ 1:2:7), was 5.86 times that of the original sample (sample A, Ag:In:Zn ¼ 1:1:7). This study demonstrated the critical role of excess indium in the (AgeIneZn)S photocatalyst.
Acknowledgment This material is based on research sponsored by the Air Force Research Laboratory, under agreement number FA2386-11-14081. The U.S. Government is authorized to reproduce and distribute reprints for Governmental purposes notwithstanding any copyright notation thereon. Disclaimer: The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the Air Force Research Laboratory or the U.S. Government. The authors would like to thank the Instrument Center at National Chung Cheng University for assistance with SEM and TEM images, the Industrial Technology Research Institute for BET measurements, and Professor YaSen Sun’s Lab at National Central University for UVeVis measurements.
references
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Enhanced photocatalytic hydrogen production over In-rich (AgeIneZn)S particles Po-Chang Lin b, Pei-Ying Wang b, Yuan-Yao Li b, Chi Chung Hua b,**, Tai-Chou Lee a,* a b
Department of Chemical and Materials Engineering, National Central University, 300 Jhongda Rd., Jhongli, Taoyuan 320, Taiwan Department of Chemical Engineering, National Chung Cheng University, 168 University Rd., Min-Hsiung, Chia-Yi 621, Taiwan
article info
abstract
Article history:
The ratio of ZnS to AgInS2 is usually adjusted to tune the band gaps of this quaternary (Ag
Received 14 January 2013
eIneZn)S semiconductor to increase photocatalytic activity. In this study, the [Zn]/[Ag]
Received in revised form
ratio was kept constant. The hydrogen production rate was enhanced by increasing the
11 April 2013
content of indium sulfide. Compared to the steady H2 evolution rate obtained with equal
Accepted 24 April 2013
moles of indium and silver ([In]/[Ag] ¼ 1, 0.64 L/m2 h), that obtained with In-rich photo-
Available online xxx
catalyst ([In]/[Ag] ¼ 2, 3.75 L/m2 h) is over 5.86 times higher. The number of nanostep structures, on which the Pt cocatalysts were loaded by photodeposition, increased with the
Keywords:
content of indium. The indium-rich samples did not induce phase separation between
Photocatalyst
AgxInxZnyS2xþy and AgIn5S8, instead forming a single-phase solid solution. Although the
Hydrogen production
photocatalytic activity decreased slightly for bare In-rich photocatalysts, Pt loading played
Water splitting
a critical role in the hydrogen production rate. This study demonstrates the significant
Solar energy
effect of In2S3 on this unique (AgeIneZn)S photocatalyst. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Due to deep concerns over global warming and increasing energy costs, an increasing number of researchers have focused on improving the efficiency of energy utilization and the development alternative energy sources. Semiconductor systems can convert solar energy into hydrogen, which is an energy carrier that produces no harmful emissions. This conversion has been considered an ideal solution for reducing the use of fossil fuels [1e6] and mitigating environmental problems [7e9]. Metal oxide photocatalysts, such as TiO2, SrTiO3, Ta2O5, LiTaO3, KTaO3, CaTa2O6, K4Nb6O7, AgTaO3, NaTaO3, and Ga2O3 [5,10e12], have been studied for decades for application in artificial photosynthesis, typically the solar splitting of water into hydrogen and oxygen. However, due to
their large band gap energies (3.6e4.7 eV), metal oxide photocatalysts are mainly limited to ultraviolet (UV) light activity [5,10,13], which account for only approximately 3e5% in the solar spectrum. Since visible light accounts for almost half of the photons, it is necessary to develop a visible-light-responsive photocatalyst [14,15]. Some metal sulfide and metal oxide materials can split water to generate hydrogen and oxygen under visible-light irradiation [16e20]. CdS is one of the photocatalysts for H2 production from water [21,22]. However, since CdS includes harmful cadmium, rigorous safety measures must be taken during the processing steps. The design of solid-solution particles and quantum dots with controlled electronic structures, including (AgeIneGa)S [23], (ZneIn)S [24], (AgeIneZn)S [25e32], (CueIneZn)S [33,34], (CueAgeIneZn)S [35], (CdeZn)S [36e41], and (ZneCueCd)S [42,43], has progressed
* Corresponding author. Tel.: þ886 3 4227151 34211; fax: þ886 3 4252296. ** Corresponding author. Tel.: þ886 5 2720411 33412; fax: þ886 5 2721206. E-mail addresses: [email protected] (C.C. Hua), [email protected] (T.-C. Lee). 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.2013.04.125
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in recent years. A series of multi-component metal sulfide solid solutions that show apparent activity for hydrogen evolution from water containing sacrificial reagents, SO2 3 and S2 under visible-light irradiation have been reported [13,15,25,44]. AgInZn7S9 exhibits a high quantum yield (19.8% at 420 nm) of H2 production, with a reported steady evolution rate of 1.8 L/m2 h under simulated sunlight [25]. The photoelectrochemical properties of (AgeIneZn)S solid solutions were investigated by immobilizing the nanoparticles on ZnO nanorod electrodes [44]. The structure of (AgeIneZn)S was investigated and the binary AgInS2eZnS phase diagram was reported [45]. The single-phase regions are: AgInS2 (with less than 2 mol.% ZnS), sphalerite ZnS (with 97e100 mol.% ZnS), and a solid solution of ZnSeAgInS2 with a hexagonal structure (60e90 mol.% ZnS) [46,47]. ZnS, with a wide band gap energy of 3.5 eV, is a photocatalyst under UV irradiation. (AgeIn)S, including AgInS2 and AgIn5S8, is a narrow-band-gap semiconductor with a band gap energy of 1.87e2.03 eV [48]. Tsuji et al. synthesized a high-efficiency photocatalyst by making a solid solution of these two materials. A theoretical basis was provided [25,35]. The band gap of ZnSeAgInS2 solid solution can be tuned continuously from 1.8 to 2.4 eV by adjusting the ZnS content [25]. With this strategy, the H2 production efficiency can be maximized by optimizing the ratio of ZnS to AgInS2 [25,29]. However, the preparation of single-phase AgInS2 with high purity is difficult. The operating window is rather small; in theory, only equal moles of Ag2S and In2S3 will suffice, according to the phase diagram of Ag2S and In2S3 [49]. Based on our experience, high-temperature (>800 C) heat treatment is also required to prepare good-quality photocatalysts [29]. Even with an optimal ratio of ZnS to AgInS2, the routine fabrication of (AgeIneZn)S solid solution will be unstable and probably not even repeatable. Therefore, a robust synthetic protocol is needed for further industrial applications. In the present study, the [In]/[Ag] ratio is tuned and the phase type and activity of the obtained photocatalysts is discussed. By fixing the [Zn]/[Ag] ratio, changes in the band gap energy are negligible because there is only a minor variation of ZnS concentration ratios during the processing steps. However, the effect of indium on the activity of Pt-loaded photocatalyst is dramatic and independent of the effect of the band gap. In our wet chemistry process, metal sulfides were generated from reactions between metal nitrate and thioacetamide (TAA), the source of S2. The precipitates of metal sulfides can be collected, when concentration products exceed the solubility constant KSP. In order to keep the [Zn]/[Ag] ratio at 7, the optimal composition for hydrogen production [25,29], the reaction was divided into two steps. Ample powders of (AgeZn)S were synthesized in the first step. The powders were then used for preparing photocatalysts with various amounts of indium. Under suitable experimental conditions and heat treatment in a tube furnace with flowing ultra-pure nitrogen, (AgeIneZn)S solid solutions were obtained, as determined by X-ray diffraction (XRD). Scanning electron microscopy (SEM) images show nanostep structures on the surface of the particles. In the photochemical study, the hydrogen production was found to be a function of [In]/[Ag], with the optical band gap kept constant. The highest H2 evolution rate ([In]/[Ag] ¼ 2)
is 4.86 times higher than that of the original sample ([In]/ [Ag] ¼ 1) under 100-mW/cm2 Xe irradiation. The excess photocatalysts were loaded to make sure no light could escape from the rear of the illumination area. Evidence of the enhanced semiconductor-assisted photoreaction is provided and a possible mechanism is proposed.
2.
Experimental details
2.1.
Preparation of (AgeIneZn)S photocatalyst
A completely aqueous solution route without toxic H2S gas was adopted. The sulfur ions were provided by the decomposition of thioacetamide in an acidic environment [29]. The detailed experimental procedure was reported previously [29]. Briefly, aqueous solutions of 0.4 M indium nitrate (In(NO3)3$xH2O, Sigma Aldrich; 99.99%), 1.6 M zinc nitrate (Zn(NO3)2$6H2O, J. T. Baker; 98%), and 1 M thioacetamide (TAA, CH3CSNH2, S2 source, Merck; 99%) were prepared separately. The zinc nitrate solution (43.75 mL; 70 mmol) and silver nitrate (1.7 g; 10 mmol) (AgNO3, Sigma Aldrich; 99.9%) were added into a 250 mL round-bottom flask, fully dissolved and mixed well. The pH value of the solution was then adjusted to be around 1 by adding concentrated sulfuric acid (H2SO4, Merck; 95e97%). 100 mL of TAA (100 mmol) was poured into the flask, mixed well, and maintained at 80 C in a thermostat bath for 8 h. The precipitates were rinsed thoroughly with deionized (DI) water. They were then dried at 80 C in an oven for 12 h. In the second stage, 0.806 g (Ag:Zn ¼ 1:7) of the dried powders with a fixed [Zn]/[Ag] ratio were mixed with various amounts of 0.4 M indium nitrate solution (2.5, 3.125, 3.75, 4.375, 5, and 5.625 mL). The precursor solutions were diluted using 20 mL of DI water in a 100-mL round-bottom flask under rigorous magnetic stirring. The pH value of the suspension was adjusted to be around 2 by adding concentrated sulfuric acid. Finally, prescribed TAA solutions (2, 2.5, 3, 3.5, 4, and 4.5 mL) were added into the suspension. After a reaction at 80 C for 5 h, the precipitates were collected and rinsed thoroughly with DI water. They were then dried at 80 C in an oven for 12 h. The obtained powders were covered with a quartz plate and heat-treated at 850 C for 5 h in an ultra-pure N2 environment in a tube furnace. The molar ratios of the precursor solutions are given in Table 1.
2.2.
Characterization of samples
The crystal structures of the samples were determined using XRD (Bruker KAPPA APEX II). The XRD patterns were recorded in the 2q range of 20 e70 at a scanning rate of 4 /min. Fieldemission scanning electron microscopy (FE-SEM, Hitachi S4800-I) was employed to investigate the surface morphology of the powders. Transmission electron microscopic images were obtained using a JEOL JEM-2010 microscope operated at 200 kV. The absorption spectra (UVeVisible (UVeVis), Jasco V670) were measured with an integrating sphere in the wavelength range of 300e800 nm at room temperature. Surface areas were determined by the BET method (Micromeritics ASAP 2020). Photoluminescence spectra were recorded using a
Please cite this article in press as: Lin P-C, et al., Enhanced photocatalytic hydrogen production over In-rich (AgeIneZn)S particles, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.04.125
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photoluminescence spectrometer (PerkinElmer LS-55). The samples were suspended in ethanol with a concentration of 1.25 mg/mL. The excitation wavelength was set at 350 nm. Inductively coupled plasma optical emission spectrometry (ICP-OES, Horiba Jobin Yvon JYULTIMA 2000, 120e180 nm) was used to analyze the composition of Zn, In, Ag, and Pt of selected samples. The standard solutions for calibration were prepared from indium nitrate (30e150 ppm), zinc nitrate (150e270 ppm), silver nitrate (20e100 ppm) and H2PtCl6$H2O (Alfa; 99.95%; 1e5 ppm). 0.1 g of each sample was weighted and soaked in aqua regia solution (20 mL) overnight. The transparent yellow-orange solution was then further diluted tenfold by adding distilled water. The concentrations of each element were in the range from 8 ppm to 400 ppm, which were suitable for ICP-OES measurement.
2.3.
Photocatalytic reaction
Photocatalytic reactions were conducted in a glass-made cell with a quartz side window with an illumination area of 23 cm2. 0.5 g of the annealed photocatalyst powders was dispersed in an aqueous solution that contained 220 mL sacrificial reagents (0.25 M K2SO3 and 0.35 M Na2S) with a magnetic stirrer under 300-W Xe lamp irradiation with an intensity of 100 mW/cm2. The water displacement method was employed to collect hydrogen gas. Bare photocatalysts and Pt-loaded photocatalysts were tested separately. First, the hydrogen production rate was measured using bare photocatalysts. The time was recorded after the first bubble appeared. The steady hydrogen evolution rate was averaged from a 5-h reaction. For the Pt-loaded photocatalyst, the aforementioned batch was used successively. After the addition of the desired weight percentage of H2PtCl6$H2O into the cell, the Pt photodeposition process was carried out under a light intensity of 400 mW/cm2 for 30 min. The corresponding hydrogen production was then recorded for the next 3 h. Throughout this paper, the average hydrogen evolution rate in units of L/m2 h is given. The compositions of the evolved gas were analyzed by gas chromatography with a thermal conductivity detector (GC-14B, Shimadzu; Molecular sieve 5 A columns; Ar carrier gas). Almost 99% of the gas evolved was hydrogen.
Table 1 e Molar ratios of metals in precursor solutions. Sample
A B C D E F B-2 E-1 E-2
Results and discussion
As mentioned in the Introduction, only the ZnSeAgInS2 binary phase diagram can be found in the literature [47]. Approximately 60e90 mol.% of ZnS can generate a single-phase AgInS2eZnS solid solution. The corresponding energy band gap can be varied from 1.8 to 2.4 eV. This unique optical property enables researchers to investigate photocatalytic hydrogen production as a function of the relative ZnS-toAgInS2 composition [13,24,25,29]. Although many factors affect photocatalytic activity, including band structure, crystal structure, and crystallinity, it appears that, in this (AgeIneZn) S system, the position of the absorption edge and energy band gap are decisive parameters [25,29]. Our previous study found that mixtures of AgInS2 and AgIn5S8 possess high photocatalytic activity [48], perhaps due to the excited charge carrier separation resulting from the band alignment. It is of
Ag
In
Zn
1 1 1 1 1 1 1 1 1
1 1.25 1.5 1.75 2 2.25 1.25 2 2
7 7 7 7 7 7 7 7 7
Pt loading (wt%) 3 3 3 3 3 3 4.5 1.5 4.5
fundamental interest to study other critical parameters that influence photocatalytic activity, specifically the effect of In content on this (AgeIneZn)S quaternary metal sulfide semiconductor. Note that AgInS2 and AgIn5S8 have similar energy band gaps. Typically, they are in the range of 1.8e2.0 eV [48]. Therefore, the band gap of the single phase of this (AgeIneZn) S quaternary compound semiconductor is primarily decided by the molar ratio of ZnS. To clarify the critical role of In content in the photocatalyst, the molar ratio of [Zn]/[Ag] should remain constant. According to the literature [25,29], a [Zn]/[Ag] ratio of 7 in the precursor solution leads to the highest photocatalytic activity. Therefore, only the results of In-rich samples with a [Zn]/[Ag] ratio of 7 are reported here. However, it is rather difficult to control the [Zn]/[Ag] ratio through one-pot chemistry using our wet process. Our synthetic strategy was to synthesize (AgeZn)S powders first. The powders served as the seeds for various In2S3 coatings. In this way, the energy band gap, or [Zn]/[Ag] ratio, of the photocatalysts can be easily controlled. Six different amounts of In2S3 were used to investigate the effect of In content. Table 1 shows the experiential conditions in the precursor solutions. With the [Zn]/[Ag] ratio kept constant (samples AeF), the [In]/ [Ag] concentration ratios were varied from 1.0 to 2.25 for samples A to F. All the samples were annealed in a tube furnace at 850 C for 5 h in an ultra-pure nitrogen environment.
3.1.
3.
Precursor solution (molar ratio)
X-ray diffraction patterns
Fig. 1 shows the diffraction patterns of all samples (A-F) before Pt loading. The diffraction peaks match the single phase of (AgeIneZn)S solid solutions with a hexagonal structure (wurtzite-type). Note that the peak intensities of sample B ([In]/[Ag] ¼ 1.25) are the strongest. The peak heights decreased with increasing [In]/[Ag] in samples C to F. These XRD patterns indicate that the crystals obtained were not mixtures of AgInZn7S9 and In2S3/AgIn5S8, but single-phase (AgeIneZn)S solid solutions. Moreover, the crystallinity of these solid solutions decreased with indium content. The cell parameter calculation was carried out by fitting the XRD patterns (Fig. 1) using the Powder Cell program. The P63mc space group for the wurtzite crystal structure was used. No peak shift due to the difference in ion radii was observed. The variation of cell parameters (a ¼ b ¼ 3.872e3.882 A; c ¼ 6.338e6.358 A) seemed to be within the experimental error. These data indicate that no significant distortion of the crystal was induced by the
Please cite this article in press as: Lin P-C, et al., Enhanced photocatalytic hydrogen production over In-rich (AgeIneZn)S particles, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.04.125
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and AgIn5S8 (spinel) can be observed. The peak intensities of AgIn5S8 increased at 550 C. With a further increase in the annealing temperature, all the peaks started to merge to those of (AgeIneZn)S solid solution with a wurtzite structure. Ternary phase diagrams of Ag2S, In2S3, and ZnS could not be found in the literature. Our results shed light on the manipulation of the compositions and crystal structures of this quaternary metal sulfide material. The proposed method can be used to tune the properties of this compound semiconductor for various optical and energy applications.
3.2.
Fig. 1 e Powder X-ray diffraction patterns of (AgeIneZn)S samples. All samples were annealed in a N2 environment at 850 C for 5 h.
formation of the solid solution. The atomic coordinates of ZnS and the AgeIneS system can be referenced to Yeh et al. [50] and Delgado et al. [51], respectively. To our best knowledge, however, the detailed crystal structures of ZnSeAgInS2 solid solutions and corresponding coordination number of each atom cannot be found in the literature. Tsuji et al. reported the band structure calculation using wurtzite ZnS and wurtzitelike AgInS2 [25]. In this case, all the ions, including Agþ, In3þ, and Zn2þ have a coordination number of 4. The corresponding ˚ , 0.76 A, and 0.74 A, radius of Agþ, In3þ, and Zn2þ are 1.14 A respectively [52]. In order to investigate the crystal structure evolution, i.e., the formation of the solid solution, various heat treatment conditions were used and ex-situ XRD experiments were performed. Fig. 2 shows XRD patterns of sample E annealed at various temperatures. After 5-h annealing at 450 C, mixed diffraction peaks of ZnS (sphalerite), AgInS2 (chalcopyrite),
Fig. 2 e Powder X-ray diffraction patterns of sample E annealed at various temperatures (450e650 C).
UVeVis spectra
The band gaps of silver indium sulfide are located in the range of 1.8e2.0 eV, for both AgInS2 and AgIn5S8. The [Zn]/[Ag] ratio can be used to tune the band gaps of the solid solutions of the ZnSeAgInS2 system. In this study, the absorption edge was kept in the same wavelength when [Zn]/[Ag] was fixed. Fig. 3 shows that the UVeVis spectra have sharp absorption edges in the visible-light region around 500 nm. The scattering due to the particulate samples was collected by the integrating sphere, resulting in spectra similar to those measured using the diffuse reflectance technique. This result indicates that the absorption edge did not vary significantly as a function of [In]/[Ag]. The second peak around 600 nm might be induced by the defects inside the materials. Since sulfur, zinc, and indium can be easily volatilized during the heat treatment process in N2 flow [25], vacancies in the local area generate various structures, changing the absorption. Several annealing processes (with various annealing durations and temperatures) were conducted, but this secondary absorption appeared random and was very difficult to remove. The dark spots can be easily distinguished from the yelloweorange colors of the samples (Sample B to Sample F) by using a regular optical microscope. Sample A, which exhibited a grayeblack color, had the weakest absorption at 500 nm and a loner tail in the near-infrared range, probably due to the defects. The defectinduced optical absorption in the visible light range was observed experimentally [28] and theoretically [53]. In summary, similar absorption edges indicate that samples B to F possess similar optical band gaps, and thus should have a
Fig. 3 e Absorption spectra of (AgeIneZn)S solid solutions.
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similar driving force for hydrogen production from water splitting.
3.3.
Hydrogen production
Photocatalytic H2 evolution from an aqueous solution of 0.25 M K2SO3 and 0.35 M Na2S (220 mL) over the In-rich photocatalysts with and without 3 wt% Pt cocatalyst was investigated (samples AeF). The sacrificial reagents served as the electron donors to decrease the photocorrosion of sulfide catalysts and to enhance photocatalytic activity [29]. Fig. 4 shows the effect of indium content on the photocatalytic activity of unloaded and Pt-loaded AgeIneZneS solid solutions under the illumination of a 300-W Xe lamp with an intensity of 100 mW/cm2. Without the Pt cocatalyst, the hydrogen production rate was relatively low, on the order of 0.1 L/m2 h. The H2 evolution rate was highest for sample B (0.374 L/m2 h). The rate then decreased gradually to 0.169 L/m2 h for sample F. The decreasing H2 generation rate of the bare solid solutions (without Pt loading) can be explained by the lower crystallinity for higher-In-content samples. From the XRD patterns in Fig. 1, sample B has the most pronounced diffraction peaks, which implies the highest crystallinity. This high crystallinity (few grain boundaries) might decrease the probability of electronehole recombination. In contrast, the hydrogen production rate depends strongly on the [In]/[Ag] ratio after loading 3 wt% Pt. Sample B showed the highest photocatalytic activity without Pt loading (0.374 L/m2 h), but it was not the highest after Pt loading (1.74 L/m2 h). The highest H2 evolution over the photocatalysts with Pt loading was observed for sample E (3.75 L/m2 h). The UVeVis spectra indicate that the optical energy band gap of the In-rich solid solutions does not significantly vary. This enhancement was thus not related to the absorption edge. In order to understand the origin of this enhancement, the morphologies, nanostructures, and compositions of the Inrich photocatalysts were studied. Fig. 5 shows SEM images of all the unloaded samples. The photocatalysts are agglomerates with micrometer-scale diameters, as determined
Fig. 4 e Photocatalytic H2 evolution over (AgeIneZn)S photocatalysts in a 0.25 M K2SO3 and 0.35 M Na2S aqueous solution. Irradiation area: 23 cm2.
5
from direct observation. It can be expected that the specific surface areas of the samples are low. However, careful observation of the images reveals nanostep structures in all samples. These nanostep structures were generated during the heat treatment process. The quantity of nanosteps increases with increasing indium content. This suggests that the amount of indium is the key to increasing the number of nanosteps in this quaternary photocatalyst. Samples E and F have the most nanostep structures. When H2PtCl6 was added, the Pt particles were rapidly photodeposited at the initial stage of the photocatalytic reaction. Fig. 6a shows a SEM image of the surface of the photocatalyst after Pt loading. It appears that the Pt was selectively deposited on the edges of the nanosteps. Fig. 6b shows a transmission electron microscopy (TEM) image, giving a clearer view of Pt deposition on the edges. Tsuji et al. demonstrated that the edges of nanostep structures facilitate electronehole charge separation [25]. The selective deposition of Pt particles also implies that photogenerated electrons migrate to the edge of the nanosteps, and these positions play a key role as reduction sites in the photocatalytic reaction [54]. Table 2 lists the atomic ratios of various metal components measured using ICP-OES, as well as the BET surface area data. The composition of the powders is a function of that of the precursor solution. The proposed two-step synthetic strategy was effective at keeping the [Zn]/[Ag] ratio constant. The standard deviation of [Zn]/[Ag] was lower than 5.1% for the various indium content levels. The BET surface area increased with increasing In content, perhaps due to more nanostep structures on the particle surface. The BET surface area varied from 0.83 to 3.82 m2/g, which is comparable to that of a similar system [13,25]. Table 2 also clearly shows that the Pt molar ratio increased with In content in the precursor solution. The same weight percentage of H2PtCl6 precursor was used to load the Pt cocatalyst onto the photocatalyst surfaces. The higher level of Pt content after loading suggests more active sites during photodeposition. Pt cocatalyst can trap electrons and provide active sites for hydrogen production from water under irradiation. Sample F had the most nanostep structures, which is consistent with the ICP-OES data (0.424 mol.% Pt and a BET surface area of 3.82 m2/g). However, sample E had the highest photocatalytic activity. This might be due to the competition among crystallinity, number of active sites, and specific surface area. To further verify the critical role of In content on Pt loading, two samples were selected to change the weight percentage of H2PtCl6 in the precursor solutions. Samples B and E had the highest hydrogen production rates of the bare and Pt-loaded photocatalysts, respectively. Since a lower Pt mol.% was observed for sample B (see Table 2), more H2PtCl6 (4.5 wt%, sample B-2) was added into the precursor. Less H2PtCl6 (1.5 wt %, sample E-1) was added into the precursor for sample E because more Pt was observed. The molar ratios of Pt after loading were 0.467 and 0.341 mol.% for samples B-2 and E-1, respectively. The amount of Pt was thus brought to similar levels for comparison. Sample B-2 had more Pt loading than that of sample E. However, the hydrogen production rate (1.565 L/m2 h) was lower than that of the latter (3.75 L/m2 h). The hydrogen production rate (3.043 L/m2 h) of less Pt-loaded sample E-1 was higher than that of sample B (1.74 L/m2 h). The
Please cite this article in press as: Lin P-C, et al., Enhanced photocatalytic hydrogen production over In-rich (AgeIneZn)S particles, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.04.125
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Fig. 5 e SEM images of bare (AgeIneZn)S solid solutions (no Pt loading) prepared with various [In]/[Ag] ratios.
hydrogen production rates obtained for various Pt loadings are plotted in Fig. 7. Furthermore, Sample E-2 (4.5 wt% Pt) shows almost the same photocatalytic activity (3.03 L/m2 h) as Sample E-1, but lower than Sample E. Considering the effective reaction surface area and the shielding for incident photons, this agrees with a study [25] that indicated that 3 wt% Pt loading is the optimal condition.
The luminescence properties of photocatalyst result from the electronehole recombination processes within the semiconductor [22]. Therefore, luminescence measurement is an efficient tool to detect the charge separation probability of our samples qualitatively. Fig. 8 shows the normalized photoluminescence spectra in two extreme cases, sample B and E. These spectra exhibit an emission peak at 500 nm,
Fig. 6 e (a) SEM and (b) TEM images of Pt-loaded sample E. Please cite this article in press as: Lin P-C, et al., Enhanced photocatalytic hydrogen production over In-rich (AgeIneZn)S particles, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.04.125
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Table 2 e Compositions and BET surface areas of In-rich photocatalysts. Atomic molar ratioa
Sample
A B C D E F B-2 E-1
Ag
In
Zn
Pt
1 1 1 1 1 1 1 1
1.05 1.34 1.51 1.82 2.1 2.25 1.32 2.09
7.23 7.32 7.27 7.43 7.39 6.88 7.35 7.42
0.0339 0.0369 0.0387 0.0419 0.044 0.043 0.0452 0.0358
Pt mol.%
BET (m2/g)
0.365 0.382 0.396 0.409 0.419 0.424 0.467 0.341
0.83 1.08 2.07 2.96 2.44 3.82 e e
a The atomic molar ratio of Ag:In:Zn was determined by ICP-OES.
Fig. 8 e Photoluminescence spectra of photocatalyst excited at 350 nm: (a) bare sample B, [In]/[Ag] [ 1.25, (b) Pt-loaded sample B, [In]/[Ag] [ 1.25, (c) bare sample E, [In]/[Ag] [ 2.00, (d) Pt-loaded sample E, [In]/[Ag] [ 2.00.
Fig. 7 e H2 evolution rate obtained for various Pt loading.
corresponding to the band-edge emission, which is consistent with the UVeVis spectra shown in Fig. 3. Curves a and c are measured from the bare samples B and E, respectively. Note that the normalized PL intensity of bare sample B is slightly lower than sample E, suggesting a lower charge recombination. This observation agrees with the hydrogen production rate shown in Fig. 4. After Pt cocatalyst loading, the PL intensities of both samples B and E decrease dramatically. This quenching effect produced by Pt loading is more pronounced for sample E, a 44.7% decrease in PL intensity. It is known that Pt particles are efficient traps for electrons [55]. This further demonstrates that Pt-loading treatment enhances the electronehole separation, especially for In-rich samples. Although the amount of Pt loading affects photocatalytic activity, the distribution of Pt cocatalyst is critical. The interaction between In and Pt contributes to the unique enhancement of hydrogen production using our In-rich photocatalysts. Nanostep structures were self-constructed during the heat treatment [25], especially for In-rich samples. Tsuji et al. proposed that this kind of solid solution consists of (Ag, In)-rich and Zn-rich planes in the wurtzite structure, and that (Ag, In)-rich planes generate nanostep structures. Our findings support their arguments. Tsuji et al.
also mentioned that excess indium is required in their recipe [25]; however, they did not discuss the effect of indium in detail. Furthermore, many factors affect the photocatalytic activity of particles, including specific surface area, crystal structure, crystallinity, and compositions of solid solutions [56]. These factors also change the charge transport properties, such as conductivity, transport mechanism, and mobility of the charge carriers. In this study, however, we first focus on the effect of In content and Pt loading on the photocatalytic hydrogen production. In fact, our results demonstrated that indium is a crucial factor for improving photocatalytic activity of the multi-component photocatalysts. In summary, the number of nanostep structures on the surface of this material depends on indium content. Although crystallinity decreases with increasing indium content, a suitable amount of Pt loading on the nanostep structures greatly improves the hydrogen production rate.
4.
Conclusion
An (AgeIneZn)S quaternary semiconductor was prepared using a two-step method in aqueous solutions to keep the band gap energy constant. After heat treatment at 850 C in a N2 environment, solid solutions with a wurtzite structure and various [In]/[Ag] ratios were obtained. The photocatalytic activity enhancement with increasing [In]/[Ag] ratio was mainly
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attributed to more nanostep structures and the dispersion of Pt cocatalysts. The highest evolution rate (3.75 L/m2 h), observed for sample E (Ag:In:Zn ¼ 1:2:7), was 5.86 times that of the original sample (sample A, Ag:In:Zn ¼ 1:1:7). This study demonstrated the critical role of excess indium in the (AgeIneZn)S photocatalyst.
Acknowledgment This material is based on research sponsored by the Air Force Research Laboratory, under agreement number FA2386-11-14081. The U.S. Government is authorized to reproduce and distribute reprints for Governmental purposes notwithstanding any copyright notation thereon. Disclaimer: The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the Air Force Research Laboratory or the U.S. Government. The authors would like to thank the Instrument Center at National Chung Cheng University for assistance with SEM and TEM images, the Industrial Technology Research Institute for BET measurements, and Professor YaSen Sun’s Lab at National Central University for UVeVis measurements.
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Please cite this article in press as: Lin P-C, et al., Enhanced photocatalytic hydrogen production over In-rich (AgeIneZn)S particles, International Journal of Hydrogen Energy (2013), http://dx.doi.org/10.1016/j.ijhydene.2013.04.125