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Original article
01 2021
:15;
103513
doi:
10.1016/j.arabjc.2021.103513

Facile synthesis of Z-scheme NiO/α-MoO3 p-n heterojunction for improved photocatalytic activity towards degradation of methylene blue

, , , , , , ,
a Experimental Practising & Teaching Center, Hebei GEO University, Shijiazhuang 050031, PR China
b Hebei Province Key Laboratory of Sustained Utilization & Development of Water Recourse, Hebei Province Collaborative Innovation Center for Sustainable Utilization of Water Resources and Optimization of Industrial Structure, School of Water Resource and Environment, Hebei Geo University, Shijiazhuang 050031, PR China
c School of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, PR China

⁎Corresponding author at: Hebei Province Key Laboratory of Sustained Utilization & Development of Water Recourse, Hebei GEO University, Shijiazhuang 050031, PR China. pzpzlxl@163.com (Changyu Lu)

Received: , Accepted: ,
© The Author(s) 2021
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

In this article, Z-scheme NiO/α-MoO3 p-n heterojunction is successfully synthesized by a facile hydrothermal route. The phase and nanostructures are researched through a series of characterizations, such as XRD, SEM, TEM, EDX, XPS and DRS. It is confirmed that the NiO nanoparticles are deposited homogeneously on one dimensional α-MoO3 nanobelts and p-n heterojuction is constructed at the interface of α-MoO3 and NiO. Photocatalytic activity of the as-synthesized photocatalysts is investigated by photodegradation of methylene blue (MB) under simulated solar light irradiation. Compared with bare α-MoO3, the NiO/α-MoO3 p-n heterojunction exhibits significantly improved photocatalytic activity and photostability for MB degradation. The improvement in the photocatalytic performance can be attributed to the optimization of the charge transport pathway offered by Z-scheme heterojunctions, which can promote the effective separation of electron-hole pairs. The results indicate that Z-scheme NiO/α-MoO3 p-n heterojunction is a novel and efficient photocatalyst with potential application for the removal of organic contaminant in wastewater.

Keywords

α-MoO3 nanobelts
NiO
p-n heterostructure
Photocatalysis
MB
1

1 Introduction

The toxic and cancerogenic organic dyes have become one of the major problems for the drinking water due to the extracts of printing, textiles and extraction mills. Confronted with the difficult degradation of multifarious organic compounds, the development and utilization of photocatalysis technology is pressing (Li et al., 2020; Wang et al., 2020; Guo et al., 2021). In the past two decades, semiconductor catalysts such as ZnO, TiO2, Fe2O3, NiO, WO3, and SnO2 have attracted significant attention owing to their low toxicity, high stability and environmentally friendly (Ali et al., 2021; Yoon et al., 2021; Nguyen-Dinh et al., 2021; Rong et al., 2021). However, for the convenient and cost-efficient single-component photocatalyst, the photogenerated carriers can easily recombine, which directly causes the low quantum efficiency and poor photocatalytic activity (Dong et al., 2019; Gui et al.,2020; Dong et al.,2020; Zuo et al., 2021). Several strategies have been investigated to promote the photocatalytic performance which include element doping, noble metal functionalization, and heterostructured composite fabrication (Zhai et al., 2020; Mehmood et al., 2017; Zha et al., 2021; Li 2022). The design of new photocatalyst materials with the desired properties has become a popular research field.

α-MoO3, a well-known n-type semiconductor, has attracted much attention on photocatalytic applications due to its unique structural and optical properties. Recently, α-MoO3 with various morphologies and modifications have been prepared and studied (Zhang et al., 2019; Szkoda et al., 2018; Chiang et al., 2015). However, the challenge for promoting the catalytic activity of bare α-MoO3 still exists, which mainly lies in enhancing the visible light absorption and reducing the recombination of charge carriers. In order to further degrade the adsorbed organic contaminants efficiently, α-MoO3 has be applied in conjunction with other elements or semiconductors to form new compounds or heterojunctions. For example, Zhang et al. (Zhang et al., 2018) reported the functionalization of AuPd nanoparticles on the surface of α-MoO3 nanowires leading to an enhanced photocatalytic activity (11.5 times higher than that of bare α-MoO3) for the degradation of trichloroethylene. Phuruangrat et al. (Phuruangrat et al., 2016; Phuruangrat et al., 2017; Phuruangrat et al., 2016) synthesized Gd, Dy and Eu-doped α-MoO3 nanobelts by hydrothermal method, which exhibited high visible-light-driven activity for photodegradation of MB. Feng et al. (Feng et al., 2017) combined α-MoO3 nanobelts with AgBr to form a Z-scheme photocatalyst, resulting in the dramatic visible light absorption and improved photocatalytic performance in degrading RhB. Xi et al. (Xi et al., 2019) constructed a dye-sensitized MoO2 incorporated α-MoO3 photocatalyst, which displayed high RhB degradation efficiency under visible-light irradiation. Thus, it is necessary to construct the α-MoO3-based photocatalysts to further shorten the degradation period and enhance photocatalytic efficiency.

P-n heterojunction constructed by combining a n-type semiconductor (electron-rich) with a p-type semiconductor (hole-rich) is one of the most effective methods to enhance the charge separation. Among the family of semiconductor materials, NiO, a p-type semiconductor, has been widely applied in the fabrication of p-n heterostructures with different n-type semiconductors. Up to now, a series of NiO-based composites have been studied, such as NiO/SnO2 (Jayababu et al., 2019), TiO2/NiO (Sun et al., 2016), NiO/Fe2O3 (Jana et al., 2018), ZnO/NiO (Li et al., 2018) and so on. However, to our best knowledge, there is rarely report on the preparation and photocatalytic performance of NiO/α-MoO3 constructed p-n heterostructures. Notably, both the valence band (VB) and conduction band (CB) of NiO are more negative than those of α-MoO3. We can introduce the appropriate band positions of NiO into α-MoO3 to create a Z-scheme photocatalyst system, which will help decrease the electron-hole recombination of α-MoO3 and enable us to obtain higher efficiency photocatalysts. In this work, we designed and successfully constructed a Z-scheme NiO/α-MoO3 p-n heterojunction by a two-step hydrothermal route. The microstructure, elemental composition and optical property of the as-fabricated heterostructures were evaluated. The photocatalytic activity was estimated by using the degradation of the MB dye under simulated solar light irradiation. Finally, the possible mechanism for the photocatalytic process was proposed and discussed in detail.

2

2 Experimental procedures

2.1

2.1 Preparation of one dimensional α-MoO3 nanobelts

In a typical procedure, 1.24 g of ammonium heptamolybdate tetrahydrate ((NH4)6Mo7O24·4H2O) was dissolved into 50 mL pure water, followed by adding 5.0 mL nitric acid (HNO3, 65%) in a dropwise manner. After continuous stirring for 30 min, the mixture was transferred to a Teflon-lined stainless-steel autoclave with capacity of 100 mL for hydrothermal treatment at 180 °C for 36 h. Upon cooling down the autoclave to room temperature, a white precipitate was collected by filtration, washed by distilled water and ethanol, and dried at 80 °C for 24 h.

2.2

2.2 Preparation of NiO/α-MoO3 heterostructure

NiO/α-MoO3 heterostructures were also prepared by a hydrothermal approach. NiO/α-MoO3 heterostructures with different mass ratios of NiO powders (1 wt%, 3 wt% and 5 wt%) were described as follows: 0.24 g of as-synthesized α-MoO3 was suspended into 40 mL pure water with ultrasonic concussion for 20 min, and then different mass ratios of Ni(NO3)2·6H2O (0.01, 0.03 and 0.05 g) and 0.4 g of D-glucose monohydrate (C6H12O6·H2O) were put into the mixed solution under strong magnetic stirring for 15 min. Then the mixture was transferred to a 50 mL Teflon-lined stainless autoclave and heated at at 160 °C for 5 h. The product was centrifugally separated, washed several times with water/ethanol and then dried in vacuum at 70 °C for 24 h. Finally, the as-obtained materials were calcined in a muffle furnace at 400 °C for 3 h. The NiO/α-MoO3 heterostructures with different mass ratios of NiO (1 wt%, 3 wt%, and 5 wt%) were labelled as 1% NiO/α-MoO3, 3% NiO/α-MoO3 and 5% NiO/α-MoO3, respectively (EDS patterns could be seen in Fig. S1). For comparison, pure NiO was synthesised without adding of α-MoO3 nanobelts.

2.3

2.3 Characterization

Characterization techniques are provided in the Supporting Information.

2.4

2.4 Photocatalytic activity

Photocatalytic experiments of the as-synthesized samples were examined by degrading MB in aqueous solution under simulated solar light irradiation of a 500 W Xe lamp. In a typical photocatalytic process, 0.01 g of each catalyst and 100 mL MB (10 mg L−1) were added into the photocatalytic reactor. To achieve the adsorption–desorption equilibrium between the catalysts and MB dye, the solution was continuously stirred in the dark for 30 min. At periodic intervals (30 min), 2 mL of suspension was collected from the solution and separated through centrifuge to remove the photocatalyst particles. Finally, the concentration of leftover solution was analysed by the UV–vis spectrophotometer (UV-3600, Shimadzu). Recovery of photocatalyst was conducted by using methanol/acetic acid (95:5, v/v) solution. After the photocatalyst was contacted with the solvent completely, the photocatalyst was separated by centrifuge. To investigate the reproducibility of the photocatalyst, the photocatalyst after photocatalytic degradation was reused in experiments and the process was duplicated five times.

3

3 Result and discussion

The crystal structures of NiO/α-MoO3 heterojunctions, α-MoO3 nanobelts and NiO nanoparticles are studied by XRD analysis. As the XRD patterns revealed in Fig. 1, the diffraction peaks for bare α-MoO3 are in accordance with JCPDS No.05–0508. The mainly characteristic peaks located in 2θ = 12.8°, 25.7° and 39.0° are indexed to (0 2 0), (0 4 0) and (0 6 0) crystal planes (Li et al., 2017), verifying the orthorhombic α-MoO3 phase. Moreover, the pattern of NiO exhibits four diffraction peaks at about 2θ of 37.2°, 43.3°, 62.9° and 75.5°, which correspond to the crystal plane (1 1 1), (2 0 0), (2 2 0) and (3 1 1) of cubic NiO phase (JCPDS No. 47–1049) (Sun et al., 2016), respectively. It is worth to mention that all diffraction peaks of NiO/α-MoO3 heterojunctions are matched well with the α-MoO3 phase and no feature peaks of NiO are detected. The reason may be that there is few NiO in the nanocomposites and the diffraction peaks of α-MoO3 can cover the NiO peaks (Li et al., 2017).

XRD patterns of (a) α-MoO3, (b) 1% NiO/α-MoO3, (c) 3% NiO/α-MoO3, (d) 5% NiO/α-MoO3 and (e) NiO.
Fig. 1
XRD patterns of (a) α-MoO3, (b) 1% NiO/α-MoO3, (c) 3% NiO/α-MoO3, (d) 5% NiO/α-MoO3 and (e) NiO.

SEM, TEM and HRTEM are acquired to confirm the successful introduction of NiO nanoparticles on the α-MoO3 nanobelts. Fig. 2(a-e) shows the typical SEM images of pure α-MoO3, NiO/α-MoO3 heterojunctions and NiO. The SEM results reveal that the pure α-MoO3 material displays a belt-like structure with the length of 2–10 μm and width of 200–300 nm, and the ratio of length/width can reach as high as 50/1 (Fig. 2(a)). Pure NiO possesses a flower-like morphology with diameters ranging from 500 to 800 nm (Fig. 2(e)). Compared with pure α-MoO3, it can be found that the surfaces of NiO/α-MoO3 turn coarser which can be ascribed to the successful loading of the secondary NiO particles onto the α-MoO3 substrates. However, with the increase of loading content, the independent NiO particles appear in the SEM image of 5% NiO/α-MoO3 (Fig. 2(d)).

SEM images of (a) α-MoO3, (b) 1% NiO/α-MoO3, (c) 3% NiO/α-MoO3, (d) 5% NiO/α-MoO3 and (e) NiO; TEM (f) and HRTEM (g) images of 3% NiO/α-MoO3; (h) Schematic illustration for the preparation of the NiO/α-MoO3 heterojunctions.
Fig. 2
SEM images of (a) α-MoO3, (b) 1% NiO/α-MoO3, (c) 3% NiO/α-MoO3, (d) 5% NiO/α-MoO3 and (e) NiO; TEM (f) and HRTEM (g) images of 3% NiO/α-MoO3; (h) Schematic illustration for the preparation of the NiO/α-MoO3 heterojunctions.

The TEM image (Fig. 2(f)) of the single NiO-functionalized α-MoO3 nanobelt further confirms that NiO nanoparticles are uniformly dispersed on the surface of α-MoO3 nanobelts. The HRTEM image of 3% NiO/α-MoO3 is also acquired (Fig. 2(g)). The interplanar spacings of 0.37 nm and 0.40 nm are clearly observed, which are indexed to the (0 0 1) and (1 0 0) planes of the α-MoO3 crystal (Li et al., 2017). As for the lattice fringe of 0.21 nm, it can be assigned to the (2 0 0) plane of NiO crystal (Hou et al., 2014; Zheng et al., 2015). The synthetic processes for the NiO/α-MoO3 heterojunctions are illustrated in Fig. 2(h). Under the hydrothermal conditions, linear or branched oligomers are formed by the partially dehydrated glucose, which decompose into a carbon shell coating on the surface of α-MoO3 (TGA and DSC analyses could be seen in Fig. S2). In addition, the carbon shell has distribution of –OH groups on the hydrophilic surface of α-MoO3 (Titirici et al., 2006; Wang et al., 2015; Demir-Cakan et al., 2009). The metal nickel ions (Ni2+) conjugate with the –OH groups by an electrostatic attraction or coordination effect. Some Ni2+ ions agglomerate together to form larger clusters with the reaction time prolonging. Finally, the NiO/α-MoO3 heterojunctions are obtained by oxidative calcination in air to remove the carbon.

Fig. 3(a-d) illustrates the corresponding elemental mappings of Mo, O and Ni of the 3% NiO/α-MoO3 heterojunction, which reveals the different distributions of the three elements in the heterostructure. The mapping shows that the signal of Mo is mainly displayed in the core region, and the signal of Ni is distributed from the outer layer, which further confirms that the NiO/α-MoO3 heterojunctions are successfully obtained. Meanwhile, the EDX analysis (Fig. 3(e)) demonstrates that the elements of the heterojunctions are composed of Mo, O, and Ni and no impurities are observed. The atomic ratios for Mo and Ni are about 17:1.

The scanning TEM image (a) and corresponding elemental mappings (b-d) of Mo, O, and Ni of 3% NiO/α-MoO3 heterojunctions, respectively; EDX image (e) of the heterojunctions.
Fig. 3
The scanning TEM image (a) and corresponding elemental mappings (b-d) of Mo, O, and Ni of 3% NiO/α-MoO3 heterojunctions, respectively; EDX image (e) of the heterojunctions.

XPS analysis is conducted to confirm the chemical states of the elements for 3% NiO/α-MoO3 and pure α-MoO3. XPS survey spectrum (Fig. 4(a)) reveals that the heterojunction mainly contains Ni, Mo and O elements, which agrees well with EDX results. Fig. 4(b) gives the XPS spectra of the Ni 2p, which reveals two edges of Ni 2p1/2 and Ni 2p3/2 (Zheng et al., 2015; Jiang et al., 2019). The Ni 2p3/2 main peak and its satellite (856.3 eV and 863.3 eV) as well as Ni 2p1/2 main peak and its satellite (873.7 eV and 880.8 eV) are ascribed to the Ni2+ ions in NiO, suggesting the Ni2+ ions are dominant in the product (Jiang et al., 2019). The Mo 3d XPS spectrum (Fig. 4(c)) exhibits two peaks at binding energies of 232.5 and 235.7 eV, which relate to Mo 3d5/2 and Mo 3d3/2 in α-MoO3, respectively (Sunu et al., 2004; Sui et al., 2015). However, the Mo 3d peaks in NiO/α-MoO3 heterojunctions shift to higher energies, with the binding energies of Mo 3d5/2 and Mo 3d3/2 shifting to 232.9 and 236.1 eV, respectively. The Gauss fitting curves of the O 1s spectra (Fig. 4(d)) of pure α-MoO3 and 3% NiO/α-MoO3 contain two peaks centered at 530.6/531.0 eV and 532.1/532.5 eV, which come from the lattice oxygen in α-MoO3 and chemisorbed oxygen site (Li et al., 2017; Yang et al.,2017), respectively. It is obvious that the O 1s binding energy peaks of 3% NiO/α-MoO3 show a shift to higher value. The slight variation of Mo or O electronic structure may be attributed to the formation of heterojunction interface between two phases which would play a critical role in the photocatalytic process of the heterojunctions.

Full survey scan spectra (a) and Ni 2p XPS spectra (b) of 3% NiO/α-MoO3 heterojunction; high-resolution XPS spectra of Mo 3d (c) and O 1s (d) of bare α-MoO3 and 3% NiO/α-MoO3 heterojunction.
Fig. 4
Full survey scan spectra (a) and Ni 2p XPS spectra (b) of 3% NiO/α-MoO3 heterojunction; high-resolution XPS spectra of Mo 3d (c) and O 1s (d) of bare α-MoO3 and 3% NiO/α-MoO3 heterojunction.

The optical absorption properties of α-MoO3, NiO and a series of NiO/α-MoO3 heterojunctions are investigated by UV–Vis diffuse reflectance spectroscopy (DRS) in Fig. 5(a). The bare NiO has a pronounced light absorption in the wavelength region 200–800 nm because of its dark color (Li et al., 2021; Sabzehparvar et al., 2021). While for pure α-MoO3, it displays a steep absorption edge at 420 nm and exhibits the intrinsic absorption in UV region and the weak absorption band in visible wavelength. By contrast, the absorption edge of the composites shows an obvious shift towards the visible range upon decoration of NiO, which might be caused by the strong interactions between α-MoO3 and NiO of the heterostructure (Sabzehparvar et al., 2021). The red shift of the absorption wavelengths is favorable for the NiO/α-MoO3 heterojunctions to absorb light in the broader region and enhance the visible light photoresponse. The optical band gap can be calculated according to the Tauc equation: (αhv)2 = A (hv-Eg). As shown in Fig. 5(b), the estimated band gaps of α-MoO3 and NiO by the plot of (αhν)2 vs. hν are 3.25 and 3.08 eV (vs. NHE), respectively, which are identical with the previous studies (Sun et al., 2016; Li et al., 2017). To further get the band position of pure α-MoO3 and NiO, XPS valence band spectra is employed to investigate the VB energy levels. As shown in Fig. 5(c), the maximum energy edges of VB density of states for pure α-MoO3 and NiO are approximately at 3.27 eV and 0.53 eV (vs. NHE), respectively. Furthermore, the position of CB is calculated according to the formula: Eg = EVB- ECB. Thus, the CB values of α-MoO3 and NiO are calculated to be 0.02 and −2.55 eV, respectively. According to the above computed results, the energy-levels of the semiconductors are worked out in Fig. 5(d). It reveals that the VB and CB positions of NiO are negative than those of α-MoO3, demonstrating the suitable energy band structures for them to construct the heterostructure composites.

(a) UV–vis DRS of α-MoO3, NiO and a series of NiO/α-MoO3 heterojunctions; (b) Eg values of α-MoO3 and NiO; (c) XPS valence band spectrum for α-MoO3 and NiO. (d) The band structure diagrams of α-MoO3 and NiO.
Fig. 5
(a) UV–vis DRS of α-MoO3, NiO and a series of NiO/α-MoO3 heterojunctions; (b) Eg values of α-MoO3 and NiO; (c) XPS valence band spectrum for α-MoO3 and NiO. (d) The band structure diagrams of α-MoO3 and NiO.

The photocatalytic activities of as-prepared samples are investigated by the MB photodegradation under simulated solar light irradiation. Fig. 6(a) shows the photodegradation efficiency of the photocatalysts. Obviously, for the blank experimental analysis, it can be seen that MB is hardly degraded under visible and catalyst-free conditions, which fully shows that MB is very stable in the aqueous solution. The MB removal efficiency of pure α-MoO3 and NiO is only 41.3% and 49.8% in 2 h, respectively, implying the poor degradation efficiency of the pristine samples. While, after the addition of different amount of NiO, the photocatalytic performance is significantly enhanced. The photoactivity of the NiO/α-MoO3 heterojunctions follows the order of 3% NiO/α-MoO3 > 5% NiO/α-MoO3 > 1% NiO/α-MoO3, with the removal efficiency of 96.5%, 77.4% and 66.5%, respectively. The results show that 3% NiO/α-MoO3 exhibits the best photocatalytic performance, and the MB is almost completely degraded after 2 h of light irradiation (Fig. 6(b)). The reaction kinetics of the MB degradation by different photocatalysts is fitted to a first order model and Fig. 6(c) displays the plot between Ln(C0/C) and reaction time. The corresponding apparent rate constants of the catalysts for MB degradation are 0.00375, 0.00515, 0.00903, 0.02685 and 0.01143 min−1, respectively (Fig. 6(d)). Noticeably, the k value for MB degradation over 3% NiO/α-MoO3 (0.02685 min−1) is 7.16 folds than that of bare α-MoO3 (0.00375 min−1). To verify the universality of our catalysts, we selected some other organic contaminants, such as RhB and phenol, for photocatalytic degradation over 3% NiO/α-MoO3. As exhibited in Fig. S3, 3% NiO/α-MoO3 exhibits good photocatalytic activity towards other organic contaminants.

(a) Photodegradation efficiency of MB over various photocatalysts; (b) Absorption spectra of MB with time using 3% NiO/α-MoO3; (c) The pseudo-first-order reaction kinetics and (d) apparent rate constants of the photocatalysts for MB degradation.
Fig. 6
(a) Photodegradation efficiency of MB over various photocatalysts; (b) Absorption spectra of MB with time using 3% NiO/α-MoO3; (c) The pseudo-first-order reaction kinetics and (d) apparent rate constants of the photocatalysts for MB degradation.

In addition to photodegradation efficiency, the photostability of the catalysts is also a very significant factor for practical applications. In order to evaluate the photostability and degradation rate for 3% NiO/α-MoO3 heterojunction, the cycling runs in the MB degradation are carried out. Fig. 7(a) displays that the photocatalytic degradation rate of 3% NiO/α-MoO3 heterojunction is retained at 94.3% after five successive experimental runs, confirming that 3% NiO/α-MoO3 heterojunction is very stable during the photocatalytic degradation processes. No apparent crystalline structure and morphology’s changes are observed in the XRD pattern (Fig. 7(b)) and SEM image (Fig. S4) of 3% NiO/α-MoO3 after cyclicing experiment, which further demonstrates the significant stability of the photocatalyst. Furthermore, when compared with other reports for photodegradation of MB (Table S1), this work displays better photocatalytic degradation rate constant. Therefore, it can be inferred that the 3% NiO/α-MoO3 heterojunction is a reusable photocatalyst for application in treatment of organic pollutants from wastewaters.

(a) Cycling runs of 3% NiO/α-MoO3 heterojunction for photodegradation of MB; (b) XRD patterns of 3% NiO/α-MoO3 before and after 5 runs.
Fig. 7
(a) Cycling runs of 3% NiO/α-MoO3 heterojunction for photodegradation of MB; (b) XRD patterns of 3% NiO/α-MoO3 before and after 5 runs.

Nitrogen adsorption–desorption isotherm curves of as-prepared photocatalysts are shown in Fig. 8(a). It can be seen that the loading of NiO nanoparticles on α-MoO3 nanobelts results in an enhancement of specific surface area (13.32 m2 g−1) in comparison with pristine α-MoO3 (7.34 m2 g−1). The heterojunction with the higher surface area can provide more active sites, thus improve the adsorption capacity and photocatalytic degradation activities (Shi et al.,2020). To investigate the influence of NiO introduction on the behavior of charge carriers, several photo-electrochemical measurements including PL, photocurrent response measurement and EIS are carried out to reveal the intrinsic charge separation characteristics. In Fig. 8(b), PL spectra is displayed to probe the photo-generated electron transfer of the α-MoO3 and 3% NiO/α-MoO3 heterojunction. Pristine α-MoO3 has a strong emission peak centered at 460 nm, which possesses the strongest PL emission intensity. After the loading of NiO, the PL emission intensity of 3% NiO/α-MoO3 exhibits a rapid decrease compared with the pure α-MoO3. Ordinarily, a weaker PL intensity implies a lower recombination rate of photoinduced electron-hole pairs (Guo et al., 2021; Wang et al., 2021). Thus, the decoration of NiO can provide more electron-hole capture sites, which leads to enhanced photocatalytic activity. The transient photocurrent response spectra of 3% NiO/α-MoO3 and bare α-MoO3 are recorded for four intermittent light on–off circulations in Fig. 8(c). The pure α-MoO3 has a poor light current response of about 0.01 μA/cm2, while the photocurrent response of 3% NiO/α-MoO3 is much higher at about 0.04 μA/cm2. This indicates the faster transfer rate of photo-generated electrons of 3% NiO/α-MoO3 heterojunction, leading to a more efficient carriers transfer process (Zhang et al., 2021; Zhou et al., 2021). Moreover, the EIS Nyquist plots of the photocatalysts are exhibited in Fig. 8(d) and 3% NiO/α-MoO3 heterojunction shows the smaller arc radius than bare α-MoO3. The result further indicates that the heterojunction possesses a lower charge-transfer resistance and thus a faster charge-transfer rate in the photodegradation process (Lu et al., 2019).

(a) Nitrogen adsorption–desorption isotherms, (b) PL spectra, (c) transient photocurrent responses and (d) EIS Nyquist spectra of bare α-MoO3 and 3% NiO/α-MoO3 heterojunction.
Fig. 8
(a) Nitrogen adsorption–desorption isotherms, (b) PL spectra, (c) transient photocurrent responses and (d) EIS Nyquist spectra of bare α-MoO3 and 3% NiO/α-MoO3 heterojunction.

For the purpose of explore the main reactive species of 3% NiO/α-MoO3 during MB photodegradation, the trapping experiments are implemented with ethylenediamine tetraacetate (EDTA-2Na) as the hole (h+) scavenger, benzoquinone (BQ) as the superoxide radical (•O2−) scavenger and isopropanol (IPA) as the hydroxyl (•OH) scavenger. As displayed in Fig. 9(a), when no scavenger is added in the system, the degradation rate of 3% NiO/α-MoO3 is still 96.5% within 120 min. In contrast, when EDTA-2Na, BQ and IPA are added, the degradation rate is significantly restrained, indicating that h+, •O2− and •OH are all the key active species in the photodegradation process. Spintrapping ESR technology is conducted to further verify the •O2− and •OH generated in the photocatalytic processes. The ESR signals of 3% NiO/α-MoO3 under xenon lamp irradiation and in the dark are presented in Fig. 9(b). No characteristic peaks of active species can be observed under dark conditions. However, six clear signals of •O2− appear under xenon lamp irradiation, which proves the presence of •O2− in the photodegradation process over 3% NiO/α-MoO3 heterojunction photocatalyst. Moreover, when 3% NiO/α-MoO3 heterojunction is irradiated under xenon lamp, four characteristic signals of •OH with intensity of 1:2:2:1 are detected. The ESR results reveal that both •O2− and •OH are involved, and they can play important roles in decomposing MB dye (Li et al., 2020).

(a) Active species trapping experiments of MB degradation over 3% NiO/α-MoO3; (b) ESR spectra of 3% NiO/α-MoO3 heterojunction: DMPO- •O2 - in methanol dispersion and DMPO- •OH in aqueous dispersion.
Fig. 9
(a) Active species trapping experiments of MB degradation over 3% NiO/α-MoO3; (b) ESR spectra of 3% NiO/α-MoO3 heterojunction: DMPO- •O2 - in methanol dispersion and DMPO- •OH in aqueous dispersion.

Considering the experimental results described above and the calculated band gap structures, the proposed mechanism of NiO/α-MoO3 heterojunction under light irradiation is presented in Fig. 10. Due to the well-matched VB and CB positions, a heterojunction can form between α-MoO3 and NiO, and two possible mechanisms are proposed: (a) traditional heterojunction-type II and (b) Z-scheme type. The CB and VB potentials of NiO (−2.55 eV vs. NHE and 0.53 eV vs. NHE) are more negative than those of α-MoO3 (0.02 eV vs. NHE and 3.27 eV vs. NHE, respectively). If the charge transfer route follows the traditional type-II mechanism in Fig. 10(a), the electrons in the CB of NiO will transform to the CB of α-MoO3 and holes in the VB of α-MoO3 move to the VB of NiO. Consequently, the electrons will accumulate on the CB of α-MoO3, and the holes will accumulate on the VB of NiO. However, the system may not have high reduction potential (only 0.02 eV for CB of α-MoO3) and oxidation potential (only 0.53 eV for VB of NiO) for generating •O2− (−0.33 eV) and •OH radicals (1.99 and 2.74 eV) (Li et al., 2020; Zeng et al., 2021). This presumption is contradictory to the ESR results, in which •O2− and •OH are detected to be the main active species during the photodegradation process. Thus, it is supposed that a more reasonable Z-scheme photoelectron migration emerges in the p-n heterojunction.

Two models of charge migration for NiO/α-MoO3 p-n heterojunction: (a) traditional heterojunction-type II (b) and Z-scheme type.
Fig. 10
Two models of charge migration for NiO/α-MoO3 p-n heterojunction: (a) traditional heterojunction-type II (b) and Z-scheme type.

NiO is a p-type material, and the Fermi level is close to the VB; α-MoO3 is a n-type material, and the Fermi level is near to the CB (Teng et al.,2017; Sun et al., 2016; Zhao et al.,2021; Wang et al., 2020). As shown in Fig. 10(b), EF of NiO is higher than that of α-MoO3. After contact of NiO with α-MoO3, electrons will transfer from NiO to α-MoO3 until unified EF is fabricated. At the equilibrium state, NiO loses electrons and is positively charged; while α-MoO3 acquires electrons and forms an electron-rich layer (Xu et al., 2019). This process forms an internal electric field at the interface and induces energy band bending (Deng et al., 2021). When NiO/α-MoO3 p-n heterojunction is irradiated by the simulated sunlight, the photoinduced electrons at the CB of α-MoO3 will rapidly transfer to the VB of NiO and recombine with the photoexcited holes at the VB of NiO. Therefore, the corresponding photogenerated electrons and holes are left at the CB of NiO and the VB of α-MoO3, respectively, which retains the high oxidation and reduction capabilities of the NiO/α-MoO3. Meanwhile, useless photogenerated electrons (from the CB of α-MoO3) and holes (from the VB of NiO) are consumed relatively, and useful charge carriers are preserved effectively through the intimate heterojunction interface, which can be sufficiently derived in the photocatalytic system (Li et al., 2019). Therefore, •O2− and •OH should be the main active species in the reaction and degrade MB, which was consistent with the result of ESR. This Z-scheme heterojunction not only effectively promote the separation of photogenerated charges, but also retain the prominent redox potential (Muñoz-Batista et al., 2016; Zheng et al., 2019).

4

4 Conclusions

In summary, the Z-scheme NiO/α-MoO3 p-n heterojunction has been prepared through a two-step hydrothermal route. And the decoration of NiO is demonstrated to be a promising approach to design highly active and stable α-MoO3 based photocatalysts. According to the SEM/TEM/EDX/XPS analysis, the NiO nanoparticles are successfully grown on α-MoO3 nanobelts, forming heterojuction at the interface of NiO and α-MoO3. Compared with pure α-MoO3 nanoblets, the NiO/α-MoO3 heterojunction exhibits enhanced photocatalytic activity, achieving 96.5% photocatalytic efficiency within 2 h under simulated solar light irradiation. Meanwhile, the heterostructures retain relatively consistent activity after five repeat cycles. A Z-scheme charge transfer process between NiO and α-MoO3 can be inferred, which significantly accelerates the charge separation. This study provides a reference for the future research of MoO3-based p-n heterojunction photocatalysts.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (21906039), Hebei Province 333 Talents Project (A202101020), Funding Project for Introduced Overseas Scholars of Hebei Province (C20190321), Science and Technology Project of Hebei Education Department (BJ2021010), Doctoral research fund of Hebei Geo University (BQ2019041), Funding for basic scientific research of universities in Hebei Province and Funding for the Science and Technology Innovation Team Project of Hebei GEO University (KJCXTD-2021-02).

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Appendix A

Supplementary material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.arabjc.2021.103513.

Appendix A

Supplementary material

The following are the Supplementary data to this article:

Supplementary data 1

Supplementary data 1


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