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The effects of amount of La on the photocatalytic performance of ZnIn2S4 for hydrogen generation under visible light
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The effects of amount of La on the photocatalytic performance of ZnIn2S4 for hydrogen generation under visible light Fei Tian a, Rongshu Zhu a,b,*, Kelin Song a, Feng Ouyang a,b,*, Gang Cao a,b,* a
Harbin Institute of Technology Shenzhen Graduate School, Shenzhen Key Laboratory of Water Resource Utilization and Environmental Pollution Control, Shenzhen 518055, PR China b Public Platform for Technological Service in Urban Waste Reuse and Energy Regeneration, Shenzhen 518055, PR China
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abstract
Article history:
A series of La modified ZnIn2S4 photocatalysts (x wt% LaeZnIn2S4, x ¼ 0, 0.02, 0.1, 0.5, 1.0,
Received 2 September 2014
2.0, 5.0, 10.0) were prepared using the hydrothermal method and characterized by various
Received in revised form
analysis techniques, such as UVeVis, XRD, SEM, EDX, TEM, BET and PL. The results indi-
4 December 2014
cated that the La element existed as the oxide La2O3 which was located on the surface of
Accepted 8 December 2014
ZnIn2S4 petals. The increase in La amount can reduce ZnIn2S4 crystallite size, inhibit
Available online xxx
ZnIn2S4 grain growth, promote ZnIn2S4 crystallite self-organization into a micro-sphere with flower-like morphology, increase ZnIn2S4 surface area and total pore volume, and
Keywords:
bring rich defects to ZnIn2S4, indicating that the properties of these photocatalysts greatly
ZnIn2S4
depended on the amount of La added. The photocatalytic activities of LaeZnIn2S4 were
La
evaluated based on photocatalytic hydrogen production from water under visible-light
Visible-light
irradiation and the hydrogen production efficiency increased by 60.6%, 110.8%, 129.8%,
Hydrogen evolution
141.6%, 103.7%, 79.3% and 83.2% after adding 0.02 wt%, 0.1 wt%, 0.5 wt%, 1.0 wt%, 2.0 wt%, 5.0 wt% and 10.0 wt% of La, respectively. On the basis of the characterization of the catalysts, the effect of the amount of La added as a modifier on the photocatalytic activity of LaeZnIn2S4 was studied. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The environmental pollution and the energy crisis have become two major problems in the development of human society [1]. Hydrogen fuel has been recognized as an
environmental friendly renewable source for human society in the future. Hydrogen (H2) production from water splitting by photocatalysis under visible light has attracted considerable attention owing to growing demand for sustainable and environmentally friendly energy [2e4].
* Corresponding authors. Harbin Institute of Technology Shenzhen Graduate School, Shenzhen Key Laboratory of Water Resource Utilization and Environmental Pollution Control, Shenzhen 518055, PR China. E-mail addresses: [email protected] (R. Zhu), [email protected] (F. Ouyang), [email protected] (G. Cao). http://dx.doi.org/10.1016/j.ijhydene.2014.12.025 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Tian F, et al., The effects of amount of La on the photocatalytic performance of ZnIn2S4 for hydrogen generation under visible light, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.12.025
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In the recent years, ZnIn2S4, the only member of the AB2X4 family of semiconductors with a layered structure, has recently been studied because of its potential applications in different fields, such as charge storage [5], thermoelectricity [6], photocatalysis [2] and so on. Porous ZnIn2S4 has been tested to have a suitable band gap corresponding to the visible-light absorption region, high photocatalytic activity and considerable chemical stability for photocatalytic H2 evolution [3]. Numerous methods have been used to synthesize ZnIn2S4 materials with various morphologies [7,8] and improve the performance of ZnIn2S4 materials for photocatalytic water reduction [9e11]. Among these methods, suitable metal ion doping, such as transition-metal [9,10], alkaline-earth metal [11] and rare-earth (RE) metal [12] doping, has been proven to be an efficient route for achieving efficient hydrogen evolution. At present, doping RE ions into photocatalyst has also become a research hotspot [12e18]. Among these researches, doping La3þ ion into photocatalyst has been demonstrated to show the best effect. Du et al. [14] investigated the effect of surface OH population on the photocatalytic activity of RE (RE ¼ La, Ce, Y, Pr, Sm)-doped P25eTiO2 in methylene blue degradation and found that RE modified catalysts performed better than unmodified P25 and La was the preferred RE. Sato Fukugami [15] studied the photocatalytic properties of doped TiO2 with various RE ions, and found the photocatalytic activity for water cleavage was in the order La > Nd > Eu. They thought that the promotion of the gas evolution might be attributed to the increase of the mobility of electrons. Parida Sahu [16] investigated the photocatalytic activity of RE (RE ¼ La, Nd, Pr) titania nanocomposites under visible light and found that 0.4 mol % La3þ-TiO2 (activated at 673 K) showed highest surface area, lowest crystallite size and highest photocatalytic activity. Zhang et al. [18] investigated the effects of RE doping on the texture and properties of nanocrystalline mesoporous TiO2, and found that the anatase-to-rutile phase transformation of nanosized TiO2 was significantly inhibited by RE doping and the inhibitory effects followed the order of La3þ > Gd3þ > Yb3þ. Recently, a series of RE ions (RE ¼ La3þ, Ce3þ, Gd3þ, Er3þ or Y3þ) modified ZnIn2S4 photocatalysts (RE-ZnIn2S4) have been reported in our previous research [12] and the results revealed that the photocatalytic activity is enhanced with the increase of the RE3þ radius and decrease of the number of electrons of RE 4f shell and the La3þ show the best effect. The amount of the foreign elements will bring great influence on the properties of the catalyst [9,19,20]. Shen et al. [9] found that the optical properties and the layered structure of Cu-doped ZnIn2S4 greatly depended on the amount of Cu. Jing et al. [19] prepared Ni doped ZnIn2S4, and found that there was an optimal Ni doping content at 0.3 wt%. Yuan et al. [20] revealed that the optimal amount of Co was 0.3 wt% for Codoped ZnIn2S4 photocatalyst. In order to understand the effects of the amount of La3þ on the performance of the catalyst, a series amounts of La3þ modified porous ZnIn2S4 were prepared, and their properties were characterized and their photocatalytic activity were evaluated by photocatalytic hydrogen production from the splitting of water under visible-light irradiation.
Experimental Materials All chemicals are of analytical grade and used as received without further purification. La(NO3)3$6H2O (purity 99.99%) were purchased from Shanghai Jingchun Industrial Co. Ltd., China. ZnSO4$7H2O, In(NO3)3$4H2O, TAA (Thioacetamide), absolute ethanol, Na2S and Na2SO3 (purity 99.99%) were purchased from Sinopharm Chemical Reagent Co. Ltd., China. H2PtCl6$6H2O (purity 99.99%) was purchased from Shanghai July Chemical Co. Ltd., China.
Preparation of photocatalyst The modified ZnIn2S4 samples were prepared by a hydrothermal method [8]. For the synthesis of unmodified ZnIn2S4, 0.735 g ZnSO4$7H2O, 1.903 g In(NO3)3$4H2O and a double excess of TAA were dissolved in 50 mL of distilled water. The mixed solution was then transferred into a 100 mL Teflonlined autoclave. The autoclave was sealed and kept at 160 C for 1 h, and then cooled to room temperature naturally. A yellow precipitate was obtained, then filtered and washed with absolute ethanol and deionized water for three times. After dried in a vacuum at 80 C, ZnIn2S4 was obtained. For the synthesis of x wt% LaeZnIn2S4 (x ¼ 0, 0.02, 0.1, 0.5, 1.0, 2.0, 5.0, 10.0), a desired amount of La(NO3)3$6H2O aqueous solution was dropped into the mixed solution before heating.
Characterization The UVevisible absorption spectra of the samples were recorded using a UVevis diffuse reflectance spectroscopy (UVe2501PC, Shimadzu, Japan) in the spectral range of 200e750 nm. The crystal phases and crystallite sizes of the asprepared samples were characterized by X-ray diffraction (XRD) (Rigaku, D/max 2500) at room temperature, and the patterns were recorded over the angular range 20 e80 (2q), using a scan rate of 5 /min and Cu-Ka1 radiation with working voltage and current of 40 kV and 20 mA, respectively. The surface morphology of the samples was obtained using a scanning electron microscopy (SEM, S-4800, Hitachi, Japan). The chemical compositions of the sample were tested by an energy dispersive X-ray detector (EDX, EMax-250, Horiba, Japan). The microstructure of the samples was further investigated by high-resolution TEM (HRTEM) using a JEM-2010(HR) microscope working at 200 kV. The BET surface area was evaluated by N2 adsorption in a constant volume adsorption apparatus (Bel sorpII, Bayer Japan Co. Ltd., Japan). The analysis of photoluminescence spectra (PL) was carried out at room temperature using a Renishaw inVia raman microscope.
Photocatalytic reaction Photocatalytic reactions were conducted in an 868 mL gasclosed stainless steel reactor [2]. The cross section area and height of the reactor were 72.3 cm2 and 12 cm, respectively. A quartz glass cover on the top of the reactor allowed irradiation to pass through. The light source was a 350-W Xe lamp
Please cite this article in press as: Tian F, et al., The effects of amount of La on the photocatalytic performance of ZnIn2S4 for hydrogen generation under visible light, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.12.025
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(Shenzhen Stone-lighting Opto Device Co. Ltd., China) and the UV part of the light was removed by a cut-off filter (l > 420 nm). The light beam was focused coherently on the surface of the 200-mL catalyst suspension (2.8 cm in depth). The suspension in the reactor was continuously stirred using a magnetic stirrer at a speed of 400 rpm. The light intensity was measured using a spectroradiometer (FZ-A, Photoelectric Instrument Factory of Beijing Normal University). In all experiments, 200 mL of deionized water containing 0.2 g of catalyst and 0.25 M Na2SO3/0.35 M Na2S mixed sacrificial agent was added into the reaction cell. Here, a sacrificial agent was used to scavenge photo-generated holes. In order to remove air prior to the reaction, argon gas was bubbled through the reaction mixture for 30 min before the reaction started. Pt as a cocatalyst for the promotion of hydrogen evolution was photodeposited in situ on the photocatalyst from the precursor of H2PtCl6$6H2O [21]. The temperature for all the photocatalytic reactions was kept at 25 ± 1 C. The concentrations of H2 were measured with a gas chromatograph (Shanghai Precision & Scientific Instrument Co. Ltd., GC-112A, High-performance carbon molecular sieve packed column (2 m 3 mm)), equipped with a thermal conductivity detector (TCD). All the measurements of the produced H2 concentrations at different irradiation time were performed three times to confirm their reproducibility. Apparent quantum yields (A.Q.Y.) defined by the Eq. (1) were measured using a 420 nm band-pass filter and an irradiatometer. The energy conversion efficiency in the visiblelight region (using 350 W Xe lamp combined with a 420 nm cut-off filter) was determined by Eq. (2). A:Q:Y:ð%Þ ¼ ¼
hc ¼
The number of reacted electrons 100 The number of incident photons
The number of evolved H2 molecules 2 100 The number of incident photons
DG0P RP ES A
(1)
(2)
In Eq. (2), DG0P is the standard Gibbs energy for the energy storage reaction generating product H2, RP is the rate (mol/s) of generation of H2 in its standard state, ES is the incident visiblelight irradiance (W/m2) and A is the irradiated area (m2) [22]. To evaluate the photostability, the reaction condition was as mentioned above except that the reaction system should be evacuated for 30 min by argon gas after each cycle and was reused in the next run (one cycle ¼ 3 h).
Results and discussion Characterization of catalysts Analysis of UVevisible diffuse reflectance spectra Fig. 1 shows the UVevisible diffuse reflectance spectra of various ZnIn2S4 samples. The corresponding absorption edge and band gap are listed in Table 1. As shown in Table 1, the absorption edge of ZnIn2S4 locates at 540 nm and the corresponding band gap calculated from the onset of absorption edge is 2.29 ev, which is consistent with our previous work
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Fig. 1 e UVevis diffuse reflectance spectra of ZnIn2S4 samples.
[12]. The absorption edge and band gaps of ZnIn2S4 modified by La3þ ion are quite close to that of unmodified ZnIn2S4. These results indicate that the band structure of ZnIn2S4 would not be changed though different amount of La has been added. Furthermore, the shapes of the absorption edges are steep and the absorption in the visible region is strong. The steep absorption edge implies that the ZnIn2S4 is a singlephase crystal. These findings reveal that the absorption band of ZnIn2S4 is attributed to the transition from the valence band to the conduction band [8] and not to the transition from the metal impurity level to the conduction band [9,21]. Therefore, the modification of different amount of La3þ ion does not cause an essential difference on the light absorption property of ZnIn2S4.
X-ray diffraction (XRD) analysis Fig. 2 shows the XRD patterns of the ZnIn2S4 samples. All samples have almost the same XRD pattern, in which all the characteristic peaks could be assigned to the crystal phase of hexagonal ZnIn2S4 (ICSD-JCPDS card No.01-072-0773). No other impurities, such as ZnS, In2S3, oxides, or organic compounds related to reactants, were observed. All these features lead to the conclusion that these samples are pure singlephase hexagonal highly crystalline materials. This result is consistent with that was obtained by Shen et al. [8] and the observation from UVevis diffuse reflectance spectra (in Fig. 1). Further investigation does not reveal any evident XRD peak shift for the modified ZnIn2S4 when compared with unmodified ZnIn2S4. The data of d (006) space of the catalysts are also listed in Table 1. As shown in Table 1, the modifications of La3þ ion do not result in evident changes for the d (006) spaces of the catalyst. If a foreign element is doped into the crystal lattices, lattice distortion will occur, and the parameters of the unit cell will change, thus resulting in the shift of characteristic peaks in XRD patterns [8,15,17]. The ionic radius of La3þ ˚ ) is significantly larger than that of Zn2þ (0.740 A ˚ ) and (0.900 A 3þ ˚ In (0.810 A). The lack of a change in d (006) spaces may be
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Table 1 e Crystal structure parameters, photocatalytic activities for hydrogen production over various ZnIn2S4 Photocatalysts. La amount (wt%) Absorption edge (nm) Band gapa (ev) ˚) d (006) spaceb (A ˚) Half peak width (A Crystal sizec (nm) Actual La amountd (wt%) BET (m2/g) Total pore volume (cm3/g) Hydrogen production (mmol g1 h1) A.Q.Ye (%) hcf (%) a b c d e f
0 540 2.29 4.19 0.648 6.4 0 95.1 0.0809 241.0 3.59 2.09
0.02 542 2.28 4.19 0.746 5.5 0.02 102.9 0.1029 387.0 5.56 3.35
0.1 541 2.29 4.19 0.759 5.4 0.09 114.7 0.1134 508.2 7.12 4.41
0.5 548 2.28 4.19 0.820 5.1 0.49 118.5 0.1198 554.1 8.31 4.81
1.0 540 2.29 4.18 0.895 4.6 1.02 122.8 0.1211 583.4 8.83 5.06
2.0 543 2.28 4.18 1.072 3.9 1.41 120.3 0.1201 491.2 7.62 4.26
5.0 535 2.30 4.19 1.204 3.4 1.79 107.9 0.1182 432.1 6.41 3.75
10.0 539 2.30 4.19 0.851 4.9 2.06 101.2 0.1018 324.2 4.35 2.81
Calculated from absorption onset. Calculated from 2d sin q ¼ l. Determined from XRD results by using the Scherrer equation. Determined from EDX results. Calculated from Eq. (1). Calculated from Eq. (2).
attributed to the fact that the La3þ ion was too large to dope into the lattices of ZnIn2S4. Fig. 2 also shows that the width of peak (006) increases gradually with the increasing amount of La3þ from 0 wt% to 5.0 wt%. Peak width is known to be inversely proportional to the crystal size according to the Scherrer equation. Apparently, La3þ modification can reduce the crystallite size of the catalyst significantly and achieve a good dispersion. The crystallite sizes calculated from the reflection (006) by using the Scherrer equation are also listed in Table 1. As shown in Table 1, the diameter of the crystallite is calculated to be approximately 3 nme6 nm. The crystallite sizes sharply decrease from 6.4 nm to 3.4 nm with the amount of La3þ increasing from 0 wt% to 5.0 wt%, and then increase slightly when the amount of La3þ continues to increase. The reduction in crystallite size can be attributed to the segregation of the dopant cations at the grain boundary, which inhibits grain growth by restricting the coalescence of some smaller
neighboring grains [13,23,24]. Catalysts with smaller crystallites are generally known to exhibit better catalytic properties [25]. The actual La amount characterized by EDX are listed in Table 1. As shown in Table 1, when the theoretical La amount is less than 2.0 wt%, the actual La amount is equal to the theoretical date. when the theoretical La amount is more than 2.0 wt%, the actual La amount is less than the theoretical date. Which means that not all the La had been actually get into the catalyst when the theoretical La amount is more than 2.0 wt%. The XRD peaks for rare earth La also could not be detected in modified ZnIn2S4, which may be attributed to the small amount and good dispersion of the La3þ ion [9].
BrunauereEmmetteTeller (BET) analysis and total pore volume The BET surface area and total pore volume of ZnIn2S4 samples are also listed in Table 1. Compared with unmodified ZnIn2S4, the BET surface area and total pore volume of modified ZnIn2S4 samples increase rapidly when the amount of La3þ increases from 0.0 wt% to 1.0 wt%. If the amount of La3þ is larger than 1.0 wt%, the BET surface area and total pore volume decrease gradually. This result indicates that the BET surface area and total pore volume of the samples are strongly dependent on the amount of La3þ.
Morphology analysis
Fig. 2 e XRD patterns of ZnIn2S4 samples.
Fig. 3 shows the scanning electron microscopy (SEM) images of ZnIn2S4 samples. As shown in Fig. 3a, c, e, g, i, k, m, and o, ZnIn2S4 crystallites were self-organized into microsphere morphology with a wide distribution of diameters ranging from 2 mm to 5 mm. The microspheres with a wide distribution of pore sizes were evidently constructed by different sizes of nanoflakes. La3þ modification did not seriously affect the diameter of the sphere. As shown in Fig. 3b, on the surface of unmodified ZnIn2S4, the nanosheets are entirely collapsed and present gully-like structures; the crystal gathered together, and the pore structure was not open. As shown in Fig. 3d, f, h, j, l, n, and p, the spherical morphology gradually has a fine pore structure with the increasing amount of La3þ, which coincides with the result in Section
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Fig. 3 e SEM images of ZnIn2S4 samples: (a, b) 0 wt% LaeZnIn2S4; (c, d) 0.02 wt% LaeZnIn2S4; (e, f) 0.1 wt% LaeZnIn2S4; (g, h) 0.5 wt% LaeZnIn2S4; (i, j) 1.0 wt% LaeZnIn2S4; (k, l) 2.0 wt% LaeZnIn2S4; (m,n) 5.0 wt% LaeZnIn2S4; (o, p) 10.0 wt% LaeZnIn2S4.
BrunauereEmmetteTeller (BET) analysis and total pore volume. This finding can be attributed to the fact that, with the addition of La3þ from 0.0 wt% to 1.0 wt%, the pore structure was gradually opened; the gap width between the petals was increased, and more pores was produced, thus providing a greater BET surface area and total pore volume. When the amount of La3þ is larger than 1.0 wt%, the chip exfoliated off the sphere, destroying the pore structure of the regular roselike microclusters, and thus the BET surface area and total pore volume decrease gradually. The morphology and layered structure of La3þ modified ZnIn2S4 greatly depended on the amount of La3þ ion. As shown in Fig. 3d, when 0.02 wt% La3þ is added, the surface of the ZnIn2S4 sphere also does not open as well as the morphology as shown in Fig. 3b, but the surface bump becomes more obvious. When the amount of La3þ increased from 0.02 wt% to 0.1 wt% (in Fig. 3f), there were some chips exposed from the surface but the pore structure is still not open yet. The microspheres were composed of numerous petals/sheets when the amount of La3þ was more than 0.5 wt %, which was similar to the results obtained by other researchers [8,9,11]. When the amount of La3þ increased to
0.5 wt% (in Fig. 3h), most of the surface of the ZnIn2S4 sphere has already opened and assembled into rose-like microclusters, and the width of the gap between the petals is 0.05 mme0.1 mm. Finally, the surface of the ZnIn2S4 sphere opened completely and showed a regular morphology when the amount of La3þ increased to 1.0 wt% (in Fig. 3j). Agglomeration is hardly observed on the surface of the ZnIn2S4 sphere, and the width of gap between the petals further increases from 0.1 mm to 0.2 mm. When the amount of La3þ continues to increase to 2.0 wt% and 5.0 wt% (in Fig. 3l and n), some peeling chips appear on the surface of the sphere, and the width of gap between the petals further increases from 1 mm to 2 mm. When 10.0 wt% La3þ is added (in Fig. 3p), particles with sizes from 2 mm to 10 mm appear on the surface of the sphere, and the sphere becomes irregular. These results indicate that the modification of La3þ ion makes the surface morphology of ZnIn2S4 more regular, stabilizes textural structures, arrests agglomeration effectively and maintains mesopores. Fig. 4 shows the TEM images of 1.0 wt% LaeZnIn2S4. As shown in Fig. 4a and b, the ZnIn2S4 petals dispersed from each other which may be due to the ultrasonic pretreatment that
Please cite this article in press as: Tian F, et al., The effects of amount of La on the photocatalytic performance of ZnIn2S4 for hydrogen generation under visible light, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.12.025
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Fig. 4 e Low-resolution TEM images of 1.0 wt%LaeZnIn2S4 (a, b), High-resolution TEM images of ZnIn2S4 (c) and 1.0 wt% LaeZnIn2S4 (d).
destroyed the structure of ZnIn2S4 sphere as shown in Fig. 3j. However, this will facilitate the observation of the ZnIn2S4 petals. The size of ZnIn2S4 petals is about 300 nme400 nm, in accordance with the SEM images (Fig. 3j). Fig. 4c shows the HRTEM of ZnIn2S4 sphere. The lattice interplanar spacing is measured to be 0.324 nm, corresponding to the (102) plane of hexagonal ZnIn2S4 [10,26]. Fig. 4d shows the HRTEM of 1.0 wt% LaeZnIn2S4 petals. The lattice interplanar spacing is measured to be 0.298 nm, corresponding to the (101) plane of the hexagonal La2O3 [27], which is in accordance with the result of our previous research [12]. La2O3 particles have a diameter of about 5 nm and are located on the surface of ZnIn2S4 petals.
% to 1.0 wt%. If the amount of La3þ is larger than 1.0 wt%, the emission intensity of the peak decreases gradually. A stronger excitonic PL spectrum results in a higher content of surface defects [9,21,28,29]. Therefore, La3þ modification provides rich defects to the surface of ZnIn2S4 samples and the amount of the defects increases with the increasing amount of La from 0.0 wt% to 1.0 wt%. In addition, The emission intensities of the peaks of 5.0 wt% and 10.0 wt% are slightly weaker than that of ZnIn2S4. This may be due to the fact that too much La2O3 occupies the surface of ZnIn2S4, leading to the weaker absorption of light.
Photoluminescence (PL) analysis The PL spectra of ZnIn2S4 samples are shown in Fig. 5. In the PL spectra, all ZnIn2S4 samples have only one PL emission peak centered at 610 nm, which is approximately 60 nm longer than that of the ZnIn2S4 absorption edge. That is, the energy of the radiative photon (610 nm) is lower than the band gap energy (550 nm). Jing et al. [28] introduced the PL performance and mechanism of nano-sized semiconductor materials. According to their report, the PL performance of photocatalyst shown in Fig. 5 is an excitonic PL process, in which the non-radiative transitions of excited electrons from the conduction band bottom to different sub-bands (or surface states) occur first and subsequent radiative transitions from the sub-band to the valence band top can take place. Therefore, the excitonic PL process in Fig. 5 means rich defects. As shown in Fig. 5, the emission intensity of the peak increases rapidly when the amount of La3þ increases from 0.0 wt
Fig. 5 e Photoluminescence spectra of ZnIn2S4 samples.
Please cite this article in press as: Tian F, et al., The effects of amount of La on the photocatalytic performance of ZnIn2S4 for hydrogen generation under visible light, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.12.025
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Evaluation of photocatalytic activity Photocatalytic activity Fig. 6 shows the time course of photocatalytic hydrogen evolution over ZnIn2S4 samples under visible light irradiation. The hydrogen production rate is also listed in Table 1. As shown in Fig. 6, the efficiency of hydrogen evolution increases rapidly when the amount of La3þ increases from 0.0 wt% to 1.0 wt% and then decreases gradually when the amount of La3þ is larger than 1.0 wt%. The average hydrogen production rate of unmodified ZnIn2S4 reaches 241 mmol/g$h. Compared with unmodified ZnIn2S4, the efficiencies of La modified ZnIn2S4 (x wt% LaeZnIn2S4, x ¼ 0.02, 0.1, 0.5, 1.0, 2.0, 5.0, 10.0) increase by 60.6%, 110.8%, 129.8%, 141.6%, 103.7%, 79.3% and 83.2% respectively. These results indicate that the hydrogen production activity of ZnIn2S4 is influenced by the amount of La significantly. The apparent quantum yields and the energy conversion efficiency are also listed in Table 1. As shown in Table 1, with the optimal amount of La added (1.0 wt%), the apparent quantum yields and the energy conversion efficiency were determined to be 8.83% and 5.06%. In contrast to the characterization results of catalysts, the activity order is closely related to the surface morphology, surface area, pore structure and PL emission intensity. With the addition of La from 0.0 wt% to 1.0 wt%, it makes the surface morphology of ZnIn2S4 more regular, stabilizes textural structures, arrests agglomeration effectively and maintains mesopores (in Fig. 3), leading to gradual increases in the BET surface area and total pore volume of the catalysts (in Table 1). A greater content of surface defects results in a stronger PL signal. During the process of photocatalytic H2 production reaction, surface defects can become capture centers for photogenerated electrons. Thus, the recombination of photogenerated electrons and holes can be effectively inhibited to achieve higher photocatalytic activity [30]. With the amount of La gradually increasing, it can also reduce the crystallite size of catalysts and make the catalysts with smaller crystallites exhibit better hydrogen production efficiency (in Table 1) except that the hydrogen production
Fig. 7 e Stability study of photocatalytic H2 evolution over ZnIn2S4 and 1.0 wt% LaeZnIn2S4 under the visible-light irradiation from an aqueous Na2SO3 and Na2S solution.
efficiency of 1.0 wt% LaeZnIn2S4 is better than 2.0 wt% LaeZnIn2S4 and 5.0 wt% LaeZnIn2S4 which have smaller crystallite size. This may be due to the fact that too much La2O3 occupies the surface of ZnIn2S4, leading to the weaker absorption of light, and that the rose-like microclusters structure has been disturbed to a withering flowers-like microclusters structure (in Fig. 3l, n). From the discussion above, all these improvements are relevant to the amount of La and eventually promote the photocatalytic activity of ZnIn2S4.
Photostability Fig. 7 shows the photostability of ZnIn2S4 and LaeZnIn2S4 in five consecutive runs for total 15 h. As shown in Fig. 7, no significant deactivation is observed during the photocatalytic reaction for at least 15 h. The slight drop in the H2 production rate could be related to the consumption of the sacrificial reagents (0.25 M Na2SO3/0.35 M Na2S) in the reaction mixture over time because the concentrations of sacrificial reagents largely affect the performance of H2 production [31].
Conclusions
Fig. 6 e Photocatalytic activities of ZnIn2S4 samples.
A series of different amounts of La (0.00 wt%, 0.02 wt%, 0.1 wt %, 0.5 wt%, 1.0 wt%, 2.0 wt%, 5.0 wt% and 10.0 wt%) modified ZnIn2S4 photocatalysts were successfully synthesized using the hydrothermal method. The photocatalytic hydrogen evolution activities of them have also been evaluated under visible-light irradiation. Compared with unmodified ZnIn2S4, the efficiencies of LaeZnIn2S4 increased by 60.6%, 110.8%, 129.8%, 141.6%, 103.7%, 79.3% and 83.2% respectively. With the increasing amount of La added, it will reduce the crystallite size, inhibit the grain growth, promote ZnIn2S4 crystallite selforganization into a micro-sphere flower-like morphology, increase the surface area and total pore volume, bring rich defects to the surface of catalyst, and inhibit the recombination
Please cite this article in press as: Tian F, et al., The effects of amount of La on the photocatalytic performance of ZnIn2S4 for hydrogen generation under visible light, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.12.025
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of photogenerated electrons and holes effectively. The effects of La amount on the photocatalytic activity of ZnIn2S4 were attributed to the interaction among the crystal size, surface area, pore structure, surface morphology and PL emission intensity.
Acknowledgments All the authors gratefully acknowledge support from the Special fund for the development of strategic and new industry in Shenzhen (No. JCYJ20120613114951217, JCYJ20130329162012793), the Fund for the Research and Development of Science and Technology in Shenzhen (No. CXZZ20130516145955144, JC201105160593A) and the National High Technology Research and Development Program of China (2012ZX07206-002).
references
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The effects of amount of La on the photocatalytic performance of ZnIn2S4 for hydrogen generation under visible light Fei Tian a, Rongshu Zhu a,b,*, Kelin Song a, Feng Ouyang a,b,*, Gang Cao a,b,* a
Harbin Institute of Technology Shenzhen Graduate School, Shenzhen Key Laboratory of Water Resource Utilization and Environmental Pollution Control, Shenzhen 518055, PR China b Public Platform for Technological Service in Urban Waste Reuse and Energy Regeneration, Shenzhen 518055, PR China
article info
abstract
Article history:
A series of La modified ZnIn2S4 photocatalysts (x wt% LaeZnIn2S4, x ¼ 0, 0.02, 0.1, 0.5, 1.0,
Received 2 September 2014
2.0, 5.0, 10.0) were prepared using the hydrothermal method and characterized by various
Received in revised form
analysis techniques, such as UVeVis, XRD, SEM, EDX, TEM, BET and PL. The results indi-
4 December 2014
cated that the La element existed as the oxide La2O3 which was located on the surface of
Accepted 8 December 2014
ZnIn2S4 petals. The increase in La amount can reduce ZnIn2S4 crystallite size, inhibit
Available online xxx
ZnIn2S4 grain growth, promote ZnIn2S4 crystallite self-organization into a micro-sphere with flower-like morphology, increase ZnIn2S4 surface area and total pore volume, and
Keywords:
bring rich defects to ZnIn2S4, indicating that the properties of these photocatalysts greatly
ZnIn2S4
depended on the amount of La added. The photocatalytic activities of LaeZnIn2S4 were
La
evaluated based on photocatalytic hydrogen production from water under visible-light
Visible-light
irradiation and the hydrogen production efficiency increased by 60.6%, 110.8%, 129.8%,
Hydrogen evolution
141.6%, 103.7%, 79.3% and 83.2% after adding 0.02 wt%, 0.1 wt%, 0.5 wt%, 1.0 wt%, 2.0 wt%, 5.0 wt% and 10.0 wt% of La, respectively. On the basis of the characterization of the catalysts, the effect of the amount of La added as a modifier on the photocatalytic activity of LaeZnIn2S4 was studied. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The environmental pollution and the energy crisis have become two major problems in the development of human society [1]. Hydrogen fuel has been recognized as an
environmental friendly renewable source for human society in the future. Hydrogen (H2) production from water splitting by photocatalysis under visible light has attracted considerable attention owing to growing demand for sustainable and environmentally friendly energy [2e4].
* Corresponding authors. Harbin Institute of Technology Shenzhen Graduate School, Shenzhen Key Laboratory of Water Resource Utilization and Environmental Pollution Control, Shenzhen 518055, PR China. E-mail addresses: [email protected] (R. Zhu), [email protected] (F. Ouyang), [email protected] (G. Cao). http://dx.doi.org/10.1016/j.ijhydene.2014.12.025 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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In the recent years, ZnIn2S4, the only member of the AB2X4 family of semiconductors with a layered structure, has recently been studied because of its potential applications in different fields, such as charge storage [5], thermoelectricity [6], photocatalysis [2] and so on. Porous ZnIn2S4 has been tested to have a suitable band gap corresponding to the visible-light absorption region, high photocatalytic activity and considerable chemical stability for photocatalytic H2 evolution [3]. Numerous methods have been used to synthesize ZnIn2S4 materials with various morphologies [7,8] and improve the performance of ZnIn2S4 materials for photocatalytic water reduction [9e11]. Among these methods, suitable metal ion doping, such as transition-metal [9,10], alkaline-earth metal [11] and rare-earth (RE) metal [12] doping, has been proven to be an efficient route for achieving efficient hydrogen evolution. At present, doping RE ions into photocatalyst has also become a research hotspot [12e18]. Among these researches, doping La3þ ion into photocatalyst has been demonstrated to show the best effect. Du et al. [14] investigated the effect of surface OH population on the photocatalytic activity of RE (RE ¼ La, Ce, Y, Pr, Sm)-doped P25eTiO2 in methylene blue degradation and found that RE modified catalysts performed better than unmodified P25 and La was the preferred RE. Sato Fukugami [15] studied the photocatalytic properties of doped TiO2 with various RE ions, and found the photocatalytic activity for water cleavage was in the order La > Nd > Eu. They thought that the promotion of the gas evolution might be attributed to the increase of the mobility of electrons. Parida Sahu [16] investigated the photocatalytic activity of RE (RE ¼ La, Nd, Pr) titania nanocomposites under visible light and found that 0.4 mol % La3þ-TiO2 (activated at 673 K) showed highest surface area, lowest crystallite size and highest photocatalytic activity. Zhang et al. [18] investigated the effects of RE doping on the texture and properties of nanocrystalline mesoporous TiO2, and found that the anatase-to-rutile phase transformation of nanosized TiO2 was significantly inhibited by RE doping and the inhibitory effects followed the order of La3þ > Gd3þ > Yb3þ. Recently, a series of RE ions (RE ¼ La3þ, Ce3þ, Gd3þ, Er3þ or Y3þ) modified ZnIn2S4 photocatalysts (RE-ZnIn2S4) have been reported in our previous research [12] and the results revealed that the photocatalytic activity is enhanced with the increase of the RE3þ radius and decrease of the number of electrons of RE 4f shell and the La3þ show the best effect. The amount of the foreign elements will bring great influence on the properties of the catalyst [9,19,20]. Shen et al. [9] found that the optical properties and the layered structure of Cu-doped ZnIn2S4 greatly depended on the amount of Cu. Jing et al. [19] prepared Ni doped ZnIn2S4, and found that there was an optimal Ni doping content at 0.3 wt%. Yuan et al. [20] revealed that the optimal amount of Co was 0.3 wt% for Codoped ZnIn2S4 photocatalyst. In order to understand the effects of the amount of La3þ on the performance of the catalyst, a series amounts of La3þ modified porous ZnIn2S4 were prepared, and their properties were characterized and their photocatalytic activity were evaluated by photocatalytic hydrogen production from the splitting of water under visible-light irradiation.
Experimental Materials All chemicals are of analytical grade and used as received without further purification. La(NO3)3$6H2O (purity 99.99%) were purchased from Shanghai Jingchun Industrial Co. Ltd., China. ZnSO4$7H2O, In(NO3)3$4H2O, TAA (Thioacetamide), absolute ethanol, Na2S and Na2SO3 (purity 99.99%) were purchased from Sinopharm Chemical Reagent Co. Ltd., China. H2PtCl6$6H2O (purity 99.99%) was purchased from Shanghai July Chemical Co. Ltd., China.
Preparation of photocatalyst The modified ZnIn2S4 samples were prepared by a hydrothermal method [8]. For the synthesis of unmodified ZnIn2S4, 0.735 g ZnSO4$7H2O, 1.903 g In(NO3)3$4H2O and a double excess of TAA were dissolved in 50 mL of distilled water. The mixed solution was then transferred into a 100 mL Teflonlined autoclave. The autoclave was sealed and kept at 160 C for 1 h, and then cooled to room temperature naturally. A yellow precipitate was obtained, then filtered and washed with absolute ethanol and deionized water for three times. After dried in a vacuum at 80 C, ZnIn2S4 was obtained. For the synthesis of x wt% LaeZnIn2S4 (x ¼ 0, 0.02, 0.1, 0.5, 1.0, 2.0, 5.0, 10.0), a desired amount of La(NO3)3$6H2O aqueous solution was dropped into the mixed solution before heating.
Characterization The UVevisible absorption spectra of the samples were recorded using a UVevis diffuse reflectance spectroscopy (UVe2501PC, Shimadzu, Japan) in the spectral range of 200e750 nm. The crystal phases and crystallite sizes of the asprepared samples were characterized by X-ray diffraction (XRD) (Rigaku, D/max 2500) at room temperature, and the patterns were recorded over the angular range 20 e80 (2q), using a scan rate of 5 /min and Cu-Ka1 radiation with working voltage and current of 40 kV and 20 mA, respectively. The surface morphology of the samples was obtained using a scanning electron microscopy (SEM, S-4800, Hitachi, Japan). The chemical compositions of the sample were tested by an energy dispersive X-ray detector (EDX, EMax-250, Horiba, Japan). The microstructure of the samples was further investigated by high-resolution TEM (HRTEM) using a JEM-2010(HR) microscope working at 200 kV. The BET surface area was evaluated by N2 adsorption in a constant volume adsorption apparatus (Bel sorpII, Bayer Japan Co. Ltd., Japan). The analysis of photoluminescence spectra (PL) was carried out at room temperature using a Renishaw inVia raman microscope.
Photocatalytic reaction Photocatalytic reactions were conducted in an 868 mL gasclosed stainless steel reactor [2]. The cross section area and height of the reactor were 72.3 cm2 and 12 cm, respectively. A quartz glass cover on the top of the reactor allowed irradiation to pass through. The light source was a 350-W Xe lamp
Please cite this article in press as: Tian F, et al., The effects of amount of La on the photocatalytic performance of ZnIn2S4 for hydrogen generation under visible light, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.12.025
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(Shenzhen Stone-lighting Opto Device Co. Ltd., China) and the UV part of the light was removed by a cut-off filter (l > 420 nm). The light beam was focused coherently on the surface of the 200-mL catalyst suspension (2.8 cm in depth). The suspension in the reactor was continuously stirred using a magnetic stirrer at a speed of 400 rpm. The light intensity was measured using a spectroradiometer (FZ-A, Photoelectric Instrument Factory of Beijing Normal University). In all experiments, 200 mL of deionized water containing 0.2 g of catalyst and 0.25 M Na2SO3/0.35 M Na2S mixed sacrificial agent was added into the reaction cell. Here, a sacrificial agent was used to scavenge photo-generated holes. In order to remove air prior to the reaction, argon gas was bubbled through the reaction mixture for 30 min before the reaction started. Pt as a cocatalyst for the promotion of hydrogen evolution was photodeposited in situ on the photocatalyst from the precursor of H2PtCl6$6H2O [21]. The temperature for all the photocatalytic reactions was kept at 25 ± 1 C. The concentrations of H2 were measured with a gas chromatograph (Shanghai Precision & Scientific Instrument Co. Ltd., GC-112A, High-performance carbon molecular sieve packed column (2 m 3 mm)), equipped with a thermal conductivity detector (TCD). All the measurements of the produced H2 concentrations at different irradiation time were performed three times to confirm their reproducibility. Apparent quantum yields (A.Q.Y.) defined by the Eq. (1) were measured using a 420 nm band-pass filter and an irradiatometer. The energy conversion efficiency in the visiblelight region (using 350 W Xe lamp combined with a 420 nm cut-off filter) was determined by Eq. (2). A:Q:Y:ð%Þ ¼ ¼
hc ¼
The number of reacted electrons 100 The number of incident photons
The number of evolved H2 molecules 2 100 The number of incident photons
DG0P RP ES A
(1)
(2)
In Eq. (2), DG0P is the standard Gibbs energy for the energy storage reaction generating product H2, RP is the rate (mol/s) of generation of H2 in its standard state, ES is the incident visiblelight irradiance (W/m2) and A is the irradiated area (m2) [22]. To evaluate the photostability, the reaction condition was as mentioned above except that the reaction system should be evacuated for 30 min by argon gas after each cycle and was reused in the next run (one cycle ¼ 3 h).
Results and discussion Characterization of catalysts Analysis of UVevisible diffuse reflectance spectra Fig. 1 shows the UVevisible diffuse reflectance spectra of various ZnIn2S4 samples. The corresponding absorption edge and band gap are listed in Table 1. As shown in Table 1, the absorption edge of ZnIn2S4 locates at 540 nm and the corresponding band gap calculated from the onset of absorption edge is 2.29 ev, which is consistent with our previous work
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Fig. 1 e UVevis diffuse reflectance spectra of ZnIn2S4 samples.
[12]. The absorption edge and band gaps of ZnIn2S4 modified by La3þ ion are quite close to that of unmodified ZnIn2S4. These results indicate that the band structure of ZnIn2S4 would not be changed though different amount of La has been added. Furthermore, the shapes of the absorption edges are steep and the absorption in the visible region is strong. The steep absorption edge implies that the ZnIn2S4 is a singlephase crystal. These findings reveal that the absorption band of ZnIn2S4 is attributed to the transition from the valence band to the conduction band [8] and not to the transition from the metal impurity level to the conduction band [9,21]. Therefore, the modification of different amount of La3þ ion does not cause an essential difference on the light absorption property of ZnIn2S4.
X-ray diffraction (XRD) analysis Fig. 2 shows the XRD patterns of the ZnIn2S4 samples. All samples have almost the same XRD pattern, in which all the characteristic peaks could be assigned to the crystal phase of hexagonal ZnIn2S4 (ICSD-JCPDS card No.01-072-0773). No other impurities, such as ZnS, In2S3, oxides, or organic compounds related to reactants, were observed. All these features lead to the conclusion that these samples are pure singlephase hexagonal highly crystalline materials. This result is consistent with that was obtained by Shen et al. [8] and the observation from UVevis diffuse reflectance spectra (in Fig. 1). Further investigation does not reveal any evident XRD peak shift for the modified ZnIn2S4 when compared with unmodified ZnIn2S4. The data of d (006) space of the catalysts are also listed in Table 1. As shown in Table 1, the modifications of La3þ ion do not result in evident changes for the d (006) spaces of the catalyst. If a foreign element is doped into the crystal lattices, lattice distortion will occur, and the parameters of the unit cell will change, thus resulting in the shift of characteristic peaks in XRD patterns [8,15,17]. The ionic radius of La3þ ˚ ) is significantly larger than that of Zn2þ (0.740 A ˚ ) and (0.900 A 3þ ˚ In (0.810 A). The lack of a change in d (006) spaces may be
Please cite this article in press as: Tian F, et al., The effects of amount of La on the photocatalytic performance of ZnIn2S4 for hydrogen generation under visible light, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.12.025
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Table 1 e Crystal structure parameters, photocatalytic activities for hydrogen production over various ZnIn2S4 Photocatalysts. La amount (wt%) Absorption edge (nm) Band gapa (ev) ˚) d (006) spaceb (A ˚) Half peak width (A Crystal sizec (nm) Actual La amountd (wt%) BET (m2/g) Total pore volume (cm3/g) Hydrogen production (mmol g1 h1) A.Q.Ye (%) hcf (%) a b c d e f
0 540 2.29 4.19 0.648 6.4 0 95.1 0.0809 241.0 3.59 2.09
0.02 542 2.28 4.19 0.746 5.5 0.02 102.9 0.1029 387.0 5.56 3.35
0.1 541 2.29 4.19 0.759 5.4 0.09 114.7 0.1134 508.2 7.12 4.41
0.5 548 2.28 4.19 0.820 5.1 0.49 118.5 0.1198 554.1 8.31 4.81
1.0 540 2.29 4.18 0.895 4.6 1.02 122.8 0.1211 583.4 8.83 5.06
2.0 543 2.28 4.18 1.072 3.9 1.41 120.3 0.1201 491.2 7.62 4.26
5.0 535 2.30 4.19 1.204 3.4 1.79 107.9 0.1182 432.1 6.41 3.75
10.0 539 2.30 4.19 0.851 4.9 2.06 101.2 0.1018 324.2 4.35 2.81
Calculated from absorption onset. Calculated from 2d sin q ¼ l. Determined from XRD results by using the Scherrer equation. Determined from EDX results. Calculated from Eq. (1). Calculated from Eq. (2).
attributed to the fact that the La3þ ion was too large to dope into the lattices of ZnIn2S4. Fig. 2 also shows that the width of peak (006) increases gradually with the increasing amount of La3þ from 0 wt% to 5.0 wt%. Peak width is known to be inversely proportional to the crystal size according to the Scherrer equation. Apparently, La3þ modification can reduce the crystallite size of the catalyst significantly and achieve a good dispersion. The crystallite sizes calculated from the reflection (006) by using the Scherrer equation are also listed in Table 1. As shown in Table 1, the diameter of the crystallite is calculated to be approximately 3 nme6 nm. The crystallite sizes sharply decrease from 6.4 nm to 3.4 nm with the amount of La3þ increasing from 0 wt% to 5.0 wt%, and then increase slightly when the amount of La3þ continues to increase. The reduction in crystallite size can be attributed to the segregation of the dopant cations at the grain boundary, which inhibits grain growth by restricting the coalescence of some smaller
neighboring grains [13,23,24]. Catalysts with smaller crystallites are generally known to exhibit better catalytic properties [25]. The actual La amount characterized by EDX are listed in Table 1. As shown in Table 1, when the theoretical La amount is less than 2.0 wt%, the actual La amount is equal to the theoretical date. when the theoretical La amount is more than 2.0 wt%, the actual La amount is less than the theoretical date. Which means that not all the La had been actually get into the catalyst when the theoretical La amount is more than 2.0 wt%. The XRD peaks for rare earth La also could not be detected in modified ZnIn2S4, which may be attributed to the small amount and good dispersion of the La3þ ion [9].
BrunauereEmmetteTeller (BET) analysis and total pore volume The BET surface area and total pore volume of ZnIn2S4 samples are also listed in Table 1. Compared with unmodified ZnIn2S4, the BET surface area and total pore volume of modified ZnIn2S4 samples increase rapidly when the amount of La3þ increases from 0.0 wt% to 1.0 wt%. If the amount of La3þ is larger than 1.0 wt%, the BET surface area and total pore volume decrease gradually. This result indicates that the BET surface area and total pore volume of the samples are strongly dependent on the amount of La3þ.
Morphology analysis
Fig. 2 e XRD patterns of ZnIn2S4 samples.
Fig. 3 shows the scanning electron microscopy (SEM) images of ZnIn2S4 samples. As shown in Fig. 3a, c, e, g, i, k, m, and o, ZnIn2S4 crystallites were self-organized into microsphere morphology with a wide distribution of diameters ranging from 2 mm to 5 mm. The microspheres with a wide distribution of pore sizes were evidently constructed by different sizes of nanoflakes. La3þ modification did not seriously affect the diameter of the sphere. As shown in Fig. 3b, on the surface of unmodified ZnIn2S4, the nanosheets are entirely collapsed and present gully-like structures; the crystal gathered together, and the pore structure was not open. As shown in Fig. 3d, f, h, j, l, n, and p, the spherical morphology gradually has a fine pore structure with the increasing amount of La3þ, which coincides with the result in Section
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Fig. 3 e SEM images of ZnIn2S4 samples: (a, b) 0 wt% LaeZnIn2S4; (c, d) 0.02 wt% LaeZnIn2S4; (e, f) 0.1 wt% LaeZnIn2S4; (g, h) 0.5 wt% LaeZnIn2S4; (i, j) 1.0 wt% LaeZnIn2S4; (k, l) 2.0 wt% LaeZnIn2S4; (m,n) 5.0 wt% LaeZnIn2S4; (o, p) 10.0 wt% LaeZnIn2S4.
BrunauereEmmetteTeller (BET) analysis and total pore volume. This finding can be attributed to the fact that, with the addition of La3þ from 0.0 wt% to 1.0 wt%, the pore structure was gradually opened; the gap width between the petals was increased, and more pores was produced, thus providing a greater BET surface area and total pore volume. When the amount of La3þ is larger than 1.0 wt%, the chip exfoliated off the sphere, destroying the pore structure of the regular roselike microclusters, and thus the BET surface area and total pore volume decrease gradually. The morphology and layered structure of La3þ modified ZnIn2S4 greatly depended on the amount of La3þ ion. As shown in Fig. 3d, when 0.02 wt% La3þ is added, the surface of the ZnIn2S4 sphere also does not open as well as the morphology as shown in Fig. 3b, but the surface bump becomes more obvious. When the amount of La3þ increased from 0.02 wt% to 0.1 wt% (in Fig. 3f), there were some chips exposed from the surface but the pore structure is still not open yet. The microspheres were composed of numerous petals/sheets when the amount of La3þ was more than 0.5 wt %, which was similar to the results obtained by other researchers [8,9,11]. When the amount of La3þ increased to
0.5 wt% (in Fig. 3h), most of the surface of the ZnIn2S4 sphere has already opened and assembled into rose-like microclusters, and the width of the gap between the petals is 0.05 mme0.1 mm. Finally, the surface of the ZnIn2S4 sphere opened completely and showed a regular morphology when the amount of La3þ increased to 1.0 wt% (in Fig. 3j). Agglomeration is hardly observed on the surface of the ZnIn2S4 sphere, and the width of gap between the petals further increases from 0.1 mm to 0.2 mm. When the amount of La3þ continues to increase to 2.0 wt% and 5.0 wt% (in Fig. 3l and n), some peeling chips appear on the surface of the sphere, and the width of gap between the petals further increases from 1 mm to 2 mm. When 10.0 wt% La3þ is added (in Fig. 3p), particles with sizes from 2 mm to 10 mm appear on the surface of the sphere, and the sphere becomes irregular. These results indicate that the modification of La3þ ion makes the surface morphology of ZnIn2S4 more regular, stabilizes textural structures, arrests agglomeration effectively and maintains mesopores. Fig. 4 shows the TEM images of 1.0 wt% LaeZnIn2S4. As shown in Fig. 4a and b, the ZnIn2S4 petals dispersed from each other which may be due to the ultrasonic pretreatment that
Please cite this article in press as: Tian F, et al., The effects of amount of La on the photocatalytic performance of ZnIn2S4 for hydrogen generation under visible light, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.12.025
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Fig. 4 e Low-resolution TEM images of 1.0 wt%LaeZnIn2S4 (a, b), High-resolution TEM images of ZnIn2S4 (c) and 1.0 wt% LaeZnIn2S4 (d).
destroyed the structure of ZnIn2S4 sphere as shown in Fig. 3j. However, this will facilitate the observation of the ZnIn2S4 petals. The size of ZnIn2S4 petals is about 300 nme400 nm, in accordance with the SEM images (Fig. 3j). Fig. 4c shows the HRTEM of ZnIn2S4 sphere. The lattice interplanar spacing is measured to be 0.324 nm, corresponding to the (102) plane of hexagonal ZnIn2S4 [10,26]. Fig. 4d shows the HRTEM of 1.0 wt% LaeZnIn2S4 petals. The lattice interplanar spacing is measured to be 0.298 nm, corresponding to the (101) plane of the hexagonal La2O3 [27], which is in accordance with the result of our previous research [12]. La2O3 particles have a diameter of about 5 nm and are located on the surface of ZnIn2S4 petals.
% to 1.0 wt%. If the amount of La3þ is larger than 1.0 wt%, the emission intensity of the peak decreases gradually. A stronger excitonic PL spectrum results in a higher content of surface defects [9,21,28,29]. Therefore, La3þ modification provides rich defects to the surface of ZnIn2S4 samples and the amount of the defects increases with the increasing amount of La from 0.0 wt% to 1.0 wt%. In addition, The emission intensities of the peaks of 5.0 wt% and 10.0 wt% are slightly weaker than that of ZnIn2S4. This may be due to the fact that too much La2O3 occupies the surface of ZnIn2S4, leading to the weaker absorption of light.
Photoluminescence (PL) analysis The PL spectra of ZnIn2S4 samples are shown in Fig. 5. In the PL spectra, all ZnIn2S4 samples have only one PL emission peak centered at 610 nm, which is approximately 60 nm longer than that of the ZnIn2S4 absorption edge. That is, the energy of the radiative photon (610 nm) is lower than the band gap energy (550 nm). Jing et al. [28] introduced the PL performance and mechanism of nano-sized semiconductor materials. According to their report, the PL performance of photocatalyst shown in Fig. 5 is an excitonic PL process, in which the non-radiative transitions of excited electrons from the conduction band bottom to different sub-bands (or surface states) occur first and subsequent radiative transitions from the sub-band to the valence band top can take place. Therefore, the excitonic PL process in Fig. 5 means rich defects. As shown in Fig. 5, the emission intensity of the peak increases rapidly when the amount of La3þ increases from 0.0 wt
Fig. 5 e Photoluminescence spectra of ZnIn2S4 samples.
Please cite this article in press as: Tian F, et al., The effects of amount of La on the photocatalytic performance of ZnIn2S4 for hydrogen generation under visible light, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.12.025
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 4 ) 1 e8
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Evaluation of photocatalytic activity Photocatalytic activity Fig. 6 shows the time course of photocatalytic hydrogen evolution over ZnIn2S4 samples under visible light irradiation. The hydrogen production rate is also listed in Table 1. As shown in Fig. 6, the efficiency of hydrogen evolution increases rapidly when the amount of La3þ increases from 0.0 wt% to 1.0 wt% and then decreases gradually when the amount of La3þ is larger than 1.0 wt%. The average hydrogen production rate of unmodified ZnIn2S4 reaches 241 mmol/g$h. Compared with unmodified ZnIn2S4, the efficiencies of La modified ZnIn2S4 (x wt% LaeZnIn2S4, x ¼ 0.02, 0.1, 0.5, 1.0, 2.0, 5.0, 10.0) increase by 60.6%, 110.8%, 129.8%, 141.6%, 103.7%, 79.3% and 83.2% respectively. These results indicate that the hydrogen production activity of ZnIn2S4 is influenced by the amount of La significantly. The apparent quantum yields and the energy conversion efficiency are also listed in Table 1. As shown in Table 1, with the optimal amount of La added (1.0 wt%), the apparent quantum yields and the energy conversion efficiency were determined to be 8.83% and 5.06%. In contrast to the characterization results of catalysts, the activity order is closely related to the surface morphology, surface area, pore structure and PL emission intensity. With the addition of La from 0.0 wt% to 1.0 wt%, it makes the surface morphology of ZnIn2S4 more regular, stabilizes textural structures, arrests agglomeration effectively and maintains mesopores (in Fig. 3), leading to gradual increases in the BET surface area and total pore volume of the catalysts (in Table 1). A greater content of surface defects results in a stronger PL signal. During the process of photocatalytic H2 production reaction, surface defects can become capture centers for photogenerated electrons. Thus, the recombination of photogenerated electrons and holes can be effectively inhibited to achieve higher photocatalytic activity [30]. With the amount of La gradually increasing, it can also reduce the crystallite size of catalysts and make the catalysts with smaller crystallites exhibit better hydrogen production efficiency (in Table 1) except that the hydrogen production
Fig. 7 e Stability study of photocatalytic H2 evolution over ZnIn2S4 and 1.0 wt% LaeZnIn2S4 under the visible-light irradiation from an aqueous Na2SO3 and Na2S solution.
efficiency of 1.0 wt% LaeZnIn2S4 is better than 2.0 wt% LaeZnIn2S4 and 5.0 wt% LaeZnIn2S4 which have smaller crystallite size. This may be due to the fact that too much La2O3 occupies the surface of ZnIn2S4, leading to the weaker absorption of light, and that the rose-like microclusters structure has been disturbed to a withering flowers-like microclusters structure (in Fig. 3l, n). From the discussion above, all these improvements are relevant to the amount of La and eventually promote the photocatalytic activity of ZnIn2S4.
Photostability Fig. 7 shows the photostability of ZnIn2S4 and LaeZnIn2S4 in five consecutive runs for total 15 h. As shown in Fig. 7, no significant deactivation is observed during the photocatalytic reaction for at least 15 h. The slight drop in the H2 production rate could be related to the consumption of the sacrificial reagents (0.25 M Na2SO3/0.35 M Na2S) in the reaction mixture over time because the concentrations of sacrificial reagents largely affect the performance of H2 production [31].
Conclusions
Fig. 6 e Photocatalytic activities of ZnIn2S4 samples.
A series of different amounts of La (0.00 wt%, 0.02 wt%, 0.1 wt %, 0.5 wt%, 1.0 wt%, 2.0 wt%, 5.0 wt% and 10.0 wt%) modified ZnIn2S4 photocatalysts were successfully synthesized using the hydrothermal method. The photocatalytic hydrogen evolution activities of them have also been evaluated under visible-light irradiation. Compared with unmodified ZnIn2S4, the efficiencies of LaeZnIn2S4 increased by 60.6%, 110.8%, 129.8%, 141.6%, 103.7%, 79.3% and 83.2% respectively. With the increasing amount of La added, it will reduce the crystallite size, inhibit the grain growth, promote ZnIn2S4 crystallite selforganization into a micro-sphere flower-like morphology, increase the surface area and total pore volume, bring rich defects to the surface of catalyst, and inhibit the recombination
Please cite this article in press as: Tian F, et al., The effects of amount of La on the photocatalytic performance of ZnIn2S4 for hydrogen generation under visible light, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.12.025
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of photogenerated electrons and holes effectively. The effects of La amount on the photocatalytic activity of ZnIn2S4 were attributed to the interaction among the crystal size, surface area, pore structure, surface morphology and PL emission intensity.
Acknowledgments All the authors gratefully acknowledge support from the Special fund for the development of strategic and new industry in Shenzhen (No. JCYJ20120613114951217, JCYJ20130329162012793), the Fund for the Research and Development of Science and Technology in Shenzhen (No. CXZZ20130516145955144, JC201105160593A) and the National High Technology Research and Development Program of China (2012ZX07206-002).
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Please cite this article in press as: Tian F, et al., The effects of amount of La on the photocatalytic performance of ZnIn2S4 for hydrogen generation under visible light, International Journal of Hydrogen Energy (2014), http://dx.doi.org/10.1016/ j.ijhydene.2014.12.025