Article Cite This: ACS Appl. Nano Mater. 2019, 2, 3654−3661
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TiO2/Au Nanoring/p-Si Nanohole Photocathode for Hydrogen Generation Liang Zhang, Xingyao Chen, Zongbin Hao, Xinyu Chen, Yang Li, Yushuang Cui, Changsheng Yuan, and Haixiong Ge* Department of Materials Science and Engineering, College of Engineering and Applied Sciences, National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China
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ABSTRACT: Nowadays, numerous electrochemical and photoelectrochemical (PEC) methods have been utilized for water-splitting hydrogen production. Herein, we designed a novel photocathode based on a TiO2/Au nanoring (AuNR)/Si nanohole (SiNH) hetero-nanostructure (HN), which can be fabricated in a programmable way. The SiNH arrays substrate was prepared by nanoimprint lithography, and then, embedded AuNRs were fabricated by sputtering deposition and subsequent ion beam etching to remove the Au layer covering the horizontal Si surface. Cylindrical AuNRs clinging to the sidewalls of SiNH arrays could maximize the horizontal exposure area of the Si substrate and have little adverse effect on its light absorption. The design is supported by theory simulation and could lead to expectable PEC performance by precisely controlling the geometry and size of the AuNR which will trigger localized surface plasmon resonance (LSPR), bringing about a prominent enhancement of the light harvesting ability and exciting vast hot electrons under light illumination. Generated electrons could transfer across the Schottky junction formed by AuNR and TiO2 to contribute to the hydrogen evolution reaction (HER). The excellent hydrogen production performance with an onset potential of 0.32 VRHE of our prepared HN electrode could be attributed to the synergetic effect of an electrochemical and PEC process, and the maximum photon-toenergy conversion efficiency reaches 13.3%. The experimental results are in good accordance with the simulation analysis and demonstrate an enhancement of the catalytic performance by optimizing the sizes of those components. This work may provide a new path to boost hydrogen production performance by designing customized HNs with a positive effect for electrocatalysis or photoelectrocatalysis. KEYWORDS: localized surface plasmon resonance, photocathode, hetero-nanostructure, nanofabrication, hydrogen production
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INTRODUCTION The huge increase in energy consumption and the overuse of fossil fuels which are intensifying air pollution and global warming urge us to develop a clean, renewable, and affordable energy. Hydrogen (H2), which has the highest energy density of all chemical fuels (142 MJ/kg), is considered as the most promising alternative to fossil fuels for the future.1−3 A greatly attractive approach to produce hydrogen is photoelectrochemical (PEC) water splitting, which could convert solar energy into chemical energy.4−7 The strategies for the development of efficient and low-cost PEC electrodes mainly lie on the utilization of suitable materials and the optimization for the structure of the electrode surface. The combination of different materials like semiconductors and metals has been a promising approach to enhance photocatalytic activity in many works.8 In addition, the nanostructured topography of the electrode− electrolyte interface is always introduced to expand the reaction area and improve the light-harvesting efficiency.9 Semiconductors with smaller band gaps (e.g., Eg < 2.5 eV) are appropriate for PEC water splitting because the significant part of solar radiation in the visible range can be utilized.10 Si with a band gap of 1.12 eV is a promising candidate for the © 2019 American Chemical Society
photoelectrode due to its efficient solar energy harvesting across the entire solar spectrum, low material cost, and abundance.11−15 In addition, Si is the material most compatible with semiconductor device fabrication techniques. Antireflective nanostructures can be conveniently manufactured on the Si surface to improve the solar absorption efficiency.16,17 However, the single-component Si photocathode suffers from low electron−hole separation efficiency and photocorrosion. Thus, the introduction of other materials to increase the PEC performance and chemical stability of the photocathode is necessary. Many works so far have focused on the utilization of metal plasmonic nanostructures which could be incorporated into semiconductor materials to extend the spectral range of light absorption to the full solar spectrum and improve the performance of the PEC electrode.18,19 These nanostructures could generate localized surface plasmon resonance (LSPR), which is accompanied by valuable physical effects such as Received: March 30, 2019 Accepted: May 14, 2019 Published: May 15, 2019 3654
DOI: 10.1021/acsanm.9b00590 ACS Appl. Nano Mater. 2019, 2, 3654−3661
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ACS Applied Nano Materials
Figure 1. Schematic of the fabrication process of TiO2/AuNR/SiNH HN arrays. (a−g) Fabrication of large-area ordered SiNH arrays. (h, i) Fabrication of AuNR arrays embedded in SiNH arrays. (j) TiO2/AuNR/SiNH HN arrays forming after TiO2 layer deposition.
spots where hot electrons could be preferentially emitted were mainly concentrated near the Schottky junction. It is helpful to design an HN photoelectrode with a more efficient transport process of electrons and better improved PEC performance.
optical near-field enhancement, heat generation, and excitation of hot electrons.20 Two main combination methods of plasmonic nanostructures are utilized in these works. One is a nanostructured metal layer covered with a continuous semiconductor layer.21,22 This approach guaranteed enough light absorption in the semiconductor but may damage the utilization of hot carriers generated by LSPR compared with discontinuous metal particles because of the easier decay of hot carriers in continuous metal structures. The other one is to decorate a structured or planar semiconductor layer with metal nanoparticles.23−25 One main challenge of this method is how to balance the catalytic activity and light absorption because the metal catalysts will reflect and absorb some of the light, decreasing the resulting efficiency. Thus, the suitable design of the geometry of the metal particle, which will simultaneously affect the LSPR properties, is fairly important. The commonly used fabrication methods of metal particles in these works are thermal annealing26 and chemical synthesis.27 By these means, metal nanoparticles sized less than 100 nm could be prepared. However, the sizes are dispersed in a wide range, and the geometry is hardly controlled. In this work, we put forward a novel design and fabrication process to achieve a customized photocathode based on hetero-nanostructure (HN) arrays composed of Si nanohole (SiNH) arrays, Au nanorings (AuNRs), and TiO2 layers. SiNH arrays were fabricated on a p-Si wafer by nanoimprint lithography. Cylindrical Au nanorings clinging to the sidewalls of the prepared SiNH arrays were formed by sputtering deposition and subsequent ion beam etching. By these means, AuNRs with uniform shapes and sizes could be obtained in a controllable way. Moreover, this design could maximize the light absorption because the horizontal exposure area of the Si substrate is nearly unchanged. The TiO2 layer evaporated on the horizontal surfaces could improve the long-term stability of Si photoelectrodes in aqueous environments as the passivation material for surface protection.28−30 Furthermore, when the TiO2 layer is in direct contact with the AuNR, a Schottky junction could be formed, and the plasmonic metal particles will directly inject hot electrons to the conduction band (CB) of the neighboring TiO2 to participate in the solar watersplitting process.31 Through simulation, it was found that hot
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EXPERIMENTAL SECTION
Fabrication of Large-Area Ordered SiNH Arrays. Large-area ordered SiNH arrays were fabricated by the combination of nanoimprint lithography (NIL) and reactive ion etching (RIE). Figure 1a−g schematically illustrates the main fabrication process of ordered SiNH arrays. First, 0.1−1 Ω cm and 500 μm thick p-type Si (100) wafers were used as substrates. The Si wafers were ultrasonically cleaned in acetone and ethanol for 5 min, respectively, followed by a DI water washing, subsequently bathing in H2SO4/ H2O2 with a volume ratio of 4:1 for 10 min. Then, an underlying 200 nm thick poly(methyl methacrylate) (PMMA) layer was spin-coated on Si substrates, and an 80 nm thick silicon-containing UV-curable resist layer was spin-coated on the PMMA layer. We used a hybrid mold consisting of a rigid features layer and an elastic poly(dimethylsiloxane) (PDMS) support, which was duplicated from a master Si mold of hexagonally arranged nanodot arrays with a pitch of 600 nm and a diameter of 400 nm.32 The hybrid PDMS mold was pressed onto the resist film, followed by a curing step by exposing the sample to the UV radiation (dose: 300 mJ/cm2) in nitrogen atmosphere. After removal of the mold, the silicon-containing resist and PMMA were etched by RIE (CE 300I, ULVAC). After the hexagonal arrays of pillars generated in the PMMA layer were etched without a residual layer, a 30 nm thick layer of Ni was evaporated on it by high-vacuum electron-beam evaporation. Thus, a continuous Ni film with a hexagonal array of holes was obtained as an etching mask after a lift-off process to remove the PMMA. Finally, an RIE process was carried out to etch the Si substrate, and ordered SiNH arrays would be prepared after removing the Ni mask by dilute nitric acid. Fabrication of AuNR Arrays Embedded in SiNH Arrays. Figure 1h,i illustrates the fabrication process of AuNR arrays which were embedded in the prepared SiNH arrays. First, the as-prepared Si sample was immersed into a solution of 5% HF for 1 min to remove the oxidation layer. Then, a conformal coating of metal was deposited on the SiNH arrays by ion beam sputtering, which has weaker directionality and better film quality compared with electron-beam evaporation. After that, we adopted an argon ion beam etching (IBE) process in the normal direction to the substrate surface to remove the metal layer on the top and bottom of the SiNH arrays and reserved the metal films on sidewalls of the SiNH arrays thus forming cylindrical nanorings embedded in nanoholes. 3655
DOI: 10.1021/acsanm.9b00590 ACS Appl. Nano Mater. 2019, 2, 3654−3661
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ACS Applied Nano Materials Fabrication of TiO2/AuNR/SiNH HN Arrays. As Figure 1j illustrates, the TiO2 layer was e-beam deposited on the SiNH arrays with embedded Au nanorings in the normal direction to the substrate surface by high-vacuum electron-beam evaporation with good deposition directionality. The TiO2 layers covered the top plane of the substrate and the bottom of the hole arrays and formed TiO2/ AuNR/SiNH HNs. Sample Characterization. The morphology of the samples was observed by scanning electron microscopy (SEM, ZEISS Corporation, ULTRA 55). Materials compositions and distribution of the samples were investigated by energy-dispersive X-ray spectrometry (EDX). A UV−vis spectrophotometer (UV-3600, Shimadzu) was utilized to measure the UV−vis reflection spectra for different samples. Ohmic Contact Formation of the Samples. On the back side of the as-prepared samples, an ohmic contact was established by embedding a Cu wire in a eutectic gallium−indium alloy. The back side was subsequently covered with silicone to insulate the entire back side of the electrode and the Cu wire from electrolyte. An area of 1 cm2 on the front side of the electrode was exposed. Electrochemical Measurements. All electrochemical measurements were performed using a CHI 660D electrochemical workstation in a standard three-electrode system with a Pt sheet serving as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. H2SO4 (0.5 M) was used as the electrolyte. The illumination of 1 sun was provided by a 300 W xenon lamp with AM filter for PEC measurements. The Faradaic efficiency was calculated by measuring the evolved gas volume from the working electrode with gas chromatography (GC, Shimadzu, gc-8a) and comparing it with the gas volume calculated on the basis of the integral of measured current over illumination time.
Figure 2. (a) Top-view SEM image of the AuNR/SiNH arrays after the IBE process. (b) Cross-sectional SEM image of the AuNR/SiNH arrays after the IBE process. (c, d) EDX elemental maps of the AuNR/SiNH arrays (Au, dark cyan; Si, green).
minimize its block to the incident light on the planar surface and maximize the horizontal exposure area of the Si substrate, i.e., maximize the light absorption of the photoelectrode. Figure 3a,b exhibits the contrastive cross-sectional SEM images of AuNR/SiNH arrays after the IBE process and TiO2/ AuNRs/SiNH arrays after the deposition of the TiO2 layer. The thickness of the TiO2 layer is about 80 nm, so we can clearly observe the TiO2/AuNRs/SiNH structures. Figure S1d
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RESULTS AND DISCUSSION Figure S1a shows a top-view SEM image of the UV-imprinted hexagonally arranged nanopillar arrays with a pitch of 600 nm and a diameter of 400 nm. The top layer containing silicon has a much higher O2 RIE resistance. This allows the low-aspectratio imprinted patterns in the top layer to be enlarged to highaspect-ratio patterns through the underlying PMMA layer by O2 RIE. The large difference in etching selectivity between PMMA and the UV-curable layer (greater than 30:1) ensures etched patterns with high aspect ratios and sufficient depths, which is necessary for the pattern transfer process.33 Figure S1b shows the top-view SEM image of the prepared SiNH arrays with a Ni etching mask layer. The hole arrays with a 600 nm pitch and a 400 nm diameter were faithfully replicated and transferred from the mold. By adjusting the etching time, we can easily control the depth of SiNH, which will further influence the light absorption ability of the sample.34 Figure S1c exhibits the cross-sectional SEM image of the sample after sputtering deposition of Au. The Au film conforming the topography of SiNH could be observed. Figure 2a,b shows the top-view and cross-sectional SEM images of the Au-coated arrays after the IBE process, respectively. As shown in Figure 2a, a bright ring surrounding the edge of the nanohole opening was observed. Au belts were clearly seen on the sidewall of the nanoholes in Figure 2b, indicating that cylindrical AuNRs were formed against the sidewall of the nanoholes. The thickness could be controlled by adjusting the sputtering thickness of the Au film. The Au distribution on the surface of the SiNH substrate was investigated by EDX mapping. Figure 2c,d are EDX elemental maps of Au and Si on the surface of the same sample in Figure 2a,b, respectively. The EDX maps confirmed that Au was mainly concentrated at the edge of the nanohole opening, and there is no obvious Au barrier covering the planar surface of SiNH arrays. The Au only covering the sidewall of SiNH could
Figure 3. Comparison of cross-sectional SEM images and highresolution SEM images (insets) of (a) the AuNR/SiNH arrays after the IBE process and (b) the TiO2/AuNRs/SiNH arrays. (c−f) EDX elemental maps of the TiO2/AuNRs/SiNH arrays (Si, red; Au, yellow; Ti, purple; O, dark cyan). 3656
DOI: 10.1021/acsanm.9b00590 ACS Appl. Nano Mater. 2019, 2, 3654−3661
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of our as-prepared structures, three-dimensional (3-D) numerical simulations were carried out using COMSOL Multiphysics based on the finite element method. Optical reflectance and absorptance spectra of the TiO2/AuNR/SiNH HN arrays were calculated. Figure S3 illustrates that the reflectance spectrum matches well with the experimental result. Thus, we can expect to design a more efficient photocathode by optimizing the structure parameters with the help of simulation. When LSPR is generated, the collective charge oscillations in the AuNRs lead to increased ohmic loss of electromagnetic energy, which could be visualized as peaks in the absorptance spectrum. Theoretical simulation is an effective tool to gain useful messages including the LSPR peaks and the microscopic insights of electric field distribution in different regions. As Figure 4b reveals, the peaks of the absorptance spectrum of TiO2/AuNR/SiNH HN arrays exist at 410, 560, and 650 nm and so on. Compared with the absorptance spectrum of the AuNR/SiNH sample, we find that not all the absorptance peaks are induced by LSPR, and the LSPR peaks’ locations are slightly shifted with the introduction of the TiO2 layer. By simulating the electric field distribution (| E/E0|) at these peaks, we could distinguish LSPR peaks and further understand the absorption enhancement and the underlying effect of our designed HN. The numerical model is established according to the geometries of the TiO2/AuNR/SiNH sample, which has a hexagonal periodicity. In the simulations, a plane wave with linear polarization is normally illuminated, and the direction of the electric field is along the most close-packed direction of the nanoholes. As shown in Figure 5a,b, the simulated electric field
is the SEM image of TiO2/AuNRs/SiNH arrays whose signal is secondary electrons, and we can distinguish the different materials more clearly from it. Figure 3c−f is EDX elemental maps of TiO2/AuNRs/SiNH arrays, which show the elemental distribution and give convincing evidence of this heterostructure. To investigate the antireflection properties and the optimization in light harvesting of our fabricated HN arrays, the UV−vis optical reflectance spectra of planar Si, SiNH arrays, and TiO2/AuNR/SiNH HN arrays in the wavelength range 300−900 nm were measured. As shown in Figure 4a, it is
Figure 4. (a) Measured UV−vis reflection spectra of planar Si, SiNH arrays, and TiO2/AuNR/SiNH HN arrays. (b) Simulated optical absorptance spectra of AuNR/SiNH arrays and TiO2/AuNR/SiNH HN arrays by COMSOL Multiphysics.
Figure 5. COMSOL Multiphysics simulated electric field (|E/E0|) distributions for TiO2/AuNR/SiNH HN arrays at 560 nm. (a) Crosssectional view normal to the direction of the electric field. (b) Topdown view of the interface between the TiO2 layer and the top surface of the Si substrate.
obvious that the SiNH arrays efficiently suppress the reflection across the whole measured spectrum as compared to the planar Si wafer. The suppressed reflection is due to the light-trapping effect and graded refractive index profile between air and the Si substrate caused by the SiNH arrays’ morphology.35 By the introduction of the embedded AuNRs and TiO2 layer, the reflection could be further suppressed as a result of the increased light harvesting. The hemispherical optical reflectance spectra of SiNH arrays and TiO2/AuNR/SiNH HN arrays are exhibited in Figure S2. It can be found that scattering induced by the introduced AuNRs and TiO2 layers has few unfavorable effects on the absorption ability of the samples. The spectral features of the light absorption spectrum can be affected by the electric field of the plasmonic resonances of the AuNRs. To understand the optical responses and mechanisms
distribution (|E/E0|) at 560 nm manifests that an intensive electric field is centralized at the interface of the TiO2 layer and the AuNRs. It is obvious that the intensive electric field is mainly attributed to the excitation of the dipolar mode associated with the AuNRs, which is the characteristic of the LSPR effect of Au rings.36 The intensive electric field means the generation of high-density hot electrons in the top part of AuNRs close to the TiO2 layer. The simulated electric field distribution (|E/E0|) at 650 nm, another peak in the absorption spectrum, is shown by Figure S4a,b. We can see that another multipolar mode appears at the bottom of the AuNRs. Meanwhile, the dipolar mode in the top part still exists though the location slightly shifts. As shown in Figure S5, at peaks of the absorptance spectrum of the TiO2/AuNR/SiNH HN arrays, we could see that the typical dipolar modes exist mostly in the visible spectrum and gradually vanish in the near3657
DOI: 10.1021/acsanm.9b00590 ACS Appl. Nano Mater. 2019, 2, 3654−3661
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Figure 6. (a) J−V curves of SiNH arrays, TiO2/SiNH arrays, AuNR/SiNH arrays, and TiO2/AuNR/SiNH arrays measured with or without light illumination of 100 mW/cm2 in a solution of 0.5 M H2SO4. (b) Nyquist plots of SiNH arrays, TiO2/SiNH arrays, AuNR/SiNH arrays, and TiO2/ AuNR/SiNH arrays with light illumination of 100 mW/cm2 in a solution of 0.5 M H2SO4. (c) J−V curves of TiO2/AuNR/SiNH HN arrays with different thicknesses of deposited Au and TiO2 layers measured with or without light illumination of 100 mW/cm2 in a solution of 0.5 M H2SO4. (d) Measured (blue) and calculated (red) amount of evolved H2 as a function of time with light illumination of 100 mW/cm2 in a solution of 0.5 M H2SO4.
ohmic contact, the dark current shows a linear relationship with applied bias. Upon comparison of the current density versus potential (J−V) curves of the AuNR/SiNH and TiO2/ AuNR/SiNH sample, it can be discovered that both dark and light currents are increased when the TiO2 layer is introduced. We think that this may be attributed to the Schottky junction formed by the AuNRs and TiO2. The TiO2 we deposited contains oxygen vacancies and presents a dark blue color.37 The defects in the TiO2 layer should also play a significant role in affecting the optical absorption and PEC activity of our photocathodes.38,39 To control these effects, TiO2 was deposited on the different samples under the same working conditions. It can be considered approximately that defects of TiO2 formed in different samples are repeatable and almost have the same effect on the optical absorption and PEC activity. The oxygen vacancies result in n-type doping and high conductivity of the TiO2 films, with a work function smaller than Au. Thus, a Schottky junction is established at the interface between the AuNR and TiO2, with upward band bending of TiO2, shown in Figure S6. The Schottky junction can effectively suppress electron/hole recombination, accelerate charge transfer, and hence lead to larger current density.40 Referring to the numerical simulations above, it is known that a multitude of hot electrons will generate near the interface under light illumination, which promotes the hot electron injection from AuNRs to TiO2 and makes the HER
infrared region. The LSPR peaks exist at 560, 650, 720, and 800 nm in the visible spectrum. The dipolar modes in the top part of AuNRs result in hot spots near the interface of TiO2 and AuNRs, which will be a beneficial contribution to the injection of hot electrons from AuNRs to the CB of the neighboring TiO2. To determine the electrochemical performance and the mechanism of the hydrogen evolution process of our designed HN photocathode, electrochemical properties of different samples were tested, separately, and are presented in Figure 6a. The thicknesses of the deposited Au layer and TiO2 layer of the prepared samples were all 40 nm. As shown in Figure 6a, upon comparison of the TiO2/SiNH sample with the bare SiNH sample, PEC performances are enhanced with the introduced TiO2 layer since a higher plateau photocurrent and more positive onset potential could be observed. This indicates the passivation effect of the TiO2 layer. The currents generated in the SiNH and TiO2/SiNH samples under dark conditions are both very small. By contrast, the obvious dark currents of the AuNR/SiNH and TiO 2 /AuNR/SiNH sample are observed. Since the work function of Au is slightly higher than p-Si, the SiNH tends to form an ohmic contact with the AuNR. Therefore, the electrically excited electrons could easily transport across the interfaces of Si and Au, contributing to the hydrogen evolution reaction (HER) during the electrochemical experiments. Corresponding to the property of 3658
DOI: 10.1021/acsanm.9b00590 ACS Appl. Nano Mater. 2019, 2, 3654−3661
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ACS Applied Nano Materials process proceed more facilely.41 Compared with the other samples, the TiO2/AuNR/SiNH sample possesses higher photocurrent and better onset potentials. At an applied bias of −0.25 VRHE, the current density of the TiO2/AuNR/SiNH sample reaches −26.2 mA/cm2, where the photocurrent density is about −20.0 mA/cm2 and arrives at a plateau, and its onset potential is 0.32 VRHE, superior to other tested samples. This clearly validates that the HN brings about a great enhancement for the catalytic process. To better understand the PEC performance, we performed electrochemical impedance spectroscopy (EIS) measurements to elucidate the charge-transfer resistances in different photocathodes. The Nyquist plots are shown in Figure 6b, and the data can be fitted to different equivalent circuit models shown in Figure S7. As we can see from Figure 6b, the chargetransfer resistance from the electrode surface to electrolyte of our HN photocathode (172.2 Ω cm2) is dramatically lower than those of the other photocathodes, confirming the facile electrode kinetics of the TiO2/AuNR/SiNH structure. The measured EIS results agree well with the trend of our measured J−V performance. The photon-to-energy efficiency of different samples is also calculated to evaluate the PEC performance of our HN arrays photocathode.42 As shown in Figure S8, the efficiencies of AuNR/SiNH and TiO2/SiNH samples are both higher than that of the SiNH sample, and the HN sample possesses the best photoconversion efficiency which reaches approximately 13.3% at an applied bias of −0.23 VRHE. To further determine the effects of the introduced AuNRs and TiO2 layer on PEC performance, the J−V curves of the TiO2/AuNR/SiNH samples with different deposition thicknesses of Au and TiO2 layers were measured in the dark and under 1 sun illumination, respectively. We prepared four samples with different combinations of the Au layer with a thickness of 20 or 40 nm and TiO2 layer with a thickness of 20 or 40 nm. We named these different samples, respectively, by AuNR thickness−TiO2 layer thickness as 20−20, 20−40, 40− 20, and 40−40. The results are presented in Figure 6c. First, as we can see, a greater thickness of the Au layer contributes to a higher current density in both dark and light illumination conditions. We think that this may be attributed to the greater amount of electrons generated in AuNRs and larger contact area formed for the Schottky junction. Next, we investigate the impact of the TiO2 layer. By comparing the dark current of 20−20 and 20−40, 40−20, and 40−40, we can clearly discover the passivation effect of the TiO2 layer which will further suppress the dark current with a larger thickness. That may be ascribed to the longer migration distance of electrons to reach the surface of TiO2, which needs a stronger driving force. However, by comparing the light current of 40−20 and 40−40, it can be observed that 40−20 with a thinner TiO2 layer exhibits a larger dark current and poorer PEC performance. This indicates that TiO2 with insufficient thickness cannot suppress the dark current effectively and will weaken the photocatalytic process. Thus, we can conclude that only a suitable TiO2 layer thickness can contribute to a better PEC performance including larger photocurrent density and better onset potential, as can be seen from the J−V curve of sample 40−40. We performed an electrochemical test on the sample 40−40 in 0.5 M H2SO4 under 100 mW/cm2 illumination, with an applied bias of −0.23 VRHE. During the process, we measured the evolved amount of H2 by gas chromatography and made a comparison with the calculated amount of H2 based on the measured current of the photocathode. The
results are shown in Figure 6d. From the experimental results, we make sure that both photocurrent and dark current contribute to hydrogen production, and the Faradaic efficiency is approximately 100%. Thus, it is made evident that our NH photocathode possesses a good ability to produce hydrogen. The chemical stability of the HN arrays cathode with a 40 nm thick Au layer and 40 nm thick TiO2 layer is characterized by a continuous 5 h PEC experiment in H2SO4 (0.5 M) under 100 mW/cm2 illumination, with an applied bias of −0.23 VRHE, where the photoconversion efficiency reaches a peak value. It is apparently exhibited in the Figure S9 that the current density varies from about 23 to 25 mA/cm2, keeping a stable value and showing no obvious decline. The morphology of the surface of the electrode after the test was investigated by SEM, as shown in Figure S10. By comparison with the sample before the test, the morphology presents no obvious change. It confirms that our HN cathode exhibits a good and stable performance of hydrogen production. The PEC performance can be seen from the photocurrent part in total current density, as shown in Figure S11. The key performances of the PEC process of some other works are listed in Table S1. We can find that the PEC performance of our HN photocathode is comparable with those of those reported photocathodes. Moreover, the electrochemical process also makes a contribution to hydrogen production. Thus, it is made evident that our HN photocathode could have an outstanding hydrogen production performance. In summary of the working mechanism of our designed TiO2/AuNR/SiNH HN in the water-splitting process, illustrated in Figure 7, first, the AuNR acts as an electron
Figure 7. Working mechanism illustration of the TiO2/AuNR/SiNH hetero-nanostructure in a hydrogen evolution process.
source which could not only generate electrons with an applied bias but also generate a host of hot electrons by LSPR under light illumination. The electrons generated electrically could transfer to the surface of the TiO2 layer to contribute to electrochemical hydrogen production. Simultaneously, with light illumination, vast hot electrons generated in the top part of the AuNR near the interface of AuNR and TiO2 will transfer across the Schottky junction and contribute to a PEC hydrogen production. Meanwhile, the photogenerated electron−hole pairs emerge in SiNH, and the electrons will be extracted to the AuNR to replenish the transferred electrons while the remaining holes will flow to the Pt electrode for the oxygen evolution reaction. The PEC process proceeds synergistically with the electrochemical process to contribute to an outstanding hydrogen production performance. 3659
DOI: 10.1021/acsanm.9b00590 ACS Appl. Nano Mater. 2019, 2, 3654−3661
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ACS Applied Nano Materials
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CONCLUSION In summary, we have designed a novel TiO2/AuNR/SiNH HN and fabricated large-scale HN arrays by a controllable nanofabrication process. The important novelty of our work is that the particular HN design could maximize the horizontal exposure area of the Si substrate and promote the efficient generation and transport process of carriers, leading to a promising PEC performance. Each component of the HN could be fabricated with predetermined geometric size by adjusting the fabricating process. Experimental results confirm that the designed HN could be applied in hydrogen production by a synergetic effect of an electrochemical and PEC process. The AuNR plays an important role as an electron source which not only generates electrons with an applied bias but also originates LSPR which excites vast hot electrons under light illumination. By simulation, we can conclude that the hot electrons are mostly generated in the top part of AuNRs near the Schottky junction formed with the TiO2 layer, which facilitates the transfer of electrons across the Schottky junction to contribute to the HER at the TiO2 surface. The great current density and onset potential indicate the enhanced PEC performance of our prepared samples, which is well-explained by the simulation results. This work develops an innovative HN photocathode which demonstrates a great hydrogen production performance and may provide a new path to fabricate cost-effective photocathodes by utilization of suitable materials in combination with a reasonable design of component structures.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.9b00590.
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SEM images, hemispherical optical reflectance spectra, simulated and measured UV−vis reflection spectra, COMSOL Multiphysics simulated electric field (|E/ E0|) distributions, energy band diagram, equivalent circuits, energy conversion efficiency, chemical stability test, PEC part of a J−V curve, and the key performance of photocathodes in the reported literature (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Liang Zhang: 0000-0001-8347-3580 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was jointly supported by the National Key R&D Program of China (Grant 2018YFB1105400) and National Natural Science Foundation of China (Grant 51721001).
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