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Assembly-promoted photocatalysis: Three-dimensional assembly of CdSxSe1−x (x = 0–1) quantum dots into nanospheres with enhanced photocatalytic performance
J Materiomics xxx (2017) 1e8
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Assembly-promoted photocatalysis: Three-dimensional assembly of CdSxSe1x (x ¼ 0e1) quantum dots into nanospheres with enhanced photocatalytic performance Lina Wang, Qiumei Di, Mingming Sun, Jia Liu**, Chuanbao Cao, Jiajia Liu, Meng Xu, Jiatao Zhang* Beijing Key Laboratory of Construction Tailorable Advanced Functional Materials and Green Applications, School of Materials Science and Engineering, Beijing Institute of Technology, Beijing, 100081, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 October 2016 Received in revised form 16 November 2016 Accepted 30 November 2016 Available online xxx
In this paper, through an emulsion-based bottom-up self-assembly method, monodisperse CdSxSe1x (x ¼ 0e1) quantum dots (QDs) with tailoring compositions have been three-dimensionally assembled into spherical architectures in sub-micrometer sizes. UVeVis absorption measurements revealed the enhanced light harvesting abilities of the assembled CdSxSe1x spheres relative to their constituting QDs. HRTEM characterizations over the CdSxSe1x assemblies suggested the existence of localized oriented adjoining of the CdSxSe1x QDs and the resulting nano-twin structures that are favorable for photogenerated electron-hole separation. The quenching of photoluminescence and the improvement in IPCE after the assembly of CdSxSe1x QDs provided a clue to the likely suppressed electron-hole recombination brought about by the unique architectures and interfaces derived from self-assembly. The above findings were coincided with the remarkably improved H2 evolution activities observed for the wellassembled CdSxSe1x nanospheres in photocatalytic water splitting, underpinning the importance of the alternative strategy to design advanced semiconductor photocatalysts based on architectural engineering. © 2017 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: CdSxSe1x quantum dots Three-dimensionally assembly Colloidal nanospheres Photocatalysis
1. Introduction Converting solar energy into renewable hydrogen fuel via photocatalytic water reduction over semiconductor photocatalysts has attracted considerable research attentions due to its great promise in addressing the growing energy demand worldwide [1e11]. IIeVI semiconductors are one of the promising catalysts for solar-driven hydrogen generation from water since many of them possess a bandgap that allows absorption in the visible region of the spectrum [7]. However, these catalyst materials usually show low H2 evolution activities owing to the insufficient light harvesting capability, and the easy recombination of photo-induced electrohole pairs before they migrate to the semiconductor surface where water splitting reaction takes place [12,13]. Therefore, enhancing
* Corresponding author. ** Corresponding author. E-mail address: [email protected] (J. Zhang). Peer review under responsibility of The Chinese Ceramic Society.
light harvesting and promoting photoexcited charge separation are still key challenges for development of efficient semiconductor photocatalysts [14e18]. Recently, self-assembly of semiconductor nanocrystals into hierarchical structures has been recognized as an alternative way to improve photocatalysis efficiency in consideration of the improved light absorption capacity [17e22]. For examples, through a selfassembly growth mechanism in the presence of ionic liquid molecule, BiOI hollow microsperes have been fabricated which displayed evidently enhanced photocatalytic activities than the BiOI nanoplates [17]. Yin et al. have synthesized mesoporous submicron clusters of anatase TiO2 based on the self-assembly of TiO2 nanocrystals (NCs) with the assistance of a silica template. The scattering and multiple reflecting of the light within the formed mesoporous TiO2 clusters was proposed to be a crucial reason accounting for their improved photocatalytic performance in decomposition of organic dyes under UV irradiation [18]. Meanwhile, Wu and coworkers have reported the light-triggered organization of colloidal semiconductor NCs into hollow-structured
http://dx.doi.org/10.1016/j.jmat.2016.11.008 2352-8478/© 2017 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: Wang L, et al., Assembly-promoted photocatalysis: Three-dimensional assembly of CdSxSe1x (x ¼ 0e1) quantum dots into nanospheres with enhanced photocatalytic performance, J Materiomics (2017), http://dx.doi.org/10.1016/j.jmat.2016.11.008
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nanospheres via the use of bubble templates generated in situ through photocatalytic water splitting reaction [20,21]. In addition to these specially defined synthetic systems, Li group has established a general emulsion-based bottom-up assembly approach to assemble various NCs building blocks, e.g. CdS, PbS, ZrO2 and NaYF4 NCs, into three-dimensional (3D) colloidal spheres [23]. This method inspired us to construct new types of integrated architectures of semiconductor QDs, particularly CdSxSe1x (x ¼ 0e1) QDs, towards creation of advanced photocatalyst materials. On the other hand, Guo and coworkers have reported that Cd1xZnxS solid solution NCs with nano-twin structures can exhibit superior photocatalytic activities for H2 evolution from water under visible light irradiation [14e16]. They unambiguously demonstrated that the formation of homojunctions derived from the parallel twining planes in the Cd1xZnxS crystals significantly boosted the separation of photogenerated electron-hole pairs and contributed to prolonged lifetime of the charge carriers. Encouraged by their interesting findings, here the crystal structure of the CdSxSe1x building blocks has been carefully engineered in the hope of generating twin-induced homojunctions for efficient charge separation. In this work, sub-micrometer sized CdSxSe1x (x ¼ 0e1) spheres have been synthesized from the bottom-up self-assembly of monodisperse CdSxSe1x QDs. The resulting CdSxSe1x nanospheres exhibited stronger light absorption and enhanced incident photonto-electron conversion efficiency (IPCE) in the visible regime than those of the CdSxSe1x QDs. Moreover, the assembled CdSxSe1x spheres afforded notably higher photocatalytic efficiency for H2 evolution from water under visible light irradiation relative to the unassembled CdSxSe1x QDs. It was conjectured that the synergy of the improved light absorbance and the formed nano-twin structures can make a contribution to the superior catalytic performance of CdSxSe1x spheres. Besides that, the CdSxSe1x spheres displayed good dispersity and stability in aqueous solutions and can be easily scaled up, which are also advantageous features in pursuit of novel structures for efficient photocatalytic applications. 1.1. Raw materials Octadecylene (chromatographically pure), Shanghai Jingchun Reagent Co., Ltd, Shanghai, China. Oleic acid (analytically pure), Shanghai Jingchun Reagent Co., Ltd, Shanghai, China. Chromic oxide (CdO, analytically pure), Shanghai Jinshantingxin Chemical Reagent Co., Ltd, Shanghai, China. Toluene (analytically pure), Beijing Chemical Works, Beijing, China. Cyclohexane (analytically pure), Beijing Chemical Works, Beijing, China. Cetyltrimethylammonium bromide (CTAB, analytically pure), Tianjin Guangfu Fine Chemical Research Institute, Tianjin, China. Sulfur powder (chemically pure), Tianjin Fuchen Chemical Reagent Co., Ltd, Tianjin, China. Selenide powder (analytically pure), Tianjin Guangfu Fine Chemical Research Institute, Tianjin, China.
QDs from non-polar solvents into water according to our previous research [12]. 1.3. Self-assembly of CdSxSe1x QDs into nanospheres CdSxSe1x QDs dispersed in toluene (3 mL) were precipitated by adding ethanol and were re-dispersed in cyclohexane (1 mL). In a general preparation of water-dispersed 3D colloidal spheres from the oil-dispersed CdSxSe1x QDs via the bottom-up assembly approach [23], first an aqueous solution of CTAB was prepared by dissolving 35 mg of CTAB into 10 mL of deionized water. The above solution (1 mL) containing CdSxSe1x QDs in cyclohexane was added to the CTAB aqueous solution. The obtained mixture was emulsified by vigorous stirring and was kept at 80 C with constant stirring for 1 h, to allow the self-assembly of the CdSxSe1x QDs into 3D spheres. After purified by centrifugation, the final products were re-dispersed in water (10 mL). 1.4. Characterization A JEOL JEM 1200EX transmission electron microscope (TEM) working at 100 kV was used to characterize the morphologies of the CdSxSe1x QDs and their assembly spheres. Samples dispersed in toluene were casted onto a carbon-coated copper grid prior to the measurement. A FEI Tecnai G2 F20 S-Twin high-resolution TEM (HRTEM) working at 200 kV were used to determine the morphologies and crystal lattice details of the samples, and to perform the energy dispersive spectra (EDS) elemental mapping and line scan elemental analysis. The UVeViseNIR absorption spectra of the as-prepared samples were recorded on a Shimadzu UV3600 spectrophotometer at room temperature (RT). The steady state luminescence spectra of colloidal samples were collected on a FluoroMax-4 spectrophotometer (HORIBAJOBINYVON USA) at RT. 1.5. Incident photon-to-electron conversion efficiency measurements The incident photon-to-electron conversion efficiency (IPCE) studies on CdSxSe1x QDs and the nanospheres assembled from the CdSxSe1x QDs were carried out on a Kit (Crowntech QTest Station 1000AD) with a tungsten halogen lamp (CT-TH-150), a calibrated silicon diode and a monochromator (Crowntech QEM24-S 1/4 m) at room temperature. The light power density was calibrated against a Si solar cell (Hamamatsu S1133) to accurately simulate the light to 100 mW cm2. All incident photon-to-electron conversion efficiency performances were carried out in a two-electrode system equipped with platinum electrode as counter. The working electrode was made by dropping samples on ITO glass and sintered at 400 C for 3 h under the atmosphere of N2. 0.1 M Na2SO4 solution was used as the electrolyte. 1.6. Photocatalytic H2 production measurements
1.2. Preparation of monodisperse CdSxSe1x (x ¼ 0e1) QDs CdSxSe1x (x ¼ 0e1) QDs were prepared by the hot-injection method [24]. Typically, 5 mL of octadecylene, 6 mL of oleic acid, and 64 mg of CdO were mixed by magnetic stirring. The resulting mixture was heated up to 170 C under the atmosphere of nitrogen, then a certain quantity of S or Se organic precursors in a desirable ratio were injected into the solution. Afterwards the reaction temperature was reset to 290 C and maintained for half an hour under magnetic stirring. The S precursor and the Se precursor were prepared and the transfer of as-prepared hydrophobic CdSxSe1x
The photocatalytic H2 production test on CdSxSe1x QDs and the colloidal spheres assembled from the oil-dispersed CdSxSe1x QDs was conducted by using a sealed circulation online analysis system (Beijing Aulight Technology Co. Ltd., China) combined with a gas chromatograph (SP7800, Beijing KeRuida Co. Ltd.). In a typical run, 0.002 g of sample was suspended in 100 mL of aqueous solution (containing 0.5 mol L1 K2SO3 and 0.5 mol L1 Na2S) under magnetic stirring. Then the sealed system was vacuumed to reach to 0.1 MPa. A 300 W Xe lamp with a 400 nm cut-off filter (CELHXF300/CEL-HXUV300) was applied to irradiate the mixture. The
Please cite this article in press as: Wang L, et al., Assembly-promoted photocatalysis: Three-dimensional assembly of CdSxSe1x (x ¼ 0e1) quantum dots into nanospheres with enhanced photocatalytic performance, J Materiomics (2017), http://dx.doi.org/10.1016/j.jmat.2016.11.008
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amount of H2 gas was determined by online gas chromatography equipped with a thermal conductivity detector. All tests were conducted at room temperature.
2. Results and discussion 2.1. Characterization of the CdSxSe1x (x ¼ 0e1) QDs and their assembled spheres Monodisperse CdSxSe1x (x ¼ 0e1) QDs were prepared by high temperature organic phase method. The morphologies of the synthesized samples were illustrated by transmission electron microscopy (TEM) images as shown in Fig. 1. The CdSxSe1x QDs obtained here showed good monodispersity with an average diameter of 5 nm independent of the S to Se molar ratios (x varied at 1, 0.86, 0.5, and 0 from Fig. 1A to D). The high monodispersity of the CdSxSe1x QDs was very beneficial for their subsequent self-assembly to form 3D structures through the bottum-up approach. Fig. 2 shows the high-resolution transmission electron microscopy (HRTEM) (2A2D) and scanning transmission electron microscopy (STEM) (2E, 2F) images of the typical samples of 3D colloidal spheres that were assembled from CdSe QDs. From Fig. 2, the dimensions of the CdSe spheres were estimated to be in the range of 50e80 nm. It is easily seen that the CdSe QDs building blocks did not retain their individual characters but rather adjoined together into larger units.
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Looking into the interfaces between two neighboring QDs, one can see that the two QDs either grew together by sharing the same lattices as exhibited in Fig. 2B and Fig. S1, or grew into twinned nanocrystals as suggested by Fig. 2D and Fig. S2. The adjoining of QDs to form an integral architecture as formed in our case can facilitate rapid charge transfer and reduce the recombination rates of electron-hole pairs [14,25e27]. Fig. 3 illustrates the TEM images of assembled CdSxSe1x spheres with verified compositions. Decreasing the S to Se molar ratios, the morphologies of assembled products from QDs did not show notable changes. This may be ascribed to the good retaining of the diameter and morphologies of CdSxSe1x QDs when altering the S to Se molar ratios as aforementioned. Fig. 4 provides the schematic illustration of the self-assembly processes of CdSxSe1x QDs. Cyclohexane solution containing monodisperse CdSxSe1x QDs, cetyltrimethylammonium bromide (CTAB) and water were mixed together at an appropriate oil-towater ratio (1:10 by volume). Under vigorous stirring, a stable oilin-water (O/W) microemulsion system was formed, where the CdSxSe1x QDs were confined in the oil microemulsion droplets stabilized by the CTAB molecules. Thereafter, cyclohexane (a lowboiling solvent) was evaporated from the system by heating at 80 C. During this process, the oil droplets gradually shrank and the QDs in the droplets became concentrated and got assembled in the 3D confined oil-emulsion droplet spaces. Through continuous
Fig. 1. TEM images of as-prepared CdSxSe1x QDs: a) CdS QDs; b) CdS0.86Se0.14 QDs; c) CdS0.5Se0.5 QDs; D) CdSe QDs. scale bar ¼ 100 nm.
Please cite this article in press as: Wang L, et al., Assembly-promoted photocatalysis: Three-dimensional assembly of CdSxSe1x (x ¼ 0e1) quantum dots into nanospheres with enhanced photocatalytic performance, J Materiomics (2017), http://dx.doi.org/10.1016/j.jmat.2016.11.008
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Fig. 2. Assembled colloidal spheres by CdSe QDs: A and C) HRTEM images of the assembled colloidal spheres of CdSe quantum dots; B) Amplification of the circled part in Fig. 2A; the inset is the image intensity analysis by Digital Micrograph software which reflects the lattice structures of the sample; D) Amplification of the circled part in Fig. 2C; the twinned nanoparticles surfaces between nanoparticles are marked with white dash lines; E) and F) STEM images of assembled colloidal spheres of CdSe QDs at different magnifications; scale bar ¼ 500 nm and 50 nm for Fig. 2E and F, respectively.
evaporation of cyclohexane, the QDs finally stuck together to form 3D colloidal spheres, and this might result in the formation of some twin-containing structures nearby the interface of two neighboring CdSxSe1x QDs. According to previous research [28e30], the hydrophobic interactions between the alkane chains of CTAB molecules can guarantee the high stability of the colloidal assembled architectures while the hydrophilic moieties of CTAB accounted for their good dispersity in aqueous solution. Based on these advantageous, we further investigated the potential of the formed
CdSxSe1x QDs colloidal spheres as photocatalysts for catalyzing hydrogen evolution reaction from water. Primarily, the light absorption abilities of the CdSxSe1x QDs assemblies were studied by UVeVis spectroscopy. Fig. 5 and Fig. S3 present the UVeVis absorption spectra of the CdSxSe1x QDs and the assembled CdSxSe1x nanospheres normalized at 400 nm. It can be seen that the absorbance was all enhanced with a red shift in the energy for the assembled CdSxSe1x spheres compared to the isolated CdSxSe1x QDs regardless of S to Se molar ratios. These results
Please cite this article in press as: Wang L, et al., Assembly-promoted photocatalysis: Three-dimensional assembly of CdSxSe1x (x ¼ 0e1) quantum dots into nanospheres with enhanced photocatalytic performance, J Materiomics (2017), http://dx.doi.org/10.1016/j.jmat.2016.11.008
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Fig. 3. TEM images of assembled colloidal spheres of CdSxSe1x QDs: A) CdS QDs; B) CdS0.86Se0.14 QDs; C) CdS0.5Se0.5 QDs. Scale bar ¼ 200 nm.
Fig. 4. Schematic illustration of CdSxSe1x (x ¼ 0e1) spheres assembled from monodisperse CdSxSe1x (x ¼ 0e1) QDs via an emulsion-based bottom-up self-assembly approach.
are similar to the previous observations acquired from the assembly of CdSe and CdTe QDs [19,21], indicating the promoted light harvesting ability of the semiconductor QDs assemblies relative to their individual constituents [26]. The band gap for bulk CdSxSe1x
should be between that of CdSe (1.7 eV) [27] and CdS (2.42 eV) [31,32]. The increase in the content of Se could cause a drop in the apparent band gap of CdSxSe1x QDs and the redshift of the absorption spectrum [12]. This is consistent with our findings
Fig. 5. UVeVis absorption spectra comparison of as-formed CdSxSe1x nanospheres and the CdSxSe1x QDs with varied S to Se molar ratios.
Please cite this article in press as: Wang L, et al., Assembly-promoted photocatalysis: Three-dimensional assembly of CdSxSe1x (x ¼ 0e1) quantum dots into nanospheres with enhanced photocatalytic performance, J Materiomics (2017), http://dx.doi.org/10.1016/j.jmat.2016.11.008
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positive to the separation of electron-hole pairs and the suppression of their recombination [14e16,33], thus can be exploited as a powerful tool for achieving higher photocatalytic activities in solar hydrogen generation. 2.2. Photocatalytic activity measurements
Fig. 6. Photoluminescence spectra comparison between as-prepared CdSe QDs and assembled CdSe spheres.
exhibited in Fig. 5AeD where the S to Se molar ratios varied at 1:0, 1:0.16, 1:1 and 0:1, correspondingly. Thereafter, CdSe QDs and their assembled nanospheres were employed to investigate the effect of self-assembly on the recombination tendency of photo-induced charge carriers by using photoluminescence spectroscopy. Generally speaking, the photoluminescence can be generated during the recombination of the charge carriers on a semiconductor. As shown in Fig. 6, in comparison with CdSe QDs which displayed a strong luminescence band centered at ~660 nm, a distinct decrease in the luminescence intensity was observed for the assembled CdSe nanospheres. These results were further corroborated by the observations under a UV lamp as provided in Fig. S4, strongly indicating the quenching of the photoluminescence for CdSe nanospheres that were constructed by closely assembled CdSe QDs. The energy transfers from the aggregated smaller QDs nanospheres (larger band gap due to the quantum confinement effect) to the larger ones (smaller band gap) can be a possible mechanism leading to the remarkably decreased luminescence intensity [20]. However, it is reasonable to presume that the reduced emission intensity might partially profit from the inhibited charge carrier recombination due to the presence of twininduced homojunctions in the assembled architectures of the CdSe QDs (Fig. 2D). Such homojunctions have been demonstrated
We further examined the activities of CdSxSe1x QDs (x ¼ 0 and 0.5) and their assembled nanospheres toward photocatalytic hydrogen evolution from water splitting. For these experiments, 2 mg of sample was dispersed in 100 mL of deionized water containing 0.5 M K2SO3 and 0.5 M Na2S as hole scavengers. As shown in Fig. 7, an induction period (about 1 h) at the initial stage of light irradiation was observed, which can be explained by the higher rate of photocatalytic degradation of oleic acid adsorbed on the surface of QDs than photocatalytic water splitting [20,34]. After this self-cleaning process, the amount of H2 evolved from photocatalytic water splitting increased with the irradiation time. It is clear to see that the assembled spheres from CdSxSe1x QDs clearly showed increased H2 production rate than their constituting building blocks at both measured S to Se molar ratios (0:1 and 1:1). Under visible light irradiation, the assembled CdSe spheres (x ¼ 0) provided the highest H2 evolution rate (7.66 mmol h1) among the four samples, which was almost fourfold higher than that of the CdSe QDs (1.98 mmol h1). At the same time, the assembled CdS0.5Se0.5 spheres (x ¼ 0.5) showed a twofold increment in H2 evolution activity (4.16 mmol h1) than the CdS0.5Se0.5 QDs (1.78 mmol h1). Several factors may account for the highest activity afforded by the assembled CdSe nanospheres. Upon irradiation by visible light, the absorption of incident photons leads to the generation of excitons in the CdSxSe1x semiconductor QDs. The evolution of H2 requires the efficient transfer of electrons from the semiconductor QDs to the surface of assembled spheres, accompanied by the fast hole consumption by the scavengers. The increased Se content (CdS0.5Se0.5 vs CdSe) can cause a reduction in the apparent band gap of the semiconductor QDs and a notable redshift of the absorption edge as confirmed by the results presented in Fig. 5C and D. Therefore, the CdSe photocatalysts possessed a higher light harvesting capability relative to the CdS0.5Se0.5 counterparts and can intrigue a larger amount of photogenerated electron-hole pairs under light illumination. When comparing the catalytic activities of the two CdSe QDs samples before and after self-assembly, the notable increment in H2 evolution rate observed for the assembled CdSe nanospheres can be explained by their enhanced light harvesting ability as a
Fig. 7. (A) H2 evolution curves as a function of irradiation time for different CdSxSe1x QDs and assembled spheres photocatalysts in photocatalytic water reduction reaction. (B) Comparison of the H2 evolution activities of different CdSxSe1x QDs and assembled spheres photocatalysts.
Please cite this article in press as: Wang L, et al., Assembly-promoted photocatalysis: Three-dimensional assembly of CdSxSe1x (x ¼ 0e1) quantum dots into nanospheres with enhanced photocatalytic performance, J Materiomics (2017), http://dx.doi.org/10.1016/j.jmat.2016.11.008
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result of self-assembly (Fig. 5D) [18,19]. The generation of the twin-induced homojunctions at the interface between neighboring QDs during the self-assembly process was assumed to be another important reason with regard to the promoted charge separation as suggested by the results in Fig. 6. Accordingly, the much improved photocatalytic performance of the assembled semiconductor QDs can be expected. The incident photon-to-electron conversion efficiency (IPCE) measurements over the assembled CdSxSe1x nanospheres and the as-formed CdSxSe1x QDs (x ¼ 0 and 0.5) were carried out using a two-electrode configuration with Pt wire as the counter electrode. The results provided useful information about the charge generation and separation properties of the catalyst samples as a function of the irradiation wavelength. As shown in Fig. S5, for all samples the IPCE values decreased with the increased irradiation wavelength, parallel to the evolution tendency of their optical absorption abilities as shown in Fig. 5. Furthermore, it is clear to see that at the same irradiation wavelength the assembled nanospheres photoanodes always displayed higher IPCE than their QDs counterparts, and CdSe QDs-assembled nanospheres photoanodes afforded the highest IPCE among the four samples, in a good agreement with their photocatalytic performance as shown in Fig. 7. 3. Conclusion In summary, an emulsion-based bottom-up self-assembly approach was utilized to synthesize well-assembled nanospheres from CdSxSe1x QDs with different S to Se molar ratios. By combining different spectroscopy technologies, we found that the assembly of CdSxSe1x QDs into larger architectures can enhance their light absorption capability in the visible range, and can be in favor of the generation of twinned QDs which have been verified propitious to efficient photogenerated electron-hole separation. The comparison of the photocatalytic H2 evolution activities and IPCE of the CdSxSe1x QDs before and after self-assembly indicated the positive impacts brought about by the assembly and adjoining of the semiconductor QDs on their photocatalysis behaviors. We believe that the findings presented here can be further developed into a useful strategy for designing advanced photocatalyst materials and structures. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 21322105, 51501010, 91323301, 51372025). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jmat.2016.11.008. References [1] Kudo A, Miseki Y. Heterogeneous photocatalyst materials for water splitting. Chem Soc Rev 2009;38:253e78. [2] Zong X, Yan HJ, Wu GP, Ma GJ, F Wen Y, Wang L, et al. Enhancement of photocatalytic H2 evolution on CdS by loading MoS2 as cocatalyst under visible light irradiation. J Am Chem Soc 2008;130:7176e7. [3] Maeda K, Teramura K, Lu DL, Takata T, Saito N, Inoue Y, et al. Photocatalyst releasing hydrogen from water. Nature 2006;440:295. [4] Hisatomi T, Kubota J, Domen K. Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting. Chem Soc Rev 2014;43: 7520e35. [5] Ma Y, Wang Xl, Jia YS, Chen XB, Han HX, Li C. Titanium dioxide-based nanomaterials for photocatalytic fuel generations. Chem Rev 2014;114: 9987e10043.
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L. Wang et al. / J Materiomics xxx (2017) 1e8 Lina Wang was born in Liaoning, China. She received her MSc degree in 2007 from the Kunming University of science and technology, and is now studying at Beijing institute of technology for her PhD. Her research interest includes synthesis and assembly of semiconductor nanocrystals.
Jiatao Zhang was born in 1975. He received his PhD degree in 2006 from the Department of Chemistry, Tsinghua University, China. Currently he is Xu Teli Professor in the School of Materials and Engineering, Beijing Institute of Technology. He was awarded Excellent Young Scientist foundation of NSFC in 2013. He also serves as the director of Beijing Key Laboratory of Construction-Tailorable Advanced Functional Materials and Green Applications. His current research interest is inorganic chemistry of semiconductor based hybrid nanostructures to possess novel optical, electron-ic properties for applications in energy conversion and storage, catalysis, optoelectronics and biology.
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Assembly-promoted photocatalysis: Three-dimensional assembly of CdSxSe1x (x ¼ 0e1) quantum dots into nanospheres with enhanced photocatalytic performance Lina Wang, Qiumei Di, Mingming Sun, Jia Liu**, Chuanbao Cao, Jiajia Liu, Meng Xu, Jiatao Zhang* Beijing Key Laboratory of Construction Tailorable Advanced Functional Materials and Green Applications, School of Materials Science and Engineering, Beijing Institute of Technology, Beijing, 100081, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 October 2016 Received in revised form 16 November 2016 Accepted 30 November 2016 Available online xxx
In this paper, through an emulsion-based bottom-up self-assembly method, monodisperse CdSxSe1x (x ¼ 0e1) quantum dots (QDs) with tailoring compositions have been three-dimensionally assembled into spherical architectures in sub-micrometer sizes. UVeVis absorption measurements revealed the enhanced light harvesting abilities of the assembled CdSxSe1x spheres relative to their constituting QDs. HRTEM characterizations over the CdSxSe1x assemblies suggested the existence of localized oriented adjoining of the CdSxSe1x QDs and the resulting nano-twin structures that are favorable for photogenerated electron-hole separation. The quenching of photoluminescence and the improvement in IPCE after the assembly of CdSxSe1x QDs provided a clue to the likely suppressed electron-hole recombination brought about by the unique architectures and interfaces derived from self-assembly. The above findings were coincided with the remarkably improved H2 evolution activities observed for the wellassembled CdSxSe1x nanospheres in photocatalytic water splitting, underpinning the importance of the alternative strategy to design advanced semiconductor photocatalysts based on architectural engineering. © 2017 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: CdSxSe1x quantum dots Three-dimensionally assembly Colloidal nanospheres Photocatalysis
1. Introduction Converting solar energy into renewable hydrogen fuel via photocatalytic water reduction over semiconductor photocatalysts has attracted considerable research attentions due to its great promise in addressing the growing energy demand worldwide [1e11]. IIeVI semiconductors are one of the promising catalysts for solar-driven hydrogen generation from water since many of them possess a bandgap that allows absorption in the visible region of the spectrum [7]. However, these catalyst materials usually show low H2 evolution activities owing to the insufficient light harvesting capability, and the easy recombination of photo-induced electrohole pairs before they migrate to the semiconductor surface where water splitting reaction takes place [12,13]. Therefore, enhancing
* Corresponding author. ** Corresponding author. E-mail address: [email protected] (J. Zhang). Peer review under responsibility of The Chinese Ceramic Society.
light harvesting and promoting photoexcited charge separation are still key challenges for development of efficient semiconductor photocatalysts [14e18]. Recently, self-assembly of semiconductor nanocrystals into hierarchical structures has been recognized as an alternative way to improve photocatalysis efficiency in consideration of the improved light absorption capacity [17e22]. For examples, through a selfassembly growth mechanism in the presence of ionic liquid molecule, BiOI hollow microsperes have been fabricated which displayed evidently enhanced photocatalytic activities than the BiOI nanoplates [17]. Yin et al. have synthesized mesoporous submicron clusters of anatase TiO2 based on the self-assembly of TiO2 nanocrystals (NCs) with the assistance of a silica template. The scattering and multiple reflecting of the light within the formed mesoporous TiO2 clusters was proposed to be a crucial reason accounting for their improved photocatalytic performance in decomposition of organic dyes under UV irradiation [18]. Meanwhile, Wu and coworkers have reported the light-triggered organization of colloidal semiconductor NCs into hollow-structured
http://dx.doi.org/10.1016/j.jmat.2016.11.008 2352-8478/© 2017 The Chinese Ceramic Society. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article in press as: Wang L, et al., Assembly-promoted photocatalysis: Three-dimensional assembly of CdSxSe1x (x ¼ 0e1) quantum dots into nanospheres with enhanced photocatalytic performance, J Materiomics (2017), http://dx.doi.org/10.1016/j.jmat.2016.11.008
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nanospheres via the use of bubble templates generated in situ through photocatalytic water splitting reaction [20,21]. In addition to these specially defined synthetic systems, Li group has established a general emulsion-based bottom-up assembly approach to assemble various NCs building blocks, e.g. CdS, PbS, ZrO2 and NaYF4 NCs, into three-dimensional (3D) colloidal spheres [23]. This method inspired us to construct new types of integrated architectures of semiconductor QDs, particularly CdSxSe1x (x ¼ 0e1) QDs, towards creation of advanced photocatalyst materials. On the other hand, Guo and coworkers have reported that Cd1xZnxS solid solution NCs with nano-twin structures can exhibit superior photocatalytic activities for H2 evolution from water under visible light irradiation [14e16]. They unambiguously demonstrated that the formation of homojunctions derived from the parallel twining planes in the Cd1xZnxS crystals significantly boosted the separation of photogenerated electron-hole pairs and contributed to prolonged lifetime of the charge carriers. Encouraged by their interesting findings, here the crystal structure of the CdSxSe1x building blocks has been carefully engineered in the hope of generating twin-induced homojunctions for efficient charge separation. In this work, sub-micrometer sized CdSxSe1x (x ¼ 0e1) spheres have been synthesized from the bottom-up self-assembly of monodisperse CdSxSe1x QDs. The resulting CdSxSe1x nanospheres exhibited stronger light absorption and enhanced incident photonto-electron conversion efficiency (IPCE) in the visible regime than those of the CdSxSe1x QDs. Moreover, the assembled CdSxSe1x spheres afforded notably higher photocatalytic efficiency for H2 evolution from water under visible light irradiation relative to the unassembled CdSxSe1x QDs. It was conjectured that the synergy of the improved light absorbance and the formed nano-twin structures can make a contribution to the superior catalytic performance of CdSxSe1x spheres. Besides that, the CdSxSe1x spheres displayed good dispersity and stability in aqueous solutions and can be easily scaled up, which are also advantageous features in pursuit of novel structures for efficient photocatalytic applications. 1.1. Raw materials Octadecylene (chromatographically pure), Shanghai Jingchun Reagent Co., Ltd, Shanghai, China. Oleic acid (analytically pure), Shanghai Jingchun Reagent Co., Ltd, Shanghai, China. Chromic oxide (CdO, analytically pure), Shanghai Jinshantingxin Chemical Reagent Co., Ltd, Shanghai, China. Toluene (analytically pure), Beijing Chemical Works, Beijing, China. Cyclohexane (analytically pure), Beijing Chemical Works, Beijing, China. Cetyltrimethylammonium bromide (CTAB, analytically pure), Tianjin Guangfu Fine Chemical Research Institute, Tianjin, China. Sulfur powder (chemically pure), Tianjin Fuchen Chemical Reagent Co., Ltd, Tianjin, China. Selenide powder (analytically pure), Tianjin Guangfu Fine Chemical Research Institute, Tianjin, China.
QDs from non-polar solvents into water according to our previous research [12]. 1.3. Self-assembly of CdSxSe1x QDs into nanospheres CdSxSe1x QDs dispersed in toluene (3 mL) were precipitated by adding ethanol and were re-dispersed in cyclohexane (1 mL). In a general preparation of water-dispersed 3D colloidal spheres from the oil-dispersed CdSxSe1x QDs via the bottom-up assembly approach [23], first an aqueous solution of CTAB was prepared by dissolving 35 mg of CTAB into 10 mL of deionized water. The above solution (1 mL) containing CdSxSe1x QDs in cyclohexane was added to the CTAB aqueous solution. The obtained mixture was emulsified by vigorous stirring and was kept at 80 C with constant stirring for 1 h, to allow the self-assembly of the CdSxSe1x QDs into 3D spheres. After purified by centrifugation, the final products were re-dispersed in water (10 mL). 1.4. Characterization A JEOL JEM 1200EX transmission electron microscope (TEM) working at 100 kV was used to characterize the morphologies of the CdSxSe1x QDs and their assembly spheres. Samples dispersed in toluene were casted onto a carbon-coated copper grid prior to the measurement. A FEI Tecnai G2 F20 S-Twin high-resolution TEM (HRTEM) working at 200 kV were used to determine the morphologies and crystal lattice details of the samples, and to perform the energy dispersive spectra (EDS) elemental mapping and line scan elemental analysis. The UVeViseNIR absorption spectra of the as-prepared samples were recorded on a Shimadzu UV3600 spectrophotometer at room temperature (RT). The steady state luminescence spectra of colloidal samples were collected on a FluoroMax-4 spectrophotometer (HORIBAJOBINYVON USA) at RT. 1.5. Incident photon-to-electron conversion efficiency measurements The incident photon-to-electron conversion efficiency (IPCE) studies on CdSxSe1x QDs and the nanospheres assembled from the CdSxSe1x QDs were carried out on a Kit (Crowntech QTest Station 1000AD) with a tungsten halogen lamp (CT-TH-150), a calibrated silicon diode and a monochromator (Crowntech QEM24-S 1/4 m) at room temperature. The light power density was calibrated against a Si solar cell (Hamamatsu S1133) to accurately simulate the light to 100 mW cm2. All incident photon-to-electron conversion efficiency performances were carried out in a two-electrode system equipped with platinum electrode as counter. The working electrode was made by dropping samples on ITO glass and sintered at 400 C for 3 h under the atmosphere of N2. 0.1 M Na2SO4 solution was used as the electrolyte. 1.6. Photocatalytic H2 production measurements
1.2. Preparation of monodisperse CdSxSe1x (x ¼ 0e1) QDs CdSxSe1x (x ¼ 0e1) QDs were prepared by the hot-injection method [24]. Typically, 5 mL of octadecylene, 6 mL of oleic acid, and 64 mg of CdO were mixed by magnetic stirring. The resulting mixture was heated up to 170 C under the atmosphere of nitrogen, then a certain quantity of S or Se organic precursors in a desirable ratio were injected into the solution. Afterwards the reaction temperature was reset to 290 C and maintained for half an hour under magnetic stirring. The S precursor and the Se precursor were prepared and the transfer of as-prepared hydrophobic CdSxSe1x
The photocatalytic H2 production test on CdSxSe1x QDs and the colloidal spheres assembled from the oil-dispersed CdSxSe1x QDs was conducted by using a sealed circulation online analysis system (Beijing Aulight Technology Co. Ltd., China) combined with a gas chromatograph (SP7800, Beijing KeRuida Co. Ltd.). In a typical run, 0.002 g of sample was suspended in 100 mL of aqueous solution (containing 0.5 mol L1 K2SO3 and 0.5 mol L1 Na2S) under magnetic stirring. Then the sealed system was vacuumed to reach to 0.1 MPa. A 300 W Xe lamp with a 400 nm cut-off filter (CELHXF300/CEL-HXUV300) was applied to irradiate the mixture. The
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amount of H2 gas was determined by online gas chromatography equipped with a thermal conductivity detector. All tests were conducted at room temperature.
2. Results and discussion 2.1. Characterization of the CdSxSe1x (x ¼ 0e1) QDs and their assembled spheres Monodisperse CdSxSe1x (x ¼ 0e1) QDs were prepared by high temperature organic phase method. The morphologies of the synthesized samples were illustrated by transmission electron microscopy (TEM) images as shown in Fig. 1. The CdSxSe1x QDs obtained here showed good monodispersity with an average diameter of 5 nm independent of the S to Se molar ratios (x varied at 1, 0.86, 0.5, and 0 from Fig. 1A to D). The high monodispersity of the CdSxSe1x QDs was very beneficial for their subsequent self-assembly to form 3D structures through the bottum-up approach. Fig. 2 shows the high-resolution transmission electron microscopy (HRTEM) (2A2D) and scanning transmission electron microscopy (STEM) (2E, 2F) images of the typical samples of 3D colloidal spheres that were assembled from CdSe QDs. From Fig. 2, the dimensions of the CdSe spheres were estimated to be in the range of 50e80 nm. It is easily seen that the CdSe QDs building blocks did not retain their individual characters but rather adjoined together into larger units.
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Looking into the interfaces between two neighboring QDs, one can see that the two QDs either grew together by sharing the same lattices as exhibited in Fig. 2B and Fig. S1, or grew into twinned nanocrystals as suggested by Fig. 2D and Fig. S2. The adjoining of QDs to form an integral architecture as formed in our case can facilitate rapid charge transfer and reduce the recombination rates of electron-hole pairs [14,25e27]. Fig. 3 illustrates the TEM images of assembled CdSxSe1x spheres with verified compositions. Decreasing the S to Se molar ratios, the morphologies of assembled products from QDs did not show notable changes. This may be ascribed to the good retaining of the diameter and morphologies of CdSxSe1x QDs when altering the S to Se molar ratios as aforementioned. Fig. 4 provides the schematic illustration of the self-assembly processes of CdSxSe1x QDs. Cyclohexane solution containing monodisperse CdSxSe1x QDs, cetyltrimethylammonium bromide (CTAB) and water were mixed together at an appropriate oil-towater ratio (1:10 by volume). Under vigorous stirring, a stable oilin-water (O/W) microemulsion system was formed, where the CdSxSe1x QDs were confined in the oil microemulsion droplets stabilized by the CTAB molecules. Thereafter, cyclohexane (a lowboiling solvent) was evaporated from the system by heating at 80 C. During this process, the oil droplets gradually shrank and the QDs in the droplets became concentrated and got assembled in the 3D confined oil-emulsion droplet spaces. Through continuous
Fig. 1. TEM images of as-prepared CdSxSe1x QDs: a) CdS QDs; b) CdS0.86Se0.14 QDs; c) CdS0.5Se0.5 QDs; D) CdSe QDs. scale bar ¼ 100 nm.
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Fig. 2. Assembled colloidal spheres by CdSe QDs: A and C) HRTEM images of the assembled colloidal spheres of CdSe quantum dots; B) Amplification of the circled part in Fig. 2A; the inset is the image intensity analysis by Digital Micrograph software which reflects the lattice structures of the sample; D) Amplification of the circled part in Fig. 2C; the twinned nanoparticles surfaces between nanoparticles are marked with white dash lines; E) and F) STEM images of assembled colloidal spheres of CdSe QDs at different magnifications; scale bar ¼ 500 nm and 50 nm for Fig. 2E and F, respectively.
evaporation of cyclohexane, the QDs finally stuck together to form 3D colloidal spheres, and this might result in the formation of some twin-containing structures nearby the interface of two neighboring CdSxSe1x QDs. According to previous research [28e30], the hydrophobic interactions between the alkane chains of CTAB molecules can guarantee the high stability of the colloidal assembled architectures while the hydrophilic moieties of CTAB accounted for their good dispersity in aqueous solution. Based on these advantageous, we further investigated the potential of the formed
CdSxSe1x QDs colloidal spheres as photocatalysts for catalyzing hydrogen evolution reaction from water. Primarily, the light absorption abilities of the CdSxSe1x QDs assemblies were studied by UVeVis spectroscopy. Fig. 5 and Fig. S3 present the UVeVis absorption spectra of the CdSxSe1x QDs and the assembled CdSxSe1x nanospheres normalized at 400 nm. It can be seen that the absorbance was all enhanced with a red shift in the energy for the assembled CdSxSe1x spheres compared to the isolated CdSxSe1x QDs regardless of S to Se molar ratios. These results
Please cite this article in press as: Wang L, et al., Assembly-promoted photocatalysis: Three-dimensional assembly of CdSxSe1x (x ¼ 0e1) quantum dots into nanospheres with enhanced photocatalytic performance, J Materiomics (2017), http://dx.doi.org/10.1016/j.jmat.2016.11.008
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Fig. 3. TEM images of assembled colloidal spheres of CdSxSe1x QDs: A) CdS QDs; B) CdS0.86Se0.14 QDs; C) CdS0.5Se0.5 QDs. Scale bar ¼ 200 nm.
Fig. 4. Schematic illustration of CdSxSe1x (x ¼ 0e1) spheres assembled from monodisperse CdSxSe1x (x ¼ 0e1) QDs via an emulsion-based bottom-up self-assembly approach.
are similar to the previous observations acquired from the assembly of CdSe and CdTe QDs [19,21], indicating the promoted light harvesting ability of the semiconductor QDs assemblies relative to their individual constituents [26]. The band gap for bulk CdSxSe1x
should be between that of CdSe (1.7 eV) [27] and CdS (2.42 eV) [31,32]. The increase in the content of Se could cause a drop in the apparent band gap of CdSxSe1x QDs and the redshift of the absorption spectrum [12]. This is consistent with our findings
Fig. 5. UVeVis absorption spectra comparison of as-formed CdSxSe1x nanospheres and the CdSxSe1x QDs with varied S to Se molar ratios.
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positive to the separation of electron-hole pairs and the suppression of their recombination [14e16,33], thus can be exploited as a powerful tool for achieving higher photocatalytic activities in solar hydrogen generation. 2.2. Photocatalytic activity measurements
Fig. 6. Photoluminescence spectra comparison between as-prepared CdSe QDs and assembled CdSe spheres.
exhibited in Fig. 5AeD where the S to Se molar ratios varied at 1:0, 1:0.16, 1:1 and 0:1, correspondingly. Thereafter, CdSe QDs and their assembled nanospheres were employed to investigate the effect of self-assembly on the recombination tendency of photo-induced charge carriers by using photoluminescence spectroscopy. Generally speaking, the photoluminescence can be generated during the recombination of the charge carriers on a semiconductor. As shown in Fig. 6, in comparison with CdSe QDs which displayed a strong luminescence band centered at ~660 nm, a distinct decrease in the luminescence intensity was observed for the assembled CdSe nanospheres. These results were further corroborated by the observations under a UV lamp as provided in Fig. S4, strongly indicating the quenching of the photoluminescence for CdSe nanospheres that were constructed by closely assembled CdSe QDs. The energy transfers from the aggregated smaller QDs nanospheres (larger band gap due to the quantum confinement effect) to the larger ones (smaller band gap) can be a possible mechanism leading to the remarkably decreased luminescence intensity [20]. However, it is reasonable to presume that the reduced emission intensity might partially profit from the inhibited charge carrier recombination due to the presence of twininduced homojunctions in the assembled architectures of the CdSe QDs (Fig. 2D). Such homojunctions have been demonstrated
We further examined the activities of CdSxSe1x QDs (x ¼ 0 and 0.5) and their assembled nanospheres toward photocatalytic hydrogen evolution from water splitting. For these experiments, 2 mg of sample was dispersed in 100 mL of deionized water containing 0.5 M K2SO3 and 0.5 M Na2S as hole scavengers. As shown in Fig. 7, an induction period (about 1 h) at the initial stage of light irradiation was observed, which can be explained by the higher rate of photocatalytic degradation of oleic acid adsorbed on the surface of QDs than photocatalytic water splitting [20,34]. After this self-cleaning process, the amount of H2 evolved from photocatalytic water splitting increased with the irradiation time. It is clear to see that the assembled spheres from CdSxSe1x QDs clearly showed increased H2 production rate than their constituting building blocks at both measured S to Se molar ratios (0:1 and 1:1). Under visible light irradiation, the assembled CdSe spheres (x ¼ 0) provided the highest H2 evolution rate (7.66 mmol h1) among the four samples, which was almost fourfold higher than that of the CdSe QDs (1.98 mmol h1). At the same time, the assembled CdS0.5Se0.5 spheres (x ¼ 0.5) showed a twofold increment in H2 evolution activity (4.16 mmol h1) than the CdS0.5Se0.5 QDs (1.78 mmol h1). Several factors may account for the highest activity afforded by the assembled CdSe nanospheres. Upon irradiation by visible light, the absorption of incident photons leads to the generation of excitons in the CdSxSe1x semiconductor QDs. The evolution of H2 requires the efficient transfer of electrons from the semiconductor QDs to the surface of assembled spheres, accompanied by the fast hole consumption by the scavengers. The increased Se content (CdS0.5Se0.5 vs CdSe) can cause a reduction in the apparent band gap of the semiconductor QDs and a notable redshift of the absorption edge as confirmed by the results presented in Fig. 5C and D. Therefore, the CdSe photocatalysts possessed a higher light harvesting capability relative to the CdS0.5Se0.5 counterparts and can intrigue a larger amount of photogenerated electron-hole pairs under light illumination. When comparing the catalytic activities of the two CdSe QDs samples before and after self-assembly, the notable increment in H2 evolution rate observed for the assembled CdSe nanospheres can be explained by their enhanced light harvesting ability as a
Fig. 7. (A) H2 evolution curves as a function of irradiation time for different CdSxSe1x QDs and assembled spheres photocatalysts in photocatalytic water reduction reaction. (B) Comparison of the H2 evolution activities of different CdSxSe1x QDs and assembled spheres photocatalysts.
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result of self-assembly (Fig. 5D) [18,19]. The generation of the twin-induced homojunctions at the interface between neighboring QDs during the self-assembly process was assumed to be another important reason with regard to the promoted charge separation as suggested by the results in Fig. 6. Accordingly, the much improved photocatalytic performance of the assembled semiconductor QDs can be expected. The incident photon-to-electron conversion efficiency (IPCE) measurements over the assembled CdSxSe1x nanospheres and the as-formed CdSxSe1x QDs (x ¼ 0 and 0.5) were carried out using a two-electrode configuration with Pt wire as the counter electrode. The results provided useful information about the charge generation and separation properties of the catalyst samples as a function of the irradiation wavelength. As shown in Fig. S5, for all samples the IPCE values decreased with the increased irradiation wavelength, parallel to the evolution tendency of their optical absorption abilities as shown in Fig. 5. Furthermore, it is clear to see that at the same irradiation wavelength the assembled nanospheres photoanodes always displayed higher IPCE than their QDs counterparts, and CdSe QDs-assembled nanospheres photoanodes afforded the highest IPCE among the four samples, in a good agreement with their photocatalytic performance as shown in Fig. 7. 3. Conclusion In summary, an emulsion-based bottom-up self-assembly approach was utilized to synthesize well-assembled nanospheres from CdSxSe1x QDs with different S to Se molar ratios. By combining different spectroscopy technologies, we found that the assembly of CdSxSe1x QDs into larger architectures can enhance their light absorption capability in the visible range, and can be in favor of the generation of twinned QDs which have been verified propitious to efficient photogenerated electron-hole separation. The comparison of the photocatalytic H2 evolution activities and IPCE of the CdSxSe1x QDs before and after self-assembly indicated the positive impacts brought about by the assembly and adjoining of the semiconductor QDs on their photocatalysis behaviors. We believe that the findings presented here can be further developed into a useful strategy for designing advanced photocatalyst materials and structures. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 21322105, 51501010, 91323301, 51372025). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jmat.2016.11.008. References [1] Kudo A, Miseki Y. Heterogeneous photocatalyst materials for water splitting. Chem Soc Rev 2009;38:253e78. [2] Zong X, Yan HJ, Wu GP, Ma GJ, F Wen Y, Wang L, et al. Enhancement of photocatalytic H2 evolution on CdS by loading MoS2 as cocatalyst under visible light irradiation. J Am Chem Soc 2008;130:7176e7. [3] Maeda K, Teramura K, Lu DL, Takata T, Saito N, Inoue Y, et al. Photocatalyst releasing hydrogen from water. Nature 2006;440:295. [4] Hisatomi T, Kubota J, Domen K. Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting. Chem Soc Rev 2014;43: 7520e35. [5] Ma Y, Wang Xl, Jia YS, Chen XB, Han HX, Li C. Titanium dioxide-based nanomaterials for photocatalytic fuel generations. Chem Rev 2014;114: 9987e10043.
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Please cite this article in press as: Wang L, et al., Assembly-promoted photocatalysis: Three-dimensional assembly of CdSxSe1x (x ¼ 0e1) quantum dots into nanospheres with enhanced photocatalytic performance, J Materiomics (2017), http://dx.doi.org/10.1016/j.jmat.2016.11.008
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L. Wang et al. / J Materiomics xxx (2017) 1e8 Lina Wang was born in Liaoning, China. She received her MSc degree in 2007 from the Kunming University of science and technology, and is now studying at Beijing institute of technology for her PhD. Her research interest includes synthesis and assembly of semiconductor nanocrystals.
Jiatao Zhang was born in 1975. He received his PhD degree in 2006 from the Department of Chemistry, Tsinghua University, China. Currently he is Xu Teli Professor in the School of Materials and Engineering, Beijing Institute of Technology. He was awarded Excellent Young Scientist foundation of NSFC in 2013. He also serves as the director of Beijing Key Laboratory of Construction-Tailorable Advanced Functional Materials and Green Applications. His current research interest is inorganic chemistry of semiconductor based hybrid nanostructures to possess novel optical, electron-ic properties for applications in energy conversion and storage, catalysis, optoelectronics and biology.
Please cite this article in press as: Wang L, et al., Assembly-promoted photocatalysis: Three-dimensional assembly of CdSxSe1x (x ¼ 0e1) quantum dots into nanospheres with enhanced photocatalytic performance, J Materiomics (2017), http://dx.doi.org/10.1016/j.jmat.2016.11.008