Surface plasmon-driven photoelectrochemical water splitting of TiO2 nanowires decorated with Ag nanoparticles under visible light illumination

Applied Surface Science 420 (2017) 286–295

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Surface plasmon-driven photoelectrochemical water splitting of TiO2 nanowires decorated with Ag nanoparticles under visible light illumination Chuchu Peng, Wenzhong Wang ∗ , Weiwei Zhang, Yujie Liang, La Zhuo School of Science, Minzu University of China, Beijing, 100081, PR China

a r t i c l e

i n f o

Article history: Received 17 February 2017 Received in revised form 7 May 2017 Accepted 12 May 2017 Available online 19 May 2017 Keywords: Surface plasmon resonance Photoelectrochemical water splitting TiO2 nanowires Ag nanoparticle Visible light photocatalyst

a b s t r a c t Here, we demonstrate that TiO2 nanowires (NWs) can be significantly driven by visible light through the decoration with Ag nanoparticles (NPs) (Ag-decorated TiO2 NWs). The Ag-decorated TiO2 NWs show remarkably photoelectrochemical (PEC) water splitting performance under illumination with ␭ > 420 visible light due to surface plasmon resonance (SPR) of Ag NPs. In this work, low power of the used light source (100 mW/cm2 ) was not capable of heating the Ag-decorated TiO2 nanowire photoanode enough to directly split water. In addition, under irradiation with ␭ > 420 nm visible light, no photocurrent was produced by TiO2 nanowire photoanode indicates that electron transitions between valence band and conduction band do not take place in prepared anatase TiO2 NWs. Meanwhile, the SPR energy (2.95–2.13 eV < 3.2 eV) is insufficient to excite TiO2 NWs to generate electro-hole pairs through SPR-enhanced electromagnetic fields. Thus the remarkably visible-light-responsive PEC water splitting activity of Ag-decorated TiO2 NWs is not attributed to local heating caused by SPR-mediated photothermal process, large enhancement of electromagnetic fields induced by SPR and scattering of resonant photons. We propose that the visible light PEC water splitting performance of Ag-decorated TiO2 NWs is attributed to electron transfer from Ag NPs to the conduction band of TiO2 NWs mediated by SPR. In addition, a Schottky barrier established at the interface of Ag NPs and TiO2 NWs prevents these transferred electrons from returning to the Ag NPs and significantly retarded the recombination of electron-hole pairs in the Ag NPs, also contributing to visible-light-driven PEC water splitting performance. So the remarkably visible-light-driven PEC water splitting performance of Ag-decorated TiO2 NWs is attributed to the synergistic effects of electron transfer mediated by SPR and the Schottky barrier between Ag NPs and TiO2 NWs. The achieved Ag-decorated TiO2 NWs can be added to these previously prepared TiO2 photocatalysts mainly driven by SPR of Au NPs for the development of new visible light photocatalysts. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Solar energy will present a promising alternative to meet the steadily growing energy demands in the future. It has been demonstrated that the photocatalytic splitting of water with semiconductor oxides is a hopeful option for the conversion of solar energy to chemical energy or to electricity in an economical way [1–6]. Among the semiconductor oxides, wide band gap TiO2 has been considerably studied for photocatalytic water splitting because of its eminent photocatalytic property, high chemical stability, nontoxicity, and relatively low cost [7–13]. Nevertheless, the main issue of using TiO2 as photoanode for photocatalytic water

∗ Corresponding author. E-mail address: [email protected] (W. Wang). http://dx.doi.org/10.1016/j.apsusc.2017.05.101 0169-4332/© 2017 Elsevier B.V. All rights reserved.

splitting is that TiO2 is not capable of absorbing visible light due to its wide band gap of ∼3.2 eV, hence there are very few solar photons (∼5%) that can be applied to drive water splitting, leading to a very low water splitting efficiency even irradiation under ultraviolent light [13]. Various attempts have therefore been made to prepare visible light TiO2 photocatalyst by extending its optical absorption to the visible light, such as doping metal and nonmetal ions [14–16], creating defects [17], and constructing heterostructures [18–20]. Recently, a very efficient strategy has been developed to construct the visible-light-responsive TiO2 -based catalysts by coupling TiO2 with plasmonic metal NPs, such as noble metal Au, Ag, and Cu NPs. The visible-light-responsive photocatalytic performance of TiO2 coupled with plasmonic metal NPs is attributed to visible light absorption of plasmonic metal nanoparticle via localized surface plasmon resonance (LSPR).

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It has been reported that surface plasmons arise from the collective oscillation of surface free electrons of conducting metals, when conducting metals interact strongly with light [21–24]. The plasmonic metal NPs absorb visible light through LSPR and then generate hot electrons. In photocatalysts of semiconductors coupled with plasmonic metal NPs, the previous studies have demonstrated that the plasmon-generated hot electrons can be transferred from the plasmonic metal NPs to the conduction band of semiconductor [25,26], making the semiconductor-based photocatalysts can be driven by visible light. Due to their intriguing plasmonic properties that can strongly interact with visible light, plasmonic metal NPs have therefore been coupled with wide band gap TiO2 , ZnO and Fe2 O3 to fabricate visible-lightresponsive photoelectrochemical (PEC) photocatalysts for water splitting [27–32]. For instance, the photocatalyst of plasmonic Au nanocrystals coupled with TiO2 nanotubes for PEC water splitting under visible light illumination has been reported recently [33]. A high photocurrent density of ∼150 ␮A/cm2 was achieved in fabricated photocatalyst [33]. Recently studies indicated that Au nanostructure-decorated TiO2 NWs exhibited the improved PEC water splitting activity within entire UV-visible region [34]. Simulation study indicated that the enhanced PEC water splitting performance of Au nanoparticle-decorated TiO2 NWs was basically attributed to the electric-field amplification effect as well as hot electron excitation of Au NPs generated by SPR [34]. It was reported that the Ag/TiO2 nanocomposites prepared by photoreduction method exhibited visible-light PEC waster splitting activity [35]. The TiO2 nanotube arrays coupled with Ag NPs exhibited the enhanced visible-light-driven [36] and solar-driven PEC water splitting activities [37]. However, the synthesis and PEC watersplitting performance of plasmonic Ag nanoparticle coupled with TiO2 NWs is limited. In this work, we fabricated the visible-light-responsive TiO2 photocatalysts by coupled TiO2 NWs with Ag NPs (Ag-decorated TiO2 NWs). The Ag-decorated TiO2 NWs are prepared by combining a facile hydrothermal method with a wet chemical deposition route. The as-fabricated Ag-decorated TiO2 NWs photocatalysts exhibit remarkable PEC water splitting performance under illumination with ␭ > 420 nm visible light. The studies demonstrate that the remarkable PEC water splitting performance is attributed to the synergistic effects of electron move from Ag NPs to TiO2 mediated by SPR, and a Schottky barrier established at the interface of Ag NPs and TiO2 NWs which prevents these transferred electrons from returning to the Ag NPs and significantly inhibits the recombination of electrons and holes in the Ag NPs. The photocatalysts reported in this work by using SPR of Ag NPs to drive photocatalytic activity of wide band gap TiO2 semiconductor under visible light can be added to these previously prepared TiO2 visible-lightresponsive photocatalysts mainly driven by SPR of Au NPs for the development of new visible light photocatalysts.

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Second, the hydrothermally heating treated Ti foil with gray color was put into HCl solution for 1 h, followed by heating treatment for 2 h at 650 ◦ C in a tube furnace to form TiO2 NWs with gray and white color. Finally, 20 mg of the obtained TiO2 NWs was ultrasonically dispersed into 10 mL deionized water, followed by adding 5 mL absolute ethanol, 2 mL NaOH solution with 1 M concentration and 2 mL AgNO3 solution with 0.014 M concentration. Then mixed solution was maintained at 50–60 ◦ C for 8 h to deposit Ag NPs on the surfaces of TiO2 NWs. The loading content of Ag NPs is 15 wt% evaluated by the amount of AgNO3 solution. The prepared sample is denoted as Ag/TiO2 NWs-15. By controlling the amount of AgNO3 solution with 0.014 M concentration, the samples with 7.5 wt% and 20 wt% Ag NPs were prepared, which were denoted as Ag/TiO2 NWs-7.5 and Ag/TiO2 NWs-20, respectively. 2.2. Fabrication of the photoanode of the Ag-decorated TiO2 NWs The photoanode film of the Ag-decorated TiO2 NWs was prepared by a drop coating method. Briefly, 1.0 mg of the obtained Ag-decorated TiO2 NWs was ultrasonically suspended in 1.0 mL water to prepare the dispersion sample solution with 1.0 mg mL−1 concentration, followed by uniformly coating 0.5 mL of the obtained dispersion sample solution on a 2.0 × 2.0 cm2 FTO glass electrode. Finally, the FTO glass electrode was dried at 60–70 ◦ C for 6 h in a vacuum furnace. 2.3. Characterization measurements The composition and phase purity of obtained nanostructures was studied by X-ray diffraction (XRD) (Rigaku (Japan) Dmax ␥A). The morphology and size of prepared nanostructures was investigated by scanning electron microscope (SEM) (Hitachi S-4800). Transmission electron microscope (TEM) and high resolution TEM (HRTEM, JEOL-2100) were used to investigate the size and microstructure features of the as-prepared nanostructures. A Lambda 950 spectrometer (Perkin-Elmer, USA) with an integrating sphere was applied to collect UV–vis absorption spectra of the prepared nanostructures. The composition and surface electronic state of elements were studied by X-ray photoelectron spectroscopy (XPS). The peak value of C 1S at 284.8 eV was used as standard binding value to calibrate binding energies of elements in the XPS spectra. A JW-BK122F instrument was applied to measure isotherms of nitrogen adsorption and desorption of prepared TiO2 NWs and Ag-decorated TiO2 . NWs . The obtained isotherms were employed to evaluate the specific surfaces of the samples using the Brunauer-Emmet-Teller (BET) method. The Barret–Joyner–Halenda (BJH) method was used to estimate the pore size distributions of the samples. 2.4. PEC water-splitting performance evaluation

2. Experimental 2.1. Synthesis of Ag-decorated TiO2 NWs The TiO2 NWs decorated with Ag NPs were fabricated by the procedures as described in the follows: First, TiO2 NWs were synthesized by a facile hydrothermal method. Briefly, a sheet of Ti foil (3.5 cm × 3.5 cm) was cleaned by ultrasonic process in a cleaning solution of ultrapure water, 2propanol and acetone (1:1:1 in volume ratio) for 30 min. Then the washed Ti foil was put against the wall of the Teflon-sealed autoclave with 50 mL volume containing 40 mL NaOH solution (1 M), sealed, and kept at 220 ◦ C for 48 h in a furnace, and cooled to room temperature.

The PEC activities of the as-fabricated TiO2 nanowire and Ag-decorated TiO2 nanowire photoanodes were evaluated in a three-electrode configuration that was placed into a quartz cell. The as-prepared photoanode film of Ag-decorated TiO2 NWs was used as working electrode. During the experiments, the fabricated working electrode was put into a quartz cell, a Pt wire was used as counter electrode and Ag/AgCl electrode was used as reference electrode, respectively. Afterwards, 100 mL Na2 SO4 buffer solution with 0.1 M concentration and pH 7 was put into cell, followed by degassing for 30 min with N2 . A 300-W Xenon lamp with wavelength of 200–2500 nm was applied as the light source during the PEC performance measurements. The chronoamperometric J–t curves of the as-fabricated TiO2 nanowire and Ag-decorated TiO2 nanowire photoanodes were recorded in Na2 SO4 electrolyte with

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Fig. 1. (A) Low- and (B) high-magnification SEM images of the as-prepared TiO2 NWs. (C) Low- and (D) high-magnification SEM images of the as-fabricated TiO2 NWs decorated with Ag NPs.

0.1 M concentration under visible light illumination (␭ > 420 nm, AM 1.5 G, 100 mW/cm2 ). The linear sweep voltammograms (VLS) of the photoanodes were collected in applied potentials ranging from 0 to −0.5 V vs Ag/AgCl at 5 mV/s under illumination with ␭ > 420 nm visible light (AM 1.5 G, 100 mW/cm2 ). 3. Results and discussion 3.1. Characterization of structure and morphology The SEM images of obtained TiO2 NWs and Ag/TiO2 NWs-15 were presented in Fig. 1. Fig. 1A shows the low-magnification SEM images of achieved TiO2 NWs, demonstrating that a large quantity of TiO2 NWs were achieved by the present method. The as-prepared NWs have uniform diameter of ca. 80 nm and the length of several micrometers. High-magnification SEM images of the obtained TiO2 NWs were shown in Fig. 1B, showing that the surfaces of TiO2 NWs are smooth. After decorating with 15 wt% loading content of Ag NPs, SEM images clearly demonstrate that large Ag NPs are grown on the surfaces of TiO2 NWs as presented in Fig. 1C and D, indicating that TiO2 NWs were successfully decorated with Ag NPs. The size and macrostructure features of the fabricated TiO2 NWs decorated with Ag NPs were further studied by TEM and HRTEM. Fig. 2A shows TEM images of the achieved TiO2 NWs decorated with Ag NPs (Ag/TiO2 NWs-15), clearly demonstrating that the surfaces of TiO2 NWs are loaded with large quantities of Ag NPs with size in the range of 5–15 nm evaluated from higher magnification TEM images as shown in Fig. 2B. Microstructure features of the fabricated TiO2 NWs decorated with Ag NPs were further investigated by HRTEM. Fig. 2C shows a TEM image of a single TiO2

nanowire decorated with Ag NPs, which is applied to study its microstructure features. Fig. 2D shows the corresponding HRTEM image of the nanowire, clearly demonstrating lattice images of TiO2 nanowire and Ag nanoparticle, respectively. The interplanar spacing of 0.33 nm is consistent with the (101) plane of anatase TiO2 (JCPDS no. 21-1272). The lattice fringe with lattice spacing of 0.23 nm corresponds to the (111) plane of face-centered cubic (fcc) Ag (JCPDS no. 04-0783). In addition, The TEM and HRTEM images clearly demonstrate that the Ag NPs are tightly grown on the surface of TiO2 NWs, indicating that the well-defined heterojunctions between TiO2 NWs and Ag NPs are fabricated. 3.2. Evaluation of composition and crystalline phase Energy dispersive X-ray spectrometer (EDS) has been further applied to confirm the appearance of Ti, O and Ag elements in the as-achieved TiO2 NWs decorated with Ag NPs. Fig. 3 shows the EDS element mapping images of the TiO2 nanowire coated with Ag NPs (Ag/TiO2 NWs-15). The mapping image of an individual Ag-decorated TiO2 nanowire (Fig. 3A) clearly demonstrates the coexistence and uniform dispersion of Ti (red) and O (green) and Ag (blue) elements across the nanowire (Fig. 3B–D), showing that the surface of TiO2 nanowire is uniformly decorated with Ag NPs. The composition and crystalline phase of the as-prepared pure TiO2 NWs and Ag-decorated TiO2 NWs has been further analyzed by XRD. XRD diffraction patterns of the as-obtained nanostructures are presented in Fig. 4. Fig. 4A demonstrates that all diffraction peaks of pure TiO2 NWs can be well-assigned to the anatase phase TiO2 (JCPDS no. 21-1272). After loading Ag NPs with different contents, the XRD pattern of nanostructures also displays main diffraction

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Fig. 2. (A) Low- and (B) high-magnification TEM images of the as-prepared Ag/TiO2 NWs-15. (C) TEM and (D) corresponding HRTEM image of an individual Ag-decorated TiO2 nanowire obtained for samples with 15 wt% loading content of Ag NPs.

peaks from the anatase phase TiO2 as presented in Fig. 4 B–D. When the loading content of Ag NPs is 7.5 wt% (samples denoted as Ag/TiO2 -7.5), XRD patterns show that no obvious diffraction peaks of Ag NPs are detected as shown in Fig. 4B, indicating that the content of Ag NPs is too low compared to TiO2 NWs. Nevertheless, when the loading content of Ag NPs is increased to 15 wt%, two weak diffraction peaks at 44.5◦ and 64.5◦ are observed as presented in Fig. 4C (Ag/TiO2 -15). These two diffraction peaks are attributed to (200) and (220) crystal planes of the crystalline fcc Ag (04-0783). In addition, there is a slight increase for the intensities of diffraction peaks from Ag NPs with further increase loading content of Ag NPs to 20 wt% (Fig. 4D). XPS was applied to further study the composition and elemental electronic state of the fabricated nanostructures. Fig. 5 shows the XPS spectra of Ag/TiO2 NWs-15 that is selected as representative sample to study the composition and elemental electronic state. The survey XPS spectrum of Ag/TiO2 NWs-15 evidently demonstrates coexistence of Ti, Ag and O elements with sharp peaks as presented in Fig. 5A. The high-resolution XPS spectrum of Ti2p binding energies exhibits two peaks at 464.2 and 458.5 eV with a 5.7 eV gap, demonstrating that the peaks can be assigned to Ti2p3/2 and Ti2p1/2 bands of lattice Ti of TiO2 crystallite [38]. The determination of valence state of Ag element in the prepared nanostructures is very important to confirm the formation of metallic Ag NPs in the nanostructures. Fig. 5C shows the high-resolution XPS spectrum of Ag3d binding energies. Two peaks at 373.9 and 367.9 eV are ascribed to the Ag3d3/2 and Ag3d5/2 bands of metallic Ag crystallite, respectively. The 6.0 eV gap between two band states also verifies the formation of metallic Ag crystallite in nanostructures [38]. Thus the results of XPS, XRD, TEM and EDS confirm that the metallic Ag NPs are formed in the Ag/TiO2 NWs. The XPS spectrum of O1 s is shown in Fig. 5D, exhibiting a weak peak (peak I) and a strong peak (peak II). The binging energy of peak II is 529.8 eV,

being attributed to Ti-O vibration mode in TiO2 crystallite, while the weak peak (peak I) at 532.0 eV is ascribed to a hydroxy (OH) absorbed on the TiO2 crystallite surface [38]. 3.3. The specific surface area and porous distribution characterization Fig. 6A and B shows the N2 adsorption/desorption isotherms of the prepared TiO2 NWs and Ag-decorated TiO2 NWs, respectively. From the measured isotherms, the specific surface areas evaluated by BET method of TiO2 NWs and Ag-decorated TiO2 NWs are 62 m2 g−1 and 70 m2 g−1 , respectively, indicating a slight increase for surface area after loading Ag NPs on TiO2 NWs. The insets in Fig. 6A and B show the pore size distribution of TiO2 NWs and Agdecorated TiO2 NWs, showing a wide pore size distribution with an average pore size of 12 nm and 8 nm, respectively. The results show that the loading of Ag NPs on the surface of TiO2 NWs decreases the pore size of NWs due to the coating of NPs. 3.4. Studies of optical properties The optical properties of the as-achieved TiO2 NWs and Agloaded TiO2 NWs (Ag/TiO2 -15) were studied by UV–vis absorption spectroscopy. As presented in Fig. 7A, the pure TiO2 NWs display a characteristic absorption peak located at about 380 nm. After loading Ag NPs, the NWs show another two absorption peaks besides the characteristic absorption edge at about 380 nm of anatase TiO2 . A weak absorption peak located at 420 nm is ascribed to SPR of discrete Ag NPs [39]. In addition, TiO2 NWs decorated with Ag NPs exhibit an obvious absorption in the visible absorption spectrum with a peak of about 580 nm. The plasmon resonance of dimers formed by two Ag NPs is responsible for this light absorption, which has been demonstrated by previous studies of low-dimensional

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Fig. 3. (A) SEM image and (B–D) high-resolution element mappings of an individual Ag-decorated TiO2 nanowire obtained for samples with 15 wt% loading content of Ag NPs. (For interpretation of the references to colour in the text, the reader is referred to the web version of this article.)

[40,41], when these Ag nanoparticle dimers are irradiated with light, the nanoparticles can be coupled with each other due to the local plasmon between two NPs. The isotropical surface plasmon coupling in uniaxial direction of metal nanoparticle dimer leads to a local field improvement along the axis of dimers, resulting in the produce of low-energy longitudinal band. Thus the plasmon resonance of dimers of Ag NPs is mainly responsible for the visible light absorption at ca. 580 nm for nanostructure of TiO2 NWs decorated with Ag NPs. Moreover, the widened light absorption peak is mainly contributed to the various morphologies and a wide size distribution of Ag NPs [42]. 3.5. PEC activity

Fig. 4. XRD patterns of (A) TiO2 NWs, (B) Ag/TiO2 NWs-7.5, (C) Ag/TiO2 NWs-7.5, (D) Ag/TiO2 NWs-20.

plasmon coupling of plasmonic metal NPs [40,41]. In order to confirm the existence of dimers, TEM has been further applied to exam the morphology of Ag NPs located on the surfaces of TiO2 NWs. As demonstrated in TEM images of Fig. 7B, there are many dimers fabricated by two Ag NPs as marked with white ellipses on the surface of TiO2 nanowire. As reported in the previously published articles

In order to investigate visible-light-driven PEC water splitting activities of achieved TiO2 NWs and Ag-decorated TiO2 NWs (Ag/TiO2 -15), the photoresponses of the TiO2 nanowire and Ag-decorated nanowire photoanodes for photocurrent (J–t) with chopped light illumination were measured in a three-electrode configuration. It has been demonstrated that the photocurrent going through an external circuit is generally considered as a credible representative for evaluating H2 generation for PEC water splitting [43–45]. Fig. 8 shows the chronoamperometric J–t curves of the as-fabricated TiO2 nanowire and Ag-decorated TiO2 nanowire photoanodes fabricated from Ag/TiO2 -15 sample, which were recorded in Na2 SO4 electrolyte with 0.1 M concentration under visible light illumination with wavelength of above 420 nm. As shown in J–t curves of Fig. 8A, the as-fabricated Ag-decorated

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Fig. 5. XPS spectra of Ag-decorated TiO2 NWs (Ag/TiO2 NWs-15). (A) the survey XPS spectrum, (B) Ti2p, (C) Ag3d and (D) O1s.

TiO2 nanowire photoanode produces the photocurrent density of about 7.5 ␮A cm−2 under visible irradiation without applied voltage to Ag/AgCl electrode. However, the TiO2 nanowire photoanode does not show any photoresponse to produce photocurrent at the same condition. At −0.2 V vs Ag/AgCl electrode, the fabricated Agdecorated TiO2 nanowire photoanode produces the photocurrent density of about 18 ␮A cm−2 in visible irradiation, while the TiO2 nanowire photoanode also does not show any photoresponse to produce photocurrent (Fig. 8B). The photocurrent obtained from the Ag-decorated TiO2 nanowire photoanode clearly demonstrates that the as-fabricated TiO2 NWs decorated with Ag NPs exhibit remarkable PEC water splitting performance under irradiation with visible light. To further evaluate the remarkable PEC performance of Agdecorated TiO2 NWs (Ag/TiO2 -15) under illumination with visible light, the liner sweep voltammograms (LSV) of the photoanodes were collected with 5 mV/s in Na2 SO4 buffer solution with 0.1 M concentration and at pH 7 in applied potentials of 0.0 to −0.5 V vs Ag/AgCl in dark and under irradiation (␭ > 420 nm, AG 1.5 G, 100 mW/cm2 ). Fig. 9 shows the LSV curve of Ag-decorated TiO2 nanowire photoanodes, clearly demonstrating that the asfabricated Ag-decorated TiO2 nanowire photoelectrode displays fast light response and yields a 18 ␮A cm−2 photocurrent density at −0.2 V vs Ag/AgCl, while the TiO2 nanowire photoanode also does not show any photoresponse to produce photocurrent. Hence the remarkable photocurrent density achieved in the linear sweep LSV further confirms that the as-fabricated Ag-decorated TiO2 NWs exhibit the remarkable PEC water splitting performance under visible light illumination. The effects of the loading contents of Ag NPs on the PEC water splitting activities for Ag/TiO2 NWs have been investigated. Fig. 10 shows the J–t curves of TiO2 NWs loaded with different contents of

Ag NPs. When the loading content of Ag NPs is 7.5 wt%, the photoanode fabricated from Ag-decorated TiO2 NWs (Ag/TiO2 NWs-7.5) produces about 2.5 ␮A cm−2 under illumination with ␭ > 420 nm at 0 V vs Ag/AgCl electrode. When the loading content of Ag NPs is increased to 15 wt%, the Ag-decorated TiO2 NWs (Ag/TiO2 NWs15) exhibit a significant increase for photocurrent density (about 7.5 ␮A cm−2 ) under irradiation with ␭ > 420 nm at 0 V vs Ag/AgCl electrode. However, when Ag nanoparticle loading contents are further increased to 20 wt%, the heterogeneous nanostructures (Ag/TiO2 NWs-20) exhibit a decrease photocurrent density (about 6.5 ␮A cm−2 ) compared to nanostructures of Ag/TiO2 NWs-15. The results indicate that overloading of Ag NPs on TiO2 NWs will decrease the PEC water splitting performance, possibly resulting from the decrease of light absorption caused by overloading Ag NPs.

3.6. Charge transfer and separation in the Ag-decorated TiO2 NWs In order to clarify the remarkable PEC water splitting activity of Ag-decorated TiO2 NWs under irradiation with visible light, a crucial issue is to understand the generation, transfer and separation of the photogenerated electron-hole pairs in photoanode system. It has been demonstrated that a Schottky barrier (denoted as ␾b ) can be established at the interface between metallic Ag and semiconductor TiO2 when Ag NPs contact with TiO2 NWs [46]. When the Ag-decorated TiO2 NWs are irradiated with ␭ > 420 nm, the strong SPR of the metallic Ag NPs is excited, thus enhanced electromagnetic fields around the Ag nanoparticle surface are generated, which remarkably raises the generation of “hot electrons” from metallic Ag NPs at the interface between Ag and TiO2 . Because the potential energy (denoted as ESPR ) of the “hot electrons” is higher than ␾b at the interface, a large potential energy difference (denoted as ESPR

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Fig. 7. (A) UV–vis absorption spectra of TiO2 NWs and Ag-decorated TiO2 NWs. (B) SEM images of the Ag/TiO2 NWs-15, showing many dimers constructed by aggregating of two Ag NPs. Fig. 6. Isotherms of N2 adsorption/desorption and pore size distribution curves of (A) TiO2 NWs and (B) Ag-decorated TiO2 NWs.

− ␾b ) between ESPR and ␾b is formed, which will drive the “hot electrons” to fast and efficiently transfer to the conduction band of TiO2 [47]. In addition, the Schottky barrier (␾b ) inhibits these transferred “hot electrons” from returning to the Ag NPs, leading to an accumulation of the transferred electrons in the conduction band of TiO2 . Since the TiO2 NWs cannot be excited to generate electrons and holes under irradiation with ␭ > 420 nm light as demonstrated in Figs. 8 and 9, there are no holes in TiO2 valence band (VB) under visible illumination. Thus the “hot electrons” accumulated in the CB of TiO2 should have longer lifetimes to take part in the PEC water splitting to generate photocurrent. Fig. 11 shows the schematic diagram of the generation and transportation of the photogenerated “hot electrons” in Ag-decorated TiO2 nanowire photocatalyst. 3.7. Mechanism for visible-light-driven PEC water splitting It has been demonstrated that SPR of plasmonic metal NPs is capable of affecting the photocatalytic performance of photocatalysts via the following four possible mechanisms: (1) local heating caused by SPR-mediated photothermal process [48], (2) large electromagnetic field enhancement induced by SPR [20,31,37,48], (3) scattering of resonant photons [21], and (4) transfer of electron of plasmonic metal NPs to semiconductor driven by SPR [49–52]. However, in the present study, the very low power of the used light source (100 mW/cm2 ) was not capable of heating the Agdecorated TiO2 nanowire photoanode enough to directly split water as reported previously [49,50,53], this was further verified by

the slight temperature raise of solution ranging from 23 to 25 ◦ C obtained within the 15 min illumination period. In addition, under irradiation of visible light with ␭ > 420 nm, no photocurrent was produced by TiO2 nanowire photoanode (Fig. 8 and 9), indicating that electron transitions between valence band and conduction band do not take place in the as-prepared anatase TiO2 NWs. Furthermore, the above obtained UV–vis absorption spectra (Fig. 7A) clearly demonstrate that the SPR wavelength of Ag NPs is located between 420 and 580 nm (2.95–2.13 eV <3.2 eV), and thus the SPR energy is insufficient to excite TiO2 NWs to generate electro-hole pairs through SPR-enhanced electromagnetic fields, therefore both large enhanced electromagnetic fields induced by SPR and scattering of resonant photons are not possible mechanism for the production of electron-hole pairs within the as-prepared TiO2 NWs. Thus observed photocurrent production from Ag-decorated TiO2 nanowire photoanode under ␭ > 420 nm illumination can mainly be ascribed to the transfer of electron of the Ag NPs to the TiO2 NWs. 4. Conclusions Plasmonic Ag NPs have been successfully applied to decorate TiO2 NWs for designing visible-light-driven photocatalyst. The as-designed Ag-decorated TiO2 nanowire photocatalyst shows remarkable PEC water splitting performance under illumination with ␭ > 420 nm visible light because of SPR of Ag NPs. Our studies clearly demonstrate that the remarkable PEC water splitting performance of the fabricated Ag-decorated TiO2 nanowire photocatalyst is attributed to synergistic effects of SPR-mediated electron

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Fig. 10. J–t curves of photoelectrodes fabricated from Ag-decorated TiO2 NWs with loading contents of 7.5 wt%, 15 wt% and 20 wt% Ag NPs under illumination with light illumination (␭ > 420 nm), recorded at 5 mV/s and pH = 7 in 0.1 M Na2 SO4 buffer solution at 0 V vs Ag/AgCl electrode.

Fig. 8. J–t curves of TiO2 nanowire and Ag-decorated TiO2 nanowire photoanodes prepared form nanostructures of Ag/TiO2 NWs-15 under visible light illumination (␭ > 420 nm) with 60 s light on/off cycles, recorded at 5 mV/s and pH = 7 in 0.1 M Na2 SO4 buffer solution at (A) 0 V and (B) −0.2 V vs Ag/AgCl.

transfer and the Schottky barrier between Ag NPs and TiO2 NWs, which can be confirmed by our experimental results. The very low power of the applied light source (100 mW/cm2 ) in the experiments

was not capable of driving PEC water splitting of the Ag-decorated TiO2 nanowire catalyst by SPR-mediated local photothermal heating. In addition, no direct electron transition between valence band and conduction band within the TiO2 NWs with ␭ > 420 nm illumination is observed, and the SPR energy of Ag NPs (420–580 nm, 2.95∼2.18 eV) is less than direct interband transition energy of TiO2 (3.2 eV), indicating that the PEC water splitting activity driven by visible light for Ag-decorated TiO2 NWs is not attributed to SPR-enhanced electromagnetic fields and scattering of resonant photons. This work has provided a facile and efficient strategy for designing visible-light-responsive wide band gap TiO2 photocatalyst by decorating plasmonic Ag NPs. The achieved Ag-decorated TiO2 photocatalyst can be added to these previously prepared TiO2 catalysts generally driven by SPR of Au NPs, for the enrichment of the available visible-light-responsive TiO2 photocatalysts and the development of new TiO2 -based visible light photocatalysts.

Fig. 9. LSV curves of the as-prepared TiO2 nanowire and Ag-decorated TiO2 nanowire photoanodes prepared form Ag/TiO2 NWs-15 samples under visible light irradiation (␭ > 420 nm), recorded at 5 mV s−1 and pH = 7 in 0.1 M Na2 SO4 buffer solution.

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Fig. 11. Scheme diagram of generation and transportation of “hot electrons” in Agdecorated TiO2 nanowire system.

Acknowledgments This work was supported by the National Natural Science Foundation of China under Grant Nos 11074312, 11374377, 11474174, 61575225 and 11404414, and the Undergraduate Research Training Program of Minzu University of China under Grant Nos. GCCX2016110009 and GCCX2016110010. References [1] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37–38. [2] M.G. Walter, E.L. Warren, J.R. McKone, S.W. Boettcher, X.Q. Mi, E.A. Santori, N.S. Lewis, Solar water splitting cells, Chem. Rev. 110 (2010) 6446–6473. [3] X.B. Chen, S.H. Shen, L.J. Guo, S.S. Mao, Semiconductor-based photocatalytic hydrogen generation, Chem. Rev. 110 (2010) 6503–6570. [4] S.C. Warren, K. Voitchovsky, H. Dotan, C.M. Leroy, M. Cornuz, F. Stellacci, C. Hebert, A. Rothschild, M. Gratzel, Identifying champion nanostructures for solar water-splitting, Nat. Mater. 12 (2013) 842–849. [5] A.A. Ismail, D.W. Bahnemann, Photochemical splitting of water for hydrogen production by photocatalysis: a review, Sol. Energy Mat. Sol. C 128 (2014) 85–101. [6] A. Kargar, K. Sun, Y. Jing, C. Choi, H. Jeong, G.Y. Jung, S. Jin, D.L. Wang, 3D branched nanowire photoelectrochemical electrodes for efficient solar water splitting, ACS Nano 7 (2013) 9407–9415. [7] G. Wang, H. Wang, Y. Ling, Y. Tang, X. Yang, R.C. Fitzmorris, C. Wang, J.Z. Zhang, Y. Li, Hydrogen-treated TiO2 nanowire arrays for photoelectrochemical water splitting, Nano Lett. 11 (2011) 3026–3033. [8] I.S. Cho, Z. Chen, A.J. Forman, D.R. Kim, P.M. Rao, T.F. Jaramillo, X. Zheng, Branched TiO2 nanorods for photoelectrochemical hydrogen production, Nano Lett. 11 (2011) 4978–4984. [9] Z.R. He, J.Z. Xiao, F. Xia, K. Kajiyoshi, C. Samart, H.B. Zhang, Enhanced solar water-splitting performance of TiO2 nanotube arrays by annealing and quenching, Appl. Surf. Sci. 313 (2014) 633–639. [10] S.Y. Noh, K. Sun, C. Choi, M. Niu, M. Yang, K. Xu, S. Jin, D. Wang, Branched TiO2 /Si nanostructures for enhanced photoelectrochemical water splitting, Nano Energy 2 (2013) 351–360. [11] Y. Lin, G. Yuan, R. Liu, S. Zhou, S.W. Sheehan, D. Wang, Semiconductor nanostructure-based photoelectrochemical water splitting: a brief review, Chem. Phys. Lett. 507 (2011) 209–215. [12] S. Girish Kumar, K.S.R. Koteswara Rao, Comparison of modification strategies towards enhanced charge carrier separation and photocatalytic degradation activity of metal oxide semiconductors (TiO2 , WO3 and ZnO), Appl. Surf. Sci. 391 (2017) 124–148. [13] Y.J. Hwang, C. Hahn, B. Liu, P. Yang, Photoelectrochemical properties of TiO2 aanowire arrays: A study of the dependence on length and atomic layer deposition coating, ACS Nano 6 (2012) 5060–5069. [14] B. Mei, H. Byford, M. Bledowski, L.D. Wang, J. Strunk, M. Muhler, R. Beranek, Beneficial effect of Nb doping on the photoelectrochemical properties of TiO2 and TiO2 -polyheptazine hybrids, Sol. Energy Mater. Sol. C 117 (2013) 48–53. [15] J.H. Park, S. Kim, A.J. Bard, Novel carbon-doped TiO2 nanotube arrays with high aspect ratios for efficient solar water splitting, Nano Lett. 1 (2006) 24–28. [16] M. Xu, P. Da, H. Wu, D. Zhao, G. Zheng, Controlled Sn-doping in TiO2 nanowire photoanodes with enhanced photoelectrochemical conversion, Nano Lett. 12 (2012) 1503–1508. [17] J. Zhang, Y. Wu, M. Xing, S.A.K. Leghai, S. Sajjad, Development of modified N doped TiO2 photocatalyst with metals, nonmetals and metal oxides, Energy Environ. Sci. 3 (2010) 715–726.

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