A 2D/2D BiPO4/g-C3N4-B Z-type heterojunction for enhanced photocatalytic degradation of dye pollutants

A 2D/2D BiPO4/g-C3N4-B nano-sheet heterojunction photocatalyst was synthesized via a simple coprecipitation method at room temperature using glacial acetic acid as solvent, which showed excellent activity toward the degradation of rhodamine B (RhB). The heterojunction showed much higher efficiency of separation and transfer of photogenerated carriers compared to that of its constituents. Moreover, the spectral response range of BiPO4 was effectively broadened after the combination of g-C3N4-B and BiPO4. Consequently, a 97.3% degradation of RhB within 25 min by BiPO4/g-C3N4-B heterojunction photocatalyst under visible light irradiation was observed. The difference in work functions of BiPO4 and g-C3N4-B was evident from UPS characterization, which led to the bending of the energy band and the establishment of an internal electric field at the interface of the heterojunction. Therefore, the synthesized direct Z-type BiPO4/g-C3N4-B heterojunction enhanced the oxidation-reduction ability by promoting the effective separation of photogenerated carriers.


INTRODUCTION
With rapid development, various dyes originating from the clothing, paper, and paint industries are continuously polluting the water resources on which humans rely for survival.These toxic dyes are difficult to degrade and adversely affect human health and safety.Modernized and developed water pollution treatment technologies have tried to solve these problems.For example, Bismuth-based materials, such as Bi 2 O 3 [1], BiPO 4 [2], BiVO 4 [3], Bi 2 WO 6 [4], and Bi 2 O 2 CO 3 [5], have been developed, which exhibit high photocatalytic activity toward water purification via degradation of harmful pollutants.They decompose pollutants into H 2 O, CO 2 , and other substances via photodegradation that are not harmful to the environment and unable to cause secondary pollution.This is an efficient, green, and sustainable solution to environmental pollution and a promising technical strategy.Among these materials, BiPO 4 exhibits high photocatalytic activity under ultraviolet light, with high photothermal stability, low cost, and non-toxic characteristics.Although it has great potential for further development [6], its wide band gap limits its utilization of solar energy [7].Therefore, it is often combined with narrow band gap semiconductors to build a heterojunction with a reduced band gap, broadened spectral response range, enhanced photo-induced interfacial charge transfer, and suppressed recombination of photogenerated carriers.For example, heterojunctions, such as Bi-BiOBr/BiPO 4 [8], Fe 3 O 4 /BiPO 4 [9], CQDs/ Ag 3 PO 4 /BiPO 4 [10], BiPO 4 /Bi 2 O 2 CO 3 [11], and BiPO 4 /Bi 4 O 5 I 2 [12], have been developed to improve the photocatalytic performance of BiPO 4 under visible light.
Recently, graphite-phase carbon nitride (g-C 3 N 4 ) has emerged as a typical two-dimensional non-metallic polymer semiconductor photocatalyst material.It has many advantages, such as excellent optical, chemical, and thermal stability, various low-cost and facile synthetic methods, a rich source of elements, environment-friendly nature (as it is free of heavy metals), visible light response, good electronic acceptance and transport capacity.Simultaneously, g-C 3 N 4 can easily be coated on the surface of other compounds, as it is a soft polymer material.It has already become a common candidate material in various application fields and is attracting more and more attention [13].Of late, scientific research has been focused on the construction of heterojunction composite photocatalysts comprising BiPO 4 and g-C 3 N 4 [14][15][16].However, most of these BiPO 4 /g-C 3 N 4 materials are micro-sized block aggregate structures, which reduces the contact area of g-C 3 N 4 and BiPO 4 and the number of active sites.The photocatalytic activity of these materials needs to be improved.On the other hand, boron (B) doped g-C 3 N 4 can stably anchor the active N atoms exposed on the surface of g-C 3 N 4 and can effectively inhibit the photo-carrier recombination of g-C 3 N 4 [17].Therefore, in this work, the B-doped g-C 3 N 4 nano-sheet (g-C 3 N 4 -B) was combined with the BiPO 4 nano-round chips to form a 2D/2D BiPO 4 /g-C 3 N 4 -B heterojunction with larger contact area and active sites.Further, its potential for catalytic reduction of RhB and methylene blue (MB) under visible light has been assessed.

Synthetic Procedure
g-C 3 N 4 -B was prepared via calcination at a high temperature.Briefly, 1.0 g of melamine, 1.0 g of boric acid, and 1.0 g of NaCl [18], were put into an agate mortar for full grinding and mixing.The obtained ground powder was dissolved in 30 mL of anhydrous ethanol, heated, stirred, and evaporated to dry most of the ethanol at 80 o C. Then it was kept in an oven to dry completely.The dried mixture was again ground into powder, placed in a covered crucible, and calcined in a muffle furnace at 550 o C for 4 h (Direct calcination without programmed temperature).The crucible was taken out after natural cooling to room temperature, and the obtained yellow product after calcination was ground in an agate mortar to obtain g-C 3 N 4 -B.
A coprecipitation technique was adopted to prepare BiPO 4 /g-C 3 N 4 -B.Briefly, 0.4851 g of Bi (NO 3 ) 3 •5H 2 O and different masses of g-C 3 N 4 -B (0.0920 g, 0.1840 g, 0.2760 g) were added into 20 mL of acetic acid and ultrasonicated for 10 min.Separately, 0.1198 g of NaH 2 PO 4 was added into 20 mL of deionized water.The two aforementioned systems were stirred for 20 min and then mixed.The stirring was further continued for 1 h, and then the mixture was centrifuged.The obtained precipitate was washed alternately with deionized water and anhydrous ethanol, then put into an oven at 60 o C for 6 h to dry it, and then ground to obtain the composites.Composites with BiPO 4 : g-C 3 N 4 -B mass ratios of 1 : 1, 1 : 2, and 1 : 3 were marked as 1 : 1BP-CB, 1 : 2BP-CB, and 1 : 3BP-CB, respectively.Additionally, pure BiPO 4 was prepared via the same method, except that the mass of g-C 3 N 4 -B was 0 in that case.

Photocatalytic Experiment
The target pollutants for photocatalytic experiments to evaluate the photocatalytic activity of different samples were RhB and MB dyes.Briefly, 50 mg of the sample was dispersed into a reaction flask containing 50 mL of dye (10 mg/L) and stirred for 1 h under dark conditions to achieve adsorption-desorption equilibrium.Then, an LED lamp was used to irradiate the reaction bottle, aliquots were taken out from the reaction mixture at different times, and the absorbance was measured after centrifugation.

Sample Characterization
The X-Ray diffraction (XRD) patterns were obtained for characterizing the bulk phase structure of the prepared samples, using a Cu K  radiation X-ray diffractometer (Rigaku Ultima IV, Japan) at a scanning range of 5 o -90 o , a scanning rate of 8 o min 1 , a voltage of 40 kV and a current of 40 mA.The microscopic morphology of the samples was investigated using FEI Inspect F50 field emission scanning electron microscope (SEM), and energy dispersal spectroscopy (EDS) was done for the composite material.The distribution of the chemical composition of the prepared sample was also studied by EDAX super octane using the same instrument.The internal lattice structure of the sample was observed using a Talos F200S G2 high-resolution transmission electron microscope (HR-TEM).A lambda 1050 UV-vis near-infrared spectrophotometer was used to obtain the UV-vis diffuse reflectance spectra (UVvis DRS) of the sample in the range of 200-800 nm with BaSO 4 as the standard.The chemical state of the elements on the surface of the sample was probed through an X-ray photoelectron spectrometer (XPS, ThermoFisher, ESCALAB Xi+) using Al K  radiation.A PL-SPV/IPCE1000 stable surface photovoltage spectrometer (Beijing Poffile Technology Co., Ltd.) was used to obtain the surface photovoltage (SPV) spectrum of the sample.A CS310H electrochemical workstation with a standard three-electrode system was used to measure the electrochemical impedance spectroscopy (EIS) and transient photocurrent response of the samples (irradiated by a 300 W xenon lamp).A Thermofly Escalab Xi+ instrument was used to measure the ultraviolet photoelectron spectroscopy (UPS) of the samples.102) Bragg peaks of 1 : 2BP-CB are significantly enhanced and narrowed, which explains why the composite material 1 : 2BP-CB still maintains a planar sheet shape (Fig. 2(c)) [19].

SEM and HRTEM Studies
The photocatalytic activity of a material depends consideranly on its morphology; therefore, the morphology and microstructure of the prepared materials were characterized using SEM, EDS, and HRTEM.

DRS and VBs Spectral Studies
The optical properties of a material must be investigated for its photocatalysis applications.Here, the optical absorption performance and band gap of the prepared samples were determined using UV-vis DRS spectra (Fig. 4(a)), which represented the light response of the sample in the wavelength range of 200-800 nm.The results suggest that the absorption edge of the pure BiPO 4 was about 310 nm, while the absorption edge of the composite BP-CB Additionally, the band edge positions of BiPO 4 could be estimated using the following empirical formulas: where E VB is the edge potential of the valence band (VB), E CB is the edge potential of the conduction band (CB), X is the average electronegativity of the semiconductor (the value of BiPO 4 is 6.49) December, 2023 [27], E e is the free electron energy on the hydrogen scale (about 4.5 eV), and E g is the band gap of the semiconductor.The E VB and E CB values of BiPO 4 were found to be 4.03 eV and 0.05 eV, respectively.Now, g-C 3 N 4 -B, due to the incorporation of B, the X value cannot be the same as the known X value of g-C 3 N 4 .Therefore, the g-C 3 N 4 -B sample was characterized using their XPS VBs spectra (Fig. 4(c)), which concluded the E VB value of 1.45 eV for g-C 3 N 4 -B.The E CB was then calculated to be 0.50 eV.

Transient Photocurrent, EIS, and SPV Studies
The transient photocurrent response curves (Fig. 5(a)) indicate that all samples could produce fast and stable photocurrent signals under visible light irradiation with periodic changes while passing through 10 intermittent switches.The photocurrent of the samples increased rapidly when it was illuminated and then decreased rapidly upon the withdrawal of the illumination.Notably, the photocurrent intensity of 1 : 2BP-CB was the highest.This indicates that the combination of g-C 3 N 4 -B and BiPO 4 improved the separation of photogenerated electrons and holes [28].Additionally, EIS (Fig. 5(b)) was also used to study the charge transfer behavior of these samples.The 1 : 2BP-CB sample had the smallest arc radius, so it showed a smaller resistance value and faster charge transfer capability [29].It may be noted that a certain voltage will be generated when the electrons are transferred from the VB to the CB.According to previous reports, the intensity of SPV optical voltage can explain the separation efficiency of electron-hole pairs [30].The SPV diagrams of the combined samples are depicted in Fig. 5(c).The g-C 3 N 4 -B and 1 : 2BP-CB samples showe higher voltage and photogenerated carrier separation efficiency compared to pure BiPO 4 .The aforementioned results of transient photocurrent response, EIS, and SPV analysis show that the combination of g-C 3 N 4 -B and BiPO 4 significantly enhanced the separation and transfer efficiency of photogenerated carriers in the 1 : 2BP-CB heterojunction.

Photocatalytic Activity
The photocatalytic degradation of RhB in the presence of different samples under visible light irradiation is shown in Fig. 6(a).Pure BiPO 4 and g-C 3 N 4 -B exhibit almost negligible photodegradation effects on RhB, whereas the degradation rate is significantly improved in the presence of the BP-CB composites.Notably, 1 : 2BP-CB shows the best degradation rate of RhB, reaching 97.3% photodegradation in only 25 min.Fig. 6(b) illustrates the first-order kinetic curves of RhB photodegradation in the presence of different samples, which concluded rate constants of 0.0005803 min 1 , 0.00132 min 1 , 0.0521 min 1 , 0.1505 min 1 , and 0.1191 min 1 , for BiPO 4 , g-C 3 N 4 -B, 1 : 1BP-CB, 1 : 2BP-CB, and 1 : 3BP-CB, respectively.The degradation rate is the fastest in the presence of 1 : 2BP-CB, which is 259.3 and 114.0 times faster than that of pure BiPO 4 and pure g-C 3 N 4 -B, respectively, indicating the significant enhancement of photocatalytic RhB degradation activity after the formation of heterojunction.
Additionally, the adsorption rates of RhB by different samples while keeping the reaction mixture in the dark for 1 h (before light irradiation) are depicted in Fig. 6(c).Although g-C 3 N 4 -B and RhB had poor adsorption effects, the adsorption performance of the BP-CB composite improved.Here, 1 : 2BP-CB exhibits the best adsorption rate, which is 7.13 times more than that of BiPO 4 .
It can be concluded, based on the above photocatalytic degradation experiments and results of the sample characterization, that the 1 : 2BP-CB sample (synthesized via the combination of BiPO 4 and g-C 3 N 4 -B) has the highest dye adsorption performance, the highest photogenerated charge-hole separation efficiency, and greatly improved absorption of visible light.These results in its superior photocatalytic degradation performance.
Furthermore, the photocatalytic RhB degradation performance of semiconductors related to BiPO 4 in recent years was extracted from the literature and summarized in Table 1.
However, 1 : 2 BP-CB could not significantly photodegrade MB.If the pH is not adjusted, the degradation of MB is relatively slow, and complete degradation js in about 200 minutes.The MB degradation efficiency of the 1 : 2 BP-CB sample at different solution pH values of 1, 3, 5, 7, 9, 11, and 13 is shown in Fig. 6(d).The degradation effect on MB exhibits some improvement at pH 11, while it reaches its best upon further increasing the pH of the MB solution to 13.It degrades MB completely in only 40 min under these conditions.It is speculated that the reason for the pH dependence is that when pH is equal to 11, beyond the isoelectric point of 1 : 2BP-CB, 1 : 2BP-CB begins to change into a negatively charged surface, thus increasing the electrostatic attraction with the MB (cationic dye) [31].Therefore alkaline conditions are favorable for the degradation of MB by the BP-CB complex.
It is necessary to determine the main active species involved in the process of photocatalytic degradation to further clarify the mechanism of photocatalysis of the BP-CB composite material.Therefore, free radical capture experiments were performed for the composite material 1 : 2BP-CB.Figs.6(e) and (f) show that the degradation of RhB by 1 : 2BP-CB was greatly inhibited after the addition of p-benzoquinone (PBQ, •O 2  scavenger) and ethylenediaminetetraacetic acid disodium salt (Na 2 EDTA, h + scavenger).This indicates that •O 2  and h + play a major role in the degradation of RhB.However, the photodegradation rate of RhB in the presence of isopropanol (IPA, •OH scavenger) decreased slightly only, which indicates that •OH also plays a role in the degradation of RhB.

Proposed Mechanism
A plausible mechanism for photocatalytic RhB degradation by BP-CB composites is proposed in Fig. 7 [32].This is inconsistent with the results of free radical trapping experiments.Therefore, the traditional photoexcited carrier transfer and charge separation in a type II heterojunction is not suitable for the BP-CP composites [33,34].Therefore, an alternate mechanism is proposed in Fig. 7 The exactness of the speculated hypothesis proposed above was verified using the work function values.The E Cutoff of the samples could be obtained from Fig. 8 when the abscissa was the binding energy.Now, the following formula [36] was used to calculate the work function () of the samples: Here, E Cutoff was the abscissa value corresponding to the intersection of the tangent of the secondary electron cut-off edge and the baseline, and E Fermi is the abscissa value corresponding to the mid- The energy band configuration diagram of BP-CB could be drawn according to the above calculation results [38].Here, both the semiconductor photocatalysts have staggered energy band structures and different Fermi energy levels (Fig. 9(a)).Therefore, upon bringing them in contact with each other (Fig. 9(b)), the difference in the work function between them induces the free electrons of g-C 3 N 4 -B to migrate to BiPO 4 , the energy band at the contact site bends, and the charge is redistributed until the Fermi energy level reaches equilibrium.This forms an internal electric field [39].This energy band bending at the interface of the heterojunction simultaneously inhibits the transfer of electrons in the g-C 3 N 4 -B CB to the BiPO 4 CB and the transfer of holes in the BiPO 4 VB to the g-C 3 N 4 -B VB.This, in turn, promotes the recombination of electrons in the BiPO 4 CB with holes in the g-C 3 N 4 -B VB [37], forming a direct Z-type heterojunction.This corroborated well with the proposed photocatalysis mechanism shown in Fig. 7(b).Therefore, the abrupt change of energy band structure at the interface of BiPO 4 / g-C 3 N 4 -B heterojunction resulted in the bending of its energy band and the formation of an internal dielectric field.Effective separation of carriers at the interface was promoted in this direct Z-type heterojunction, and an enhanced oxidation-reduction ability was also found simultaneously.Furthermore, semiconductor materials BiPO 4 and g-C 3 N 4 -B could easily be assembled into 2D/2D BiPO 4 / g-C 3 N 4 -B nanochips to achieve an effective combination of different multi-functional materials.Therefore, this work suggests that the 2D/2D BiPO 4 /g-C 3 N 4 -B direct Z-type heterojunction has a considerable potential from the perspective of a broader photocatalytic application.

CONCLUSION
A 2D/2D BiPO 4 /g-C 3 N 4 -B nano-sheet heterojunction was designed via the epitaxial growth of BiPO 4 on g-C 3 N 4 -B to obtain a broadened absorption spectrum and improved separation efficiency of photogenerated electron-hole pairs.It was further applied as a photocatalyst for the degradation of dyes such as RhB and MB in water.The dye degradation experiments showed that the photocatalytic degradation efficiency was greatly improved for the heterojunction comprising g-C 3 N 4 -B and BiPO 4 , compared to the individual constituents.The experimental results of SPV, EIS, instantaneous current response, free radical capture, UPS, and DRS comprehensively concluded that the efficient photocatalytic effect of 2D/2D BiPO 4 /g-C 3 N 4 -B complex originated mainly from its highly improved visible light absorption performance and the enhanced separation efficiency of electron-hole pairs in this direct Z-type heterostructure.

Fig. 1
depicts the XRD patterns of the pure BiPO 4 and BP-CB composites with different proportions.The diffraction peaks of December, 2023 pure BiPO 4 at 2 values of 14.6 o , 20.1 o , 25.5 o , 29.5 o , 31.3 o , 41.9 o , and 49.6 o were evident, which was attributed to the (100), (101), (110), (200), (102), (211), and (113) crystal planes of hexagonal BiPO 4 , respectively (JCPDS 15-0766).Although the XRD patterns of the BP-CB composites with different proportions mainly represent the characteristic diffraction peaks of BiPO 4 , a different peak was found at 2 value of 32.5 o , which corresponded to the (200) crystal plane of g-C 3 N 4 (JCPDS 50-1512).Fig. 1(b) shows the amplification of Bragg peaks (200) and (102) in the range of 28-32 o , The solid line represents the experimental data of using Jade for Rietveld refinement, and the dashed line is the difference between experimental data and refined experimental data.Obviously, compared to BiPO 4 , the (200) and (

Fig. 6 .
Fig. 6.(a) The photocatalytic degradation effect and (b) degradation kinetics of RhB; (c) The adsorption rate of RhB on the sample; (d) Degradation effect of 1 : 2BP-CB on MB at different pH values; (e) Photocatalytic performance and (f) degradation kinetics of 1 : 2BP-CB to RhB under the presence of different inactivators.
(a).Upon irradiation of BP-CP composites with LED light, the photogenerated electrons (e  ) on g-C 3 N 4 -B and BiPO 4 jump to their respective CB and left holes (h + ) on their VB.Now, the electrons in the g-C 3 N 4 -B CB are transferred to the CB of BiPO 4 , and the holes in the VB of BiPO 4 are transferred to the VB of g-C 3 N 4 -B.However, as the potential of the BiPO 4 CB (0.05 eV) is more positive than the redox potential of O 2 /•O 2  (0.33 eV), the electrons of the BiPO 4 CB are unable to reduce the O 2 adsorbed on BiPO 4 to •O 2  .Moreover, the g-C 3 N 4 -B VB potential value (1.45 eV) is lower than the redox potential of •OH/H 2 O (2.68 eV), and the holes accumulated in the g-C 3 N 4 -B (b).Upon illumination of BP-CB by the LED lamp, electrons on the surface of g-C 3 N 4 -B and BiPO 4 are photoexcited, and electrons (e  ) and holes (h + ) are generated in their CB and VB, respectively.Next, the electrons in the CB of BiPO 4 and the holes in the VB of g-C 3 N 4 -B are quenched together [35], which ultimately concentrates electrons in the CB of g-C 3 N 4 -B and holes in the VB of BiPO 4 .As the potential of the CB of g-C 3 N 4 -B is 0.50 eV, these photoelectrons can reduce the O 2 adsorbed on g-C 3 N 4 -B to •O 2  .On the other hand, the high VB potential of 4.03 eV of BiPO 4 empowered the holes with strong oxidation ability and oxidizes H 2 O on the sur-face of BiPO 4 to generate •OH.This is consistent with the results of the free radical capture experiment.Therefore, the BiPO 4 /g-C 3 N 4 -B composite synthesized by this strategy must be a direct Ztype heterojunction photocatalyst.

Fig. 7 .
Fig. 7. Proposed mechanisms for photogenerated charge transfer and photocatalysis on the surface of the composite.