BiOBr nanoplate-wrapped ZnO nanorod arrays for high performance photoelectrocatalytic application

Abstract

BiOBr nanoplates (NPs) decorated vertically aligned ZnO nanorod arrays (NRAs) have been successfully synthesized by a facile and cost-effective solvothermal process. The obtained ZnO/BiOBr heterostructured photoanode shows an enhanced water splitting performance compared to pure ZnO and BiOBr photoanodes, which may be mainly attributed to the formation of a heterojunction that facilitates the separation and transfer efficiency of photoinduced charges to generate a higher current. Moreover, Rhodamine B (RhB) was chosen as a model pollutant to evaluate the photoelectrocatalytic activity of the electrodes. As expected, the ZnO/BiOBr photoanode exhibits a much higher degradation ability than pure ZnO and BiOBr photoanodes, and achieves the highest photoelectrocatalytic degradation efficiency (95.4%) after irradiation for 100 min. Additionally, main reactive species trapping trials demonstrate that ˙OH and/or ˙O₂⁻ radicals play significant roles in the photocatalytic degradation process. The above results indicate that the novel photoanode may serve as a promising catalyst toward the practical application of photoelectrochemical water splitting and organic pollutant degradation.

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1. Introduction

As one of the most widely investigated semiconductors, zinc oxide (ZnO) nanostructures have been greatly employed in photochemical and optoelectronic applications due to their many merits such as ease of synthesis,¹ high electron mobility² and large exciton binding energy of 60 meV at room temperature.³ In particular, one-dimensional ZnO nanorod arrays (NRAs), with a large surface area, high aspect ratio, short diffusion length and direct electrical pathway for electrons, represent a superior photoanode material that hold vast applications for photocatalysis,⁴ solar cells,⁵ supercapacitors,⁶ photodetectors⁷ and diodes.⁸ However, they can only absorb about 5% of the solar spectrum in the ultraviolet region because of a wide band gap (E_g = 3.2 eV). Moreover, the high recombination rate of photoinduced charges inevitably results in an unsatisfactory photocatalytic activity and photocurrent response. Recently, considerable efforts have been employed to improve their visible light absorption ability, including coupling with other narrow band gap semiconductors,⁹ noble metal deposition,¹⁰ transition metal ion doping¹¹ and incorporation with carbon materials.¹² It is worth noting that, compared to other methods, coupling with other narrow band gap semiconductors, such as Fe₂O₃,¹³ CdS,¹⁴ WO₃ ¹⁵ etc. has gained increasing interest due to the synergistic effects on the photoelectrochemical and photocatalytic performances. The constructed heterojunction between semiconductors with matching energy band gaps can not only promote the visible light absorption ability and improve the overall energy conversion efficiency, but also facilitate the separation and transfer efficiency of photoinduced charges.¹⁶ Therefore, it is indispensable to search for highly efficient and novel photocatalysts.

Recently, increasing attention have been focused on the new-type bismuth-based semiconductor catalysts (BiOX, X = Cl, Br, I). The layered structure of BiOX can provide space large enough to polarize the related atoms and orbitals. Meanwhile, the induced dipole can efficiently separate the charges, which is beneficial to enhance photocatalytic activity.¹⁷ Moreover, Bi 6s and O 2p levels can form a preferable hybridized valence band to meet the potential demand of organic oxidation.¹⁸ Among these bismuth oxyhalides, BiOBr is a well-known photocatalyst due to its active and stable photocatalytic properties under visible light irradiation.¹⁹ However, the photocatalytic activity of BiOBr lacks enough efficiency owing to a low quantum field. Thus, it still remains a great challenge to further improve its photocatalytic efficiency for practical application.

Constructing a heterostructure is universally acknowledged to be an effective strategy to improve photocatalytic activity. Notably, the heterojunction has great potential in tuning the desired electronic properties of photocatalysts and efficiently separates the photoinduced charges.²⁰ Compared with the impurity doping approach, the formation of a heterojunction between two semiconductors with different band gaps is more flexible for broadening the visible light absorption and less sensitive to the component homogeneity.²¹ In this work, we developed a BiOBr NPs-wrapped vertically aligned ZnO NRAs as a model photoanode for highly efficient photoelectrocatalytic application. Scheme 1 illustrates the synthetic process of the ZnO/BiOBr heterostructured photoanode. The introduction of BiOBr has turned out to be an effective approach to extend the visible light absorption range of pure ZnO. On the other hand, the heterojunction between ZnO and BiOBr significantly improves the separation efficiency and transport rate of charges, thus resulting in an enhanced water splitting performance and photoelectrocatalytic activity. Based on the estimated energy band positions and the results of experiments, a possible photoelectrocatalytic mechanism of the ZnO/BiOBr photoanode was also proposed.

Scheme 1 Schematic illustration for the fabrication process of ZnO/BiOBr heterostructured photoanode.

2. Experimental section

Chemicals and materials

Zn(NO₃)₂·6H₂O, C₆H₁₂N₄ (HMT), NH₄Ac, Bi(NO₃)₃·5H₂O, KBr, ethylene glycol monomethyl ether, Rhodamine B (RhB), isopropyl alcohol (IPA), triethanolamine (TEOA) and nitrogen (N₂) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All the reagents are of analytical grade and used directly without further purification.

ZnO NRAs deposition method

ZnO NRAs were synthesized by an electrochemical deposition process as in our previous work.²² In a typical process, ZnO NRAs were grown on a clean fluorine tin oxide (FTO) substrate in a two-electrode system containing an aqueous solution of 0.02 M Zn(NO₃)₂, 0.01 M NH₄Ac and 0.01 M HMT, with a current density of −2.0 mA cm⁻² and a reaction temperature of 90 °C; the deposition time was fixed to 50 min. Subsequently, the FTO substrate with a sheet resistance of 14 Ω □⁻¹ and a graphite rod were used as the working electrode and counter electrode, respectively. The obtained ZnO NRAs were washed with deionized water and ethyl alcohol several times, then used in the following experiments.

Fabrication of the ZnO/BiOBr photoanode

The ZnO/BiOBr photoanode was fabricated using a simple solvothermal method. In a typical process, 0.05 mM KBr was slowly added into 20 mL of ethylene glycol monomethyl ether containing the same stoichiometric amount of Bi(NO₃)₃·5H₂O with stirring. Subsequently, the resulting mixed solution was poured into a 25 mL stainless steel Teflon lined autoclave with a ZnO NRAs covered FTO substrate immersed. The autoclave was sealed and then subjected to solvothermal treatment at 120 °C for 4 h. Upon completion of the reaction, the autoclave was cooled to room temperature and the substrate was removed from the solution, washed thoroughly with distilled water and ethyl alcohol, then dried at 70 °C in air. For comparison, a pure BiOBr sample was also prepared via the same procedure without the addition of the ZnO NRAs covered FTO substrate, and a BiOBr thin film was prepared as a reference sample.

Characterization

Powder X-ray diffraction (XRD) measurements were conducted on a PANalytical, PW3040/60 diffractometer with monochromatized Cu Kα radiation (λ = 0.15418 nm). The surface morphology and crystal microstructure of the samples were examined using a field emission scanning electron microscopy instrument (FE-SEM, JEOL JSM-7001F) equipped with an energy-dispersive spectroscopy (EDS) device and a transmission electron microscopy instrument (TEM, JEM2010-HR). Surface electronic states and compositions of the sample were analyzed by X-ray photoelectron spectroscopy (XPS, ESCALab250). The UV-vis diffuse reflectance spectra (UV-vis DRS) of the samples were obtained using a UV-vis spectrophotometer (Cary 300) with BaSO₄ as a reference and Raman spectra were recorded on a Renishaw inVia spectrometer with a visible laser excitation of 514.5 nm at room temperature. The final products of the degradation were analyzed using an electrospray ionization (ESI) source for mass spectrometry (MS) detection.

Water splitting performance

The water splitting performance was assessed using an electrochemical workstation (CHI 760D, Shanghai Chenhua) in a single compartment quartz cell with a three-electrode system. The as-prepared photoanodes, Ag/AgCl electrode and Pt sheet (1 cm²) served as the working electrode (the working area was fixed to 2.25 cm²), reference electrode and counter electrode, respectively. The applied potential of photocurrent density response (I–t) curves was the open circuit potential for each sample. Moreover, a 0.5 M Na₂SO₄ aqueous solution was employed as the electrolyte, a 300 W Xe lamp equipped with a 420 nm cut-off optical filter was placed parallel with the photoelectrochemical system. Moreover, the distance was kept at 10 cm from the quartz cell to the light source.

Photoelectrocatalytic degradation measurements

The photoelectrocatalytic performance of the photoanodes was evaluated by degrading a Rhodamine B (RhB) solution. Typically, 50 mL of a 0.1 M Na₂SO₄ solution containing 5 mL of a 0.1 mM RhB solution was used as the electrolyte. Prior to irradiation, the photoanode was fixed and immersed vertically into the solution for 20 min in the dark to establish an adsorption/desorption equilibrium of dye molecules on the catalysts. Moreover, a 300 W Xe lamp equipped with a 420 nm cut-off optical filter was introduced to the system, and a constant bias potential of 1.0 V was employed to impel the photoinduced electron transfer from the working electrode to the counter electrode. At a regular time interval of 20 min, 3 mL of the solution was collected and analyzed with a UV-vis absorption spectrometer to determine the conversion rate of the RhB solution.

Detection of main reactive species and analysis of hydroxyl radicals (˙OH)

The main reactive species detection process is similar to the photocatalytic process. Various scavengers were added into the RhB solution prior to the addition of photoanodes. Furthermore, photoluminescence (PL) spectra with terephthalic acid (TA) as a probe molecule was used to disclose the formation of ˙OH radicals on the surface of ZnO/BiOBr under visible light irradiation. In brief, the ZnO/BiOBr sample was immersed into a 40 mL aqueous solution containing 5 mM NaOH and 3 mM TA at room temperature. The above solution was subjected to evaluation of the photocatalytic activity and a corresponding sample of 2 mL was taken out every 20 min. The PL intensity was measured using a fluorescence spectrophotometer with an excitation wavelength of 325 nm.

3. Results and discussion

The typical XRD patterns are shown in Fig. 1 to examine the phase structure and composition of the as-prepared products. It can be seen that all the ZnO diffraction peaks can be well indexed to the hexagonal wurtzite phase of ZnO (JCPDS card no. 36-1451, lattice parameters: a = b = 3.25 Å, c = 5.21 Å), the diffraction peaks of BiOBr are consistent with the tetragonal phase of BiOBr (JCPDS card no. 73-2061, lattice parameters: a = b = 3.92 Å, c = 8.08 Å), suggesting that a pure BiOBr crystal phase was formed. The XRD pattern of ZnO remains unchanged when it was decorated with BiOBr, indicating that the decoration of BiOBr did not give rise to the development of new crystal orientations. Furthermore, the sharp and intense diffraction peaks of both ZnO and BiOBr indicate their good crystalline nature. No traces of other peaks were observed, confirming the high purity of the two products.

Fig. 1 XRD patterns of pure ZnO, BiOBr and the ZnO/BiOBr heterostructure.

The morphology and detailed microstructure information of the as-prepared samples were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The hexagonal and vertically oriented ZnO NRAs, with a smooth surface and a diameter of about 200 nm, were obtained by means of electrodeposition synthesis and uniformly distributed on the FTO substrate (Fig. S1†). After a hydrothermal process with a BiOBr precursor solution, the hexagonal morphology of ZnO disappeared and several BiOBr NPs were decorated on the ZnO nanorod (Fig. 2a), forming a branched and leaf-like structure, which could facilitate the transfer of photoinduced carriers and improve the photoelectrochemical performance. The morphology variation can also be observed in Fig. 2b, the ZnO nanorod surface became rough after the solvothermal treatment and some BiOBr NPs were anchored on it. Fig. 2c exhibits a high-resolution transmission electron microscopy (HRTEM) image, the distinct lattice fringe reveals the highly crystalline nature of ZnO/BiOBr. A lattice spacing of 0.260 nm corresponds to the (002) crystallographic plane of the ZnO wurtzite phase. Notably, other lattice fringe spacings of 0.276, 0.281 and 0.351 nm can be indexed to the inter-planar distances of the (110), (012) and (011) planes of tetragonal BiOBr. To further elucidate the heterostructure, EDX elemental mapping technique was employed for the ZnO/BiOBr sample. As presented in Fig. 2d–g, there are only Zn, O, Br and Bi elements distributing on the nanostructure, which matches well with the EDS spectrum (Fig. S2†). The results further demonstrate that the as-prepared samples are composed of ZnO and BiOBr.

Fig. 2 (a) SEM images, (b) TEM image and (c) high-resolution TEM image of the ZnO/BiOBr heterostructured nanorod, and (d–g) EDX elemental mapping images of Zn, O, Br and Bi, respectively.

Fig. 3a and b demonstrates that the hierarchical BiOBr microspheres, with an average diameter of 1 μm, are made up of two-dimensional BiOBr NPs. The HRTEM image in Fig. 3c reveals good crystallinity, and the lattice fringe with an inter-planar spacing of 0.281 and 0.352 nm matches well with the (012) and (011) atomic plane of tetragonal BiOBr. The corresponding selected area electron diffraction (SAED) pattern of BiOBr is also presented in Fig. 3d. In addition, Fig. 3e–g shows element mapping images including Bi, O and Br elements, further demonstrating the high purity of BiOBr.

Fig. 3 (a) SEM images, (b) TEM image, (c) high-resolution TEM image and (d) SAED pattern of the BiOBr hierarchical microsphere, and (e–g) EDX elemental mapping images of Bi, O and Br, respectively.

The chemical state and composition of the ZnO/BiOBr heterostructure were further examined by X-ray photoelectron spectroscopy (XPS). The typical XPS survey spectrum (Fig. S3†) also reveals that the heterostructure is composed of Bi, Zn, Br, and O elements. The two symmetric peaks at binding energies of 1021.40 and 1044.45 eV shown in Fig. 4a are assigned to Zn 2p_3/2 and Zn 2p_1/2, indicating the existence of Zn²⁺ in the ZnO/BiOBr heterostructure.^23,24 The two strong peaks located at 158.95 eV and 164.35 eV in Fig. 4b are attributed to Bi 4f_7/2 and Bi 4f_5/2, respectively, which are characteristic of Bi³⁺ in BiOBr.²⁵ The peaks located at 68.01 and 69.03 eV presented in Fig. 4c are associated with Br 3d_5/2 and Br 3d_3/2, which are consistent with the value state of Br⁻ in BiOBr.²⁶ Meanwhile, the O 1s peaks of ZnO/BiOBr can be deconvoluted into three peaks at 530.18 eV, 530.90 eV and 531.95 eV in Fig. 4d, corresponding to the Bi–O bonds in [Bi₂O₂]²⁺ slabs of the BiOBr layered structure,²⁷ Zn–O bonds of ZnO²⁸ and O–H, H₂O or O₂ adsorbed on the surface,²⁹ respectively. Additionally, the Raman spectrum of ZnO/BiOBr (Fig. S4†) is also consistent with the results of XRD and XPS analysis.

Fig. 4 XPS spectra of the ZnO/BiOBr heterostructure: (a) Zn 2p, (b) Bi 4f, (c) Br 3d and (d) O 1s.

UV-Vis diffuse reflectance spectra of the as-prepared samples are presented in Fig. 5. ZnO exhibits almost no absorption in the visible light region due to its wide band gap, with an absorption edge of around 390 nm. BiOBr shows strong photo-absorption from UV light to visible light shorter than 500 nm, indicating a smaller band gap. After BiOBr was decorated with ZnO, the absorption edge of the heterostructure shifts red to around 420 nm. Moreover, the absorption intensity in the visible light region is higher than that of pure ZnO and BiOBr. For a semiconductor, the band gap (E_g) can be calculated from the following equation:

αhν = A(hν − E_g)^n/2 (1)

where α, hν and A is the absorption coefficient, photoenergy and a constant, respectively. In addition, n is dependent on the optical transition type of a semiconductor, i.e., n = 1 for a direct transition and n = 4 for an indirect transition. As reported in previous literature, both ZnO and BiOBr are indirect transition semiconductors with the same n value of 4.^30,31 Therefore, E_g can be calculated from the plots of (αhν)^1/2 versus photoenergy (hν). The approximate E_g value of ZnO and BiOBr is about 3.16 and 2.64 eV, respectively (Fig. S5†). The enhanced visible light absorption may be mainly attributed to the formation of a heterojunction, which is in favour of charges transportation. In this case, ZnO NRAs and BiOBr NPs provide unimpeded paths for the transfer of photoinduced charges, thus effectively inhibiting the recombination between electrons and holes, which leads to the increased response in the visible light region.

Fig. 5 UV-vis diffuse reflectance spectra of pure ZnO, BiOBr and the ZnO/BiOBr heterostructure.

The water splitting performance of the ZnO/BiOBr photoanode was evaluated by measuring the photocurrent in a 0.5 M Na₂SO₄ electrolyte under Xe lamp irradiation. To obtain the optimal content of BiOBr NPs, different ZnO/BiOBr photoanodes were prepared. And the corresponding current–potential (I–V) curves of ZnO/BiOBr photoanodes prepared from different raw material amounts are also presented (Fig. S6a†). The current density at 1.2 V first increases with increasing BiOBr NPs content (Fig. S6b†), reaching a maximum value at the raw material amount of 0.05 mM [KBr + Bi(NO₃)₃], and then decreases. The SEM images of ZnO/BiOBr photoanodes prepared from different raw material amounts are displayed (Fig. S7†). Initially, the content of BiOBr NPs decorated on the surface of ZnO NRAs increases with raw material amount, leading to better visible light absorption and thus more efficient exciton generation. However, if the raw material amount is too large, an excessive content of BiOBr NPs decoration may cause the aggregation of BiOBr NPs, which increases the recombination rate of photoinduced electron–hole pairs (charges have to diffuse longer to reach the ZnO/BiOBr interface to separate). As a result, the optimal raw material amount would be 0.05 mM [KBr + Bi(NO₃)₃], and further experiments were carried out using the as-prepared ZnO/BiOBr photoanode. By comparison, pure ZnO and BiOBr photoanodes were also measured.

Fig. 6a shows the photocurrent–potential (I–V) curves of pure ZnO, BiOBr and ZnO/BiOBr photoanodes under visible light illumination in a potential range of 0.2 to 1.2 V vs. Ag/AgCl. Generally, photocurrent begins to increase with increasing applied potential which is regarded as a sign that a water oxidation reaction occurred, and the corresponding applied potential is the actual needed potential to drive the water oxidation reaction, which is called the photocurrent onset potential. A more negative photocurrent onset potential is significantly important because it reduces the external bias needed for the water oxidation reaction, and thus increases the overall efficiency of photoelectrochemical cells. It can be seen that ZnO/BiOBr photoanode produced an obvious photocurrent at the applied potential of about 0.64 V, which is more negative than that of pure ZnO (0.80 V) and BiOBr (0.79 V) photoanodes, indicating a lower required external bias to drive the water oxidation reaction. These negative shifts of the photocurrent onset potential can be mainly ascribed to the negatively shifted Fermi level and decreased electron–hole recombination near the flat band potential.³² Moreover, the ZnO/BiOBr photoanode displays a much higher photocurrent density than pure ZnO and BiOBr photoanodes under the same potential, indicating the enhanced separation of photoinduced electron–hole pairs, which can also be observed in the I–V curves in a dark condition (Fig. S8†). Fig. 6b shows the photocurrent density response (I–t) curves of the photoanodes measured at an open circuit potential under chopped light irradiation. With the repeated on–off cycles, the photocurrent density generates instantly and increases sharply, then reduces to zero promptly as soon as irradiation is stopped. The nearly vertical rising and falling of the photocurrent indicates that the charge transport in the photoanode proceeds very quickly, which can be attributed to the direct electron transfer pathway provided by the ZnO NRAs. It is noteworthy that the ZnO/BiOBr photoanode exhibits a much higher photocurrent density than pure ZnO and BiOBr photoanodes, indicating that a more effective separation of photoinduced charges and a faster interfacial charge transfer were achieved in the heterostructure. Moreover, the photocurrent density with time is quite stable for the ZnO/BiOBr photoanode, as shown in Fig. S9,† about 90.88% remains at the end of illumination, demonstrating high stability in practical application.

Fig. 6 (a) Current density versus applied potential curves for pure ZnO, BiOBr and the ZnO/BiOBr photoanodes under visible light (≥420 nm). (b) Photocurrent response of the photoanodes prepared from pure ZnO, BiOBr and the ZnO/BiOBr heterostructure to light on–off under visible light illumination (≥420 nm).

Mott–Schottky (M–S) analysis is generally based on the Schottky barrier formed between the semiconductor and electrolyte, and commonly used in photoanode characterization to determine the carrier density.^33,34 The slope of such plots is usually used to estimate the donor density (N_d) for n-type semiconductors or acceptor density (N_a) for p-type semiconductors. The M–S plots of ZnO, BiOBr and ZnO/BiOBr photoanodes are expressed as 1/C² vs. potential, where C is the space charge capacitance of the semiconductor electrode (Fig. S10†). It can be seen that all the electrodes show a positive slope, revealing that n-type behaviours with electrons represents the majority of carriers. Moreover, ZnO/BiOBr photoanode displays a much smaller slope than pure ZnO and BiOBr photoanodes, suggesting that a greater donor density appears after the construction of the heterostructure. The donor density of the electrodes can be estimated from the following equation:

N_d = (2/εε₀e₀)[d(1/C²)/dV]⁻¹ (2)

where ε is the dielectric constant of the semiconductor, ε₀ is the permittivity of a vacuum (8.854 × 10⁻¹² F m⁻¹), e₀ is the electronic charge unit (1.602 × 10⁻¹⁹ C) and V is the potential applied at the electrode. With a ε value of 10 for ZnO and BiOBr, the electron densities of ZnO, BiOBr and ZnO/BiOBr photoanodes are calculated to be 4.07 × 10¹⁹, 5.54 × 10¹⁹ and 1.73 × 10²⁰ cm⁻³, respectively. The carrier density of the ZnO/BiOBr heterostructure increases about one order compared to pure ZnO and BiOBr. Although the M–S equation is derived from the planar electrode model, the result of the carrier densities is still qualitatively comparable as they have a similar morphology and surface area. The compact combination of ZnO and BiOBr to form a heterostructure promotes the transfer of charge carriers, which is considered as the major contributing factor for the enhanced photocurrent density.

The photoelectrocatalytic performances of ZnO, BiOBr and ZnO/BiOBr photoanodes were evaluated by degrading a 0.1 mM RhB solution under visible light illumination with an anodic bias of 1.0 V vs. Ag/AgCl. As displayed in Fig. 7a, compared with electrocatalytic (EC), photocatalytic (PC) and photoelectrocatalytic (PEC) degradation efficiencies, the self-degradation effect of the RhB solution is negligible. Notably, ZnO shows a moderate degradation ability toward visible light illumination of RhB during the PC process, indicating the existence of a weak dye sensitization effect (DSE). Compared to pure ZnO and BiOBr photoanodes, the EC, PC and PEC performances of the ZnO/BiOBr photoanode are significantly improved. Notably, the combination of illumination and external potential can effectively enhance the catalytic activity, and the external potential can improve the separation and transfer efficiency of photoinduced electron–hole pairs. Thus, the degradation efficiencies of ZnO, BiOBr and ZnO/BiOBr photoanodes comply with the following order: ZnO/BiOBr-PEC > BiOBr-PEC > ZnO-PEC > ZnO/BiOBr-PC > BiOBr-PC > ZnO-PC > ZnO/BiOBr-EC > BiOBr-EC > ZnO-EC. In other words, the ZnO/BiOBr photoanode exhibits the highest PEC degradation efficiency, i.e., 95.4% after 100 min of irradiation. In order to obtain an intuitive comparison, the degradation rates of other degradation process are also displayed (Fig. S11†). Furthermore, the degradation kinetics of the RhB solution was also investigated, and the results are shown in Fig. 7b. Clearly, the degradation rates of the RhB solution match well with a pseudo-first-order reaction according to the simplified Langmuir–Hinshelwood model, −ln (C_t/C₀) = kt, where C₀ is the initial concentration of the RhB solution, C_t is the concentration of that at different intervals during the degradation process, and k is the reaction rate constant (min⁻¹). The rate constants and relative coefficients were obtained by fitting the experimental data (Table S1†). It is obvious that the rate constant of ZnO/BiOBr-PEC is significantly higher than other degradation processes, which is consistent with our expectation.

Fig. 7 (a) EC, PC and PEC degradation rates of a RhB solution on pure ZnO, BiOBr and ZnO/BiOBr NRAs photoanodes; (b) the corresponding plots of −ln(C_t/C₀) versus irradiation time. (c) Photocatalytic degradation rates of a RhB solution for the main reactive species trapping experiments. (d) ˙OH radical trapping PL spectra in a TA solution of ZnO/BiOBr under visible light irradiation.

A series of reactive species were excited by photoinduced charges, such as ˙O₂⁻ or ˙OH radicals, which are considered to be involved in the PC process. In a typical procedure, 10 mmol of triethanolamine (TEOA) and isopropyl alcohol (IPA) were added to the solution as scavengers of h^{+ 35} and ˙OH³⁶ radicals, respectively. Meanwhile, N₂ purging (50 mL min⁻¹) was conducted as an ˙O₂⁻ radical scavenger.³⁷ As shown in Fig. 7c, the degradation of RhB is almost unchanged in the presence of TEOA, indicating that h⁺ does not play a major role in the degradation process under visible light illumination. In contrast, an apparent decline was observed after the N₂ purging treatment, and the degradation rate was drastically inhibited by the addition of IPA, suggesting that ˙O₂⁻ and/or ˙OH radicals were the main reactive species in the PC process. Fig. 7d shows the ˙OH trapping PL spectra detected from changes in the TA solution under visible light irradiation. It can be seen that the PL signal was observed at 425 nm for each time point, and the intensity increased gradually with prolonged irradiation time, suggesting that ˙OH radicals were generated in the PC system. To further confirm the above inference and the decomposition products of the RhB molecule, ESI-MS analysis was carried out for the photocatalytic system (Fig. S12†). The results more forcefully suggested that ˙O₂⁻ and/or ˙OH radicals played vital roles in the degradation process of RhB, and the RhB molecule suffered from de-ethylation, hydroxylation and oxidation processes to achieve the decomposition procedure.

Further insights for the PEC mechanism of the ZnO/BiOBr photoanode were discussed as follows. The conduction band (CB) and valence band (VB) edge potential of a semiconductor at the point of zero charge can be estimated by the Mulliken electronegativity theory:

E_VB = X − E^e + 0.5E_g (3)

where E^e is the energy of free electrons on the hydrogen scale (about 4.5 eV) and E_VB is the VB edge potential. X is the absolute electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms, and E_g is the band gap energy of the semiconductor. Meanwhile, the CB edge potential (E_CB) can be calculated by the equation:³⁸ E_CB = E_VB − E_g. Here, the X values for ZnO and BiOBr are about 5.95 and 6.18 eV,^39,40 the E_VB of ZnO and BiOBr are calculated to be 3.03 and 3.00 eV vs. NHE, respectively. Moreover, E_CB of ZnO and BiOBr are estimated to be −0.13 and 0.36 eV vs. NHE. The band structure variation of the two semiconductors before and after contact is displayed (Fig. S13†). Generally, pure ZnO and BiOBr possess their own band structures and Fermi levels before contact. When BiOBr is coupled on the surface of ZnO, a novel heterojunction would be formed. The ZnO with a higher Fermi level is filled with holes and the BiOBr with a lower one is filled with electrons, subsequently, an intrinsic electric field could be constructed until the Fermi levels for both phases are equal, thus leading to the CB of BiOBr lying more negative than that of ZnO, and making the photoinduced electron transfer to the CB of ZnO achievable.^16,26,39 Fig. 8 illustrates the proposed mechanism for the superior catalytic property of the PEC process. The pure ZnO NRAs decorated with BiOBr NPs greatly enhance the contact areas as well as the visible light absorption capacity, which drives more organic pollutants and photons to participate in the PC reaction, thus leading to a higher PC efficiency. On the other hand, an applied potential accelerates the separation and transfer efficiency of photoinduced charges and promotes the electron density in the CB of BiOBr and ZnO, thus resulting in high PEC degradation efficiency. Based on the above analysis, we put forward the possible action trails for reactive species as follows:

ZnO (e⁻_CB) + O₂ → ˙O₂⁻ (6)

˙O₂⁻ + H₂O → ˙OH (7)

RhB + ˙OH → CO₂ + H₂O +… (8)

RhB + ˙O₂⁻ → CO₂ + H₂O +… (9)

RhB (dye sensitization effect) → CO₂ + H₂O +… (10)

Fig. 8 Schematic diagram for the PEC degradation process, magnified view and proposed working mechanism of the ZnO/BiOBr heterostructured NRAs photoanode.

4. Conclusions

In summary, BiOBr NPs decorated vertically aligned ZnO NRAs heterostructured photoanode has been successfully fabricated through a facile and low-cost solvothermal route. The obtained ZnO/BiOBr photoanode shows a significantly enhanced water splitting performance and a much higher degradation ability than pure ZnO and BiOBr photoanodes under the same conditions, which may be mainly attributed to the heterojunction effectively facilitating the separation and transfer efficiency of photoinduced charges. Moreover, ˙OH and/or ˙O₂⁻ radicals are considered to play major roles in the PC degradation process under visible light irradiation. The above results indicate that the novel photoanode may serve as a promising catalyst toward the practical application of photoelectrochemical water splitting and organic pollutant degradation.

Acknowledgements

This work was supported by Natural Science Foundation of China (Grant No. 21306030, 21576056, and 21576057), Guangdong Natural Science Foundation (Grant No. 2014A030313520 and 2015A030313503), Program Foundation of the second batch of innovation teams of Guangzhou Bureau of Education (Grant No. 13C04), Scientific Research Project of Guangzhou Municipal Colleges and Universities (Grant No. 1201410618).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra27310b

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