Construction of direct Z-scheme g-C3N4/BiYWO6 heterojunction photocatalyst with enhanced visible light activity towards the degradation of methylene blue

Construction of the Z-scheme heterojunction photocatalyst achieved highly improved photocatalytic ability by its high redox ability of the photoinduced e−-h+ pairs. In the study, Z-scheme g-C3N4/BiYWO6 heterojunction photocatalyst is prepared by the single-step hydrothermal method. Further, its photocatalytic ability was assessed by degrading methylene blue under visible light exposure. Particularly, the optimized 30 wt% of g-C3N4 in the g-C3N4/BiYWO6 composite exposes almost complete degradation after 90 min, that is ~ 3.0 times greater than the bare BiYWO6 and g-C3N4 with the rate constant value 0.032 min−1. Experimentally, the radical trapping studies indicate O2·− and ·OH radicals are playing a vital role in the photocatalytic degradation process. Also, the Z-scheme g-C3N4/BiYWO6 heterojunction photocatalyst exhibits excellent photoelectrochemical property and it is stable after 5 cycles, which indicates its good reusability nature. These enhancements are due to the newly formed heterostructure that facilitates the migration and separation efficiency of the photoproduced e−-h+ pairs. Hence, the synthesized Z-scheme g-C3N4/BiYWO6 heterostructure could be an excellent material for wastewater remediation works.


Introduction
Over thriving concerns about the environmental impact, imprudent mortal activities like discharging of organic and inorganic contaminants from the modern industries like paper, textile, cosmetics, and printing might end up in the destruction of aquatic environment. These industries use a number of highly noxious synthetic organic compounds for their production to improve the product value (Hao et al. 2012;Abdelkader et al. 2019;Shi et al. 2021). Hence, these compounds exert high toxicity when left untreated into the aquatic system. The industrial effluent-contaminated waterbodies will impact a significant effect on human health, terrestrial lands, and the aquatic life. Therefore, devising of effective and economically sustainable technologies is at present a vital requirement to rectify the issues previously discussed (Yi et al. 2018;Priya et al. 2020a, b;Malathi et al. 2018;Liu et al. 2021a). With respect to this, semiconductor photocatalysis is considered as a greener method for the removal of toxic organic contaminants, due to its operational process that directly use sun/solar energy. In the advanced oxidation process (AOPs), the noxious contaminants are converted into nontoxic (CO 2 and H 2 O) products which are a complete mineralization, without producing any secondary pollutants (Senthil et al. 2018;Vinitha et al. 2021). In general, though the traditional photocatalysts such as TiO 2 and ZnO possess nontoxic, highly stable, and inexpensive activities, unexpectedly, their wide bandgap (E g ) (~ 3.2 eV) energy seems unfavorable for the visible light absorption (VLA) Rosset et al. 2021;Alanazi et al. 2021;Liu et al. 2021b). To address this issue, an absolute strategic factor is considered as an affordable design for a good visible light-responsive photocatalyst.
Recently, the bismuth-based semiconductor metal oxides are receiving worldwide attention due to its suitable bandgap and excellent absorption in the visible spectral region. Among bismuth-based metal oxides, BiYWO 6 is considered as an eminent photocatalyst for the wastewater remediation, because of its economical, easily synthesized, nontoxic, discrete surface structure and high stability (Han et al. 2018;Pasternak and Paz 2016). However, the photocatalytic performance of the BiYWO 6 is restricted, owing to the rapid reconnection of the photoproduced charge carriers (e − -h + ) which may reduce the photocatalytic performance of BiYWO 6 in large-scale applications. The formation of Z-scheme heterojunction with other semiconductors is considered as the most prospective approach to tackle this issue. This ensures the direct transition of charge carriers in the beginning of the conduction band (CB) of the oxidative photocatalyst to the valence band (VB) of the reductive photocatalyst, resulting in an improved photocatalytic performance towards the degradation of the organic pollutant under visible light illumination (VLI) . Recently, Luo et al. (2021) designed a AgI/Zn 3 V 2 O 8 -based Z-scheme heterojunction photocatalyst using the facile in situ precipitation process to degrade tetracycline under VLI. Likewise, Ma et al. (2021), too, used a simple hydrothermal method to prepare a-Fe 2 O 3 / BiVO 4 Z-scheme heterojunction photocatalyst, to assess the photocatalytic activity of the photocatalyst over the degradation of tetracycline under VLI. Similarly, Zhao et al. (2021) synthesized the ZnCdS/Bi 2 WO 6 Z-scheme heterojunction photocatalyst for the decomposition of malachite green under the VLI. Based on the results of the literature, the creation of the Z-scheme heterojunction has been shown to be effective in enhancing the photocatalytic performance of the photocatalyst under VLI.
In this study, we have created a Z-scheme heterojunction between BiYWO 6 and g-C 3 N 4 using one-step hydrothermal route under VLI. A sequence of g-C 3 N 4 /BiYWO 6 composite photocatalysts is prepared via differentiating the amount of g-C 3 N 4 (10 wt%, 30 wt%, 50 wt%), to assess the photocatalytic degradation activity of the composites towards the decomposition of methylene blue (MB) under VLI. This Z-scheme-based g-C 3 N 4 /BiYWO 6 heterojunction with schematic illustration of the possible charge transfer mechanism photocatalyst has not yet been reported so far for the wastewater pollutant degradation. Hence, the g-C 3 N 4 / BiYWO 6 heterojunction photocatalyst could be a potential material for the wastewater remediation process.

Preparation of g-C 3 N 4 nanosheet
g-C 3 N 4 nanosheets are prepared by self-condensation method, in which melamine was directly taken from a silica crucible and was heated under nitrogen atmosphere at a high temperature of 550 °C with a heating rate of 15 °C/min in a tubular furnace for 5 h. Then, obtained pale yellow-colored g-C 3 N 4 was collected as a fine powder.

Preparation of g-C 3 N 4 /BiYWO 6 nanocomposite
The g-C 3 N 4 /BiYWO 6 nanocomposite photocatalyst was synthesized through a one-step hydrothermal method. Herein, 1.2 mmol of Y(NO 3 ) 3 ·6H 2 O and 2.5 mmol of Bi(NO 3 ) 2 ·5H 2 O were dissolved in 0.5 N HNO 3 solution. Then, NaOH (1 N) solution containing 1.2 mmol of Na 2 WO 4 ·2H 2 O solution was slowly dripped into the abovementioned blend followed by 30 min of stirring. The pH of the solution was adjusted from 13.0 to 9.0 using HNO 3 (1 N) solution. Later, different amounts (10 wt%, 30 wt%, 50 wt%) of g-C 3 N 4 were added in the abovementioned mixture and then moved into a Teflon-lined autoclave for heat treatment at 160 °C for 18 h. Then, the obtained g-C 3 N 4 /BiYWO 6 nanocomposite was filtered and dried at 60 °C for 3 h. Furthermore, the g-C 3 N 4 / BiYWO 6 nanocomposite with different amounts (10 wt%, 30 wt%, 50 wt%) of g-C 3 N 4 is indicated as BC-10, BC-30, and BC-50, respectively. Similarly, the pure BiYWO 6 was prepared without the addition of g-C 3 N 4 . Figure 1 shows the schematic illustration of the one-step synthesis of g-C 3 N 4 / BiYWO 6 composite photocatalyst.

Material characterization
The crystal phase purity of the photocatalytic samples was analyzed by an X-ray diffractometer using the Mini-Flex II instrument with Cu Kα radiation (λ = 0.1542 nm). The surface morphologies of the photocatalytic materials were investigated by scanning electron microscopy (SEM; JEOL JSM-7600F, USA) equipped with an energy-dispersive X-ray spectroscopy (EDAX) and transmission electron microscopy (TEM; JEOL JSM-6330, USA). The UV-vis diffuse reflection spectra (DRS) were performed in Jasco V-670, and photoluminescence (PL) was investigated in the Jobin Yvon Fluorolog-3-11 spectrofluorometer.

Photocatalytic degradation studies
Here, the photocatalytic ability of all the as-prepared photocatalytic materials was examined by degrading MB in the presence of VLI and was tested by a 250-W tungsten halogen lamp. In the typical experiment, 75 mL of MB (2 × 10 −4 M) dye solution with 75 mg of the photocatalyst was added and continuously stirred for 45 min before VLI. In the presence of VLI, 5 mL of aliquots was taken at regular time distances and suspended particles were removed by centrifugation. Finally, the dye's absorption maximum at 664 nm was noted by a UV-visible spectrometer (Jasco V-630, Japan).

Photoelectrochemical studies
The photoelectrochemical (PEC) studies were investigated by a CHI608E electrochemical workstation with three typical electrodes: platinum (Pt), Ag/AgCl used as counter and reference electrodes, and the working electrode as the photocatalyst sample coated onto a fluorine-doped tin oxide (FTO) plate. A typical preparation of working electrode is as follows: 5 mg of the sample, 20 μL water, and 10 μL of Triton X-100 were well mixed to form a slurry. Subsequently, 0.5 × 0.5 cm 2 area of the FTO plate was coated with the abovementioned prepared slurry followed by 6 h of drying at 80 °C; 0.1 N Na 2 SO 4 was used as electrolyte in PEC studies.
The SEM, TEM, and EDAX are used to study the structural features of the as-synthesized photocatalysts. Figure 3 illustrates the SEM images of pure g-C 3 N 4 , pure BiYWO 6 , and BC-30 photocatalysts. As displayed in Fig. 3a, the bare g-C 3 N 4 exposes the 2D layered sheet-like morphology and BiYWO 6 exposes the nanoflake-like structure as presented in Fig. 3b, whereas in the BC-30 nanocomposite photocatalyst, the interconnection between the BiYWO 6 nanoflakes with the g-C 3 N 4 nanosheet is clearly presented in Fig. 3c. In addition, the elemental compositions of the photocatalytic samples are displayed in EDAX spectrum (Fig. 3). In Fig. 3d and e, the elemental composition of bare g-C 3 N 4 and pure BiYWO 6 displays the existence of C, N and Bi, Y, W, and O elements, respectively, whereas in the BC-30 composite photocatalyst, the appearance of Bi, Y, W, C, N, and O elements revealing both the elements of g-C 3 N 4 and BiYWO 6 in the composite, with no more impurity peaks observed, is shown in Fig. 3f. Figure 4 displays the elemental mapping images of the optimized BC-30 photocatalyst.
Further structural evaluations are presented in Fig. 5a, which shows the TEM images for 2D nanosheets of g-C 3 N 4 , and Fig. 5b shows the nanoflakes of BiYWO 6 . Likewise, Fig. 5c displays the TEM image of the optimized BC-30 composite photocatalyst and their closer interrelation between BiYWO 6 and g-C 3 N 4 , confirming the formation of g-C 3 N 4 /BiYWO 6 heterojunction between them. Figure 5d shows the HR-TEM image of the optimized BC-30 composite photocatalyst, which too clearly shows the existence of g-C 3 N 4 /BiYWO 6 heterojunction, with the distance of the lattice fringes being 0.295 nm and 0.352 nm for (002) and (040) planes of g-C 3 N 4 and BiYWO 6 , respectively. Additionally, selective area electron diffraction (SAED) pattern of the optimized BC-30 photocatalyst is also shown in Fig. 5e.
The XPS was acquired to analyze the elemental composition and oxidation states of the elements present in the sample, and the observed XPS spectra of pure g-C 3 N 4 , pure BiYWO 6 , and g-C 3 N 4 /BiYWO 6 composite photocatalysts are shown in Fig. 6. From Fig. 6a (survey spectra), the pure g-C 3 N 4 possesses C and N elements and the pure BiYWO 6 has Bi, Y, W, and O elements. As expected, the g-C 3 N 4 / BiYWO 6 composite is composed of C, N, Bi, Y, W, and O elements. The high-resolution spectra of C 1 s for both pure g-C 3 N 4 and g-C 3 N 4 /BiYWO 6 composite show two peaks at 284.9 eV and 287.7 eV corresponding to the C-C bond and N-C = N bond, respectively. The N 1 s spectra (Fig. 6c) show peaks at 397.7 eV, 399.9 eV, and 403.5 eV for sp 2 -hybridized N (C-N = C), N-(C) 3 , and C = N-H, respectively. A doublet at 158.7 eV and 164.1 eV corresponding to Bi 4f 7/2 and 4f 5/2 observed in BiYWO 6 and g-C 3 N 4 /BiYWO 6 composite indicates the presence of Bi 3+ in these compounds. Two   peaks of W 4f which appeared at 35.4 eV (4f 7/2 ) and 37.5 eV (4f 5/2 ) indicate the existence of tungsten in the W 6+ state. The high-resolution XPS spectra of Y 3d (Fig. 6f) exhibit two peaks at 158.7 eV and 164.3 eV, respectively, related to 3d 5/2 and 3d 3/2 states of Y 3+ . In the O 1 s spectra (Fig. 6g), the peak observed at 532.1 eV is attributed to the surface hydroxyl group (-O-H) and the peaks located at 530.2 eV, 524.3 eV, and 516.2 eV are ascribed to metal oxygen bonds in BiYWO 6 and g-C 3 N 4 /BiYWO 6 composite. Therefore, the XPS results reveal the coexistence of BiYWO 6 and g-C 3 N 4 in the g-C 3 N 4 /BiYWO 6 composite and in well accordance with XRD and EDAX results. UV-DRS is generally used to analyze the absorption ability of the photocatalyst in the visible region, which aids in calculating the bandgap (E g ) of the samples. As seen in Fig. 7a, the absorption edges as 489 nm, 452 nm, 482 nm, 474 nm, and 469 nm are noted for pure BiYWO 6 , g-C 3 N 4 , and BC-10, BC-30, and BC-50 composite photocatalysts, correspondingly. It clearly shows that all the prepared photocatalytic samples show a good VLA. Furthermore, the E g values of 2.84 eV, 3.10 eV, 2.87 eV, 2.90 eV, and 2.92 eV are noted for pure BiYWO 6 , pure g-C 3 N 4 , and BC-10, BC-30, and BC-50 photocatalysts individually as calculated by the Kubelka-Munk function, using the plot between (αhν) ½ and photon energy (hν) (Fig. S1) (Bavani et al. 2021a).
Generally, PL and PEC studies were used to investigate the separation and transfer of photoexcited e − -h + pairs. Figure 7b displays the PL spectra of the synthesized materials at an exciting wavelength of 320 nm. In Fig. 7b, all the prepared photocatalytic materials are showing the emission peak around 464 nm, and it is further noted that the g-C 3 N 4 / BiYWO 6 nanocomposite photocatalysts exhibit lower peak intensities than the pure BiYWO 6 . The addition of bare g-C 3 N 4 in the g-C 3 N 4 /BiYWO 6 nanocomposite is found to considerably suppress the reconnection rate of the e − -h + pairs by newly formed Z-scheme heterojunction. Among all prepared photocatalytic materials, the optimized BC-30 nanocomposite photocatalyst has a reduced rejoining rate of e − -h + pairs.
In Fig. 7c, photocurrent-time (I-t) curve exposes the transient properties of the photoinduced charge carriers with information on the separation ability during repeated runs by switching light. Generally, the higher photocurrent intensity manifests the higher separation and transfer efficiency of the e − -h + pairs. Hence, the optimized BC-30 photocatalyst possesses higher photocurrent intensity than the individual g-C 3 N 4 and BiYWO 6 and might be expected to exhibit superior photocatalytic activity. Furthermore, the charge transfer resistance of the samples was studied using electrochemical impedance spectra (EIS). In Fig. 7d, the semicircular radius of the optimized BC-30 photocatalyst seems much smaller than its individuals as this exemplifies the reduced charge transfer resistance, thus improving the charge transfer with higher separation ability and the rate of photocatalytic reaction (Tayyab et al. 2022).

Photocatalytic activities
The photocatalytic performance of the as-synthesized BiYWO 6 and g-C 3 N 4 and g-C 3 N 4 /BiYWO 6 heterojunction photocatalysts was evaluated based on the photodegradation of MB in the presence of VLI. The self-degradation of the MB without photocatalyst after 90 min in VLI shows no considerable change in the dye concentration which exposes the stability of MB dye. In Fig. 8a, the photocatalytic degradation plots of pure BiYWO 6 and g-C 3 N 4 show 30% and 33.2% of photodegradation ability, whereas the g-C 3 N 4 / BiYWO 6 heterojunction photocatalysts expose an excellent photocatalytic activity of 88.1%, 99.5%, and 72.2% that corresponded to BC-10, BC-30, and BC-50, respectively, after 90 min of VLI. Considerably, an increasing amount of g-C 3 N 4 (10-50%) in the g-C 3 N 4 /BiYWO 6 composite shows an increase in degradation efficiency to a certain content, i.e., up to 30 wt% of g-C 3 N 4 in the composite, and above this, the degradation seems to decrease, as it possibly acts as an e − /h + recombination center. So, it could be estimated that an optimized 30 wt% of g-C 3 N 4 in the g-C 3 N 4 /BiYWO 6 composite would exhibit superior photocatalytic degradation activity, i.e., complete degradation of MB in 90 min. This increase in photocatalytic performance is due to the effective transfer and separation of e − and h + pairs, thereby avoiding the rapid recombination of charges. In such a way, the Z-scheme heterojunction seems to increase the photocatalytic ability of the g-C 3 N 4 /BiYWO 6 composite photocatalyst.
In addition, the photocatalytic degradation results as in Fig. 8b follow the first-order kinetic equation (Malathi et al. Fig. 6 a The XPS survey spectra (b-g) and high-resolution XPS of C 1 s (b), N 1 s (c), Bi 4f (d), W 4f (e), Y 3d (f), and O 1 s (g) elements in pure g-C 3 N 4 , pure BiYWO 6 , and optimized BC-30 composite 2017), lnC 0 /C = kt, where t, k, and C and C 0 are the time (min), rate constant (min −1 ), and concentration of the MB at initial and various time intervals, respectively. From Fig. 7b, the rate constant values of the pure BiYWO 6 , pure g-C 3 N 4 , BC-10, BC-30, and BC-50 are calculated as 0.00472 min −1 , 0.0036 min −1 , 0.022 min −1 , 0.032 min −1 , and 0.013 min −1 . The BC-20 has a higher rate constant value which is 6.7 times and 8.8 times greater than the pure BiYWO 6 and g-C 3 N 4 photocatalysts. Furthermore, Fig. 8c displays the influence of catalyst amount on the removal of MB on exposure to VLI. In increasing the catalyst amount, the photocatalytic activity seems to gradually increase up to 1.0 g/L, and later, it shows a decreasing trend due to the increasing content of the catalyst that does not allow the light to pass through the solution. Table 2 displays the results of the comparative study of the various nanocomposite photocatalysts towards the degradation of different organic pollutants under various experimental conditions. Figure 9a displays the reusability and photostability characteristics of the optimized BC-30 photocatalyst. For every cycle, the catalyst is separated and washed with water and dried at 80 °C for 6 h in a hot air oven. After complete drying, the catalyst was reintroduced to the next cycling run. In such instance, the optimized BC-30 photocatalyst seems stable after 5 cycles and also negligible changes in the photocatalytic performances of the cycling runs are noted. Figure 9b displays the XRD pattern of before and after five recycling runs of the optimized BC-30 photocatalyst, which reveals that there are small changes between before and after recycling runs were observed, thus exposing the photocatalytic stable nature. So, the optimized BC-30 photocatalyst proved to be a capable material for remediation of wastewater.
The involvement of more active radical in the photocatalytic decomposition of MB under VLI using the optimized BC-30 photocatalyst was investigated by trapping the radicals. Various radical quenchers such as AO (1 mM), BQ (1 mM), and IPA (1 mM) are employed to trap h + , O 2 · − , and ·OH radicals, respectively (Bavani et al. 2021b). Accordingly, Fig. 10 clearly shows that no notable changes in the degradation efficiency while using AO, but a large difference was observed on using BQ and IPA. It reveals that the O 2 · − and ·OH are the major species in the decomposition of MB.  (c), and Nyquist plot with equivalent circuit (d) of bare g-C 3 N 4 , bare BiYWO 6 , and g-C 3 N 4 / BiYWO 6 composites Photocatalytic degradation mechanism Figure 11 presents the pictorial representation of the Z-scheme of the g-C 3 N 4 /BiYWO 6 photocatalyst. The valence and conduction band potentials are calculated by using the Butler-Ginley equations (Eqs. 1 and 2) (1) E VB = X − E e + 0.5E g where the E CB and E VB are the CB and VB potentials, respectively, and X and E e are the electronegativity of the photocatalyst and the energy of free electrons (~ 4.5 eV) on the hydrogen scale, respectively. The VB and CB edge potential values of the pure g-C 3 N 4 and BiYWO 6 are recorded as 1.775 eV and − 1.33 eV and as 2.96 eV and (2) E CB = E VB − E g Fig. 8 a Photodegradation plot. b First-order kinetic plot of bare g-C 3 N 4 , bare BiYWO 6 , and g-C 3 N 4 /BiYWO 6 composite photocatalysts over the degradation of MB under VLI. c Influence of the amount of BC-30 composite photocatalyst towards the degradation of MB (all measurements were conducted in triplicate) 0.123 eV, respectively. As per the mechanism, under VLI, the photoinduced e − on the CB of the BiYWO 6 gets easily transferred into the VB of the g-C 3 N 4 by the formation of an electrostatic interaction between the e − on the CB of BiYWO 6 and the h + on the VB of g-C 3 N 4 , as it expectedly inhibits the rapid reconnection of the e − -h + pairs. At the same time, the photoexcited e − on the CB of BiYWO 6 reduces the O 2 and creates O 2 · − radicals. This disrupts the chromophores of the pollutant molecules, resulting in the formation of degradation products that finally connected into the small molecules like CO 2 and H 2 O. The photogenerated h + that is left behind the VB of the g-C 3 N 4 simultaneously oxidizes the dye to non-noxious and biodegradable products. So, the Z-scheme heterojunction favors the separation and transferring ability of the photoinduced charge carriers with a strong redox ability. This is found to be advantageous in enhancing the photocatalytic efficiency towards the removal of the MB under VLI.

Summary
In summary, the g-C 3 N 4 /BiYWO 6 Z-scheme heterojunction photocatalyst was prepared by the facile hydrothermal process. Then, the synthesized g-C 3 N 4 /BiYWO 6 composite photocatalyst was optimized and evaluated towards the degradation of MB under VLI. Further, the optimized BC-30 photocatalyst showed an improved photocatalytic ability than the individual BiYWO 6 and g-C 3 N 4 . This enhancement is mainly credited by the formation of the Z-scheme heterojunction that reduced the rapid reconnection and improved the separation and transfer efficiency of the e − -h + pairs. Besides, the reusability test proved the stable nature of the synthesized optimized photocatalyst. Thus, the developed g-C 3 N 4 /BiYWO 6 Z-scheme heterojunction has proven to be a potential photocatalyst with improved photocatalytic activity under VLI and will also surely serve as a promising material for the wastewater remediation processes.