Design of Au@Ag/BiOCl–OV photocatalyst and its application in selective alcohol oxidation driven by plasmonic carriers using O2 as the oxidant

Huiqin An *a, Congying Deng b, Yang Sun a, Zhaotao Lv b, Lifang Cao a, Shunyuan Xiao a, Lizhi Zhao c and Zhen Yin *d
aState Key Laboratory of Separation Membranes and Membrane Processes & School of Chemistry and Chemical Engineering, Tiangong University, Tianjin 300387, China. E-mail:
bSchool of Environmental Science and Engineering & State Key Laboratory of Separation Membranes and Membrane Processes, Tiangong University, Tianjin 300387, China
cSchool of Materials Science and Engineering, Tiangong University, Tianjin 300387, China
dCollege of Chemical Engineering and Materials Science, Tianjin University of Science and Technology, Tianjin 300457, China. E-mail:

Received 25th August 2020 , Accepted 16th September 2020

First published on 17th September 2020

The hot holes produced by plasmonic noble metals have a mild oxidizing ability and provide an ideal alternative for photocatalytic selective oxidation. In this work, BiOCl with oxygen vacancies (BiOCl–OV) was photosensitized to the visible spectrum using plasmonic Au@Ag nanoparticles (NPs) to study photocatalytic selective alcohol oxidation using O2 as the oxidant. In the Au@Ag/BiOCl–OV system, the Au@Ag NPs consisting of a plasmonic Au core and covered by a very thin Ag shell were distributed on BiOCl spheres with the assistance of 3-aminopropyl-triethoxysilane (APTES). The combination of Au@Ag NPs and BiOCl–OV into one system endowed it with remarkable advantages in photocatalytic selective oxidation. Firstly, strong O2-adsorption capacity was achieved on the BiOCl–OV surface due to the presence of oxygen vacancies that serve as active sites. Secondly, the electron–hole generation capability is enhanced due to the extended optical absorption properties and amplified electromagnetic (EM) field effect that originated from the surface plasmon resonance (SPR) coupling effect of Au and Ag. Thirdly, the enhanced separation efficiency of hot electrons and holes was achieved through the trapping of hot electrons by the oxygen vacancies. The aforesaid, resulted in the remarkable enhancement of Au@Ag/BiOCl–OV as a photocatalyst in the selective oxidation of benzyl alcohol to benzaldehyde. We believe that the present strategy based on the plasmonic effect would be a significant contribution in the design and preparation of efficient photocatalysts for the selective oxidation.

1. Introduction

Aromatic aldehydes, one of the important fine chemical intermediates, possess intriguing application potential in various fields, such as pharmaceuticals, perfumes, and beverage industries.1,2 Traditionally, aromatic aldehydes are synthesized via the selective oxidation of aromatic alcohols at high temperature and high pressure using stoichiometric oxygen donors (chromate and permanganate) as oxidants, which not only have toxicity or corrosivity, but also produce large number of harmful byproducts accompanied by energy consumption during the reaction.1–3 The aforementioned are environment unfriendly from the standpoint of green and sustainable chemistry. Therefore, the development of an efficient and green method for the selective oxidation of aromatic alcohols to aromatic aldehydes under mild conditions is of importance from the viewpoint of energy conservation and environmental protection.

Solar energy has great potential as a green, abundant and renewable energy source.4 Since a breakthrough on the photocatalytic water splitting over TiO2 electrodes was reported by Fujishima and Honda,5 the semiconductor photocatalysis technology is considered an ideal alternative for solar-energy storage and utilization and has subsequently aroused great research interest worldwide because of the mild and green reaction properties.6–9 All of these have made it a highly popular research topic in various photocatalysis fields, such as photocatalytic H2 evolution,6 CO2 reduction,7 pollutant degradation,8 and synthesis of chemicals.9 On the other hand, O2 is a benign and abundant oxidant that is readily available in the natural environment. Moreover, the produced byproduct of O2 oxidation is clear water, which is non-toxic and environment friendly in comparison with other stoichiometric oxygen donors. Therefore, the photocatalysis technique when employed with O2 as the oxidant offers a sustainable avenue for the green synthesis of aromatic aldehydes through the selective oxidation of aromatic alcohols under mild conditions. While substantial efforts have been devoted towards selective photocatalytic oxidation using O2,10–12 improving its efficiency remains a huge challenge, and is mainly limited by two drawbacks. One is the strong oxidizing ability of the photoinduced holes produced by the semiconductor photocatalysts, which usually leads to thorough mineralization of the organics.13,14 The other is the weak adsorption of O2 on the photocatalysts.15 It has been reported that O2 can act as a reagent or an electron acceptor during the selective oxidation process,15–17 and thus the O2-adsorbing capacity of the photocatalyst is essential for activating the catalytic reactions. Therefore, the rationale behind the design and construction of a photocatalytic material with strong O2-adsorbing capacity without producing strong oxidizing holes is the key factor.

Based on the first issue, exploiting photocatalysts with mild oxidation properties and efficient solar-energy-harvesting ability is an overarching concern in selective photocatalytic oxidation. In the past decades, accompanied by the booming development of localized SPR of noble metals, NPs of plasmonic metals, such as Au and Ag, excel in the transformation of solar energy to chemical energy due to their strong light-absorption characteristics.18,19 More importantly, the much milder oxidizing ability of their hot holes in comparison with those of semiconductors provide a feasible pathway for selective oxidation using the photocatalysis technology.15,20,21 SPR features collective oscillation in the noble-metal nanocrystals as a result of the interaction with light, accompanied by the production of strong EM fields localized around the particles.22 Generally, the SPR of a noble metal is closely related to its composition, size, and morphology.23 For example, the SPR peak of spherical Au is located at about 500 nm, and the SPR peak of spherical Ag usually lies at about 400 nm.24 Compared with the limited light absorption of single Au or Ag used in our previous report, extended light absorption across the visible-light region was obtained through the construction of an elaborate Au@Ag core–shell structure by the SPR coupling of Au and Ag, in which the core of Au NP was coated by a very thin Ag shell.24 Moreover, a tunable absorption band over a broad range of visible light was achieved through the fine control of Au size and Ag shell thickness. In addition, the SPR coupling effect further amplifies the EM energy around NPs24–26 and therefore, generates various near-field enhancement effects and is responsible for their superior plasmonic catalytic activity. However, the hot electrons and holes of plasmonic metals suffer from rapid decay,21 which seriously limits their applications. The integration of plasmonic metals with semiconductors is an effective strategy to separate the hot carriers by transferring the hot electrons from the plasmonic metal to the semiconductor.19,27 Though a lot of work has been done towards the construction of a “plasmon-semiconductor” photocatalyst, a semiconductor with strong O2 adsorption ability and non-excitation nature under visible light is pivotal for selective photocatalytic oxidation by employing O2 as the oxidant.

Bismuth oxychloride (BiOCl), a typical V–VII ternary semiconductor, possesses a characteristic layered structure with a self-built internal static electric field along the [001] direction,28 which greatly benefits the separation of photoinduced carries after the integration of a plasmonic metal with BiOCl. The bandgap of BiOCl is usually above 3.2 eV,29 which determines the effectiveness of this semiconductor photocatalyst under UV light and that it cannot produce strong oxidizing holes under visible-light illumination. Moreover, oxygen vacancies on BiOCl can be easily obtained by simple light illumination treatment.28 It has been reported that supports with defects, such as oxygen vacancies, are more inclined to adsorbing O2 compared to those with no defects.15 Therefore, BiOCl with oxygen vacancies is an ideal candidate for integration with plasmonic metals to form a new type of photocatalysts for selective photocatalytic oxidation.

Inspired by the above results, Au@Ag/BiOCl–OV was designed and constructed to study photocatalytic selective alcohol oxidation by using O2 as the oxidant in this paper; the Au@Ag core–shell-structured plasmonic NPs consisted of a plasmonic Au core covered by a very thin Ag shell and were uniformly distributed on the BiOCl–OV microspheres. Combining the advantages of BiOCl–OV and Au@Ag in one system, the prepared Au@Ag/BiOCl–OV catalyst possessed strong O2-adsorption capacity, extended optical absorption properties, enhanced electron–hole generation ability and improved separation efficiency in one system, all of which endowed it with remarkable activity in photocatalytic selective oxidation.

2. Experimental methods

2.1. Preparation of colloidal Au@Ag NPs

The Au NPs were synthesized by the citrate reduction method reported in our previous work.24 The Au@Ag NPs were synthesized by a seed-mediated approach with some modifications.24 Typically, 100 μL of an AgNO3 solution (0.032 M) was added to 50 mL of sodium citrate solution (0.0031 M) under stirring. Then, 5 mL of Au NPs was injected into the above solution quickly. Subsequently, 10 mL of ascorbic acid solution (0.0114 M) was added dropwise in about 5 min, and the suspension was stirred for another 1 h. The Ag shell thickness in Au@Ag NPs was adjusted by changing the AgNO3 amount, and three types of Au@Ag NPs with different Ag thicknesses were prepared and named as Au@Ag-1, Au@Ag-2 and Au@Ag-3, respectively. The detailed preparation data are shown in Table S1. The photos of colloidal Au NPs and the series of Au@Ag NPs are displayed in Fig. S1.

2.2. Preparation of Au@Ag/BiOCl–OV

BiOCl–OV was synthesized by the solvothermal method.15 In a typical procedure for Au@Ag/BiOCl–OV preparation, 0.2 g of BiOCl–OV was dispersed into 50 mL of diethylene glycol under sonication for 0.5 h. Then, 1 mL of APTES was added to the above solution and kept at 60 °C for 4 h under stirring. After washing and centrifugation, the obtained APTES-modified BiOCl–OV was dispersed in 200 mL of water, and then, 260 mL of Au@Ag-1 was added to the above suspension. After stirring for 2 h, Au@Ag/BiOCl–OV was obtained by centrifugation and drying. The Au@Ag content in Au@Ag/BiOCl–OV was 2.36 wt%. Au/BiOCl–OV and Ag/BiOCl–OV were prepared through similar procedures except that Au@Ag-1 was replaced by Au NPs and Ag NPs, respectively.

In addition, Au@Ag/BiOCl was also prepared by the same procedure except that BiOCl–OV was replaced by the defect-free BiOCl, which was obtained by calcination of BiOCl–OV in an O2 atmosphere at 300 °C for 4 h.

2.3. Characterization

The crystallographic phases of the products were tested by X-ray diffraction using a Rigaku D/Max-2500 V diffractometer. The morphology of the products was investigated using a high-resolution transmission electron microscope (HRTEM, TecnaiG2 F20) equipped with a high-angle annular dark-field (STEM-HAADF) detector and energy-dispersive X-ray spectroscopy (EDX). X-ray photoelectron spectroscopy (XPS) was performed on a K-alpha spectrometer. The optical properties were measured on a UV/vis spectrophotometer (2700). The photoluminescence spectra were obtained using a fluorospectro photometer (PL, RF-6000, Shimadzu). The low-temperature EPR properties were tested with a JES-FA200 EPR spectrometer.

The photoelectrochemical properties were tested on a CHI 760E electrochemical analyzer. In detail, 5 mg of the sample was added into 1 mL of ethanol with ultrasound. Then, the colloidal sample was dropped on a glass substrate and used as the working electrode after vacuum-drying. Platinum and saturated calomel were used as the opposite electrode and reference electrode, respectively. The electrolyte was a 0.5 mol L−1 Na2SO4 solution.

2.4. Photocatalytic oxidation of benzyl alcohol to benzaldehyde

50 mg of the catalyst was suspended in 10 mL of acetonitrile containing 0.1 mmol benzyl alcohol, and the mixture was bubbled with O2 for 0.5 h, following which this reactor was equipped with 1 atmosphere of oxygen. After stirring for 1 h, the suspension was subsequently irradiated by a 300 W xenon lamp (PLS-SXE300). At regular intervals, 2 mL of the suspension was extracted and centrifuged, and the concentrations of alcohol and aldehyde were determined by an Agilent 6890 gas chromatograph. The conversion percentage of benzyl alcohol and the selectivity for benzaldehyde were calculated using the following equations:
Conversion (%) = [(C0Cr)/C0] × 100% Selectivity (%) = [Ct/(C0Cr)] × 100%
where C0 is the initial concentration of benzyl alcohol, and Cr and Ct are the concentrations of benzyl alcohol and benzaldehyde at regular intervals, respectively.

3. Results and discussion

The morphologies of Au NPs and the series of Au@Ag NPs are displayed in Fig. 1. The Au NPs had a diameter of ∼15 nm and were dispersed uniformly without any aggregation (Fig. S2). When the Ag shell was grown on the Au core by the seed-mediated method, a clear contrast between the two sequential components could be detected in the Au@Ag core–shell configuration (Fig. 1A and B). Moreover, the thickness of the Ag shell was adjusted by modulating the AgNO3 amount (Fig. 1C–F). The STEM-HAADF image and EDX maps (Fig. 1G) were used to further confirm the distribution of Au and Ag and the core–shell structure, and the red color of the Ag shell was uniformly visible around the green color of the Au core, revealing a core–shell configuration. Due to the plasmonic coupling effect of Au and Ag, a tunable absorption band over a broad range of visible light was achieved due to the construction of an elaborate Au@Ag core–shell structure (Fig. 1H). In contrast to the single SPR peak of Au at 521 nm for Au NPs and that of Ag at 390 nm for Ag NPs, Au@Ag-1 displayed two typical SPR peaks at 500 and 382 nm, originating from the Au core and Ag shell, respectively. Moreover, the SPR peak intensities of Au decreased or disappeared on increasing the thickness of the Ag shell, suggesting that the Au core was partially or completely shielded by the Ag shell (Au@Ag-2 and Au@Ag-3). Considering the efficient light-harvesting ability of Au@Ag NPs, Au@Ag-1 with a broader light-absorption range was selected to be deposited on BiOCl–OV.
image file: d0ce01246g-f1.tif
Fig. 1 TEM and HRTEM images of (A and B) Au@Ag-1, (C and D) Au@Ag-2 and (E and F) Au@Ag-3, respectively; (G) STEM-HAADF images of Au@Ag-1; (H) UV-vis spectra of Au NPs, Ag NPs and the series of Au@Ag NPs.

Fig. 2A and B are the SEM and TEM images of BiOCl–OV, respectively. Both showed a microsphere structure that was self-assembled by the random adhesion of the BiOCl nanosheet. This characteristic superstructure does not only make it a popular support for noble metal deposition, but also endows it with excellent light-utilization ability due to the multiple reflections of the BiOCl nanosheet. When Au@Ag NPs were loaded on BiOCl–OV, the overall morphology of Au@Ag/BiOCl–OV was similar to that of BiOCl–OV (Fig. 2C), and many Au@Ag NPs were distributed on the surface of BiOCl–OV microspheres (Fig. 2D and E). From the HRTEM image (Fig. 2F), it can be clearly confirmed that the Au@Ag NPs are anchored on BiOCl–OV successfully.

image file: d0ce01246g-f2.tif
Fig. 2 SEM (A) and TEM (B) images of BiOCl–OV, TEM (C and D) and HRTEM (E and F) images of Au@Ag/BiOCl–OV.

Fig. 3 shows the XRD patterns of BiOCl–OV and Au@Ag/BiOCl–OV. The diffraction peaks located at about 11.79°, 25.88°, 32.62°, 33.41° and 40.92° correspond to the (001), (101), (110), (102) and (112) crystal planes of BiOCl (No. 73-2060), respectively. In addition, some additional peaks assigned to the (111) crystal plane of Au and the (200) crystal plane of Ag were detected for Au@Ag/BiOCl–OV, which are seen clearly in the enlarged view, indicating the successful preparation of Au@Ag/BiOCl–OV. The XPS spectra further confirmed this result (Fig. 4A and B). In Fig. 4, two peaks at 367.6 and 373.5 eV corresponding to Ag 3d5/2 and Ag 3d3/2 of metallic Ag and two peaks at 83.5 and 87.2 eV ascribed to Au 4f7/2 and Au 4f5/2 of metallic Au30 were detected in Au@Ag/BiOCl–OV.

image file: d0ce01246g-f3.tif
Fig. 3 XRD patterns of BiOCl–OV and Au@Ag/BiOCl–OV (the inset is the magnified view of the selected region).

image file: d0ce01246g-f4.tif
Fig. 4 XPS spectra of (A) Ag 3d and (B) Au 4f in Au@Ag/BiOCl–OV.

Low-temperature EPR measurement was used to verify the presence of oxygen vacancies on BiOCl–OV, and the result is exhibited in Fig. 5. Obviously, a characteristic signal attributed to oxygen vacancies could be observed in the BiOCl–OV and Au@Ag/BiOCl–OV spectra,15 whereas no signal was detected in the BiOCl and Au@Ag/BiOCl spectra. Oxygen vacancy, a popular defect, plays a key role in photocatalytic selective oxidation. It did not only improve the O2 adsorption efficiency by providing active sites, but also trapped and separated the hot electrons of Au@Ag NPs since its localized electronic states were below the conduction band of BiOCl.15–17 The presence of localized electronic states for oxygen vacancies was proven by the UV-vis spectra, and the light absorption properties of the prepared samples are exhibited in Fig. 6. Different from the absorption edge of BiOCl was at 377 nm in the UV region, BiOCl–OV exhibited an exponentially attenuated tail in the visible region, corresponding to the oxygen vacancy absorption.15 When the Au@Ag NPs were deposited on BiOCl–OV, the obtained Au@Ag/BiOCl–OV integrated the light absorption properties of BiOCl–OV and Au@Ag in one system, which was confirmed by the existence of a long absorption tail and the SPR coupling absorption of Au and Ag. In order to see the SPR coupling peak of Au and Ag in the Au@Ag/BiOCl–OV system clearly, the magnified view of the Au@Ag SPR peak is displayed as an inset in Fig. 6. It could be seen that Au@Ag/BiOCl–OV exhibited an obvious SPR coupling effect of Au and Ag, which led to light absorption in a broader visible region compared with BiOCl–OV, indicating high light-utilization efficiency.

image file: d0ce01246g-f5.tif
Fig. 5 Low-temperature EPR spectra of the obtained catalysts.

image file: d0ce01246g-f6.tif
Fig. 6 UV-vis absorption spectra of the obtained catalysts (the inset is an enlarged view of the SPR peaks).

The photocatalytic oxidation of benzyl alcohol to benzaldehyde was used as a probe reaction to estimate the performance of the prepared samples, and the results are shown in Fig. 7. The photocatalytic activities of Au/BiOCl–OV and Ag/BiOCl–OV are also listed in Fig. 7 for comparison. For BiOCl–OV, the conversion was 40.9%, and its selectivity was 76% due to the mineralization of the organics. However, the conversion rates over Au/BiOCl–OV and Ag/BiOCl–OV increased to 73% and 61%, and their selectivity were at 88% and 84%, respectively, which was ascribed to the introduction of plasmonic Au or Ag on BiOCl–OV. When Au@Ag NPs were deposited on BiOCl–OV, its conversion and selectivity reached the highest compared with those of the other samples owing to the SPR coupling effect of Au and Ag, and its values of conversion and selectivity were 92% and 99%, respectively. Hence, the integration of BiOCl–OV and plasmonic Au@Ag in one system offers a promising opportunity to accomplish high-efficiency selective oxidation by photocatalysis.

image file: d0ce01246g-f7.tif
Fig. 7 Photocatalytic oxidation of benzyl alcohol to benzaldehyde over BiOCl–OV, Au/BiOCl–OV, Ag/BiOCl–OV, Au@Ag/BiOCl and Au@Ag/BiOCl–OV after 10 h of irradiation under O2 and N2 atmospheres.

The differences in the photocatalytic performances of the prepared samples were collectively attributed to three reasons. One is the O2 adsorption capacity of the catalysts, which was confirmed by comparison of the photocatalytic results of Au@Ag/BiOCl and Au@Ag/BiOCl–OV (Fig. 7). Obviously, the conversion and selectivity over Au@Ag/BiOCl were much lower than those over Au@Ag/BiOCl–OV due to the absence of oxygen vacancies. Actually, the oxygen vacancies present on BiOCl–OV impart a strong adsorption capacity for O2 by serving as active sites, which act as a reagent or an electron acceptor to produce ·O2 and facilitate the reaction process.15 In order to further clarify the role of O2 in the reaction, photocatalytic benzyl alcohol oxidation was also performed under an N2 atmosphere instead of an O2 atmosphere (Fig. 7). For Au@Ag/BiOCl–OV, both conversion and selectivity reduced greatly in the N2 atmosphere due to the absence of O2, which indicated that O2 takes part in the detailed process and plays a crucial part in the reaction process. The next reason is the hot electron–hole generation capability caused by the optical absorption properties and the EM field effect. It has been reported that the hot holes from plasmonic metals and ·O2 produced by the reaction of absorbed O2 and trapped hot electrons synergistically promote photocatalytic selective oxidation.15 Therefore, the amount of hot holes and electrons produced is pivotal to improving the photocatalytic efficiency. In Au/BiOCl–OV and Ag/BiOCl–OV, an extension of the visible light absorption range is achieved due to the SPR absorption of Au or Ag. It leads to the production of hot electrons and holes from plasmonic Au or Ag, which subsequently participates in the photocatalytic reaction. Furthermore, a broader SPR absorption has been achieved over Au@Ag/BiOCl–OV as a result of the plasmonic coupling effect of Au and Ag, which further produces amplified EM energy around the Au@Ag NPs.24–26 Enhanced light absorption and EM energy collectively lead to more hot electrons and holes under light illumination,18,23 and further improve the photocatalytic efficiency. The amplification of EM field intensity over Au@Ag NPs has been confirmed by FDTD methods in our previous report,24 in which a significant increase in the EM fields over Au@Ag NPs was observed in comparison with those around single Au or Ag NPs.

The third reason that affects the catalytic activity is the separation efficiency of hot electrons and holes that originate from the noble metal NPs. In the BiOCl–OV-based system, the oxygen vacancies on BiOCl–OV produce a localized electronic state below the conduction band of BiOCl, which was confirmed by the UV-vis result. The produced electronic state can trap the hot electrons of noble metal NPs and result in the efficient separation of hot electrons and holes.15 The detailed schematic diagram is shown in Fig. 8. In Au@Ag/BiOCl, due to the mismatched energy levels between the conduction band of BiOCl and the Fermi energy of the noble metal, the hot electrons produced by Au@Ag NPs are not transferred to the conduction band of BiOCl. However, in Au@Ag/BiOCl–OV, the hot electrons of Au@Ag NPs are trapped by the oxygen vacancies on BiOCl–OV because the localized electronic state of oxygen vacancies lies below the conduction band of BiOCl and matches with the Fermi energy of noble metal.15 Thus, the efficient separation of hot electrons and holes can be achieved over the Au@Ag/BiOCl–OV system. EPR measurement (Fig. 9) was used to confirm this viewpoint. Obvious ·O2 signals could be observed over Au@Ag/BiOCl–OV compared with those of Au@Ag/BiOCl, because of the reaction of absorbed O2 on the oxygen vacancies and the trapped electrons from noble metal NPs. In addition, the O2 adsorption capacity of BiOCl and BiOCl–OV was also verified by the EPR results of Au@Ag/BiOCl–OV and Au@Ag/BiOCl. As seen in Fig. 9, Au@Ag/BiOCl–OV exhibited much higher ·O2 signals than Au@Ag/BiOCl, indicating the stronger O2 adsorption capacity of Au@Ag/BiOCl–OV as the oxygen vacancies serve as active sites.

image file: d0ce01246g-f8.tif
Fig. 8 The transfer process of plasmon-induced hot electrons over the (A) Au@Ag/BiOCl and (B) Au@Ag/BiOCl–OV systems (CB and VB represent the conduction band and valence band, respectively).

image file: d0ce01246g-f9.tif
Fig. 9 EPR spectra of DMPO spin-trapping ·O2 under visible light.

The photocurrent measurements were used to collectively evaluate the amount of hot electron and holes produced and their separation efficiency in various samples (Fig. 10A). The highest photocurrent produced by Au@Ag/BiOCl–OV indicated the most effective production and separation of electrons and holes, which is consistent with the above discussion. This conclusion was further supported by the photoluminescence (Fig. 10B) spectra, in which Au@Ag/BiOCl–OV exhibited the lowest peak.

image file: d0ce01246g-f10.tif
Fig. 10 (A) Photocurrent–time and (B) photoluminescent tests of the prepared samples.

The photocatalytic performances of Au@Ag/BiOCl–OV with different Au@Ag loadings are listed in Fig. 11A. The strong influence of Au@Ag content on the conversion and selectivity can be seen in the figure. Too little Au@Ag NPs would not produce sufficient hot electrons and holes in the photocatalytic process; however, too much Au@Ag NPs might occupy the oxygen vacancies in BiOCl–OV and subsequently reduce the O2 absorption capacity of the BiOCl–OV surface.15 The corresponding photocurrent and photoluminescence results were used to further support the photocatalytic performance results (Fig. S3).

image file: d0ce01246g-f11.tif
Fig. 11 (A) Photocatalytic oxidation of benzyl alcohol to benzaldehyde over Au@Ag/BiOCl–OV with different Au@Ag contents after 10 h of irradiation; (B) the reusability performance of Au@Ag/BiOCl–OV after 6 h of irradiation.

The reusable stability of Au@Ag/BiOCl–OV was studied by the cycling experiment, and the results are shown in Fig. 11B. As seen, after four cycles, Au@Ag/BiOCl–OV retained a high conversion rate and selectivity. No changes in the element valence states were detected on comparing the XRD patterns before and after the cycling experiment (Fig. S4).

Based on the above results and the literature,15–17 a mechanism is proposed in Fig. 12. Firstly, Au@Ag NPs generate hot electrons and holes due to the SPR effect. The photoinduced hot electrons are trapped by O2 to produce ·O2 at the oxygen vacancies of BiOCl–OV, whereas the hot holes of noble metal adsorb the α-H of benzyl alcohol and ruptured it from benzyl alcohol. Then, the carbon-centered radical and ·O2 form an oxygen-bridged structure, from which the C–O bond of alcohol and the O–O bond of O2 are broken simultaneously, leading to the formation of benzaldehyde. Finally, the BiOCl-bound peroxide-bridge structure reacts with adsorbed H+ or H2O to generate H2O2 and complete the catalytic cycle.

image file: d0ce01246g-f12.tif
Fig. 12 The mechanism of photocatalytic oxidation of benzyl alcohol to benzaldehyde on Au@Ag/BiOCl–OV.

4. Conclusions

BiOCl–OV was photosensitized to the visible spectrum using plasmonic Au@Ag NPs to study photocatalytic selective alcohol oxidation using O2 as the oxidant. In the Au@Ag/BiOCl–OV system, the Au@Ag NPs consisting of a plasmonic Au core and covered by a very thin Ag shell were uniformly distributed on BiOCl spheres with the assistance of APTES. Au@Ag/BiOCl–OV possessed strong O2 adsorption capacity, extended optical absorption properties, enhanced electron–hole generation ability and improved separation efficiency in one system, which endowed it with remarkable activity in photocatalytic selective oxidation. The conversion and selectivity over Au@Ag/BiOCl–OV were 92% and 99%, respectively, which is much higher than those of BiOCl–OV.

Conflicts of interest

There are no conflicts of interest to declare.


This work is supported by the National Natural Science Foundation of China (No. 21872104 and 21501131) and Science and Technology Commission Foundation of Tianjin (18JCQNJC76200).


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Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ce01246g

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