n-Si photoanode for water oxidation

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Enhancing the photovoltage of Ni/n-Si photoanode for water oxidation through a rapid thermal process Shengyang Li, Guangwei She, Cheng Chen, Shaoyang Zhang, Lixuan Mu, Xiangxin Guo, and Wensheng Shi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16986 • Publication Date (Web): 26 Feb 2018 Downloaded from http://pubs.acs.org on February 27, 2018

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ACS Applied Materials & Interfaces

Enhancing the Photovoltage of Ni/n-Si Photoanode for Water Oxidation through a Rapid Thermal Process Shengyang Li,a,b Guangwei She,a* Cheng Chen,b,c Shaoyang Zhang,a,b Lixuan Mu,a Xiangxin Guo,c and Wensheng Shi a,b* a

Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute

of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China b

University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100049,

China c

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai

Institute of Ceramics, Chinese Academy of Science, Shanghai 200050, China. KEYWORDS: Ni/n-Si photoanode, water oxidation, interface states, Schottky barrier height, photovoltage

ACS Paragon Plus Environment

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ABSTRACT: The Ni in the Ni/n-Si photoanode can not only protect Si from corrosion, but also catalyze the water oxidation reaction. However, the high density of interface states at the Ni/n-Si interface could pin the Fermi level of silicon, which will lower the Schottky barrier height of the Ni/n-Si. As a result, a low photovoltage and consequent high onset potential of Ni/n-Si photoanode for water oxidation were generated. In this study, the interfacial states of the Ni/n-Si photoanodes were efficiently diminished through a rapid thermal process (RTP). Calculated from the Mott-Schottky plots, the Schottky barrier height of Ni/n-Si was increased from 0.58 eV to 0.78 eV after RTP. Under the illumination of 100 mW cm-2 of Xe lamp the onset potential of the Ni/n-Si photoanode for water oxidation was negatively shifted for 150 mV after RTP. Besides, the RTP-treated Ni/n-Si photoanode exhibited a high stability during the PEC water oxidation of 8 hours in 1 M KOH solution.


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Introduction Silicon (Si) is considered as a nearly ideal photoanode material for efficient solar-driven water splitting due to its optimal band gap (1.12 eV) and high carrier mobility.1-4 However, Si is easy to be photocorroded or photopassivated in contact with electrolytes.5-7 In addition, the dynamics of the oxygen evolution reaction on the bare Si surface is quite sluggish.8 It was found that decorating a layer of metal, such as Pt,9 Ir,10 Co11, or Ni12, 13 on the surface of Si, can not only protect Si from corrosion, but also efficaciously catalyze the oxygen evolution reaction. In 2013, Kenney et al.12 deposited an ultrathin (2 nm) nickel layer onto n-Si surface to realize Ni/nSi photoanodes for water oxidation. Actually the as-prepared Ni/n-Si photoanode was a metalinsulator-semiconductor (MIS) structure since the existence of native SiOx on the surface of Si. In brief, the Ni/SiOx/n-Si was abbreviated to Ni/n-Si. Although the Ni (2 nm)/n-Si exhibited good performance, the Ni film is too thin to afford extended stability, particularly in alkaline media.12 Seriously, there are numerous defects at the Ni/Si interface, which inevitably cause a high density of interface states.14 Consequently, the strong Fermi level pinning resulted from these interface states would lower the Schottky barrier between the Ni and n-Si.11, 15, 16 It is known that the photovoltage produced from a MIS device strongly depends on the height of Schottky barreier.12, 17-20 A low Schottky barrer height will drop the photovoltage. As a result, the onset potential for water oxidation has to be high. In this regard, to diminish the interface states and thus the Fermi level pinning strength from the Ni/n-Si photoanode is essential for the achievement of higher photovoltage and better photoelctrochemical (PEC) performance. In this study, a rapid thermal process (RTP) was utilized to effectively reduce the interface states from the Ni/n-Si photoanodes. The Schottky barrier height of Ni/n-Si can be increased


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about 200 meV after RTP. Under illumination of 100 mW cm-2 of Xe lamp, onset potential of the Ni/n-Si photoanode in 1 M KOH solution was negatively shifted nearly 150 mV after RTP. Experimental Details Fabrication and characterization of Ni/n-Si photoanodes Single polished n-Si (111) wafers (0.8–1 Ω·cm) were cleaned by standard Radio Corporation of the America (RCA) clean and immersed into a dilute HF (4%) solution for 20 s to etch the surface oxide layer. Then the cleaned wafers as the substrate were transferred into the chamber with pressure of 2×10-4 Pa and temperature of 50 °C immediately, and the Ni of about 20 nm in thickness was deposited onto the Si substrate with electron beam evaporation. Subsequently, the silicon wafers with 20 nm Ni films were transferred to an AW-610 RTP oven (Allwin21 Corp). The oven was heated to 450 °C from room temperature with a rate of 30 °C/s and then maintained at this temperature in a pure N2 ambience for 30 s. The crystallization of deposited Ni film was analyzed by X-ray diffraction (XRD, Bruker D8 PHASER). The structures of Ni/n-Si were investigated by cross-sectional transmission electron microscopy (TEM, JEOL 2100F). The samples were cleaved into small pieces of 2×2 mm2, polished and thinned by ion milling for TEM observation. The morphologies of Ni/n-Si photoanodes surface were observed by scanning electron microscopy (SEM, Hitachi S-4800). PEC measurements PEC measurements were performed in a three-electrode system controlled by a CHI660C workstation. The Hg/HgO (1 M KOH) electrode and a platinum foil were used as the reference


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and counter electrodes, respectively. The conversion of the potential of Hg/HgO electrode to reversible hydrogen electrode (RHE) used the following equation E(RHE) = E(Hg/HgO) + 0.098 V + 0.059*pH

(1)

The prepared Ni/n-Si photoanodes were used as the working electrode. Indium-gallium eutectic was scratched onto the backside of the Ni/n-Si photoanodes to form an ohmic contact with Si. A copper foil was placed on the indium-gallium eutectic to conduct current. The Ni/n-Si photoanodes with active area of 0.38 cm2 were placed in a homemade electrode holder. The electrolyte of aqueous 1 M KOH (pH=13.6) was degassed with high purity argon for 30 min prior to each measurement. The light emitted from a Xenon lamp was calibrated to 100 mW cm-2 at the surface of the working electrode. The scan rate of all the cyclic voltammetry curves was 50 mV/s. Fabrications and measurements of solid-state cells The solid-state cells were fabricated as follows. The Ni/n-Si samples were sliced into small pieces of 1×1.5 cm2 in dimension. The Ni film gate layer acted as the working electrode. Indiumgallium eutectic was scratched onto the backside of the Si to form an ohmic contact with Si, which was used simultaneously as the counter as well as reference electrode. The measurements of the as-prepared solid-state cells were performed in dark on the CHI660C workstation at room temperature. The Mott-Schottky plots were measured with the AC potential frequency of 100 KHz. The scan rate of the current-voltage curves was 100 mV/s. Results and discussion


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In order to ascertain the phase and microstructure of the RTP-treated and untreated Ni/n-Si samples, the glancing angle XRD and cross-sectional TEM analysis of the samples were carried out. It can be determined from the glancing angle XRD patterns (Figure 1) that the deposited films on the silicon are crystalline cubic nickel with space group Fm3m (JCPDS card no. 040850) before and after RTP. Furthermore, it can be found that the RTP enable the diffraction peaks from nickel to become sharp, which could be attributed to the enhancement of the crystallinity and grain growth of nickel film. These could be further verified by the following TEM observation. As shown in Figure 2a, a continuous and uniform polycrystalline nickel film with thickness of about 20 nm can be found from the untreated sample, and the orientation of these numerous small grains is random. Moreover, there is an amorphous native SiOx layer with about 1.5 nm in thickness between nickel film and silicon. This thin SiOx layer on silicon surface was due to the inevitable encounter of silicon with air during the processing of silicon devices. In brief, the Ni/SiOx/Si was abbreviated to Ni/Si in this article. Although little change in the thicknesses of nickel film and SiOx layer was detected after the RTP, the enhancement effect of the RTP on the grains growth and crystallinity of Ni film is obvious (Figure 2b). In order to further demonstrate the effect of the RTP, the fast Fourier transformation (FFT) was performed from the TEM images of untreated and RTP-treated Ni (insets of Figure 2a and 2b). The FFT pattern of RTP-treated Ni film is mainly composed of periodically arranged diffraction spots. As a comparison, only diffraction rings were observed from the untreated Ni film, indicating the enhancement of crystallinity by the RTP. It has been reported that the RTP for the Ni film deposited on the silicon substrates may result in the formation of NiSi film due to the diffusion of Ni atoms into the silicon substrates.21, 22 However, without NiSi phase was detected from the


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RTP-treated sample in present work. Maybe the 1.5 nm SiOx is thick enough to prevent the Ni from diffusing into the silicon substrate.

Figure 1 XRD patterns of deposited Ni films before (black) and after (red) RTP.

Figure 2 Cross-sectional TEM images of untreated Ni/n-Si (a) and RTP-treated Ni/n-Si (b); insets are the corresponding FFT patterns of Ni. The RTP-treated and untreated Ni/n-Si samples were used as photoanodes for PEC water splitting in 1 M aqueous KOH solution, and the cyclic voltammograms of the samples are shown


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in Figure 3. The untreated Ni/n-Si exhibits an onset potential of ~0.55 V vs. Hg/HgO (corresponding to ~1.45 V versus RHE). However, the onset potential of the RTP-treated Ni/n-Si sample was shifted to ~0.40 V vs. Hg/HgO (corresponding to ~1.30 V versus RHE). Obviously, the RTP has enabled the onset potential of the Ni/n-Si photoanode to negatively shift about 150 mV. It is know that the onset potential of photoanode is largely determined by the photovoltage generated from photoanode.23, 24 Generally, the higher the photovoltage is, the lower the onset potential become. 16, 23 Accordingly, the 150 mV negative shift of the onset potential implies that the photovoltage of photoanode was enhanced by the RTP. The enhancement of the photovoltage could also be further demonstrated by around 150 mV negative shift of the oxidation peak of Ni after RTP.16

Figure 3 Cyclic voltammograms of untreated Ni/n-Si (black) and RTP-treated Ni/n-Si (red) in 1 M KOH solution. (solid line, under 100 mW cm-2 illumination; dashed line, in the dark.) It was suggested that the photovoltage of the Ni/n-Si photoanodes linearly depends on its Schottky barrier height (Φ ) and a higher photovoltge could be resulted from a higher Schottky barrier.12,

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In order to determine the Φ of the RTP-treated and untreated Ni/n-Si samples,

solid-state cells were fabricated. The schematic diagram of the cell is shown as the inset of


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Figure 4a. The Ni film acts as a gate layer and the backside of the Si was coated with the indium-gallium eutectic. As shown in Figure 4a, the Mott-Schottky plots of both RTP-treated and untreated Ni/n-Si exhibits good linearity to a reverse bias of 0.8 V. The flat band potentials ( ) of the RTP-treated and untreated Ni/n-Si can be determined from the x-intercept of linear

fitted line to be 0.56 V and 0.36 V, respectively. The Φ can be calculated from the following equation Φ = +

(2)

Where Vn is the difference in energy between Fermi level and the bottom of the conduction band of n-Si, which can be achieved from following equation =

/

(3)

Where Nc is the effective density of states in the conduction band of silicon and is 2.85×1019 cm3

at temperature of 300 K.25 Nd, the dopant concentration of silicon substrate, can also be

determined from the slope of the linear fitted line of Mott-Schottky plots in Figure 4a according to the following equation

=

−

!

(4)

With the relative dielectric constant (ε) of 11.68, Nd of both samples were calculated to be 4.5 ± 0.2×1015 cm-3 according to Eq. (4). This value means a corresponding resistivity of 1.08 Ω·cm, which is close to the range of 0.8–1 Ω·cm sated by the company of the Si wafer. Accordingly, the Vn is calculated to be approximately 0.22 eV for both samples according to Eq. (3). Therefore, with the of both samples, the Φ of the RTP-treated and untreated Ni/n-Si can be ACS Paragon Plus Environment

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calculated to be 0.78 eV and 0.58 eV, respectively, according to Eq. (4). Obviously, the Φ of Ni/n-Si was increased about 200 meV after RTP. Due to the linear dependence of the photovoltage on the Φ , the increase of the Φ would enhance the phtovoltage of the photoanode, and finally resulted in the obviously negative shift of the onset potential.

Figure 4 (a) Mott-Schottky plots of untreated Ni/n-Si and RTP-treated Ni/n-Si solid-state cell measured with the AC potential frequency 100 KHz; inset is the test geometry. (b) Currentvoltage curves of the untreated Ni/n-Si and RTP-treated Ni/n-Si solid-state cell in dark. The enhancement of the Schottky barrier height and photovoltage after RTP was attributed to the decrease of interface defects. The decrease of the interface defects can be proved by the results obtained from the measurement of current-voltage curves of the Ni/n-Si solid cells. As shown in Figure 4b, the current-voltage curves of the RTP-treated and untreated Ni/n-Si solidstate cells both present a certain rectifying effect. However, the reverse saturation current of the RTP-treated Ni/n-Si is lower than that of the untreated Ni/n-Si. Because the interface states act as recombination centers, a lower interface states density result in a lower reverse saturation current.26, 27 The lower reverse saturation current of RTP-treated Ni/n-Si implies the decrease of the interface states at the Ni/Si interface after RTP. In order to better explain the enhancement of Φb as a result of decrease of interface states, we plot energy band diagram of Ni/n-Si before and


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after RTP as shown in Scheme 1. Under the ideal cases, the Φb of Ni/n-Si is the difference of the electron affinity of Si and the work function (ΦM) of Ni.28 However, when the nickel is deposited onto the silicon by e-beam evaporation, the initial nickel atoms may introduce a lot of damage centers on the silicon surface.29, 30 These damage centers as defects at the Ni/SiOx interface. In addition there are also large number of defects at the Si/SiOx interface and within SiOx.31 This defects cause high density of interface states at Ni/Si interface, which tend to strongly pin the Fermi level of silicon and result in a low Φ" . 11,15,16,25 After RTP in nitrogen gas, we can significantly reduce these interface states31-33 and thus alleviate the Fermi level pinning, and as a result obtain a higher Φ" compared with untreated Ni/n-Si.16,25,32,33

Scheme 1 Energy band diagram of the untreated Ni/n-Si (a) and RTP-treated Ni/n-Si. The PEC stability of the photoanodes is an important issue for their applications. In order to investigate the stability of the RTP-treated Ni/n-Si as photoanode, we observed the change of photocurrent densities at an applied constant potential of 0.6 V vs. Hg/HgO for 8 hours. It can be found from Figure 5a that the photocurrent densities of the RTP-treated Ni/n-Si are quite stable during the stability test. Furthermore, the cyclic voltammogram of RTP-treated Ni/n-Si after


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stability tests was measured. As shown in Figure 5b, the RTP-treated Ni/n-Si retained its high PEC activity after 8 hours tests. That means the RTP-treated Ni/n-Si owns a high PEC stability. We also observed the surface morphologies of the RTP-treated Ni/n-Si photoanode before and after stability tests. As shown in Figure 5c and 5d, some platelets were formed on the surface of Ni films after the stability tests. These platelets are considered to be nickel oxyhydroxide, which were formed during the oxidation of Ni film in the PEC stability tests in aqueous KOH solution as reported by others.11, 13 In Kenney’s research they found that the etch holes were existed on the electrode surface and the silicon substrate inside the hole were exposed after 5 hours PEC stability tests for Ni (2nm)/n-Si photoanode.12 However, in our study, no etch holes were observed from the RTP-treated Ni/n-Si photoanode after 8 hours stability tests. The thicker Ni films of 20 nm in present configuration may improve the protection for silicon substrates. Moreover, enhancing the crystallinity of the Ni films via the RTP may be other critical factor to improve the corrosion resistance of the samples.


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Figure 5 (a) Current density-time curves obtained at 0.6 V vs Hg/HgO for RTP-treated Ni/n-Si in 1M KOH solution. (b) Cyclic voltammograms taken before and after 8 hours stability tests for RTP-treated Ni/n-Si. Surface SEM images of RTP-treated Ni/n-Si photoanode before (c) and after (d) 8 hours stability tests. Conclusions In conclusion, the photovoltage of the Ni/n-Si photoanode was successfully promoted by the facile and low cost RTP. The onset potential of the Ni/n-Si photoanode was negatively shifted for about 150 mV after RTP. The RTP could efficiently decrease the defects at the Ni/Si interface, and the Schottky barrier height of the Ni/ n-Si photoanode was resultantly increased. From the Mott-Schottky plots, it can be determined that the Schottky barrier height of the Ni/n-Si photoanode was increased 200 meV after RTP, which lead to the enhancement of the photovoltage of the photoanode. Present results provide a promising route to enhance the photovoltage of silicon-based MIS photoanode by the RTP.


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AUTHOR INFORMATION Corresponding Author Guangwei She*. Email: [email protected]. Wensheng Shi*. E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by National Basic Research Program of China (Grant No. 2016YFA0200801); NSFC (Grant No. 51672284); Chinese Academy of Sciences (Grant QYZDJ-SSW-JSC032 and XDB17000000), and Youth Innovation Promotion Association CAS (2014022). REFERENCES (1) Sun, K.; Shen, S.; Liang, Y.; Burrows, P. E.; Mao, S. S.; Wang, D. Enabling Silicon for Solar-fuel Production. Chem. Rev. 2014, 114 (17), 8662-8719. (2) Yang, J.; Walczak, K.; Anzenberg, E.; Toma, F. M.; Yuan, G.; Beeman, J.; Schwartzberg, A.; Lin, Y.; Hettick, M.; Javey, A.; Ager, J. W.; Yano, J.; Frei, H.; Sharp, I. D. Efficient and Sustained Photoelectrochemical Water Oxidation by Cobalt Oxide/Silicon Photoanodes with Nanotextured Interfaces. J. Am. Chem. Soc. 2014, 136 (17), 6191-6194. (3) Wang, X.; Peng, K. Q.; Hu, Y.; Zhang, F. Q.; Hu, B.; Li, L.; Wang, M.; Meng, X. M.; Lee, S. T. Silicon/hematite Core/shell Nanowire Array Decorated with Gold Nanoparticles for Unbiased Solar Water Oxidation. Nano Lett. 2014, 14 (1), 18-23.


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(26) Esposito, D. V.; Levin, I.; Moffat, T. P.; Talin, A. A. H2 Evolution at Si-based Metal-Insulator-Semiconductor Photoelectrodes Enhanced by Inversion Channel Charge Collection and H Spillover. Nat Mater 2013, 12 (6), 562-8. (27) Green, M. A. Effects of Pinholes, Oxide Traps, and Surface-state on MIS Solar-cells. Applied Physics Letters 1978, 33 (2), 178-180. (28) Sze, S. M. Physics of Semiconductor Devices, 2nd ed; Wiley: New York, 1981: pp 270-278 (29) Mullins, F. H.; Brunnschweiler, A. Effects of Sputtering Damage on Characteristics of Molybdenum-Silicon Schottky-Barrier Diodes. Solid-State Electron. 1976, 19 (1), 4750. (30) Berg, S.; Andersson, L. P.; Norström, H.; Grusell, E. 2.2 Substrate Surface Damages by Rf-sputtering. Vacuum. 1977, 27 (3), 189-191. (31) Zhang X. G. Electrochemistry of Silicon and Its Oxide; Kluwer Academic Publishers, New York, 2001: chapter 3, pp 120-125 (32) Andersson, L. P.; Evwaraye, A. O. Electrical Characteristics of Sputtering-Induced Defects in Type-n Silicon. Vacuum. 1978, 28 (1), 5-7. (33) Koshy, J. Effects of Annealing on Resistivity and on Schottky-Barrier Heights of Sputter-deposited MoSi2 Films. J. Appl. Phys. 1989, 66 (10), 4818-4820.


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Table of Contents Graphic:


19


Table of Contents graphic 33x13mm (600 x 600 DPI)


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Figure 1 XRD patterns of deposited Ni films before (black) and after (red) RTP. 66x55mm (300 x 300 DPI)



Figure 2 Cross-sectional TEM images of untreated Ni/n-Si (a) and RTP-treated Ni/n-Si (b); insects are the corresponding FFT patterns of Ni. 40x20mm (300 x 300 DPI)


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Figure 3 Cyclic voltammograms of untreated Ni/n-Si (black) and RTP-treated Ni/n-Si (red) in 1 M KOH solution. (solid line, under 100 mW cm-2 illumination; dashed line, in the dark.) 66x54mm (300 x 300 DPI)



Figure 4 (a) Mott-Schottky plots of untreated Ni/n-Si and RTP-treated Ni/n-Si solid-state cell measured with the AC potential frequency 100 KHz; inset is the test geometry. (b) Current-voltage curves of the untreated Ni/n-Si and RTP-treated Ni/n-Si solid-state cell in dark. 33x13mm (300 x 300 DPI)


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Figure 5 (a) Current density-time curves obtained at 0.6 V vs Hg/HgO for RTP-treated Ni/n-Si in 1M KOH solution. (b) Cyclic voltammograms taken before and after 8 hours stability tests for RTP-treated Ni/n-Si. Surface SEM images of RTP-treated Ni/n-Si photoanode before (c) and after (d) 8 hours stability tests. 64x50mm (300 x 300 DPI)



Scheme 1 Energy band diagram of the untreated Ni/n-Si (a) and RTP-treated Ni/n-Si. 51x32mm (300 x 300 DPI)


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n-Si photoanode for water oxidation

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