Photocurrent Enhancements of TiO2-Based Nanocomposites with

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials 2

Photocurrent Enhancements of TiO-Based Nanocomposites with Gold Nanostructures/Reduced Graphene Oxide on Nanobranched Substrate Hsin-Chia Ho, Kai Chen, Tadaaki Nagao, and Chun-Hway Hsueh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b03714 • Publication Date (Web): 08 Aug 2019 Downloaded from pubs.acs.org on August 10, 2019

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The Journal of Physical Chemistry

Photocurrent Enhancements of TiO2-Based Nanocomposites with Gold Nanostructures/Reduced Graphene Oxide on Nanobranched Substrate Hsin-Chia Ho,† Kai Chen,‡,§ Tadaaki Nagao,§,⊥,* and Chun-Hway Hsueh†,* †Department ‡Institute

of Materials Science and Engineering, National Taiwan University, Taipei 10617, Taiwan

of Photonics Technology, Jinan University, Guangzhou 510632, PR China

§International

Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba,

Ibaraki 305-0044, Japan ⊥Department

of Condensed Matter Physics, Graduate School of Science, Hokkaido University, Sapporo

060-8010, Japan

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: In this work, we studied three-dimensional (3D) highly branched TiO2 nanorod (b-NR) arrays decorated with plasmonic Au triangular nanoprisms (TNPs) and reduced graphene oxide (rGO) sheets for photoelectrochemical (PEC) water oxidation. The photocurrent densities of rGO/TiO2 b-NRs and Au TNPs/rGO/TiO2 b-NRs exhibited 21% and 90% enhancements, respectively, compared with bare TiO2 b-NRs at 0.5 V (vs. Ag/AgCl). Incident photon-toelectron conversion efficiency measurements revealed the synergistic effects of plasmonic Au TNPs and rGO on the enhancement of photocurrent response, especially in the visible region. The electrochemical impedance spectroscopy analysis provided further evidence that the charge transport resistance between TiO2 photoanode and electrolyte was greatly reduced with the incorporation of Au TNPs and rGO. It is suggested that the plasmonic Au TNPs exhibiting localized surface plasmon resonances at 560 and 660 nm to enhance the visible light absorption, rGO with superior electron conductivity, and 3D TiO2 b-NR arrays with large surface area and high carrier mobility synergistically contribute to the high PEC performance. Our results of the facile decoration of Au TNPs and rGO sheets could arouse great interest in PEC water splitting reaction and aid in the evaluation of other photoanodes.


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INTRODUCTION Following the first demonstration of water photoelectrolysis by Fujishima and Honda,1 researchers have investigated a variety of material systems to improve the conversion efficiency from solar energy to storable fuels; e.g., hydrogen gas.2-9 The capability of photocatalytic materials to harvest and convert the sunlight into chemical energy is dependent on various factors, including the energy bandgap, charge carrier mobility, defect, crystallinity, etc.10 The methods conducted for optimizing the reaction efficiency are also of great importance.11 TiO2 has been extensively used for photoelectrochemical (PEC) photoanodes to generate H2 from water with different configurations due to its superior chemical stability, preferable band edge positions, and high photocorrosion resistance.12-15 One-dimensional (1D) TiO2 nanostructures, such as nanorods and nanotubes, possess the superior photocatalytic activity and PEC property with high charge transport efficiency and low charge recombination rate compared with granular films composed of zero-dimensional (0D) TiO2 nanoparticles.16-17 Recently, three-dimensional (3D) TiO2 nanostructures were reported to exhibit even better performance than their 1D building blocks since they provided remarkably larger contact areas with the electrolytes and efficient diffusion paths for photogenerated holes to reach the semiconductor-electrolyte interface reducing the charge recombination rate.18-21 While 3D TiO2 has been regarded as one of the most promising photoelectrodes, the intrinsic large band gap of TiO2 limits the light absorption to the UV range, which is less than 5% of the solar radiation on the ground level. In order to extend the light harvesting ability of TiO2 to the visible or even infrared region, numerous attempts have been conducted including elements doping,22-25 chemical modification,7,26 quantum dots decoration,20,27-28 and tandem architectures with other narrower band gap semiconductors.29 Plasmonic nanoparticles with distinctive localized surface plasmon resonance (LSPR) behaviors have been employed to enhance the 3 ACS Paragon Plus Environment


photoactivity of conventional semiconductors.30-38 LSPR describes the collective oscillations of free electrons in metal nanoparticles triggered by the incident electromagnetic field.39 Upon excitation of LSPR, these nanoparticles could transfer the concentrated energy to the adjacent semiconductor through radiation with increased optical scattering as well as near-field nanofocusing effect of light.40 In addition, they could also enhance the photocatalytic performance in a non-radiative way via hot electron generation and injection to the conduction band of the semiconductor.40 Among these metallic nanoparticles, Au nanoparticles have been widely used to improve the response of TiO2 in the visible region.41-47 For example, Pu et al.41 demonstrated that TiO2 nanowire arrays decorated with a mixture of Au nanospheres and nanorods (NRs) showed an enhanced photoresponse in the entire UV-visible region due to electric-field amplification and hot electron generation by the Au nanoparticles.41 In addition, Su et al.42 reported that Au nanoparticles attached to TiO2 branched nanorod (b-NR) arrays could achieve prominent photocurrent compared with pristine TiO2 under either the visible light (≥ 420 nm) or the simulated solar light. The improved performance was mainly attributed to the LSPR effect of Au nanoparticles enhancing the visible light absorption as well as the large surface area/efficient charge separation of TiO2 b-NR arrays.42 In addition to plasmonic nanostructures, graphene has also attracted tremendous interest in the fields of photocatalytic degradation of pollutants and PEC water splitting.6,48-50 For example, the large surface area with flexibility could improve the adsorption capacity of substrate toward the pollutants,49 the formation of Ti-O-C chemical bonds between graphene and TiO2 substrate could result in bandgap narrowing of TiO2,48 and the excellent electrical conductivity of graphene makes it a photoelectron acceptor and mediator to enhance the photocurrent.6,50 The coupling of metal nanostructures and graphene with TiO2 nanoparticles or macroporous structures has been realized as a feasible system in PEC water splitting, and it benefits from the 4 ACS Paragon Plus Environment

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plasmonic-induced hot electron transfer and the high electron mobility through graphene to boost the overall photocatalytic efficiency.51-54 Herein, we proposed a ternary heterojunction architecture composed of Au triangular nanoprisms (TNPs), reduced graphene oxide (rGO) sheets and 3D TiO2 b-NR arrays. Specifically, the ternary architecture was processed using spin coating followed by N2 annealing for rGO sheets and decoration via molecular linker for Au TNPs, respectively, onto TiO2 b-NR arrays, which were fabricated by a two-step hydrothermal method. We studied the PEC performances by photocurrent measurements under simulated solar light illumination, incident photon-to-electron conversion efficiency (IPCE) evaluation, and electrochemical impedance spectroscopy (EIS). A mechanism describing the process of charge transportation in this ternary architecture was also proposed. We believed the prominent improvement of TiO2 photoactivity by the unique plasmonic nanostructures and rGO sheets, especially in the visible region, could give rise to great interest in photocatalysis and PEC reaction.

METHODS Synthesis of TiO2 b-NR Arrays. A two-step hydrothermal method was conducted to synthesize 3D TiO2 b-NR arrays.55 In this case, FTO (fluorine-doped tin oxide, 550 nm) glass (6−8 Ω/cm2) was cut into 2×1 cm2 and cleaned by acetone, isopropyl alcohol, and distilled water sequentially for 20 min each and then dried with N2 gas. Meanwhile, 1 mL of titanium butoxide was dissolved in a solution containing the same volume of distilled water (30 mL) and hydrochloric acid (30 mL). The washed FTO glass was then loaded into a Teflon-lined stainless steel vessel with the prepared growth solution. While 1×1 cm2 of the surface area of FTO was reserved for the electrode connection, another 1×1 cm2 was used to grow TiO2 NRs. The vessel was heated at 180 °C for 3 h to grow vertically aligned rutile TiO2 NRs on the FTO surface. 5 ACS Paragon Plus Environment


After that, the sample was removed and incubated in a 0.1 M TiCl4 solution (iced) for 1 h to coat the surface of rutile TiO2 NRs with a thin Ti-precursor film, which functioned as a seed layer to promote the growth of branched structure. Then the second hydrothermal synthesis was conducted by immersing the sample into a 35 mL aqueous solution containing 0.5 mL sulfuric acid and 0.2 mL titanium butoxide, followed by heating at 180 °C for 3 h to grow the TiO2 bNRs. To increase the crystallinity of TiO2 NRs and TiO2 b-NRs, annealing at 500 °C for 2−6 h was performed after each hydrothermal treatment. Decoration of Reduced Graphene Oxide (rGO). Graphene oxide (GO) solution was prepared by the modified Hummer’s method using graphite flake as the stock material.56 After a series of oxidation, exfoliation, cleaning and drying, the as-prepared GO powder was redispersed in distilled water and sonicated for 1 h prior to spin-coating. Then, GO solution was spin-coated onto the TiO2 b-NR arrays at 2000 rpm for 30 s. Herein, three different concentrations of GO solutions (0.1, 0.3 and 0.5 mg/mL) were used. Finally, the sample was heated at 400 °C for 2 h in N2 atmosphere to reduce GO to rGO. During the reduction process, GO with the carboxylic acid functional groups would interact with the surface hydroxyl groups of TiO2 and form the chemical bonding.48 Synthesis of Au Triangular Nanoprisms (TNPs). Au TNPs were synthesized by a rapid reduction method.57 Typically, 0.256 g of CTAC was dissolved in distilled water at 50 °C. Then the binding agent KI(aq) (0.375 mL, 0.01 M) was added. The mixture of Au3+ (0.4 mL, 25.4 mM) and NaOH (0.1 mL, 0.1 M) was injected into the solution with gently stirring. The solution color changed from light yellowish to transparent after the addition of reducing agent of ascorbic acid (0.4 mL, 0.08M). Then the solution was kept rested for 3 min to assure all Au3+ ions were reduced to Au+. Finally, NaOH (60 μL, 0.05M) was quickly injected and the solution was rapidly stirred for 10 s. The solution gradually changed from colorless to pink, purple, and ultimately 6 ACS Paragon Plus Environment

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dark blue color. The final Au TNPs solution with the concentration of 1 mM was rested for 1 h before usage. Fabrication of Au TNPs/rGO/TiO2 b-NRs Heterostructure. The fabrication process of the ternary heterojunction sample is schematically shown in Figure 1. Briefly, after the two-step hydrothermal process, shorter branched NRs protruded out from the surface of trunk structures, and 3D TiO2 b-NRs with large surface area were then obtained. TiO2 b-NRs sample coated with GO solution was then annealed in reducing N2 gas to form conductive rGO sheets. It is anticipated that the flexible rGO sheets could intercalate into the spaces between TiO2 b-NRs and bridge them to convey charge carriers more efficiently. Finally, Au TNPs were attached to the rGO/TiO2 b-NRs photoelectrode using 1 M of 3-mercaptopropionic acid (3-MPA) as the linker with its thiol group attached to Au and carboxylic acid group attached to TiO2.46,58 It is worth noting that in order to fabricate a Schottky junction with intimate contact between metal and semiconductor, a post heat treatment could be performed to remove the linker, MPA, before the evaluation of photoresponse.46 Such ternary heterojunction architectures could both enhance the visible light response and reduce the charge recombination rate, leading to higher PEC water splitting efficiency.46 Both without and with this post heat treatment were adopted in our study for comparison.



Figure 1. Schematic drawing depicting the fabrication of ternary heterostructure Au TNPs/rGO/TiO2 b-NRs on FTO glass substrate. Characterization. The crystallographic patterns were acquired by X-ray diffraction (XRD, Rigaku SmartLab) with Cu Kα as the radiation source. The morphologies were characterized using field-emission scanning electron microscope (FESEM, Hitachi, SU 8230). Transmission electron microscope (TEM) and high resolution transmission electron microscope (HRTEM) images of the Au TNPs/rGO/TiO2 b-NRs heterostructure were taken with a JEOL JEM 2100F microscope. To examine the degree of reduction of GO sheets, Raman spectra were collected by the confocal Raman imaging system (WITec alpha 300) with a 532 nm laser as the excitation source. The laser power out of the objective lens was controlled at ~10 mW to avoid the damage of rGO sheets. The PEC response was measured using a three-electrode system with a homemade glass reactor coupled with a quartz window. In this system, we used Ag/AgCl (saturated in KCl) as the reference electrode (RE), Pt wire as the counter electrode (CE), and the 8 ACS Paragon Plus Environment

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fabricated sample as the working electrode (WE). Measurements were performed in 1 M KOH (pH = 13.7) electrolyte. A potentiostat (VersaSTAT 4 Potentiostat Galvanostat) was utilized to simultaneously control the voltage between WE and RE and collect the current between WE and CE. A solar simulator (XES-40S1, San-Ei Electric) with Air Mass value of 1.5 G (100 mW/cm2) was employed as the light source shining through the quartz window onto the front side of the sample. Before measurements, the electrolyte was deaerated by Ar purging for 20 min to remove the dissolved oxygen gas. Electrochemical impedance spectroscopy (EIS) was performed using the same system. To evaluate the wavelength-dependent incident photon-to-electron conversion efficiency (IPCE), tunable light source (Bunkou-Keiki, NIJI-2) with wavelength ranging from 340 to 800 nm was used.

RESULTS AND DISCUSSION Micrographs and Element Mapping. The SEM images of TiO2 NRs and TiO2 b-NRs are shown in Figure 2. From the top view (Figure 2(a)), it could be seen that TiO2 NRs grew with a well-defined rectangular cross section and each NR was actually composed of a bundle of small nanowires during the first hydrothermal growth. The NRs with smooth side-surfaces grew in the direction normal to the FTO surface (inset of Figure 2(a)). The width and length of the NRs were 78.16 ± 14.26 nm and 3.10 ± 0.10 μm, respectively. Following the second hydrothermal synthesis, shorter TiO2 branches with length of 131.87 ± 20.77 nm were formed on the surface of TiO2 NRs grown in the first hydrothermal step (Figure 2(b)). Compared to the bottom parts of the TiO2 NRs, the upper parts of the NRs show more densely distributed b-NRs (inset of Figure 2(b) and Figure S1 for a higher magnification view). The formation of these protrusions is attributed to a uniform TiCl4 seed layer coated on the surface of TiO2 NRs leading to crystallization of the branched nanostructures during the second hydrothermal and annealing 9 ACS Paragon Plus Environment


processes.

Figure 2. SEM images of (a) top view and side view (inset) of TiO2 NRs after the first hydrothermal synthesis, and (b) top view and side view (inset) of TiO2 b-NRs after the second hydrothermal fabrication. (c) SEM image of Au TNPs/rGO/TiO2 b-NRs heterostructure, (d) HRTEM image of Au TNP/TiO2 b-NR and (e) HAADF-STEM image and EDS elemental mapping of Au TNP/rGO/TiO2 b-NR, showing the distributions of Ti (green), Au (red), and C (blue). While TiO2 NRs were pure rutile after the first hydrothermal synthesis (see the XRD patterns in Figure S2), anatase phase showed up as the shorter branched NRs protruded out from


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the surface of rutile trunk after the second hydrothermal process which is revealed at 2 = 25.28° for (101) plane (Figure S2). It is known that anatase TiO2 exhibits better photocatalytic performance than rutile one owing to the electronic band structure of the former favorably retarding the charge carrier recombination rate.59 The presence of branched anatase phase was also confirmed using Raman spectroscopy, as described in Supporting Information and Figure S3. The branched nanostructures could increase the surface area, and it was expected to provide more reactive sites for the reactions, leading to the improved photoresponse. To resolve the issue of fast charge recombination (~10–9 s) of semiconductor photocatalysts,60 rGO has been used to accelerate the transportation of charge carriers and consequently to enhance the photocurrent.6,20,48 We adopted the spin-coating method to apply GO solution onto the surfaces of TiO2 b-NRs followed by N2 annealing to successfully transform GO into rGO sheets. The detailed analysis is provided in Supporting Information and Figure S3. After deposition of rGO sheets, Au TNPs were attached onto the samples. Figure 2(c) presents the SEM image of the ternary heterojunction architecture (Au TNPs/rGO/TiO2 b-NRs). It could be seen that rGO sheets cover some regions and fold along the surface of b-NRs with the distinct winkle structures (see the left and upper right regions of the image). The b-NRs underneath rGO become slightly blurred. Well-defined Au TNPs with the average size of 43.25 ± 8.38 nm (Figure S4) were anchored to b-NRs through the MPA monolayer with thiol group on one end attaching to Au and carboxylic acid group on the other end connecting to Ti.46 Au TNPs can also bind to rGO sheets via the organic functional groups of rGO. It is worth noting that the X-ray diffraction peaks of Au TNPs could not be detected, and it was due to the low concentration and overlapping with TiO2 rutile peak at 2 ~ 44°. Based on TEM observation, we found that both Au and TiO2 presented well-resolved features in the Au TNP/TiO2 b-NR sample, and the HRTEM image shown in Figure 2(d) reveals a clear interface between Au and TiO2. The 11 ACS Paragon Plus Environment


observed lattice spacing of 0.234 nm corresponds to (111) crystal plane of Au fcc structure, and those of 0.238 nm and 0.345 nm are, respectively, (004) and (101) planes of TiO2 anatase phase. Figure 2(e-1) shows the high-angle annular dark-field scanning TEM (HAADF-STEM) of Au TNP/rGO/TiO2 b-NR along with the energy dispersive X-ray spectroscopic (EDS) elemental mapping (Figure 2(e-2)). It clearly indicates the presence of Ti (Figure 2(e-3), from TiO2), Au (Figure 2(e-4), from Au TNP), and C (Figure 2(e-5), from rGO), which confirms that Au TNPs could be strongly bound to TiO2 b-NRs. Photocurrent Under Simulated Solar Light. To explore the effects of the addition of rGO sheets and Au TNPs to TiO2 b-NRs framework, the photocurrent generated as a function of applied potential; i.e., linear sweep voltammetry (LSV), upon the illumination of simulated solar light was measured and the results for TiO2 NRs, TiO2 b-NRs, rGO/TiO2 b-NRs, Au TNPs/TiO2 b-NRs, and Au TNPs/rGO/TiO2 b-NRs are shown in Figure 3(a). To evaluate the effects of rGO coverage on PEC performance, three concentrations (0.1, 0.3, 0.5 mg/mL) of rGO were tested, and it was found that rGO with 0.3 mg/mL showed the highest photocurrent (Figure S5). Lower concentration (0.1 mg/mL) could result in sparse distribution of rGO sheets with no connected network formed for the efficient transportation of the generated charge carriers. On the other hand, higher concentration (0.5 mg/mL) could increase the chance for rGO sheets to stack into multilayered structure, which could diminish the conductivity.54 Moreover, thick graphitic sheets could hinder the underneath TiO2 b-NRs from light absorption and decrease the photocurrent.6 Therefore, the rGO decoration with concentration of 0.3 mg/mL was adopted for the following measurements. Also, the Au TNPs/TiO2 b-NRs and Au TNPs/rGO/TiO2 b-NRs samples were subjected to heat treatment at 160 °C for 1 h to remove the molecular linker and to form a close Schottky junction between Au and TiO2 before the photoactivity measurements. It was found that the photocurrent could be enhanced with this heat treatment (see Figure S6). As the potential 12 ACS Paragon Plus Environment

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(vs. Ag/AgCl) slowly (2.5 mV/s) swept from −1.00 to +0.75 V, the photocurrents gradually increased and became relatively stable beyond the potential of −0.25 V. Dark current was also collected with the incident light blocked by a shutter, and it revealed a negligible background signal. Upon the simulated solar light illumination, it could be clearly seen that the photoresponse of TiO2 b-NRs was greatly enhanced compared to that of TiO2 NRs. This was attributed to the distinctive branched feature with larger surface areas that greatly increased the reaction sites between the TiO2 photoanode and electrolyte. On the other hand, despite it has been reported that the photogenerated electrons should be transferred from rutile to anatase,61-63 the band alignments between these two phases in vacuum condition are different from that in aqueous medium, in which TiO2 is immersed for the redox reaction, and the direction of charge transfer between anatase and rutile is still in debate. However, based on the experimental and theoretical results in the literature,64-65 we believed that the branched anatase TiO2 with higher reduction potential of photogenerated electrons than rutile phase is capable of suppressing charge recombination more efficiently at the interface region between anatase and rutile.59,66 The increment of surface area could be verified by the comparison of electrochemically active surface area (ECSA), which was estimated from the double-layer capacitance (Cdl) of the electrode, and the results are shown in Figure S7 with the detailed calculation shown in the Supporting Information.67-68 Generally, a higher ECSA value implies a larger surface area. The calculated results clearly indicated that the branched nanorods provided more active sites leading to higher photocurrent density than TiO2 NRs (Figure S7).



Figure 3. (a) LSV response of bare TiO2 NRs, TiO2 b-NRs, rGO/TiO2 b-NRs, Au TNPs/TiO2 bNRs, and Au TNPs/rGO/TiO2 b-NRs recorded in 1 M aqueous KOH electrolyte in the dark and under simulated solar light illumination. (b) Photocurrent density collected with three chopped light on-off cycles at 0.5 V (vs. Ag/AgCl) irradiated with simulated solar light for the samples described in (a).

With the addition of rGO sheets, the photocurrent density showed an enhancement of ~21% than that of TiO2 b-NRs at 0.50 V (vs. Ag/AgCl). The graphene platform could play an important role in transferring the charge carriers more efficiently to prolong their lifetime.19,54 It was conjectured that the oxygen vacancy and the corresponding Ti3+ could form during annealing in reducing atmosphere. To investigate the change of chemical state, X-ray photoelectron spectroscopy (XPS) was performed and the results are shown in Figure S8. In the presence of 14 ACS Paragon Plus Environment

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Ti3+, a red shift of Ti 2p signals toward the lower binding energy should appear.69 However, a clear blue shift for both Ti 2p3/2 and Ti 2p1/2 peaks was observed in our rGO/TiO2 sample in comparison with bare TiO2, indicating the change of chemical state and/or coordination environment of Ti4+ possibly due to strong electronic interactions leading to the efficient charge transfer from TiO2 to rGO.54,70 With the addition of Au TNPs, the photocurrent density was improved further by ~33%. This is attributed to the LSPR produced at the sharp tips of the nanotriangles, which transfers the concentrated energy to the adjacent TiO2 b-NRs through nonradiative decay with the hot electron generation and injection to the conduction band of semiconductor. It has been demonstrated that the hot electrons transferred to the conduction band of TiO2 from Au nanoparticles possessing lifetimes that are two-order of magnitude longer than those of excited electrons generated within TiO2 under UV excitation.71 It requires substantially high energy for these transferred hot electrons to traverse back to Au nanoparticles through the depletion layer and Schottky barrier (~1.0 eV for Au/TiO2 interface).72-73 Therefore, the electronhole pairs recombination in Au TNPs could be suppressed, and the holes left in Au TNPs could participate in the water oxidation reaction. Finally, the Au TNPs/rGO/TiO2 b-NRs sample showed the highest photocurrent density of 0.76 mA/cm2 at 0.5 V (vs. Ag/AgCl), which was ~90% enhancement compared to bare TiO2 b-NRs sample at the same potential. In this case, once the photogenerated hot electrons from Au TNPs are injected into the conduction band of TiO2, some of them could be further transferred to rGO sheets, and accordingly the electron-hole recombination rate would be largely suppressed. The photocurrent response with three consecutive cycles of on-and-off illumination at 0.5 V (vs. Ag/AgCl) was also performed (Figure 3(b)). It could be seen that the photocurrent values were quite stable during the measurements, and the recorded photocurrent densities were basically consistent with that obtained from the LSV measurements. All the samples responded to the illumination instantaneously without time 15 ACS Paragon Plus Environment


delay implying the electrodes were highly conductive and photoresponsive. The intrinsic PEC activity attributed to the active surface area is worth further studying, where the effect of geometrical surface area could be deconvoluted from the driving forces for the charge carrier formation by taking account of ECSA values of each sample.67-68 The comparison of the photocurrent normalized by the geometrical area and the ECSA at 0.5 V (vs. Ag/AgCl) revealed that the photocurrent densities contributed from ECSA were higher than that from the geometrical counterpart (see Figure S9). Also, the improvement trend of photocurrent density for each photoanode was similar, evidencing the important role of rGO and Au TNPs in enhancing the PEC performance. In addition, the theoretical photoconversion efficiency ()35,44,67 of each photoanode based on the current−voltage curve as a function of applied voltage was calculated and is shown in Figure S10 with the detailed calculation shown in Supporting Information. The Au TNPs/rGO/TiO2 b-NRs electrode exhibited a maximum photoconversion efficiency of 0.33% at 0.6 V (vs. RHE), which was remarkably higher than that of Au TNPs/TiO2 b-NRs (0.26%), rGO/TiO2 b-NRs (0.22%), TiO2 b-NRs (0.13%) and TiO2 NRs (0.05%) at the same potential. The optimal  of Au TNPs/rGO/TiO2 b-NRs yielded 250% enhancement compared to that of TiO2 b-NRs, manifesting the superior performance of the ternary heterojunction architecture in water oxidation reaction. Photocurrent Under Visible Light and IPCE. The effects of rGO sheets and Au TNPs on the PEC performances in the visible light region were further investigated using a filter (420 nm <  < 800 nm) fitted to the solar simulator. It can be seen in Figure 4(a) and (b) that the photocurrents of TiO2 NRs and TiO2 b-NRs are nearly negligible. This poor photoresponse is due to the large bandgap of rutile and anatase structures that the charge carriers could not be photogenerated by the visible light. On the other hand, rGO/TiO2 b-NRs shows a slight


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improvement compared to TiO2 b-NRs which is attributed to the presence of Ti-O-C chemical bond leading to bandgap narrowing of TiO2.48 However, Au TNPs with LSPR effects in the visible region greatly enhanced the photoresponse, with the photocurrent densities of ~90 and ~140 μA/cm2 at 0.5 V (vs. Ag/AgCl), respectively, for Au TNPs/TiO2 b-NRs and Au TNPs/rGO/TiO2 b-NRs.

Figure 4. (a) LSV response of bare TiO2 NRs, TiO2 b-NRs, rGO/TiO2 b-NRs, Au TNPs/TiO2 bNRs, and Au TNPs/rGO/TiO2 b-NRs under illumination of visible light (420 nm <  < 800 nm). (b) Photocurrent density collected with three chopped light on-off cycles at 0.5 V (vs. Ag/AgCl). (c) IPCE measured at 0.5 V (vs. Ag/AgCl) with tunable light source from 340 to 800 nm. (d) Enlarged IPCE in the wavelength range between 440 and 800 nm, corresponding to the dashed box in (c), and the extinction spectrum of as-prepared Au TNPs solution.



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To reveal the effects of rGO and Au TNPs on the photoactivity behavior of TiO2 under the monochromatic light illumination, incident photon-to-electron conversion efficiency (IPCE) was compared. IPCE has been commonly adopted to characterize the optoelectronic efficiency of different photoelectrodes, and the value could be calculated based on the following equation: IPCE = (1240I) / (Jlight)

(1)

where I refers to the photocurrent density measured at a given wavelength () of the incident light, and Jlight is the measured irradiance at . From Figure 4(c), the IPCE value was greatly enhanced in the UV region as the rGO sheets and Au TNPs were added compared to bare TiO2 b-NRs. This enhancement is attributed to the synergistic effects of the following two mechanisms. First, the formation of Ti-O-C bond between TiO2 and graphene found in other TiO2/rGO systems might also occur in our system.48 This unique chemical bonding not only causes a red-shift in the absorption edge of TiO2 with band gap narrowing (see UV-Vis reflectance spectra in Figure S11), but also increases the light absorption intensity. Secondly, Au TNPs decoration could improve the photoconversion efficiency of TiO2 in the UV region by facilitating the charge separation in TiO2 coupled with the bandgap transition of TiO2 itself. Specifically, the Schottky junction between Au and TiO2 could effectively passivate the surface states of TiO2 and act as electron traps,41 reducing the charge recombination and thereby, the photoresponse and IPCE value in the UV region could be enhanced. These two mechanisms function synergistically to enhance the IPCE in the UV region. In the visible region (Figure 4(d)), in contrast to bare TiO2 b-NRs with negligible photoresponse, the photocurrent of rGO/TiO2 b-NRs shows slight enhancement for the wavelength up to ~660 nm, which is attributed to the extension of absorption edge of TiO2 by Ti-O-C bond as mentioned above.48 Upon Au TNPs incorporation, the photoresponse exhibits further enhancement with its spectral features matching well with the extinction spectrum of the Au TNPs solution in the wavelength 18 ACS Paragon Plus Environment

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range from 450 to 800 nm. Since the scattering and near-field electromagnetic enhancement are important when the spectrum of the illumination source, LSPR of metal nanoparticles and semiconductor absorbance are overlapped, it was suggested that the IPCE enhancements in the visible region were predominantly attributed to the non-radiative decay of LSPR followed by hot electrons injection from Au TNPs to TiO2.74 The sample with the ternary structure has the most prominent response, which is attributed to concomitant contributions of (1) rGO with absorption extension and high conductivity and (2) Au TNPs with LSPR effects increasing the sensitivity of photoelectrode in the visible region. The LSPR band at around 660 nm and a broader band at around 560 nm correspond, respectively, to the in-plane dipole and out-of-plane dipole resonant modes associated with Au TNPs.57 It is worth noting that despite the IPCE intensity of rGO/TiO2 b-NRs seemed to be comparable to that of Au TNPs/TiO2 b-NRs in the visible region, the photocurrent of the latter obtained from LSV and I-t measurements under visible light illumination was remarkably higher than that of the former (see Figure 4(a) and (b)). We compared the IPCE enhancements for each sample by dividing the IPCE values with that of bare TiO2 b-NRs, as shown in Figure S12. It could be seen that the enhancement factor of Au TNPs/TiO2 b-NRs was much higher than that of rGO/TiO2 b-NRs, especially in the region corresponding to the position of Au TNPs LSPR, which was believed to be responsible for the improved photocurrent in the visible region. Additionally, the hot electrons injected to the conduction band of TiO2 from Au TNPs could prolong their lifetime and lead to the further enhancements of photocurrent.71 The PEC and IPCE performances could also be evaluated from the flat-band potential values determined from the Mott-Schottky plots. As shown in Figure S13, the flat band potentials of TiO2 b-NRs, rGO/TiO2 b-NRs, and Au TNPs/rGO/TiO2 b-NRs deduced from the Mott-Schottky plots were, respectively, −0.895, −0.910, and −0.926 V vs. Ag/AgCl. Compared 19 ACS Paragon Plus Environment


with bare TiO2, the negative shift of flat band potentials as the rGO and Au TNPs were added implied that the Fermi level was progressively moved toward the conduction band, and thus reducing the photocurrent onset potential (see Figure S14). The decrease in the photocurrent onset potential suggested that the maximum photocurrent could be obtained at a lower applied bias and led to the improved PEC and IPCE performances.54 The roles of rGO and Au TNPs in facilitating the charge carriers separation and transportation could also be understood from the photoluminescence (PL) behaviors, as shown in Figure S15. All of the samples displayed strong PL emission peaks at ~420 and ~467 nm, which were resulted from the electron-hole pairs migration and the charge transfer transition of oxygen vacancy trapped electrons in TiO2, respectively.75 More importantly, the PL emission intensity gradually decreased as the rGO and Au TNPs were incorporated into TiO2 b-NRs, and reached the lowest intensity in Au TNPs/rGO/TiO2 b-NRs. The reduction of PL emission intensity signifies the suppression of the charge carriers recombination in TiO2 which, in turn, improves the electron-hole pairs separation and prolongs their lifetime.76 Simulations. The interaction between the unpolarized incoming light and the ternary heterostructure at the above two resonant wavelengths was simulated by finite-difference timedomain (FDTD) method for Au TNP/rGO/TiO2 system to study the electric field enhancements, and the results are shown in Figure 5. For the out-of-plane dipole mode (at  = 560 nm), the enhanced electric fields were distributed at the edges of the Au TNP. On the other hand, the electric fields at the tips of the Au TNP and at the interface between TiO2 and Au TNP were greatly enhanced for the in-plane dipole mode (at  = 660 nm). This suggests that the LSPRinduced near-field nanofocusing effect might provide a higher probability for the energy transfer from the plasmonic nanoparticle to semiconductor. Most importantly, the LSPR effect promotes the hot electron generation and injection to the conduction band of TiO2 boosting the redox 20 ACS Paragon Plus Environment

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reaction, which accounts for the better performance of Au TNPs-decorated photoelectrode over rGO/TiO2 b-NRs and pristine TiO2 b-NRs in the visible region. Based on the IPCE analysis stated above, it is suggested that these two distinct sensitizers (rGO and Au TNPs) concomitantly contribute to the enhanced photoconversion efficiency of TiO2 b-NRs extending from UV to visible region, which corroborates the superior photocurrent enhancement observed under simulated solar light irradiation (Figure 3).

Figure 5. Electric filed intensity distribution simulated by FDTD method for Au TNP/rGO/TiO2 upon irradiation with an unpolarized light source with a wavelength of (a) 560 and (b) 660 nm. The rGO and TiO2 are underneath the Au TNP. The propagation direction of incident light is perpendicular to the top surface of Au TNP.

Stability of Photoelectrodes. To assess the long-term stability of photoelectrodes, photocurrents at 0.5 V (vs. Ag/AgCl) under illumination of the simulated solar light and visible light (420 nm <  < 800 nm) were acquired for 1 h, and the results are shown in Figure 6(a) and (b), respectively. In both cases, the incident light was turned on after 1 min of dark current measurement to confirm again the photosensitivity of the samples. The results clearly indicated that the photocurrents for all the samples were quite stable up to 1 h, evidencing the high efficiency and high quality of our photoelectrodes without severe degradation after long time 21 ACS Paragon Plus Environment


PEC reactions.

Figure 6. Photoresponses of bare TiO2 NRs, TiO2 b-NRs, rGO/TiO2 b-NRs, Au TNPs/TiO2 bNRs, and Au TNPs/rGO/TiO2 b-NRs under illumination of (a) simulated solar light and (b) visible light for 1 h at 0.5 V (vs. Ag/AgCl).

Charge Transfer Dynamics. The charge carrier dynamics at the interface between the photoanode and the electrolyte could be characterized via the electrochemical impedance spectroscopy (EIS).19,42,46 In Figure 7, Nyquist plot and Bode plot are presented with the variation of the impedance, |𝑍|, as a function of frequency, where 𝑍 = 𝑍′ +𝑖𝑍′′. The diameter of the semicircle in Nyquist plot could be an indicator of the resistance of charge transfer from photoanode to electrolyte. From Figure 7(a), it is clear that rGO decoration exhibits a noticeable increase of charge transfer efficiency that results in a smaller arc than that of bare TiO2 b-NRs. Furthermore, Au TNPs/rGO/TiO2 b-NRs shows the smallest semicircle compared with the other two configurations, implying that the charge transfer is drastically accelerated by both Au TNPs and rGO. The representative equivalent circuit for a simple redox reaction was used to fit the data in Nyquist plot and is shown in the inset of Figure 7(a), where Rs is the internal resistance including the intrinsic components, the connections and the electrolyte, Rct is the interfacial charge transfer resistance between the photoanode and the electrolyte, and Cdl is the double layer 22 ACS Paragon Plus Environment

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capacitance on the surface of photoanode. The fitted Rs values were 308.89, 56.48, and 10.05 ohm, Rct were 12419.0, 8634.8, and 2596.3 ohm, Cdl were 5.89 × 10–6, 2.46 × 10–5, and 2.01 × 10–4 F, respectively, for TiO2 b-NRs, rGO/TiO2 b-NRs, and Au TNPs/rGO/TiO2 b-NRs. In addition, Bode plot could give another indication of the charge transfer resistance of different photoelectrodes by evaluating the slope and the intercept of the plot. Similar to the results in Nyquist plot, the AC impedance of Au TNPs/rGO/TiO2 b-NRs is lower than those of rGO/TiO2 b-NRs and bare TiO2 b-NRs (Figure 7(b)). Based on the EIS measurements, we reiterated that the photocatalytic performance of TiO2 could be remarkably enhanced by rGO sheets and Au TNPs.

Figure 7. (a) Nyquist plot and (b) Bode plot with the frequency-dependence of the impedance of 23 ACS Paragon Plus Environment


TiO2 b-NRs, rGO/TiO2 b-NRs, and Au TNPs/rGO/TiO2 b-NRs under AM 1.5 G solar illumination. Mechanism. Based on the above results and analyses, we proposed a mechanism to illustrate the process of electron transfer in the Au TNPs/rGO/TiO2 b-NRs system upon light irradiation (UV or visible light). Under UV light illumination (process 1 in Figure 8), electronhole pairs are immediately photogenerated in TiO2, and the electrons in the conduction band could be either transferred directly to the underneath FTO substrate or accepted by the rGO. Under visible light irradiation (process 2 in Figure 8), if the incident light coincides with the LSPR resonances of the plasmonic metal nanoparticles (Au TNPs), the electrons near the Fermi level (Ef) are excited to the higher energy states by gaining the energy from the plasmon resonances and become the so-called “hot electrons” via non-radiative Landau damping.71 These hot electrons with sufficient energy could overcome the Schottky barrier formed at the interface between Au and TiO2, and are then injected into the conduction band of TiO2. It has been well documented in ultrafast spectroscopic studies that the hot electron injection occurs within 240 fs.77 The Schottky barrier promotes efficient electron transfer by surmounting extremely fast electron−electron scattering relaxation (< 100 fs) in photoexcited Au nanoparticles in the presence of sufficiently strong electronic coupling between the hot electron and electronaccepting levels (i.e., conduction band of TiO2).78-79 Besides, there is also a probability for a portion of hot electrons to be accepted and shuttled via the rGO. Either in the UV or visible light, rGO with superior conductivity could play a role to accept and transfer the electrons generated from TiO2 or Au to the respective electrode,54,67 efficiently hindering the charge recombination and thereby enhancing the photocurrent. The electrons transferred to the Pt counter electrode via external circuit could reduce water into hydrogen gas, while the photogenerated holes (from both TiO2 and Au) could participate in the water oxidation process. 24 ACS Paragon Plus Environment

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Figure 8. Schematic of charge transfer mechanism for the Au TNPs/rGO/TiO2 system under the UV light and visible light illumination. Finally, merits of our ternary architecture with higher photocatalytic efficiency compared with bare TiO2 NRs could be accordingly concluded: (1) The large surface area of TiO2 b-NR arrays creates more reaction sites between the photoanode and the electrolyte, and the anatase branches exhibit higher photocatalytic activity. (2) rGO sheets with extraordinary charge transport property mediate and shuttle the electrons (either coming from metal Au or semiconductor TiO2) and mitigate the electron-hole recombination rate. (3) rGO sheets increase the light absorption efficiency of TiO2 in the UV region and extend the absorption edge to the visible region. (4) Optical nanofocusing effect of Au TNPs contributes to injecting the light energy to TiO2 and rGO. (5) The LSPR-induced energetic hot electrons transferred from Au TNPs to TiO2 raises the redox reaction rate.

CONCLUSIONS In summary, 3D highly branched TiO2 NR arrays synthesized by the two-step hydrothermal method were decorated with prominent electron acceptor rGO and distinct plasmonic Au TNPs. 25 ACS Paragon Plus Environment


The PEC response of rGO/TiO2 b-NRs displayed a ~21% enhancement compared with bare TiO2 b-NRs, and Au TNPs/rGO/TiO2 b-NRs showed an even higher enhancement of ~90% under AM 1.5 G illumination. IPCE analysis demonstrated that both Au TNPs and rGO contributed to the enhanced photocurrent of TiO2 in the entire UV-Vis range. The LSPRs of the Au NTPs not only increased the visible response of TiO2, but also created hot electrons to facilitate the photocurrent generation. The rGO sheets with exceptional conductivity was believed to accept and transfer the photogenerated electrons from TiO2 and Au, and thereby effectively prolonged the lifetime of charge carriers. The designed ternary heterostructure Au TNPs/rGO/TiO2 b-NRs and the results in this work suggested that this system could be a promising candidate for PEC water-splitting application.

ASSOCIATED CONTENT Supporting Information List of abbreviations; materials used for the fabrication of photoelectrode; SEM cross-sectional image of TiO2 branched nanorods; XRD spectra of TiO2 NRs, TiO2 b-NRs and Au TNPs/rGO/TiO2 b-NRs; Raman spectra evaluation of transformation of GO into rGO; TEM image of synthesized Au TNPs; LSV responses of rGO/TiO2 b-NRs with different rGO concentrations; transient photoresponse of Au TNPs/TiO2 b-NRs with and without post heat treatment; calculation and comparison of ECSA; cyclic voltammetry curves and charging current differences against scan rate for TiO2 NRs and TiO2 b-NRs; XPS spectra; normalized photocurrent density; calculated photoconversion efficiency as a function of applied potential; UV-Vis reflectance spectra; IPCE enhancement as a function of the wavelength; Mott-Schottky plots; LSV data obtained under simulated solar light illumination showing the onset potential of


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each sample; photoluminescence emission spectra for the photoelectrodes. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (T. Nagao), *E-mail: [email protected] (C. H. Hsueh)

ORCID Chun-Hway Hsueh: 0000-0002-6477-7148 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This work was jointly supported by the Ministry of Science and Technology, Taiwan under Contract no. MOST 107-2221-E-002-013, JSPS Kakenhi project (Grant Nr. 16H 06364) and JST CREST (JPMJCR 13C3). The experiments were partly performed in the MANA Foundry, National Institute for Materials Science, Japan and is gratefully acknowledged.



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xMoxO4

core/shell heterojunction array for efficient photoelectrochemical water splitting.

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TOC Graphic


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Figure 1, Schematic drawing depicting the fabrication of ternary heterostructure Au TNPs/rGO/TiO2 b-NRs on FTO glass substrate.

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Figure 2. SEM images of (a) top view and side view (inset) of TiO2 NRs after the first hydrothermal synthesis, and (b) top view and side view (inset) of TiO2 b-NRs after the second hydrothermal fabrication. (c) SEM image of Au TNPs/rGO/TiO2 b-NRs heterostructure, (d) HRTEM image of Au TNP/TiO2 b-NR and (e) HAADF-STEM image and EDS elemental mapping of Au TNP/rGO/TiO2 b-NR, showing the distributions of Ti (green), Au (red), and C (blue).


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Figure 3. (a) LSV response of bare TiO2 NRs, TiO2 b-NRs, rGO/TiO2 b-NRs, Au TNPs/TiO2 b-NRs, and Au TNPs/rGO/TiO2 b-NRs recorded in 1 M aqueous KOH electrolyte in the dark and under simulated solar light illumination. (b) Photocurrent density collected with three chopped light on-off cycles at 0.5 V (vs. Ag/AgCl) irradiated with simulated solar light for the samples described in (a).



Figure 4. (a) LSV response of bare TiO2 NRs, TiO2 b-NRs, rGO/TiO2 b-NRs, Au TNPs/TiO2 b-NRs, and Au TNPs/rGO/TiO2 b-NRs under illumination of visible light (420 nm < λ < 800 nm). (b) Photocurrent density collected with three chopped light on-off cycles at 0.5 V (vs. Ag/AgCl). (c) IPCE measured at 0.5 V (vs Ag/AgCl) with tunable light source from 340 to 800 nm. (d) Enlarged IPCE in the wavelength range between 440 and 800 nm, corresponding to the dashed box in (c), and the extinction spectrum of as-prepared Au TNPs solution.


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Figure 5. Electric filed intensity distribution simulated by FDTD method for Au TNP/rGO/TiO2 upon irradiation with an unpolarized light source with a wavelength of (a) 560 and (b) 660 nm. The rGO and TiO2 are underneath the Au TNP. The propagation direction of incident light is perpendicular to the top surface of Au TNP.



Photoresponses of bare TiO2 NRs, TiO2 b-NRs, rGO/TiO2 b-NRs, Au TNPs/TiO2 b-NRs, and Au TNPs/rGO/TiO2 b-NRs under illumination of (a) simulated solar light and (b) visible light for 1 h at 0.5 V (vs. Ag/AgCl).


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Figure 7. (a) Nyquist plot and (b) Bode plot with the frequency-dependence of the impedance of TiO2 b-NRs, rGO/TiO2 b-NRs, and Au TNPs/rGO/TiO2 b-NRs under AM 1.5 G solar illumination.



Figure 8. Schematic of charge transfer mechanism for the Au TNPs/rGO/TiO2 system under the UV light and visible light illumination.


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Graphic abstract 84x38mm (300 x 300 DPI)


Photocurrent Enhancements of TiO2-Based Nanocomposites with

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