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Efficient visible light driven photocatalytic hydrogen production from water using attapulgite clay sensitized by CdS nanoparticles
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2013 Nanotechnology 24 505401 (http://iopscience.iop.org/0957-4484/24/50/505401) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 131.155.2.68 This content was downloaded on 03/06/2014 at 14:27
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IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 24 (2013) 505401 (7pp)
doi:10.1088/0957-4484/24/50/505401
Efficient visible light driven photocatalytic hydrogen production from water using attapulgite clay sensitized by CdS nanoparticles Jian Zhang, Ru He and Xiaoheng Liu Key Laboratory for Soft Chemistry and Functional Materials of Ministry of Education, and Key Laboratory of Jiangsu Province for Chemical Pollution Control and Resources Reuse, Nanjing University of Science and Technology, Nanjing 210094, People’s Republic of China E-mail: [email protected]
Received 30 June 2013, in final form 17 October 2013 Published 27 November 2013 Online at stacks.iop.org/Nano/24/505401 Abstract Hydrogen production through water splitting using photocatalysts with solar energy can produce clean fuel from renewable resources. In this study, CdS nanoparticle sensitized attapulgite (ATP) nanocomposites were successfully prepared by a facile approach. Under visible-light irradiation, the as-prepared photocatalysts were used for photocatalytic water splitting for hydrogen evolution from aqueous solutions containing Na2 SO3 and Na2 S as sacrificial reagents even without the noble metals. Photocatalytic hydrogen production activity is ascribed to the presence of CdS nanocrystals that alter the energy levels of the conduction band and valence band in the coupled semiconductor system. Furthermore, the theoretical calculations show that the natural Fe doping (two ATP cells sharing one Fe atom) can promote the photocatalytic process. S Online supplementary data available from stacks.iop.org/Nano/24/505401/mmedia (Some figures may appear in colour only in the online journal)
1. Introduction
of novel catalysts to visible light and increasing their photocatalytic efficiencies are important for future research goals. So far, as an effective approach using visible light, semiconductor nanocrystals such as CdS [11], CdTe [12], PbS [13], InP [14], and Ag2 S [15] have been used as sensitizers of semiconductor photocatalysts. Among these sensitizers, CdS [16–18] is most promising due to its narrow band gap and sufficiently negative conduction band edge. Recently, intensive studies have been performed on CdS nanocrystal sensitized photocatalysts. For example, Yu et al [19] reported significant improvement in the efficiency of photocatalytic hydrogen production by incorporating CdS quantum dots onto the TiO2 nanosheets with exposed (001) facets. Kang et al [20] reported that CdS nanocrystal sensitization improved the visible spectrum absorption of NiO
Photocatalytic water splitting over semiconductor nanomaterials has been regarded as one of the most promising means of hydrogen production from renewable resources [1]. Alternatively, photocatalytic water splitting using metal oxides such as TiO2 [2, 3], ZnO [4], Fe2 O3 [5] and WO3 [6] offers a promising route for non-toxic, abundant and stable production of hydrogen. Among various semiconductors, TiO2 is currently one of the most promising photocatalysts because of its biological and chemical inertness, cost effectiveness, and long-term stability against photochemical and chemical corrosion [7–10]. However, anatase TiO2 is only effective under ultraviolet irradiation (λ < 387 nm) due to its large band gap (3.2 eV). Usually, sunlight contains about 4% ultraviolet light only. Therefore, extending the photo-response 0957-4484/13/505401+07$33.00
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c 2013 IOP Publishing Ltd Printed in the UK & the USA
Nanotechnology 24 (2013) 505401
J Zhang et al
nanoparticle photoelectrodes. Hou’s group [21] fabricated a core–shell heterostructure CdS@TaON nanocomposite with enhanced visible-light water splitting efficiency. Indeed, so far there have been many reports related to different approaches for the water splitting, but to our knowledge there have been hardly any investigations on natural products as splitting agents. As a mass of natural silicate clay mineral, attapulgite (ATP) has a wide variety of industrial applications such as absorbent, catalyst carrier, densifying agent, adhesive, and food additive. The ideal composition of ATP is Mg5 Si8 O20 (OH)2 (OH2 )4 [22]. However, according to previous reports from different countries in the world [22], it is very important for our research that actual composition varies because of partial replacement of magnesium by iron elements. Actually, metal oxide-based minerals naturally contain doped transition metal ions isomorphically substituted into the structure that can alter the structural as well as electronic properties [23]. In our research, for ATP, we dope with Fe3+ , as it is isovalent with Mg2+ and forms an –O–Fe–O– structure that is isostructural with the –O–Mg–O– structure. Based on the above analysis, effects of natural Fe-doped ATP were analyzed by density functional theory (DFT) calculation and the astonishing results presented in this study demonstrate high performance for the as-prepared photocatalyst in the visible-light range for water splitting. Herein, we use chemical impregnation to combine CdS nanoparticles with ATP (CdS–ATP nanocomposites) and investigate the effect of the CdS content on the rate of photocatalytic hydrogen evolution using Na2 S and Na2 SO3 as sacrificial agents under visible light. It is shown that the activity of the CdS–ATP nanocomposite is significantly enhanced even without using any noble metals as co-catalysts. In addition, to obtain further insight into the high photocatalytic activity of the photocatalyst, the transient photocurrent responses of CdS–ATP electrodes were recorded for several on–off cycles of visible-light irradiation. Under visible-light irradiation, these CdS–ATP photoanodes are highly stable in photoelectrochemical conversion and thus can serve as potential candidates for ATP-based materials in a variety of solar energy driven applications.
filtered, washed with water and ethanol successively, and then held under vacuum at 60 ◦ C for 8 h. 2.2. Characterization Transmission electron microscopy (TEM) imaging and energy dispersive x-ray (EDX, EDAX Genesis 2000) spectroscopy were performed on a JEOL JEM-2100 microscope operating at 200 kV, by depositing a drop of sample dispersion onto 300 mesh Cu grids coated with a carbon layer. The obtained samples were characterized by XRD on a Bruker D8 Advance x-ray powder diffractometer with Cu Kα radiation (λ = ˚ UV–vis absorption spectra were obtained using 1.5418 A). a Ruili UV-1201 spectrophotometer. Valence band x-ray photoelectron spectra (XPS) were acquired on an ESCALAB 250 with Al Kα (hυ = 1486.6 eV) as the excitation source. The photocurrent was measured on an electrochemical system (Chenhua CHI-660B). The visible light (λ > 420 nm) was obtained using a 300 W Xe lamp with a 420 nm cutoff filter to completely remove any radiation below 420 nm. 2.3. Photocatalytic tests Typically, reactions were carried out in a closed gascirculation system. A 300 W xenon lamp (XL-300, Yirda) was used as the light source, and visible-light irradiation was realized by attaching a 420 nm cutoff filter. In a typical photocatalytic experiment, 0.1 g catalyst was dispersed with a constant stirring in 150 ml of mixed aqueous solution containing 0.35 M Na2 S and 0.25 M Na2 SO3 . The system was evacuated several times prior to irradiation. The evolved hydrogen was analyzed by online gas chromatography (GC-1690, Jiedao, TCD, Ar carrier). Quantum efficiency (QE) was calculated under identical photocatalytic reaction conditions with irradiation light at λ = 420 nm using a 300 W xenon lamp and combined band-pass and cutoff filters (the FWHM of the filter is 5 nm). A model 1916-R Newport power meter was used for the intensity measurement of light. The QE was calculated according to number of reacted electrons × 100 number of incident photons number of evolved H2 molecules × 2 = number of incident photons × 100.
QE (%) =
2. Experimental section 2.1. Preparation of CdS–ATP nanocomposite
(1)
The TON was calculated according to
All the reagents used in this work were of AR grade from the Shanghai Chemical Factory of China and were used without further purification. ATP was obtained from Xuyi of Jiangsu Province in China. In a typical synthesis of the CdS–ATP nanocomposite, an aqueous solution of Na2 S·9H2 O and a pre-determined amount of ATP was added slowly to Cd(Ac)2 ·2H2 O solution under vigorous stirring. The weight ratios of CdS to ATP were fixed at the following values: 0.5 wt%, 1.0 wt%, 3.0 wt%, and 5.0 wt%, and the obtained samples were labeled as Cd0.5, Cd1, Cd3, and Cd5, respectively. The resulting mixture, which was yellow in color, was stirred for 2 h. After this, the yellow solid was
number of product molecules number of catalyst molecules number of evolved H2 molecules = . number of catalyst molecules
TON =
(2)
2.4. Photoelectrochemical measurements The working electrodes were prepared as follows: 5 mg of photocatalyst was suspended in 1 ml ethanol under ultrasonic treatment to make a slurry. The slurry was then coated onto 2
Nanotechnology 24 (2013) 505401
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a 2.0 cm × 1.0 cm F-doped SnO2 -coated glass electrode by the doctor blade method. Next, the resulting electrodes were dried in an oven and calcined at 250 ◦ C for 30 min in a N2 gas flow. The photoelectrochemical properties were investigated in a conventional three-electrode cell using an electrochemical analyzer (CHI610B, Shanghai Chenhua, China) and the prepared samples as the working electrodes, a Pt wire as the counter-electrode, and Ag/AgCl (saturated KCl) as a reference electrode. A 300 W xenon lamp (XL-300, Yirda) was used as the light source, and visible-light irradiation was realized by attaching a 420 nm cutoff filter. 0.1 M Na2 S and 0.02 M Na2 SO3 mixed aqueous solution was used as the electrolyte. 2.5. Theoretical calculation details
Figure 1. XRD patterns of the as-prepared CdS–ATP composites and bare ATP.
The electronic structure calculations were performed by using the Vienna ab initio simulation package (VASP). In all calculations, the ion–electron interaction is described with the projected augmented wave (PAW) [24, 25] method, the exchange–correlation functional is PW91 [26] based on the generalized gradient approximation (GGA), and the convergence threshold is set to be 10−5 eV in energy and ˚ −1 in force. 10−3 eV A
the EDX spectra, as shown in figure 2(d), which confirms that the ATP is decorated by CdS nanocrystals. The UV–vis absorption spectra of the as-prepared CdS–ATP nanocomposite samples are shown in figure 3. Compared to the UV–vis absorption spectrum of bare ATP, a red-shift for the Cd0.5–Cd5 samples indicates the presence of a strong quantum confinement effect in comparison to CdS nanoparticles, which also influences their redox potentials in relation to the normal hydrogen electrode potential (NHE) [21]. The appropriate modification in the energy levels of the conduction band edge and valence band edge is evidence that CdS nanocrystals are acting as sensitizers [31], which makes CdS–ATP nanocomposites active under visible-light irradiation. The inset of figure 3 shows a Tauc equation plot for bare ATP sample (figure 3, curve of ATP). The best fit was obtained for a plot of (αhν)2 versus photon energy (hν), where α is the absorption coefficient. The extrapolation of the linear regions in the Tauc plot suggests one-regime optical absorption by ATP (marked with a dotted line), where the associated direct optical band gap is about 3.75 eV.
3. Results and discussion 3.1. Characterization of CdS–ATP nanocomposites The crystal structures of bare ATP and CdS–ATP nanocomposites were analyzed by XRD patterns as shown in figure 1. We cannot find structure information of ATP in the Bruker XRD database, but it is quite similar to a previous report [27]. It is evident that there is almost no change in the crystal structure of ATP after the coating of CdS nanoparticles (samples Cd0.5, Cd1, Cd3, and Cd5). Moreover, no evident shift in the peak positions is observed in each of the as-prepared CdS–ATP samples, suggesting that the deposited CdS nanocrystals are not incorporated into the lattice of ATP. Thus, it was clear that the coating of no more than 5 wt% CdS nanocrystals had a negligible effect on the crystal structure of ATP due to the small amount of CdS with low crystallinity and relatively strong diffraction intensity of ATP. TEM was used to characterize the morphology and microstructures of ATP in the absence and presence of CdS nanoparticles. The bare ATP exhibits the bundles of nanorod structures (figure 2(a)) with average length of 100–1000 nm, and the nanorods of ATP have smooth surfaces with average diameter of about 20–40 nm (figure 2(b)). As compared with the bare ATP, no change was observed on the morphology of the CdS–ATP (Cd3) sample except for the dispersion of CdS nanoparticles with an average diameter of 8 nm throughout the surface of the ATP (figure 2(c)), attributed to high surface area [22, 28] as well as strong abilities to absorb nanoparticles [29] and molecules [30]. The elemental composition of the Cd3 sample was analyzed by the EDX spectra. For the red-squared areas in figure 2(c), peaks associated with O, Fe, Mg, Al, Si, S, and Cd were observed in
3.2. Photocatalytic hydrogen production Photocatalytic hydrogen evolution over different CdS–ATP samples was evaluated in an aqueous solution containing Na2 SO3 and Na2 S as sacrificial reagents under the simulated visible-light irradiation (>420 nm). Control experiments indicated that no hydrogen evolution is observed in the absence of either light irradiation or photocatalyst. Figure 4 shows the hydrogen evolution rate over the samples with different amounts of CdS nanocrystal loading under visiblelight irradiation. It can be seen that CdS has exhibited a significant influence on the photocatalytic activity of CdS–ATP nanocomposites: first, no hydrogen was detected when bare ATP was used as the photocatalyst, suggesting that ATP is not active for photocatalytic hydrogen evolution under visible-light irradiation because its band gap is too large (3.75 eV, figure 3) to absorb the visible light. Second, impregnation with CdS nanocrystals improves the evolution 3
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Figure 2. (a) TEM and (b) low-magnification TEM images of ATP. (c) Low-magnification TEM image of Cd3 and (d) EDX elemental analysis of the selected area shown by the red square in (c).
Figure 3. UV–vis absorption spectra of the as-prepared CdS–ATP nanocomposites using different amounts of CdS nanocrystal loading. Inset: Tauc equation plot for ATP.
Figure 4. Comparison of the visible-light photocatalytic activity of CdS–ATP nanocomposites using various amounts of CdS nanocrystal loading.
rate up to a loading of 3 wt% (sample Cd3), where the highest hydrogen generation rate was found at 32 µmol h−1 (which corresponds to a quantum efficiency (QE) of 8.2% at 420 nm), and a turnover number of 79 was obtained while the catalyst was still active. This exceeds the hydrogen evolution rate of a pure CdS nanocrystal sample of 9 µmol h−1 under the same irradiating conditions, which is similar to a large number of previous investigations [32]. Third, when the CdS content was increased beyond 3 wt% (sample
Cd5, figure S1 available at stacks.iop.org/Nano/24/505401/ mmedia), the photocatalytic activity dramatically decreased, with a hydrogen production rate of 22 µmol h−1 . As a consequence, the suitable CdS content causes good dispersion on the ATP surface, which favors the transfer and separation of the charge carriers. A similar observation has been reported for CdS/TiO2 composites [33]. Besides activity, the stability of a photocatalyst is important for its application. To demonstrate the stability of 4
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Figure 6. Transient photocurrent responses of the ATP (a), Cd0.5 (b), Cd1 (c), Cd3 (d), and Cd5 (e) samples in 0.1 M Na2 S + 0.02 M Na2 SO3 mixed aqueous solution under visible-light irradiation at 0.4 V versus Ag/AgCl.
Cd3, and Cd5 is 0.02, 0.25, 0.28, 0.34, and 0.33 mA cm−2 , respectively. The CdS–ATP nanocomposites exhibited a higher photoinduced current due to the enhancement in light absorption in the visible-light region and additionally due to the performance improvements of charge separation and charge injection [34]. The Cd3 photocatalyst shows the highest photocurrent intensity. However, on further increasing the CdS concentration to 5 wt% (Cd5), the photocurrent decreases due to some CdS particle agglomeration (figure S1 available at stacks.iop.org/Nano/24/505401/mmedia). Figure 5. (a) Cycling measurements of hydrogen generation through direct photocatalytic water splitting with CdS–ATP (Cd3) under visible light and (b) XRD patterns of Cd3 (before and after the standard cycling measurement).
3.3. Discussion of the photocatalytic mechanism The ideal composition of ATP is Mg5 Si8 O20 (OH)2 (OH2 )4 [22] (figure 7(a)). However, according to previous reports from other groups [22, 35, 36], research [22, 26] indicated the ATP contains 3% Fe element, which suggested a structure of two ATP crystal cells sharing one Fe atom. Figure 2(d) confirms remarkably the existence of Fe, and about 3% content is estimated within the error of EDX. Thus, by using VASP [37], we optimized the Fe-doped structures: Mg4.5 Fe0.5 Si8 O20 (OH)2 (OH2 )4 . By comparison of three different structures in which Fe doped in different sites, the most stable structure with lowest total energy is determined, as shown in figure 7(b). Based on the above optimized structures, we conducted DFT calculations as shown in figures 7(c) and (d). There is a wide energy gap of about 5 eV and a high valence band level in ideal ATP (figure 7(c)). However, after doping with Fe, the ATP levels are lowered to 1.6 eV, as shown in figure 7(d), which is 2.15 eV smaller than the experimental value (3.75 eV) as a result of the well known band gap underestimation within the framework of standard DFT. The wide valence band was also measured by the XPS valence band spectrum of ATP (figure 8(a)). The valence band maximum of ATP was determined to be 2.6 eV as shown in figure 8(a). This is 0.6 eV bigger than that of anatase TiO2 [38, 39], suggesting that ATP has a lower valence band maximum than anatase TiO2 by about 0.6 eV relative to an
our composite catalysts, we cycled the hydrogen evolution for sample Cd3 (figure S2 available at stacks.iop.org/Nano/24/ 505401/mmedia). Figure 5(a) displays the hydrogen evolution curve in a cycling photocatalytic run. Measurement was conducted for four consecutive days and each day the sample was irradiated for 6 h and then stored in darkness overnight before testing the next day. Continuous hydrogen evolution with no noticeable degradation of the Cd3 sample was clearly observed from the beginning of the reaction. In addition, XRD analysis of the nanocomposite before and after the experiment (figure 5(b)) also illustrates that the crystal structure has not changed and no photocorrosion is detected in the running period. The high stability could be attributed to the better coupling between the structures and electronic configurations of CdS and ATP together, and the formation of junctions between CdS and ATP, which improved the charge separation. The transient photocurrent responses of as-synthesized CdS–ATP samples as well as the bare ATP were recorded for several on–off cycles of visible-light irradiation. Figure 6 shows a comparison of photocurrent variation curves for a series of CdS–ATP nanocomposites. From figure 6, we can observe that the photocurrent of bare ATP, Cd0.5, Cd1, 5
Nanotechnology 24 (2013) 505401
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Figure 7. Computational model of ATP unit crystal cell on the (001) plane (a) and the best position of Fe doping (b). Atoms: red—O, green—Mg, gray—H, yellow—Si, purple—Fe. Calculated electronic band structures of the ideal ATP (c) and natural Fe-doping ATP (d).
Figure 8. Valence band XPS spectra of the ATP (a) and electronic potential diagram for anatase TiO2 and ATP (b).
NHE. Because the band gap of the ATP is 3.75 eV from the optical absorption spectrum (figure 3), the electronic potentials of the ATP could be determined (figure 8(b)). Obviously, the conduction band minimum of ATP is more negative than the redox potentials for H+ /H2 . Besides, the conduction band potential and valence band potential of CdS were determined to be −0.87 and 1.5 eV [40], as shown in figure 9. As a result, the conduction band minimum of CdS is more negative than that of ATP, and under visible-light irradiation photoinduced electrons from the CdS can easily transfer to ATP.
On the basis of the above results, a proposed mechanism for the photocatalytic activity of the CdS–ATP nanocomposite is shown schematically in figure 9. Under visible-light irradiation, CdS nanocrystals are activated. The photogenerated electrons transfer from the conduction band of CdS into ATP and accumulate at the lower-lever conduction band of ATP to generate hydrogen, while holes accumulate at the valence band of CdS. Hence, the CdS–ATP nanocomposites can increase photoactivity as a result of the efficient separation of the photogenerated electron–hole pairs. 6
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[5] Mayer M T, Du C and Wang D 2012 J. Am. Chem. Soc. 134 12406–9 [6] Jiao Z, Wang J, Ke L, Sun X W and Demir H V 2011 ACS Appl. Mater. Interfaces 3 229–36 [7] Yu J, Qi L and Jaroniec M 2010 J. Phys. Chem. C 114 13118–25 [8] Chowdhury P, Malekshoar G, Ray M B, Zhu J and Ray A K 2013 Indust. Eng. Chem. Res. 52 5023–9 [9] Hwang Y J, Boukai A and Yang P 2009 Nano Lett. 9 410–5 [10] Lai Y, Gong J and Lin C 2012 Int. J. Hydrog. Energy 37 6438–336 [11] Wang H, Bai Y, Zhang H, Zhang Z, Li J and Guo L 2010 J. Phys. Chem. C 114 16451–5 [12] Chen H M, Chen C K, Chang Y, Tsai C, Liu R, Hu S, Chang W and Chen K 2010 Angew. Chem. Int. Edn 49 5966–9 [13] Kang Q, Liu S, Yang L, Cai Q and Grimes C A 2011 ACS Appl. Mater. Interfaces 3 746–9 [14] Charron G, Stuchinskaya T, Edwards D R, Russell D A and Nann T 2012 J. Phys. Chem. C 116 9334–42 [15] Wu J, Chang R, Chen D and Wu C 2012 Nanoscale 4 1368–72 [16] Shin K, Yoo J and Park J H 2013 J. Power Sources 225 263–8 [17] Park H, Chio W and Hoffmann M R 2008 J. Mater. Chem. 18 2379–85 [18] Kim H N, Kim T W, Kim I Y and Hwang S 2011 Adv. Funct. Mater. 21 3111–8 [19] Qi L, Yu J and Jaroniec M 2011 Phys. Chem. Chem. Phys. 13 8915–23 [20] Kang S H, Zhu K, Neale N R and Frank A J 2011 Chem. Commun. 47 10419–21 [21] Hou J, Wang Z, Kan W, Jiao S, Zhu H and Kumar R V 2012 J. Mater. Chem. 22 7291–9 [22] Cao E, Bryant R and Williams D J A 1996 J. Colloid Interface Sci. 179 143–50 [23] Baltrusaitis J, Hatch C and Orlando R 2012 J. Phys. Chem. C 116 18847–56 [24] Bl¨ochl P E 1994 Phys. Rev. B 50 17953–79 [25] Kresse G and Joubert D 1999 Phys. Rev. B 59 1758–75 [26] Perdew J P and Wang Y 1992 Phys. Rev. B 45 13244–9 [27] Chen L F, Liang H W, Lu Y, Cui C H and Yu S H 2011 Langmuir 27 8998–9004 [28] Haden W L and Schwint J I A 1967 Indust. Eng. Chem. 9 58–69 [29] Wang X W, Liu G, Lu G Q and Cheng H M 2010 Int. J. Hydrog. Energy 35 8199–205 [30] Jin X, Li Y, Yu C, Ma Y, Yang L and Hu H 2011 J. Hazard. Mater. 198 247–56 [31] Wang X, Liu G, Chen Z, Li F, Wang L, Lu G Q and Cheng H 2009 Chem. Commun. 45 3452–4 [32] Zong X, Yan H, Wu G, Ma G, Wen F, Wang L and Li C 2008 J. Am. Chem. Soc. 130 7176–7 [33] Sun W, Yu Y, Pan H, Gao Q C and Peng L 2008 J. Am. Chem. Soc. 130 1124–5 [34] Banerjee S, Mohapatra S K, Das P P and Misra M 2008 Chem. Mater. 20 6784–91 [35] Zhao D, Zhou J and Liu N 2006 Appl. Clay Sci. 33 161–70 [36] Neaman A and Singer A 2004 Appl. Clay Sci. 25 121–4 [37] Kresse G and Hafner J 1993 Phys. Rev. B 47 558–61 [38] Liu G, Wang L, Sun C, Yan X, Wang X, Chen Z, Smith S C, Cheng H and Lu G Q 2009 Chem. Mater. 21 1266–74 [39] Liu G, Niu P, Yin L and Cheng H 2012 J. Am. Chem. Soc. 134 9070–3 [40] Osterloh F E 2008 Chem. Mater. 20 35–54
Figure 9. Schematic illustration for the charge transfer and separation in CdS nanocrystals sensitizing ATP under visible light.
4. Conclusion In summary, a high efficiency of photocatalytic hydrogen production from water splitting under visible-light irradiation has been achieved over a CdS–ATP photocatalyst synthesized by a facile method. The 3 wt% CdS nanocrystal sensitized ATP composites (Cd3) showed a high hydrogen production rate of 32 µmol h−1 even without the deposition of noble metal co-catalyst. The red-shift in the absorption spectrum confirms the quantum size effect of CdS; the presence of CdS nanocrystals alters the energy levels of the conduction and valence bands in the coupled system, which favors the electron transfer and enhances photoactivity. Moreover, the theoretical calculations show that natural Fe doping (two ATP cells sharing one Fe atom) can also promote the photocatalytic process. Although the absolute activity of CdS–ATP nanocomposites is still not very high, this work opens up new possibilities for the development of highly efficient ATP-based photocatalysts that utilize visible light.
Acknowledgments This project is supported financially by the Natural Science Foundation of China (NSFC, No 51272107), and the Natural Science Foundation of Jiangsu Province, China (No BK2011024).
References [1] Fujishima A and Honda K 1972 Nature 238 37–8 [2] Sharma R, Das P P, Misra M, Mahajan V, Bock J P, Trigwell S, Biris A S and Mazumder M 2009 Nanotechnology 20 075704 [3] Zhang Z, Zhang L, Hedhili M N, Zhang H and Wang P 2013 Nano Lett. 13 14–20 [4] Wei Y, Ke L, Kong J, Liu H, Jiao Z, Lu X, Du H and Sun X W 2012 Nanotechnology 23 235401
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Efficient visible light driven photocatalytic hydrogen production from water using attapulgite clay sensitized by CdS nanoparticles
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2013 Nanotechnology 24 505401 (http://iopscience.iop.org/0957-4484/24/50/505401) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 131.155.2.68 This content was downloaded on 03/06/2014 at 14:27
Please note that terms and conditions apply.
IOP PUBLISHING
NANOTECHNOLOGY
Nanotechnology 24 (2013) 505401 (7pp)
doi:10.1088/0957-4484/24/50/505401
Efficient visible light driven photocatalytic hydrogen production from water using attapulgite clay sensitized by CdS nanoparticles Jian Zhang, Ru He and Xiaoheng Liu Key Laboratory for Soft Chemistry and Functional Materials of Ministry of Education, and Key Laboratory of Jiangsu Province for Chemical Pollution Control and Resources Reuse, Nanjing University of Science and Technology, Nanjing 210094, People’s Republic of China E-mail: [email protected]
Received 30 June 2013, in final form 17 October 2013 Published 27 November 2013 Online at stacks.iop.org/Nano/24/505401 Abstract Hydrogen production through water splitting using photocatalysts with solar energy can produce clean fuel from renewable resources. In this study, CdS nanoparticle sensitized attapulgite (ATP) nanocomposites were successfully prepared by a facile approach. Under visible-light irradiation, the as-prepared photocatalysts were used for photocatalytic water splitting for hydrogen evolution from aqueous solutions containing Na2 SO3 and Na2 S as sacrificial reagents even without the noble metals. Photocatalytic hydrogen production activity is ascribed to the presence of CdS nanocrystals that alter the energy levels of the conduction band and valence band in the coupled semiconductor system. Furthermore, the theoretical calculations show that the natural Fe doping (two ATP cells sharing one Fe atom) can promote the photocatalytic process. S Online supplementary data available from stacks.iop.org/Nano/24/505401/mmedia (Some figures may appear in colour only in the online journal)
1. Introduction
of novel catalysts to visible light and increasing their photocatalytic efficiencies are important for future research goals. So far, as an effective approach using visible light, semiconductor nanocrystals such as CdS [11], CdTe [12], PbS [13], InP [14], and Ag2 S [15] have been used as sensitizers of semiconductor photocatalysts. Among these sensitizers, CdS [16–18] is most promising due to its narrow band gap and sufficiently negative conduction band edge. Recently, intensive studies have been performed on CdS nanocrystal sensitized photocatalysts. For example, Yu et al [19] reported significant improvement in the efficiency of photocatalytic hydrogen production by incorporating CdS quantum dots onto the TiO2 nanosheets with exposed (001) facets. Kang et al [20] reported that CdS nanocrystal sensitization improved the visible spectrum absorption of NiO
Photocatalytic water splitting over semiconductor nanomaterials has been regarded as one of the most promising means of hydrogen production from renewable resources [1]. Alternatively, photocatalytic water splitting using metal oxides such as TiO2 [2, 3], ZnO [4], Fe2 O3 [5] and WO3 [6] offers a promising route for non-toxic, abundant and stable production of hydrogen. Among various semiconductors, TiO2 is currently one of the most promising photocatalysts because of its biological and chemical inertness, cost effectiveness, and long-term stability against photochemical and chemical corrosion [7–10]. However, anatase TiO2 is only effective under ultraviolet irradiation (λ < 387 nm) due to its large band gap (3.2 eV). Usually, sunlight contains about 4% ultraviolet light only. Therefore, extending the photo-response 0957-4484/13/505401+07$33.00
1
c 2013 IOP Publishing Ltd Printed in the UK & the USA
Nanotechnology 24 (2013) 505401
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nanoparticle photoelectrodes. Hou’s group [21] fabricated a core–shell heterostructure CdS@TaON nanocomposite with enhanced visible-light water splitting efficiency. Indeed, so far there have been many reports related to different approaches for the water splitting, but to our knowledge there have been hardly any investigations on natural products as splitting agents. As a mass of natural silicate clay mineral, attapulgite (ATP) has a wide variety of industrial applications such as absorbent, catalyst carrier, densifying agent, adhesive, and food additive. The ideal composition of ATP is Mg5 Si8 O20 (OH)2 (OH2 )4 [22]. However, according to previous reports from different countries in the world [22], it is very important for our research that actual composition varies because of partial replacement of magnesium by iron elements. Actually, metal oxide-based minerals naturally contain doped transition metal ions isomorphically substituted into the structure that can alter the structural as well as electronic properties [23]. In our research, for ATP, we dope with Fe3+ , as it is isovalent with Mg2+ and forms an –O–Fe–O– structure that is isostructural with the –O–Mg–O– structure. Based on the above analysis, effects of natural Fe-doped ATP were analyzed by density functional theory (DFT) calculation and the astonishing results presented in this study demonstrate high performance for the as-prepared photocatalyst in the visible-light range for water splitting. Herein, we use chemical impregnation to combine CdS nanoparticles with ATP (CdS–ATP nanocomposites) and investigate the effect of the CdS content on the rate of photocatalytic hydrogen evolution using Na2 S and Na2 SO3 as sacrificial agents under visible light. It is shown that the activity of the CdS–ATP nanocomposite is significantly enhanced even without using any noble metals as co-catalysts. In addition, to obtain further insight into the high photocatalytic activity of the photocatalyst, the transient photocurrent responses of CdS–ATP electrodes were recorded for several on–off cycles of visible-light irradiation. Under visible-light irradiation, these CdS–ATP photoanodes are highly stable in photoelectrochemical conversion and thus can serve as potential candidates for ATP-based materials in a variety of solar energy driven applications.
filtered, washed with water and ethanol successively, and then held under vacuum at 60 ◦ C for 8 h. 2.2. Characterization Transmission electron microscopy (TEM) imaging and energy dispersive x-ray (EDX, EDAX Genesis 2000) spectroscopy were performed on a JEOL JEM-2100 microscope operating at 200 kV, by depositing a drop of sample dispersion onto 300 mesh Cu grids coated with a carbon layer. The obtained samples were characterized by XRD on a Bruker D8 Advance x-ray powder diffractometer with Cu Kα radiation (λ = ˚ UV–vis absorption spectra were obtained using 1.5418 A). a Ruili UV-1201 spectrophotometer. Valence band x-ray photoelectron spectra (XPS) were acquired on an ESCALAB 250 with Al Kα (hυ = 1486.6 eV) as the excitation source. The photocurrent was measured on an electrochemical system (Chenhua CHI-660B). The visible light (λ > 420 nm) was obtained using a 300 W Xe lamp with a 420 nm cutoff filter to completely remove any radiation below 420 nm. 2.3. Photocatalytic tests Typically, reactions were carried out in a closed gascirculation system. A 300 W xenon lamp (XL-300, Yirda) was used as the light source, and visible-light irradiation was realized by attaching a 420 nm cutoff filter. In a typical photocatalytic experiment, 0.1 g catalyst was dispersed with a constant stirring in 150 ml of mixed aqueous solution containing 0.35 M Na2 S and 0.25 M Na2 SO3 . The system was evacuated several times prior to irradiation. The evolved hydrogen was analyzed by online gas chromatography (GC-1690, Jiedao, TCD, Ar carrier). Quantum efficiency (QE) was calculated under identical photocatalytic reaction conditions with irradiation light at λ = 420 nm using a 300 W xenon lamp and combined band-pass and cutoff filters (the FWHM of the filter is 5 nm). A model 1916-R Newport power meter was used for the intensity measurement of light. The QE was calculated according to number of reacted electrons × 100 number of incident photons number of evolved H2 molecules × 2 = number of incident photons × 100.
QE (%) =
2. Experimental section 2.1. Preparation of CdS–ATP nanocomposite
(1)
The TON was calculated according to
All the reagents used in this work were of AR grade from the Shanghai Chemical Factory of China and were used without further purification. ATP was obtained from Xuyi of Jiangsu Province in China. In a typical synthesis of the CdS–ATP nanocomposite, an aqueous solution of Na2 S·9H2 O and a pre-determined amount of ATP was added slowly to Cd(Ac)2 ·2H2 O solution under vigorous stirring. The weight ratios of CdS to ATP were fixed at the following values: 0.5 wt%, 1.0 wt%, 3.0 wt%, and 5.0 wt%, and the obtained samples were labeled as Cd0.5, Cd1, Cd3, and Cd5, respectively. The resulting mixture, which was yellow in color, was stirred for 2 h. After this, the yellow solid was
number of product molecules number of catalyst molecules number of evolved H2 molecules = . number of catalyst molecules
TON =
(2)
2.4. Photoelectrochemical measurements The working electrodes were prepared as follows: 5 mg of photocatalyst was suspended in 1 ml ethanol under ultrasonic treatment to make a slurry. The slurry was then coated onto 2
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a 2.0 cm × 1.0 cm F-doped SnO2 -coated glass electrode by the doctor blade method. Next, the resulting electrodes were dried in an oven and calcined at 250 ◦ C for 30 min in a N2 gas flow. The photoelectrochemical properties were investigated in a conventional three-electrode cell using an electrochemical analyzer (CHI610B, Shanghai Chenhua, China) and the prepared samples as the working electrodes, a Pt wire as the counter-electrode, and Ag/AgCl (saturated KCl) as a reference electrode. A 300 W xenon lamp (XL-300, Yirda) was used as the light source, and visible-light irradiation was realized by attaching a 420 nm cutoff filter. 0.1 M Na2 S and 0.02 M Na2 SO3 mixed aqueous solution was used as the electrolyte. 2.5. Theoretical calculation details
Figure 1. XRD patterns of the as-prepared CdS–ATP composites and bare ATP.
The electronic structure calculations were performed by using the Vienna ab initio simulation package (VASP). In all calculations, the ion–electron interaction is described with the projected augmented wave (PAW) [24, 25] method, the exchange–correlation functional is PW91 [26] based on the generalized gradient approximation (GGA), and the convergence threshold is set to be 10−5 eV in energy and ˚ −1 in force. 10−3 eV A
the EDX spectra, as shown in figure 2(d), which confirms that the ATP is decorated by CdS nanocrystals. The UV–vis absorption spectra of the as-prepared CdS–ATP nanocomposite samples are shown in figure 3. Compared to the UV–vis absorption spectrum of bare ATP, a red-shift for the Cd0.5–Cd5 samples indicates the presence of a strong quantum confinement effect in comparison to CdS nanoparticles, which also influences their redox potentials in relation to the normal hydrogen electrode potential (NHE) [21]. The appropriate modification in the energy levels of the conduction band edge and valence band edge is evidence that CdS nanocrystals are acting as sensitizers [31], which makes CdS–ATP nanocomposites active under visible-light irradiation. The inset of figure 3 shows a Tauc equation plot for bare ATP sample (figure 3, curve of ATP). The best fit was obtained for a plot of (αhν)2 versus photon energy (hν), where α is the absorption coefficient. The extrapolation of the linear regions in the Tauc plot suggests one-regime optical absorption by ATP (marked with a dotted line), where the associated direct optical band gap is about 3.75 eV.
3. Results and discussion 3.1. Characterization of CdS–ATP nanocomposites The crystal structures of bare ATP and CdS–ATP nanocomposites were analyzed by XRD patterns as shown in figure 1. We cannot find structure information of ATP in the Bruker XRD database, but it is quite similar to a previous report [27]. It is evident that there is almost no change in the crystal structure of ATP after the coating of CdS nanoparticles (samples Cd0.5, Cd1, Cd3, and Cd5). Moreover, no evident shift in the peak positions is observed in each of the as-prepared CdS–ATP samples, suggesting that the deposited CdS nanocrystals are not incorporated into the lattice of ATP. Thus, it was clear that the coating of no more than 5 wt% CdS nanocrystals had a negligible effect on the crystal structure of ATP due to the small amount of CdS with low crystallinity and relatively strong diffraction intensity of ATP. TEM was used to characterize the morphology and microstructures of ATP in the absence and presence of CdS nanoparticles. The bare ATP exhibits the bundles of nanorod structures (figure 2(a)) with average length of 100–1000 nm, and the nanorods of ATP have smooth surfaces with average diameter of about 20–40 nm (figure 2(b)). As compared with the bare ATP, no change was observed on the morphology of the CdS–ATP (Cd3) sample except for the dispersion of CdS nanoparticles with an average diameter of 8 nm throughout the surface of the ATP (figure 2(c)), attributed to high surface area [22, 28] as well as strong abilities to absorb nanoparticles [29] and molecules [30]. The elemental composition of the Cd3 sample was analyzed by the EDX spectra. For the red-squared areas in figure 2(c), peaks associated with O, Fe, Mg, Al, Si, S, and Cd were observed in
3.2. Photocatalytic hydrogen production Photocatalytic hydrogen evolution over different CdS–ATP samples was evaluated in an aqueous solution containing Na2 SO3 and Na2 S as sacrificial reagents under the simulated visible-light irradiation (>420 nm). Control experiments indicated that no hydrogen evolution is observed in the absence of either light irradiation or photocatalyst. Figure 4 shows the hydrogen evolution rate over the samples with different amounts of CdS nanocrystal loading under visiblelight irradiation. It can be seen that CdS has exhibited a significant influence on the photocatalytic activity of CdS–ATP nanocomposites: first, no hydrogen was detected when bare ATP was used as the photocatalyst, suggesting that ATP is not active for photocatalytic hydrogen evolution under visible-light irradiation because its band gap is too large (3.75 eV, figure 3) to absorb the visible light. Second, impregnation with CdS nanocrystals improves the evolution 3
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Figure 2. (a) TEM and (b) low-magnification TEM images of ATP. (c) Low-magnification TEM image of Cd3 and (d) EDX elemental analysis of the selected area shown by the red square in (c).
Figure 3. UV–vis absorption spectra of the as-prepared CdS–ATP nanocomposites using different amounts of CdS nanocrystal loading. Inset: Tauc equation plot for ATP.
Figure 4. Comparison of the visible-light photocatalytic activity of CdS–ATP nanocomposites using various amounts of CdS nanocrystal loading.
rate up to a loading of 3 wt% (sample Cd3), where the highest hydrogen generation rate was found at 32 µmol h−1 (which corresponds to a quantum efficiency (QE) of 8.2% at 420 nm), and a turnover number of 79 was obtained while the catalyst was still active. This exceeds the hydrogen evolution rate of a pure CdS nanocrystal sample of 9 µmol h−1 under the same irradiating conditions, which is similar to a large number of previous investigations [32]. Third, when the CdS content was increased beyond 3 wt% (sample
Cd5, figure S1 available at stacks.iop.org/Nano/24/505401/ mmedia), the photocatalytic activity dramatically decreased, with a hydrogen production rate of 22 µmol h−1 . As a consequence, the suitable CdS content causes good dispersion on the ATP surface, which favors the transfer and separation of the charge carriers. A similar observation has been reported for CdS/TiO2 composites [33]. Besides activity, the stability of a photocatalyst is important for its application. To demonstrate the stability of 4
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Figure 6. Transient photocurrent responses of the ATP (a), Cd0.5 (b), Cd1 (c), Cd3 (d), and Cd5 (e) samples in 0.1 M Na2 S + 0.02 M Na2 SO3 mixed aqueous solution under visible-light irradiation at 0.4 V versus Ag/AgCl.
Cd3, and Cd5 is 0.02, 0.25, 0.28, 0.34, and 0.33 mA cm−2 , respectively. The CdS–ATP nanocomposites exhibited a higher photoinduced current due to the enhancement in light absorption in the visible-light region and additionally due to the performance improvements of charge separation and charge injection [34]. The Cd3 photocatalyst shows the highest photocurrent intensity. However, on further increasing the CdS concentration to 5 wt% (Cd5), the photocurrent decreases due to some CdS particle agglomeration (figure S1 available at stacks.iop.org/Nano/24/505401/mmedia). Figure 5. (a) Cycling measurements of hydrogen generation through direct photocatalytic water splitting with CdS–ATP (Cd3) under visible light and (b) XRD patterns of Cd3 (before and after the standard cycling measurement).
3.3. Discussion of the photocatalytic mechanism The ideal composition of ATP is Mg5 Si8 O20 (OH)2 (OH2 )4 [22] (figure 7(a)). However, according to previous reports from other groups [22, 35, 36], research [22, 26] indicated the ATP contains 3% Fe element, which suggested a structure of two ATP crystal cells sharing one Fe atom. Figure 2(d) confirms remarkably the existence of Fe, and about 3% content is estimated within the error of EDX. Thus, by using VASP [37], we optimized the Fe-doped structures: Mg4.5 Fe0.5 Si8 O20 (OH)2 (OH2 )4 . By comparison of three different structures in which Fe doped in different sites, the most stable structure with lowest total energy is determined, as shown in figure 7(b). Based on the above optimized structures, we conducted DFT calculations as shown in figures 7(c) and (d). There is a wide energy gap of about 5 eV and a high valence band level in ideal ATP (figure 7(c)). However, after doping with Fe, the ATP levels are lowered to 1.6 eV, as shown in figure 7(d), which is 2.15 eV smaller than the experimental value (3.75 eV) as a result of the well known band gap underestimation within the framework of standard DFT. The wide valence band was also measured by the XPS valence band spectrum of ATP (figure 8(a)). The valence band maximum of ATP was determined to be 2.6 eV as shown in figure 8(a). This is 0.6 eV bigger than that of anatase TiO2 [38, 39], suggesting that ATP has a lower valence band maximum than anatase TiO2 by about 0.6 eV relative to an
our composite catalysts, we cycled the hydrogen evolution for sample Cd3 (figure S2 available at stacks.iop.org/Nano/24/ 505401/mmedia). Figure 5(a) displays the hydrogen evolution curve in a cycling photocatalytic run. Measurement was conducted for four consecutive days and each day the sample was irradiated for 6 h and then stored in darkness overnight before testing the next day. Continuous hydrogen evolution with no noticeable degradation of the Cd3 sample was clearly observed from the beginning of the reaction. In addition, XRD analysis of the nanocomposite before and after the experiment (figure 5(b)) also illustrates that the crystal structure has not changed and no photocorrosion is detected in the running period. The high stability could be attributed to the better coupling between the structures and electronic configurations of CdS and ATP together, and the formation of junctions between CdS and ATP, which improved the charge separation. The transient photocurrent responses of as-synthesized CdS–ATP samples as well as the bare ATP were recorded for several on–off cycles of visible-light irradiation. Figure 6 shows a comparison of photocurrent variation curves for a series of CdS–ATP nanocomposites. From figure 6, we can observe that the photocurrent of bare ATP, Cd0.5, Cd1, 5
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Figure 7. Computational model of ATP unit crystal cell on the (001) plane (a) and the best position of Fe doping (b). Atoms: red—O, green—Mg, gray—H, yellow—Si, purple—Fe. Calculated electronic band structures of the ideal ATP (c) and natural Fe-doping ATP (d).
Figure 8. Valence band XPS spectra of the ATP (a) and electronic potential diagram for anatase TiO2 and ATP (b).
NHE. Because the band gap of the ATP is 3.75 eV from the optical absorption spectrum (figure 3), the electronic potentials of the ATP could be determined (figure 8(b)). Obviously, the conduction band minimum of ATP is more negative than the redox potentials for H+ /H2 . Besides, the conduction band potential and valence band potential of CdS were determined to be −0.87 and 1.5 eV [40], as shown in figure 9. As a result, the conduction band minimum of CdS is more negative than that of ATP, and under visible-light irradiation photoinduced electrons from the CdS can easily transfer to ATP.
On the basis of the above results, a proposed mechanism for the photocatalytic activity of the CdS–ATP nanocomposite is shown schematically in figure 9. Under visible-light irradiation, CdS nanocrystals are activated. The photogenerated electrons transfer from the conduction band of CdS into ATP and accumulate at the lower-lever conduction band of ATP to generate hydrogen, while holes accumulate at the valence band of CdS. Hence, the CdS–ATP nanocomposites can increase photoactivity as a result of the efficient separation of the photogenerated electron–hole pairs. 6
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Figure 9. Schematic illustration for the charge transfer and separation in CdS nanocrystals sensitizing ATP under visible light.
4. Conclusion In summary, a high efficiency of photocatalytic hydrogen production from water splitting under visible-light irradiation has been achieved over a CdS–ATP photocatalyst synthesized by a facile method. The 3 wt% CdS nanocrystal sensitized ATP composites (Cd3) showed a high hydrogen production rate of 32 µmol h−1 even without the deposition of noble metal co-catalyst. The red-shift in the absorption spectrum confirms the quantum size effect of CdS; the presence of CdS nanocrystals alters the energy levels of the conduction and valence bands in the coupled system, which favors the electron transfer and enhances photoactivity. Moreover, the theoretical calculations show that natural Fe doping (two ATP cells sharing one Fe atom) can also promote the photocatalytic process. Although the absolute activity of CdS–ATP nanocomposites is still not very high, this work opens up new possibilities for the development of highly efficient ATP-based photocatalysts that utilize visible light.
Acknowledgments This project is supported financially by the Natural Science Foundation of China (NSFC, No 51272107), and the Natural Science Foundation of Jiangsu Province, China (No BK2011024).
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