Synthesis of Au/CdSe Janus Nanoparticles with Efficient Charge Transfer for Improving Photocatalytic Hydrogen Generation
- 155 Downloads
Metal-semiconductor heterostructures integrate multiply functionalities beyond those of their individual counterparts. Great efforts have been devoted to synthesize heterostructures with controlled morphologies for the applications ranging from photocatalysis to photonic nanodevices. Beyond the morphologies, the interface between two counterparts also significantly influences the performance of the heterostructures. Here, we synthesize Au/CdSe Janus nanostructures consisting of two half spheres of Au and CdSe separated by a flat and high-quality interface. Au/CdSe with other morphologies could also be prepared by adjusting the overgrowth conditions. The photocatalytic hydrogen generation of the Au/CdSe Janus nanospheres is measured to be 3.9 times higher than that of the controlled samples with CdSe half-shells overgrown on the Au nanospheres. The highly efficient charge transfer across the interface between Au and CdSe contributes to the improved photocatalytic performance. Our studies may find the applications in the design of heterostructures with highly efficient photocatalytic activity.
KeywordsAu/CdSe Janus nanospheres Interface Highly efficient photocatalysis
X-ray powder diffraction
Energy-dispersive X-ray spectroscopy
Transmission electron microscopy
High-resolution transmission electron microscope
Metal-semiconductor colloidal heterostructures have attracted extensive interests due to their extraordinary optical behaviors and functionalities far beyond those of their individual counterparts and have exhibited great potential in solar energy conversion [1, 2], photocatalysis [3, 4, 5, 6, 7, 8], photoelectric devices [9, 10, 11], and photothermal therapy [12, 13, 14, 15], etc. Especially, plasmon-based hybrid nanostructures become a promising candidate for photocatalytic water splitting or hydrogen generation with excellent photocatalytic performance [16, 17, 18, 19]. Colloidal nanoparticles of metal chalcogenide semiconductors (sulfide, selenide, and telluride) have received significant attention in photocatalytic application due to their suitable and tunable band gap matched with solar spectrum as well as their chemical properties. However, the low absorption efficiency in visible light region and the quick recombination of photo-induced charge carriers have limit the application of pure semiconductor nanoparticles. To overcome these issues, many efforts have been devoted to integrate plasmonic metal nanocrystals (nanospheres , nanorods , nanoplates , etc.) and chalcogenide semiconductors (CdX [23, 24, 25, 26, 27, 28], Ag2X [29, 30, 31, 32, 33], Cu2X [12, 13, 14, 15], PbX  etc. (X = S, Se, Te)) to build hybrid nanostructures with intriguing properties.
As for the plasmon-enhanced photocatalytic performance, many possible mechanisms have been discussed in previous works, including effectively harvesting light energy through surface plasmon resonances, concentrating local electromagnetic field in adjacent semiconductors, promoting photoexcited charge generation and transfer, suppressing electron-hole recombination and plasmon-induced hot-electron transfer from metals to semiconductors [35, 36, 37, 38, 39]. Besides that, several structural factors such as morphology, size, hybrid configuration, and contact interface have been reported to be crucial for photocatalytic activity [40, 41, 42, 43]. Zhao et al. have finely tuned the structural symmetry of the Au/CdX (X = S, Se, Te) hybrid nanoparticles with controllable spatial distribution between the two components by a non-epitaxial synthetic route and demonstrated the dependence of photocatalysis on the structural symmetry . The interfacial charge transfer and the exposure of active materials to reaction solution are the important factors for determining the performance of heterodimer type and core-shell-type hybrids [41, 44]. The possibility of charge transfer between the metal and the chalcogenide semiconductors has been exhibited in several types of hybrids [41, 44, 45, 46]. Additionally, the charge transfer also depends significantly on the interfacial conditions, such as interfacial energy and quality between the two counterparts [41, 44]. There remain great challenges to obtain a good heterogeneous interface for metal-semiconductor hybrid nanostructures due to the large lattice mismatch between two components. Therefore, it is meaningful to finely tailor the interface and contact to achieve the tunable properties and electronic mobility in the metal-semiconductor hybrid nanostructures.
In this paper, we report a particular approach to synthesize water-dispersed asymmetric Au/CdSe Janus heterostructures with a flat and high-quality interface between Au and CdSe. By manipulating the pH value of the reaction solution, CdSe with different morphologies and coverages are grown on the Au nanoparticles. The results show the pH value is crucial for the formation of Janus morphology with the flat and high-quality interface. Hydrogen generation measurements show that the Janus Au/CdSe heterostructures has a significantly higher efficiency than those of the other types of hybrid structures due to the low interface energy and the improved electron transfer efficiency on the interface of Au and CdSe.
Chloroauric acid (HAuCl4·4H2O, 99.99%), silver nitrate (AgNO3, 99.8%), glycine acid (99.5%), selenium powder (Se, 99.5%), L-ascorbic acid (99.7%), sodium hydrate (NaOH, 96.0%), cadmium nitrate tetrahydrate (Cd(NO3)2·4H2O, 99.0%), hydrochloric acid (HCl, 36–38%), hexamethylenetetramine (HMT, 99.0%), and sodium borohydride (NaBH4, 96%) were all purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Cetyltrimethylammonium-bromide (CTAB, 99.0%) was obtained from Amresco, Inc. (America). All chemicals were used as received and without further purification.
Synthesis of Au Nanoparticles
The CTAB-stabilized Au nanoparticles were synthesized at room temperature by a seed-mediated growth method reported previously . Firstly, 4.5 mL aqueous solution was prepared by mixing 500 μL of 5 mM HAuCl4 and 5 mL of 0.2 mM CTAB, and then 600 μL of 10 mM ice-cooled NaBH4 solution was added. The brownish solution of Au seeds was left undisturbed for 2 h for further use. Next, 120 μL Au seed solution was added into a aqueous mixture including 190 mL of H2O, 4 mL of 10 mM HAuCl4, 9.75 mL of 0.1 M CTAB, and 15 mL of 100 mM ascorbic acid. The solution was well mixed by a slight shaking and then was allowed to stand overnight for the growth of Au nanoparticles.
Synthesis of Au-Ag Bimetallic Nanoparticles
Firstly, the pH value of a aqueous mixture including 5.0 mL of the Au nanoparticles (8.0 nM) and 5.0 mL of 200 mM glycine acid was respectively adjusted to 2.5, 4.5, 7.2 or 8.1 by the dropwise addition of HCl solution (VHCl:VH2O = 1:9) or NaOH solution (2 M). The mixture was kept at 30 °C under stirring for 1 min. Then, 15 µL of 100 mM AgNO3 solution was injected. The mixture was kept at 30 °C without stirring for 10 h. The products of Au-Ag bimetallic nanoparticles were directly used for the growth of Au-CdSe hybrid nanoparticles.
Synthesis of Au/CdSe Janus Heterostructures
The Au/CdSe Janus heterostructures was prepared by mixing 2 mL of the as-prepared Au-Ag nanoparticles, 6 mg of selenium powder, 0.01 mL of 100 mM Cd(NO3)2 solution, and 40 µL of 10 mM NaBH4 solution. The mixed reaction was vigorously stirred at 90 °C for 2 h. The products were centrifuged at 9500 rpm for 5 min and washed by water twice. The controlled samples with other morphologies were prepared by the same procedure except for the pH value of the growth of Au-Ag nanoparticles.
Evaluation of Photocatalytic Activities
The visible light photocatalytic hydrogen evolution tests were carried out in a quartz tube reactor with a rubber diaphragm. One hundred milligrams of Au/CdSe photocatalyst powder was dispersed in 50 mL of an aqueous solution containing 5 mL of lactic acid as a sacrificial agent in a quartz tube reactor. The reactor was pumped off with stirring for 30 min to remove any dissolved air. The light source is a 300-W xenon lamp with an ultraviolet cutoff filter (λ > 420 nm). During the entire photocatalytic test, the temperature of the suspension was maintained at 6 °C with an external water-cooling system to withstand the temperature rise of the optical radiation. The content of hydrogen was automatically analyzed by on-line gas chromatography (Tianmei GC-7806).
TEM studies were done with a JEOL 2010 HT microscope operated at 200 kV by drop casting the sample dispersions on carbon-coated copper grids. HRTEM, TEM, and EDX analyses were performed using a JEOL 2010 FET microscope operated at 200 kV accelerating voltage. The UV-Vis spectra were recorded with a TU-1810 (Purkinje General Instrument Co. Ltd. Beijing, China) and Cary 5000 (Agilent) spectrometer. All optical measurements were performed at room temperature under ambient conditions.
Results and Discussion
In summary, we presented a precise synthesis of water-dispersed Au/CdSe Janus nanospheres with controlled interfacial condition and quality. Four types of Au/CdSe hybrids of Janus nanospheres, heterodimers, symmetric double-headed nanoparticles, and multi-headed nanoparticles could be produced by manipulating the pH value. The evaluation of photocatalytic hydrogen generation showed that the Au/CdSe Janus nanospheres exhibit at least 3.9 times higher H2 evolution rate than other Au/CdSe counterparts. The improved photocatalytic performance is owing to the flat and high-quality interface between Au and CdSe, which promotes the charge transfer across the interface and accelerates the interfacial charge separation.
Special thanks to Wei Wang from Wuhan University for providing support of checking references.
XDL and QQW designed the experiments and drafted this manuscript. XDL performed the experiments. KC and SM perform the structural characterization of samples. SL, LZ, and ZHH helped in data analysis and the manuscript modification. All authors contributed to the data analysis and scientific discussion. All authors read and approved the final manuscript.
This work was supported by the National Key R&D Program of China (Grant No. 2017YFA0303402), the National Natural Science Foundation of China (Grant Nos. 11674254, 91750113, 11874293, and 11504105), the Hubei Provincial Natural Science Foundation of China, and the Natural Science Foundation of Hunan Province (Grant Nos. 2019JJ50367).
The authors declare that they have no competing interests.
- 44.Liu J, Feng J, Gui J, Chen T, Xu M, Wang H, Dong HF, Chen HL, Li XW, Wang L, Chen Z, Yang ZZ, Liu JJ, Hao WC, Yao Y, Gu L, Weng YX, Huang Y, Duan XF, Zhang JT, Li YD (2018) Metal@semiconductor core-shell nanocrystals with atomically organized interfaces for efficient hot electron-mediated photocatalysis. Nano Energy 48:44–52CrossRefGoogle Scholar
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.