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Nano-CdS confined within titanate nanotubes for efficient photocatalytic hydrogen production under visible light illumination
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 035603 (http://iopscience.iop.org/0957-4484/25/3/035603) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 93.180.53.211 This content was downloaded on 27/12/2013 at 03:53
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 25 (2014) 035603 (6pp)
doi:10.1088/0957-4484/25/3/035603
Nano-CdS confined within titanate nanotubes for efficient photocatalytic hydrogen production under visible light illumination Lizhen Long1,2 , Xiang Yu3 , Liangpeng Wu1 , Juan Li1 and Xinjun Li1 1
Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, People’s Republic of China 2 University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China 3 Analytical and Testing Center, Jinan University, Guangzhou 510632, People’s Republic of China E-mail: [email protected] Received 24 September 2013, revised 19 November 2013 Accepted for publication 28 November 2013 Published 20 December 2013 Abstract
CdS nanoparticles were confined within titanate nanotubes (TNTs) by an ion-exchange reaction and a subsequent sulfurization process. Prior to the ion-exchange reaction, the exterior surfaces of the TNTs were modified by a silane coupling agent to make CdS nanoparticles selectively deposit on the inner wall. The composites were characterized by high-resolution transmission electron microscopy, powder x-ray diffraction, inductively coupled plasma atomic emission spectrometry, N2 adsorption–desorption and UV–vis absorption spectra. The results confirm that CdS in the range of 2–3 nm in diameter are confined within the inner cavity of the TNTs. CdS confined within TNTs shows a significant blue-shift of the absorption band edge compared with CdS nanoparticles deposited on the exterior surface of TNTs. Also the TNTs-confined CdS composite exhibits enhanced photocatalytic activity and photostability for hydrogen evolution under visible light illumination due to the quantum size effect of CdS as a result of the spatial confinement effect of the TNTs. Keywords: CdS nanoparticles, titanate nanotubes, confinement effect, hydrogen production, quantum size effect (Some figures may appear in colour only in the online journal)
1. Introduction
light irradiation due to its suitable band position. However, the photocatalytic efficiency over bare CdS is generally low, mainly due to the fast recombination of electron/hole pairs and photocorrosion problems [1]. Currently, the effective ways to improve the photocatalytic activity and photostability of CdS are coupling with wide-bandgap semiconductors, such as ZnO [2] and TiO2 [3], forming a solid solution with ZnS [4], and incorporating CdS nanoparticles into the interlayer of the photocatalysts [5]. Confining CdS into nanospaces could be another approach to improve the photocatalytic activity and photostability of CdS.
Photocatalytic water splitting over semiconductors is a green chemistry pathway for converting solar energy into clean and renewable hydrogen fuel. In the solar spectrum, ultraviolet radiation accounts for approximately only 7% of the solar energy, while visible light accounts for more than 50% of the solar energy. Considerable efforts have been focused on developing various efficient visible-lightsensitive photocatalysts. Cadmium sulfide (CdS), with a bandgap energy of 2.4 eV, has become one of the most important semiconductors for water splitting under visible 0957-4484/14/035603+06$33.00
1
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Figure 1. Schematic illustration of CdS nanoparticles deposited on the inner cavity of TNTs.
Recently, the novel structure of confining active components into porous materials [6] or tubular materials [7] has been attracting much attention in the field of catalysis. An increasing number of studies have demonstrated that confining metal or metal oxide nanoparticles inside carbon nanotubes (CNTs) often leads to an improved catalytic activity [8, 9]. Like CNTs, TNTs with well-defined nanosized channels are anticipated to provide an intriguing confinement environment for catalysts. Wang et al [10] reported that TNTsconfined CeO2 exhibited superiority in selective catalytic reduction of NO with respect to TiO2 nanoparticle-supported CeO2 . Kazumoto et al [11] found that TNTs with Pt nanoparticles loaded on the inside surface exhibited higher photocatalytic activity for the oxidation of acetaldehyde than that with Pt nanoparticles on the outside surface. Bavykin et al [12] studied methods for the deposition of noble metals Pt, Pd, Ru and Au on the surface of TNTs, and suggested that blocking the external surface of TNTs using suitable surfactants could be a viable strategy to immobilize metal nanoparticles inside the inner cavities of TNTs. In view of the unique chemical and physical properties of the confined catalysts, it is suggested that TNTs-confined CdS nanoparticles should present enhanced photocatalytic activity for hydrogen production from water splitting. However, few achievements have been witnessed so far in confining metal sulfides within TNTs. Balkus et al [13, 14] reported that PbS QDs can be selectively decorated on the inner or outer surface of TiO2 nanotubes using a double-chain cationic surfactant. CdS decorated TNTs have been prepared for photocatalytic degradation of Rhodamine B [15] and photocatalytic hydrogen production [16]. Herein, CdS nanoparticles are confined within modified TNTs by an ion-exchange reaction followed by a sulfurization process. A silane coupling agent is used as modifier to block the external surface of the TNTs to make the CdS nanoparticles deposit selectively on the inner wall. The performance of photocatalytic hydrogen production based on TNTs-confined CdS composite was studied in comparison with the composite of CdS nanoparticles deposited on the exterior surface of TNTs.
ric acid (HCl, 36%–38%), ethanol (C2 H5 OH, ≥99.7%), glacial acetic acid (CH3 COOH, ≥99.5%), ammonia solution (NH3 , 25%) and cadmium chloride (CdCl2 ·2.5H2 O, ≥99%) were obtained from Guangzhou Chemical Reagent Factory. Thiourea (H2 NCSNH2 , 99%) was purchased from Tianjin Fuchen Chemical Reagent Factory. γ Methacryloxpropyltrimethoxysilane (KH-570, 99%) was obtained from Sinopharm Chemical Reagent Co., Ltd. 2.2. Material synthesis 2.2.1. Hydrothermal synthesis of titanate nanotubes (TNTs).
A commercial, anatase TiO2 powder (obtained from Panzhihua Iron and Steel Group) was used as a starting material. The hydrothermal synthesis of TNTs was according to the literature procedure [17]. In a typical procedure, 100 ml of a 10 M NaOH solution and 2 g of TiO2 powder were put into a 250 ml Teflon-lined flask. The flask was maintained at 110 ◦ C under atmospheric pressure and stirred for 24 h. After cooling down to room temperature, the product was rinsed with deionized water until the pH value of the filtrate was about 7. Then the filter cake was bathed with 0.1 M HCl solution for 5 h, and subsequently rinsed with deionized water until the pH value of the filtrate was about 7. Finally, the TNTs were obtained after drying at 80 ◦ C for 24 h in air. The experimental procedures for preparing TNT-confined CdS composite are schematically illustrated in figure 1. Firstly, the external surface of the TNTs was modified with a silane coupling agent (KH-570) to form a hydrophobic surface, to make the following in situ growth of CdS take place on the inner wall of the TNTs. The modification method is as follows: 1 g of as-prepared TNTs were ultrasonically dispersed in 50 ml deionized water for 10 min, then the pH was adjusted to 3–4 using 5 wt% acetic acid. After stirring at room temperature for 1 h, 50 ml of ethanol dispersed with 0.2 ml KH-570 was added into the solution and the mixture was stirred at 80 ◦ C for 5 h. The precipitate was washed with deionized water followed by ethanol, and dried at 80 ◦ C for 24 h in air. Then, in situ growth of CdS was carried out by an ion-exchange reaction followed by a sulfurization process. The ion-exchange reaction of [Cd(NH3 )4 ]2+ substituting for H+ was carried out according to the following method. A certain amount of ammonia solution was dropwise added 2.2.2. Preparation of TNT-confined CdS composite.
2. Experimental details 2.1. Chemicals and materials
All reagents were of analytical grade and used as received. Sodium hydroxide (NaOH, ≥96%), hydrochlo2
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into a 0.2 M CdCl2 aqueous solution until the blue solution became clear. Then, 1 g of the modified TNTs were added into the [Cd(NH3 )4 ]2+ solution and stirred at room temperature for 24 h. After that, the product was washed with deionized water several times to remove the residual [Cd(NH3 )4 ]2+ (using Na2 S as an inspection reagent), H+ , Cl− and NH+ 4. The sulfurization process was performed by ultrasonically treating the white product in 0.2 M thiourea solution for 1 h under vacuum conditions. Light yellow products (designated as in-TNTs) were obtained after washing and drying at 80 ◦ C for 24 h in air. In order to make a comparison, CdS nanoparticles deposited on the exterior surface of TNTs were prepared according to the following method. 1 g of as-prepared TNTs were added into a 0.2 M cadmium–ammonia complex ion solution and stirred at room temperature for 30 min to carry out the ion-exchange reaction of [Cd(NH3 )4 ]2+ substituting for H+ . After washing with deionized water, the product was added into 0.2 M thiourea solution and stirred at room temperature for 1 h. Yellow products (designated as ex-TNTs) were obtained after washing and drying at 80 ◦ C for 24 h in air. Pure CdS particles were also prepared as follows: appropriate amounts of 0.2 M [Cd(NH3 )4 ]2+ aqueous solution and 0.2 M thiourea were mixed and stirred at room temperature for 1 h. The yellow precipitate was washed with deionized water several times and dried at 80 ◦ C for 24 h in air. 2.3. Characterization
High-resolution transmission electron microscopy (HRTEM) was carried out using a JEM-2100F microscope (JEOL Co. Ltd) with an acceleration voltage of 200 kV. X-ray diffraction (XRD) patterns were recorded using an X’ Pert-PRO MPD diffractometer equipped with Cu Kα radiation (λ = 0.154 056 nm, Holland). The absorption spectrum was recorded by a UV–visible spectrophotometer (LAMBDA 750) equipped with an integration sphere. The elemental analysis was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES, OPTIMA 8000). The nitrogen adsorption–desorption isotherm was recorded at 77 K in a SI-MP-10/Pore Master 33 instrument and all samples were degassed at 190 ◦ C for 16 h prior to the measurement. 2.4. Photocatalytic hydrogen production
Photocatalytic hydrogen production was performed in an outer irradiation Pyrex glass reactor at room temperature and atmospheric pressure. A 300 W Xenon lamp was used as the light source, with the UV part of its spectrum removed by a cutoff filter (λ ≥ 420 nm). The photocatalyst (0.1 g) was dispersed in an 80 ml aqueous solution containing 0.35 M Na2 S/0.25 M Na2 SO3 and 5.3 mg H2 PtCl6 ·6H2 O. Before measurement, the solution was illuminated for 30 min to in situ photo-deposit Pt on the photocatalysts. The evolved gases were extracted periodically and analyzed by gas chromatography (Agilent 6890) by means of a thermal conductivity detector equipped with a 5A molecular sieve using Ar as the carrier gas.
Figure 2. TEM images of (a) TNTs, (b) ex-TNTs and (c) in-TNTs.
3. Results and discussion
Figure 2 shows the TEM images of as-prepared TNTs, ex-TNTs and in-TNTs. As presented in figure 2(a), TNTs prepared by the hydrothermal process have a hollow cavity and open ends. Also the TNTs obtained are multilayered nanotubes with diameters of about 10 nm (see the inset of figure 2(a)). For the ex-TNTs (see figure 2(b)), CdS with 3
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Figure 3. XRD patterns of as-prepared TNTs, ex-TNTs and
in-TNTs.
particle sizes of more than 5 nm are mostly loaded on the exterior surface of the TNTs, with only several small CdS particles being embedded within the cavity of the TNTs. For the in-TNTs (see figure 2(c)), most of the CdS nanoparticles with diameters in the range of 2–3 nm are confined inside the multilayered TNTs. The HRTEM image (the inset of figure 2(c)) of in-TNTs recorded from the region marked with the white arrow shows clear crystal lattices with an interplanar distance of 0.335 nm, which corresponds to the CdS(111) plane. Figure 3 shows the XRD patterns of TNTs, in-TNTs and ex-TNTs. The as-synthesized TNTs appear to be trititanate nanotubes [18], which have been proposed to form by scrolling of nanosheets [19, 20]. Apart from the characteristic diffraction peaks of H2 Ti3 O7 nanotubes, in-TNTs and ex-TNTs show peaks at 26.5◦ , 43.9◦ and 51.9◦ , which can be attributed to cubic CdS (JCPDS card No. 01-001-0647). The lower CdS peak intensity for in-TNTs with respect to ex-TNTs could be ascribed to the smaller particle size and more homogeneous dispersion when the CdS nanoparticles were confined within the TNTs. The actual chemical compositions of the samples were determined from ICP-AES analysis. The atom ratios of Cd/Ti in samples of in-TNTs and ex-TNTs are 47.5:100 and 60:100, respectively, indicating that the CdS content of the ex-TNTs sample is higher than that of in-TNTs. Figure 4 shows the nitrogen adsorption/desorption isotherms and pore-size distribution curves (estimated using the desorption branch of the isotherm) for the TNTs, ex-TNTs and in-TNTs. The isotherms for all the samples exhibit a typically BDDT Type III isotherm with a large type H3 hysteresis hoop, indicating mesoporosity of the photocatalyst [21]. A large uptake is observed when close to the saturation pressure, where capillary condensation commences in the large voids among the aggregates of nanotubes [16]. The BET surface area is 322 m2 g−1 for TNTs, 190 m2 g−1 for ex-TNTs and 187 m2 g−1 for in-TNTs. The above results indicate that the deposited CdS on the surface of the TNTs decreases the specific surface area of the TNTs. For the TNTs sample, the pore-size distribution
Figure 4. (a) Nitrogen adsorption/desorption isotherm, (b) the
corresponding pore-size distribution of TNTs, ex-TNTs and in-TNTs.
(analyzed using BJH method) exhibits a broad peak in the range of 4–20 nm, with the center at 9 nm, which could be attributed to the inner cavity of the nanotubes. For the ex-TNTs sample, the pore-size distribution is similar to that of the TNTs, with the pore volume of this portion being only slightly decreased, indicating that only a few CdS nanoparticles are loaded on the inner surface of the TNTs. However, for the in-TNTs sample, the pore volume in the range of 4–20 nm is dramatically reduced, suggesting that most of the CdS nanoparticles are confined within the inner cavity of the TNTs. Figure 5(a) shows the UV–vis absorption spectra of the as-prepared TNTs, ex-TNTs and in-TNTs. No absorption in the visible region for the as-prepared TNTs is observed. For ex-TNTs and in-TNTs, the broad absorption band extends to the visible region. Compared with ex-TNTs, the absorption band edge of in-TNTs is blue-shifted and the absorption intensity is lower. Since the absorption intensity is proportional to the amount of absorption materials, it also indicates a greater loading content of CdS for ex-TNTs than for in-TNTs. 4
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Figure 5. (a) UV–vis absorption spectra, (b) the corresponding
(αhν)1/2 versus hν curves of TNTs, ex-TNTs, in-TNTs.
Figure 6. (a) The courses of hydrogen evolution on pure CdS,
ex-TNTs and in-TNTs, and (b) the corresponding amounts of hydrogen evolution for each one-hour period under 300 W Xe lamp visible light irradiation (λ > 420 nm) in a mixed 0.25 M Na2 SO3 /0.35 M Na2 S solution and 5.3 mg H2 PtCl6 ·6H2 O with 0.1 g of catalyst.
The band gaps of the corresponding samples were calculated using the following equation [22]: ahν = C1 (hν − Eg )2
(1)
where α is the absorbance coefficient and hν is the energy of the incident photons. Figure 5(b) shows (αhν)1/2 plotted versus hν and the tangent segment of the spectra is extended to intersect with the X-axis to obtain the indirect bandgap value of the measured sample. The indirect bandgap energies (Eg ) of as-prepared TNTs, ex-TNTs and in-TNT are about 3.4 eV, 2.1 eV and 2.4 eV, respectively. The larger bandgap energy for in-TNTs relative to ex-TNTs suggests the presence of a quantum size effect, further confirming that in-TNTs have smaller CdS particles inside the cavity due to the spatial confinement effect of TNTs. Figure 6 shows the hydrogen evolution rates of ex-TNTs, in-TNTs and pure CdS under visible light irradiation (λ > 420 nm). The pure CdS exhibits a low activity of hydrogen production. For both ex-TNTs and in-TNTs, however, an increase in the hydrogen production activities is observed, as shown in figure 6(a). It can be concluded that CdS coupled with TNTs promotes the transfer of photoexcited electrons and results in improved activities of hydrogen production. It is noticeable that the in-TNTs sample exhibits a higher activity of hydrogen production than the ex-TNTs sample. The hydrogen evolution rate over in-TNTs shows a linear
increase during 5 h reaction and the total amount of hydrogen evolution is 1438 µmol. In comparison, for ex-TNTs, the hydrogen evolution rate starts to decrease after 2 h reaction and the total amount of hydrogen evolution in 5 h reaction is only 786 µmol. The amount of hydrogen evolution in each one-hour period is summarized in figure 6(b). The amount of hydrogen evolution of in-TNTs obviously increases from 241 µmol for the first hour to 295 µmol for the second hour, which is accompanied by photoreduction of the H2 PtCl6 ·6H2 O on photocatalyst. The amount of hydrogen evolution in each one-hour period starts to stabilize at about 308 µmol after 3 h reaction. In contrast, for ex-TNTs, the amount of hydrogen evolution increases slightly from 172 µmol for the first hour to 178 µmol for the second hour, but dramatically decreases to 147 µmol for the third hour and then slowly decreases to 141 µmol for the fifth hour, which can be ascribed to the photocorrosion of CdS. The phenomenon of photocorrosion on pure CdS is more obvious, and the amount of hydrogen evolution monotonically decreases from 60 µmol for the first hour to 33 µmol for the fifth hour. 5
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The photocatalytic processes (taking in-TNTs as an example) can be summarized as follows. CdS absorbs visible light, giving rise to e− /h+ pairs, then the photogenerated electrons migrate from the conduction band (CB) of CdS to the CB of TNTs, as described by equation (2). The photogenerated electrons in the in-TNTs reduce water molecules to form hydrogen and the photogenerated holes in 2− CdS oxidize SO2− 3 and S , according to equations (3), (4), (5) and (6), respectively. λ>420 nm
CdS/TNTs + hν −−−−−−→ CdS(h+ )/TNTs(e− ) TNTs(2e ) + H2 O → H2 + 2OH −
SO2− 3
−
+ + 2OH + 2h → SO2− 4 + 2H S2− + 2h+ → S2− 2 2− 2− + S2 + 2SO3 + 2h → 2S2 O2− 3 . −
+
nanotubes-confined CdS exhibits enhanced photocatalytic activity and photostability for hydrogen production, which can be ascribed to the quantum size effect of CdS as a result of the spatial confinement effect of the titanate nanotubes. Acknowledgments
The work is supported by the National Natural Science Foundation of China (No. 51172233), the Key Laboratory of Water and Air Pollution Control of Guangdong Province, China (No. GD2012A05), and the Science & Technology Plan Project of Guangzhou City, China (No. 2013J4300035).
(2) (3) (4)
References
(5) [1] Meissner D, Memming R and Kastening B 1988 J. Phys. Chem. 92 3476–83 [2] Wang X W, Liu G, Chen Z G, Li F, Wang L Z, Lu G Q and Cheng H M 2009 Chem. Commun. 3452–4 [3] Yu J C, Wu L, Lin J, Li P S and Li Q 2003 Chem. Commun. 1552–3 [4] Xing C J, Zhang Y J, Yan W and Guo L J 2006 Int. J. Hydorg. Energy 31 2018–24 [5] Shangguan W F and Yoshida A 2002 J. Phys. Chem. B 106 12227–30 [6] Jing D W and Guo L J 2007 J. Phys. Chem. C 111 13437–41 [7] Pan X L and Bao X H 2011 Acc. Chem. Res. 44 553–62 [8] Chen W, Fan Z L, Pan X L and Bao X H 2008 J. Am. Chem. Soc. 130 9414–9 [9] Pan X L, Fan Z L, Chen W, Ding Y J, Luo H Y and Bao X H 2007 Nature Mater. 6 507–11 [10] Wang H Q, Chen X B, Weng X L, Liu Y, Gao S and Wu Z B 2011 Catal. Commun. 12 1042–5 [11] Nishijima K, Fukahori T, Murakami N, Kamai T-A, Tsubota T and Ohno T 2008 Appl. Catal. A 337 105–9 [12] Bavykin D V, Lapkin A A, Plucinski P K, Torrente-Murciano L, Friedrich J M and Walsh F C 2006 Top. Catal. 39 151–60 [13] Ratanatawanate C, Xiong C R and Balkus K J 2008 ACS Nano 2 1682–8 [14] Ratanatawanate C, Tao Y and Balkus K J 2009 J. Phys. Chem. C 24 10755–60 [15] Xiao M W, Wang L S, Wu Y D, Huang X J and Dang Z 2008 Nanotechnology 19 015706 [16] Li C L, Yuan J, Han B Y, Jiang L and Shangguan W F 2010 Int. J. Hydorg. Energy 35 7073–9 [17] Zhang Y Y, Chen J Z and Li X J 2010 Catal. Lett. 139 129–33 [18] Chen Q, Du G H, Zhang S and Peng L M 2002 Acta Crystallogr. B 58 587–93 [19] Zhang S, Peng L M, Chen Q, Du G H, Dawson G and Zhou W Z 2003 Phys. Rev. Lett. 91 256103 [20] Zhang S, Chen Q and Peng L M 2005 Phys. Rev. B 71 014104 [21] Xu S P, Du A J H, Liu J C, Ng J W and Sun D D 2011 Int. J. Hydorg. Energy 36 6560–8 [22] Ng J, Xu S, Zhang X, Yang H Y and Sun D D 2010 Adv. Funct. Mater. 20 4287–94 [23] Zhang Z, Wang C-C, Zakaria R and Ying J Y 1998 J. Phys. Chem. B 102 10871–8 [24] Kongkanand A, Tvrdy K, Takechi K, Kuno M and Kamat P V 2008 J. Am. Chem. Soc. 130 4007–15 [25] Jing D W and Guo L J 2007 Catal. Commun. 8 795–9
(6)
Compared with ex-TNTs, the in-TNTs sample exhibits a higher photocatalytic hydrogen evolution activity, which can be explained by the quantum size effect of CdS as a result of the spatial confinement effect of the TNTs. The tubular inner cavity of the TNTs can hamper the growth and aggregation of embedded CdS nanoparticles in the process of preparation, and result in the formation of smaller CdS nanoparticles inside the TNTs. Thus, electron–hole volume recombination, which is a dominant process in the large CdS particles, can be reduced and more electrons and holes reach the surface of the photocatalyst [23]. On the other hand, according to the quantum size effect, the excitonic band increases with decreasing particle size, resulting in a decrease of light-harvesting capability, whereas the shift in the conduction band to more negative potentials increases the driving force and favors fast electron injection from CdS into the TNTs [24], as described by equation (2). As a result, more electrons can participate in the photocatalytic reaction through equation (3), resulting in the enhanced hydrogen production rate. The higher photostability of hydrogen production for in-TNTs relative to ex-TNTs could be ascribed to the unique tubular structure of TNTs, which could protect CdS from photocorrosion by confining them inside the TNTs, similar to the important role of mesopore substrates for effective confinement of CdS nanoparticles [25]. 4. Conclusion
Nano-CdS with diameters of 2–3 nm were confined within the inner cavity of titanate nanotubes by an ion-exchange reaction and a subsequent sulfurization process. Prior to the ion-exchange reaction, the titanate nanotubes were modified by a silane coupling agent to make the CdS nanoparticles selectively deposit on the inner wall. CdS confined within titanate nanotubes shows a significant blue-shift of the absorption band edge compared with CdS nanoparticles deposited on the exterior surface of TNTs. The titanate
6
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Nano-CdS confined within titanate nanotubes for efficient photocatalytic hydrogen production under visible light illumination
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 035603 (http://iopscience.iop.org/0957-4484/25/3/035603) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 93.180.53.211 This content was downloaded on 27/12/2013 at 03:53
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 25 (2014) 035603 (6pp)
doi:10.1088/0957-4484/25/3/035603
Nano-CdS confined within titanate nanotubes for efficient photocatalytic hydrogen production under visible light illumination Lizhen Long1,2 , Xiang Yu3 , Liangpeng Wu1 , Juan Li1 and Xinjun Li1 1
Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, People’s Republic of China 2 University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China 3 Analytical and Testing Center, Jinan University, Guangzhou 510632, People’s Republic of China E-mail: [email protected] Received 24 September 2013, revised 19 November 2013 Accepted for publication 28 November 2013 Published 20 December 2013 Abstract
CdS nanoparticles were confined within titanate nanotubes (TNTs) by an ion-exchange reaction and a subsequent sulfurization process. Prior to the ion-exchange reaction, the exterior surfaces of the TNTs were modified by a silane coupling agent to make CdS nanoparticles selectively deposit on the inner wall. The composites were characterized by high-resolution transmission electron microscopy, powder x-ray diffraction, inductively coupled plasma atomic emission spectrometry, N2 adsorption–desorption and UV–vis absorption spectra. The results confirm that CdS in the range of 2–3 nm in diameter are confined within the inner cavity of the TNTs. CdS confined within TNTs shows a significant blue-shift of the absorption band edge compared with CdS nanoparticles deposited on the exterior surface of TNTs. Also the TNTs-confined CdS composite exhibits enhanced photocatalytic activity and photostability for hydrogen evolution under visible light illumination due to the quantum size effect of CdS as a result of the spatial confinement effect of the TNTs. Keywords: CdS nanoparticles, titanate nanotubes, confinement effect, hydrogen production, quantum size effect (Some figures may appear in colour only in the online journal)
1. Introduction
light irradiation due to its suitable band position. However, the photocatalytic efficiency over bare CdS is generally low, mainly due to the fast recombination of electron/hole pairs and photocorrosion problems [1]. Currently, the effective ways to improve the photocatalytic activity and photostability of CdS are coupling with wide-bandgap semiconductors, such as ZnO [2] and TiO2 [3], forming a solid solution with ZnS [4], and incorporating CdS nanoparticles into the interlayer of the photocatalysts [5]. Confining CdS into nanospaces could be another approach to improve the photocatalytic activity and photostability of CdS.
Photocatalytic water splitting over semiconductors is a green chemistry pathway for converting solar energy into clean and renewable hydrogen fuel. In the solar spectrum, ultraviolet radiation accounts for approximately only 7% of the solar energy, while visible light accounts for more than 50% of the solar energy. Considerable efforts have been focused on developing various efficient visible-lightsensitive photocatalysts. Cadmium sulfide (CdS), with a bandgap energy of 2.4 eV, has become one of the most important semiconductors for water splitting under visible 0957-4484/14/035603+06$33.00
1
c 2014 IOP Publishing Ltd Printed in the UK
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Figure 1. Schematic illustration of CdS nanoparticles deposited on the inner cavity of TNTs.
Recently, the novel structure of confining active components into porous materials [6] or tubular materials [7] has been attracting much attention in the field of catalysis. An increasing number of studies have demonstrated that confining metal or metal oxide nanoparticles inside carbon nanotubes (CNTs) often leads to an improved catalytic activity [8, 9]. Like CNTs, TNTs with well-defined nanosized channels are anticipated to provide an intriguing confinement environment for catalysts. Wang et al [10] reported that TNTsconfined CeO2 exhibited superiority in selective catalytic reduction of NO with respect to TiO2 nanoparticle-supported CeO2 . Kazumoto et al [11] found that TNTs with Pt nanoparticles loaded on the inside surface exhibited higher photocatalytic activity for the oxidation of acetaldehyde than that with Pt nanoparticles on the outside surface. Bavykin et al [12] studied methods for the deposition of noble metals Pt, Pd, Ru and Au on the surface of TNTs, and suggested that blocking the external surface of TNTs using suitable surfactants could be a viable strategy to immobilize metal nanoparticles inside the inner cavities of TNTs. In view of the unique chemical and physical properties of the confined catalysts, it is suggested that TNTs-confined CdS nanoparticles should present enhanced photocatalytic activity for hydrogen production from water splitting. However, few achievements have been witnessed so far in confining metal sulfides within TNTs. Balkus et al [13, 14] reported that PbS QDs can be selectively decorated on the inner or outer surface of TiO2 nanotubes using a double-chain cationic surfactant. CdS decorated TNTs have been prepared for photocatalytic degradation of Rhodamine B [15] and photocatalytic hydrogen production [16]. Herein, CdS nanoparticles are confined within modified TNTs by an ion-exchange reaction followed by a sulfurization process. A silane coupling agent is used as modifier to block the external surface of the TNTs to make the CdS nanoparticles deposit selectively on the inner wall. The performance of photocatalytic hydrogen production based on TNTs-confined CdS composite was studied in comparison with the composite of CdS nanoparticles deposited on the exterior surface of TNTs.
ric acid (HCl, 36%–38%), ethanol (C2 H5 OH, ≥99.7%), glacial acetic acid (CH3 COOH, ≥99.5%), ammonia solution (NH3 , 25%) and cadmium chloride (CdCl2 ·2.5H2 O, ≥99%) were obtained from Guangzhou Chemical Reagent Factory. Thiourea (H2 NCSNH2 , 99%) was purchased from Tianjin Fuchen Chemical Reagent Factory. γ Methacryloxpropyltrimethoxysilane (KH-570, 99%) was obtained from Sinopharm Chemical Reagent Co., Ltd. 2.2. Material synthesis 2.2.1. Hydrothermal synthesis of titanate nanotubes (TNTs).
A commercial, anatase TiO2 powder (obtained from Panzhihua Iron and Steel Group) was used as a starting material. The hydrothermal synthesis of TNTs was according to the literature procedure [17]. In a typical procedure, 100 ml of a 10 M NaOH solution and 2 g of TiO2 powder were put into a 250 ml Teflon-lined flask. The flask was maintained at 110 ◦ C under atmospheric pressure and stirred for 24 h. After cooling down to room temperature, the product was rinsed with deionized water until the pH value of the filtrate was about 7. Then the filter cake was bathed with 0.1 M HCl solution for 5 h, and subsequently rinsed with deionized water until the pH value of the filtrate was about 7. Finally, the TNTs were obtained after drying at 80 ◦ C for 24 h in air. The experimental procedures for preparing TNT-confined CdS composite are schematically illustrated in figure 1. Firstly, the external surface of the TNTs was modified with a silane coupling agent (KH-570) to form a hydrophobic surface, to make the following in situ growth of CdS take place on the inner wall of the TNTs. The modification method is as follows: 1 g of as-prepared TNTs were ultrasonically dispersed in 50 ml deionized water for 10 min, then the pH was adjusted to 3–4 using 5 wt% acetic acid. After stirring at room temperature for 1 h, 50 ml of ethanol dispersed with 0.2 ml KH-570 was added into the solution and the mixture was stirred at 80 ◦ C for 5 h. The precipitate was washed with deionized water followed by ethanol, and dried at 80 ◦ C for 24 h in air. Then, in situ growth of CdS was carried out by an ion-exchange reaction followed by a sulfurization process. The ion-exchange reaction of [Cd(NH3 )4 ]2+ substituting for H+ was carried out according to the following method. A certain amount of ammonia solution was dropwise added 2.2.2. Preparation of TNT-confined CdS composite.
2. Experimental details 2.1. Chemicals and materials
All reagents were of analytical grade and used as received. Sodium hydroxide (NaOH, ≥96%), hydrochlo2
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into a 0.2 M CdCl2 aqueous solution until the blue solution became clear. Then, 1 g of the modified TNTs were added into the [Cd(NH3 )4 ]2+ solution and stirred at room temperature for 24 h. After that, the product was washed with deionized water several times to remove the residual [Cd(NH3 )4 ]2+ (using Na2 S as an inspection reagent), H+ , Cl− and NH+ 4. The sulfurization process was performed by ultrasonically treating the white product in 0.2 M thiourea solution for 1 h under vacuum conditions. Light yellow products (designated as in-TNTs) were obtained after washing and drying at 80 ◦ C for 24 h in air. In order to make a comparison, CdS nanoparticles deposited on the exterior surface of TNTs were prepared according to the following method. 1 g of as-prepared TNTs were added into a 0.2 M cadmium–ammonia complex ion solution and stirred at room temperature for 30 min to carry out the ion-exchange reaction of [Cd(NH3 )4 ]2+ substituting for H+ . After washing with deionized water, the product was added into 0.2 M thiourea solution and stirred at room temperature for 1 h. Yellow products (designated as ex-TNTs) were obtained after washing and drying at 80 ◦ C for 24 h in air. Pure CdS particles were also prepared as follows: appropriate amounts of 0.2 M [Cd(NH3 )4 ]2+ aqueous solution and 0.2 M thiourea were mixed and stirred at room temperature for 1 h. The yellow precipitate was washed with deionized water several times and dried at 80 ◦ C for 24 h in air. 2.3. Characterization
High-resolution transmission electron microscopy (HRTEM) was carried out using a JEM-2100F microscope (JEOL Co. Ltd) with an acceleration voltage of 200 kV. X-ray diffraction (XRD) patterns were recorded using an X’ Pert-PRO MPD diffractometer equipped with Cu Kα radiation (λ = 0.154 056 nm, Holland). The absorption spectrum was recorded by a UV–visible spectrophotometer (LAMBDA 750) equipped with an integration sphere. The elemental analysis was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES, OPTIMA 8000). The nitrogen adsorption–desorption isotherm was recorded at 77 K in a SI-MP-10/Pore Master 33 instrument and all samples were degassed at 190 ◦ C for 16 h prior to the measurement. 2.4. Photocatalytic hydrogen production
Photocatalytic hydrogen production was performed in an outer irradiation Pyrex glass reactor at room temperature and atmospheric pressure. A 300 W Xenon lamp was used as the light source, with the UV part of its spectrum removed by a cutoff filter (λ ≥ 420 nm). The photocatalyst (0.1 g) was dispersed in an 80 ml aqueous solution containing 0.35 M Na2 S/0.25 M Na2 SO3 and 5.3 mg H2 PtCl6 ·6H2 O. Before measurement, the solution was illuminated for 30 min to in situ photo-deposit Pt on the photocatalysts. The evolved gases were extracted periodically and analyzed by gas chromatography (Agilent 6890) by means of a thermal conductivity detector equipped with a 5A molecular sieve using Ar as the carrier gas.
Figure 2. TEM images of (a) TNTs, (b) ex-TNTs and (c) in-TNTs.
3. Results and discussion
Figure 2 shows the TEM images of as-prepared TNTs, ex-TNTs and in-TNTs. As presented in figure 2(a), TNTs prepared by the hydrothermal process have a hollow cavity and open ends. Also the TNTs obtained are multilayered nanotubes with diameters of about 10 nm (see the inset of figure 2(a)). For the ex-TNTs (see figure 2(b)), CdS with 3
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Figure 3. XRD patterns of as-prepared TNTs, ex-TNTs and
in-TNTs.
particle sizes of more than 5 nm are mostly loaded on the exterior surface of the TNTs, with only several small CdS particles being embedded within the cavity of the TNTs. For the in-TNTs (see figure 2(c)), most of the CdS nanoparticles with diameters in the range of 2–3 nm are confined inside the multilayered TNTs. The HRTEM image (the inset of figure 2(c)) of in-TNTs recorded from the region marked with the white arrow shows clear crystal lattices with an interplanar distance of 0.335 nm, which corresponds to the CdS(111) plane. Figure 3 shows the XRD patterns of TNTs, in-TNTs and ex-TNTs. The as-synthesized TNTs appear to be trititanate nanotubes [18], which have been proposed to form by scrolling of nanosheets [19, 20]. Apart from the characteristic diffraction peaks of H2 Ti3 O7 nanotubes, in-TNTs and ex-TNTs show peaks at 26.5◦ , 43.9◦ and 51.9◦ , which can be attributed to cubic CdS (JCPDS card No. 01-001-0647). The lower CdS peak intensity for in-TNTs with respect to ex-TNTs could be ascribed to the smaller particle size and more homogeneous dispersion when the CdS nanoparticles were confined within the TNTs. The actual chemical compositions of the samples were determined from ICP-AES analysis. The atom ratios of Cd/Ti in samples of in-TNTs and ex-TNTs are 47.5:100 and 60:100, respectively, indicating that the CdS content of the ex-TNTs sample is higher than that of in-TNTs. Figure 4 shows the nitrogen adsorption/desorption isotherms and pore-size distribution curves (estimated using the desorption branch of the isotherm) for the TNTs, ex-TNTs and in-TNTs. The isotherms for all the samples exhibit a typically BDDT Type III isotherm with a large type H3 hysteresis hoop, indicating mesoporosity of the photocatalyst [21]. A large uptake is observed when close to the saturation pressure, where capillary condensation commences in the large voids among the aggregates of nanotubes [16]. The BET surface area is 322 m2 g−1 for TNTs, 190 m2 g−1 for ex-TNTs and 187 m2 g−1 for in-TNTs. The above results indicate that the deposited CdS on the surface of the TNTs decreases the specific surface area of the TNTs. For the TNTs sample, the pore-size distribution
Figure 4. (a) Nitrogen adsorption/desorption isotherm, (b) the
corresponding pore-size distribution of TNTs, ex-TNTs and in-TNTs.
(analyzed using BJH method) exhibits a broad peak in the range of 4–20 nm, with the center at 9 nm, which could be attributed to the inner cavity of the nanotubes. For the ex-TNTs sample, the pore-size distribution is similar to that of the TNTs, with the pore volume of this portion being only slightly decreased, indicating that only a few CdS nanoparticles are loaded on the inner surface of the TNTs. However, for the in-TNTs sample, the pore volume in the range of 4–20 nm is dramatically reduced, suggesting that most of the CdS nanoparticles are confined within the inner cavity of the TNTs. Figure 5(a) shows the UV–vis absorption spectra of the as-prepared TNTs, ex-TNTs and in-TNTs. No absorption in the visible region for the as-prepared TNTs is observed. For ex-TNTs and in-TNTs, the broad absorption band extends to the visible region. Compared with ex-TNTs, the absorption band edge of in-TNTs is blue-shifted and the absorption intensity is lower. Since the absorption intensity is proportional to the amount of absorption materials, it also indicates a greater loading content of CdS for ex-TNTs than for in-TNTs. 4
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Figure 5. (a) UV–vis absorption spectra, (b) the corresponding
(αhν)1/2 versus hν curves of TNTs, ex-TNTs, in-TNTs.
Figure 6. (a) The courses of hydrogen evolution on pure CdS,
ex-TNTs and in-TNTs, and (b) the corresponding amounts of hydrogen evolution for each one-hour period under 300 W Xe lamp visible light irradiation (λ > 420 nm) in a mixed 0.25 M Na2 SO3 /0.35 M Na2 S solution and 5.3 mg H2 PtCl6 ·6H2 O with 0.1 g of catalyst.
The band gaps of the corresponding samples were calculated using the following equation [22]: ahν = C1 (hν − Eg )2
(1)
where α is the absorbance coefficient and hν is the energy of the incident photons. Figure 5(b) shows (αhν)1/2 plotted versus hν and the tangent segment of the spectra is extended to intersect with the X-axis to obtain the indirect bandgap value of the measured sample. The indirect bandgap energies (Eg ) of as-prepared TNTs, ex-TNTs and in-TNT are about 3.4 eV, 2.1 eV and 2.4 eV, respectively. The larger bandgap energy for in-TNTs relative to ex-TNTs suggests the presence of a quantum size effect, further confirming that in-TNTs have smaller CdS particles inside the cavity due to the spatial confinement effect of TNTs. Figure 6 shows the hydrogen evolution rates of ex-TNTs, in-TNTs and pure CdS under visible light irradiation (λ > 420 nm). The pure CdS exhibits a low activity of hydrogen production. For both ex-TNTs and in-TNTs, however, an increase in the hydrogen production activities is observed, as shown in figure 6(a). It can be concluded that CdS coupled with TNTs promotes the transfer of photoexcited electrons and results in improved activities of hydrogen production. It is noticeable that the in-TNTs sample exhibits a higher activity of hydrogen production than the ex-TNTs sample. The hydrogen evolution rate over in-TNTs shows a linear
increase during 5 h reaction and the total amount of hydrogen evolution is 1438 µmol. In comparison, for ex-TNTs, the hydrogen evolution rate starts to decrease after 2 h reaction and the total amount of hydrogen evolution in 5 h reaction is only 786 µmol. The amount of hydrogen evolution in each one-hour period is summarized in figure 6(b). The amount of hydrogen evolution of in-TNTs obviously increases from 241 µmol for the first hour to 295 µmol for the second hour, which is accompanied by photoreduction of the H2 PtCl6 ·6H2 O on photocatalyst. The amount of hydrogen evolution in each one-hour period starts to stabilize at about 308 µmol after 3 h reaction. In contrast, for ex-TNTs, the amount of hydrogen evolution increases slightly from 172 µmol for the first hour to 178 µmol for the second hour, but dramatically decreases to 147 µmol for the third hour and then slowly decreases to 141 µmol for the fifth hour, which can be ascribed to the photocorrosion of CdS. The phenomenon of photocorrosion on pure CdS is more obvious, and the amount of hydrogen evolution monotonically decreases from 60 µmol for the first hour to 33 µmol for the fifth hour. 5
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The photocatalytic processes (taking in-TNTs as an example) can be summarized as follows. CdS absorbs visible light, giving rise to e− /h+ pairs, then the photogenerated electrons migrate from the conduction band (CB) of CdS to the CB of TNTs, as described by equation (2). The photogenerated electrons in the in-TNTs reduce water molecules to form hydrogen and the photogenerated holes in 2− CdS oxidize SO2− 3 and S , according to equations (3), (4), (5) and (6), respectively. λ>420 nm
CdS/TNTs + hν −−−−−−→ CdS(h+ )/TNTs(e− ) TNTs(2e ) + H2 O → H2 + 2OH −
SO2− 3
−
+ + 2OH + 2h → SO2− 4 + 2H S2− + 2h+ → S2− 2 2− 2− + S2 + 2SO3 + 2h → 2S2 O2− 3 . −
+
nanotubes-confined CdS exhibits enhanced photocatalytic activity and photostability for hydrogen production, which can be ascribed to the quantum size effect of CdS as a result of the spatial confinement effect of the titanate nanotubes. Acknowledgments
The work is supported by the National Natural Science Foundation of China (No. 51172233), the Key Laboratory of Water and Air Pollution Control of Guangdong Province, China (No. GD2012A05), and the Science & Technology Plan Project of Guangzhou City, China (No. 2013J4300035).
(2) (3) (4)
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Compared with ex-TNTs, the in-TNTs sample exhibits a higher photocatalytic hydrogen evolution activity, which can be explained by the quantum size effect of CdS as a result of the spatial confinement effect of the TNTs. The tubular inner cavity of the TNTs can hamper the growth and aggregation of embedded CdS nanoparticles in the process of preparation, and result in the formation of smaller CdS nanoparticles inside the TNTs. Thus, electron–hole volume recombination, which is a dominant process in the large CdS particles, can be reduced and more electrons and holes reach the surface of the photocatalyst [23]. On the other hand, according to the quantum size effect, the excitonic band increases with decreasing particle size, resulting in a decrease of light-harvesting capability, whereas the shift in the conduction band to more negative potentials increases the driving force and favors fast electron injection from CdS into the TNTs [24], as described by equation (2). As a result, more electrons can participate in the photocatalytic reaction through equation (3), resulting in the enhanced hydrogen production rate. The higher photostability of hydrogen production for in-TNTs relative to ex-TNTs could be ascribed to the unique tubular structure of TNTs, which could protect CdS from photocorrosion by confining them inside the TNTs, similar to the important role of mesopore substrates for effective confinement of CdS nanoparticles [25]. 4. Conclusion
Nano-CdS with diameters of 2–3 nm were confined within the inner cavity of titanate nanotubes by an ion-exchange reaction and a subsequent sulfurization process. Prior to the ion-exchange reaction, the titanate nanotubes were modified by a silane coupling agent to make the CdS nanoparticles selectively deposit on the inner wall. CdS confined within titanate nanotubes shows a significant blue-shift of the absorption band edge compared with CdS nanoparticles deposited on the exterior surface of TNTs. The titanate
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