Au–Ag alloy nanoparticle-incorporated AgBr plasmonic photocatalyst

A solid-phase photochemical method produces Au–Ag alloy nanoparticles (NPs) with a sharp size distribution and varying composition in AgBr crystals (Au–Ag@AgBr). These features render Au–Ag@AgBr promising as a material for the plasmonic photocatalyst further to provide a possibility of elucidating the action mechanism due to the optical tunability. This study shows that the visible-light activity of Au–Ag@AgBr for degradation of model water pollutant is very sensitive to the alloy composition with a maximum at the mole percent of Au to all Ag in AgBr (y) = 0.012 mol%. Clear positive correlation is observed between the photocatalytic activity and the quality factor defined as the ratio of the peak energy to the full width at half maximum of the localized surface plasmon resonance band. This finding indicates that Au–Ag@AgBr works as a local electromagnetic field enhancement-type plasmonic photocatalyst in which the Au–Ag NPs mainly promotes the charge separation. This conclusion was further supported by the kinetic analysis of the light intensity-dependence of external quantum yield.


Results and discussion
Photocatalyst characterization. Au-Ag alloy nanoparticles (NPs)-incorporated AgBr crystals (Au-Ag@ AgBr) with varying composition were synthesized by the solid-phase photochemical method recently developed (Scheme 1) 16 . The amount of Au and Ag in Au-Ag@AgBr were determined by inductively coupled plasma (ICP) spectroscopy. The dose of Au was expressed by y (mol%) = (Au mole/all Ag mole in Au-Ag@AgBr) × 100, and controlled at 0 ≤ y ≤ 0.159 mol% by the HAuCl 4 concentration. In the X-ray diffraction (XRD) patterns of the samples with varying y, no diffraction peak of Au and/or Ag metal is observed because of the small amount, while the formation of AgBr is clearly confirmed (Supplementaly Fig. S1). The AgBr matrix of Au-Ag@AgBr was selectively dissolved by NaCl aqueous solution with octadecyltrimethylammonium chloride, and the metal particles were observed by TEM. High resolution (HR)-TEM image and energy dispersive X-ray spectroscopy (EDS) line analysis indicate that Au-Ag alloy NP with high crystallinity is generated in the AgBr matrix (Supplementaly Fig. S2). The TEM images of the metal NPs obtained from Au-Ag@AgBr with different Au-dose levels show that uniform nearly spherical metal NPs are produced (Fig. 1, Supplementaly Fig. S3). The mean sizes of metal NPs (d) were determined to be d = 2.90 nm at y = 0, d = 2.95 nm at y = 0.012 mol%, and d = 2.84 nm at y = 0.159 mol%, indicating that the d is almost independent of y. The standard deviation (σ) is smaller than 0.75 nm, while the values for the alloy nanoparticles prepared by usual liquid-phase methods are in the range from 1.2 to 2.1 nm 17, 18 . The uniformity of Au-Ag metal NPs in the present solid-phase photochemical method probably results from the restriction of the metal particle growth via coalescing particles in the AgBr solid matrix 19 . The sharp size distribution of Au-Ag NPs enables the experimental scrutiny of the action mechanism of the Au-Ag@AgBr plasmonic photocatalyst (vide infra).
Photocatalytic activity. 2-Naphthol was selected as a model water pollutant since it is industrially used for the production of azo-dyes, also having no absorption in the visible region. After Au-Ag@AgBr particles (20 mg) were added to 10 µM 2-naphthol aqueous solution (20 mL, pH 6.73) and stirred in the dark for 30 min, time evolution for the degradation was traced under visible-light irradiation (λ > 400 nm) at 298 K (Fig. 2a). The light intensity was adjusted so that the intensity integrated from 420 to 485 nm was 4.0 mW cm −2 using an actinometer sensitive to the wavelength range. The degradation of 2-naphthol proceeds with irradiation, while its concentration is almost invariant in the dark. The photocatalytic activity of Au-Ag@AgBr strongly depends  www.nature.com/scientificreports/ on the alloy composition. Strikingly, the 2-naphthol disappears within only 15 min at y = 0.012 mol%. Several intermediates containing 2,2′-dihydroxy-1,1′-binaphthyl were generated during the irradiation to be finally oxidized to CO 2 in 68% yield after 24 h irradiation. In this manner, the Au-Ag@AgBr photocatalyst has a strong oxidation power to completely mineralize 2-naphthol by long-time irradiation. To check the catalyst stability, the photocatalytic degradation of 2-naphthol for 15 min was repeated 5 times using Au-Ag@AgBr (y = 0.012 mol%) (Supplemental Material Fig. S4). The high degradation yield is maintained after 5-times repetition of the reaction. The photocatalytic activity is almost maintained within an experimental error. Further, ion chromatography and ICP measurements confirmed that no leaching of Ag + and Br − ions from the catalysts to the reaction solution occurs. The initial rate of reaction (v 0 ) in each system was calculated from the 2-naphthol concentration at the first sampling time (15 or 30 min). The plot of v 0 vs. log (y/mol%) shows a volcano-shaped curve with a maximum at y = 0.012 mol% (Fig. 2b). The highest v 0 value is greater than that for non-dose sample (y = 0) by a factor of ~ 3, while further increase in y rapidly decreases the photocatalytic activity.
Action mechanism of Au-Ag@AgBr. AgBr is a semiconductor with the indirect band gap of 2.6 eV 19 .
In the absorption spectrum of Au-Ag@AgBr (y = 0.012 mol%) (Fig. 3a), the strong absorption band due to the interband transition rises at λ ≈ 480 nm, and the broad absorption around 600 nm results from the LSPR of Au-Ag alloy NPs. To gain information about the origin for the Au-Ag alloying effect, the action spectrum analysis was performed for the photocatalytic degradation of 2-naphthol using LEDs with different emission wavelengths. The action spectrum of the apparent quantum yield or external quantum yield (ϕ ex ) bears good resemblance to the absorption spectrum of AgBr with no clear peak corresponding to the LSPR of the Au-Ag NPs (Fig. 3a). This result excludes the hot-electron transfer mechanism from the major route in this reaction because in the case, the action spectrum should have a peak corresponding to the LSPR absorption 20 . The reaction was further carried out under excitation by light with λ ex (1) = 420 ± 40 nm or/and λ ex (2) = 640 ± 40 nm using two LEDs (Fig. 3b). Irradiation of Au-Ag@AgBr at λ ex (1) and λ ex (2) almost selectively excites the band gap of AgBr and the LSPR of Au-Ag NPs, respectively. The comparison of the v 0 values for the excitation at λ ex (1), λ ex (2), and λ ex (1) + λ ex (2) indicates that the excitation at λ ex (1) causes rapid degradation of 2-naphthol, while the reaction is sluggish under the excitation at λ ex (2) (Fig. 3c). Simultaneous excitation (λ ex (1) + λ ex (2)) greatly enhances the reaction, i.e., the v 0 (λ ex (1) + λ ex (2)) is much larger than the sum of v 0 (λ ex (1)) and v 0 (λ ex (2)). A similar synergetic effect was previously observed in the Ag NP-deposited AgI system 21 . Further, λ ex (2) was changed from 470 to 850 nm with the λ ex (1) fixed at 420 ± 40 nm. Plots of the enhancing ratio defined as v 0 (λ ex (1) + λ ex (2))/v 0 (λ ex (1)) vs. λ ex (2) shows that the profile is good resemblance to the LSPR absorption of the Au-Ag NPs (Fig. 3d). Evidently, the Au-Ag@AgBr photocatalyst is driven by the band-gap excitation of AgBr with the activity boosted by the LSPR of incorporated Au-Ag alloy NPs, i.e., Au-Ag@AgBr works as a LEFEtype plasmonic photocatalyst 16 .
The quality factor (Q-factor) defined as the ratio of the peak energy (E res ) to the full width at half maximum (Γ) of the LSPR band (Eq. 1) expresses the enhancement of the oscillation amplitude of an oscillating system with respect to the driving amplitude 22 .
For example, the intensity of surface enhanced Raman scattering is proportional to Q 4 23-25 , and thus, the Q-factor can be an experimental quantitative indicator for the LEFE-type plasmonic photocatalyst. The absorption spectra of Au-Ag alloy colloids with varying y were measured by selectively removing the AgBr matrix, and the Q-factor was calculated from Eq. (1) from each E res and Γ. Interestingly, the plot of v 0 vs. Q-factor in the Au-Ag@AgBr systems with varying y exhibits good positive correlation between them (Fig. 4a). Thus, the unique (1) Q-factor = E res / Ŵ  The plot of ln v 0 vs ln I in the same reaction as Fig. 3a where I is the intensity of incident light. www.nature.com/scientificreports/ y-dependence of the photocatalytic activity (Fig. 2b) can be explained in terms of the LEFE effect of the Au-Ag alloy NPs. However, the origin for the increase in Q-factor with a slight dose of Au into Ag NP remains unclear since simulations for Au x -Ag 1−x alloy NPs at 0 ≤ x ≤ 1 by finite-difference time-domain method indicated that the increase in x decreases the LEFE factor with the LSPR peak wavelength redshifted [26][27][28][29][30] . Based on these results, we discuss the action mechanism of Au-Ag@AgBr in the photocatalytic degradation of 2-naphthol (Scheme 2). Visible light (λ > 400 nm) excites the electrons in the valence band (vb) of AgBr to the conduction band (cb) simultaneously with the LSPR of Au-Ag alloy NPs. The excited electrons in the cb of AgBr possess a high energy of − 3.40 eV vs. vacuum 19 to cause oxygen reduction reaction with a redox potential of E(O 2 /O 2 − ) = − 4.16 V vs. vacuum at pH 6.73 31 . The remaining holes in the vb and superoxide radicals can oxidize the adsorbed 2-naphthol finally to CO 2 . The LSPR-induced LEFE effect of Au-Ag alloy NPs can enhance the photoexcitation of AgBr and the charge separation to boost the photocatalytic activity. A previous study on hollow AgI-Ag plasmonic photocatalyst suggests that its high visible-light activity for Rhodamine B degradation results from the increase in the rate of electron-hole pair generation in AgI by the amplified near-field of Ag NP-LSPR 32 . For this scheme to effectively work, significant overlap between the interband transition band of semiconductor and the LSPR of metal NPs is necessary 33 . However, in the Au-Ag@AgBr system, the spectral overlap between them is small, and thus, the charge separation enhancement effect would mainly contribute to the increase in the photocatalytic activity.
In the photocatalytic degradation of 2-naphthol, the application of the steady-state approximation to the excited electrons and holes yields Eq. (2) relating ϕ ex and incident light intensity I under the conditions of ϕ ex < < α (light absorption by photocatalyst) (see Supplemental Material Method). In this case, αI expresses the generating rate of electron-hole pairs.
where k rec is the rate constant of recombination, k red is the rate constant of oxygen reduction reaction, and k ox is the rate constant for 2-naphthol oxidation.
Linear relation can be expected between 1/ϕ ex and I 1/2 , and the smaller slope (or smaller ratio of k rec /k red k ox [2naphthol ad ][O 2ad ]α) indicates the greater charge separation efficiency in the system. We have recently reported that CuO cluster-surface modified BiVO 4 (CuO/BiVO 4 ) semiconductor photocatalyst 34 is also active for the degradation of 2-naphthol. As predicted, the plots of ϕ ex vs. I 1/2 for the Au-Ag@AgBr, and CuO/BiVO 4 systems provide straight lines with the slopes of 1.4 × 10 4 and 3.5 × 10 5 einstein −1/2 s 1/2 , respectively (Fig. 4b). The much smaller slope in the Au-Ag@AgBr system further supports the conclusion that the Au-Ag NPs in AgBr greatly enhance the charge separation.

Conclusions
The solid-phase photochemical method yields Au-Ag alloy NPs with a sharp size distribution and varying composition in AgBr crystals (Au-Ag@AgBr). A slight amount of Au-dosing gives rise to a drastic increase in the photocatalytic activity for the degradation of 2-naphthol used as a model water pollutant. Clear positive correlation between the photocatalytic activity and the LSPR-quality factor of Au-Ag NPs indicates that the www.nature.com/scientificreports/ striking Au-Ag alloy effect is mainly induced by the charge separation enhancement due to the intense local electric field generated at the interface between Au-Ag NP and AgBr. These conclusions would also be valid for other silver halide-based systems, underpinning their further development as the plasmonic photocatalysts for the solar-to-fuel conversion and environmental purification.

Methods
Catalyst preparation and characterization. The catalysts were prepared by the solid-phase photochemical method recently developed 14 . KBr (12 mmol) and HAuCl 4 was dissolved into distilled water (80 mL).
To the solution, AgNO 3 aqueous solution (0.1 M, 40 mL) was dropwise slowly by means of perista pump with 0.5 mL min −1 . After stirring at room temperature for 1 h, the generating particles were collected by centrifugation. The sample was washed with distilled water several times, and dried in vacuo to form AgBr:Au + . The particles (0.1 g) were added to MeOH (20 mL), and re-dispersed by ultrasonication. UV-light (λ = 365 nm, the light intensity integrated from 310 to 420 nm (I 310-420 ) = 4.0 mW cm −2 ) was irradiated by using LED at room temperature for 15 min. The resulting particles were washed with distilled water several times, and dried in vacuo to yield Au-Ag@AgBr. The Au amount was determined by inductively coupled plasma spectroscopy (ICPS-7000, Shimadzu). The y value was varied in the range at 0 ≤ y ≤ 0.159 mol% by changing the concentration of Received: 16 September 2020; Accepted: 5 November 2020