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Enhanced Photocatalytic Hydrogen Evolution by Loading Cd0.5Zn0.5S QDs onto Ni2P Porous Nanosheets
Nanoscale Research Letters volume 13, Article number: 31 (2018)
Ni2P has been decorated on CdS nanowires or nanorods for efficient photocatalytic H2 production, whereas the specific surface area remains limited because of the large size. Here, the composites of Cd0.5Zn0.5S quantum dots (QDs) on thin Ni2P porous nanosheets with high specific surface area were constructed for noble metal-free photocatalytic H2 generation. The porous Ni2P nanosheets, which were formed by the interconnection of 15–30 nm-sized Ni2P nanoparticles, allowed the uniform loading of 7 nm-sized Cd0.5Zn0.5S QDs and the loading density being controllable. By tuning the content of Ni2P, H2 generation rates of 43.3 μM h− 1 (1 mg photocatalyst) and 700 μM h− 1 (100 mg photocatalyst) and a solar to hydrogen efficiency of 1.5% were achieved for the Ni2P-Cd0.5Zn0.5S composites. The effect of Ni2P content on the light absorption, photoluminescence, and electrochemical property of the composite was systematically studied. Together with the band structure calculation based on density functional theory, the promotion of Ni2P in charge transfer and HER activity together with the shading effect on light absorption were revealed. Such a strategy can be applied to other photocatalysts toward efficient solar hydrogen generation.
As an efficient strategy to produce H2 by utilizing solar energy, photocatalytic hydrogen production has attracted extensive attention since TiO2 was reported as a photocatalyst in 1972 . Compared with TiO2, CdxZn1−xS shows excellent visible-light driven catalytic activity because of the narrower band gap and good photochemical stability. A H2 production rate as high as 1097 μM h− 1 g− 1 has been achieved by using Cd0.5Zn0.5S as photocatalyst , which composition has been proven to be the optimum for photocatalytic property. To decrease carrier recombination and prompt carrier separation for hydrogen evolution reaction (HER), noble metals such as Pt, Co-Pt, Ru, Au, and Pd have been used as co-catalysts [3,4,5,6,7,8]. For example, when co-catalyzed with Co-Pt, the photocatalytic H2 generation rate of Cd0.5Zn0.5S quantum dots (QDs) could be increased by 4.7-folds . A H2 production as high as ~ 6.3 mM h− 1 mg− 1 was achieved when CdZnS was combined with Au . However, the high cost of noble-metals greatly limits the future application in large scale, which makes the non-precious co-catalysts to be good candidates of precious ones for photocatalytic H2 generation.
Among the various non-noble co-catalysts including carbon family (graphene, carbon nanotubes, reduced grapheme oxide, carbon nanodots) [10,11,12,13,14,15], phosphides [16,17,18,19,20,21,22], and TiO2 [23, 24] and sulfides [25,26,27,28,29,30,31,32], Ni2P and CoP have been extensively composited with CdS nanowires and/or nanorods for efficient photocatalytic H2 production [16,17,18, 33,34,35,36]. In these composites, one-dimensional (1D) CdS was always decorated by smaller phosphides’ nanoparticles or nanosheets with HER activity, and carrier recombinations can be greatly reduced because of the long carrier diffusion length of the 1D structure and its well-defined hetero-interface with the co-catalysts. Considering the advantages of QDs such as its high solar energy to fuel conversion efficiency, low fabrication costs [37, 38], and HER mainly occurs at co-catalyst/electrolyte interface, it is rational to construct hetero-nanostructures with plenty of specific surface area of active sites while still maintaining fast carrier separation. In this case, a reverse structure with photocatalysts loaded onto co-catalysts was reported for efficient photocatalytic H2 generation [10, 13]. For instance, hydrogen generation rates of 2.08 and ~ 33.4 mM h− 1 mg− 1 were established by loading Cd0.5Zn0.5S QDs onto onion-like carbon and 2D graphitic carbon nitride (g-C3N4) microribbons, respectively. These make it highly expectable for photocatalytic H2 generation if phosphide nanostructures were decorated by Cd0.5Zn0.5S QDs. However, such a reverse structure has been rarely reported up to now.
Here, a reverse structure of Cd0.5Zn0.5S QDs on Ni2P nanosheet arrays was synthesized by thermal solution method for enhanced photocatalytic H2 generation. A hydrogen generation rate of 700 μM h− 1 (with 100 mg feeding catalyst) and a solar to hydrogen efficiency (STH) of 1.5% were achieved at 1.5 wt% of Ni2P. The effect of Ni2P on the H2 generation rate, optical, and electrochemical property of the composite was systematically studied. Moreover, the band structure of Ni2P was calculated based on density functional theory, together with the photo-electrochemical property, the detailed role of Ni2P for the H2 generation was revealed.
Synthesis of Co-catalyst
Firstly, 20 mL deionized water containing 2.61 g nickel nitrate and 2.52 g hexamethylenetetramine was transferred to a Teflon autoclave and heated at 120 °C for 10 h for the formation of NiOOH. After cooled down to room temperature, the NiOOH product was washed by alcohol and deionized water via centrifugation at 2000 rpm for three times and each time for 5 min. Then, a mixture of 0.22 g NiOOH and 0.44 g sodium hypophosphite was put into a tube furnace and heated at 500 °C for 2 h for phosphorizing. When it naturally cooled down to room temperature, black Ni2P powder was obtained and collected.
Synthesis of Ni2P-Cd0.5Zn0.5S Nanocomposites
To prepare Ni2P-Cd0.5Zn0.5S composite, 100 mg Ni2P powder was dispersed into 20 mL ethanol via ultrasonic processing for 1 h. Then x mL (x = 0.48, 0.96, 1.4, 3, 5) well-dispersed Ni2P solution was added into a 20 mL ethylene glycol solution containing 272.6 mg ZnCl2 and 456.7 mg CdCl2∙2.5H2O, and was heated to 170 °C with continuous stirring under nitrogen protection. After the addition of 20 mL ethylene glycol solution dissolving 960.7 mg Na2S∙9H2O, the solution was heated to 180 °C and held for 1 h for the growth of Cd0.5Zn0.5S on Ni2P. Finally, the samples were washed by alcohol and deionized water respectively for three times. By weighing the final xNi2P-Cd0.5Zn0.5S composites, the weight percents (wt%) were determined to be 0.5 (x = 0.48), 1 (x = 0.96), 1.5 (x = 1.4), 3 (x = 3), 5 (x = 5). As a comparison, pure Cd0.5Zn0.5S QDs were synthesized via the similar method except the addition of Ni2P.
Morphology, Structure, and Optical Properties Characterization
The morphology, microstructure, and composition were characterized by field emission scanning electron microscopy (FESEM, JSM-7100F, JEOL) and transmission electron microscopy (TEM, FEI Tecnai 20) equipped with scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDX). Powder X-ray diffraction (XRD) patterns were recorded on a Bruker AXS D8 X-ray diffractometer with Cu Kα (λ = 1.54056 Å). Elemental composition, chemical, and valence states were studied by (valence band) X-ray photoelectron spectroscopy (XPS) measurements (XPS, Escalab 250Xi) with Al Kα radiation. UV-Vis absorption was investigated by an UV-Vis spectrophotometer (UV-3600, Shimadzu) equipped with an integrating sphere device, and the weight/volume ratio of sample to deionized water was kept at 1 mg/10 mL. Photoluminescence (PL) measurements were carried out on a 7000 FL spectrophotometer (Hitachi, F7000) with an excitation wavelength of 400 nm. Before the PL measurements, pure Cd0.5Zn0.5S QDs and the composites were well dispersed in ethanol, and the concentration of Cd0.5Zn0.5S was maintained at 0.5 mg/mL for all the samples.
Linear Sweep Voltammetry (LSV) and Electrochemical Impedance Spectra (EIS) Measurements
LSV measurements were conducted in 1 M NaOH electrolyte (pH = 14) in an electrochemical work station (CHI 760E, CH Instruments) with a typical three-electrode configuration. A Pt foil and a saturated Ag/AgCl were used as the counter and reference electrode, respectively. The potentials were converted to those vs reversible hydrogen electrode (RHE) by the equation E(vs RHE) = E(vs Ag/AgCl) + EAg/AgCl (ref) + 0.0591 V × pH, where (EAg/AgCl (ref) = 0.1976 V vs NHE (normal hydrogen electrode) at 25 °C) . Electrochemical impedance spectra (EIS) measurements were carried out in darkness at 0.5 V vs RHE with an amplitude of 5 mV and the electrolyte of 0.35 M Na2SO3 and 0.25 M Na2S aqueous solution by using a similar three-electrode system. The working electrode was made via spreading ~ 2 mg product (dispersed in 5 mL ethanol) over 4 cm2 area FTO substrate and dried at 70 °C for 5 h. The frequency range was kept within 0.1 Hz ~ 100 kHz, and the spectra were analyzed by the Z-View program (Scribner Associates Inc.).
Photocatalytic (PC) H2 Generation
Before H2 production, the photocatalysts with different mass (1, 5, and 10 mg) were dispersed in a sealed quartz reactor (volume 40 mL, 5 cm × 5 cm × 1.6 cm) with 15 mL 0.75 M Na2S and 1.05 M Na2SO3 aqueous solution. After degassing for 30 min by nitrogen, the photocatalytic experiment was performed under the irradiation of a 300 W Xe (PLS-SXE300/300UV, Perfect Light) lamp with a cut-off filter of 420 nm and an incident power of 300 mW/cm2. The catalytic solution was kept continually stirring during the whole PC experiment. In every hour, 1-mL gas production was collected and analyzed by a gas chromatograph (GC-2018, Shimadzu, Japan, TCD). Further cycling stability experiment was performed under the same condition. Paralleling experiments with the feeding dosage of photocalysts from 15 to 100 mg were conducted in 100 mL electrolyte of Na2S and Na2SO3 in a larger reactor (volume 150 mL) under the same illumination. The solar to hydrogen efficiency (STH) was calculated by the flowing equation:
The energy and electronic properties of bulk Ni2P were calculated using density functional theory (DFT) method. Vienna Ab-initio Simulation Package (VASP)  was adopted during the calculations with the projector augmented wave pseudo potentials (PAW) , and the Perdew-Burke-Ernzerhof type (PBE) generalized gradient approximation (GGA)  exchange–correlational functional methods. A Brillouin zone with a 9 × 9 × 9 Monkhorst−Pack Γpoint grid , a kinetic energy cut off with 450 eV, and an energy criterion of 10− 6 eV were applied for geometric optimization until the residual forces were converged to less than 0.01 eV/Å. The bulk model of hexagonal Ni2P with P-62M symmetry was taken into account. After fully structure optimized, the lattice parameter of Ni2P (a = b = 5.86918 Å, and c = 3.37027 Å) can be obtained, which is well consistent with the reported values .
Results and Discussion
Figure 1a, b show the morphology of Ni2P before and after the composition with Cd0.5Zn0.5S QDs (Ni2P wt%: 1.5%). Pure Ni2P has a flower-like morphology which is composed of many crossed nanosheets with the thickness less than 20 nm and planar size from several tens nanometer to micrometer scope. From the XRD pattern of pure Ni2P in Fig. 1c, diffraction peaks of (111), (201), (210), and (300) planes can be clearly observed at 40.7°, 44.6°, 47.4°, and 54.2°, respectively, which correspond to hexagonal Ni2P (JCPDF no. 89-2742). After loaded by Cd0.5Zn0.5S QDs, the surface of the nanosheets become rather rough, and plenty of nanoparticles with size less than 10 nm can be distinguished on the pristine Ni2P skeleton. At the same time, the XRD refraction peaks of Cd0.5Zn0.5S (JCPDF no. 89-2943) (100), (002), (101), and (110) planes can be clearly found at 26.0°, 27.8°, 29.6°, and 45.9°, respectively [6, 45], while the diffraction signal of Ni2P is greatly depressed because of the low weight ratio (1.5 wt%) of Ni2P to Cd0.5Zn0.5S. The coexistence of Cd0.5Zn0.5S and Ni2P was demonstrated by the X-ray photoelectron spectrometer (XPS) fine and survey spectra in Fig. 1d–f. Except the oxygen and carbon signals arising from the air absorption, only Ni, P, Cd, Zn, and S can be detected, which rules out the possibility of other impurities. The peaks at 855.5 and 873.9 eV can be assigned to Ni 2p3/2 and 2p1/2, respectively, and the peak of P 2p3/2 can be found at 133.6 eV [16, 46]. Concurrently, the doublet peaks of Zn 2p, Cd 3d, and S 2p suggest the bivalent Zn2+, Cd2+, and S2− from Cd0.5Zn0.5S QDs [3, 34, 47]. In brief, the growth of Cd0.5Zn0.5S on Ni2P nanosheets has been established for the formation of Ni2P-Cd0.5Zn0.5S nanocomposites.
The microstructure and elemental composition of the samples were further investigated by TEM-related techniques. From the different magnification TEM images of pure Ni2P (Fig. 2a, b), the nanosheets are porous and composed of cross-linked irregular nanoparticles with size of ~ 15–30 nm. The selected area electron diffraction pattern (SAED) in Fig. 2c shows the diffraction ring of Ni2P (111), (201), (210), and (300) planes. The diffractive signals of high-index planes such as (222), (402), and (420) can also be detected due to the strong multi-scattering of the high-energy electrons. After composited with Cd0.5Zn0.5S, the intercrossed Ni2P nanosheets were covered by plenty of smaller nanoparticles with size of ~ 7 nm (Fig. 2d). The EDX spectra (inset, Fig. 2f) clearly shows the signal of Ni, P, Cd, Zn, and S, indicative of the coexistence of Ni2P and Cd0.5Zn0.5S. From the SAED pattern (Fig. 2f), strong diffractive rings of Cd0.5Zn0.5S (002), (110), and (200) planes (denoted by yellow dash lines) can be clearly distinguished along with the weak signals of Ni2P (300), (402), and (420) (marked by white dash lines), suggesting the good composition of Ni2P with QDs. It is noticeable that Ni2P (300) ring overlaps with Cd0.5Zn0.5S (110) and (200) planes, making it hard to be distinguished. The high-resolution TEM image of Ni2P-Cd0.5Zn0.5S sample in Fig. 2e further shows the lattice fringes with spacing of 0.34 and 0.22 nm, which corresponds to the Cd0.5Zn0.5S (002) and Ni2P (111) crystal planes, respectively. The elemental EDX mappings (Fig. 2h–l) taken from the region shown by the high-angle annular dark field (HAADF) image (Fig. 2g) exhibit that Ni, P, Cd, Zn, and S are distributed uniformly among the sample, further demonstrating the successful composition of Cd0.5Zn0.5S QDs with the porous Ni2P nanosheets.
Figure 3a shows the H2 evolution rate of Ni2P-Cd0.5Zn0.5S nanocomposites varied with the content of Ni2P at the feeding dosage of 1 mg in a 40 mL reactor. Pure Cd0.5Zn0.5S shows a photocatalytic H2 evolution rate of 12.6 μM h− 1 mg− 1, and pure Ni2P shows negligible hydrogen generation. With the addition of Ni2P, the photocatalytic activity of the Ni2P-Cd0.5Zn0.5S composites has been obviously enhanced and reaches the highest value of 43.3 μM h− 1 mg− 1 at 1.5 wt% Ni2P, nearly 3.4 times higher than pure Cd0.5Zn0.5S. Further addition of Ni2P (≥ 3 wt%) will result in fast degradation of property, and the H2 evolution rate is less than pure Cd0.5Zn0.5S when Ni2P increases to 5 wt%. Such a non-linear behavior suggests that there exist an optimum Ni2P content, namely, an appropriate loading density of Cd0.5Zn0.5S on Ni2P for the photocatalytic property. At the same time, the stability of 1.5 wt% Ni2P-Cd0.5Zn0.5S was studied by cycling test (Fig. 3b). During four successive cycles that lasted for totally 16 h, the H2 generation maintained relative stable with negligible degradation, indicating the good photocatalytic stability of the composite.
The effect of the amount of catalyst on STH efficiency and H2 generation was systematically studied (Fig. 3c–d) for 1.5 wt% Ni2P-Cd0.5Zn0.5S sample. Two typical reactors with volume of 40 and 150 mL were adopted at the same illumination power density. When tested in the smaller reactor (40 mL), though both the STH and H2 generation rate increase with the catalyst’s dosage from 1 to 10 mg, the increased step is far less than that of the dosage. The STH and H2 generation rate are only 0.45% and 166 μM h− 1 when the dosage of the catalyst increased to 10 mg, nearly 3.8 times of the 1 mg sample. For the larger reactor (150 mL), distinct increase in STH and H2 generation can be found with the dosage increased from 15 to 100 mg, and a 1.53% STH and a 700 μM h− 1 of H2 generation can be achieved at the dosage of 100 mg, nearly 3.1 times of the 15 mg catalyst. Considering that the incident light has longer path when it passes through a deeper reactor, such a result shows that larger reactor will be more beneficial for the utilization of the incident light. However, the STH efficiency will be saturated once the dosage increased to about 100 mg, suggesting there exists an optimum dosage for the light utilization. The optimum H2 generation rate is superior than CdZnS QDs-2D g-C3N4 microribbons (H2 generation rate 33.4 mM h− 1 g− 1) , Cd0.1Zn0.9S nanoparticles-carbon nanotubes (rate: 1563 μM h− 1 g− 1) , a sandwich-structured C3N4/Au/CdZnS photocatalyst (rate 6.15 mM h− 1 g− 1) , and CdS QDs-sensitized Zn1−xCdxS solid solutions (rate 2128 μM h− 1 g− 1) .
To reveal the mechanism for the enhanced photocatalytic property and detailed role of Ni2P, both the optical and electrochemical property of pure Ni2P, Cd0.5Zn0.5S, and the composites were studied by Fig. 4. From the absorption spectra (Fig. 4a), pure Cd0.5Zn0.5S exhibits an absorption edge at 506 nm, corresponding to the band gap of 2.45 eV [13, 49]. For pure Ni2P (the inset), wide absorption over the whole visible range can be found. After the composition, besides the absorption in range < 506 nm, obvious tails over the visible wavelength > 506 nm can be found, which can be attributed to the contribution from Ni2P. As the visible absorption in longer wavelength increases with Ni2P, the composite shows reduced absorption of Cd0.5Zn0.5S (< 506 nm). At the same time, the photoluminescence spectra (Fig. 4b) exhibit that pure Cd0.5Zn0.5S has intensive band edge luminescence at ~ 620 nm when excited at the wavelength of 400 nm. After composition, it will be degraded gradually with the addition of Ni2P. Considering that higher content of Ni2P will induce more Ni2P/Cd0.5Zn0.5S interfaces which help to enhance charge transfer and suppress charge recombination, the decrease of PL intensity can be understood by the reduced carrier recombination and enhanced charge transfer at the Ni2P/Cd0.5Zn0.5S interface.
The effective role of Ni2P in prompting charge transfer can also be reflected by the EIS spectra depending on Ni2P content (Fig. 4c). As shown by the equivalent circuit (inset, Fig. 4c), the charge transfer resistance (Rct) at catalyst/electrolyte interface can be evaluated by the semicircle radius of the Nyquist plots based on R-C equivalent circuit. The equivalent series resistance (ESR) can be obtained from the intersection of the curve and the real resistance (Z’) axis, while the charge-transfer resistance (Rct) corresponds to the width of the semicircle plotted at higher frequencies. The RCT of pure Cd0.5Zn0.5S is 17,320 Ω, indicative of its semiconductor nature. After the composition with 1, 1.5, and 3 wt% Ni2P, RCT decreases gradually to 8432, 7721, and 5473 Ω, respectively, suggesting the enhancement of Ni2P in the electrical conductivity. Indeed, Ni2P has been considered as a good electrocatalyst toward HER [44, 50, 51]. From the LSV curve of pure Ni2P on Ni foam shown in Fig. 4d, the Ni2P has good HER activity with overpotentials of 84 mV and 201 mV to attach the current density of 10 and 50 mA/cm2 (without iR-correction), respectively. The EIS spectrum (inset Fig. 4d) exhibits that Ni2P has a very low RCT (~ 7.3 Ω), indicating the metallic character of Ni2P. Therefore, Ni2P can not only increase the electrical conductivity at Cd0.5Zn0.5S/Ni2P interface, but also supply effective active sites for HER, then leading to enhanced photocatalytic property of the composite.
Considering that the addition of Ni2P decreased the absorption at wavelength < 506 nm, it is necessary to demonstrate whether the light absorption of Ni2P can be utilized to generate hydrogen. The band structure of Ni2P was then studied by DFT calculation. Figure 5a, b presents the ball and stick model of bulk Ni2P and the calculated band structure. From Fig. 5b, no band gap can be detected, suggesting the metallic characteristic of Ni2P, which agrees well with the above EIS result. This indicates that the photoelectrons are mainly attributed to the photo-excitation of Cd0.5Zn0.5S rather than Ni2P. Moreover, the Fermi level of Ni2P (obtained from out car file) locates at 1.03 V vs. NHE, much lower than the conductive band minimum (CBM) level (− 1.04 V vs NHE) of Cd0.5Zn0.5S QDs .
Accordingly, the schematic mechanism was demonstrated for the photocatalytic H2 evolution of the composite by Fig. 5c. The location of Fermi level of Ni2P makes it energetically favorable for the transfer of photo-generated electrons from Cd0.5Zn0.5S to Ni2P, then prompts the separation of photo-excited electrons and holes at the interface, resulting in the depression of charge recombination. Concurrently, H2 will evolve efficiently at the active sites of Ni2P due to the good HER activity and large specific surface area of the composites. The positive roles of Ni2P in charge transfer and HER activity will dominate at the lower content of Ni2P (≤ 1.5 wt%). When the content surpasses 1.5 wt%, the shading effect of Ni2P in light absorption will overcome the positive aspect, leading to the degradation of H2 generation. An optimum photocatalytic property will be achieved at 1.5 wt% Ni2P when the two effects reach a balance.
A reverse structure of Cd0.5Zn0.5S QDs on Ni2P porous nanosheets were fabricated for efficient photocatalytic H2 production. The Ni2P porous nanosheets were composed of 15–30-nm-sized nanoparticles that allows the effective loading of 7-nm-sized Cd0.5Zn0.5S QDs. As the charge separation and transfer property is enhanced with the addition of Ni2P from 0 to 5 wt%, a competitive shading effect that unbeneficial for the light absorption of Cd0.5Zn0.5S is induced. An optimum photocatalytic H2 generation of 43.3 μM h− 1 (dosage 1 mg) will be achieved at 1.5 wt% Ni2P. Based on the optimum content, the photocatalytic dependence on feeding dosage of catalyst shows that the STH efficiency will reach the highest value of 1.5% at the dosage of 100 mg. The high HER activity and band structure of Ni2P were revealed, confirming the effective role of Ni2P in prompting photocatalytic H2 evolution dynamics from both experimental and theoretical aspects. The heterostructure of Cn0.5Zn0.5S QDs-Ni2P porous nanosheets can not only help to prompt the photo-excited charge separation and transfer, but also speed up the dynamics of hydrogen evolution reaction via the co-catalytic role of Ni2P, thus enhances the photocatalytic hydrogen generation property. Such a method can be applied to other catalysts toward efficient photocatalytic property.
Conductive band minimum
Density functional theory
Energy dispersive X-ray spectroscopy
Electrochemical impedance spectra
Field emission scanning electron microscopy
Fluorine-doped tin oxide
Generalized gradient approximation
Hydrogen evolution reaction
Linear sweep voltammetry
Normal hydrogen electrode
Reversible hydrogen electrode
Scanning transmission electron microscopy
Solar to hydrogen
Transmission electron microscopy
Vienna Ab-initio Simulation Package
X-ray photoelectron spectroscopy
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This work was supported by the National Natural Science Foundation of China (nos. 51472080, 51602094, 11104097), Open Research Fund Program of the State Key Laboratory of Low-dimensional Quantum Physics (no. KF201705), and Excellent Youth Foundation of Hubei Province (no. 2017CFA038). We thank the helps on DFT part from Prof. Zhongbin Huang and Dr. Hui Yang from Center for Computational Science, HUFPET.
National Natural Science Foundation of China (51472080, 51602049, and 11104097) and Open Research Fund Program of the State Key Laboratory of Low-dimensional Quantum Physics.
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Xiao, L., Su, T., Wang, Z. et al. Enhanced Photocatalytic Hydrogen Evolution by Loading Cd0.5Zn0.5S QDs onto Ni2P Porous Nanosheets. Nanoscale Res Lett 13, 31 (2018) doi:10.1186/s11671-018-2438-0
- Quantum dot
- Hydrogen evolution