The Synthesis of h-BN-Modified Z-Scheme WO3/g-C3N4 Heterojunctions for Enhancing Visible Light Photocatalytic Degradation of Tetracycline Pollutants.

The photocatalytic performance of common photocatalysts is limited by their low surface area, insufficient absorption of light energy, and fast photogenerated electron-hole recombination rate. The introduction of Z-scheme photocatalysts decorated with hexagonal boron nitride (h-BN) has already been confirmed to be an effective way to extend the surface area and increase the charge separation, thereby enhancing the photocatalytic performance. In this study, a hexagonal boron nitride (h-BN)-decorated WO3/g-C3N4 heterojunction photocatalyst was successfully synthesized via an in situ method using tungstic acid, melamine, and hexagonal boron nitride as the precursors. The physical and chemical properties of the resulting samples were thoroughly characterized. The surface, morphological, and optical properties of the resulting materials were thoroughly characterized by XRD, XPS, SEM, TEM, UV-vis DRS, BET surface areas, PL, and ESR analysis. The WO3/g-C3N4/BN composite exhibited a much higher photocatalytic activity for tetracycline degradation under visible light irradiation than pure g-C3N4, WO3, and BN. The favorable photocatalytic activity of WO3/g-C3N4/BN composites can be ascribed to the increased surface area and enhanced separation efficiency of photogenerated electron-hole pairs by adding h-BN nanosheets and forming the WO3/g-C3N4 heterojunction. This work indicates that the WO3/g-C3N4/BN photocatalyst is a promising material in wastewater treatment.

Introduction

In recent years, severe water contamination, especially from poisonous and hazardous pollutants, has aroused a significant and wide impact concern of the scientific community.[1] Semiconductor-based photocatalysis using solar energy has been rapidly developed in sewage treatment, which receives increasing attention.[2,3] As is known, two problems must be resolved in the field of photocatalyst utilization. First, it should extend the excitation wavelength range of photocatalysts to take full advantage of solar energy. In addition, another key is how to improve efficiently the photoexcited electron–hole separation.[4] Nowadays, conventional photocatalysts of metal oxide semiconductors, such as TiO2, respond to narrow spectral widths, which wastes a lot of light energy.[5−7] Single-component photocatalysts were still confined by the intrinsic drawbacks of poor visible light harvesting and low quantum yield, which results in a poor application prospect in a large scale.[8] How to obtain photocatalysts with efficient photocatalytic activity and optimized high efficiency is very important. In this regard, various efforts including element doping, copolymerization, surface sensitization, and heterojunction engineering improve the utilization of photocatalysts.[9−12] Among these methods, heterojunction engineering of photocatalysts has been widely used as a method to solve the problems of photocatalysts.[13,14] In general, the construction of heterostructure photocatalysts has become an impressive and feasible method to reduce the recombination of photogenerated electron–hole pairs and retain the prominent redox ability.[15,16] This makes heterojunction photocatalysts a potential way to solve global energy shortage and environmental pollution problems. Recently, various photocatalysts have been designed, such as BiOBr-Bi2MoO6,[17] Bi2Fe4O9/Bi2WO6,[18] BiOI/Ag@AgI,[19] and Ag@ AgCl/BiVO4.[20] Among these catalysts, the resulting relatively high cost and low degradation rate were common problems for practical application. Thus, finding the right combination of photocatalysts has become a hotspot of environmental governance.[21,22]

Currently, graphitic carbon nitride (g-C3N4) is a hot research topic for photocatalytic degradation of organic pollutants due to its non-toxicity, good physical and chemical stability, low cost, and high visible light response (band gap of 2.7 eV).[23,24] However, due to its high complexation rate of photogenerated electron–hole pairs, the photocatalytic activity of g-C3N4 under visible light irradiation is low, thus severely limiting the application of g-C3N4.[25] In order to overcome these defects, researchers have performed a lot of work on the design of straight Z-type heterojunctions. For example, Zhang et al. successfully synthesized Bi2O3/g-C3N4 heterostructures by a simple heat treatment method, in which a much higher visible light activity than pure g-C3N4 was obtained.[26] Wang et al. prepared indirect Z-scheme BiOI/g-C3N4 photocatalysts to facilitate efficient separation of photogenerated carriers. Their photoreduction CO2 activity under visible light irradiation exhibited a more efficient photocatalytic activity than pure g-C3N4 and BiOI, among others.[27] It is well known that tungsten oxide (WO3) is a promising catalyst for the photocatalytic degradation of pollutants.[28] The ECB (conduction band edge) and EVB (valence band edge) of WO3 are about 0.74 and 3.44 eV, respectively.[4] It has interesting properties such as being an n-type material with direct and finite band gaps.[4] This allows the combination of tungsten oxide with g-C3N4 to greatly enhance the photocatalytic performance. This conclusion was then verified in a study by Chen et al.[4] However, binary star systems are always limited by the rate of light absorption and charge complexation.[29]

Recently, several scientists have carried out attempts to apply two-dimensional carbon-based nanomaterials such as graphene, fullerenes, carbon dots, and carbon nanotubes (CNTs) as electron transfer channels for g-C3N4 to decelerate the recombination rate of photogenerated electron–holes and obtain efficient photocatalysts.[30,31] For example, the metal-free hybrid g-C3N4/rGO prepared by Wang et al. has an enhanced photodegradation rate. The reason is that GO acts as an excellent separating electron acceptor and transporter in nanocomposites, which contributes to enhancing the photocatalytic performance.[32] Chen et al. produced CNT-modified g-C3N4 with enhanced visible light photocatalytic activity because the light-generated charge carriers can be separated efficiently.[33] Pan et al. prepared carbon quantum dot-modified porous g-C3N4/TiO2 nanoparticle heterojunctions, in which the carbon quantum dots can promote the separation of photogenerated carriers and improve the utilization of visible light.[34]

Hexagonal boron nitride (h-BN) has a similar structure to graphene and has good properties similar to the graphene structure. In addition, it is chemically stable and has strong oxidation resistance and good thermal conductivity.[35,36] It can be used as a carrier for photocatalysts to enhance their activity.[37,38] More importantly, h-BN in the form of two-dimensional nanosheets is highly electronegative. Attracting photoexcited holes will help prevent the rapid recombination of light-induced electron–hole pairs.[35] In addition to this, it has been found that some conductive carbon materials, especially graphene and carbon nanotubes, are used as electron transfer channels due to accelerated separation of photoexcited charge carriers.[39,40] These carbon materials can provide more active sites, improve visible light absorption, and facilitate charge transfer to effectively hinder the recombination of photoexcited charge carriers.[41] In particular, the photocatalytic activity of h-BN can be greatly improved by increasing the transfer of h+. Theoretical work revealed that the characteristic recombination time of surface h+ is much shorter than e–, about 10 ns vs 100 ns.[42] Due to electrostatic effects, the charged h-BN can facilitate the migration of h+ out of the main body of the photocatalyst. Moreover, the alteration of B and N atoms leads to the ionic properties of BN crystals. The negatively charged BN comes from stable defects associated with nitrogen vacancies (N-side triangular defects) or carbon impurities.[38] However, the construction of h-BN nanosheet-decorated g-C3N4-WO3 as efficient photocatalysts for TC degradation using visible light irradiation has been rarely reported.

A direct h-BN nanosheet-decorated Z-scheme g-C3N4-WO3 was synthesized by a facile co-calcination strategy in this work, which displays excellent photocatalytic activity. The chemical compositions, morphology structures, photocatalytic activities, and photostability of the resulting samples were thoroughly explored by XRD, XPS, SEM, TEM BET, UV–vis DRS, PL, ESR, and so on. The photocatalytic performance of WO3/g-C3N4/h-BN was investigated by the degradation of tetracycline (TC) under visible light irradiation. It could be found that the WO3/g-C3N4/h-BN composite showed a much higher photocatalytic performance than pure g-C3N4, WO3, and h-BN under visible light irradiation (λ > 420 nm). Furthermore, the probable photocatalytic mechanism for the key role of h-BN with regard to the Z-scheme photocatalyst was proposed and the main active substances in photocatalytic activity were determined.

Experimental Section

Synthesis of the WO3/g-C3N4/h-BN Composite

g-C3N4 was prepared using a modified version of a previously reported thermal treatment method (see the Supporting Information for specific detailed operations).[26]

The WO3/h-BN, g-C3N4/h-BN, g-C3N4/WO3, and WO3/g-C3N4/h-BN photocatalysts were prepared by a solid-phase calcination method (see the Supporting Information for the specific detailed operations). After cooling down to room temperature, the resulting products were milled into powder and labeled as BW, g-CB, CW, and WCB, respectively.

Results and Discussion

XRD Analysis

The crystalline phases of the as-prepared samples were investigated by X-ray diffraction (XRD) (Figure and Figure S1). Two characteristic peaks of g-C3N4 at 13.1° for (100) and 27.6° for (002) (JCPDS 87-1526) were obvious. These two diffraction peaks are in good accordance with the found results of g-C3N4 in ref (43). The WO3 XRD patterns of all the diffraction peaks were in good agreement with those of orthorhombic WO3 (JCPDS 20-1324).[44] For the hexagonal structure of pure h-BN, the distinct diffraction peaks at 2θ of 26.8, 41.6, 43.9, 50.1, 55.2, 71.4, and 75.9° were attributed to the (002), (100), (101), (102), (004), (104), and (110) planes (JCPDS 34-0421). As for WO3/g-C3N4/h-BN, the characteristic peaks are observed in Figure b. The peaks that belong to monoclinic WO3 can be obviously found in the XRD pattern of WO3/g-C3N4/h-BN curves. However, no clear boron nitride peaks can be detected in WO3/g-C3N4/h-BN composites, which might be due to the limited amount of h-BN on these samples or high dispersion of h-BN. Similar results were found in other studies.[45,46] The same XRD patterns as g-C3N4, which were shown as the typical (100) and (002) diffraction planes, were also found. These results indicated that a relatively pure composite material was synthesized.

Figure 1

(a) XRD patterns of the as-prepared samples. (b) Detailed information of WCB.

Chemical State Analysis

The surface elemental composition and electronic state of the WCB composite were determined by the XPS measurements (Figure ). Figure a presents the full spectra of the acquired WCB composite. Five elements, C, N, W, B, and O, were detected in the XPS survey spectra (Figure a). Moreover, high-resolution XPS spectra of C 1s, N 1s, W 4f, B 1s, and O 1s were obtained. As revealed in Figure b, a peak in the high-resolution spectra of C 1s at 284.8 eV is assigned to the C–C bonds.[47] At 288.1 eV, the main peak of C 1s could be observed, which is attributed to the tertiary carbon C-(N)3 in the graphitic-like carbon nitride.[48] Two characteristic peaks located at 399.8 and 398.2 eV (Figure c) are attributed to the N atoms sp2-bonded to two carbon atoms (C=N–C) and N3– in the BN layer, respectively.[49,50] There are two peaks at 35.5 and 37.7 eV (Figure d), which correspond to the characteristic W 4f 7/2 and W 4f 5/2 peaks for WO3, respectively.[51] As displayed in Figure e, the typical B 1s spectrum shows the predominant peak at 190.6 eV, which could be attributed to B–N bonds.[46] The O 1s spectrum in Figure f can be deconvoluted into two peaks at 532.2 and 530.2 eV, which are ascribed, respectively, to hydroxyl groups and lattice oxygen in WO3.[52] Therefore, the results of XPS confirm that the successful composite of WO3, h-BN, and g-C3 N4 exists in the WCB heterostructures.

Figure 2

XPS spectra of the WCB composite. (a) Survey of the sample; (b) C 1s; (c) N 1s; (d) W 4f; (e) B 1s; and (f) O 1s.

Morphology Characterization

The morphologies and shapes of g-C3N4, WO3, h-BN, and the as-synthesized WCB composites were further investigated by TEM and HRTEM techniques. Figure a displays that the bulk g-C3N4 are composed of a large number of crystal stacking layers with a large size and lamellar structure, which is the typical characteristic structure of g-C3N4 synthesized by a polymerization method.[53] As shown in Figure b, it can be observed that “because of aggregation and gathering of the h-BN sheets”, h-BN displays a thin layered accumulation sheet-like structure. Indeed, as seen from Figure f, h-BN presented a sheet-like morphology, which is confirmed by TEM observations.[54] WO3 mostly displayed a bulky morphology composed of many irregular agglomerates (Figure c). As for WCB, it revealed that some small particles were dispersedly intercalated within the bulk g-C3N4 (Figure d). In addition, the specific morphology was further determined by TEM. The enlarged view in Figure h showed that the WO3 nanobulk and h-BN sheet were partially wrapped within the g-C3N4 microstructure. Particularly, the TEM image of the WCB sample displays that the surface of layered g-C3N4 has randomly distributed highly dispersed WO3 pieces in nanometer size and small sheet-like h-BN from Figure h.[55] To further investigate the particular crystal structure of the ternary system, the samples were observed by HRTEM. From Figure i, due to aggregation and accumulation of the g-C3N4 layers, it is seen that the composites possessed a worm-like pore structure, consisting of wrinkles.[56] As for Figure k, the lattice spacing of 0.377 nm corresponds to the (020) plane of WO3. Moreover, the HRTEM of WCB showed that small sheet-like h-BN and the typical (020) plane of WO3 were found covering the g-C3N4 surface or embedded in g-C3N4 layers without fewer agglomeration, which not only confirm the coexistence of WO3, h-BN, and g-C3N4 phases (Figure l) but also provide more photocatalytic reaction sites.

Figure 3

SEM images of (a) g-C3N4, (b) BN, (c) WO3, and (d) WCB. TEM micrographs of (e) g-C3N4, (f) BN, (g) WO3, and (h) WCB. (d) High-resolution images of (i) g-C3N4, (j) BN, (k) WO3, and (l) WCB.

Specific Surface Areas and Optical Properties

The textural properties of the as-prepared samples were investigated using nitrogen gas porosimetry measurement. Figure depicts the nitrogen adsorption–desorption isotherms and pore size distribution curves of the four samples. Apparently, it can be seen that all of them have type IV isotherms with type H3 hysteresis loops, suggesting the presence of mesopores within the composites.[57,58] As depicted in Table , the specific surface areas and the pore size distribution of the samples were calculated by BET and BJH methods.[59] Intriguingly, compared with single WO3, g-C3N4, and CW, the BET surface area of WCB composites increases. Therefore, it suggests that the WCB composite owns a better photocatalytic activity compared with a single material for the degradation of organic pollutants.

Figure 4

N2 adsorption–desorption isotherms of the as-prepared samples.

Table 1

BET Surface Areas, Pore Diameters, and Pore Volumes of the As-prepared Samples

photocatalysts	SBET (m2 g–1)	pore diameter (nm)	pore volume (cm3 g–1)
CN	25.7	17.7	0.114
WO3	13.9	20.8	0.072
CW	29.1	9.0	0.056
WCB	34.9	14.71	0.119

The optical properties of the as-prepared ternary composite and single WO3, g-C3N4, and h-BN were investigated by UV–vis DRS. As shown in Figure a, the bare WO3 and pure g-C3N4 display a similar intensive absorption edge nearby 480 nm, while h-BN shows no obvious wavelength absorption edge in the visible spectrum. As we know, its electronic structure affects the light absorption performance of optical materials.[53] The absorption edge of WCB only had a slight red shift and the visible absorbance appropriately decreased, which may be ascribed to the interfacial interaction between different single materials, affecting the light absorption properties.[60] However, all WCB composites still can be applied for visible light photocatalysis due to significant visible light absorption. Generally, it should be considered that the enhancement of the photocatalytic activity of the synthesized sample may be due to the charge separation and migration. The PL emission spectra were employed to investigate the combination and separation of the photoinduced carriers, which played a crucial role in photocatalytic reactions.[61] Commonly, the low PL emission intensity corresponds to the high photocatalytic performance due to the lower recombination efficiency of the photoinduced electron–hole pairs.[53]Figure S2 exhibits the PL spectra of pure g-C3N4, g-C3N4/WO3, g-C3N4/h-BN, and WCB at an excitation wavelength of 350 nm. In addition to the WCB sample, all the other samples display similar shapes and high intensity around 445–470 nm due to the high recombination rate of electron–hole pairs. Apparently, the WCB sample exhibit a dramatically diminished PL intensity in comparison to the pure g-C3N4 and the composites, demonstrating that the interaction of WO3 nanopieces, h-BN sheets, and g-C3N4 can be favorable for the separation of photogenerated electron–hole charges.

Figure 5

(a) UV–vis spectrum and (b) band gaps of g-C3N4, WO3, BN, and WCB.

Generally, the band gap energies (Eg) of synthetic optical materials can be estimated by the following relation:[56]where h, α, ν, A, and Eg are the Planck constant, the absorption coefficient, the light frequency, a constant, and the band gap energy, respectively. The optical band gaps can be estimated by a plot of (αhν)1/2 vs hν (n = 4 for indirect transition). For the pure g-C3N4, it was found that the band gap is sure to be 2.54 eV by extrapolating the tangent line of the curve to the hν axis intercept. Similarly, the band gap of WO3 was estimated to be about 2.51 eV. In addition, it can be discovered that the band gap energy of WCB was 2.43 eV. Thus, one can conclude that the band gap of WCB is slightly diminished.

Photocatalytic Activity and Stability Evaluation

The photodegradation of TC under visible light irradiation was measured to evaluate the photocatalytic activity of the as-prepared samples. According to the results, the photolysis process can be ignored because TC is stable as a molecule and can only be slightly degraded without a photocatalyst. The as-prepared single, binary, and ternary samples exhibited quite different performances. All of the prepared samples showed certain catalytic capacities as a result of the gradually decreased concentration of TC along with exposure time. The photocatalytic activities of all samples were in the order of WCB > pure g-C3N4 > pure h-BN > pure WO3 under visible light irradiation (λ > 420 nm), in which the WCB composite displayed much better photocatalytic activities than other samples (Figure a). The poor visible light absorption capability and fast electron–hole recombination of pure WO3 led to the lowest TC removal efficiency of only ca. 19.2% under visible light irradiation for 60 min as compared to the 62.7% of g-C3N4 and 55.5% of BN. In addition, the photocatalytic activities of pairwise combination of the three materials were also investigated. CW exhibited a degradation efficiency of approximately 65.9% while the degradation efficiency of g-CB was 68.9%, indicating that introduction of h-BN significantly improved the catalytic performance. The photocatalytic activity was expected to be enhanced when WO3, h-BN, and g-C3N4 were combined. As a matter of fact, the WCB nanocomposite indeed displayed the highest degradation efficiency (ca. 81.4%). It has been confirmed by UV–vis spectrum, N2 adsorption–desorption isotherms, and PL emission that the combination of WO3, h-BN, and g-C3N4 enhanced the photocatalytic activity by improving the visible light absorption, increasing the surface area, and enhancing the separation efficiency of the photogenerated electron–hole charges of the photocatalyst. All the facts suggested that the formation of the WCB ternary composite and the synergistic effects between each component could cause the formation of an h-BN hole attraction or a Z-scheme junction at the interface between different components, which is consistent with the previous reports (Figure b),[54,62] drastically enhancing the photocatalytic performance.

Figure 6

(a) Photocatalytic activities of the as-prepared samples for TC degradation under visible light (λ > 420 nm). (b) Apparent rate constants for TC degradation.

The reaction kinetics of WCB samples for photodegradation of TC are modeled by the pseudo-first-order kinetics model: ln(C/C0) = −kt, where C0 and C are the initial concentration and instant concentration at reaction time t and k is the rate constant. The corresponding plot of ln(C/C0) ∼ t exhibits a good linearity,[63] as shown in Figure S3. The values of the rate constant k for all the samples are more intuitively displayed in Figure d, in which WCB showed the highest rate constant. All results well demonstrated that the formation of the WO3/g-C3N4/h-BN composite appreciably promoted the photocatalytic activity of g-C3N4.

Figure 8

Species trapping experiments for the degradation of TC over WCB photocatalysts under visible light irradiation.

For practical application, the stability of the catalyst, that is, its lifetime, has to be evaluated. Therefore, under the same conditions, degradation experiments of TC by the WCB composite were repeated five times to evaluate the stability of the as-prepared photocatalysts. Between each cycle, the photocatalysts were collected by centrifugation, then washed, and dried before the next run. After five successive recycles, the photocatalytic activities of WCB composites dropped to only 8.2% (Figure a). Furthermore, the XRD patterns of the WCB composite before and after the fifth cycle in Figure b indicated that, compared with the non-irradiated composites, the phase structure of the regenerated WCB composites has no obvious difference except that two diffraction peaks were missing. There is no significant diffraction intensity at 13° and 36°. It is because there is so little of it (crystal preferred orientation). The results revealed its excellent stability and great potential in environmental purification.

Figure 7

(a) Repeated photocatalytic experiments of the WCB photocatalyst for the degradation of TC under visible light irradiation. (b) XRD pattern of the WCB sample after the fifth cycle of photocatalytic experiments.

Meanwhile, the catalytic activity of WCB samples was compared with various photocatalysts reported in the literature for the photocatalytic degradation of TC in this paper (see Table S1 for the detailed data). Obviously, the present catalytic system also revealed to have a higher degradation efficiency than the others reported for the catalytic removal of TC. Specifically, the TC removal and rate constants of the WCB composite were 81.4% and 0.01201 min–1 within 60 min, which is superior to other common and relevant photocatalysts such as TiO2,[64] WO3/g-C3N4,[65] N-CNT/mpg-C3N4,[66] CQDs/g-C3N4,[67] and GQDs/mpg-C3N4.[68] This verifies that WCB composites are more promising for TC removal from wastewater.

Photocatalytic Mechanism

First, the degradation pathway of TC was discussed. It is generally believed that the mechanism of the photochemical degradation reaction lies in the absorption of light energy by molecules into excited states to initiate various reactions. Antibiotic molecules directly absorb photons for a photochemical reaction called direct photolysis. The photocatalytic degradation of tetracycline (TC) involves degradation and mineralization to carbon dioxide and water.[69,70] Subsequently, the predominant active species in the photocatalytic TC degradation in the process over the WCB sample were identified. In the free radical trapping tests (Figure ), the photocatalytic degradation of TC was least affected (only suppressed to 57.5%) when isopropanol (IPA, a scavenger of hydroxyl radicals (·OH)) was added, indicating that ·OH is not the main reactive species. However, it can vastly suppress the photocatalytic degradation of TC (suppressed to 29.9 and 49.9%) when BQ (1,4-benzoquinone) and TEA (triethanolamine) were added, in which TEA,[49] BQ,[71] and IPA[72] were used as the scavengers of holes (h+), superoxide radicals (·O2–), and hydroxyl radicals (·OH), respectively. These results demonstrate that ·O2–, h+, and ·OH are the active species for the WO3/g-C3N4/h-BN sample in the photocatalytic process, while ·O2– makes a major contribution to it. Moreover, the sequence of effects is as follows: ·O2– > h+ > ·OH.

Furthermore, the ESR spin-trap technique was employed to confirm the emergence of ·OH and ·O2– radicals during the optical radiation process with the DMPO radical scavenger. The signals of the formed radical species for the WCB sample are shown in Figure ; there are no distinct characteristic peaks for both DMPO-·O2– and DMPO-·OH under dark conditions. However, when under visible light irradiation, the prominent characteristic peaks of DMPO-·O2– appeared in the methanol system of WCB. The result showed that the ·O2– radical species were produced in the photocatalytic degradation. The larger amount and long lifetime of ·O2– were beneficial to the outstanding photocatalytic activity of WCB. Furthermore, we can also find the typical characteristic peaks of DMPO-·OH (Figure b), which indicates that the ·OH radicals are also present and worked in WCB reaction systems.

Figure 9

DMPO spin-trapping ESR spectra with the WCB sample (a) in methanol dispersion (for DMPO-·O2–) and (b) in aqueous dispersion (for DMPO-·OH) under visible light irradiation.

A simple approach can be used to measure the band edge positions of CB and VB of a semiconductor.[56] The EVB and ECB of a semiconductor at the point of zero charge can be calculated by the equations as follows:where X is the electronegativity of the semiconductor and Ee is the energy of free electrons on the hydrogen scale (∼4.5 eV vs NHE). According to the above formulas, the CB and VB potentials of g-C3N4 are calculated to be −1.04 and +1.50 eV, respectively. The CB and VB potentials of WO3 are estimated to be +0.74 and +3.25 eV, respectively. The results are in accordance with the reported results.[73]

Based on the analysis mentioned, a possible mechanism for TC degradation using the WCB composite is proposed and illustrated in Scheme according to the band gap structures. A direct Z-scheme mechanism was more logical and was suggested for the WCB composite photocatalysts. Under visible light irradiation, both WO3 and g-C3N4 were excited to produce electrons and holes in the system. The photoexcited electrons in the CB of WO3 could easily migrate to the photoexcited holes in the VB of g-C3N4. At the same time, the holes on the VB of WO3 and electrons on the CB of g-C3N4 with high redox ability were reserved to produce ·OH and ·O2–. In other words, a typical Z-scheme photocatalyst is beneficial to the production of ·O2– and ·OH reactive species. In addition, the holes photoexcited by g-C3N4 and WO3 are transferred rapidly to its surface due to the electrostatic attraction between the holes and the negatively charged BN and then participate in the photocatalytic reaction. The results were in accordance with the active species investigation.[38] The electrons in g-C3N4 could reduce molecular oxygen to yield ·O2– because the CB edge potential (−1.04 eV vs NHE) of g-C3N4 is more positive than that the reduction potential (−0.33 eV vs NHE) of O2/·O2–.[74] Interestingly, there are similar results in the analyzed ·OH because we can observe the typical characteristic peaks of DMPO-·OH adducts (Figure b), which indicates that the ·OH radicals are also formed by water molecules. Furthermore, the active radicals (·O2– and ·OH) participate in the removal of TC. Meanwhile, the holes accumulated on the VB of WO3 directly oxidize TC to harmless products. In this way, the electron–hole pairs could be separated effectively by adding h-BN. It was worth noticing that the combination of WO3 and g-C3N4 in the photocatalytic process would also facilitate Z-scheme structure construction and restrain charge recombination, further enhancing the visible light photoactivity of the WCB sample. Analyses of the above synthesis, including BET, DRS PL, and ESR, showed that the high photocatalytic activity of WCB composites could be due to the higher separation efficiency of electron–hole pairs and the larger surface area.[25,75]

Scheme 1

Schematic of the Separation and Transfer of Photogenerated Charges Combined with the Possible Reaction Mechanism of the Photocatalytic Procedure

Conclusions

In summary, the hexagonal boron nitride (h-BN)-decorated WO3/g-C3N4 heterojunction photocatalyst was successfully synthesized via an in situ method. The obtained WO3/g-C3N4/h-BN samples display a more efficient photocatalytic performance for TC degradation than pure g-C3N4 (50.91%),[76] WO3 (12.14%),[76] h-BN (4.4%)[25] under visible light irradiation. 81.4% TC was photodegraded under visible light irradiation for 60 min. Based on structure and electrochemical characterizations results, the enhanced photocatalytic activity is mainly attributed to the increased surface area and enhanced separation efficiency of photogenerated electron–hole pairs. The radical trapping experiment and ESR analysis confirmed that the photocatalytic active component is the ·O2–, hole, and ·OH. On the whole, under the premise that WO3 and g-C3N4 form the Z-scheme, the addition of h-BN can better promote the separation of electrons and holes and realize the degradation of tetracycline hydrochloride by free radicals.