Article pubs.acs.org/JACS
Positioning the Water Oxidation Reaction Sites in Plasmonic Photocatalysts Shengyang Wang,†,‡,∥ Yuying Gao,†,‡,∥ Shu Miao,†,‡ Taifeng Liu,†,‡ Linchao Mu,†,‡,∥ Rengui Li,†,‡ Fengtao Fan,†,‡,§ and Can Li*,†,‡,§ †
State Key Laboratory of Catalysis, ‡Dalian National Laboratory for Clean Energy, and §The Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Zhongshan Road 457, Dalian 116023, China ∥ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *
ABSTRACT: Plasmonic photocatalysis, stemming from the effective light absorbance and confinement of surface plasmons, provides a pathway to enhance solar energy conversion. Although the plasmonic hot electrons in water reduction have been extensively studied, exactly how the plasmonic hot holes participate in the water splitting reaction has not yet been well understood. In particular, where the plasmonic hot holes participate in water oxidation is still illusive. Herein, taking Au/TiO2 as a plasmonic photocatalyst prototype, we investigated the plasmonic hot holes involved in water oxidation. The reaction sites are positioned by photodeposition together with element mapping by electron microscopy, while the distribution of holes is probed by surface photovoltage imaging with Kelvin probe force microscopy. We demonstrated that the plasmonic holes are mainly concentrated near the gold−semiconductor interface, which is further identified as the reaction site for plasmonic water oxidation. Density functional theory also corroborates these findings by revealing the promotion role of interfacial structure (Ti−O−Au) for oxygen evolution. Furthermore, the interfacial effect on plasmonic water oxidation is validated by other Au−semiconductor photocatalytic systems (Au/SrTiO3, Au/BaTiO3, etc.).
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INTRODUCTION
the role of hot holes has been mostly overlooked, which hinders a comprehensive understanding of the PICS process. Very recently, attention has been growing on how to harness plasmon-induced hot holes for organic transformations,25,26 water oxidation,27−29 and even overall water splitting.30−32 Unfortunately, the detailed mechanism of plasmon-induced photooxidation reactions triggered by hot holes has not been clarified yet, including the localization and oxidizing ability of hot holes.33−35 Recently, several strategies have been proposed to solve these problems. For instance, the oxidation ability of plasmon-generated hot holes has been evaluated through coordination dissolution of metals,36 hydroxylation of plasmonic metals,37 or site-selective plasmonic etching of silver nanocubes.38 Another example is photoinduced polymerization in a photoelectrochemical system to detect the distribution of the plasmon-induced electric field.39,40 Nevertheless, visualizing the distribution of hot holes and positioning the reaction sites of plasmonic hole-induced oxidation reaction still remain very challenging due to the lack of effective probe reactions and detection methods with high sensitivity and spatial resolution. Of particular interest is the water oxidation reaction, which is
Plasmonic nanostructures characterized by strong light absorption through excitations of collective free electron oscillations, known as surface plasmon resonance (SPR), have recently shown great potential for solar energy conversion.1−4 In a typical SPR-enhanced process, the plasmon-induced hot charge separation (PICS) mechanism is widely studied due to its possibility for extending absorption spectra of photocatalysts by controlling the size and shape of metal particles.5,6 In PICS, the plasmon resonance is damped nonradiatively into hot electron−hole pairs within the metal via Landau damping on a time scale ranging from 1 to 100 fs.7,8 Such a short lifetime of hot charges severely restricts the application of plasmonic photocatalysis. An efficient approach for promoting hot carrier separation is to form a Schottky contact between the plasmon metal nanoparticles and an appropriate semiconductor support (e.g., TiO2).9−11 Currently, it is manifested that the hot electrons in the metal can be injected to the adjacent semiconductors in a conventional way or an interfacial way confirmed through transient spectroscopy analysis.12,13 Thereafter, the transferred hot electrons have been demonstrated to be available for various reactions, such as H2 dissociation,14,15 O2 activation,16,17 and water reduction.18−24 On the contrary, © 2017 American Chemical Society
Received: May 1, 2017 Published: August 4, 2017 11771
DOI: 10.1021/jacs.7b04470 J. Am. Chem. Soc. 2017, 139, 11771−11778
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Journal of the American Chemical Society
deposition and Pb(NO3)2 for PbO2 deposition) were mixed in an aqueous solution (100 mL) with electron donors or acceptors, respectively (methanol was used as an electron donor for Cr2O3 deposition and NaS2O8 was used as an electron acceptor for PbO2 deposition). After air was exhausted, the suspension was irradiated by a 300 W Xe lamp with different cutoff filters (UV or visible light) under continuous stirring. After 5 h of photodeposition, the suspension was filtered, washed with deionized water more than three times, and finally dried at 333 K overnight. A portion of photocatalysts after photodeposition was annealed in muffle furnaces at 473 K for 1 h under air. The as-obtained powder was used for characterizations and/ or activity tests. The location of deposited species was detected by high angle annular dark field (HAADF) and energy-dispersive X-ray spectrometer (EDS) images. The HAADF images were taken on a probe-corrected JEM-ARM200F electron microscope. Compositional maps were acquired with EDS fitted on the microscope. Both HAADF and EDS data were collected at an operation voltage of 200 kV. KPFM Measurement. In order to conduct the KPFM experiment, samples must be in high conductivity, so the TiO2 samples were doped with 0.05% Nb as in most surface science research. The doped-rutile TiO2 (100) single-crystal substrate (Shinkosha) was first cleaned with acetone (30 min) and ethanol (30 min) by ultrasound. Then the wafers were immersed in 10% HF solution for 5 min, thoroughly rinsed with deionized water, and dried in a nitrogen stream. Finally, the rutile crystal was annealed at 1173 K for 1 h under atmospheric conditions to obtain a flat surface. Au nanoparticles prepared by citrate reduction were deposited on the rutile (100) crystal by spin coating using a Au colloid solution, followed by drying at 353 K under vacuum for 2 h. The final Au/TiO2 sample was treated by 5 min of oxygen plasma etching and annealed at 673 K for 2 h to form an excellent interface between Au and TiO2. The KPFM and AFM images were taken with a Bruker Dimension V SPM system using Pt/Ir-coated tips (resonant frequency 72 kHz). It is worth noting that the KPFM measurement was carried out at ambient conditions with a moisture content of 50%. In such a case, a thin layer of water was formed at the surface of sample, and we believe that it should be representative of the real conditions of the photocatalytic reaction. Images were acquired at scan rates of 0.5 Hz, and the tip lift height was 60 nm at tapping mode for potential mapping. The contact potential difference (CPD) is defined as the difference between the work function of the tip and the sample. The CPD of each sample was measured with the same tip in the dark and with light illumination. The surface photovoltage (SPV) image is the CPD change after visible light irradiation, which can be calculated as SPV = ΔCPD = CPDlight − CPDdark, where CPDdark and CPDlight are the CPD measured in the dark and in light. A 532 nm laser (5 mW/ cm2) was used to excite the Au surface plasmon resonance. Theoretical Calculations. All the spin-polarized calculations have been performed using the Vienna ab Initio Simulation Package (VASP) code. The Perdew−Burke−Ernzerh (PBE) parametrization of the generalized gradient approximation (GGA) was adopted for the exchange and correlation potential. The projector-augmented wave method was applied to describing electron−ion interactions. The electron wave function was expanded in plane waves up to a cutoff energy of 400 eV. During geometry optimization the residual forces on the atoms are allowed to converge to a value smaller than 0.01 eV/Å, and the self-consistent iteration continues until the tolerance for total energy reaches 10−4 eV. In order to simulate a gold cluster on the TiO2 surface, we chose the Au8 cluster as a model because of its stability. For the TiO2 surface, rutile (110) and anatase (101) were selected. The rutile (110) surface slab includes 6 Ti layers and 18 O layers, with 2 bottom Ti layers and 6 bottom O layers being fixed at bulk-truncated positions. The anatase (101) surface slab contains 12 Ti layers and 24 O layers, with 4 bottom Ti layers and 8 O layers fixed. The Γ-centered k-point mesh of the Brillouin zone sampling for the rutile (110) and anatase (101) were set at (3 × 2 × 1) and (3 × 3 × 1) based on the Monkhorst− Pack scheme. All the electronic property calculations have been carried out using the HSE hybrid density functional. For oxygen evolution reaction (OER) calculation, we simplify the Au nanoparticle (NP) as
considered as the bottleneck for both natural and artificial photosynthesis, offering a great challenge to uncover the underlying mechanism of the plasmon-induced water splitting reaction. As a result, studying plasmonic hole-related issues by adopting the water oxidation reaction as a probe reaction is a significant subject for understanding the SPR-induced solar energy conversion process. Photodeposition is an effective method to unveil the distribution of photogenerated charges in a photocatalytic system. In such a case, reduced (with hot electrons) and oxidized (with hot holes) products can be respectively formed on the places where electrons and holes accumulate, which can be visualized through an elemental resolution electron microscope.41,42 In this regard, the location of reaction sites can be mapped by taking advantage of photodeposition, which is capable of converting transient plasmonic hot charges into permanent sediment. Kelvin probe force microscopy (KPFM) is a scanning technique capable of acquiring information about the separation, transportation, and recombination of plasmoninduced charge carriers with nanometer-scale spatial resolution and millivolt sensitivity.43,44 By taking advantage of these methods, we are in a position to disclose the reaction sites of plasmonic hot-hole-driven water oxidation reactions. Herein, taking Au/TiO2 as a plasmonic photocatalyst prototype, we conducted selective photodeposition to position the location of the reaction sites, which were visualized by an electron microscope equipped with element mapping. The separation of plasmonic hot carriers and the location of the hot charge carriers were also directly imaged by KPFM. We demonstrated that the hot holes are mainly accumulated near the interfacial regions and the gold−semiconductor interface is identified as the catalytic reaction site responsible for water oxidation. The density functional theory (DFT) calculations further validate the pivotal role of the interface in water oxidation.
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EXPERIMENTAL SECTION
Photocatalyst Preparation. The Au/TiO2 (the gold loading amount is 2 wt %) was prepared by the deposition−precipitation method. In a typical experiment, 1.0 g of TiO2 (rutile or anatase) was added to 100 mL of an aqueous solution of HAuCl4 (1 mM) and urea (0.42 M). The suspension was kept under stirring conditions with a magnetic stirrer at 353 K by a thermostat for 4 h in the absence of light. Then the solution was centrifuged, washed with ultrapure water several times, and dried at 333 K overnight. After drying, the powdered sample was calcined at 673 K for 4 h in a muffle furnace. Photodeposition of Cr2O3 and PbO2. In the photodeposition experiments, four different methods were used: photoreduction deposition of Cr2O3 under either UV (440 nm) irradiation and photooxidation deposition of PbO2 under either UV or visible light irradiation. The schematic preparation process is shown in Figure S5. The reaction equation of photodeposition is shown as follows: Photoexcitation:
Au/TiO2 + 6hν → 6e− + 6h+
(1)
Photoreduction:
2CrO4 2 − + 5H 2O + 6e− → Cr2O3 ↓ + 10OH−
(2)
Photooxidation:
3Pb2 + + 6H 2O + 6h+ → 3PbO2 ↓ +12H+
(3)
In a typical experiment, 0.20 g of Au/TiO2 (rutile) powder and a calculated amount of precursors (K2CrO4 was used for Cr2O3 11772
DOI: 10.1021/jacs.7b04470 J. Am. Chem. Soc. 2017, 139, 11771−11778
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Journal of the American Chemical Society one surface atom in order to reduce computational demand. An oxygen atom was replaced by a Au atom on top of the rutile (110) and anatase (101) surface.
and Au/anatase samples with various TiO2 sizes. Despite the size of TiO2, the activity of Au/rutile is always higher than that of Au/anatase (Figure S2). Furthermore, the possibility for the plasmon-enhanced activity due to the enhanced near-field effect is also excluded through comparing the activities between Au/ rutile and Au@SiO2/rutile (Figures S3 and S4). The above results lead us to the conclusion that water oxidation can take place on the TiO2 surface, and the different performance in plasmonic water oxidation between Au/TiO2 (rutile) and Au/ TiO2 (anatase) may arise from the two different phases of TiO2 having different activities in water oxidation.45 So the question is where exactly are the water oxidation sites, on the TiO2 or Au surface? To locate the water oxidation sites, photodeposition (both oxidation and reduction) methods are employed.42,46,47 According to the reactions 2 and 3 in the Experimental Section, the place where PbO2 deposited is the site where photogenerated holes accumulated, while the deposited Cr2O3 can indicate the position where photogenerated electrons are located (the proposed charge transfer mechanism considering the solution Fermi level is discussed in Supporting Information Figure S6). Because the deposited PbO2 and Cr2O3 NPs are insoluble and can be reliably mapped by high-resolution STEM equipped with EDS, we are able to locate the sites where the holes are available for water oxidation. First of all, photoreduction deposition of Cr(III) oxide was carried out for TiO2 excited with UV light (440 nm). From the EDS mapping images (Figure 2B), the PbO2 species are deposited on both the surface of Au NPs and the interface between Au NPs and the TiO2 surface, indicating that the plasmonic holes are mainly distributed on the Au NPs and the interface. Correspondingly, the Cr2O3 nanoparticles were distributed on the TiO2 surface when Cr(VI) ions were photoreduced by the plasmonic electrons (Figure S7). Additionally, there is a sharp difference when comparing the deposition of CrOx and PbO2 (Figure S8). The Cr species are mainly located at the top surface of the Au and the interface is deposited with only little CrOx. In contrast, many PbO2 species are assembled near the interface, indicating more holes are located at the interface. This is the main reason for their activity difference (see below for details). These results reasonably propose that plasmonic electrons transfer from Au NPs to TiO2, while the plasmonic holes are left behind in the Au NPs. Interestingly, these photodeposited nanoparticles on the Au surface migrate easily under thermal annealing at temperatures above 473 K. The amorphous Cr2O3 species on Au NPs were shrunk into island-like particles on the surface of Au due to the thermal treatment, and some Cr2O3 particles are aggregated at the interface between gold and TiO2 (Figure 2C). More obviously, as shown in Figure 2D, some PbO2 particles deposited on Au NPs migrated to the interface between Au NPs and the TiO2 surface, forming a PbO2 circle belt around
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RESULTS AND DISCUSSION The plasmonic photocatalyst Au/TiO2 (rutile) shows two characteristic optical absorption bands edged at ∼400 nm and centered at 550 nm, respectively (Figure S1) attributed to the band−band electronic transition absorption of TiO2 (rutile) and plasmon resonance absorption of gold nanoparticle with sizes in the range 4.0−5.0 nm (Figure S1). As a reference, both TiO2 and Au/SiO2 are inactive for water oxidation under visible light irradiation (>440 nm), while Au/TiO2 shows quite high activity in water oxidation, clearly indicating that the SPR excitation of Au/TiO2 photocatalysts is responsible for the water oxidation (Figure 1A). The action spectrum of Au/TiO2
Figure 1. (A) Comparisons of Au/rutile, Au/SiO2, and bare rutile in water oxidation under visible light illumination (>440 nm). (B) Absorption spectrum (right axis) and action spectrum (left axis) of Au/rutile in plasmonic water oxidation. (C) Plasmonic water oxidation activities of Au/TiO2 in rutile and anatase phases. (D) Photocatalytic water oxidation activities of bare TiO2 in rutile and anatase phases under UV light (
