Mechanochemically synthesized CuO/m-BiVO₄ composite with enhanced photoelectrochemical and photocatalytic properties

Abstract

The present work aims to prepare a CuO/monoclinic-BiVO₄ (m-BiVO₄) composite with large surface area for enhanced photocatalytic performance. Nano-sized m-BiVO₄ photocatalysts with a large BET surface area (10 m² g⁻¹) were first synthesized by a mechanochemical high energy ball milling approach. To further improve the photo-efficiency, 2–10 at% of a p-type CuO co-catalyst was introduced to the surface of BiVO₄ by impregnation to constitute the heterostructured composite. The physicochemical properties of the as-prepared composite were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, UV-vis diffuse reflectance spectroscopy, photoluminescence and surface photovoltage responses. The composite exhibited much higher photocurrents in photoelectrochemical (PEC) tests and faster photocatalytic degradation of rhodamine B upon visible light irradiation in comparison with pristine BiVO₄, appearing as a promising visible light active photocatalyst suitable for environmental remediation. The improved PEC and photocatalytic properties can be attributed to the simultaneous enhancement on photon absorption and charge separation. The band structure and possible charge transfer process of the CuO/BiVO₄ composite were also elucidated to explain the enhanced photo-activities.

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1. Introduction

Monoclinic bismuth vanadate (m-BiVO₄), with a narrow bandgap of 2.4–2.5 eV, low toxicity and excellent photocatalytic activity,¹ has attracted considerable attention as a promising visible light photocatalyst for environmental remediation,^2,3 artificial photosynthesis for H₂ generation⁴ and CO₂ conversion.⁵ Nevertheless, the practical applications of the pure m-BiVO₄ are hindered by the poor solar energy conversion efficiency below 10% as limited by its bandgap⁶ as well as the low quantum efficiency due to the fast charge recombination.⁷ Many attempts have therefore been made to improve the photocatalytic efficiency of m-BiVO₄.

The separation efficiency of photogenerated electron–hole pairs can be enhanced by increasing the specific surface area via size reduction or morphology control of m-BiVO₄. Nano-sized m-BiVO₄ with large BET surface area has therefore been prepared by modified hydrothermal method,⁸ reverse-microemulsion process,⁹ microwave-assisted approach¹⁰ and high energy ball milling approach.¹¹ In particular, the last mechanochemical process representing a simple and effective mean with relatively high yield to obtain fine particles with large surface area as compared to other complex approaches involving many steps or with a low throughput has recently been investigated by our group for the synthesis of m-BiVO₄ nanoparticles with BET surface area of ca. 10 m² g⁻¹. The good balance between crystallization and particle size distribution for m-BiVO₄ annealed at 400 °C led to its superior photodegradation rate of rhodamine B (RhB) under visible light irradiation.¹² Other than different synthesis approaches to achieve size reduction, distinctive morphologies including microspheres,¹³ sheets,¹⁴ plates,¹⁵ fibers¹⁶ and tubes¹⁷ have also been studied so as to increase the specific surface area as well as to prolong the optical path to enhance the charge separation.¹⁸

Coupling m-BiVO₄ with metals,^19,20 carbon materials²¹ or other semiconductors^2,22–25 to construct junctions is another important strategy to improve the photocatalytic efficiency of m-BiVO₄. Especially, with m-BiVO₄ as an n-type semiconductor, different p-type semiconductors with suitable band edge positions have been explored to fabricate p–n heterojunctions. Long et al. found that n-BiVO₄/p-Co₃O₄ composite with 0.8 wt% cobalt content significantly enhanced and stabilized the generation of photocurrent as compared to the bare BiVO₄.² Jiang et al. demonstrated that p-CuO as a co-catalyst on surface of n-type m-BiVO₄ exhibited enhanced photocatalytic degradation on methylene blue under visible light irradiation.²² Wang et al. showed that the assembly of p-type Cu₂O on the surface of the BiVO₄ nanocrystals improved the photocatalytic performance of individual BiVO₄ nanocrystals.²³ Our group previously investigated γ-Bi₂O₃/m-BiVO₄ core–shell p–n heterogeneous nanostructure with enhanced photocatalytic performance through a novel alkaline “etching” process.²⁴ More recently, BiOCl/BiVO₄ p–n heterojunction structure was prepared and the composite exhibited markedly improved efficiency for photodegradation of methyl orange in comparison with pure BiVO₄.²⁵ These p–n heterojunctions result in efficient separation of electron–hole pairs and fast interfacial charge transfer due to the presence of inertial electric field.¹⁸ On top of that, the region of visible light utilization is often extended simultaneously to significantly enhance the visible light photocatalytic activities.

Cupric oxide (CuO), a p-type semiconductor with bandgap of 1.70 eV, exhibits poor photocatalytic performance by itself due to its low charge-transfer rate. Yet, it was commonly employed as an excellent co-catalyst with other semiconductors including TiO₂,²⁶ ZnO,²⁷ and SnO₂ (ref. 28) to effectively improve the charge separation. Although there have been few works reporting the preparation of CuO/m-BiVO₄ composite in literature,^22,29–31 the obtained particles were usually gross (>100 nm) with small surface area, limiting the photocatalytic efficiency significantly. On top of that, direct experimental evidence to confirm the proposed enhancement of charge separation is also lacking in these studies. Therefore, the main objective of the present study is to prepare nano-sized CuO/m-BiVO₄ composite with large surface area as well as to experimentally elucidate the origin of its improved photocatalytic performance in comparison with pristine BiVO₄ via various characterization tools. Specifically, based on our previous work on mechanochemical synthesis of nano-sized m-BiVO₄,¹² p-type CuO will be further impregnated onto the surface of mechanochemically synthesized n-type m-BiVO₄ nanoparticles to construct CuO/m-BiVO₄ composite with large surface area. Special interests were also devoted to confirming the simultaneous enhancement on photon absorption and charge separation of the heterostructured composite in comparison with pristine BiVO₄ via diffuse reflectance spectroscopy (DRS), photoluminescence (PL) and surface photovoltage (SPV) study. The band structures and associated charge transfer processes were proposed for in-depth understanding of the enhanced photoelectrochemical (PEC) and photocatalytic activities under visible light irradiation.

2. Experimental

2.1 Materials preparation

The mechanochemical synthesis of nano-sized BiVO₄ powder has been detailed in¹² by subjecting a stoichiometric mixture of commercially available Bi₂O₃ (99.9%, Sinopharm) and V₂O₅ (99.8%, Alfa Aesar) powders to mechanical activation using Retsch PM100 planetary high energy ball milling system in air at room temperature for 8 h at 200 rpm. The collected powder was then carried on with wet-milling process to reduce the particle size with the weight ratio of 1 mm YSZ (Yttria-Stabilized Zirconia) ball : BiVO₄ powder : ethanol = 20 : 1 : 0.5 in an YSZ bowl, and a milling speed of 550 rpm for 8 h. The powder was finally separated from YSZ balls using sieves and subject to annealing at 400 °C in dry air for 1 h to eliminate the defects induced during the ball milling process.

The CuO/BiVO₄ composites with 2 at%, 5 at% and 10 at% of CuO were prepared by mixing the mechanochemically synthesized BiVO₄ powder with 1 M Cu(NO₃)₂ solution. The mixtures were wet milled at 100 rpm for 3 h in ethanol, and then dried and annealed at 300 °C for 1 h to decompose Cu(NO₃)₂ to form CuO on the surface of BiVO₄.

2.2 Materials characterization

The crystallographic structures were studied using a X-ray diffractometer (XRD, D5000, Siemens) with an X-ray source of 1.5406 Å Cu Kα in the range of 2θ = 10–70°. The BET surface areas were measured on an adsorption analyzer (ASAP 2020, Micromeritics). Chemical states and valence band analyses were performed using X-ray photoelectron spectrometer (XPS, Thermo Fisher Scientific) equipped with monochromatic Al Kα X-ray source. All XPS spectra were referenced to the C1s peak of adventitious hydrocarbon contamination located at 285 eV to correct the charging effect. Peak fitting of the XPS data was accomplished with a Shirley-type background subtraction using the spectra deconvolution software CasaXPS. The heterostructure was observed using a transmission electron microscopy with a field emission gun with an accelerating voltage of 200 kV (TEM, JEM-2100F, JEOL). The TEM samples were prepared by sonication of the powders in ethanol for 15 minutes and subsequently dropping the dispersion onto carbon/nickel grids until drying. A low background beryllium holder is used to maximize the EDX signal. UV-vis spectrometer (Shimadzu UV-2450) operating in diffuse reflectance mode using BaSO₄ as the reference was used to record the absorbance spectra after Kubelka–Munk transformation. The PL measurement was carried out on Shimadzu RF-5301 with excitation wavelength at 340 nm. SPV response was characterized by measuring the change in contact potential difference of the sample with respect to the reference probe in dark and upon irradiation. The measurement system consists of a commercial UHV Kelvin probe unit (KP Technology Ltd) incorporated with high-power LED sources (Mightex LED) with switchable wavelength from UV (365 nm) to near infrared (850 nm) through a quartz window. The maximum output power for λ = 455 nm is 350 mW. The SPV particulate film was prepared by dispersing the sample powders with propylene carbonate binder and coating them onto the FTO glass by doctor-blade method. All samples were kept in dark for more than 16 h to stabilize their surface charges, followed by illumination for 700 s.

2.3 PEC measurements

The PEC measurement was carried out in 0.5 M Na₂SO₄ electrolyte in an electrochemistry potentiostat (Solartron 1287) with a platinum plate and an Ag/AgCl (0.210 V vs. NHE) acting as the counter and reference electrodes, respectively. It was purged with nitrogen for 5 min prior to experiment. The wavelength of the high-power LED sources (Mightex LED) was selected as 455 nm and illuminated from backside on the working electrode (1 cm in diameter) at a power density of 20 mW cm⁻². The current–potential curves were obtained in the dark or under illumination at a potential sweep rate of 5 mV s⁻¹. The dependence of photocurrent on applied potential for BiVO₄ or Cu/BiVO₄ electrodes was measured with light and dark phases of 10 s.

2.4 Evaluation of photocatalytic performance

RhB as an important organic dye pollutant difficult to be degraded under visible light was used as a model contaminant to evaluate the photocatalytic activities of the CuO/BiVO₄ composite in comparison with the mechanochemically synthesized BiVO₄. The visible light source used was a white LED (λ > 420 nm, Prizmatix optical devices) having an output powder density of 50 mW cm⁻², calibrated and measured by broadband energy/power meter (Melles Griot). The irradiance reaching to the sample surface was 1.5 mW cm⁻². In each experiment, 0.1 g of photocatalyst was added into 50 mL RhB solution with an initial concentrate of 15 μM and was kept in dark condition for 3 h to establish absorption/desorption equilibrium. At each time point, the solution was sampled, centrifuged to remove photocatalyst particles and measured using UV-vis spectrometer (Shimadzu UV-2450) to monitor the change in absorbance of RhB upon irradiation.

3. Results and discussions

3.1 Crystalline structure

The phase structures of the CuO/BiVO₄ composite with 2–10 at% CuO were investigated by XRD and Raman analysis in comparison with BiVO₄. The XRD patterns were presented in Fig. 1. All the samples indexed well with the pure phase of monoclinic sheetlite BiVO₄ (JCPDS no. 04-1688) with the distinct characteristic peaks identified. It was noted that the main characteristic peaks of CuO at 2θ = 38.759° and 35.558° corresponding to the (111) and (1̄11) planes (JCPDS no. 89-5899) were not observed, which might be due to that: (a) the amount of CuO at ca. 2–10 at% was too low to be detected; (b) the size of the CuO nanoparticles were too small and might be uniformly scattered on the surface of BiVO₄; (c) the low crystallinity of CuO nanoparticles at a decomposition temperature of 300 °C. In the work on CuO-loaded BiVO₄ by Xu et al.³² and on fly ash cenospheres supported CuO–BiVO₄ by Zhang et al.,³³ distinct CuO peaks could be identified only when the concentration of CuO reached 40 at% at calcination temperature of 300 °C³² and 40 wt%,³³ respectively. The mean crystallite size of the BiVO₄ powder was estimated to be 26.0 nm according to Scherrer's equation: D = 0.9λ/(β cos θ), where λ is the wavelength of the X-ray (1.54 Å), β is the full width at half maximum (FWHM) in radians for peak centered at 2θ = 28.8° and θ is the Bragg angle in the diffraction pattern. The associated BET surface area of 10 m² g⁻¹ was much higher than most of the reported values in literature, affirming the effectiveness of mechanochemical high energy ball milling process in producing nano-sized particles. With loading of CuO, the FWHM remains the same, indicating that the annealing temperature of 300 °C to decompose Cu(NO₃)₂ into CuO had minimal impact on the grain growth of BiVO₄. Furthermore, no peak shifting was observed, suggesting that the CuO was deposited on the surface of BiVO₄ instead of incorporated into the lattice. Therefore, the loading of CuO did not alter the local structure of BiVO₄.

Fig. 1 XRD spectra of mechanochemically synthesized BiVO₄ loaded with 2–10 at% of CuO.

3.2 Chemical states study

The oxidation states of elements and total density of states (DOS) for the valence band (VB) in the BiVO₄ with 5 at% CuO were determined by XPS. Survey scan in Fig. 2(a) confirms that the main elements on the surface were Bi, V, O, Cu and C only. The well-deconvoluted spin–orbit split positioned at 953.5 eV and 933.7 eV corresponded to Cu 2p_1/2 and Cu 2p_3/2 orbits of the Cu²⁺ respectively. Moreover, the shake-up peaks located in the range of 965.0–960.0 eV and 950.0–938.0 eV were also noted as a feature for the identification of Cu in the form of Cu(II),³⁴ confirming the presence of CuO. The fact that XRD did not show presence of CuO phase while XPS did re-affirm that CuO was dispersed only on the surface of BiVO₄ instead of penetrating into the bulk, on top of the higher resolution with XPS technique. This hypothesized surface enrichment of Cu species can be confirmed by the calculated atomic ratio of 0.18 : 1 : 1.03 for Cu : Bi : V based on relative sensitivity factors (RSF) of 5.321, 9.14, and 2.116, for Cu 2p, Bi 4f, and V 2p respectively, which was more than three times of the designated concentration. The doublets positioned at 164.6 eV and 159.3 eV corresponded to Bi 4f_5/2 and Bi 4f_7/2 orbits of the Bi³⁺ while those centered at 524.5 eV and 517.0 eV were assigned to V 2p_1/2 and V 2p_3/2 orbits respectively, and attributed to V⁵⁺ of the BiVO₄ particles.³⁵ The intense O 1s peak at ca. 530.1 eV was due to the lattice oxygen in crystalline BiVO₄ cell, while another relatively lower peak at ca. 532 eV was usually assigned to surface adventitious species such as hydrocarbon (C–H), carbonate species (C–O, C=O) and adsorbed water (OH_ads) bonds due to contamination from environment during characterization.³⁶ Overall, on the basis of the analyses of XRD, Raman and XPS, it was confirmed that CuO was successfully loaded on the surface of BiVO₄ without altering the local crystal structure.

Fig. 2 XPS spectra of survey scan (a), Cu2p (b), Bi4f (c), V2p (d), O1s (e) and VB scan (f) of 5 at% CuO/BiVO₄ composite.

For the VB scan as shown in Fig. 2(f), the peak positions of the pristine BiVO₄ were consistent with those calculated and demonstrated by Cooper et al.³⁷ The broad photoemission between 9 and 2 eV in the pristine BiVO₄ was ascribed to a combined DOS of unhybridized O 2p_π mixed with Bi 6s states at ca. 2.65 eV, O 2p_π state at ca. 3.67 eV, the hybridized O sp²/V 3d state at ca. 5.63 eV, and the hybridized O sp²/Bi 6p state near 7.45 eV. The VB edge was 2.0 eV below the Fermi energy level from the photoemission onset, suggesting the pristine BiVO₄ was an n-type semiconductor considering the overall bandgap energy of ca. 2.4 eV. With CuO loaded, additional DOS in the range between 2 and 1 eV was observed, suggesting the shifting of VB edge upper nearer to the Fermi level. This could be due to the incorporation of p-type CuO whose Fermi level lies closer to the valence band or might signify the bandgap narrowing due to the introduction of CuO with small bandgap of 1.7 eV, which might contribute to enhanced visible light absorption to improve the photocatalytic activities.

3.3 Surface morphology

Fig. 3(a) shows the TEM image of the BiVO₄ with 5 at% CuO. The nanoparticles aggregated at some level with non-uniform size distribution in the range of 50–120 nm. From the HRTEM images in Fig. 3(b) and (c), the lattice fringe of 0.291 nm can be clearly identified, matching well with the d-spacing of (040) plane of monoclinic BiVO₄,¹² consistent with XRD analysis. The CuO shell with spherical shape and average diameter of 5 nm was also seen uniformly decorated on the surface of BiVO₄, assuring the formation of heterostructures. The Annular Dark Field (ADF) STEM imaging with element mapping in Fig. 3(d) along with the simultaneously performed EDS profiles in Fig. 3(e) further confirmed that the CuO nanoparticles were uniformly anchored on the surface of BiVO₄ particles, which could form nano-composites with the BiVO₄ to conduct away the photogenerated holes for better charge carrier separation.

Fig. 3 TEM (a) and HRTEM (b–c) images of 5 at% CuO/BiVO₄ composite. STEM image with EDX element mappings of Bi, V, O, and Cu, respectively (d) and associated EDS profiles of 5 at% CuO/BiVO₄ composite (e).

3.4 UV-vis DRS photon absorption

The UV-vis DRS absorption spectra of all obtained samples are depicted in Fig. 4(a). The CuO/BiVO₄ composites exhibited much more enhanced absorption in both UV and visible light range in comparison with pristine BiVO₄, whereby the broad absorption peak centered at 730 nm originating from the surface loaded CuO with a bandgap of 1.70 eV became more manifested with increasing CuO loading. This additional absorption peak is beneficial to enhance the visible light absorption so as to improve the photocatalytic quantum efficiency significantly. It is well known that the optical absorption property is closely related to the electronic configuration. BiVO₄ was commonly reported as a direct bandgap semiconductor material with the conduction band minimum and valence band maximum comprised primarily of unoccupied V 3d orbital and hybrid orbitals of Bi 6s and O 2p, respectively.³⁷ Thus, the optical bandgap energy can be extrapolated from the linear part of (αhν)²  versus photon energy plot as shown in Fig. 4(b) based on the relationship:

α(hv) = A(hv − E_g)^1/2 (1)

where a, h, v, A, and E_g are the absorption coefficient, Planck's constant, the incident light frequency, constant and the bandgap energy, respectively. The estimated bandgap energy of 2.61 eV was slightly larger than the commonly reported value of 2.40–2.50 eV for BiVO₄,³⁸ which can be ascribed to the confinement effects for its small particle size produced by mechanochemical milling. Loading of narrow bandgap CuO could lower the bandgap of BiVO₄.^32,33 However, in present study, no distinct shift on absorption edge was identified for CuO/BiVO₄ composites as compared with the pristine BiVO₄, similar to the observations by Jiang et al. on their CuO/BiVO₄ approaches.²² This could be explained by the low concentration of CuO in the composite to barely influence the bandgap structure. Therefore, the additional DOS noted within the bandgap of BiVO₄ in XPS VB scan was more likely attributed to incorporation of p-type CuO whose Fermi level lies closer to the valence band rather than bandgap narrowing.

Fig. 4 UV-vis absorption spectra of mechanochemically synthesized BiVO₄ loaded with 2–10 at% of CuO (a) and their extrapolated bandgap (b).

3.5 Recombination and separation of photogenerated charge carriers

PL emission spectra and SPV responses were used to probe the fate of the photogenerated electron–hole pairs in BiVO₄ and CuO/BiVO₄ composites as shown in Fig. 5. The PL luminescence was resulted from the emission of photons due to the recombination of the photogenerated holes in the O 2p band with the electrons in the V 3d band. The PL peak of the pristine BiVO₄ was observed at around 500 nm, exhibiting 40 nm blue shift as compared to the typical peak location centered at 540 nm,³⁹ which might be ascribed to the small particle size of the mechanochemically synthesized BiVO₄ nanoparticles. In the presence of 2–5 at% of CuO, despite the enhanced visible light absorption, a significant decrease in PL intensity was observed as compared to pristine BiVO₄, signifying the greatly suppressed recombination of the photogenerated charge carriers. This could be attributed to the formation of p–n heterojunction to utilize CuO as effective hole scavengers to facilitate separation of charge carriers in BiVO₄. With further loading of CuO at 10 at%, the amount of CuO could be in excess to introduce unfavourable interband states that serve as recombination centers, giving rise to increased PL intensity again.

Fig. 5 PL spectra of mechanochemically synthesized BiVO₄ loaded with 2–10 at% of CuO (a) and transient SPV response of 5 at% CuO/BiVO₄ in comparison with pristine BiVO₄ (b).

SPV measures the contact potential change as a result of illumination. Its magnitude relies on the number of photogenerated charge carriers as well as the diffusion of these carriers to surface states, which in turn, also depends on the charge separation efficiency.³⁶ A stronger SPV signal therefore indicates that more charge carriers can diffuse to the particle surface, corresponding to higher efficiency of charge carrier separation.⁴⁰ As shown in Fig. 5(b), the pristine BiVO₄ exhibited a typical n-type semiconductor behavior with positive SPV values upon illumination. With loading of 5 at% CuO, a much sharper and higher SPV response was clearly observed for the composite, confirming the better charge carrier separation as compared to bare BiVO₄. This could reasonably lead to higher photon efficiency, and thus a higher photodegradation rate.

3.6 PEC activities

Cyclic voltammetry was adopted to assess the electrocatalytic capability of the catalysts under visible light illumination at λ = 455 nm as shown in Fig. 6(a). The cyclic voltammograms of both BiVO₄ and CuO/BiVO₄ composite demonstrate hysteresis characteristics. As expected for n-type semiconductor, BiVO₄ electrode generates anodic photocurrent with increasing positive potential, indicative of electrons migration to the ITO substrate while holes oxidizing water to generate peroxo species.² The onset potential of the anodic photocurrent was about −0.52 V vs. Ag/AgCl, which was close to the flat band potential reported.^2,41 With reverse potential sweep towards negative direction, the anodic photocurrent turned to a weak cathodic photocurrent at about +0.1 V vs. Ag/AgCl, resembling the p-type semiconductor behavior. Similar observations have been reported by Long et al. on Co₃O₄/BiVO₄² as well as Jing et al. on ZnO.⁴² This cathodic photocurrent was believed to be induced by adsorbed O₂/surface peroxo species serving as electron scavengers to promote hole transfer to the ITO substrate.² With loading of CuO, the onset potential of the anodic photocurrent remained similar. However, obvious oxidation and reduction peaks were observed at ca. 0 V and −0.1 V respectively, indicative of stronger redox activities as compared to pristine BiVO₄. The small peak-to-peak separation was also a good indication of fast charge transfer,³⁹ which is beneficial to the photocatalytic activities.

Fig. 6 Cyclic voltammograms of BiVO₄ and 5 at% CuO/BiVO₄ composite electrodes in 0.5 M Na₂SO₄ upon chopped irradiation with 455 nm light at a scan rate of 5 mV s⁻¹ (a); and transient photocurrent density of BiVO₄ and 5 at% CuO/BiVO₄ composite electrodes (b).

Fig. 6(b) shows the results of chopped irradiation chronoamperometric photocurrent measurements at an applied potential of 0.6 V vs. Ag/AgCl electrode. It was obvious that with the loading of 5 at% CuO, the current density was enhanced significantly by 15 times as a result of effective charge separation. This high efficiency of current generation agreed well with the SPV responses and could indicate enhanced catalytic activities. It was also noted that the photocurrent decayed over time, similar to the observations on Co₃O₄/BiVO₄² and CoB_i/BiVO₄.⁴³ Long et al. has attributed the decrease of the photocurrent to the formation of surface peroxo species to serve as the recombination centers and it was suggested that the photocurrent could be restored by CV starting with the cathodic scan² or dark experiment⁴⁴ to reduce the peroxo species back to water. Other than peroxo species, accumulation of holes or protons on the electrode surface could also result in photocurrent decay through corrosion of the CuO/BiVO₄ composite.⁴³

3.7 Photocatalytic activities

The photocatalytic performance of the BiVO₄ and CuO/BiVO₄ composites were evaluated by monitoring the degradation of RhB (C/C₀) (Fig. 7). RhB in the absence of any photocatalysts showed minimum degradation over time, excluding the possibility of photolysis. RhB showed the highest degradation efficiency of ca. 60% in the presence of BiVO₄ loaded with 5 at% of CuO within three hours under weak LED irradiation of 1.5 mW cm⁻². Based on the first-order kinetic ln(C/C₀) ∝ kt shown in the inset, the apparent reaction constants k were calculated to be 0.00675, 0.08363, 0.07837, 0.29460 and 0.22650 h⁻¹ for RhB, BiVO₄, 2 at% CuO/BiVO₄, 5 at% CuO/BiVO₄ and 10 at% CuO/BiVO₄, respectively. The reaction rate for 5 at% CuO/BiVO₄ was close to three times of that for pristine BiVO₄.

Fig. 7 RhB degradation in the presence of BiVO₄ loaded with 2–10 at% of CuO under white LED irradiation of 1.5 mW cm⁻² with the inset showing the first order reaction kinetics.

The decreased photocatalytic efficiency with further increase in CuO loading to 10 at% could be attributed to the excess CuO to either shield the active sites on BiVO₄ surface or serve as recombination centers as evidenced by its increased PL signals (Fig. 5(a)).

3.8 Enhancement mechanisms

The much enhanced PEC and photocatalytic activities of CuO/BiVO₄ composites as compared to the pristine BiVO4 can be attributed to the simultaneous enhancement on visible light absorption and charge separation with the formation of p–n heterojunction.

It is well-known that the photocatalytic efficiency is strongly related to the generation of electron–hole pairs upon light irradiation on catalysts. Based on UV-vis DRS study, BiVO₄ with a bandgap of 2.61 eV exhibits an absorption tail up to 550 nm in the visible light range. As compared to BiVO₄, CuO with a narrower bandgap introduces additional absorption peak centered at 730 nm, further improving the visible light absorption and thus the quantum efficiency to enable the generation of more charge carriers. However, these charge carriers can recombine easily in the volume or on the surface of the catalysts to dissipate the energy as light, which often hinders the overall quantum efficiency. For the prepared CuO/BiVO₄ composite, the effective separation of the photogenerated charge carriers was evidenced by the suppression of PL signals and enhancement of SPV responses and can be best rationalized from the band structures of the heterojunction formed by p-type CuO and n-type BiVO₄ semiconductors as depicted in Fig. 8.

Fig. 8 Proposed band structure of the CuO/BiVO₄ p–n heterostructured composites before contact (a) and after contact and the associated visible light photocatalytic mechanism (b).

The relative positions of the conduction band (CB) and valence band (VB) of semiconductors can be determined based on the following equations:³²

E_CB = χ − E^e − 0.5E_g (2)

E_VB = E_CB + E_g (3)

where χ as the absolute electronegativity is ca. 5.81 and 6.035 eV for CuO and BiVO₄,^45,46 respectively; E^e is the energy of free electrons on the hydrogen scale (ca. 4.5 eV) while E_g as the bandgap energy is 1.70 and 2.61 eV for CuO and BiVO₄ respectively. From the above equations, the band structures for CuO and BiVO₄ are shown in Fig. 8(a), with the predicted band edge positions indicated. Before contact, the Fermi energy level is closer to the VB in p-type CuO due to the presence of more holes in the VB than electrons in the CB and vice versa for n-type BiVO₄ with Fermi energy level closer to the CB. Upon contact, the abundant holes in p-type CuO diffuse across the junction and cancel with the excess electrons from n-type BiVO₄ to create a depletion region with hardly any mobile carriers remaining. At thermal equilibrium, the Fermi energy levels in both CuO and BiVO₄ align as shown in Fig. 8(b). Accordingly, an internal electric field is established from BiVO₄ to CuO, which can facilitate the prompt transfer of photogenerated holes from the VB of BiVO₄ to VB of CuO. On the other hand, the electrons are deterred to migrate from the CB of CuO to CB of BiVO₄ due to the presence of an energy barrier. Therefore, the photogenerated electron–hole pairs can be effectively separated in BiVO₄ with the formation of p–n heterojunctions, and the energy consuming charge recombination can be substantially reduced to lead to higher PEC and photocatalytic efficiency. As illustrated in Fig. 8(b), if these separated charge carriers manage to migrate to the surface of CuO and BiVO₄, they can react with the adsorbed oxygen and water molecules to produce reactive oxygen species (ROS) including O⁻₂˙ and OH˙. These ROS species have strong reduction-oxidation power, which can further undergo a host of electron-transfer processes with the adsorbed pollutants of suitable redox potentials and finally get them degraded.

4. Conclusions

In this study, CuO/BiVO₄ composite was successfully synthesized by mechanochemical high energy ball milling in conjunction with impregnation approach. The obtained nanoparticles retain the monoclinic sheelite BiVO₄ structure and possess a large BET surface area of ca. 10 m² g⁻¹. The fact that CuO diffraction peak was not detected in XRD patterns yet CuO concentration was found in excess in XPS analysis confirms the dispersing of CuO on the surface instead of into the bulk of BiVO₄. Based on UV-vis absorption and XPS VB analyses, it was confirmed that the loading of narrow bandgap CuO on BiVO₄ helps to enhance the absorption in visible light range up to 800 nm. PL spectra and SPV studies further confirm the effective charge carrier separation due to formation of heterojunctions between the p-type CuO and n-type BiVO₄. Cyclic voltammograms and chronoamperometric photocurrent measurements show that the as-prepared composite with 5 at% of CuO demonstrates excellent photoelectrocatalytic performance with stronger photocurrent of 15 times and faster electron transfer. The composite also exhibited three times faster photocatalytic degradation of RhB in comparison with pristine BiVO₄ upon weak visible light irradiation of 1.5 mW cm⁻². It can be concluded that the much improved photo-efficiency over pristine BiVO₄ is due to the simultaneous enhancement in the photon absorption and charge separation, rendering the composite a promising visible light active photocatalyst suitable for environmental remediation.

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Footnote

† These authors contributed equally to this work.

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