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Stable hydrogen generation from vermiculite sensitized by CdS quantum dot photocatalytic splitting of water under visible-light irradiation Jian Zhang,a Wenfeng Zhua and Xiaoheng Liu*a,b CdS quantum dot/vermiculite (CdS/VMT) nanocomposites have been synthesized via a facile one-step method and characterized by X-ray diffraction, UV-vis diffuse reflection spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy. The photocatalytic hydrogen generation activities of these samples were evaluated using Na2S and Na2SO3 as sacrificial reagents in water under visible-light illumination (λ ≥ 420 nm). The most important aspect of this work is the use of natural products (VMT) as host photocatalysts. The effect of CdS content on the rate of visible light photocatalytic hydrogen generation was investigated for different CdS loadings. The synergistic effect of VMT and CdS quantum dots (QDs) leads to efficient separation of the photogenerated charge carriers and, con-

Received 26th March 2014, Accepted 10th April 2014 DOI: 10.1039/c4dt00897a www.rsc.org/dalton

sequently, enhances the visible light photocatalytic hydrogen production activity of the photocatalyst. The CdS/VMT composite with an optimal ratio of 5% exhibits the highest hydrogen evolution rate of 92 μmol h−1 under visible light irradiation and the highest apparent quantum efficiency of 17.7% at 420 nm. A possible photocatalytic mechanism of the CdS/VMT nanocomposite is proposed and corroborated by photoelectrochemical curves.

Introduction Using solar photons to drive fuel-generating reactions, such as splitting water for hydrogen generation, will allow storage of solar energy necessary for on-demand availability.1–3 Photocatalytic water splitting is one possible means of generating hydrogen in an environmentally friendly manner because it requires only semiconductors, water, and sunlight. The creation of simple and efficient photocatalysts utilizing visible light (∼43% of the solar spectrum) as opposed to utilizing UV light (∼4% of the solar spectrum) is of great importance for practical applications.4–6 Hence, the development of visiblelight responsive photocatalysts has attracted tremendous attention due to their broad application in water splitting. Sensitizers, such as dyes,7,8 noble nanoparticles,9–11 and narrow band gap semiconductors,12–15 have been utilized to sensitize wide band gap semiconductors, providing an efficient approach for producing hybrid visible-light active photocatalysts. In such hybrid catalyst systems, the catalyst-decorated semiconductor nanocrystals generate hydrogen via light

a Key Laboratory of Soft Chemistry and Functional Materials, Nanjing University of Science and Technology, Ministry of Education, Nanjing 210094, China. E-mail: [email protected]; Fax: +86-25-84315943; Tel: +86-25-84315943 b Key Laboratory of Jiangsu Province for Chemical Pollution Control and Resources Reuse, Nanjing University of Science and Technology, Nanjing 210094, China

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absorption followed by photoelectron transfer to the catalyst in the presence of a sacrificial hole scavenger.16 Semiconductor quantum dots (QDs), such as CdS,17 CdSe,18 CdTe,19 Ag2S,20 PbS,21 and CuInS2,22 have been attached to photocatalysts to improve their photoactivity in the visible spectrum. Among various candidates, CdS, as a narrow direct bandgap semiconductor (2.4 eV), has been regarded as a promising semiconductor because of its appropriate bandgap, sufficiently negative conduction band edge, and easy fabrication.23,24 Hoffmann et al.25 reported a CdS QD-sensitized TiO2/ Pt photocatalyst with a hydrogen generation rate of 0.8 mmol h−1 g−1 under visible-light illumination. Sang et al.26 developed a CdS/TiO2 heterojunction structure using CdS QDs as sensitizers, thereby enhancing the photoactivity of TiO2 nanotube photoelectrodes. Dutta et al.27 prepared a Pt activated CdS/TiO2 that produces hydrogen at a high quantum efficiency (QE) from a simulated solar spectrum. Indeed, so far there have been many reports related to different approaches to water splitting, but to the best of our knowledge, there are hardly any reports on the use of natural products for hydrogen evolution via photocatalytic water splitting. Recently, our group28 discovered that CdS-sensitized attapulgite (a natural 1D silicate mineral) as a photocatalyst exhibited very high light harvesting efficiency and excellent photocatalytic activity for hydrogen evolution under visiblelight irradiation. Since the discovery of graphene, intensive

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efforts have been directed to the synthesis of ultrathin 2D nanomaterials including layered and non-layered compounds because they show unusual chemical, electronic, and physical properties compared with the corresponding bulk counterparts.29–31 Inspired by the work above, we extended the host photocatalyst from 1D natural nanomaterials to 2D natural nanomaterials. In this study, we report for the first time a novel and simple route to synthesis of CdS/vermiculite32,33 nanocomposites with controlled coverage density. The visible light photocatalytic tests show that the present CdS/vermiculite possesses excellent photocatalytic activity for water splitting (QE: 17.7%) under visible light irradiation, much higher than those of CdS/attapulgite samples (QE: 8.2%).28 Our study provides new insights that vermiculite nanosheets function as good supports to develop high performance vermiculite-based photocatalysts and facilitate their practical applications in energy conversion.

Experimental Sample preparation The reagents were of analytical grade from the Shanghai Chemical Factory of China and were used without purification. VMT was obtained from Yili of Xinjiang Province in China. The CdS–VMT hybrid nanocomposites were prepared as follows: the Na2S·9H2O solution was added slowly into an aqueous solution containing Cd(Ac)2·2H2O and a pre-determined amount of VMT under vigorous stirring. The weight ratios of CdS to VMT were fixed at the following values: 1 wt%, 5 wt%, and 10 wt%, and the obtained samples were labeled as 1%CdS/VMT, 5%CdS/VMT, and 10%CdS/VMT respectively. After reaction for 2 h at room temperature, the CdS/VMT hybrid nanocomposites were obtained by washing with ethanol, filtration, and drying at 60 °C for 12 h. The CdS/VMT nanocomposites were finally produced after thermal treatment at 300 °C for 2 h under a N2 protective atmosphere. The working electrodes were prepared as follows: 5 mg of the sample was suspended in 1 ml ethanol under ultrasonic treatment to make a slurry. The slurry was then coated onto a 2.0 cm × 1.0 cm F-doped SnO2-coated glass substrate by the doctor blade technique. Next, the resulting electrodes were dried in an oven and calcined at 250 °C for 30 min.

Paper

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. Photocatalytic tests The photocatalytic hydrogen production experiments were performed in a Pyrex round-bottom flask. A 300 W xenon arc lamp (XL-300, Yirda Co. Ltd) with a UV-cutoff filter (≥420 nm) was used as a visible light source to trigger the photocatalytic reaction. In a typical photocatalytic experiment, 100 mg of the as-prepared CdS–VMT nanocomposites were dispersed in a mixed solution of Na2S (0.1 mol L−1), Na2SO3 (0.04 mol L−1) and water (190 mL). Prior to irradiation, the system was vacuumed for 30 min to remove the dissolved air under constant stirring. The generated hydrogen was analyzed using a gas chromatograph (GC-1690, Jiedao, TCD, argon as a carrier gas). The apparent quantum efficiency (QE) was measured under the same photocatalytic reaction conditions. A model 1916-R Newport power meter was used for the intensity measurement of light and the irradiation area was controlled as 1 cm2. The QE was calculated according to eqn (1): number of reacted electrons  100 number of incident photons number of evolved H2 molecules  2  100 ¼ number of incident photons

QE½% ¼

ð1Þ

Photoelectrochemical measurements The photoelectrochemical performance of the electrodes was evaluated in a three-electrode configuration using a potentiostat (CHI-610B, Shanghai Chenhua, China), a Pt wire as the counter electrode, and Ag/AgCl (saturated KCl) as a reference electrode. The films were irradiated with a 300 W Xe lamp solar. The light intensity was tuned to 200 mW cm−1 as measured by a power detector (Newport, 1916-R) and the electrolyte was a mixed solution of 0.1 M Na2S and 0.02 M Na2SO3. The surface area coated sample is 1 × 1 cm2.

Results and discussion Morphology and phase structures

Sample characterization The obtained samples were characterized by XRD on a Bruker D8 Advance X-ray powder diffractometer with Cu Kα radiation (λ = 1.541 Å). UV-vis absorption spectra were obtained using a Ruili UV-1201 spectrophotometer. 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. 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

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The transmission electron microscopy (TEM) image of the bare VMT displayed in Fig. 1a clearly shows a 2D sheet-like structure. Close examination of TEM (Fig. 1b–d) reveals that CdS nanoparticles were absorbed on the surface of VMT sheets after the assembly process, and almost no free CdS nanoparticles were found outside of VMT sheets. As the ratio of CdS to VMT nanosheets increased from 1% to 5%, the density of CdS nanoparticles on the surface of VMT nanosheets increased (Fig. 1b and 1c). Further increase of CdS QDs to VMT nanosheets ratio from 5% to 10% resulted in a similar dense packing of CdS as shown in Fig. 1c and 1d. Moreover, more CdS (10%) nanoparticles lead to agglomeration because of the

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Fig. 2 XRD patterns of samples CdS, VMT, and CdS/VMT composites using different amounts of CdS: 1%CdS/VMT, 5%CdS/VMT, and 10%CdS/ VMT.

ditions, corresponding to the diffractions of the (100), (002) and (110) planes of CdS (JCPDS 41-1049), were observed in the composites with increasing the amount of CdS nanocrystals above 5%, as shown in Fig. 2, which is consistent with the previous report.36 The XRD result further confirms that CdS QDs were deposited onto the surface of VMT nanosheets. UV-vis diffuse reflectance spectra Fig. 1 TEM of (a) bare VMT, and (b–d) CdS/VMT prepared with the CdS volume of 1%, 5%, and 10%, respectively; (e) high-resolution TEM image of 5%CdS/VMT and (f ) EDX elemental analysis of the selected area shown by the red square in (c).

absence of a surfactant, as shown in Fig. 1d. The HRTEM image (Fig. 1e) shows that highly crystalline CdS nanoparticles are randomly distributed on the surface of VMT. A well-defined crystalline lattice can be identified with a d spacing of 0.34 nm corresponding to the (002) plane of CdS.34 In order to better understand the chemical composition of the prepared composite, the energy dispersive X-ray (EDX) spectrum of CdS/VMT was recorded. For the red-squared areas in Fig. 1c, peaks associated with O, Fe, Mg, Al, Si, S, and Cd were observed in the EDX spectra, as shown in Fig. 1f, which confirms that the VMT is decorated by CdS nanocrystals. Fig. 2 shows a comparison of XRD patterns of samples CdS, VMT, and CdS/VMT nanocomposites with different CdS QDs addition levels. The weak peak at 2θ = 6.2° probably corresponds to packets of VMT platelets. The intense broad band at 2θ = 7.5° is interpreted as the interstratification of the VMT platelets. We cannot find structure information of the standard VMT in the Bruker XRD database, but it is quite similar to previous reports.35 Several additional peaks at 2θ of 24.7, 26.4 and 43.6° for CdS/VMT nanocomposites with different ratios of CdS QDs to VMT under the same experimental con-

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The UV-vis absorption spectrum of VMT shows strong semiconductor-like absorption in the UV region as well as intrinsic and wide absorption in the visible region. After comparison, we find a red-shift for the CdS/VMT samples. This indicates that there is a decrease in the energy band gap of these samples. The direct band gap value of the VMT sample was estimated from the (αhν)2 versus photon energy (hν) plot as shown in the insert of Fig. 3. The extrapolation of the linear

Fig. 3 UV-vis absorption spectra of VMT, 1%CdS/VMT, 5%CdS/VMT, and 10%CdS/VMT. The inset is band gap evaluation of VMT from the plots of (αhν)2 versus photon energy (hν).

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Fig. 4 Nitrogen adsorption–desorption isotherms and the corresponding pore-size distribution curves (inset) for the VMT and 5%CdS/ VMT.

regions in the Tauc plot suggests one-regime optical absorption by VMT (marked with a dotted line), where the associated direct optical band gap is about 3.5 eV.28 BET surface area and pore size analysis Fig. 4 shows the nitrogen adsorption–desorption isotherms and the corresponding pore size distribution curves (inset in Fig. 4) for the VMT and 5%CdS/VMT samples. The isotherms are of type IV with a narrow hysteresis loop at high relative pressures, characteristic of mesopore (mesopores have widths between 2 and 50 nm) and macropore (>50 nm) features.37 These isotherms exhibit H3 hysteresis loops reflecting mesopores formed due to the deposition of the CdS.38 The above results are consistent with the TEM analysis (Fig. 1). The BET surface area of 5%CdS/VMT was found to be 20.13 m2 g−1, which is much higher than the value for the VMT sample (9.88 m2 g−1). A photocatalyst with a larger specific surface area can accommodate more surface active sites and facilitate the transport of charge carriers, which is beneficial for enhancing the photocatalytic performance.39 XPS analysis The surface element composition of the as-prepared CdSsensitized VMT and the chemical status of the Cd element in the sample were analyzed by XPS. Fig. 5a presents a comparison of the XPS spectra of the VMT and 5%CdS/VMT samples. For the VMT, the Mg, Al, Si, O, K, and C elements are observed and the corresponding photoelectron peaks appear at binding energies of 73.41 (Mg 2s), 87.82 (Al 2p), 101.15 (Si 2p), 530.82 (O 1s), 305.23 (K 2p), and 258.20 (C 1s), respectively. The photoelectron peak of the C element is due to the adventitious hydrocarbon from the XPS instrument itself. Further comparison indicates that the 5%CdS/VMT sample contains not only Mg, Al, Si, O, K, and C, but also Cd and S elements with binding energies at 405.25 (Cd 3d) and 162.12 (S 2p), respectively.40 The atomic ratio of Cd : S is ca. 1, confirming the

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Fig. 5 XPS survey spectra (a) of samples VMT and 5%CdS/VMT and high-resolution XPS spectrum (b) of Cd 3d for sample 5%CdS/VMT.

formation of CdS. Fig. 5b shows the high-resolution XPS spectrum of Cd3d for 5%CdS/VMT. The binding energies corresponding to Cd3d5/2 and Cd3d3/2 are 405.3 and 412.2 eV, respectively, indicative of +2 Cd in CdS. The 6.9 eV difference between the binding energies of Cd 3d5/2 and Cd 3d3/2 peaks is also characteristic of +2 Cd 3d states.40 Moreover, the binding energies corresponding to S 2p3/2 and S 2p1/2 are 161.2 and 162.6 eV, respectively, indicating the presence of −2 S in CdS. The XPS results are consistent with the TEM and XRD results. Photocatalytic activity The visible-light photocatalytic hydrogen generation activity of VMT, 1%CdS/VMT, 5%CdS/VMT, and 10%CdS/VMT is compared in Fig. 6. Obviously, CdS QDs deposition has a significant influence on the photocatalytic activity of VMT. In the absence of CdS QDs, VMT shows a negligible photocatalytic activity because its band gap is too large (3.5 eV) to absorb the visible light. Pure CdS also shows a low activity due to the rapid recombination of photogenerated electrons and holes. In general, under visible-light irradiation, the VB electrons of CdS are excited to CB, creating holes in the VB. In the absence of VMT, these charge carriers quickly recombine and only a very

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Fig. 6 Comparison of the photocatalytic hydrogen generation rate of VMT, CdS, 1%CdS/VMT, 5%CdS/VMT, and 10%CdS/VMT samples under visible light. Fig. 8 (a) Digital pictures of bare VMT, 1%CdS/VMT, 5%CdS/VMT and 10%CdS/VMT on FTO/glass. (b) SEM image of the selected area shown by the red square in (a). (c) Schematic diagram of the photoelectrochemical water splitting process with the CdS/VMT hybrid photoanode.

Fig. 7 A typical time course of hydrogen generation over 5%CdS/VMT sample, eight runs is one continuous reaction.

small fraction of the electrons and holes is involved in the photocatalytic reaction process. In this work, though a small amount of CdS QDs was deposited, the hydrogen generation activity of VMT is significantly enhanced (Fig. 6) because photogenerated electrons transfer from the CB of CdS into VMT and accumulate at the lower-level CB of VMT to generate hydrogen. In particular, the 5%CdS/VMT sample shows the highest photocatalytic activity; its hydrogen generation rate is as high as 92 μmol h−1 (which corresponds to a QE of 17.7% at 420 nm), and the rate exceeds that of pure CdS by more than 10 times. Time-course hydrogen generation over the 5%CdS/VMT sample was conducted for 72 h with the evacuation every 8 h and the corresponding curve is illustrated in Fig. 7. Remarkably, the photocatalytic hydrogen generation reaction on the as-prepared 5%CdS/VMT proceeds without any noticeable decrease in the activity over 72 h. This observed improvement on the photocatalytic activity and such excellent stability of CdS/VMT should be related not only to the fast separation of

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the photo-induced electron/hole pairs on the CdS QDs, but also to the unique paper-like structure of the supporting silicate materials. To demonstrate the advantages of the CdS/VMT hybrid in photoelectrochemical applications, we compared CdS/VMT films with different amounts of CdS QDs. As shown in Fig. 8(a), the color of the CdS/VMT composite films depends on the amounts of CdS QDs. The bare VMT film is light browncolored, suggesting that it can absorb visible light. When combined with CdS QDs, it changes from light brown to yellowish (1%CdS/VMT) and finally to orange-yellow (10%CdS/VMT). 5% CdS/VMT film exhibits a multilayer structure, with a thickness of 10 μm by the doctor blade technique, as shown in Fig. 8(b). Fig. 8(c) illustrates the relevant processes at the photoanode of the photoelectrochemical cell leading to hydrogen generation. When hole scavengers are present in the aqueous solution (Na2S and Na2SO3), the photogenerated hole in the CdS absorber is quickly transferred to the solution and the electron can be transported toward VMT. Vectorial electronic transport is expected for a nanosheet structure as shown in the cartoon. Then, the electron is driven toward the catalytic cathode by the external circuit, where the hydrogen evolution reaction takes place. Fig. 9 shows a set of linear-sweep current–potential plots for the films acting as photoanodes with chopped solar light. Here, a mixed solution of 0.1 M Na2S and 0.02 M Na2SO3 was used as an electrolyte to decrease the O2 overpotential for photoelectrochemical water splitting.41 Upon illumination, the bare VMT film shows an observable photocurrent in the range of several μA cm−2 at 1.45 V which continues to increase to 0.08 mA cm−2 at 1.95 V. In comparison with the bare VMT film, the CdS/VMT films show much higher photocurrents at 1.90 V (Fig. 9). The 5%CdS/VMT film reaches a photocurrent

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Conclusions

Fig. 9 Light chopped current–potential plots for bare and CdS QDs loaded VMT films.

density of 0.4 mA cm−2 at 1.90 V, which is 5 times higher than that of the bare VMT film.

Discussion of the photocatalytic mechanism On the basis of the above results, a proposed mechanism for the photocatalytic activity of the CdS/VMT composites is shown schematically in Fig. 10. Under visible-light irradiation, the photogenerated electrons are excited from the VB to the CB of CdS QDs, creating positive holes in the VB of CdS QDs. Compared to bulk CdS, the band gaps of CdS QDs increase, and the redox potentials of CdS QDs change accordingly.42 Fig. 10 shows the redox potentials of VMT33 and CdS QDs43 in relation to the normal hydrogen electrode potential (NHE). The energy levels of the CB and VB edges assure that CdS QDs act as sensitizers, which makes VMT highly active under visible light.

Fig. 10 Schematic illustration for the charge transfer and separation in CdS QDs-sensitized VMT under visible light.

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In conclusion, the CdS QDs-sensitized vermiculite (VMT) was found to be a highly efficient and low-cost for photocatalytic hydrogen generation from an aqueous solution containing Na2S and Na2SO3 under visible light without a noble metal as a co-catalyst. It was found that this nanocomposite had the ability to photocatalytically split water into hydrogen without noticeable degradation over 70 h. According to our discussion, the enhanced photocatalytic activity and intensified photostability may be ascribed to the synergic effect between VMT and CdS QDs originating from their well-matched overlapping band structures, which can effectively accelerate the charge separation to robust VMT. Although the QE of the CdS/VMT photocatalyst is still not very high (17.7%, 420 nm), this investigation opens up new possibilities for the development of highly efficient VMT-based photocatalysts that utilize visible light as an energy source.

Acknowledgements This project is supported financially by the Natural Science Foundation of China (no. 51272107), the Natural Science Foundation of Jiangsu Province, China (no. BK2011024, BK2012035), a project funded by the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions and the Specialized Research Fund for the Doctoral Program of Higher Education, China (20133219110015).

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Stable hydrogen generation from vermiculite sensitized by CdS quantum dot photocatalytic splitting of water under visible-light irradiation.

CdS quantum dot/vermiculite (CdS/VMT) nanocomposites have been synthesized via a facile one-step method and characterized by X-ray diffraction, UV-vis...
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