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Pt nanoparticle decorated InP nanopore arrays for enhanced photoelectrochemical performance
Accepted Manuscript Pt nanoparticle decorated InP nanopore arrays for enhanced photoelectrochemical performance Qiang Li, Maojun Zheng, Yuxiu You, Pengjie Liu, Li Ma, Wenzhong Shen PII:
S0925-8388(17)33829-X
DOI:
10.1016/j.jallcom.2017.11.082
Reference:
JALCOM 43775
To appear in:
Journal of Alloys and Compounds
Received Date: 22 September 2017 Revised Date:
6 November 2017
Accepted Date: 7 November 2017
Please cite this article as: Q. Li, M. Zheng, Y. You, P. Liu, L. Ma, W. Shen, Pt nanoparticle decorated InP nanopore arrays for enhanced photoelectrochemical performance, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.11.082. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Pt nanoparticle decorated InP nanopore arrays for enhanced photoelectrochemical performance
a
a, b
a, c,*
Maojun Zheng,
a
a
Yuxiu You, Pengjie Liu, Li Ma,
d
Wenzhong Shen
a
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Qiang Li,
Key Laboratory of Artificial Structure and Quantum Control, Ministry of Education, Department of
Physics and Astronomy, Shanghai Jiao Tong University, Shanghai, 200240, People’s Republic of China
c
College of Physics and Electronic Information, Huaibei Normal University, Huaibei, 235000, P.R. China
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b
Collaborative Innovation Center of Advanced Microstructures, Nanjing,210093, People’s Republic of
d
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China
School of Chemistry and Chemical Technology, Shanghai Jiao Tong University, Shanghai,
200240,People’s Republic of China
E-mail:[email protected]
Abstract
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Here, we report the scalable preparation of Pt nanoparticles decorated indium phosphide nanopore arrays (Pt@InP NPAs) using a facile two-step etching method followed by a dipping and electrodeposition process. The morphology and element composition of the
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Pt@InP heterostructures were characterized. Photoelectrochemical properties of the
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Pt@InP electrodes were investigated under simulated sunlight irradiation. Compared to pristine InP NPAs, the resulting Pt@InP NPAs show a greatly increased photocurrent density and a largely improved onset potential for photoelectrochemical (PEC) hydrogen evolution. Electrochemical impedance spectroscopy reveals that the improved PEC performance is attributed to the enhanced charge transfer and the improved charge separation at Pt@InP NPAs/electrolyte interfaces.
KEY WORDS
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Pt@InP nanopore arrays; photoelectrochemical; hydrogen evolution
1. Introduction
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Spurred by the gradual depletion of fossil fuel reserves and pressing environmental issues, effective utilization of green and renewable energy sources becomes urgent [1-4]. Solar light is an inexpensive, clean and abundant renewable source available on earth.
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The amount of energy that strikes the Earth yearly is approximately ten thousand times the total energy that is consumed on this planet [5]. However, sunlight’s intermittent
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nature is one of the issues that limit widespread harvesting of solar energy for societal power infrastructure [6]. Much research efforts have been devoted to enabling practical conversion of solar energy into usable energy fuels. Hydrogen is a good candidate since it releases energy by reacting with oxygen leaving the only product of water. Among
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possible ways to produce hydrogen, photoelectrochemical (PEC) water splitting using semiconducting electrodes is one of the most promising methods [7-12]. A variety of semiconductor electrodes in different configurations have been demonstrated to obtain solar-to-hydrogen
conversion
[13-20].
Although
many
remarkable
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efficient
improvements have been achieved in this field, PEC hydrogen evolution, however, still
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suffers from unsatisfactory conversion efficiencies. Among Ⅲ-Ⅴsemiconductors, InP has a narrow band gap value of 1.34 eV, which is
optimal for maximum efficiency in single junction solar cells [21, 22]. The specific properties of InP are also attractive as efficient photoelectrodes for PEC H2 generation [23-26]. In order to reduce material cost of InP-based photoelectrodes, recently textured surfaces have been suggested [27-32]. Nanostructured electrode not only dramatically
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increases light absorption but also enlarges electrode/electrolyte surface area, and thus achieve high conversion efficiency. However, the dilemma is that the PEC efficiency of nanostructured InP electrodes is limited by charge carrier recombination at the electrode
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surface trapped states. The high recombination rate of photogenerated charge-carriers before arriving at the photocatalyst surfaces largely limits the efficiency of almost all nanostructured semiconductor photocatalysts.
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Several strategies have been proposed to promote charge separation such as the formation of semiconductor heterostructures and the use of metal doping [33-38]. Among
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them, surface platinization has been regarded as one of the most common but important strategies. In general, Pt/semiconductor heterogeneous composites can offer two benefits. First, a Schottky junction is formed between the Pt nanoparticles and the semiconductor. The equilibrium alignment of the Fermi level of the Pt and semiconductor creates a built-
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in electric field near the interface, which can enhance the separation of photo-generated electrons and holes, and thus improve the performance of devices for solar energy-related applications [39, 40]. This makes the Pt act as the electron sinks for the photo-induced
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electron [41]. Second, the Pt nanoparticle can function as an effective co-catalyst to reduce the overpotential for surface electrochemical reactions or gas evolution processes.
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For example, deposition of Pt nanoparticles onto TiO2 was reported to greatly improve the photocatalytic hydrogen production [42]. Here, we report scalable preparation of Pt nanoparticles modified indium phosphide
nanopore arrays (Pt@InP NPAs) using a facile two-step etching method followed by a dipping and electrodeposition process. The highly ordered InP NPAs films as backbone were fabricated by a two-step etching method. The InP NPAs films were selected in this
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work for Pt deposition due to their high surface area, single crystallinity, and superior electron-transfer performance originating from highly oriented one-dimensional (1D) structures, as well as non-reflectivity owing to strong light scattering and absorption. Pt
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nanoparticles were deposited onto the InP nanopore using a dipping and electrodeposition method. This technique makes Pt nanoparticles successfully plated in the InP NPAs, thus greatly improve the PEC performance. Under simulated sunlight (AM 1.5G, 100 mWcm), the modified InP NPAs electrode yielded a anodic photocurrent of 3.44 mAcm-2 at -0.8
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2
V versus Ag/AgCl, which was about 5.5 times higher than that for the unmodified InP
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NPAs electrode. Furthermore, the onset potential of Pt-modified electrode exhibits 150 mV of cathodic shift relative to the pristine InP. These results indicates that surface modification of Pt was an effective approach to enhance the photocurrent and onset potential of the InP NPAs photoanodes.
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2. Experimental section
2.1. Preparation of the Pt@InP NPAs.
The Pt@InP NPAs were synthesized using a facile two-step etching method
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followed by a dipping and electrodeposition process as diagramed in figure 1. The first step was the formation of the ordered InP NPAs with a two-step etching method [29].
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The starting material was Sn-doped InP wafer, which was firstly etched at a constant voltage in 1 M HCl aqueous solution. Next, the specimen was immersed in a mixture of pure HCl and H3PO4 (HCl:H3PO4= 1:3 v/v) for a few minutes to remove the top irregular layer to obtain ordered porous InP templates with uniform and square pore arrays. Second, Pt nanoparticles were decorated onto the InP NPAs by a modified dipping and electrodeposition method [43]. In a particular synthesis, the as-prepared InP NPAs were
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immersed in a 2 g L-1 H2PtCl6 solution for 7 min, then moved into 0.1 M NaCl solution to undergo pulse electrodepositing at -2 V for 1 s, achieving deposition of Pt nanoparticles within the InP NPAs internal surface. This procedure of dipping and electrodepositing is
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denoted as one cycle, and cycles of varied number were tested. The electrodeposition was carried out in a standard three-electrode electrochemical cell, employing as-obtained InP
2.2. Characterization of the Pt@InP NPAs.
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NPAs working electrode, Ag/AgCl reference electrode and Pt mesh counter electrode.
The morphological structure of the Pt@InP NPAs was examined with a field-
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emission scanning electrode microscope (FE-SEM, FEISirion200). The chemical state of component elements in the samples was investigated by X-ray photoelectron spectroscope (XPS), which was carried out on Kratos AXIS Ultra DLD XPS instrument equipped with an Al Kα source. Energy-dispersive X-ray (EDX) spectroscopy equipped
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to the SEM was also performed to confirm their chemical compositions. 2.3. Photoelectrochemical study.
The photoelectrochemical experiments were performed under simulated AM 1.5
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(100 mW cm-2) illumination provided by a solar simulator, which is equipped with a xenon lamp (300 W) and an air mass (AM) 1.5 G filter. The mixture of 0.35 M Na2S and
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0.5 M Na2SO3 aqueous solution was used as an electrolyte after saturation with N2 gas for 30 min. A three-electrode configuration was used in the measurement, with the Pt@InP NPAs electrode serving as the working electrode, an Ag/AgCl as the reference electrode, and a square platinum mesh as the auxiliary electrode. The scan rate for all currentpotential (J-E) studies was 10 mVs−1.
3. Results and discussion
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Figure 2 depicts the top-view and cross-sectional SEM images of as-prepared pristine InP NPAs and Pt@InP NPAs. Figure 2(a) and (b) show a typical SEM image of pure InP NPAs in which highly ordered, compact, one-dimensional architechture is
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clearly evident. It is noteworthy that the nanopores were very smooth, and that these ordered InP NPAs possessed large porosity, which is highly favorable for uniform deposition of Pt particles. Fig. 2(c) and (d) is the corresponding magnified picture of
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the top-view, cross-sectional images after Pt deposition. The top view shows the absence of Pt bulks deposited on top of the InP pores (Fig. 2(c)), indicating that this
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method effectively suppresses deposition at the pore entrances, thus preventing pore clogging. The cross-sectional SEM image shows that Pt NPs were uniformly dispersed over the entire surface of the nanopore (Fig. 2(d)).
The element distributions were analyzed using mapping analysis, shown in figure 3
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(a-c). The corresponding size distribution of the Pt NPs (Fig. 3(d)) with 4 cycles of Pt deposition shows the size of Pt nanocrystallites ranges from 15 to 20 nm. The energy dispersive X-ray spectroscopy (EDX) analysis (Figure 4) shows a molar ratio of In to P
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of 1:1 in Pt@InP (4 cycles electrode), confirming the stoichiometric formation of InP. EDX analysis also shows the presence of 4.40 at% Pt in the Pt@InP NPAs.
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To determine the oxidation state of Pt in Pt@InP, XPS analysis was performed, and
the results are shown in figure 5.The survey spectrum in figure 5a shows that In, P, and Pt elements coexisted in the films. The In 3d5/2 (444.3 eV) and P 2p (129.3 eV for P 2p1/2and 128.5 eV for P 2p3/2) core levels exhibit the typical features for bulk InP except an additional component in the P 2p region at ca. 133.3 eV, which could be ascribed to native oxide. The peaks at 71.2 and 74.2 eV can be assigned to the 4f7/2 and 4f5/2 Pt metal
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states, respectively [44]. XRD pattern of Pt-decorated InP NPAs are shown in figure 6. Two diffraction peaks emerge at 31.6◦, 63.5◦ corresponding to the cubic InP (JCPDS
Pt lattice is displayed in the inset of figure 6.
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Card 01-070-2513). The relatively weak (111) diffraction peak of the face-centered cubic
Figure 7a shows the transient photocurrent-potential (J-V) curve obtained from Pt@InP NPAs photoelectrode in 0.35 M Na2S and 0.5 M Na2SO3 aqueous solution swept
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in the anodic direction from the potentials of -1.3 to -0.5V (vs Ag/AgCl). When the light is switched off, the anodic current density maintained at a very low level in the scanned
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potential range, implying a non-Faradic reaction. Upon illumination with light, a spike due to the transient effect in power excitation observed in photocurrent appears, followed by a fast decrease to steady state photocurrent. These results indicate the fabricated photoelectrodes display fast light response and excellent switching performance. The
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photocurrents increased with increasing applied potential representing a typical n-type semiconductor behavior. The current spikes indicate that photogenerated charges accumulate at the electrode-electrolyte interface and without inject to electrolyte [45],
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which could be due to surface traps and/or slow charge transfer kinetics [46]. Figure 7b shows the photocurrent-voltage (J-V) curves of the pure InP, Pt@InP
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NPAs in dark and under light illumination (AM 1.5G, 100 mW cm−2). Without illumination, both electrodes show negligible anodic current. However, in contrast to bare InP, the J-V curves of InP that was functionalized with Pt nanoparticles revealed an earlier and much more abrupt onset with strongly enhanced hydrogen evolution. This can be ascribed to the catalytic activity of the Pt nanoparticles, and also confirms the existence of Pt on the Pt@InP sample. Upon illumination, the photocurrent density of
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platinized photoanode was much higher than that of pure InP photoanode from -1.0 V to 0.55 V (vs.Ag/AgCl). Moreover, the photocurrent onset (defined as the potential at which Jp= 1 mA cm−2) of pure InP was around -0.77 V (vs. Ag/AgCl) and the onset potentials of
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composite films were negatively shifted to ca. -0.92 V (vs. Ag/AgCl) by introducing the Pt on InP films. The significantly enhanced photocurrent and negative shift of onset potentials of Pt-modified InP photoanode, can be ascribed to the improved holes transfer separation
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to the electrode surface. These results indicate that the
efficiency
of
photogenerated electron-hole pairs was enhanced in the presence of Pt particles,
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possibly by the fast trapping of photogenerated electrons, which likely results from high Schottky barrier formation between Pt and InP [47, 48].
The photocurrent measurement revealed that the amount of Pt nanoparticles loading in
the
resulting
Pt@InP
NPAs
nanocomposites
was
extremely
crucial
to
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photoelectrochemical activities. Figure 7c shows J-V curves for Pt@InP electrodes prepared with different deposition cycles. It was found that the PEC performance was best for a deposition number of 4cycles.The photocurrent density of Pt@InP loaded with
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different amount of Pt measured at a potential of -0.8 V vs Ag/AgCl was shown in figure 7d. The photocurrent firstly increased with Pt nanoparticles loading, which
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indicates that the Pt nanoparticles facilitate charge separation. However, as the Pt loading further increased, the photocurrent decreased possibly owing to covering effect of Pt NPs, which may reduce the incident light intensity as well as the contact area between InP and electrolyte. Therefore, Pt@InP NPAs with 4 cycles for Pt deposition exhibited the highest photocurrent density, 3.44 mA cm−2 at -0.8 V vs Ag/AgCl, approximately 5.5 times higher than that of pure InP NPAs. The
superior
PEC
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properties as compared with pristine InP indicate that decoration of Pt NPs in InP NPAs is an effective strategy for forming a Pt@InP heterostructure for PEC applications.
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To explain the reason for the enhancement of PEC performance, figure 8a schematically illustrates the band diagrams of individual Pt and n-InP. The Fermi level of Pt with respect to the vacuum level is 5.65 eV [48]. The positions of conduction band
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(ECB) and valence band (EVB) of InP lie 4.4 eV and 5.74 eV below the vacuum level (bandgap Eg = 1.34 eV) [49], respectively. Considering the heavily doped n-InP, its
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Fermi level is close to the conduction band. Since the Femi level of InP is lower than that of metal Pt, the energy band of the InP will bend up while they contact and a metalsemiconductor Schottky junction formed under equilibrium, as shown in figure 8b. When they are irradiated by simulated solar light (illustrated in figure 8c as process 1), electrons
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and holes are generated in the conduction and valence bands of InP, respectively (process 2). The photo-generated electrons accumulate and hence the Femi level of InP is elevated, which leads to electrons irreversible transferring from InP to metal Pt (process 3) [50].
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This charge separation process effectively reduces the chances of the recombination of photogenerated electron-hole pairs. Therefore, the lifetime of charges in Pt@InP is
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greatly extended. These accumulated charges with longer lifetimes make more holes participate in the oxidation reaction of S2- and SO32-, resulting in the enhancement of PEC performance.
To gain more insight into the principle of the enhancement of PEC performance,
electrochemical impedance spectroscopy (EIS) measurements were performed to scrutinize the interfacial properties between the electrode (i.e., Pt@InP) and the
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electrolyte. A semicircle in the Nyquist plot at high frequency represents the chargetransfer process, while the diameter of the semicircle reflects the charge-transfer resistance. A smaller arc radius implies a faster interfacial charge transfer and a more
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efficient charge separation [51]. As evident in Figure 9, the arch for Pt@InP NPAs obtained under light illumination was much smaller than that for pure InP NPAs, indicating that decoration with Pt nanoparticles significantly enhanced the electron
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mobility by reducing the recombination of electron-hole pairs.
The greatly enhanced photoelectrochemical activity of the Pt@InP NPAs was a
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direct consequence of synergetic effects of the highly ordered nanoporous InP matrix and the well-dispersed Pt nanoparticles (Scheme S1). First, the crystalline nature of InP together with its nanoporous geometry provided a large surface area for fast and efficient transfer of photogenerated electrons to Pt nanoparticles. Second, it is well known that the
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photoexcited electrons and holes can easily recombine to decrease the photocatalytic efficiency; in the present study, the Pt nanoparticles, acting as electron sinks, reduced the recombination of electrons and holes and extended their lifetime, leading to a greatly
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improved photocurrent and efficiency of H2 generation.
4. Conclusions
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In summary, we have developed a promising and efficient strategy of crafting Pt
nanoparticles loaded on ordered InP nanopore arrays by capitalizing on a modified dipping and electrodeposition method. The well-dispersed
Pt
nanoparticles
at
diameters of 15-20 nm were confirmed by SEM. Pt-decorated InP NPAs electrodes had greater separation efficiency of photogenerated electron-hole pairs compared to pristine InP NPAs electrodes. These characteristics resulted in a 5.5-fold enhancement
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in photocurrent density and greater onset potential shifts. The much improved performance demonstrates the superior ability of metal nanoparticles to modify the electronic properties of InP NPAs, and the potential of Pt-decorated InP NPAs as a
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hybrid system for solar energy conversion.
Acknowledgments
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This work was supported by the Natural Science Foundation of China (Grant nos. 11174197 and 11574203).
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Figure captions
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Figure 1. A schematic illustration for the synthetic route of the Pt@InP NPAs.
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Figure 2. (a) Top and (b) cross-sectional SEM images of InP nanopore arrays obtained from a two-step etching. SEM images of Pt nanoparticles deposited (4 cycles) on InP nanopore arrays: (c) top view (d) cross-sectional view (a high magnification is shown in the inset).
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Figure 3. (a-c) EDX elemental mapping of P, In and Pt. (d) Corresponding size distribution graph, which shows that the average size of the Pt nanoparticles is 15-20 nm.
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Figure 4. EDS pattern of Pt@InP NPAs
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Figure 5. Chemical composition analyses. (a) XPS survey spectra. (b-d) High-resolution In 3d, P 2p and Pt 4f spectra.
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Figure 6. XRD pattern of Pt@InP NPAs. The inset presents a higher magnified view of the bottom part of the Figure.
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Figure 7. PEC measurements of the photoelectrodes in 0.35 M Na2S and 0.5 M Na2SO3 aqueous solution. (a) Current density−voltage behavior of Pt@InP under interrupted illumination. (b) Linear sweep voltammetry for direct comparison of two types of InP electrodes. (c) PEC Performances of the Pt@InP photoelectrodes with different amount of Pt loading. (d) Comparison of photocurrent at -0.8 V (Ag/AgCl) with different amount of Pt loading.
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Figure 8. Band diagram analysis of (a) individual InP and Pt, (b) InP and Pt in contact, and (c) InP and Pt in contact under solar light illumination (green and red balls represent electrons and holes, respectively).
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Figure 9. Nyquist plots of Pt@InP NPAs and pristine InP NPAs obtained under light illumination conditions.
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Scheme 1. Charge transfer mechanism between Pt nanoparticles and InP under solar light.
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Highlights 1.Pt@InP nanopore arrays (NPAs) heterostructure is fabricated via facile procedures.
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2.Photoelectrochemical (PEC) property of the Pt@InP NPAs electrodes are investigated. 3.The Pt@InP NPAs dramatically improve PEC performance compared to pristine InP
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NPAs.
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4.The enhancement of the PEC performance is due to the improved charge separation.
S0925-8388(17)33829-X
DOI:
10.1016/j.jallcom.2017.11.082
Reference:
JALCOM 43775
To appear in:
Journal of Alloys and Compounds
Received Date: 22 September 2017 Revised Date:
6 November 2017
Accepted Date: 7 November 2017
Please cite this article as: Q. Li, M. Zheng, Y. You, P. Liu, L. Ma, W. Shen, Pt nanoparticle decorated InP nanopore arrays for enhanced photoelectrochemical performance, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.11.082. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Pt nanoparticle decorated InP nanopore arrays for enhanced photoelectrochemical performance
a
a, b
a, c,*
Maojun Zheng,
a
a
Yuxiu You, Pengjie Liu, Li Ma,
d
Wenzhong Shen
a
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Qiang Li,
Key Laboratory of Artificial Structure and Quantum Control, Ministry of Education, Department of
Physics and Astronomy, Shanghai Jiao Tong University, Shanghai, 200240, People’s Republic of China
c
College of Physics and Electronic Information, Huaibei Normal University, Huaibei, 235000, P.R. China
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b
Collaborative Innovation Center of Advanced Microstructures, Nanjing,210093, People’s Republic of
d
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China
School of Chemistry and Chemical Technology, Shanghai Jiao Tong University, Shanghai,
200240,People’s Republic of China
E-mail:[email protected]
Abstract
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Here, we report the scalable preparation of Pt nanoparticles decorated indium phosphide nanopore arrays (Pt@InP NPAs) using a facile two-step etching method followed by a dipping and electrodeposition process. The morphology and element composition of the
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Pt@InP heterostructures were characterized. Photoelectrochemical properties of the
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Pt@InP electrodes were investigated under simulated sunlight irradiation. Compared to pristine InP NPAs, the resulting Pt@InP NPAs show a greatly increased photocurrent density and a largely improved onset potential for photoelectrochemical (PEC) hydrogen evolution. Electrochemical impedance spectroscopy reveals that the improved PEC performance is attributed to the enhanced charge transfer and the improved charge separation at Pt@InP NPAs/electrolyte interfaces.
KEY WORDS
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Pt@InP nanopore arrays; photoelectrochemical; hydrogen evolution
1. Introduction
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Spurred by the gradual depletion of fossil fuel reserves and pressing environmental issues, effective utilization of green and renewable energy sources becomes urgent [1-4]. Solar light is an inexpensive, clean and abundant renewable source available on earth.
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The amount of energy that strikes the Earth yearly is approximately ten thousand times the total energy that is consumed on this planet [5]. However, sunlight’s intermittent
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nature is one of the issues that limit widespread harvesting of solar energy for societal power infrastructure [6]. Much research efforts have been devoted to enabling practical conversion of solar energy into usable energy fuels. Hydrogen is a good candidate since it releases energy by reacting with oxygen leaving the only product of water. Among
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possible ways to produce hydrogen, photoelectrochemical (PEC) water splitting using semiconducting electrodes is one of the most promising methods [7-12]. A variety of semiconductor electrodes in different configurations have been demonstrated to obtain solar-to-hydrogen
conversion
[13-20].
Although
many
remarkable
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efficient
improvements have been achieved in this field, PEC hydrogen evolution, however, still
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suffers from unsatisfactory conversion efficiencies. Among Ⅲ-Ⅴsemiconductors, InP has a narrow band gap value of 1.34 eV, which is
optimal for maximum efficiency in single junction solar cells [21, 22]. The specific properties of InP are also attractive as efficient photoelectrodes for PEC H2 generation [23-26]. In order to reduce material cost of InP-based photoelectrodes, recently textured surfaces have been suggested [27-32]. Nanostructured electrode not only dramatically
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increases light absorption but also enlarges electrode/electrolyte surface area, and thus achieve high conversion efficiency. However, the dilemma is that the PEC efficiency of nanostructured InP electrodes is limited by charge carrier recombination at the electrode
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surface trapped states. The high recombination rate of photogenerated charge-carriers before arriving at the photocatalyst surfaces largely limits the efficiency of almost all nanostructured semiconductor photocatalysts.
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Several strategies have been proposed to promote charge separation such as the formation of semiconductor heterostructures and the use of metal doping [33-38]. Among
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them, surface platinization has been regarded as one of the most common but important strategies. In general, Pt/semiconductor heterogeneous composites can offer two benefits. First, a Schottky junction is formed between the Pt nanoparticles and the semiconductor. The equilibrium alignment of the Fermi level of the Pt and semiconductor creates a built-
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in electric field near the interface, which can enhance the separation of photo-generated electrons and holes, and thus improve the performance of devices for solar energy-related applications [39, 40]. This makes the Pt act as the electron sinks for the photo-induced
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electron [41]. Second, the Pt nanoparticle can function as an effective co-catalyst to reduce the overpotential for surface electrochemical reactions or gas evolution processes.
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For example, deposition of Pt nanoparticles onto TiO2 was reported to greatly improve the photocatalytic hydrogen production [42]. Here, we report scalable preparation of Pt nanoparticles modified indium phosphide
nanopore arrays (Pt@InP NPAs) using a facile two-step etching method followed by a dipping and electrodeposition process. The highly ordered InP NPAs films as backbone were fabricated by a two-step etching method. The InP NPAs films were selected in this
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work for Pt deposition due to their high surface area, single crystallinity, and superior electron-transfer performance originating from highly oriented one-dimensional (1D) structures, as well as non-reflectivity owing to strong light scattering and absorption. Pt
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nanoparticles were deposited onto the InP nanopore using a dipping and electrodeposition method. This technique makes Pt nanoparticles successfully plated in the InP NPAs, thus greatly improve the PEC performance. Under simulated sunlight (AM 1.5G, 100 mWcm), the modified InP NPAs electrode yielded a anodic photocurrent of 3.44 mAcm-2 at -0.8
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2
V versus Ag/AgCl, which was about 5.5 times higher than that for the unmodified InP
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NPAs electrode. Furthermore, the onset potential of Pt-modified electrode exhibits 150 mV of cathodic shift relative to the pristine InP. These results indicates that surface modification of Pt was an effective approach to enhance the photocurrent and onset potential of the InP NPAs photoanodes.
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2. Experimental section
2.1. Preparation of the Pt@InP NPAs.
The Pt@InP NPAs were synthesized using a facile two-step etching method
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followed by a dipping and electrodeposition process as diagramed in figure 1. The first step was the formation of the ordered InP NPAs with a two-step etching method [29].
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The starting material was Sn-doped InP wafer, which was firstly etched at a constant voltage in 1 M HCl aqueous solution. Next, the specimen was immersed in a mixture of pure HCl and H3PO4 (HCl:H3PO4= 1:3 v/v) for a few minutes to remove the top irregular layer to obtain ordered porous InP templates with uniform and square pore arrays. Second, Pt nanoparticles were decorated onto the InP NPAs by a modified dipping and electrodeposition method [43]. In a particular synthesis, the as-prepared InP NPAs were
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immersed in a 2 g L-1 H2PtCl6 solution for 7 min, then moved into 0.1 M NaCl solution to undergo pulse electrodepositing at -2 V for 1 s, achieving deposition of Pt nanoparticles within the InP NPAs internal surface. This procedure of dipping and electrodepositing is
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denoted as one cycle, and cycles of varied number were tested. The electrodeposition was carried out in a standard three-electrode electrochemical cell, employing as-obtained InP
2.2. Characterization of the Pt@InP NPAs.
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NPAs working electrode, Ag/AgCl reference electrode and Pt mesh counter electrode.
The morphological structure of the Pt@InP NPAs was examined with a field-
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emission scanning electrode microscope (FE-SEM, FEISirion200). The chemical state of component elements in the samples was investigated by X-ray photoelectron spectroscope (XPS), which was carried out on Kratos AXIS Ultra DLD XPS instrument equipped with an Al Kα source. Energy-dispersive X-ray (EDX) spectroscopy equipped
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to the SEM was also performed to confirm their chemical compositions. 2.3. Photoelectrochemical study.
The photoelectrochemical experiments were performed under simulated AM 1.5
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(100 mW cm-2) illumination provided by a solar simulator, which is equipped with a xenon lamp (300 W) and an air mass (AM) 1.5 G filter. The mixture of 0.35 M Na2S and
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0.5 M Na2SO3 aqueous solution was used as an electrolyte after saturation with N2 gas for 30 min. A three-electrode configuration was used in the measurement, with the Pt@InP NPAs electrode serving as the working electrode, an Ag/AgCl as the reference electrode, and a square platinum mesh as the auxiliary electrode. The scan rate for all currentpotential (J-E) studies was 10 mVs−1.
3. Results and discussion
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Figure 2 depicts the top-view and cross-sectional SEM images of as-prepared pristine InP NPAs and Pt@InP NPAs. Figure 2(a) and (b) show a typical SEM image of pure InP NPAs in which highly ordered, compact, one-dimensional architechture is
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clearly evident. It is noteworthy that the nanopores were very smooth, and that these ordered InP NPAs possessed large porosity, which is highly favorable for uniform deposition of Pt particles. Fig. 2(c) and (d) is the corresponding magnified picture of
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the top-view, cross-sectional images after Pt deposition. The top view shows the absence of Pt bulks deposited on top of the InP pores (Fig. 2(c)), indicating that this
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method effectively suppresses deposition at the pore entrances, thus preventing pore clogging. The cross-sectional SEM image shows that Pt NPs were uniformly dispersed over the entire surface of the nanopore (Fig. 2(d)).
The element distributions were analyzed using mapping analysis, shown in figure 3
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(a-c). The corresponding size distribution of the Pt NPs (Fig. 3(d)) with 4 cycles of Pt deposition shows the size of Pt nanocrystallites ranges from 15 to 20 nm. The energy dispersive X-ray spectroscopy (EDX) analysis (Figure 4) shows a molar ratio of In to P
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of 1:1 in Pt@InP (4 cycles electrode), confirming the stoichiometric formation of InP. EDX analysis also shows the presence of 4.40 at% Pt in the Pt@InP NPAs.
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To determine the oxidation state of Pt in Pt@InP, XPS analysis was performed, and
the results are shown in figure 5.The survey spectrum in figure 5a shows that In, P, and Pt elements coexisted in the films. The In 3d5/2 (444.3 eV) and P 2p (129.3 eV for P 2p1/2and 128.5 eV for P 2p3/2) core levels exhibit the typical features for bulk InP except an additional component in the P 2p region at ca. 133.3 eV, which could be ascribed to native oxide. The peaks at 71.2 and 74.2 eV can be assigned to the 4f7/2 and 4f5/2 Pt metal
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states, respectively [44]. XRD pattern of Pt-decorated InP NPAs are shown in figure 6. Two diffraction peaks emerge at 31.6◦, 63.5◦ corresponding to the cubic InP (JCPDS
Pt lattice is displayed in the inset of figure 6.
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Card 01-070-2513). The relatively weak (111) diffraction peak of the face-centered cubic
Figure 7a shows the transient photocurrent-potential (J-V) curve obtained from Pt@InP NPAs photoelectrode in 0.35 M Na2S and 0.5 M Na2SO3 aqueous solution swept
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in the anodic direction from the potentials of -1.3 to -0.5V (vs Ag/AgCl). When the light is switched off, the anodic current density maintained at a very low level in the scanned
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potential range, implying a non-Faradic reaction. Upon illumination with light, a spike due to the transient effect in power excitation observed in photocurrent appears, followed by a fast decrease to steady state photocurrent. These results indicate the fabricated photoelectrodes display fast light response and excellent switching performance. The
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photocurrents increased with increasing applied potential representing a typical n-type semiconductor behavior. The current spikes indicate that photogenerated charges accumulate at the electrode-electrolyte interface and without inject to electrolyte [45],
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which could be due to surface traps and/or slow charge transfer kinetics [46]. Figure 7b shows the photocurrent-voltage (J-V) curves of the pure InP, Pt@InP
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NPAs in dark and under light illumination (AM 1.5G, 100 mW cm−2). Without illumination, both electrodes show negligible anodic current. However, in contrast to bare InP, the J-V curves of InP that was functionalized with Pt nanoparticles revealed an earlier and much more abrupt onset with strongly enhanced hydrogen evolution. This can be ascribed to the catalytic activity of the Pt nanoparticles, and also confirms the existence of Pt on the Pt@InP sample. Upon illumination, the photocurrent density of
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platinized photoanode was much higher than that of pure InP photoanode from -1.0 V to 0.55 V (vs.Ag/AgCl). Moreover, the photocurrent onset (defined as the potential at which Jp= 1 mA cm−2) of pure InP was around -0.77 V (vs. Ag/AgCl) and the onset potentials of
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composite films were negatively shifted to ca. -0.92 V (vs. Ag/AgCl) by introducing the Pt on InP films. The significantly enhanced photocurrent and negative shift of onset potentials of Pt-modified InP photoanode, can be ascribed to the improved holes transfer separation
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to the electrode surface. These results indicate that the
efficiency
of
photogenerated electron-hole pairs was enhanced in the presence of Pt particles,
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possibly by the fast trapping of photogenerated electrons, which likely results from high Schottky barrier formation between Pt and InP [47, 48].
The photocurrent measurement revealed that the amount of Pt nanoparticles loading in
the
resulting
Pt@InP
NPAs
nanocomposites
was
extremely
crucial
to
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photoelectrochemical activities. Figure 7c shows J-V curves for Pt@InP electrodes prepared with different deposition cycles. It was found that the PEC performance was best for a deposition number of 4cycles.The photocurrent density of Pt@InP loaded with
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different amount of Pt measured at a potential of -0.8 V vs Ag/AgCl was shown in figure 7d. The photocurrent firstly increased with Pt nanoparticles loading, which
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indicates that the Pt nanoparticles facilitate charge separation. However, as the Pt loading further increased, the photocurrent decreased possibly owing to covering effect of Pt NPs, which may reduce the incident light intensity as well as the contact area between InP and electrolyte. Therefore, Pt@InP NPAs with 4 cycles for Pt deposition exhibited the highest photocurrent density, 3.44 mA cm−2 at -0.8 V vs Ag/AgCl, approximately 5.5 times higher than that of pure InP NPAs. The
superior
PEC
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properties as compared with pristine InP indicate that decoration of Pt NPs in InP NPAs is an effective strategy for forming a Pt@InP heterostructure for PEC applications.
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To explain the reason for the enhancement of PEC performance, figure 8a schematically illustrates the band diagrams of individual Pt and n-InP. The Fermi level of Pt with respect to the vacuum level is 5.65 eV [48]. The positions of conduction band
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(ECB) and valence band (EVB) of InP lie 4.4 eV and 5.74 eV below the vacuum level (bandgap Eg = 1.34 eV) [49], respectively. Considering the heavily doped n-InP, its
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Fermi level is close to the conduction band. Since the Femi level of InP is lower than that of metal Pt, the energy band of the InP will bend up while they contact and a metalsemiconductor Schottky junction formed under equilibrium, as shown in figure 8b. When they are irradiated by simulated solar light (illustrated in figure 8c as process 1), electrons
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and holes are generated in the conduction and valence bands of InP, respectively (process 2). The photo-generated electrons accumulate and hence the Femi level of InP is elevated, which leads to electrons irreversible transferring from InP to metal Pt (process 3) [50].
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This charge separation process effectively reduces the chances of the recombination of photogenerated electron-hole pairs. Therefore, the lifetime of charges in Pt@InP is
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greatly extended. These accumulated charges with longer lifetimes make more holes participate in the oxidation reaction of S2- and SO32-, resulting in the enhancement of PEC performance.
To gain more insight into the principle of the enhancement of PEC performance,
electrochemical impedance spectroscopy (EIS) measurements were performed to scrutinize the interfacial properties between the electrode (i.e., Pt@InP) and the
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electrolyte. A semicircle in the Nyquist plot at high frequency represents the chargetransfer process, while the diameter of the semicircle reflects the charge-transfer resistance. A smaller arc radius implies a faster interfacial charge transfer and a more
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efficient charge separation [51]. As evident in Figure 9, the arch for Pt@InP NPAs obtained under light illumination was much smaller than that for pure InP NPAs, indicating that decoration with Pt nanoparticles significantly enhanced the electron
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mobility by reducing the recombination of electron-hole pairs.
The greatly enhanced photoelectrochemical activity of the Pt@InP NPAs was a
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direct consequence of synergetic effects of the highly ordered nanoporous InP matrix and the well-dispersed Pt nanoparticles (Scheme S1). First, the crystalline nature of InP together with its nanoporous geometry provided a large surface area for fast and efficient transfer of photogenerated electrons to Pt nanoparticles. Second, it is well known that the
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photoexcited electrons and holes can easily recombine to decrease the photocatalytic efficiency; in the present study, the Pt nanoparticles, acting as electron sinks, reduced the recombination of electrons and holes and extended their lifetime, leading to a greatly
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improved photocurrent and efficiency of H2 generation.
4. Conclusions
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In summary, we have developed a promising and efficient strategy of crafting Pt
nanoparticles loaded on ordered InP nanopore arrays by capitalizing on a modified dipping and electrodeposition method. The well-dispersed
Pt
nanoparticles
at
diameters of 15-20 nm were confirmed by SEM. Pt-decorated InP NPAs electrodes had greater separation efficiency of photogenerated electron-hole pairs compared to pristine InP NPAs electrodes. These characteristics resulted in a 5.5-fold enhancement
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in photocurrent density and greater onset potential shifts. The much improved performance demonstrates the superior ability of metal nanoparticles to modify the electronic properties of InP NPAs, and the potential of Pt-decorated InP NPAs as a
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hybrid system for solar energy conversion.
Acknowledgments
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This work was supported by the Natural Science Foundation of China (Grant nos. 11174197 and 11574203).
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Figure captions
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Figure 1. A schematic illustration for the synthetic route of the Pt@InP NPAs.
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Figure 2. (a) Top and (b) cross-sectional SEM images of InP nanopore arrays obtained from a two-step etching. SEM images of Pt nanoparticles deposited (4 cycles) on InP nanopore arrays: (c) top view (d) cross-sectional view (a high magnification is shown in the inset).
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Figure 3. (a-c) EDX elemental mapping of P, In and Pt. (d) Corresponding size distribution graph, which shows that the average size of the Pt nanoparticles is 15-20 nm.
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Figure 4. EDS pattern of Pt@InP NPAs
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Figure 5. Chemical composition analyses. (a) XPS survey spectra. (b-d) High-resolution In 3d, P 2p and Pt 4f spectra.
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Figure 6. XRD pattern of Pt@InP NPAs. The inset presents a higher magnified view of the bottom part of the Figure.
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Figure 7. PEC measurements of the photoelectrodes in 0.35 M Na2S and 0.5 M Na2SO3 aqueous solution. (a) Current density−voltage behavior of Pt@InP under interrupted illumination. (b) Linear sweep voltammetry for direct comparison of two types of InP electrodes. (c) PEC Performances of the Pt@InP photoelectrodes with different amount of Pt loading. (d) Comparison of photocurrent at -0.8 V (Ag/AgCl) with different amount of Pt loading.
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Figure 8. Band diagram analysis of (a) individual InP and Pt, (b) InP and Pt in contact, and (c) InP and Pt in contact under solar light illumination (green and red balls represent electrons and holes, respectively).
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Figure 9. Nyquist plots of Pt@InP NPAs and pristine InP NPAs obtained under light illumination conditions.
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Scheme 1. Charge transfer mechanism between Pt nanoparticles and InP under solar light.
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Highlights 1.Pt@InP nanopore arrays (NPAs) heterostructure is fabricated via facile procedures.
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2.Photoelectrochemical (PEC) property of the Pt@InP NPAs electrodes are investigated. 3.The Pt@InP NPAs dramatically improve PEC performance compared to pristine InP
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4.The enhancement of the PEC performance is due to the improved charge separation.