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. 2021 Oct 18;6(42):28347–28355. doi: 10.1021/acsomega.1c04536

Strong Visible Light Absorption and Abundant Hotspots in Au-Decorated WO3 Nanobricks for Efficient SERS and Photocatalysis

Jing-Wen Zou †, Zhi-Di Li †, Hao-Sen Kang †, Wen-Qin Zhao †, Jing-Chuang Liu †, You-Long Chen †, Liang Ma †,*, Hua-Yi Hou †,*, Si-Jing Ding ‡,*
PMCID: PMC8552476  PMID: 34723031

Abstract

graphic file with name ao1c04536_0007.jpg

Metal/semiconductor hybrids show potential application in fields of surface-enhanced Raman spectroscopy (SERS) and photocatalysis due to their excellent light absorption, electric field, and charge-transfer properties. Herein, a WO3-Au metal/semiconductor hybrid, which was a WO3 nanobrick decorated with Au nanoparticles, was prepared via a facile hydrothermal method. The WO3-Au hybrids show excellent visible light absorption, strong plasmon coupling, high-performance SERS, and good photocatalytic activity. In particular, on sensing rhodamine B (RhB) under 532 nm excitation, bare WO3 nanobricks have a Raman enhancement factor of 2.0 × 106 and a limit of detection of 10–8 M due to the charger-transfer property and abundant oxygen vacancies. WO3-Au metal/semiconductor hybrids display a largely improved Raman enhancement factor compared to pure Au and WO3 components owing to the synergistic effect of electromagnetic enhancement and charge transfer. The Raman enhancement factor and limit of detection are further improved, reaching 5.3 × 108 and 10–12 M, respectively, on increasing the content of Au to 2.1 wt %, owing to the strong plasmon coupling between the Au nanoparticles. Additionally, the WO3-Au hybrids also exhibit excellent photocatalytic activity toward degradation of RhB under visible light irradiation. WO3-Au (2.1 wt %) possesses the fastest photocatalytic rate, which is 6.1 and 2.0 times that of pure WO3 nanobricks and commercial P25, respectively. The enhanced photocatalytic activity is attributed to the strong plasmon coupling and the efficient charge transfer between Au and WO3 nanobricks. The as-prepared materials show great potential in detecting and degrading pollutants in environmental treatment.

1. Introduction

Nowadays, the problem of pollution is becoming more and more serious along with the rapid development of industries. Many kinds of pollutants are being generated, such as atmospheric pollutants, water pollutants, and soil pollutants, which can mix with drinking water and food and harm the environment, greatly threatening the health of living things and human beings’ survival. Thus, effective treatment of pollutants is particularly important. As advanced technologies, surface-enhanced Raman spectroscopy (SERS) and photocatalysis show great potentials in detecting and degrading pollutants and have been widely used in environmental treatment since their discovery.1−5 In these two methods, Raman substrate materials and photocatalysts are essential prerequisites to achieve high-performance SERS and photocatalysis, respectively. Hence, the development of high-efficiency Raman substrates and photocatalysts becomes the most important issue.

In SERS measurements, the Raman signals of molecules and pollutants can be greatly amplified at a very low concentration in the presence of suitable substrate materials. Two well-known mechanisms for SERS are proposed, namely, electromagnetic mechanism and chemical mechanism.6−9 The electromagnetic mechanism is mainly based on the enhancement of the local electric field originating from plasmon excitation. Generally, plasmonic metals (such as Au, Ag, Cu) are the commonly used substrate materials to achieve high-performance SERS due to their tunable and large electromagnetic field.10−15 However, the high cost and low stability and biocompatibility of plasmonic metals limit their utilization in practical applications. The chemical mechanism is mostly supposed to be caused by the charge transfer between the molecule and the substrate. Semiconductor materials with excellent optical and electrical properties are regarded as Raman substrates enabling the charge-transfer mechanism. Various semiconductor materials, such as TiO2, WO3, and MoS2, have been developed for SERS-based applications due to their low cost and excellent stability and biocompatibility.16−22 However, the Raman enhancement factor and the limit of detection of semiconductor-based substrates are too low compared with those of plasmonic metals.

Semiconductor-based photocatalysis is a green technology that is widely applied in solar-light-driven pollutant treatment.23−25 In the past few years, large numbers of semiconductor materials have been used for photocatalytic decomposition of organic pollutants, while the real catalytic efficiency is quite low due to their poor visible light absorption and fast charge recombination rate. Besides being efficient SERS substrates, plasmonic metal nanocrystals with outstanding light absorption and strong electric field are also regarded as ideal antennas to achieve efficient photocatalysis.26−30 The incorporation of semiconductor with plasmonic metal nanocrystal can dramatically enhance photocatalytic activity, owing to the improved light absorption, accelerated charge separation, and efficient plasmon energy transfer. Likewise, metal/semiconductor hybrids have also been proven to generate giant SERS signals due to the synergistic effect of electromagnetic enhancement and charge transfer.31−36 There are several advantages when metal/semiconductor hybrids are used as SERS substrates. The charge-transfer efficiency is promoted due to the electron transfer between the plasmonic metal and the semiconductor. Meanwhile, the Raman enhancement factor is highly enhanced compared to the metal and semiconductor components, and it can be further enhanced by size control and morphological manipulation.37−40 Therefore, it is worth constructing metal/semiconductor hybrids with high SERS sensitivity and photocatalytic activity that can efficiently detect and degrade pollutants.

In the present work, WO3-Au metal/semiconductor hybrids were prepared and used as SERS substrates and photocatalysts, aiming to efficiently detect and degrade organic pollutants. Uniform WO3 nanobricks and Au-decorated WO3 hybrids were prepared through simple hydrothermal methods. The WO3 nanobricks show excellent SERS sensitivity on detecting rhodamine B (RhB) under a 532 nm laser excitation. The Raman enhancement factor of the WO3 semiconductor reaches 2.0 × 106 and the limit of detection reaches 10–8 M due to the charge-transfer property and abundant oxygen vacancies. Moreover, the SERS properties of the WO3-Au hybrids with different contents of Au were investigated. The Raman enhancement factor is largely improved owing to the synergistic effect of electromagnetic enhancement and charge transfer. The Raman enhancement factor and the limit of detection are further improved, reaching 5.3 × 108 and 10–12 M, respectively, on increasing the content of Au to 2.1 wt %, owing to the strong plasmon coupling between the adjacent Au nanoparticles. Additionally, the WO3-Au hybrids also exhibit excellent photocatalytic activity toward degradation of RhB under visible light irradiation. WO3-Au (2.1 wt %) possesses the fastest photocatalytic rate, which is 6.1 and 2.0 times that of pure WO3 nanobricks and commercial P25, respectively. Finally, the possible physical mechanism behind the enhanced photocatalysis is proposed based on the simulation and experimental results.

2. Results and Discussion

WO3 nanobricks were prepared by a one-step hydrothermal method, in which sodium tungstate was used as a precursor. The morphological characterization of WO3 nanobricks was performed using scanning electron microscopy (SEM). Figures 1a and S1 show the SEM images of WO3, indicating that WO3 displays a uniform brick morphology and a smooth surface. Au-decorated WO3 nanobricks were synthesized by growing Au nanoparticles on the WO3 nanobricks, and chloroauric acid and ascorbic acid were used as the precursor and the reductant, respectively. In Figure 1b, the SEM image of the Au-decorated WO3 nanobricks is shown. The Au nanoparticles were randomly deposited on WO3, and the brick morphology of WO3 was well-maintained. The content of Au is determined to be about 0.7 wt % by inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis. Transmission electron microscopy (TEM) was performed to further study morphology and the component of the WO3-Au hybrids, and the results are shown in Figure 1c–f. The black spots with high contrast on the low-contrast WO3 surface correspond to Au nanoparticles (see Figure 1c). The Au nanoparticles show a uniform flowerlike morphology and have an average size of 34 nm. High-resolution TEM (HRTEM) analysis is performed to detect the crystalline structure of the WO3-Au hybrids. In Figure 1d, the HRTEM image of a Au-decorated WO3 nanobrick is shown. Two different crystal lattices are clearly observed, indicating the possible presence of Au and WO3. Figure 1e,f displays further magnified HRTEM images of WO3 and Au. The WO3 nanobrick shows continuous and ordered lattice fringes with a lattice spacing of 0.38 nm (see Figure 1e), corresponding to the (002) plane of triclinic WO3. The spacing of the lattice fringe is 0.23 nm (see Figure 1f), which can be indexed to the (111) plane of Au. Figure S2 shows the energy-dispersive spectroscopy (EDS) spectrum of the WO3-Au hybrids, and the presence of Au, W, and O can be observed.

Figure 1.

Figure 1

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Morphology characterizations of WO3 and WO3-Au hybrids. SEM images of WO3 nanobricks (a) and WO3-Au (b). TEM (c) and HRTEM (d) images of WO3-Au hybrids. Magnified HRTEM images of WO3 (e) and Au (f).

Figure 2b shows the X-ray diffraction (XRD) patterns of WO3 and WO3-Au hybrids. The WO3 pattern matches well with the triclinic-phase WO3 (PDF # 20-1323). The peaks located at 23.1, 23.6, 24.1, 32.9, and 47.2° are ascribed to the (002), (020), (200), (202), and (004) planes of triclinic WO3 (PDF # 20-1324), respectively. For WO3-Au hybrids, the diffraction peaks located at 38.2, 44.6, 62.3, and 76.7° are indexed to the (111), (200), (220), and (311) planes of Au (PDF # 65-8601), respectively. Meanwhile, the diffraction peaks of the triclinic-phase WO3 are still observed, indicating the successful formation of hybrids. No characteristic peaks of any impurities are detected, suggesting high crystalline purity. X-ray photoelectron spectroscopy (XPS) analysis is further carried out to study the chemical state and the composition of WO3-Au hybrids. The XPS survey spectrum clearly suggests that the sample mainly consists of Au, W, and O (see Figure S3). The high-resolution XPS spectra of O 1s, Au 4f, and W 4f peaks are shown in Figure 2b,c. In Figure 2b, the W 4f core level peak shown splits into two peaks at 37.3 (W 4f7/2) and 35.2 eV (W 4f5/2), indicating the existence of W6+. Figure 2c shows the peaks of Au 4f5/2 (86.8 eV) and Au 4f 7/2 (83.1 eV), proving the existence of Au in the hybrids. Figure 2d shows the high-resolution XPS spectrum of O 1s peaks. The main peak located at 529.3 eV is assigned to O2– in the hybrid. Notably, an additional broad peak at 531.6 eV appears in the spectrum, which is attributed to the oxygen species chemisorbed at oxygen vacancies. This reveals that the oxygen vacancies are stabilized by the adsorbed oxygen species, demonstrating the abundant oxygen defects in WO3 nanobricks.

Figure 2.

Figure 2

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Component characterizations of WO3-Au hybrids. (a) XRD patterns of WO3 nanobricks and WO3-Au hybrids. High-resolution XPS spectra of W 4f (b), Au 4f (c), and O 1s (d).

The morphology of the WO3-Au semiconductor/metal hybrids was tuned by adding different amounts of chloroauric acid in the synthesis process. Figure 3a,b shows the SEM images of WO3-Au with 1.4 and 2.1 wt % Au, respectively. Noticeably, larger surface areas of the WO3 nanobrick are covered with Au nanoparticles. The distance between the Au particles is obviously shortened as the content of Au is increased. The optical properties of WO3-Au hybrids were further investigated. Figure 3c shows the Raman spectra of WO3 nanobricks and WO3-Au with different contents of Au. The characteristic peaks located at 275 and 328 cm–1 are ascribed to the bending vibration mode of O–W–O. The peaks at 712 and 810 cm–1 originate from the stretching vibration mode of W–O. Noticeably, both types of modes are improved in the presence of Au, owing to the plasmon-induced electromagnetic enhancement. The intensities of these two modes are further amplified by increasing the amount of Au. This may be caused by more electromagnetic hotspots being generated by strong plasmon coupling from the surface-deposited Au nanoparticles. Figure 3d shows the extinction spectra of WO3 and WO3-Au hybrids with different contents of Au. Pure WO3 nanobricks only have an extinction band covering the visible light region, which may be caused by the adjustment of the band gap in the presence of oxygen vacancies. In the presence of Au, an arresting plasmon resonance band at around 580 nm was found in the extinction spectrum of WO3-Au (0.7 wt %), demonstrating the improved visible light-harvesting ability compared with pure WO3. The plasmon peak red-shifts and broadens as the content of Au increases, revealing the prominent visible light-harvesting ability, laying the foundation for excellent SERS and photocatalytic applications.

Figure 3.

Figure 3

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SEM images of WO3-Au hybrids with 1.4 wt % (a) and 2.1 wt % (b) Au. Raman (c) and extinction spectra (d) of WO3 and WO3-Au with different contents of Au.

The SERS behaviors of these materials are tested using RhB as a target molecule under the excitation of a 532 nm laser. Figure 4a shows the Raman spectra of RhB with different concentrations absorbed on WO3 nanobricks. The Raman peak located at 613 cm–1 is attributed to the C–C–C ring in-plane vibration movement of the RhB molecule. The peaks located at 1360, 1512, and 1647 cm–1 are ascribable to C–C stretching modes. Decreasing the concentration of RhB is highly conducive for Raman signal recognition, and the main characteristic peaks (1647 cm–1) in each spectrum can be clearly identified. Even when the concentration of the RhB solution is decreased to 10–8 M, the SERS signal is still detectable, indicating the good detection limit of the WO3 nanobricks. To quantize the SERS performance, the enhancement factor (EF) is calculated using the formula

2.

where ISERS and IRS represent the intensity of SERS and normal Raman signals, respectively. NSERS and NRS refer to the average number of molecules within the laser spot excited by SERS and normal Raman spectroscopy, respectively. The value of NRS/NSERS was estimated with the ratio of the respective molecule concentrations according to previous studies.41,42 At the concentration of 10–8 M, the EF reaches 2.0 × 106 for the WO3 nanobricks calculated with the intensity of 1647 cm–1, compared with the normal Raman spectrum of pure RhB tested on a glass slide. The excellent SERS performance may be because of the charge transfer between RhB and the WO3 nanobricks. Meanwhile, the abundant oxygen vacancies of WO3, demonstrated by XPS analysis, can play an important role in improving surface activity and promoting the charge transfer,43−45 thereby further improving the SERS activity. Especially, the surface oxygen vacancies on WO3 can offer more active sites for the adsorption of RhB, thereby increasing the affinity for the adsorbent–adsorbate interaction, which further increases the SERS signals via the charge-transfer mechanism. In addition, the energy level of oxygen vacancies in the band gap of WO3 may adjust the energy match and optimize the photoinduced charge-transfer process between the semiconductor and RhB, thus improving the SERS performance.

Figure 4.

Figure 4

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SERS activities of WO3 and WO3-Au hybrids. (a) Raman spectra of RhB with different concentrations of the substrates deposited on WO3 nanobricks. (b) Raman spectra of RhB (10–6 M) absorbed on WO3 and WO3-Au hybrids with different contents of Au. (c) SERS intensity at the characteristic peak of 1647 cm–1 in the presence of WO3 and WO3-Au hybrids with different contents of Au. (d) Concentration-dependent SERS spectra for detection of RhB absorbed on WO3-Au (2.1 wt %). (e) Plot of the peak intensity at 1647 cm–1 as a function of concentration; the inset represents the linear relationship. (f) Calculated electric field distributions of WO3 and WO3-Au hybrids deposited with one, two, and three Au nanoparticles.

We further investigated the SERS performance of WO3-Au semiconductor/metal nanobricks. Figure 4b shows the Raman spectra of RhB (10–6 M) absorbed on Au nanoparticles and WO3-Au with different contents of Au, revealing that the WO3-Au hybrids show a much higher SERS signal than bare WO3 nanobricks. Particularly, the Raman intensity at 1647 cm–1 of the WO3-Au (0.7%) hybrids is 1.98 and 5.86 times that of Au nanoparticles and WO3 (see Figure 4c), respectively. The enhancement mechanism is attributed to the synergistic effect of electromagnetic enhancement and charge transfer between Au and WO3. Most interestingly, the SERS signal of the WO3-Au hybrids is very sensitive to the content of Au, which is greatly enhanced as the content of Au increases to 2.1 wt % (see Figure 4b). The Raman intensity at 1647 cm–1 of WO3-Au (2.1 wt %) is 10.4 and 3.51 times that of pure WO3 and Au nanoparticles, respectively (see Figure 4c). To evaluate the reproducibility, 15 different points on the as-prepared WO3-Au (2.1 wt %) were chosen for SERS measurements. Figure S4a shows the Raman spectra collected at 15 random points, which indicate the good stability of the SERS signals. The relative standard deviation (RSD) at 1647 cm–1 is about 5.6% (see Figure S4b), indicating excellent uniformity and high reproducibility of the as-prepared substrates. To evaluate the detection sensitivity of the WO3-Au hybrids, RhB solutions with different concentrations ranging from 10–6 to 10–12 M were used as the probe molecules. Figure 4d shows that the WO3-Au hybrids have excellent sensitivity in a wide concentration range. The lowest detectable limit reaches 10–12 M, and the maximum EF is calculated to be 5.3 × 108 (at 1647 cm–1). The variation curve based on the concentration of RhB and the Raman intensity at 1647 cm–1 is shown in Figure 4e. The logarithmic curve shows good linearity as the concentration changes from 10–6 to 10–12 M (the inset), indicating the high sensitivity and the wide quantitation range of the WO3-Au (2.1 wt %) hybrids. To further understand the physical mechanism behind the excellent SERS performance, the interfacial electric field of the WO3 and WO3-Au hybrids were calculated. Figure 4f shows the calculated electric field distributions of the WO3 and WO3-Au hybrids with different amounts of Au nanoparticles excited at 532 nm. Compared with the bare WO3 nanobrick, the electric field around the interface between Au and the WO3 substrate shows an obvious enhancement. This interfacial electromagnetic enhancement can not only amplify the SERS signals but also accelerate the charge transfer between Au and WO3, further enhancing the SERS sensitivity. The interfacial hotspots can be further increased by adding more Au nanoparticles. Moreover, a strong enhancement of the electromagnetic field around the interface of two adjacent Au nanoparticles is observed, resulting from the plasmon coupling of Au nanoparticles. This reveals that the high SERS activity of the WO3-Au hybrids with a large content of Au is caused by the more interfacial hotspots between Au and WO3 and the strong plasmon coupling-induced electric field enhancement. The more the Au nanoparticles deposited on WO3 nanobricks, the more and stronger the plasmon coupling-induced electromagnetic hotspots, thus endowing WO3-Au (2.1 wt %) with excellent SERS sensibility. In addition, on further increasing the mass ratio of Au to 2.9 wt %, the SERS signals are decreased compared with WO3-Au (2.1 wt %) (see Figure S5). This may be caused by the excessive aggregation of Au, which may weaken the plasmonic coupling.43

We thereby applied the WO3 and WO3-Au hybrids to photocatalytic degradation of the RhB dye under visible light irradiation. The photocatalytic activities were evaluated by monitoring the absorption intensity of the characteristic peak (552 nm) of RhB. Figure 5a shows the degradation curves in the presence of WO3, Au, commercial P25, and WO3-Au with different contents of Au. It can be seen that about 39.5% of the dyes were degraded by pure WO3 nanobricks, owing to the weak visible light absorption and the fast recombination rate of the photogenerated carriers. For WO3-Au (0.7 wt %) hybrids, about 75.6% of the dyes were reduced under the same conditions, and the photocatalytic rate reached 0.025 min–1 (see Figure 5b), which is 2.9 times that of pure WO3 nanobricks. Noticeably, the photocatalytic activity of WO3-Au hybrids is further improved by increasing the content of Au. WO3-Au (2.1 wt %) has the fastest photocatalytic rate, reaching 0.053 min–1, which is 2.1, 6.1, and 2.0 times that of WO3-Au (0.7 wt %), pure WO3 nanobricks, and P25, respectively, indicating its potential commercial application. Figure 5c displays the photocatalytic stability test of the WO3-Au (2.1 wt %) hybrids. The photocatalytic rate remains at about 90% after 30 h of the reaction, indicating good photocatalytic stability. The SEM image of the WO3-Au hybrids obtained after 30 h of the reaction is shown in Figure S6; the morphology changes little, demonstrating the prominent resistance to photocorrosion. To show the physical mechanism of the enhanced photocatalytic activity, the band structure and the possible catalytic reaction of the WO3-Au hybrids are proposed in Figure 5d. Under visible light irradiation, the plasmon resonance of Au nanoparticles can be excited, which can improve the light-harvesting ability of the WO3-Au hybrids. Meanwhile, the strong electric field of Au can enhance the band gap absorption and charge generation of WO3. Moreover, the charge transfer between Au and WO3 can speed up the carrier separation, thereby enhancing the photocatalytic performance. Upon light irradiation, electrons in the valence band (VB) of WO3 are excited to the conduction band (CB), and the same number of holes are generated in the VB. Driven by potential energy, the photogenerated electrons in the CB of WO3 can be transferred to Au, which has been reported in previous studies.46,47 In the photocatalytic processes, the electrons on Au can directly react with the adsorbed O2 on the surface of samples to produce O2–.29 Additionally, the photogenerated electrons on WO3 are transferred to Au to offer more electrons for the reduction reaction. Meanwhile, the holes generated on WO3 directly react with OH– to produce hydroxyl radicals,29 and then efficiently degrade RhB molecules. For the WO3-Au (2.1 wt %) hybrids, the strong plasmon coupling between Au nanoparticles occurs due to their shortened distance, which can result in wide spectral light absorption, strong electric field, and abundant hot electrons, thereby largely improving the photocatalytic activity. Additionally, it is found that the photocatalytic activity of WO3-Au (2.9 wt %) is decreased compared with WO3-Au (2.1 wt %) (see Figure S5). The excess Au nanoparticles on WO3 may act as recombination centers, which can lower the rate of charge separation, thereby weakening the photocatalytic performance.29

Figure 5.

Figure 5

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Photocatalytic activities of WO3 and WO3-Au hybrids. (a) Photodegradation curves of RhB in the presence of WO3, commercial P25, and WO3-Au hybrids with different amounts of Au. (b) Logarithm of the absorption at 550 nm vs reduction time irradiated at visible light. (c) Photocatalytic cycling stability of WO3-Au (2.1 wt %) hybrids. (d) Proposed photocatalytic mechanism of WO3-Au hybrids.

3. Conclusions

In summary, WO3 nanobricks and WO3-Au hybrids were prepared through a facile hydrothermal method, which show excellent SERS sensitivity and photocatalytic activity toward detection and degradation of RhB. The pure WO3 nanobricks have a SERS enhancement factor of 2.0 × 106 and a limit of detection of 10–8 M under a 532 nm laser extrication, due to the charger-transfer property and abundant oxygen vacancies. Owing to the synergistic effect of electromagnetic enhancement and charge transfer, WO3-Au hybrids display much higher SERS signals than the Au and WO3 components. The SERS enhancement can be further promoted by increasing the content of Au from 0.7 to 2.1 wt %. The maximum EF of WO3-Au (2.1 wt %) reaches 5.3 × 108 and the limit of detection is lowered to 10–12 M, caused by the strong plasmon coupling-induced electromagnetic field enhancement. Additionally, equipped with strong visible light absorption and electromagnetic fields, the WO3-Au hybrids also exhibit excellent photocatalytic activity toward degradation of RhB under visible light irradiation. WO3-Au (2.1 wt %) possesses the fastest photocatalytic rate, which is 6.1 and 2.0 times that of pure WO3 nanobricks and commercial P25, respectively. Our findings offer potential strategies for enhancing the SERS and photocatalytic performances, and the materials we prepared may be used in the fields of solar cells and water evaporation.

4. Experiment Section

4.1. Reagent

Sodium tungstate (99.5%), chloroauric acid (99.9%), l-ascorbic acid (99.7%), and hydrochloric acid (30%) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Deionized water with a resistivity of about 18.25 MΩ·cm was used as the solvent in all experiments.

4.2. Synthesis of WO3 and Au-Decorated WO3 Nanobricks

The WO3 nanobricks were synthesized by a simple hydrothermal method. Typically, 20 mL of sodium tungstate (0.01 M) and 6 mL of chloroauric acid were added to a reactor. The mixture solution was transferred to an oven and kept at 180 °C for 24 h. The final products were centrifuged at 5000 rpm for 5 min to remove impurities. To synthesize Au-decorated WO3 nanobricks, 0.01 mg of WO3 nanobricks and 2 mL of sodium iodide (0.05 M) were added to a 50 mL test tube. Then, 0.5 mL of chloroauric acid (10 mM) and 0.5 mL of l-ascorbic acid (0.1 M) were added. The mixture solution was transferred to an oven and kept at 70 °C for 4 h. The final products were centrifuged and dried at 60 °C to obtain WO3-Au powders.

4.3. SERS Measurements

RhB molecules were deposited on WO3 and WO3-Au hybrids as substrates. The substrates were prepared by adding 1 mL of RhB of a given concentration to 0.5 mL of WO3 and WO3-Au (0.5 mg mL–1). The mixture solution was stirred and kept in the dark for 2 h to ensure adequate absorption of RhB. Finally, 0.02 mL of the suspension was dropped onto a clean glass slide and dried at 70 °C for 2 h. Raman spectra were obtained on a Raman spectrometer under excitation at 532 nm (1 mW) for 10 s of acquisition time. Raman spectra from three different positions were obtained for each sample.

4.4. Photocatalytic Measurements

The photocatalytic performances of WO3 and WO3-Au hybrids were tested by the photodegradation of RhB under irradiation of a 300 W xenon lamp equipped with an ultraviolet cutoff filter (λ > 420 nm). Briefly, 10 mL of photocatalysts (0.2 mg mL–1) was added to 20 mL of RhB (10–5 M). The mixture solution was stirred continuously during irradiation, and samples were taken every 10 min for further tests. The concentration of RhB was calculated by measuring the extinction density at 552 nm.

4.5. Numerical Simulation

Commercial software (COMSOL Multiphysics) was used for the finite-element method (FEM) simulations. The refractive index of water is 1.33, and refractive indices of Au and WO3 were taken from refs (48, 49). Perfectly matched layers were used in the simulations. The detailed structures of WO3 and WO3-Au hybrids are taken from the SEM and TEM images.

4.6. Sample Characterization

The TEM images were obtained using a JEOL 2100 operated at 200 kV. HRTEM and EDS were performed using a JEOL 2010 FET microscope operated at a 200 kV accelerating voltage. SEM images were obtained using a FEG SEM Sirion 200 operated at an accelerating voltage of 25.0 kV. XPS spectra were measured using a Thermo Scientific ESCALAB 250Xi system. The extinction spectra were obtained using UV–vis–NIR spectrophotometry (TU1810, Beijing Pgeneral).

Acknowledgments

This research was funded by the National Natural Science Foundation of China (11904270) and Hubei Key Laboratory of Optical Information and Pattern Recognition (202010), Wuhan Institute of Technology.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c04536.

  • Low-magnification SEM image of WO3 nanobricks; EDS spectrum of Au-decorated WO3 hybrids; XPS survey of WO3-Au hybrids; measurements of SERS spectral reproducibility; SEM image, SERS, and photocatalytic performances of WO3-Au (2.1 wt %) hybrids; and SEM image of WO3-Au (2.1 wt %) after 30 h of photocatalytic reaction (PDF)

Author Contributions

§ J.-W.Z., Z.-D.L., and H.-S.K. contributed equally to this work.

The authors declare no competing financial interest.

Supplementary Material

ao1c04536_si_001.pdf (475.7KB, pdf)

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