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Triboelectric Nanogenerator Driven Self-Powered Photoelectrochemical Water Splitting based on Hematite Photoanodes Aimin Wei, Xinkai Xie, Zhen Wen, Hechuang Zheng, Huiwen Lan, Huiyun Shao, Xuhui Sun, Jun Zhong, and Shuit-Tong Lee ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b04363 • Publication Date (Web): 23 Jul 2018 Downloaded from http://pubs.acs.org on July 24, 2018

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ACS Nano

Triboelectric Nanogenerator Driven Self-Powered Photoelectrochemical Water Splitting Based on Hematite Photoanodes

Aimin Wei†, #, Xinkai Xie†, #, Zhen Wen†, ‡, *, Hechuang Zheng†, Huiwen Lan†, Huiyun Shao†, Xuhui Sun†,*, Jun Zhong†,* and Shuit-Tong Lee†

†

Institute of Functional Nano and Soft Materials (FUNSOM), Jiangsu Key Laboratory for

Carbon-Based Functional Materials and Devices, and Joint International Research Laboratory of Carbon-Based Functional Materials and Devices, Soochow University, Suzhou 215123, China. ‡

State Key Laboratory of Silicon Materials, School of Materials Science and Engineering,

Zhejiang University, Hangzhou, 310027, China. #

A. Wei and X. Xie contributed equally to this work.

* Corresponding Author E-mail: Z. Wen: [email protected]; X. Sun: [email protected]; J. Zhong: [email protected]

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Abstract Hematite is one of the most promising photoanodes for photoelectrochemical (PEC) solar water splitting. However, due to the low conduction band position for water reduction, an external bias is necessarily required with the consumption of extra power. In this work, a titanium modified hematite (Ti-Fe2O3) photoanode based self-powered PEC water splitting system in tandem with a rotatory disc-shaped triboelectric nanogenerator (RD-TENG) has been developed. It is a fantastic strategy to effectively drive the hematite-based PEC water splitting by using the environmental mechanical energy through a TENG. When the rotation speed is 65 rpm (water flowing rate ~0.61 m/s), the peak current reaches to 0.12 mA under illumination contrast to that in the dark with almost zero. As for 80 rpm, the peak currents are 0.17 and 0.33 mA in the dark or under illumination, respectively, indicating the simultaneous occurrence of electrolysis and PEC water splitting. When higher than 120 rpm, the peak current in the dark is nearly equal to that under illumination, which can be attributed to the high enough peak voltage for direct electrolysis of water. Such a self-powered PEC water splitting system provides an alternative strategy that enables to convert both solar and mechanical energies into chemical energies.

KEYWORDS: photoelectrochemical water splitting; triboelectric nanogenerator; hematite; external bias; hydrogen evolution


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Extracting hydrogen energy from abundant and available water resource has become one of the hottest research areas for clean energies due to the excessive consumption of fossil fuels with serious green-house effect and environmental pollution.1-3 Since the first observation of hydrogen generation by illuminating TiO2 electrode in the laboratory, the photoelectrochemical (PEC) systems have been widely developed to capture solar energy and convert it into chemical energy in the form of hydrogen.4, 5 Hematite (α-Fe2O3) is one of the most promising photocatalytic materials for PEC systems.6-8 It is an attractive material due to its ample abundance in the earth, suitable bandgap for significant light absorption and excellent chemical stability in aqueous environments.9, 10 However, some limitations strongly hinder the practical application of hematite in PEC systems. The major drawback, namely, too low conduction band position for water reduction, needs to be overcome through adding an external bias. That usually means more extra power to release hydrogen.11 However, in real applications, how to provide an additional electric field necessary for the hydrogen evolution in a hematite photoanode based PEC system still poses great challenges. Since the discovery of nanogenerators, such as piezoelectric, electret and triboelectric nanogenerators, a great deal of development and breakthroughs have been springing up.12-14 Piezoelectric nanogenerators have been well applied in numerous areas with great energy conversion efficiency and extended device lifetime, especially in self-powered sensor networks.15, 16 Based on electret nanogenerators, a bi-functional smart face mask and a self-recovering flexible device were successfully fabricated to harvest ambient energy and further promoted the progress of self-powered wearable electronics.17, 18 As self-sufficient power sources, triboelectric nanogenerators (TENGs) adept in harvesting various types of mechanical energies, can be used to provide an external bias for driving many different electrochemical processes, such as air pollution


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cleaning,19 degradation of organic pollutant,20-22 removal of heavy metal ions,23 sea water desalination24 and many more.25 Since a hybrid energy cell for self-powered water splitting was fabricated in 2013, the application of TENG in water splitting has gained much concern and lots of researches.26 Tang and coworkers developed a hybrid system constituted by coupling a TENG and a water splitting unit, and achieved fully self-powered water splitting for hydrogen generation.27 Soon after, Li et al. fabricated a coupled TENG-PEC hybrid cell based on a TiO2 photoanode, where the PEC water splitting was distinctly boosted by TENG-charged Li-ion battery.28 Rational utilization of universal hydroenergy provides an advisable strategy for sustainable energy development. The emergence of TENG gives rise to a cost-effective and environmentally friendly approach to convert mechanical energy of water to electricity.29-33 Charge transfer between two triboelectric materials of the TENG induces a potential difference, operating as an external bias in the PEC system. Given those, hematite that can absorb most part of the solar light would be a favorable material to cooperate with TENG in a PEC cell for harvesting solar energy and hydrogen energy. In this work, we have developed a self-powered PEC water splitting system based on hematite photoanodes driven by a rotatory disc-shaped TENG (RD-TENG). It is a fantastic strategy to effectively drive the PEC water splitting of hematite by using the environmental mechanical energy through a TENG. Ti modified hematite (Ti-Fe2O3) photoanode was fabricated to enhance the charge separation. In the process, the generated electricity by the RD-TENG after transformation and rectification acted as an external bias to achieve the overall PEC water splitting. The output peak voltage and current in the dark or under illumination at different rotation speeds have been measured systemically. The peak current under illumination observed at a relatively low rotation speed significantly increased when compared to that in the dark, while no obvious


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change could be observed at a high rotation speed. The rate of hydrogen evolution is indeed accelerated once light is on at the practical rotation speeds. The as-designed selfpowered PEC water splitting system would not only offer a fantastic route to deal with the disadvantage of hematite, but also convert solar and mechanical energies simultaneously into chemical energy so as to effectively release hydrogen.

RESULTS AND DISCUSSION

Figure 1. Systematic configuration of the self-powered photoelectrochemical (PEC) water splitting system based on hematite photoanode driven by a rotatory discshaped triboelectric nanogenerator (RD-TENG). Here, we have proposed a self-powered PEC water splitting system driven by TENG with hematite as photocatalytic electrode, as illustrated in Figure 1. In this system, a RD-TENG functions as an alternating current (AC) power source with high voltage and relatively low current. After transforming and rectifying, AC is converted into direct current (DC) with relatively low voltage and high current, which can supply sufficient external bias to drive the water spitting process based on hematite photoanode and accelerate the rate of production evolution. For PEC solar water splitting procedure, the photo-generated holes in the bulk of hematite will migrate to the hematite/electrolyte interface for water oxidation to produce oxygen. Simultaneously,


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driven by the external electric field, the photo-generated electrons can be transferred to the counter electrode (Pt electrode) and reduce the water to release hydrogen.

Figure 2. Schematic illustration and electrical output performance of RD-TENG. (a) Schematic illustration and (b) photograph of the RD-TENG, which consists of two parts, a disk-shape print circuit board (PCB) deposited with radial Cu as the rotator and a PTFE film coated interdigital Cu electrodes as the stator (scale bar, 2 cm). SEM image inset illustrates the nanostructured surface of PTFE (scale bar, 2 μm). (c) Schematics of operating mechanism of the RD-TENG. Two sections from top to bottom illustrate the three-dimensional schematic and charge distribution when the rotator spins from the margin of electrode 1 (E1) to that of electrode 2 (E2). Electrical output performance of the RD-TENG, including open circuit voltage (Voc) and short-circuit current (Isc) under various rotation speeds in the range from 40 to 120 rpm (d) before transformation and (e) after transformation. In the proposed self-powered PEC water splitting system, the RD-TENG is composed of a multilayered structure, including a disk-shape rotator and a corresponding stator, as shown in Figure 2a. For the rotator (up), considering the output and technical requirements, radially copper segments with a central angle of 1.5°and a


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thickness of ~70 µm were preferably deposited on the print circuit board (PCB) made from stiff glass epoxy, performing as a triboelectrification layer. An acrylic sheet was adhered to the PCB acting as the supporting substrate. For the stator (bottom), arrayed copper segments are differently patterned with two complementary structures disconnected by fine trenches. A polytetrafluoroethylene (PTFE) thin film is coated on the copper electrodes, working as another triboelectrification layer. The PTFE film was treated by the one-step plasma reactive ion etching process to enhance the surface charge density. The SEM image shows the PTFE surface with the nanowires of ~100 nm in diameter and ~1 µm in length, respectively (inset of Figure 2a). The rotator and stator are fitted coaxially in operation. A photograph in Figure 2b displays the core parts of the rotator and the stator, respectively. Radially patterned copper segments were deposited with 176 mm in diameter on the PCB, which has 184 mm in conjugate diameter. Figure 2c illustrates the operating mechanism of the RD-TENG, which depends on alternating flow of electrons between electrodes by the coupling effects of triboelectrification and electrostatic induction.34-37 The three-dimensional schematic and charge distribution in short-circuit condition are demonstrated. In the initial state, the rotator is in alignment with electrode 1 (E1) on the stator, resulting in accumulation of induced charges on electrode 1 (E1) and electrode 2 (E2). As the rotation begins, triboelectrification occurs and generates equivalent quantity of positive and negative charges on the copper surface and the PTFE surface. The potential difference between E1 and E2 arises, and current flowing in the opposite direction is generated. The final state is defined as the moment when the rotator is lined up with E2 on the stator, where the polarity of charge density on both electrodes is reversed in comparison with the initial state. The following rotation circulation after the final state generates a current in the opposite direction, and AC emerges due to continuously periodic rotation. To


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measure the output performance of the RD-TENG, a programmable rotating machinery was employed and connected to the rotator to control the rotation speed. Figure 2d shows the electric output of the RD-TENG under various rotation speeds ranging from 40 to 120 rpm. At a rotation speed of 120 rpm, the open-circuit voltage (Voc) has a peakto-peak value of ~180 V, and the short-circuit current (Isc) appears a consecutive AC at an average amplitude of 0.12 mA. The electric output performance has also measured after transformation, as revealed in Figure 2e. Interestingly, with the increase of the rotation speed, both the Voc and Isc increase at the same time. At the rotation speed of 120 rpm, the peak voltage reaches 6 V and the peak current increases up to 1.6 mA, respectively. The inset of Figure 2e shows the circuit diagram of the system with a connecting transformer. The corresponding power outputs of the RD-TENG are shown in Figure S1 before transformation and Figure S2 after transformation, respectively.

Figure 3. Characterizations and photoelectrochemical properties of hematite photoanodes. (a) XRD spectra of Fe2O3 and Ti-Fe2O3. (b) High-revolution TEM image of Ti-Fe2O3. Inset shows SEM image of the top view. (c) XPS spectra at the Ti 2p edge and (d) Mott-schottky plots of Fe2O3 and Ti-Fe2O3. (e) Electrochemical impedance spectra (EIS) of Fe2O3 and Ti-Fe2O3 measured at 1.0 V vs. RHE in 1 M NaOH


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electrolyte under illumination. (f) J-V curves of Fe2O3 and Ti-Fe2O3. Inset shows highrevolution curves with the potential ranging from 0.8 V to 1.8 V vs. RHE. As one of the most promising photocatalytic materials, hematite possesses a suitable bandgap with the ability of absorbing most part of the solar light. Here, hematite was prepared as a photoanode for PEC water splitting and Ti modification was taken to enhance the performance,38, 39 as illustrated in Figure 3. The XRD spectra of as-prepared Fe2O3 and Ti-Fe2O3 in Figure 3a shows that all the diffraction peaks can be attributed to Bragg reflections from the hematite phase (JCPDS 33-0664) or from the FTO substrate. There is only one strong peak corresponding to the α-Fe2O3 (110) reflection, besides other much weaker peaks belonging to (113), (024), (214) and (300) planes. From the SEM image of Ti-Fe2O3 (inset of Figure 3b), it is clear that the nanorod arrays are well grown on the FTO glass with high uniformity. High-revolution TEM image (Figure 3b) shows the two interplanar distances of 0.37 and 0.27 nm, which are consistent with the (012) and (104) planes of α-Fe2O3, respectively. Also the similar results can be obtained for the pristine hematite (Figure S3). Therefore, Ti-modification has not changed the morphology feature or formed new other phase compared to that of the pristine α-Fe2O3, which agrees to the analysis of XRD spectra. Though, XPS spectra at the Ti 2p edge presented in Figure 3c show two obvious peaks at 464.0 and 458.2 eV, which could be assigned to Ti 2p1/2 and Ti 2p3/2 spin-orbit components of Ti4+.40 XPS spectra of other elements are displayed in Figure S4. Furthermore, the TEM elemental mapping images in Figure S3c confirm the existence of Ti element with a uniform distribution over the nanorod. PEC properties of the photoanodes were carried out in 1 M NaOH solution with a three-electrode cell. Figure 3d shows the Mottschottky plots of Fe2O3 and Ti-Fe2O3, whose slopes could be used to calculate the carrier density.41 The charge carrier densities after calculation for Fe2O3 and Ti-Fe2O3


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are 4.22 × 1020 cm-3 and 8.40 × 1020 cm-3, respectively. The slightly higher carrier density of the Ti-Fe2O3 photoanode compared with the bare Fe2O3 could partly contribute to the enhanced performance. The electrochemical impedance spectroscopy (EIS) curves for Fe2O3 and Ti-Fe2O3 were also obtained to study the interfacial charge transfer process. Figure S5 shows the equivalent circuit to simulate the Nyquist plots and Table S1 shows the fitting parameters. Compared to the pristine Fe2O3, the TiFe2O3 sample exhibits a plot with an obviously smaller diameter of semicircle with lower resistance for the interfacial charge transfer (Rct) and resistance for trapping holes by the surface states (Rtrap). The significantly decreased values of the both resistances indicate much faster charge transfer kinetics at the electrode interface and surface, which could be attributed to the formation of Fe2TiO5 by Ti modification with a favorable band position to accelerate the charge separation.42 Actually, the present synthesis method for Ti-Fe2O3 is very similar to our previous report.38 As a result, TiFe2O3 shows a higher photocurrent density than that of Fe2O3 with a value of 1.50 mA/cm-2 at 1.23 V vs. RHE (the inset in Figure 3f). It is observed that the photocurrent does not appear until the potential is up to ~0.8 V vs. RHE, indicating the necessity of considerable external bias for triggering solar water splitting. In addition, the Ti-Fe2O3 sample exhibits a good stability over 8 hours, as shown in Figure S6. Notably, the current density under illumination is higher than that in the dark all the time from 0.8 to 2.1 V vs. RHE, while the both are almost equal when the potential is greater than 2.2 V vs. RHE or even. This indicates that when the potential between the two electrodes is high enough, the electrolysis of water plays a dominant role and solar light makes no contribution to increase the current density. Such tendency and phenomenon are also observed in the following experiments of the self-powered PEC water splitting system.


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Figure 4. Fabrication and performance of RD-TENG driven self-powered PEC water splitting system. (a) Schematic diagram, (b) photograph and (c) equivalent circuit of the self-powered PEC water splitting system. (d) Current output at different rotation speeds in the dark or under illumination. (e) Peak current as a function of time at the different rotation speeds in the dark or under illumination. (f) Current as a function of voltage in the dark or under illumination. Inset shows the peak current as a function of different rotation speeds in the dark or under illumination. Considering such a necessary bias can be easily provided by TENG, thus a RDTENG driven self-powered PEC water splitting system was constructed, as illustrated in Figure 4. Figure 4a shows the schematic diagram of the RD-TENG driven selfpowered PEC water splitting system based on Ti-Fe2O3 photoanode. Figure 4b and c show the photograph and equivalent circuit of the system, respectively. The electricity generated by the RD-TENG is firstly transformed with a step-down transformer, followed by full-wave rectification to output direct current. Then, the positive pole of the rectifier is connected to the Ti-Fe2O3 photoanode, while the negative one is connected to the Pt electrode. The PEC water splitting process occurred in an electrolytic cell containing 1 M NaOH solution and hydrogen bubbles appeared around the Pt electrode. Figure 4d shows the current output of the self-powered system at different rotation speeds in the dark or under illumination. Clearly note that when the


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rotation speed is 65 rpm (equal to 0.61 m/s of the water flowing rate), the current suddenly increases to 0.12 mA at peak under illumination contrast to that in the dark with almost zero. The voltage between the two electrodes was measured about 0.9 V in the dark, which was hardly able to decompose water but could make it with the help of light. As for 80 and 115 rpm, the peak currents in the dark are approximately 0.17 and 0.47 mA, and increase to 0.33 and 0.58 mA under illumination, respectively. It indicates that electrolysis and photoelectrochemical water splitting simultaneously occur when the rotation speed increases. In order to well understand the process, the schematic diagrams for energy bands of these two water splitting processes are displayed in Figure S7 and some discussion follows it consequently. With the increase of the rotation speed, the peak voltage and peak current between the two electrodes also increase, which is consistent with that after transformation in Figure 2e. Another point deserved to be mentioned is that when the rotation speed is higher than 120 rpm (see the inset in Figure 4f), the peak current in the dark is nearly equal to that under illumination, which can be attributed to the peak voltage (measured about 2.2 V or higher) high enough for direct electrolysis of water in NaOH solution.43 Similar to the result of high potential (greater than 2.2 V vs. RHE or even) in Figure 3f, solar power cannot be stored at such peak voltages. However, since the voltage output of RD-TENG does not always keeping at the peak, most of the part are lower than the peak value and solar illumination would play significant roles on such situations. Connect the peak current points with a line, we can draw out a graph of peak current as a function of time at the different rotation speeds, as shown in Figure 4e. More intuitively, the peak current under illumination increases much compared to that in the dark at relatively low rotation speeds. With the increased rotation speed, both the peak currents under illumination and in the dark increase at the same time. When the speed is high enough, the peak


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current under illumination is almost equal to that in the dark. According to the peak currents and the voltages under illumination or in the dark recorded at different rotation speeds, the relationship between peak current and peak voltage can be found in Figure 4f. With the increase of the peak voltage, the peak photocurrent increases significantly and the peak dark current increases obviously until the voltage is higher than ~1.5 V. Especially, the peak photocurrent reaches 0.55 mA at ~1.5 V, while the corresponding peak dark current is just about 0.04 mA. When the voltage up to ~2.2 V or higher, the peak photocurrent is almost equal to the dark current. Such results are similar to the tendency and phenomenon of Ti-Fe2O3 photoanode in Figure 3f.

Figure 5. Demonstration of RD-TENG driven self-powered PEC water splitting system. Photographs of the electrolytic cell of hybrid system at 120 rpm (a) in the dark and (b) under illumination. (c) GC test of cathodic production. (d) Photographs of the gas collection tube at different time in NaOH solution. (e) Cathodic production rate as a function of different rotation speeds. To evaluate the ability of hydrogen evolution of the self-powered PEC water splitting system, gas production from the cathodic Pt electrode was collected by a tube (with a division of 20 μL) and extracted by an injector for gas chromatography (GC) test, as demonstrated in Figure 5. A typical test was carried out in the dark or under


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illumination with the RD-TENG continuously rotating at the speed of 120 rpm. Hydrogen bubbles can be observed after several seconds once light on, while few appeared for some time without light (Figure 5a and b). More details for such phenomenon can be watched from Supporting Movie S1, consistent with the lower current and significantly increased value on condition of light off and on. From the result in Figure 5c, there is a saturated peak attributed to hydrogen at 0.997 min and no other obvious peaks turn up, indicating that hydrogen is the most dominating production. To estimate the rate of hydrogen evolution roughly, a typical record at 120 rpm was displayed in Figure 5d, where the time was recorded once the gas volume was up to 20 μL. Figure 5e shows the final results at three different rotation speeds in the dark or under illumination. At lower rotation speeds, both the rates before or after illuminating are very low, resulting from the relatively low peak current and voltage. However, the light indeed accelerates the rate of hydrogen evolution compared to that in the dark, which indicates that solar power can be stored by tandem RD-TENG to provide external bias. When the rotation speed reaches 140 rpm, the rates of hydrogen evolution are up to 5.56 μL min-1 and 6.67 μL min-1 before and after illuminating, respectively. Herein, such a self-powered PEC water splitting system enables to convert both solar and mechanical energies into chemical energies. While there are some packaging and collecting problems desired to be solved, such system would be applied in real environment in prospect.

CONCLUSION In conclusion, we successfully developed a RD-TENG driven self-powered PEC water splitting system based on Ti-Fe2O3 photoanode to produce hydrogen energy. The external bias provided by RD-TENG enables hematite to overcome the disadvantage of unsuitable conduction band position for fully solar water splitting. The generated


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electricity is primarily transformed with a step-down transformer, followed by the fullwave rectification to output direct current as the necessary bias. At a relatively low rotation speed of 65 rpm, the peak current significantly increases to 0.12 mA under illumination compared to that in the dark with almost zero. With the rotation speed increase to higher than 120 rpm, the peak current in the dark is nearly equal to that under illumination, indicating the direct electrolysis of water. The rates of hydrogen evolution are up to 5.56 μL min-1 and 6.67 μL min-1 before and after illuminating at 140 rpm. This fantastic strategy would be an alternative and efficient route to collect solar power and mechanical energy simultaneously in the form of hydrogen energy.

EXPERIMENTAL METHODS Preparation of Photoanodes. A modified hydrothermal method was used to prepare the pristine hematite photoanode grown on a fluorine-doped SnO2 (FTO, Nippon Sheet Glass, Japan, 14 ohm/sq) glass substrate.44 Briefly, A cleaned FTO glasss (50 mm×30 mm×2 mm) was put into a Teflon-lined stainless steel autoclave filled with 80 mL aqueous solution containing 0.08 M ferric chloride (FeCl3.6H2O, Sinopharm Chemical Reagent Co., Ltd) and 0.08 M urea (CO(NH2)2, Sinopharm Chemical Reagent Co., Ltd) and then heated at 95 °C for 4 h. Then the product was sintered in air at 550 °C for 2 h and further annealed at 750 °C for additional 15 min. The final product was labeled as the pristine hematite (Fe2O3). Titanium-modified hematite was prepared through a twostep hydrothermal method.45 Ti precursor solution was obtained by mixing 80 mL deionized water with 35 μL titanium tetrachloric (TiCl4, Aladdin). A cleaned FTO glasss was firstly immersed into the mixed solution at 75 °C for 20 min and then heated by a hot plate in air at 180 °C for 15 min. The Ti-pretreated FTO glass was then used to grow hematite following the same steps as that for the pristine hematite. The final product was labeled as Ti-Fe2O3.


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Fabrication of the RD-TENG. For the rotator, cut an acrylic sheet with 184 mm in diameter and 3 mm in thickness by a laser cutter (Huitian Laser 4060) to perform as the supporting substrate. Then stick the PCB with radially patterned copper onto the acrylic substrate. Each copper segment possesses the central angle of 1.5°. For the stator, cut a corresponding acrylic sheet as the supporting substrate. Afterwards, adhere the PCB with interdigital Cu electrodes onto the acrylic sheet. After that, stick a PTFE thin film onto the Cu electrodes. Finally, for fabricating the RD-TENG, fit the rotator and the stator coaxially, and employ two lead wires to the two sets of Cu electrodes. The corresponding value of the water flowing rate is calculated according to the following formula: v = πnd where v is the water flowing rate, π = 3.14 is a constant, n is the rotation speed, d is the diameter of the disc-shaped PCB. Characterizations. Scanning Electron Microscope (SEM, FEI Quanta 200F) and Transmission Electron Microscopy (TEM, FEI Tecnai G2 F20 S-TIWN) were used for the morphology characterization. X-ray Diffraction (XRD, PANalytical, Zmpyrean) and X-ray photoelectron Spectrometer (XPS, Kratos AXIS UltraDLD) were used for the structure characterization. Gas chromatography (GC7900) was used to determine the species of the cathodic production. PEC Measurements. All PEC measurements were carried out by using an electrochemical workstation (CHI 660D) in a three-electrode cell with the working area about 0.15 cm2 and the left part covered by non-conductive Hysol epoxy. The electrolyte was 1 M NaOH purged with N2 for about 20 min and the pH value was controlled at 13.6. According to the Nernst equation, the measured voltage could be converted into the potential vs. reversible hydrogen electrode (RHE). Xenon High


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Brightness Cold Light Sources (XD-300) were employed as the light source with a power density of 100 mW/cm2 under AM 1.5G ﬁlter. Photocurrent vs. voltage (J-V) curves were recorded by sweeping the potential from 0.6 V to 2.4 V vs. RHE at a scan rate of 50 mV/s. Mott-Schottky plots were generated from the capacitance values derived from the electrochemical impedance 100 kHz frequency in the dark. Electrochemical impedance spectra (EIS) measurements were performed by applying 1.0 V vs. RHE at a frequency range of 10000 Hz to 0.1 Hz with an amplitude of 10 mV under illumination.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano. Additional information (PDF) Movie S1: Hydrogen evolution driven by the RD-TENG under illumination or in the dark at the rotation speed of 120 rpm. (AVI)

AUTHOR INFORMATION Corresponding Authors *E-mail (Z. Wen): [email protected] *E-mail (X. Sun): [email protected] *E-mail (J. Zhong): [email protected] ORCID Zhen Wen: 0000-0001-9780-6876 Xuhui Sun: 0000-0003-0002-1146 Jun Zhong: 0000-0002-8768-1843 Shuit-Tong Lee: 0000-0003-1238-9802 Author Contributions


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A. Wei and X. Xie contributed equally to this work.

ACKNOWLEDGMENT The work was supported by Natural Science Foundation of China (NSFC) (Grant No. U1432249, U1732110), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the Collaborative Innovation Center of Suzhou Nano Science & Technology, the Soochow University-Western University Centre for Synchrotron

Radiation

Research,

China

Postdoctoral

Science

Foundation

(2017M610346) and Natural Science Foundation of Jiangsu Province of China (BK20170343). Dr. Z. Wen specially acknowledge the support from State Key Laboratory of Silicon Materials, Zhejiang University (SKL2018-03).

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