Rational Design of Wide Spectral-Responsive Heterostructures of Au

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Rational Design of Wide Spectral-Responsive Heterostructures of Au Nanorod Coupled Ag3PO4 with Enhanced Photocatalytic Performance Zening Liu,†,⊥ Yongcheng Liu,†,⊥ Piaopiao Xu,† Zhonghua Ma,† Jingyu Wang,*,‡ and Hong Yuan*,†,§ †

College of Science, Huazhong Agricultural University, Wuhan 430070, Hubei, PR China Key Laboratory of Materials Chemistry for Energy Conversion and Storage, Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, Hubei, PR China § Key Laboratory of Pesticide and Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal University, Wuhan, 430079 Hubei, PR China

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ABSTRACT: Noble metallic nanomaterials with surface plasmon resonance (SPR) effects and hot electron cell effects open new opportunities for designing efficient visible-light-driven hybrid photocatalysts. In this work, we reported a broadband visible-light responsive photocatalyst by incorporating Au nanorods (AuNRs) into Ag3PO4 nanostructures. The longitudinal plasma of AuNRs enabled AuNRs/Ag3PO4 heterostructures to harvest light energy up to 800 nm. The obtained AuNRs/Ag3PO4 hybrid exhibited enhanced photocatalytic efficiency toward the degradation of rhodamine B (RhB) under solar irradiation. Ag3PO4, RhB, and AuNRs played different roles according to the distinct optical properties of each individual component. The dominant photocatalytic process in the different light regions were divided as follows: direct excitation of Ag3PO4 for λ ≥ 420 nm, RhB sensitization for λ ≥ 550 nm, and SPR effect for λ ≥ 600 nm. The relationship between the pathway of charge transfer and the photocatalytic activity of the AuNRs/Ag3PO4 heterostructures was investigated systematically, revealing the specific role of AuNRs in regulating the photocatalytic activity. This work presents an innovative strategy for determining the comprehensive function of the SPR effect in relevant semiconductor-based photocatalysis and functional nanodevices with a broadband light responses. KEYWORDS: AuNRs, Ag3PO4, RhB, photocatalysis, solar irradiation

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INTRODUCTION With increasing requirements for environmental remediation and alternative energy exploration, tremendous effort has been dedicated to the utilization of solar energy. As a green and sustainable technology, semiconductor photocatalysis is one of the most-promising approaches to directly harvest solar light and decompose organic pollutants under ambient temperature.1−3 To better harness a wide range of solar energy, researchers have exploited many applicable photocatalysts with suitable bandgap and redox power. Among the reported semiconductors, silver orthophosphate (Ag3PO4), which has an absorption edge at ∼530 nm, has been found to be an attractive candidate for visible-light-driven photocatalysis.4−8 Similar to other narrow-bandgap semiconductors, such as CdS and g-C3N4, the Ag3PO4 photocatalyst also suffers from a high recombination rate of photogenerated charge carriers and limited responsive range within the wide sunlight spectrum.9,10 Although there are plenty of reports on improving the photocatalytic performance of Ag3PO4 materials,11−15 the photocatalysis in the long wavelength region has been rarely reported so far. © 2017 American Chemical Society

The noble-metal-modified semiconductor photocatalysts have attracted great attention for pollutant degradation. The noble metals (such as Au and Ag) could be used as multifunctional materials positively contributed to the photocatalytic reactions, especially for visible-light driven photocatalysis.16,17 The excitation of surface plasmon resonance (SPR) occurs in the plasmonic nanostructures with high mobility of the free electron when the photon energy matches the resonance energy of its oscillation.18,19 The resonance energy locations for Ag and Au nanospheres are centered at ∼410 and ∼520 nm, respectively.20 When the size, shape, aggregation state, or local environment of nanoparticles are changed, their optical properties can be tuned from the visible to near-infrared region.21−23 Ye’s group reported a wide-range visible light harvesting of TiO2 based on the broadband absorption of Au nanorods (NRs) from visible to near-infrared light, including the transversal plasma as AuNPs and Received: May 15, 2017 Accepted: June 1, 2017 Published: June 1, 2017 20620

DOI: 10.1021/acsami.7b06824 ACS Appl. Mater. Interfaces 2017, 9, 20620−20629

Research Article

ACS Applied Materials & Interfaces

(RhB) whose light absorption is centered between the absorption edge of Ag3PO4 and the longitudinal SPR absorption onset of AuNRs. As displayed in Figure 1, such a rational design enabled the photocatalytic system to be applicable over the whole visible-spectral region. In this way, the influence of AuNRs incorporation could be determined simply by settling the cutoff light filter to 420, 550, 600, or 645 nm to selectively excite Ag3PO4, RhB, or AuNRs. Combined with the reactive oxidative species trapping test, the chargetransfer pathway in this photocatalytic system was investigated to discern the underlying mechanism under the irradiation of different photon energies, providing new insights for plasmonic metals involved in semiconductor photocatalysis and functional nanodevices.

longitudinal plasma (centered from 630 to 810 nm) depending on its aspect ratio.24 Therefore, the wide visible-spectra of solar energy could be achieved by introducing AuNRs as antennas into the Ag3PO4 nanostructures. Different mechanisms describing the role of noble metals in semiconductor-based photocatalytic systems have appeared in the literatures. For instance, some groups have proposed that the noble metals would generate the near-field electromagnetic radiative energy to enhance the electron transition of semiconductors from the valence band (VB) to the conduction band (CB).16,25 Meanwhile, it was also demonstrated that the SPR-excited energetic electrons are produced on noble metals, which can then be directly injected into the CB of semiconductors.26,27 Based on these two mechanisms, the photocatalytic activity of semiconductors is improved when the photon energy matched the light absorption of the noble metallic nanoparticles. The third possible mechanism for the improved activity involves the formation of a Schottky barrier between the semiconductor and noble metal, resulting in the increased separation efficiency of photogenerated electron− hole pairs.28,29 In addition, it was speculated that the enhancement of the photocatalytic activity may arise from a combined effect of direct electron transfer and plasmoninduced resonant energy transfer from the metal to the semiconductor.30 Very recently, Schmuki and Bi et al. illustrated a novel phenomenon of noble-metal-assisted photoelectrochemical water splitting of semiconductors. The SPR excitation enabled the Au NPs to serve as “hole-depletion” layer through providing electrons to neutralize holes of the semiconductor.31,32 As a result, the modification by the noble metal definitely improved the photocatalytic performance of the semiconductor. However, the mechanism for the enhancement of the photocatalytic activity in the above-mentioned systems was in conflict with each other. A systematic study on the interaction of noble metals with semiconductors that induced by different region of photon energies is of great importance for developing photocatalysts responsive over a wide visible range. To clarify the specific effect of plasmonic metal modification, the heterostructure of AuNRs/Ag3PO4 was constructed due to its wide-range visible response range and the lack of overlap between the plasmon resonance absorption of AuNRs and the bandgap energy of Ag3PO4 (Figure 1). Its photocatalytic performance was evaluated by the degradation of rhodamine B

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EXPERIMENTAL SECTION

Preparation of AuNRs. AuNRs were synthesized by a seedmediated growth method in light of the literatures.24,33,34 First, Au seed solution was prepared by adding 0.6 mL of 0.01 M precooled NaBH4 solution to the mixture of 5 mL of 0.2 M CTAB and 5 mL of 0.5 mM HAuCl4. The seed solution was aged for 2 h at 27 °C after the color of the reaction system changed to brown. Next, the solutions were mixed under continuously stirring in the following order: 25 mL of 0.2 M CTAB solution, 1.25 mL of fresh 0.004 M AgNO3 solution, 25 mL of 1 mM HAuCl4 solution, and 350 μL of 0.0788 M fresh ascorbic acid aqueous solution. Finally, 60 μL of the as-prepared Au seed solution was added, and the reactants were kept undisturbed for 36 h at 27 °C. After the reaction, AuNRs were obtained by centrifuging and repeatedly washing with water. Preparation of AuNRs/Ag 3 PO 4 Heterostructures. The AuNRs/Ag3PO4 heterostructures were prepared by a co-precipitation procedure. The stoichiometric AuNRs were dispersed in 80 mL of distilled water and mixed with 10 mL of 0.5 M Na2HPO4 under stirring for 5 min in the dark. Subsequently, 10 mL of 0.5 M fresh AgNO3 solution were added with stirring for another 20 min in the dark. The resultant AuNRs/Ag3PO4 powders were obtained by centrifuging, thoroughly washing, and drying at 75 °C for 24 h. The theoretical molar ratio of Au to Ag in the reactants was 0.005:1. For comparison, Ag3PO4 was also prepared as thus: 80 mL of distilled water and 10 mL of 0.5 M Na2HPO4 were added into a 100 mL beaker and stirred for 5 min in the dark. Subsequently, 10 mL of 0.5 M fresh AgNO3 solution was added with stirring for another 20 min in the dark. Last, the yellow powders obtained were dried at 75 °C for 24 h. Characterizations. The products were characterized by X-ray diffraction (XRD) patterns with Cu Kα irradiation at 40 kV and 40 mA. The morphology of the photocatalysts was observed using a SU8010 scanning electron microscope (SEM) and Tecnai G2 20S-Twin transmission electron microscope (TEM). The specific surface area of the synthesized samples was characterized by the Brunauer−Emmett− Teller (BET) method with the use of a Micromeritics (ASAP2460) instrument. X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo Scientific ESCLAB 250Xi multifunctional spectrometer using Al Kα radiation. All XPS spectra were referenced to the C 1s peak at 284.6 eV from the adventitious carbon standard. The UV−vis diffuse-reflectance absorption spectra (UV−vis DRS) were obtained by a Shimazu UV-3100 spectrophotometer equipped with an integrating sphere using BaSO4 as the reference sample. The photoluminescence (PL) spectra were measured at room temperature on a RF-5301PC fluorescence spectrophotometer with the excitation wavelength at 275 nm. Inductively coupled plasma-mass spectrometry (ICP-MS, Agilent 7900) was used to test the content of Au in AuNRs/Ag3PO4. The photoluminescence (PL) spectra were measured at room temperature on a RF-5301PC fluorescence spectrophotometer with the excitation wavelength at 275 nm. Electrochemical and Photoelectrochemical Measurements. Electrochemical characteristics of the photocatalysts were measured using a CHI660D electrochemical workstation in a standard three-

Figure 1. UV−vis absorption spectra of RhB, AuNRs, Ag3PO4, and AuNRs/Ag3PO4. The spectra are normalized against their peak intensities. The result of AuNR sample was obtained by directly measuring the sol solution. The results of the Ag3PO4 and AuNRs/ Ag3PO4 was obtained by UV−vis DRS measurement. 20621

DOI: 10.1021/acsami.7b06824 ACS Appl. Mater. Interfaces 2017, 9, 20620−20629

Research Article

ACS Applied Materials & Interfaces Table 1. Experimental Parameters of Photocatalytic Reaction Systems samples

amount of catalysts (mg)

concentration of RhB (μM)

rime intervals (min)

irradiation wavelength (nm)

irradiation intensity (mW/cm2)

AuNRs/Ag3PO4 AuNRs/Ag3PO4 AuNRs/Ag3PO4 AuNRs/Ag3PO4

10 25 25 25

10 10 10 10

2 30 60 60

≥420 ≥550 ≥600 ≥645

204 138 107 72

Figure 2. Low-magnification SEM image (a) and high-magnification SEM image (b) of AuNRs/Ag3PO4 composite, directly obtained from the SEM test without gold sputtering. The overlapping of these elemental maps (c) and the corresponding EDS elemental mapping images of each element (d−g). Trapping Test of Primary Active Species. The role of the primary active species in the photocatalytic reaction could be tested by adding their specific scavengers, such as CH3OH for •OH, Na2C2O4 for h+, and benzoquinone (BQ) for O2−•. The changes in the phototcatalytic efficiency after the addition of scavengers could reflect the inhibition degree of these active species. The trapping experiment conditions were 50 mL of reaction suspensions containing 10 mM CH3OH, 10 mM Na2C2O4, or 0.1 mM BQ. The detection method is similar to the above phototcatalytic activity measurement.

electrode configuration with the as-prepared samples modified ITO as the working electrode, a Pt foil as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. Na2SO4 (0.1 M) solution was used as the electrolyte. The working electrode was prepared as follows: 20 mg of photocatalysts were dispersed in 5 mL of absolute ethanol, and then 0.5 mL of Nafion solution was added to form the homogeneous dispersion by ultrasonic treatment. The resulting suspension was drop-cast onto the 1 cm × 4 cm ITO conductive glass. The electrode was dried in air at room temperature. Electrochemical impedance spectroscopy (EIS) was measured by AC impedance method with 1.2 V of external voltage, 10 mV of amplitude, and 0.1−500 kHz of frequency. The transient photocurrent response was tested under the irradiation of a 300 W xenon lamp with a cutoff light filter. Photocatalytic Activity Evaluations. The photocatalytic activities of the as-prepared composites were evaluated through the degradation of RhB solution under different visible light region. A 300 W xenon lamp was equipped with a cutoff light filter of 420, 550, 600, or 645 nm to remove the undesired photons. The photocatalytic reaction occurred in a 100 mL beaker containing a certain amount of photocatalysts and 50 mL of 10 μM RhB solution. Before the photodegradation was initiated, the suspensions were stirred in the dark for 30 min to reach an adsorption−desorption equilibrium between RhB molecules and catalysts. At given time intervals, 4 mL of suspensions were sampled and centrifuged at 10 000 rpm for 5 min to remove the photocatalysts. The detailed experimental parameters were provided in Table 1. The changes of RhB concentration during light irradiation were determined by recording the maximum absorption of RhB at 550 nm. In addition, the solar light was used as a light sources to simulate practical application.

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RESULTS AND DISCUSSION Morphology and Structural Characterizations. The size and morphology of the as-prepared AuNRs were investigated by TEM (Figure S1a). The AuNRs possessed a uniform size distribution with a longitudinal length of approximately 80 nm and an aspect ratio of ∼2.7. In this case, the AuNRs displayed two absorption peaks at 513 and 698 nm (Figure 1). The much-stronger peak at 698 nm was attributed to the longitudinal SPR absorption.24,33,34 Figure S1b shows that the Ag3PO4 sample was composed of polyhedral particles with a few of parasitic spherical particles.35 In comparison to the SEM image of Ag3PO4, the AuNRs/ Ag3PO4 heterostructure mainly displayed the spherical morphology with a much-smaller particle size in the presence of AuNRs (Figure 2a,b). The decreased particle size resulted in a larger specific surface area of 7.1 m2·g−1 compared to that of pure Ag3PO4 (3.2 m2·g−1). The corresponding elemental distribution maps of the AuNRs/Ag3PO4 composite were 20622

DOI: 10.1021/acsami.7b06824 ACS Appl. Mater. Interfaces 2017, 9, 20620−20629

Research Article

ACS Applied Materials & Interfaces achieved by energy dispersive spectrometer (EDS). As shown in Figure 2c−g, there exists a homogeneous distribution of Au in the AuNRs/Ag3PO4 heterostructures. By the overlapping of the elemental maps, it can be observed that all Ag elements exist in the Ag3PO4 form, which possesses much-richer zones than the AuNRs. In the synthetic procedure, the HPO42− ions are preadsorbed on the surface of the AuNRs so that the Ag+ ions are inclined to reacting with the HPO42− ions near the AuNRs. AuNRs acted as nucleation centers for the in situ growth of Ag3PO4, which favored the formation of spherical Ag3PO4. That is, AuNRs incorporation hinders the transformation of spherical Ag3PO4 to a larger polyhedral morphology. The crystal structure of the as-synthesized Ag3PO4 and AuNRs/Ag3PO4 catalysts were analyzed by XRD patterns (Figure 3). All the sharp characteristic diffraction peaks were in

Figure 4. XPS survey spectrum (a), high-resolution Ag 3d spectrum (b), high-resolution P 2p spectrum (c), and high-resolution Au 4f spectrum (d) of AuNRs/Ag3PO4 composites.

photocatalysis. The changes in the UV−vis DRS and the energy gap of Ag3PO4 upon Au incorporation are shown in Figure 1. The absorption edge of the pure Ag3PO4 was about 544 nm. It had been reported that the polyhedral Ag3PO4 absorbed more visible light than the spherical Ag3PO4.35 Obviously, the Au incorporation brought about a blue shift of the absorption edge of the Ag3PO4 to ∼520 nm, presumably due to the quantum confinement effect by decreasing particle size. Interestingly, the AuNRs/Ag3PO4 hybrid exhibited the light absorption in the range of 400−800 nm, covering the whole visible-light region. The strong light absorption with the wavelength longer than 600 nm indicated that the longitudinal SPR absorption of the AuNRs was not shielded by the surrounded Ag3PO4 crystals. The bandgap energy (Eg) of a semiconductor can be calculated by the following Kubelka−Munk equation:41−43

Figure 3. XRD patterns of as-prepared Ag3PO4 and AuNRs/Ag3PO4. The inset is the magnified pattern in the 2θ range of 36° to 40°.

accordance with the crystalline planes of the body-centered cubic Ag3PO4 (JCPDS no. 06-0505). The AuNRs/Ag3PO4 showed similar XRD diffraction peaks to the Ag3PO4, indicating that AuNRs incorporation did not alter the crystal structure of the Ag 3 PO 4 . The additional peak appeared at 38.2°, corresponding to the (111) plane of the metallic Au, could only be observed in the magnified pattern due to the relatively low content of the AuNRs in the composite. The chemical states of the elements were further characterized by XPS (Figure 4). The survey spectrum in Figure 4a indicated the existence of Au, Ag, P, and O elements in the composites. A pair of symmetric peaks that located at 367.87 and 373.84 eV were ascribed to the binding energy values of the 3d5/2 and 3d3/2 of Ag+ in the Ag3PO4 (Figure 4b). The binding energy value of the P 2p is determined to be 133.14 eV (Figure 4c), indicating the valence state of +5 in the Ag3PO4.9,36 The highresolution Au 4f spectrum in Figure 4d displays two distinct peaks at 84.86 and 88.48 eV with a spin−orbital doublet splitting (D = Au 4f5/2 − Au 4f7/2) of 3.62 eV, confirming the chemical state of the Au. The locations of the Au 4f peaks in the composites shifted to higher energy compared to the standard values,37 implying that the strong interfacial interaction with Ag3PO4 led to a decrease in the electron intensity of the Au atoms.38 Such an interaction is beneficial for its intimate interfacial contact and subsequent facilitation of charge-carrier transfer.39,40 Optical Property. The absorption property of the photocatalysts played an important role in determining their photocatalytic efficiency, especially for visible-light-driven

[F(R ∞)hν]1/ n = C2(hν − Eg )

The value of 1/n is decided by the characteristics of the transition in a semiconductor, e.g., 1/2 for an indirect transition. Through extrapolating the linear part to the hν axis intercept (Figure 5a), the Eg values of the pure Ag3PO4 and Ag3PO4 in the heterostructures were estimated to be 2.17 and 2.24 eV, being in accordance with the literatures.4,6,7 Based on the Eg estimation, the band potentials could be calculated through the following equation:44,45

Figure 5. (a) Converted Tauc plots of (F(R)hυ)1/2 versus hυ according to the UV−vis diffuse reflectance spectra in Figure 1. (b) VB-XPS spectra of AuNRs/Ag3PO4. 20623

DOI: 10.1021/acsami.7b06824 ACS Appl. Mater. Interfaces 2017, 9, 20620−20629

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respectively. The photodegradation curves were fitted according to the pseudo-first-order kinetic model (Figure S3). The values of the reaction rate constant (k) and linear fitting coefficients (R2) are given and compared in Table 2. In the case of region I, the slight degradation of RhB in the blank test for a given irradiation time indicated that the direct photolysis could be ignored as compared with photodegradation over the AuNRs/Ag3PO4 catalysts (Figure 6a). It had been reported that the polyhedral Ag3PO4 crystals exhibited a higher photocatalytic activity than that of the spherical crystals of smaller size.35 Although spherical Ag3PO4 possessed a narrower bandgap, all AuNRs/Ag3PO4 composite catalysts displayed improved photocatalytic efficiency toward RhB degradation. The degree of improvement is dependent on the proportion of AuNRs incorporation (Figure S4). With an increase in the AuNRs ratio from 0.001 to 0.005, the photocatalytic activity of the AuNRs/Ag3PO4 composite increased gradually. After that, excessive AuNRs may play a role in the recombination of photogenerated electron−hole pairs.9 The k value of the optimal composite was approximately 2.2 times that of pure Ag3PO4. The real molar ratio of Au to Ag was measured to be 0.0042 by the ICP-MS technique. If the photons were used in region II, the photolysis of RhB in the blank test was noticeable after 150 min of irradiation (Figure 6b). Because the photons in this range were incapable of producing electron−hole (e−−h+) pairs within the Ag3PO4 semiconductor, the photocatalytic reaction occurs via a dyesensitized process.2,45,46 The photodegradation of RhB over pure Ag3PO4 increased to 2.2 times as that of the blank system. Then, AuNRs incorporation brought about a further 1.4× increase. The degree of improvement by AuNRs was not as notable as that of using photos from region I, implying that the weak transversal SPR absorption by AuNRs played a lessdominant role than the direct absorption by the RhB dye, which has a much higher light-absorption coefficient.3 Under the irradiation of photons from region III, neither Ag3PO4 nor RhB could be excited to generate charge carriers due to their optical properties. Consequently, there was no significant difference in RhB degradation between the blank and pure Ag3PO4 photocatalytic system (Figure 6c). Instead, the introduction of AuNRs dramatically enhanced the degradation rate, achieving a rate 4.4 times higher than that of pure Ag3PO4. Because there are reports in the literature on the self-catalysis of the plasmonic metals,47−49 the RhB degradation over pure AuNRs was also presented in Figure S5. It should be noted that the introduction of AuNRs did not bring about a distinct change in the photodegradation rate compared to the direct photolysis in the blank test under region I and II irradiation. In contrast, the RhB degradation caused by the self-catalysis of

E VB = x − 0.5E c + 0.5Eg

where x was the absolute electronegativity of the semiconductors, determined by the geometric mean of the electronegativity of the atoms; Ec is the energy of free electrons at the hydrogenscale (∼4.5 eV versus the vacuum level). The VB edge of the Ag3PO4 in the hybrid composites could be predicted to be 2.66 eV.45 To provide further evidence, the VBXPS spectrum of the AuNRs/Ag3PO4 was recorded in Figure 5b. The VB position of the Ag3PO4 is measured to be 2.72 eV, close to the maximum theoretical prediction. The CB edge thereby obtained was 0.48 eV. Photocatalytic Activity. The photocatalytic degradation of RhB was used to evaluate the influence of Au incorporation on the activity of Ag3PO4. Prior to light irradiation, the adsorption capability of the catalyst was tested after reaching adsorption− desorption equilibrium (Figure S2). The difference in RhB adsorption over the Ag3PO4 and AuNRs/Ag3PO4 catalysts can be ignored due to the low specific surface area. It was noted that the relatively weak absorption of AuNRs/Ag3PO4 between 500−600 nm could be well compensated by the strong light absorption of RhB. Such design provided the possibility for systematically adjusting the incident photon energy to distinguish the specific function of the AuNRs. Figure 6

Figure 6. RhB degradation curves under the irradiation of light with different wavelength. (a) Region I (≥420 nm), (B) region II (≥550 nm), (c) region III (≥600 nm), and (d) region IV (≥645 nm).

displays changes in the photocatalytic degradation rate under the irradiation of light of different wavelength, corresponding to the photons with energy lower than 420 nm (region I), 550 nm (region II), 600 nm (region III), and 645 nm (region IV),

Table 2. Reaction Constants (k) and Regression Coefficients (R2) of RhB Photocatalytic Degradation over Ag3PO4 and Au NRs/Ag3PO4 samples no catalyst AuNRs Ag3PO4 AuNRs/Ag3PO4

k (min−1) R2 k (min−1) R2 k (min−1) R2 k (min−1) R2

I (≥420 nm)

II (≥550 nm)

III (≥600 nm)

IV (≥645 nm)

0.00589 0.99 0.00819 0.97 0.119 0.99 0.263 0.97

0.00157 0.99 0.00160 0.99 0.00349 0.99 0.00485 0.99

0.000142 0.99 0.000765 0.99 0.000258 0.99 0.00113 0.98

0.0000979 0.98 0.000466 0.99 0.000159 0.99 0.000529 0.99

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DOI: 10.1021/acsami.7b06824 ACS Appl. Mater. Interfaces 2017, 9, 20620−20629

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ACS Applied Materials & Interfaces AuNRs was much higher than that of direct photolysis and comparable to that of the hybrid system under region III irradiation. This indicates that the plasmonic self-catalysis, which is negligible under region I and II irradiation, played a dominant role under region III irradiation. The experiment under region IV irradiation further confirms the improvement in photocatalytic activity upon AuNRs incorporation (Figure 6d). The results indicated that the photocatalytic reaction definitely originated from the longitudinal SPR absorption of AuNRs, which functioned well, even under the irradiation of light longer than 600 nm. The photocatalytic process involving plasmonic metal nanoparticles is generally confused with the thermal catalytic process.50−53 In this regard, we conducted the dark reaction at varied temperatures under constant light intensity. The decrease in the RhB concentration in the dark could be ascribed to the contribution of thermal catalysis.53 The amount of RhB degradation within 150 min is negligible, and no obvious change is observed at the elevated temperature (Figure S6a). To further assess the light-driven photocatalytic process in this work, the effect of light intensity on the catalytic efficiency was investigated using identical reaction temperatures. Under region III irradiation, the disturbance of light absorption by Ag3PO4 and RhB can be excluded from SPR absorption to ensure the sole contribution of AuNRs to the photocatalysis. As shown in Figure S6b, the amount of RhB degradation increased with increasing the light intensity. When the light intensity increased from 50 to 150 mW cm−2, the enhancement in the k value was approximately 2.8 times. Due to the SPR effect, the stronger light intensity would induce a larger amount of electrons with high energy that can participate in the reaction.53 It could be concluded that AuNRs mainly regulated the photocatalytic efficiency of the Ag3PO4 semiconductor through a light-driven mechanism. The direct use of solar energy is an important issue for the realization of practical applications. The photocatalysis under solar light irradiation was carried out to demonstrate the effect of AuNRs incorporation. After 50 min of irradiation, the RhB dye was completely decomposed by AuNRs/Ag3PO4, while Ag3PO4 achieved only a 78% decrease in the RhB concentration (Figure S7). This remarkable improvement further verified the more efficient utilization of solar energy via AuNRs/Ag3PO4. Regulatory Mechanism upon AuNRs Incorporation. It was noted that the degradation rate steeply decreased when narrowing the light region from I to III. The difference in the k value implies the distinct difference in efficiency among the direct excitation of Ag3PO4 (pathway i), RhB sensitization (pathway ii), and the SPR effect of AuNRs (pathway iii). The photocatalysis induced by SPR effect of AuNRs and RhB sensitization is not comparable to the direct degradation by the Ag3PO4 semiconductor under region I irradiation. Does this imply that the AuNRs do not contribute to the photocatalysis via pathway i? In fact, the remarkable improvement in the degradation rate occurs after AuNRs incorporation under light from region I. To understand the role of AuNRs in the heterostructures, Nyquist plots of pure Ag3PO4 and AuNRs/ Ag3PO4 were constructed (Figure 7a). In general, the decreased diameter of the semicircle diameter at a high frequency indicated the low charge-transfer resistance (Ret) across the contact interface between electrode and electrolyte.54 Obviously, the introduction of AuNRs brought about a dramatically decrease in the semicircle diameter, suggesting a more-efficient charge transfer in AuNRs/Ag3 PO 4 . The

Figure 7. EIS Nynquist plots (a), room-temperature PL spectra (b), and transient photocurrent response (λ ≥ 420 nm) (c) of as-prepared Ag3PO4 and AuNRs/Ag3PO4.

interaction between the AuNRs and Ag3PO4 in AuNRs/ Ag3PO4 was further examined by monitoring the changes in the fluorescence emission. The PL spectra of semiconductors resulted from the recombination of photogenerated charge carriers at the trapping sites. As displayed in Figure 7b, pure Ag3PO4 exhibited a broad photoluminescence band with two emission peaks. The strong emission at approximately 370 nm was reported as the recombination of the charge-transfer transition between the O 2p orbital and empty Ag 5d orbitals or self-trap excitons in PO43−. Another peak at approximately 470 nm arose from the radiative recombination of photogenerated e−−h+ pairs at the trapping sites.55,56 The overall PL intensity of Ag3PO4 was obviously quenched after incorporating AuNRs. The higher electrical conductivity of AuNRs/Ag3PO4 facilitated the electron transfer from Ag3PO4 to the AuNRs, and thus inhibited the recombination of charge carriers. As a result of the reduced recombination loss at the interface, AuNRs/ Ag3PO4 exhibited a much-stronger photocurrent response than did pure Ag3PO4 under wide-visible-range irradiation (Figure 7c). It should be mentioned that slight declination is observed in the photocurrent signals. As pointed out by the previous studies, the light irradiation would most likely lead to the reduction of Ag(I) into metallic Ag particle.11,12 Therefore, the introduction of AuNRs into Ag3PO4 not only lowered the charge-transfer resistance but also acted as electron reservoirs to separate the photogenerated charge carriers. The efficient charge transfer and separation could lengthen the lifetime of the charge carriers and thus enhanced the photocatalytic activity. To reveal the specific role of AuNRs in AuNRs/Ag3PO4, the contributions of the main active species in the photocatalytic process were compared through trapping experiments under light of different regions. In this work, three kinds of scavengers were introduced into the photocatalytic system to capture the corresponding active species, such as CH3OH for •OH, and benzoquinone (BQ) for O2−•, and Na2C2O4 for h+ (Figure 8).57,58 When photons from region I were used, the photodegradation rate of RhB somewhat decreased with the addition of CH3OH and greatly declined upon the introduction of BQ, or Na2C2O4 (Figure 8a). The results imply that •OH, O2−•, and h+ were the main active species with the order of h+ 20625

DOI: 10.1021/acsami.7b06824 ACS Appl. Mater. Interfaces 2017, 9, 20620−20629

Research Article

ACS Applied Materials & Interfaces

would quickly recombine at the excited state, resulting in a small amount of e− and h+ participating in the photocatalytic reaction. The photogenerated e− in the CB of Ag3PO4 was readily flowing to the vacancies of the AuNRs, facilitating the space separation of the photogenerated charge carriers. In this way, the holes were accumulated in the VB of Ag3PO4 to oxidize OH− and the dye molecules; the energetic electrons kept at the higher-energy states of AuNRs activated O2 to produce O2−•. The schematic diagram reflects the dual contribution of AuNRs to the RhB photodegradation of improving charge-carrier separation and activating molecular oxygen. When the light was switched to region II, the addition of CH3OH and Na2C2O4 slightly influenced the photocatalytic reactivity of AuNRs/Ag3PO4. The absence of •OH and h+ for the RhB degradation indicated that Ag3PO4 could not be excited by these photons to generate e−−h+ pairs. Instead, adding BQ obviously suppressed the degradation efficiency, suggesting the predominant role of O2−• in the RhB sensitization process (Figure 8b). The AuNRs modified dyesensitized photocatalytic pathway II is proposed in Figure 9. In this three-component photocatalytic system, RhB dyes were dominantly responsible for absorbing this portion of photons, and thus, the direct photolysis could not be ignored (Figure 6b). The dye molecules gained energy from the light and migrated to the excited state, which followed by the decomposition to dye radicals and electrons (dye* → dye+• + e−). As reported, the electrons in the excited state could be quenched by molecular oxygen to produce O2−•.61,62 In most cases, the energy decayed directly to the ground state. This was called direct de-excitation. In the presence of the Ag3PO4 photocatalyst, the electrons were inclined to transfer to the CB of Ag3PO4 because of the negative potential of RhB*/ RhB+• (−1.09 eV),6 leading to the reduced recombination with RhB +• . The improved conductivity in AuNRs/Ag 3 PO 4 facilitated the electrons transferring to the hybrid photocatalyst. Thus, the electrons at higher energy bands were sufficiently energetic for generating O2−• radicals. The enhanced photoactivity by AuNRs in this case was mostly ascribed to the improvement in the electronic conductivity. In the cases of region III and region IV, it was mentioned that both the Ag3PO4 photocatalyst and RhB dye showed no light response above 600 nm, and thus, only the SPR of AuNRs worked within this range (see Figure 1). Under light (λ > 600 nm) irradiation, due to gaining energy, the conducting electrons (6sp electrons for gold) were redistributed to higher-energy states from the lower levels. The energetic electrons in AuNRs would activate absorbed reactant molecules and induce chemical reactions. It should be noted that when AuNRs were coupled with Ag3PO4, the photocatalytic activity was enhanced (Figure S2). Such an enhanced performance was due to increasing nonhomogeneous oscillating electromagnetic fields in the neighborhood of the Ag3PO4 semiconductor.47 Based on the negligible influence of h+ and •OH scavengers, it was confirmed that only O2−• radicals were the main active species for RhB degradation (Figure 8c,d). It could be deduced that the hot electrons in the excited state of AuNRs by the SPR effect were capable of inducing the reduction of absorbed oxygen molecules to the generation of activated O 2 −• radicals.47−51 Figure 9 illustrates the SPR mediated photocatalytic mechanism in this specific region (pathway III).

Figure 8. Effects of scavengers on the degradation efficiency of RhB over AuNRs/Ag3PO4 under the irradiation of different light wavelength. (a) Region I (≥420 nm), (b) region II (≥550 nm), (c) region III (≥600 nm), and (d) region IV (≥645 nm).

> O2−• > •OH.59 Under region I irradiation, Ag3PO4 was excited to generate e−−h+ pairs. According to the band positions of Ag3PO4, h+ on the VB could oxidize OH− to produce •OH, but e− on the CB could not reduce O2 to produce O2−• due to the negative potential of O2/O2−• (−0.046 eV versus NHE) and •OH/OH− (1.99 eV versus NHE). It has been discovered that the electrons photogenerated in the plasmonic metals are energetic enough to activate molecular oxygen via a one-electron reduction process.60 The results are in accordance with the above analysis of PL, EIS, and photocurrent, which indicated the electron transfer to the AuNRs to form the Schottky barrier. The electron-transfer pathway I for RhB degradation over AuNRs/ Ag3PO4 heterostructure under region I is presented in Figure 9.

Figure 9. Mechanism for degrading dyes over AuNRs/Ag3PO4. (a) Region I (≥420 nm), (b) region II (≥550 nm), and (c) regions III and IV (≥600 nm).

The SPR effect induced an intraband migration of the electrons in the 6sp band of AuNRs to the higher-energy states, simultaneously leaving positively charged vacancies in the lower-energy states. The electrons with sufficient energy were captured by oxygen molecules to generate O2−• radicals; meanwhile, the positive vacancies could directly oxidize the organic molecules.47−49 Generally, the electrons and vacancies 20626

DOI: 10.1021/acsami.7b06824 ACS Appl. Mater. Interfaces 2017, 9, 20620−20629

ACS Applied Materials & Interfaces

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CONCLUSIONS

The rational design of AuNRs/Ag3PO4 heterostructures was successfully achieved to harvest solar energy from a wide region of the visible spectrum. AuNRs acted as nucleation centers for the in situ growth of Ag3PO4 crystals, favoring the formation of uniformly spherical Ag3PO4 crystals rather than larger polyhedral particles. To investigate the plasmonic regulatory mechanism, RhB dye was selected as the model pollutant because its light absorption was different from that of AuNRs and of Ag3PO4. The introduction of AuNRs enhanced the photocatalytic activity of Ag3PO4 to various degrees under the irradiation of a distinct light region. The degree of enhancement was dependent on the contribution of AuNRs to the dominant photocatalytic process (for example, the direct excitation of Ag3PO4 in region I (≥420 nm), RhB sensitization in region II (≥550 nm), and the SPR effect in region III (≥600 nm)). Based on the optical, electrochemical, and photoelectrochemical analyses and the trapping tests performed to explain the AuNRs mediated photocatalytic mechanism, the different charge-transfer pathways were proposed as follows: (1) the dual function of improving the charge carriers separation and activating molecular oxygen in region I; (2) the improved conductivity facilitating the electron transfer from the dye molecules to the hybrid photocatalyst and the activation of molecular oxygen in region II; and (3) the direct injection of SPR-induced hot electrons from AuNRs to absorbed molecular oxygen to produce O2−• in region III or IV. In summary, this study not only highlights the utilization of AuNRs for developing broadband photoresponse photocatalysts but also provides comprehensive insights for the development of photofunctional nanodevices.

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ACKNOWLEDGMENTS

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REFERENCES

The research work was supported by the National Nature Science Foundation of China (grant nos. 21277055 and 21571071), the Young Outstanding Talent Foundation of Hubei Province of China (grant no. 2012FFA032), Natural Science Foundation of Hubei Scientific Committee (grant no. 2016CFA001), and the Fundamental Research Funds for the Central Universities (grant nos. 2013PY018 and CCNU16A02003).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b06824. Figures showing the morphology of Au NRs and Ag3PO4, adsorption capability test in the dark reaction, the first-order kinetic constants fitting of degrading RhB, RhB degradation over AuNRs/Ag3PO4 catalysts with different AuNRs proportion, a comparison of RhB degradation over AuNRs catalysts with no catalyst, dark reactions over AuNRs/Ag3PO4 at varied temperature, the change of reaction rate constant (k) in AuNRs/Ag3PO4 photocatalytic system by adjusting the irradiance of incident light, and the photocatalytic activities under solar light irradiation. (PDF)

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Research Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Hong Yuan: 0000-0001-9685-2440 Author Contributions ⊥

Z.L. and Y.L. contributed equally to this work.

Notes

The authors declare no competing financial interest. 20627

DOI: 10.1021/acsami.7b06824 ACS Appl. Mater. Interfaces 2017, 9, 20620−20629

Research Article

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