Evaluation of Electroless Pt Deposition and Electron Beam Pt

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Evaluation of electroless Pt deposition and electron beam Pt evaporation on p-GaAs as a photocathode for hydrogen evolution Keorock Choi, Kyungwhan Kim, In Kyu Moon, Ilwhan Oh, and Jungwoo Oh ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01838 • Publication Date (Web): 06 Dec 2018 Downloaded from http://pubs.acs.org on December 10, 2018

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ACS Applied Energy Materials

Evaluation of Electroless Pt Deposition and Electron Beam Pt Evaporation on p-GaAs as a Photocathode for Hydrogen Evolution Keorock Choi1,2, Kyunghwan Kim1,2, In Kyu Moon1,2, Ilwhan Oh3*, and Jungwoo Oh1,2* 1School

of Integrated Technology, Yonsei University, Incheon 21983, Republic of Korea

2Yonsei 3Department

Institute of Convergence Technology, Incheon 21983, Republic of Korea

of Applied Chemistry, Kumoh National Institute of Technology, 61 Daehak-ro, Gumi, 39177, Republic of Korea

KEYWORDS: Gallium arsenide, Platinum, Electrocatalyst, Electron beam evaporation, Electroless deposition, Water splitting.

ACS Paragon Plus Environment

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ABSTRACT: This study examines the changes in the photoelectrochemical (PEC) properties with Pt morphology after wet (electroless Pt deposition) and dry (e-beam Pt evaporation) deposition of Pt on p-GaAs. The Pt morphology and composition of the p-GaAs surface differed depending on the Pt deposition method, which in turn affected the optical and PEC properties of Pt on the GaAs electrode. Thus, the findings of this study can help in gaining a clearer understanding of the manner in which these changes affect the operation of a GaAs PEC water-splitting electrode.


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1. INTRODUCTION Hydrogen production using photoelectrochemical (PEC) water splitting has attracted significant attention as a hot research topic to solve environmental problems resulting from fossil fuel use and greenhouse gas generation. In PEC water splitting, hydrogen and oxygen are obtained by a chemical reaction that occurs after charge-separated electrons and holes are generated when the semiconductor absorbs sunlight. Among the components of a PEC water-splitting cell, the photoabsorber, which directly absorbs sunlight, and electrocatalysts, which activate the carrier transfer process, are very important for achieving a high level of efficiency. Therefore, many researchers have studied various electrocatalysts and photoabsorbers with the aim of increasing the performance1-5 and stability6-8 of photoelectrodes. Owing to their high stability in aqueous environments, transition-metal oxides have recently attracted significant attention from researchers.9-11 However, because of their large bandgap, transition-metal oxides can only utilize ultraviolet sunlight, which constitutes a very small part of the solar spectrum. In addition, transition-metal oxides have other drawbacks such as small diffusion length, low electrical conductivity, and short carrier lifetime. Many attempts have been made to address such drawbacks, such as anionic or cationic element doping and hydrogen treatment.12-14 However, it can be difficult to make the appropriate adjustments in this regard. PEC water-splitting cells with singlecrystal semiconductors are considered promising candidates because they have the appropriate bandgap for utilizing visible light while avoiding the high cost associated with complicated fabrication. Unlike Si, which is a typical single-crystal semiconductor, gallium arsenide (GaAs), a group IIIV compound semiconductor, is advantageous for light absorption owing to its large minority carrier diffusion length and direct bandgap. In addition, the conduction band of GaAs is at a more


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negative position than the hydrogen reduction potential, making this compound suitable for use as a photocathode. Despite these advantages, p-GaAs has not been studied as a PEC electrode owing to cost and stability issues. However, studies on cost reduction and stability improvement when applying GaAs have recently been conducted, and the results have indicated the possibility of solving these problems.15, 16 In general, even if the band edge position of a semiconductor is appropriate for a chemical reaction, a kinetic limitation occurs in a bare semiconductor, and a substantial overpotential is required to overcome this. To solve this problem, electrocatalysts can be deposited on the surface of a semiconductor to improve the kinetics of the reaction. Studies on electrocatalysts for photocathodes have mainly focused on finding inexpensive materials as replacements to Pt.17-20 However, Pt remains a difficult material to replace because it shows excellent catalytic activity.21 The catalytic activity of electrocatalysts varies depending not only on the type of material but also on the morphology. In particular, the pinch-off effect observed when electrocatalysts are deposited onto islands that are smaller than the depletion width at the interface results in good PEC water splitting characteristics; this is because the electrocatalysts lower the overpotential but not the effective barrier height.22, 23 For this reason, many studies on the PEC characteristics of Pt deposited on semiconductors have been conducted using various methods to observe the morphology. Among them, electroless deposition (ED), which is a solution-phase deposition method, has significant advantages in terms of its simplicity and low cost. Therefore, it has been applied in various PEC water-splitting studies using single-crystal semiconductors.24-26 However, the study by Gray’s group on the characteristics of PEC water splitting through the deposition of Pt onto Si revealed that higher


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catalytic activity is achieved when using e-beam evaporation (EE) rather than ED, which is most often employed for electrocatalyst deposition.17 Because each deposition method involves a specific mechanism, there is a significant difference in the manner in which the morphology changes. However, the effects of the Pt morphology on GaAs PEC water splitting according to the deposition method are yet to be thoroughly studied. Therefore, to utilize p-GaAs as a photocathode, it is necessary to conduct a detailed comparison of the manner in which the Pt morphology changes depending on the deposition method, and to examine the differences in the resulting PEC water-splitting properties.

2. EXPERIMENTAL SECTION p-type GaAs substrates ((100), Zn-doped, 1017cm3), which were used as the light absorber, were divided into 1 cm2 units. Next, p-GaAs was immersed in acetone, isopropyl alcohol (IPA), and deionized water (DI) for 5 min each. The cleaned p-GaAs was then immersed in a mixed hydrochloric acid (HCl):DI = 1:1 solution for 4 min and stirred for 15 min to remove native oxide. The ohmic contact of the backside was deposited through DC sputtering at 10-6 torr with 30 nm Au, 20 nm Zn, and 100 nm Au, and annealed at 400 °C for 1 min. Next, Pt was deposited at a deposition rate of 1Å/s for 2, 5, 8, 15, and 25 s using EE under a vacuum (10-5 torr.) ED was conducted by depositing Pt for 10, 20, 80, 130, and 160 s in a solution of hydrogen fluoride (HF) (4.6 M) and chloroplatinic acid (H2PtCl6) (1 mM). The wires were connected to the backside using Ag epoxy and sealed using an epoxy adhesive (Hysol 1C, Loctite). For the measurement, a saturated calomel electrode (SCE) (0.242 V versus NHE) as a reference electrode, and a Pt counter electrode were used. The electrolyte was prepared by mixing K2SO4 (0.5 M) and H2SO4 (0.5 M).


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The experiments were conducted under AM1.5G (1 SUN) conditions using a solar simulator (YSS100C-C500H, Yamashita Denso).

3. RESULTS AND DISCUSSION Figure 1 shows the particle size and morphology of Pt deposited on GaAs using atomic force microscopy (AFM). The particle size increased with the deposition time for both deposition methods. When Pt was deposited for (A) 10, (B) 80, and (C) 160 s through ED, the average particle size was slightly increased to 10, 20, and 40 nm, respectively. However, when Pt was deposited through EE, the particle size increased more quickly to approximately 10, 50, and 90 nm in (D) 2, (E) 8, and (F) 25 s. RMS increased in both cases, but more rapidly when depositing Pt through ED. In general, when a metal is deposited through evaporation, it is deposited in the form of a bulk film. According to the pinch-off theory, when the size of the Pt particle is larger than the depletion width of the semiconductor, the semiconductor–metal contact becomes dominant, and the effective barrier height of the interface decreases as the Pt size increases. In contrast, when the Pt particle size is smaller than the depletion width, the smaller the Pt particle, the larger the effective barrier height of the interface.22, 23 In this case, PEC water splitting becomes more favorable as the effective barrier height of the interface increases because a higher effective barrier height prevents recombination of the minority carriers generated by sunlight. Differences in the morphology of the Pt film can also be observed. In EE, Pt particles are initially dispersed. Over time, they become clustered together and increase in size, finally forming a bulk film. In contrast, when Pt is deposited using ED, Pt particles increase in size


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but do not coalesce, allowing them to maintain narrow gaps between each other, and preserving the structural inhomogeneity of the film as a whole.27 Figure 2 shows cross-sections of Pt on GaAs observed using transmission electron microscopy (TEM). Images A and B are TEM images obtained after 10 and 160 s long ED of Pt, and C and D are TEM images obtained after 2 and 25 s long deposition through EE, respectively. When the ED method is used, an oxide layer is formed between the Pt and GaAs because the metal is deposited through a reduction while the semiconductor is oxidized.27 In the case of A and B, an oxide layer is formed between the Pt and GaAs, and when the deposition time is prolonged, the oxide layer is divided into two layers. The TEMEDS results show that image A has a layer of Ga oxide, whereas B has two oxide layers: As oxide and Ga oxide. The thickness of each layer increases gradually as the deposition time increases. In images C and D, the films were deposited through EE. Initially, in image C, Pt was deposited as small particles on GaAs, and in D, it was deposited as a bulk film. In these images, no changes were observed in the composition of GaAs with respect to the length of the deposition time. As shown in Figure 3, we measured the change in reflectance according to the wavelength and solar-weighted total according to the Pt deposition time. When the reflectance was measured after the ED of Pt, the reflectance of wavelengths below 550 nm was decreased as the deposition time increased. This was because Ga oxide with a band-gap of about 4.8 eV was generated through the deposition mechanism of ED, and Ga oxide, which thickened as the deposition time increased, absorbed the sunlight within this range of wavelength. It could be seen that the difference in reflectance greatly increased in the ultraviolet region from 260 nm corresponding to 4.8 eV (Supplemental Information, Figure S2). At other


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wavelengths, the reflectance was almost the same regardless of the deposition time. In the case of Pt deposition using EE, the reflectance across the entire wavelength range increased as the deposition time increased. In a study conducted by Vadimsky’s group, as the area occupied by the Pt increased, the reflectance increased. However, when the film maintained its structural inhomogeneity, the reflectance remained unaffected, regardless of the surface area occupied by Pt.28 By comparing the solar-weighted reflectance (SWR) of the two different deposition methods, it is possible to gain a clear understanding of the differences between these methods regarding the absorbance of sunlight. When Pt was deposited using ED, a structurally inhomogeneous Pt film was formed on GaAs, which showed no significant differences in reflectance from that of bare GaAs. Structurally inhomogeneous Pt films, made up of Pt particles that are several tens of nanometers smaller than the wavelength of light, have optical transparent properties, unlike bulk films. Even if the reflectance differs at a wavelength of 550 nm or smaller during ED, it accounts only for a small portion of the solar spectrum, and hence, there is little difference in the SWR in terms of the deposition time. In contrast, when Pt is deposited using EE, the reflectance increases rapidly over the entire wavelength range. The Pt particles became larger as the deposition time increased. Unlike during the ED of Pt, the Pt particles maintained their bulk morphology, and as the deposition time increased, the Pt particles grew larger and their light reflection increased. As a result, the increase in the amount of light reflection hindered the efficient utilization of sunlight, thereby reducing the photovoltage and photocurrent. Figure 4 (A) shows the linear sweep voltammetry (LSV) graph for hydrogen evolution on a GaAs photoelectrode with electroless Pt deposition. Figure 4 (B) shows a graph


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indicating the change in the open-circuit voltage (Voc) and saturation current density (Jst) according to the deposition time. After electroless Pt deposition on GaAs, the current density did not increase under dark conditions. Under light conditions, Voc was negatively shifted to 0.38, 0.30, 0.24, 0.19, and 0.11 V as the Pt deposition time increased from 10 to 160 s. With even longer deposition times, there was no major shift in Voc. This is related to the pinch-off effect, for which the size of the Pt particles is the most important factor. The pinch-off effect can be exploited if the deposited Pt has a size equal to or less than the space charge region formed in the semiconductor at the interface of the semiconductor and electrolyte. The depletion width of the p-GaAs used in the experiment was about 50 nm. The depletion width is calculated using the expression

w=

2𝜀0𝜀𝑟(ф𝑆𝐶 ―

𝐾𝑇 ) 𝑒

𝑒𝑁𝐴 where ε0 is the vacuum permittivity, εr is the dielectric constant, фsc is the apotential drop across the space charge, and NA is the concentration of the acceptor. Figure 1 shows the gradual increase in the size of the Pt particles deposited onto GaAs. As a result, the pinch-off effect was gradually reduced, and Voc was negatively shifted. As the amount of the deposited electrocatalysts increased, the catalytic activity increased. As shown in the graph of (B), the open-circuit potential became more negative with elapsed time, allowing a sufficient amount of Pt to be deposited in only 10 s. When Pt was deposited for 160 s, particles with a diameter of about 40 nm were formed, which is slightly smaller than the depletion width of the p-GaAs used in the experiment. Even when Pt was deposited for a longer time, Voc did not change significantly. This is because as the ED progresses, more oxides are formed on the GaAs surface, gradually slowing down the redox reaction,


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and the deposited Pt does not change in size but becomes saturated.27 In addition, Figure 3 shows that almost no change occurred in the solar reflectance owing to the structural inhomogeneity of the Pt film. To detect the changes that occurred from sunlight alone, the average value of the saturation interval (-0.4 V to -0.3 V) for the current density (Jst) was obtained by subtracting the dark current density from the light current density, as shown in (B). The current density decreased slightly from -20.32 to -17.58 mA/cm2 as the deposition time increased, but no significant decrease was observed. As shown in Figure 1, a structurally inhomogeneous film whose scale is smaller than the wavelength of sunlight is formed, and it maintains optical transparency; thus, the reflectance does not increase with the increase in deposition time. However, the current density is slightly reduced, even when the reflectance is slightly reduced from the Ga oxide. The oxide layer formed during ED grew thicker with the increase in the deposition time. With the increase in the recombination resulting from the reduction of the pinch-off effect, this thickened oxide layer seemed to work as an insulator, preventing excited carriers created by sunlight from transferring to the electrolyte. In Figure 5, (A) shows a graph of the LSV of the hydrogen evolution on the GaAs photoelectrode with e-beam Pt evaporation, and (B) shows a graph indicating the change in Voc and Jst according to the deposition time. The saturation current density in Fig. 5 (B) was calculated using the same method as that in Fig. 4 (B). When Pt was deposited through EE, Voc

became more positive in the dark over time because the size of the Pt particles increases gradually during the deposition process, thereby maintaining the bulk Pt. As the Pt deposition progressed, the semiconductor–metal interface occupied more of the surface than the semiconductor–electrolyte interface, and the effective barrier height of the


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electrode interface decreased. Under light conditions, as the deposition time was increased from 2 to 25 s, Voc shifted more negatively when Pt was deposited through EE at 0.29, 0.33, 022, 0.16, and 0.03 V, as in the case of ED. This is because the size of the Pt particles increased to a greater extent than the depletion width, and the pinch-off effect was sharply reduced. In addition, the increase in the reflectance of sunlight owing to the increase in the area of bulk Pt affected the negative shift of Voc. Figure 3 shows that the reflectance increases with time during Pt deposition when using EE. As a result, the effective barrier height at the interface decreased with the progress of deposition, the photovoltage generated in the light was reduced, and Voc shifted more toward the negative direction. When a complete continuous bulk film was formed, Voc in the light state was reduced to about 0.17 V versus a reversible hydrogen electrode (RHE). The current density was drastically decreased from -20.20 to -4.07 mA/cm2 in comparison to that in the case of ED. This was the result of the increase in reflectance, as well as the increase in recombination owing to the decrease in the pinch-off effect. In the case of Pt deposition by EE, the area occupied by the bulk Pt increased over time, and a sharp increase in reflectance occurred. As a result, the absorbance of sunlight decreased, leading to a decrease in the current density as well. Therefore, sunlight can be used more effectively during ED than during EE. Figure 6 shows the inductively coupled plasma-mass spectrometry (ICP-MS) plot for the pGaAs electrodes after operation for 10 h. Samples of Pt were deposited using EE for 2 s, other samples of Pt were deposited using ED for 10 and 160 s, and bare samples were operated for 10 h with a flow of -15 mA/cm2. As see in the graph, smaller amounts of Ga and As were detected during EE than during ED. In previous studies, Nakato and colleagues reported that depositing Pt on semiconductors the increases stability.29-31 However, in the case of ED, Ga and As oxides


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generated during the Pt deposition process are not stable in solution, and corrosion occurs, thus lowering the stability of the photocathodes. On the other hand, bare GaAs or GaAs with Pt deposited using EE does not form an oxide on the surface, and is therefore able to operate more reliably in an electrolyte. Deutsch’s group reported that bare p-GaAs used as a photocathode in an acidic solution shows long-term stability.16 Thus, the experimental results show that there is less corrosion in the reaction when there is no composition change on the surface of GaAs during the Pt deposition process.

4. CONCLUSION We studied the characteristics of p-GaAs photocathodes for two different Pt deposition methods. EE, a physical vapor deposition method, and ED, a method involving the reduction of metal ions through the oxidation of semiconductors, were used to deposit Pt, and the structural and optical properties of the GaAs photoelectrodes were then measured at different points during the deposition time. Finally, we also observed the open-circuit potential and change in saturation current density. When using ED, Pt was deposited while maintaining the structural inhomogeneity, and this minimized the recombination of excited carriers owing to the pinch-off effect. In addition, Pt was formed into a structurally inhomogeneous film with optical transparency, effectively generating a photovoltage and photocurrent. However, the oxide layer on the surface was relatively thick because the deposition method involved the oxidation-reduction reaction, thus slightly decreasing the photocurrent. In addition, the generated oxide caused corrosion in the acid solution, which led to stability problems. When EE was used, Pt catalysts gradually formed bulk Pt, which reduced the effective barrier height at the interface and increased the light reflectance. As a result, the pinch-off effect was reduced, and the sunlight absorption was reduced, leading to a negative


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shift of the open-circuit potential and a decrease in the photocurrent. However, there was no change in the composition of the surface of GaAs, and it functioned stably as a PEC photocathode. Each deposition method can be considered to be suitable depending on the structure of the semiconductor substrate. Since ED is a solution-based method, it is appropriate when depositing Pt on a three-dimensional structure. On the other hand, since EE is a vacuum-based method, it can be used on planar substrates. In addition, it is necessary to understand the effects of the two deposition methods on the chemical stability and reflectance. The findings of this study can be a useful reference for research on the application

of GaAs and metal catalysts as PEC electrodes.


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ASSOCIATED CONTENT Supporting Information Reflectance, X-ray photoelectron spectroscopy (XPS) with Pt deposition time. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected] Author Contributions Keorock Choi, Kyungwhan Kim, and Inkyu Moon carried out the experiments. All authors contributed to the design of the experiments, the data analysis, and the writing of the manuscript.

ACKNOWLEDGMENT This research was supported by the Ministry of Science and ICT (MSIT), Korea, under the “ICT Consilience Creative Program” (IITP-2018-2017-0-01015) supervised by the Institute for Information & communications Technology Promotion (IITP). This research was also supported by the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education (NRF-2018R1D1A1B07045703).


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Ueda, K.; Nakato, Y.; Suzuki, N.; Tsubomura, H. Silicon electrodes coated with extremely

small platinum islands for efficient solar energy conversion. J. Electrochem. Soc. 1989, 136, 22802285.


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of photovoltages for n-Si electrodes modified with LB layers of ultrafine platinum particles. Electrochim. Acta 1997, 42, 431-437.


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Figure 1 AFM image of the Pt on GaAs deposited using ED (A, 10 s; B, 80 s; C, 160 s) and Ebeam evaporation (D, 2 s; E, 8 s; F, 25 s). The particle size of Pt (A, 10 nm; B, 20 nm; C, 40 nm; D, 15 nm; E, 50 nm; F, 90 nm) and RMS (A, 0.196; B, 0.651; C, 2.237; D, 0.216; E, 0.413; F, 0.581) increased with the deposition time.


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Figure 2 TEM cross-section images of Pt on GaAs deposited using ED (A, 10 s; B, 160 s) and EE (C, 2 s; D, 25 s), and TEM-EDS profiles of electroless Pt deposition on GaAs. TEM-EDS data are measured along the dashed line (a-b, c-d) in TEM images A and B.


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Figure 3 Reflectance spectra and calculated SWR of Pt on GaAs deposited using (A) ED and (B) EE as a function of the deposition time.


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Figure 4 (A) LSV characterization of p-GaAs photocathodes after electroless Pt deposition and (B) change in open circuit potential (Voc) and saturation current density (Jst) according to the deposition time for bare and 10, 20, 80, 120, and 160 s operations. The solid and dashed lines indicate the current measured under 1SUN illumination and without illumination in (A), respectively.


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Figure 5 (A) LSV characterization of p-GaAs photocathodes after e-beam Pt evaporation and (B) change in open-circuit potential (Voc) and saturation current density (Jst) according to the deposition time for bare, and 2, 5, 8, 15, and 25 s, operations. The solid and dashed lines indicate the current measured under 1SUN illumination and without illumination in (A), respectively.


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Figure 6 Quantification using ICP-MS of Ga and As in electrolyte after operating for 10 h under 1SUN illumination. To confirm the influence of the compositional change of the GaAs surface on the stability, Pt is deposited onto GaAs using ED for 10 and 160 s, and EE for 2 s.


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TABLE OF CONTENTS


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Evaluation of Electroless Pt Deposition and Electron Beam Pt

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