3D/3D Bamboo Charcoal/Bi2WO6 Bifunctional Photocatalyst for Degradation of Organic Pollutants and Efficient H2 Evolution Coupling with Furfuryl Alcohols Oxidation

Photocatalysis is one of the most promising pathways to relieve the environmental contamination caused by the rapid development of modern technology. In this work, we demonstrate a green manufacturing process for the 3D/3D rod-shaped bamboo charcoal/Bi2WO6 photocatalyst (210BC-BWO) by controlled carbonization temperature. A series of morphology characterization and properties investigations (XRD, SEM, UV–vis DRS, transient photocurrent response, N2 absorption-desorption isotherms) indicate a 210BC-BWO photocatalyst with higher charge separation efficiency, larger surface area, and better adsorption capacity. The excellent photocatalytic performance was evaluated by degrading rhodamine B (RhB) (98.5%), tetracycline hydrochloride (TC-HCl) (77.1%), and H2 evolution (2833 μmol·g−1·h−1) coupled with furfuryl alcohol oxidation (3097 μmol·g−1·h−1) under visible light irradiation. In addition, the possible mechanisms for degradation of organic pollutants, H2 evolution, and furfuryl alcohol oxidation were schematically investigated, which make it possible to exert photocatalysis by increasing the active radical. This study shows that the combination of bamboo charcoal and bismuth tungstate can be a powerful photocatalyst that rationally combines H2 evolution coupled with furfuryl alcohol oxidation and degradation of pollutants.


Introduction
Environmental pollution and energy shortage is brought about by energy consumption and waste [1], and is also caused by discharge of pollutants containing a variety of organic compositions such as antibiotics [2], dyes [3], phenols [4], and heavy metals [5].The use of photocatalytic degradation of pollutants and waste biomass to generate value-added products has attracted much attention, contributing significantly to low-cost renewable energy evolution, as well as solving environmental problems.Among various renewable green technologies, photocatalytic H 2 evolution is one of the most promising candidates due to its low consumption, low environmental impact, and easy sustainable recycling [6].Comparatively, photocatalytic biomass conversion has the advantages of high selectivity and high efficiency [7].Abundant biomass and its derivatives (e.g., furfuryl alcohol (FFA), 5-hydroxymethylfurfural, etc.) arise from photosynthesis in natural plants, and further transform into high-value chemicals.They have attracted extensive interest in the field of energy conversion [8,9].The results of this study showed that biomass has a stronger hole trapping capability compared to the usual sacrificial agents (methanol, lactic acid, and triethanolamine), and that the protons can consistently gain more electrons to produce H 2 with excellent photocatalytic activity [10].Yang et al. found that the production rates of H 2 and furfural reached 471.35 µmol•g −1 •h −1 and 206.19 µmol•g −1 •h −1 on the Ni-Au/CN photocatalyst, respectively, which were 3.2 and 2.9 times higher than those on the Au/CN photocatalyst [11].Hence, photocatalytic overall reaction of biomass and its derivatives is an ideal way to simultaneously produce H 2 reduction products and value-added oxidation products.
In this regard, it is important to explore heterophase photocatalysts with high performance to realize such photocatalytic reactions.However, most of the traditional photocatalysts (TiO 2 , ZnO, etc.) have a wider band gap, high photogenerated electron-hole composite rate, and less solar absorption efficiency [12,13], which limits these photocatalysts' practical applications.Consequently, constructing composites can improve the separation efficiency of photogenerated electron-hole pairs, and further improve the photocatalysts performance.Biochar is a carbonized material with the most potential, exhibiting a high carbon yield (27-45% wt%) [14], rich void structure [15], high specific surface area, high porosity [16], and excellent electrical conductivity [17].The formation of biochar is affected by the feedstock type, hydrothermal temperature, reaction pressure and holding time regulation [18].China is the world's largest producer of bamboo resource possessor.The development and utilization of bamboo charcoal (BC) resources is of great significance and is in line with the current needs of environmental protection, energy saving, and low carbon economy [19].Moreover, a remarkable feature of BC is that its pores become higher after high-temperature carbonization, which can effectively improve the separation efficiency of electron-hole pairs and greatly increase the specific surface area of BET [20].Therefore, it is chosen as a favorable carrier for photocatalysts.The green photocatalyst obtained by the combination of BC and semiconductor materials is a hot topic in today's research [21].Zhang et al. [22] synthesized TiO 2 -loaded BC composites and found that TiO 2 /BC shows better photocatalytic performance than pure TiO 2 , which provided more active sites for the degradation of pollutants.However, bismuth tungstate (Bi 2 WO 6 ) is one of the most promising semiconductor materials with narrow bandgap, low bandgap energy, high chemical stability, and high photoreactivity [23].In addition, the different morphologies (nanosheets, bird's nests, hollow spheres, octahedrons and bouquets) of Bi 2 WO 6 were obtained by different synthetic methods, which show obvious differences in photocatalytic performance.The three-dimensional (3D) bouquet-shaped structure, Bi 2 WO 6 , possesses a larger specific surface.This key feature significantly enhances its opportunities for contact with organic pollutants [24].Liang et al. [25] designed biomass carbon modified flower-like Bi 2 WO 6 microspheres, in which the removal rate of TC-HCL by a Bi 2 WO 6 /C (6:1) sample was increased by 29.7% compared with that of pure Bi 2 WO 6 .Therefore, biochar has electron transport properties in the composite catalysts that favor the separation of charge carriers.Wu et al. [26] investigated the effects of different doping ratios of biochar on the performance of Bi 2 WO 6 .They revealed that the composite photocatalysts with a mass ratio of biochar to BWO of 5% were the most effective, with the kinetic rate of RhB solution degradation being 5.27 times higher than pure BWO.Up to now, there are no reports on photocatalytic H 2 evolution coupled with waste biomass oxidation and simultaneous degradation of organic pollutants by simple assembly using BC as the carrier and Bi 2 WO 6 as the semiconductor photocatalyst.
Here, we have developed a novel, green and efficient 3D/3D rod/floral spherical T-BC-BWO photocatalyst based on 3D rod-shaped BC and 3D floral spherical BWO by simple hydrothermal method, which has excellent conductivity and high specific surface area.The photocatalytic composites were characterized by a series of morphological characterization and performance studies (XRD, SEM, UV-vis DRS, transient photocurrent response, and N 2 absorption-adsorption isotherms).The excellent photocatalytic performance was evaluated by the degradation of RhB (98.5%) and TC-HCl (77.1%), and photocatalytic H 2 evolution (2833 µmol•g −1 •h −1 ) coupled with FFA oxidation (3097 µmol•g −1 •h −1 ) under visible light irradiation.The results showed that the performance of 210BC-BWO was significantly improved compared with that of pure BWO.This study contributes to the construction of efficient rod/flower sphere composite photocatalysts for the removal of organic pollutants from water, while also realizing efficient photocatalytic H 2 evolution coupled with FFA oxidation.It is a novel and meaningful strategy to kill two birds with one stone and has applications in both energy-and environment-related fields.Schematic diagram of the T-BC-BWO photocatalysts is shown in Scheme 1.
Molecules 2024, 29, x FOR PEER REVIEW 3 of 18 was significantly improved compared with that of pure BWO.This study contributes to the construction of efficient rod/flower sphere composite photocatalysts for the removal of organic pollutants from water, while also realizing efficient photocatalytic H2 evolution coupled with FFA oxidation.It is a novel and meaningful strategy to kill two birds with one stone and has applications in both energy-and environment-related fields.Schematic diagram of the T-BC-BWO photocatalysts is shown in Scheme 1.

XRD Analysis
Figure 1a shows the XRD spectrum for three T-BC samples.As we know, cellulose has characteristic peaks at 16.3°, 22.3°, and 35°, corresponding to the crystalline structures of type II as (101), (002) and (040) [27].The presence of characteristic peaks of cellulose in the diffraction pattern of 170BC indicated that the bamboo powder was not completely charred at 170 °C.The disappearance of the characteristic peaks of 210BC and 250BC indicate that the hydrothermal carbonization reaction destroys the crystalline structure of cellulose to form amorphous carbon at a higher temperature, and generates biomass carbon materials that possess high specific surface area, endowing the carbon material with high adsorption.
The XRD pattern of the T-BC-BWO composite sample is shown in Figure 1b.T-BC has no effect on the crystal structure of the composite photocatalysts, with peaks located at 28.2°, 32.7°, 47.1°, 55.8°, 58.6°, and 69.0°, corresponding to the (131), ( 200

XRD Analysis
Figure 1a shows the XRD spectrum for three T-BC samples.As we know, cellulose has characteristic peaks at 16.3 • , 22.3 • , and 35 • , corresponding to the crystalline structures of type II as (101), (002) and (040) [27].The presence of characteristic peaks of cellulose in the diffraction pattern of 170BC indicated that the bamboo powder was not completely charred at 170 • C. The disappearance of the characteristic peaks of 210BC and 250BC indicate that the hydrothermal carbonization reaction destroys the crystalline structure of cellulose to form amorphous carbon at a higher temperature, and generates biomass carbon materials that possess high specific surface area, endowing the carbon material with high adsorption.

SEM Analysis
The T-BC material has a fractured structure and a rougher surface with the increase in carbonization temperature (Figure 2).A pattern of 170BC (Figure 2a) maintains an intact rod structure, while 210BC (Figure 2b) basically maintains an overall rod structure but with localized fracture in the charcoal material.As for 250BC (Figure 2c), the BC underwent pyrolytic fracture under high-temperature conditions, and eventually formed a large accumulation of crumbly charcoal.133), (262), and (400) crystallographic planes of the orthorhombic BWOs [28].For the following two reasons, the diffraction peaks of T-BC are not observed in all composites.Firstly, T-BC belongs to an amorphous structure and there is no obvious crystal diffraction information.Secondly, the diffraction peak intensity of T-BC is distinctly low compared with BWO.

SEM Analysis
The T-BC material has a fractured structure and a rougher surface with the increase in carbonization temperature (Figure 2).A pattern of 170BC (Figure 2a) maintains an intact rod structure, while 210BC (Figure 2b) basically maintains an overall rod structure but with localized fracture in the charcoal material.As for 250BC (Figure 2c), the BC underwent pyrolytic fracture under high-temperature conditions, and eventually formed a large accumulation of crumbly charcoal.

SEM Analysis
The T-BC material has a fractured structure and a rougher surface with the increase in carbonization temperature (Figure 2).A pattern of 170BC (Figure 2a) maintains an intact rod structure, while 210BC (Figure 2b) basically maintains an overall rod structure but with localized fracture in the charcoal material.As for 250BC (Figure 2c), the BC underwent pyrolytic fracture under high-temperature conditions, and eventually formed a large accumulation of crumbly charcoal.3a).The carbonization temperature has a large influence on the morphology of the T-BC-BWO composite photocatalyst.The morphology of 3D/3D 170BC-BWO consists of 3D rod-shaped 170BC and 3D flower-spherical BWO (Figure 3b).The 3D rodshaped carbon material can improve the dispersion of catalysts in this 3D/3D structure.The morphology of 3D/3D 210BC-BWO showing the BWO flower ball is firmly fixed on the surface of 210BC, and the morphology and structure of the BWO flower bulbs were not changed by the incorporation of bamboo charcoal, but the size of the flower bulb decreased to 2 μm (Figure 3c).In contrast, 250BC-BWO is a composite of blob-shaped BWO and crumbled 250BC, the rod structure is crumbled to form a 3D/3D blob/blob-shaped 250BC-BWO (Figure 3d), which provides more uniform dispersion in treating organic pollutants.Therefore, the optimal hydrothermal temperature can be locked at 210 °C for   3a).The carbonization temperature has a large influence on the morphology of the T-BC-BWO composite photocatalyst.The morphology of 3D/3D 170BC-BWO consists of 3D rod-shaped 170BC and 3D flower-spherical BWO (Figure 3b).The 3D rod-shaped carbon material can improve the dispersion of catalysts in this 3D/3D structure.The morphology of 3D/3D 210BC-BWO showing the BWO flower ball is firmly fixed on the surface of 210BC, and the morphology and structure of the BWO flower bulbs were not changed by the incorporation of bamboo charcoal, but the size of the flower bulb decreased to 2 µm (Figure 3c).In contrast, 250BC-BWO is a composite of blob-shaped BWO and crumbled 250BC, the rod structure is crumbled to form a 3D/3D blob/blob-shaped 250BC-BWO (Figure 3d), which provides more uniform dispersion in treating organic pollutants.Therefore, the optimal hydrothermal temperature can be locked at 210 • C for bamboo charcoal composites, taking into account that the stabilized heterogeneous structure could exhibit excellent photocatalytic performance.In addition, as shown in the elemental mapping results C, O, Bi, and W elements are uniformly distributed in the hybrids, indicating the close coupling between BC and BWO, and the successful formation of 210BC-BWO composites (Figure S2).

BET Analysis
The N 2 absorption-desorption isotherms for T-BC and T-BC-BWO are presented in Figure 4, and the associated pore information values can be found in Table 1.The pore sizes of the samples were almost distributed between 1 and 20 nm (Figure S4).All T-BC samples exhibit type III isotherms and H 4 hysteresis loops (Figure 4a), which suggests that the increase in temperature did not change the pore structure of T-BC.All T-BC-BWO composites exhibit type III isotherms and H 3 hysteresis loops and are similar to those of BWO (Figure 4b).This result indicates that the incorporation of BC under different hydrothermal temperatures does not alter the microporous and mesoporous structures.As expected, T-BC-BWO composites have a high specific surface area of 170BC-BWO (20.1 m 2 •g −1 ), 210BC-BWO (22.8 m 2 •g −1 ), and 250BC-BWO (29.8 m 2 •g −1 ).From the perspective of photo- catalysis, a higher specific surface area tends to adsorb more target pollutants and form a dense target cover layer [29], which leads to a weakening of photocatalytic ability due to a reduction in visible light absorption performance.At the same time, their inappropriate band position and large overpotential for active radicals can also lead to slower photodegradation efficiency.This undesirable phenomenon been observed by the subsequent RhB degradation and H 2 evolution coupled with FFA oxidation experiments [30].

BET Analysis
The N2 absorption-desorption isotherms for T-BC and T-BC-BWO are presented in Figure 4, and the associated pore information values can be found in Table 1.The pore sizes of the samples were almost distributed between 1 and 20 nm (Figure S4).All T-BC samples exhibit type III isotherms and H4 hysteresis loops (Figure 4a), which suggests that the increase in temperature did not change the pore structure of T-BC.All T-BC-BWO composites exhibit type III isotherms and H3 hysteresis loops and are similar to those of BWO (Figure 4b).This result indicates that the incorporation of BC under different hydrothermal temperatures does not alter the microporous and mesoporous structures.As expected, T-BC-BWO composites have a high specific surface area of 170BC-BWO (20.1 m 2 •g −1 ), 210BC-BWO (22.8 m 2 •g −1 ), and 250BC-BWO (29.8 m 2 •g −1 ).From the perspective of photocatalysis, a higher specific surface area tends to adsorb more target pollutants and form a dense target cover layer [29], which leads to a weakening of photocatalytic ability due to a reduction in visible light absorption performance.At the same time, their inappropriate band position and large overpotential for active radicals can also lead to slower photodegradation efficiency.This undesirable phenomenon been observed by the subsequent RhB degradation and H2 evolution coupled with FFA oxidation experiments [30].

UV-Vis Spectroscopy
The absorption edge of the T-BC-BWO composite material extends into the visible light region, indicating the potential for enhanced photocatalytic activity under visible light irradiation (Figure 5a).The E g values for BWO and T-BC-BWO composites can be determined using Equation (1) in Supporting Information (Figure 5b), in which the E g value (2.50 eV) of BWO is consistent with that listed in the relevant literature [31,32].E g values of the T-BC-BWO were reduced to 2.34 eV (170BC-BWO), 1.89 eV (210BC-BWO), and 0.85 eV (250BC-BWO), which may be attributed to the coupling effect between T-BC and BWO, reducing forbidden bandwidths [33].

Raman Spectra Analysis
The Raman spectra of BC, BWO, and 210BC-BWO samples are shown in Figure 6d.In the BC sample, carbon-specific D and G bands were observed at 1324 cm −1 and 1550 cm −1 , respectively.This suggests that the hydrothermal carbonization reaction results in the formation of amorphous carbon.Since pure BWO does not contain carbon, no D and G peaks were observed.For 210BC-BWO, D and G bands were also observed at 1324 cm −1 and 1550 cm −1 .Generally, the intensity of the G band is usually associated with the crystalline grains in the sample, while the intensity of the D band is typically related to structural defects.It can be demonstrated that its amorphous structure gives amorphous carbon a higher specific surface area and adsorption capacity.Therefore, the presence of amorphous structures endows the composite photocatalyst with a higher specific surface area and adsorption capacity.

Electrochemical Analysis
In order to investigate the effect of charge separation efficiency on catalytic activity, the transient photocurrent response and electrochemical impedance spectroscopy (EIS) of BWO and T-BC-BWO were tested using an electrochemical workstation (Figure 6).Under visible light irradiation, the charge separation of the catalyst was characterized by transient photocurrent responses (Figure 6a).Upon light excitation, a remarkably significant photocurrent was instantly generated, with the photocurrent intensity of all T-BC-BWO samples being noticeably enhanced, while the continuous cycles of photocurrent density for 210BC-BWO are higher than those of other samples, and the photocurrent response is increased by approximately three times compared to pure Bi2WO6.This is attributed to the conjugated π electron system of carbon increasing the electron export capability, further forming an observable photocurrent on the photoelectrode surface [34].The presence of T-BC in the photocatalyst provides better carrier stability and carrier migration effi-

Raman Spectra Analysis
The Raman spectra of BC, BWO, and 210BC-BWO samples are shown in Figure 6d.In the BC sample, carbon-specific D and G bands were observed at 1324 cm −1 and 1550 cm −1 , respectively.This suggests that the hydrothermal carbonization reaction results in the formation of amorphous carbon.Since pure BWO does not contain carbon, no D and G peaks were observed.For 210BC-BWO, D and G bands were also observed at 1324 cm −1 and 1550 cm −1 .Generally, the intensity of the G band is usually associated with the crystalline grains in the sample, while the intensity of the D band is typically related to structural defects.It can be demonstrated that its amorphous structure gives amorphous carbon a higher specific surface area and adsorption capacity.Therefore, the presence of amorphous structures endows the composite photocatalyst with a higher specific surface area and adsorption capacity.

Electrochemical Analysis
In order to investigate the effect of charge separation efficiency on catalytic activity, the transient photocurrent response and electrochemical impedance spectroscopy (EIS) of BWO and T-BC-BWO were tested using an electrochemical workstation (Figure 6).Under visible light irradiation, the charge separation of the catalyst was characterized by transient photocurrent responses (Figure 6a).Upon light excitation, a remarkably significant photocurrent was instantly generated, with the photocurrent intensity of all T-BC-BWO samples being noticeably enhanced, while the continuous cycles of photocurrent density for 210BC-BWO are higher than those of other samples, and the photocurrent response is increased by approximately three times compared to pure Bi 2 WO 6 .This is attributed to the conjugated π electron system of carbon increasing the electron export capability, further forming an observable photocurrent on the photoelectrode surface [34].The presence of T-BC in the photocatalyst provides better carrier stability and carrier migration efficiency, thus facilitating the separation and transfer of photogenerated charges.The performance of interfacial charge separation efficiency can be clearly displayed by EIS Nengquis mapping (Figure 6b).The smaller the radius of the arc, the higher the efficiency of charge carrier transfer in the catalyst material [35].All T-BC-BWO photocatalysts have smaller arc radii than BWO (BWO > 250BC-BWO > 170BC-BWO > 210BC-BWO), indicating that the participation of T-BC enhances the efficiency of the charge transfer at the complex interface between BWO and T-BC and inhibits the recombination of electron-hole pairs.Simultaneously, the Mott-Schottky (M-S) curves were measured to investigate the effect of introducing T-BC on the electrochemical properties.The 210-BC-BWO photocatalyst show positive slopes, thus this photocatalyst could be ascribed to an n-type semiconductor [36].All flat band potentials of raw materials and composites were determined through analysis of the M-S curves combined in Equation ( 3

Photocatalytic Performance for Degradation
The degradation kinetics of RhB are presented by BC, T-BC-BWO and 210BC+BWO photocatalyst under visible light (Figure 7).Generally speaking, the higher adsorption capacity creates more opportunities for the photocatalytic active site contact with the pollutant; however, 210BC-BWO with high charge separation efficiency and suitable band

Photocatalytic Performance for Degradation
The degradation kinetics of RhB are presented by BC, T-BC-BWO and 210BC+BWO photocatalyst under visible light (Figure 7).Generally speaking, the higher adsorption capacity creates more opportunities for the photocatalytic active site contact with the pollutant; however, 210BC-BWO with high charge separation efficiency and suitable band structure produces more active free radicals, which play a more important role for the RhB degradation (Figure S3).Based on the above analyses, 210BC-BWO displays good performance for the visible-light-driven photocatalytic degradation of organic dyes.The first-order kinetic simulation plot (Figure 7c,d) has been proposed according to the photocatalytic degradation curves (Figure 7a,b) and Equation ( 2).The rate of degradation for RhB (10 mg L −1 and 15 mg L −1 ) solutions are in the following order: 210BC-BWO The first-order kinetic simulation plot (Figure 7c,d) has been proposed according to the photocatalytic degradation curves (Figure 7a,b) and Equation (2).The rate of degradation for RhB (10 mg L −1 and 15 mg L −1 ) solutions are in the following order: 210BC-BWO > 170BC-BWO > 250BC-BWO > 210BC + BWO > BWO.The above results demonstrate that the introduction of T-BC materials can significantly improve the photocatalytic performance of BWO (Figure 7).
The stability of the 210BC-BWO photocatalyst was investigated by recycling experiments.Figure 7e shows that the degradation rate of RhB (10 mg L −1 ) in five rounds was 98.53%, 93.52%, 92.55%, 91.31%, and 90.32%, respectively, which proves that the photocatalyst 210BC-BWO has excellent cycling performance and stability.It can be observed that the characteristic diffraction peaks of the reused 210BC-BWO composite material are nearly unchanged.There are no disappearing peaks, nor are there any additional peaks, demonstrating that the prepared composite photocatalyst exhibits strong stability and can be reused repeatedly, as shown in Figure 1b.In the same pattern as that observed for the degradation of RhB, the initial concentration of the TC-HCl solution had a different degree of decrease after 30 min of dark adsorption, which was related to the adsorption capacity of T-BC-BWO (Figure 8).The dark adsorption results (Figure 8a) and photocatalytic efficiency (Figure 8b) together prove the feasibility and practicality of using T-BC-BWO photocatalysts for the removal of organic pollutants from wastewater [37][38][39].
Molecules 2024, 29, x FOR PEER REVIEW 10 of 18 > 170BC-BWO > 250BC-BWO > 210BC + BWO > BWO.The above results demonstrate that the introduction of T-BC materials can significantly improve the photocatalytic performance of BWO (Figure 7).The stability of the 210BC-BWO photocatalyst was investigated by recycling experiments.Figure 7e shows that the degradation rate of RhB (10 mg L −1 ) in five rounds was 98.53%, 93.52%, 92.55%, 91.31%, and 90.32%, respectively, which proves that the photocatalyst 210BC-BWO has excellent cycling performance and stability.It can be observed that the characteristic diffraction peaks of the reused 210BC-BWO composite material are nearly unchanged.There are no disappearing peaks, nor are there any additional peaks, demonstrating that the prepared composite photocatalyst exhibits strong stability and can be reused repeatedly, as shown in Figure 1b.In the same pattern as that observed for the degradation of RhB, the initial concentration of the TC-HCl solution had a different degree of decrease after 30 min of dark adsorption, which was related to the adsorption capacity of T-BC-BWO (Figure 8).The dark adsorption results (Figure 8a) and photocatalytic efficiency (Figure 8b) together prove the feasibility and practicality of using T-BC-BWO photocatalysts for the removal of organic pollutants from wastewater [37][38][39].In order to identify the main effect of active species in degradation organic pollutants, the various trapping agents been investigated in detail in the photocatalytic process In order to identify the main effect of active species in degradation organic pollutants, the various trapping agents been investigated in detail in the photocatalytic process (Figure 8c).The photocatalytic degradation of RhB with reduced activity was shown in presence of different trapping agents.Specifically, the addition of triethanolamine (TEOA) or benzoquinone (BQ) significantly decreased the photocatalytic degradation efficiency, while the degradation efficiency was only slightly inhibited by the participation of tert-butanol (TBA).Together, these results suggest that the main active substances in the photodegradation of organic pollutants were photogenerated holes (h + ) and strong superoxide radicals (•O 2 − ), rather than hydroxyl radicals (•OH).To further demonstrate the active species participating in the photocatalytic degradation of organic pollutants, in situ ESR spin trapping experiments were performed on 5,5-Dimethyl-1-pyrroline-N-oxide (DMPO) under visible light irradiation.The •O 2 − signal was observed after light irradiation (Figure 8d), which indicates that •O 2 − contributes during the degradation of organic pollutants.

Photocatalytic Performance for H 2 Production Coupled with Selective Oxidation of Furfural Alcohol
The photocatalytic performance of the prepared catalysts was evaluated by a photooxidation-reduction double reaction under simulated sunlight irradiation (Figure 9a).In this way, FFA can not only replace the traditional hole sacrificers with excellent H 2 evolution, but also enable the production of value-added oxidation chemicals.Figure 9b summarizes the H 2 yields of the same photocatalyst under different reaction conditions.The optimal reaction condition can be locked at 3 mg catalyst, 5 µmol FFA, and 20 mL acetonitrile.All the samples exhibited satisfactory H 2 evolution capacity except for pure BC, while the H 2 -evolution capacity of T-BC-BWO was higher than that of pure BWO (332 µmol•g −1 •h −1 ) (Figure 9c).Notably, the H 2 yield of T-BC-BWO was further improved by regulating the hydrothermal temperature of composites.Among them, 210BC-BWO exhibits an ultra-high hydrogen generation rate of 2833 µmol•g −1 •h −1 , which is 2.12 times higher than that of the pristine BWO; this result further demonstrates the important role of the smaller forbidden band width.We deduce that the excellent H 2 evolution ability of 210BC-BWO is due to the moderately narrowed forbidden band width, which favors the generation of more photogenerated electron-hole pairs (Figure S3).The linear relationship between H 2 yield and time shows the stability of the catalyst (Figure 9d).Compared with T-BCs, all composite samples show satisfactory H 2 generation capability, while 210BC-BWO has a higher H 2 evolution capability than 170BC-BWO and 250BC-BWO, which further proves that 210BC, with its moderately narrow forbidden bandwidth, is favorable for the generation of more photogenerated electron-hole pairs.In addition, the cycling experiments were completed in the same system, Ar was introduced to remove the air and the residual H 2 after each reaction, and the H 2 yield was almost unchanged after six cycles, which further indicates that the binary composites have greater stability (Figure 9e).It is worth noting that the present study is in the upper-middle range of performance compared to the previous literature reports on furfural (FAL) photocatalytic oxidation (Table 2).In addition, the ratio of electrons to holes consumed in the redox reaction was calculated to be ca.1.0, based on the co-production rates of H 2 and FAL.This indicates that the dehydrogenation reaction proceeds at one stoichiometry, as shown in Equation ( 6) in Supporting Information.The photocatalytic H 2 yields of T-BC-Bi 2 WO 6 photocatalysts and the previously relevant FFA and H 2 production coupled with aldehyde generation rates (including other composite photocatalysts and sacrificial agents) are shown in (Table 2).

Reaction Mechanism
Based on the relationship between the conduction band potential (CB), and the band gap energy (Eg) values, the valence band (VB) positions of the BWO and T-BC-BWO are calculated according to the empirical Equation ( 5) and are listed in Table S1 [47].
A possible mechanism for the photocatalytic degradation of organic pollutants and photocatalytic H 2 evolution coupled with FFA oxidation was proposed according to the above result (Scheme 2).The CBs of 170BC-BWO and 210BC-BWO are more negative than the O 2 /•O 2 − redox potential (−0.33 V vs. NHE) and their VBs are more positive than the FFA/FAL redox potential (0.10 V vs. NHE), which is feasible for the formation of active radicals in the reaction system for the degradation of organic pollutants [48,49] and H 2 evolution coupled with FFA oxidation [41].The VB values of 170BC-BWO and 210BC-BWO are not required for •OH/OH − redox potential, which hinders the reaction between OH − and h + in producing •OH.Therefore, •OH does not contribute in the photocatalytic reaction system.The forbidden bandwidth value of 210BC-BWO is smaller than that of 170BC-BWO (Figure 7e), and the moderately narrowed forbidden bandwidth effectively improves the catalytic activity by light to produce more photogenerated electron-hole pairs.The CB and VB of 250BC-BWO are inadequate for the O 2 /•O 2 − reduction potential and the •OH/OH − oxidation potential; thus, 250BC-BWO shows the lowest photocatalytic performance.Apparently, the formation of e − , h + , and •O 2 − active species play a key role in the degradation of pollutants and H 2 evolution coupled with FFA oxidation by 210BC-BWO.Finally, the dual photo-oxidation-reduction reaction was carried out simultaneously, and the construction of the reaction system effectively prevented the accumulation of e − and h + on the surface of the catalyst with the excellent photocatalytic activity.

Reaction Mechanism
Based on the relationship between the conduction band potential (CB), and the band gap energy (Eg) values, the valence band (VB) positions of the BWO and T-BC-BWO are calculated according to the empirical Equation ( 5) and are listed in Table S1 [47].
A possible mechanism for the photocatalytic degradation of organic pollutants and photocatalytic H2 evolution coupled with FFA oxidation was proposed according to the above result (Scheme 2).The CBs of 170BC-BWO and 210BC-BWO are more negative than the O2/•O2 − redox potential (−0.33 V vs. NHE) and their VBs are more positive than the FFA/FAL redox potential (0.10 V vs. NHE), which is feasible for the formation of active radicals in the reaction system for the degradation of organic pollutants [48,49] and H2 evolution coupled with FFA oxidation [41].The VB values of 170BC-BWO and 210BC-BWO are not required for •OH/OH − redox potential, which hinders the reaction between OH − and h + in producing •OH.Therefore, •OH does not contribute in the photocatalytic reaction system.The forbidden bandwidth value of 210BC-BWO is smaller than that of 170BC-BWO (Figure 7e), and the moderately narrowed forbidden bandwidth effectively improves the catalytic activity by light to produce more photogenerated electron-hole pairs.The CB and VB of 250BC-BWO are inadequate for the O2/•O2 − reduction potential and the •OH/OH − oxidation potential; thus, 250BC-BWO shows the lowest photocatalytic performance.Apparently, the formation of e -, h + , and •O2 − active species play a key role in the degradation of pollutants and H2 evolution coupled with FFA oxidation by 210BC-BWO.Finally, the dual photo-oxidation-reduction reaction was carried out simultaneously, and the construction of the reaction system effectively prevented the accumulation of e -and h + on the surface of the catalyst with the excellent photocatalytic activity.The above experimental data combined with the literature indicate the contribution of carbon materials to improving charge separation efficiency.The modification of BC in 210BC-BWO leads to a narrowing of the band gap, thereby facilitating the rapid generation of electron-hole pairs under visible light irradiation.Under visible light irradiation, the e − of BWO are easily liberated from the confinement of electron-hole pairs (Equation ( 1)), and they swiftly transition from the VB state to the CB state within the band gap of BWO.Importantly, the incorporation of the electron acceptor BC into BWO is crucial, as it can rapidly shuttle the excited electrons from BWO's CB state to the surface of BC (Equation ( 2)), thus preventing the recombination of photo-induced charge carriers in the composite system.Consequently, this leaves h + at the VB position, which can directly oxidize adsorbed TC molecules (Equation ( 3)).Due to the close contact and reaction of a large number of e − with oxygen molecules adsorbed on active sites of BC, more •O 2 − is produced (Equation ( 4)).Finally, the formation of •O 2 − strongly oxidizes organic pollutants (RhB, TC-HCl) into non-toxic small molecules (Equation ( 5)) [50,51].Additionally, in the hydrogen production system, the strong oxidation hole from BWO promotes FFA to rapidly convert into FAL and release protons, and the photogenerated e − from BC can readily reduce surface-adsorbed protons to generate H 2 (Equations ( 1), ( 2), ( 4), ( 6) and ( 7)).Based on the above analysis, the stepwise mechanisms are proposed as follows: e − (Bi 2 WO 6 ) → e − (biochar) h + (Bi 2 WO 6 ) + RhB/TC-HCl → degraded products •O 2 − (biochar) + RhB/TC-HCl → degraded products h + (Bi 2 WO 6 ) + FFA → FAL + 2H + 2H + + 2e − (biochar) → H 2 (7)

Preparation of T-BC-BWO Composite
Bamboo charcoal was synthesized by hydrothermal method using bamboo powder as carbon source.The method was as follows: 2.000 g of bamboo powder was washed with deionized water and ethanol, respectively, and subsequently mixed with 60 mL of deionized water, transferred to an autoclave lined with Teflon, and kept in a blast oven at a set temperature of T for 24 h.After the hydrothermal reaction, the product was rinsed 2-3 times with deionized water and anhydrous ethanol, filtered through a vacuum pump, and the solid product was dried in a vacuum oven at 60 • C.After thorough grinding in a mortar, the T-BC (T is 170 • C, 210 • C and 250 • C) powder was obtained.
Typically, 1.5000 g of Bi(NO 3 ) 3 •5H 2 O was dissolved in 10 mL of 1.6 mol L −1 nitric acid solution.After Bi(NO 3 ) 3 •5H 2 O was completely dissolved, 10 mL of deionized water was added and the solution was diluted to 20 mL to obtain the precursor solution A. Meanwhile, 0.4728 g of Na 2 WO 4 •2H 2 O was dissolved in 20 mL of 2.0 mol L −1 NaOH solution by magnetic stirring to obtain the precursor solution B. After that, precursor solution B was added drop by drop under magnetic stirring to precursor solution A, and the pH of the resulting mixed solution was adjusted to 1 with an aqueous solution of 1.6 M HNO 3 and 2 M NaOH.Subsequently, 0.09 g of T-BC was added to the above-mixed solution, and after magnetic stirring for 10 min and ultrasonic dispersing for 30 min, a homogeneous mixed solution was formed.This mixed solution was then transferred to a 100 mL Teflon-lined reactor and hydrothermally reacted at 170 • C for 12 h.After completion of the process, the reactor was removed and allowed to cool down naturally to room temperature.The reaction precipitate was collected by filtration through a Brinell funnel, washed several times and dried overnight.A brown colored T-BC-BWO composite photocatalyst powder was obtained.Depending on the preparation temperature of the added charcoal, the obtained products were labeled as 170BC-BWO, 210BC-BWO, and 250BC-BWO.In addition, 210BC was mechanically mixed with BWO, and recorded as 210BC + BWO.

Photocatalytic Experiments
The photocatalytic experiments were carried out in a 100 mL glass beaker containing a certain amount of the target solution and the prepared photocatalyst powder.The visible light source was a 500 W Xe lamp with a 420 nm cutoff filter placed vertically at a fixed distance of 15 cm from the glass beaker.Prior to the photocatalytic experiments, 0.05 g of the prepared photocatalyst powder was dispersed in 50 mL of RhB solution (10 mg L −1 ), the magnetic stirrer was turned on and kept in the dark for dark adsorption, with the aim of reaching adsorption equilibrium to exclude the effect of powder adsorption on the photocatalytic efficiency.During the photocatalytic experiments, approximately 4.0 mL of solution was withdrawn at predetermined time intervals with a pipette gun, fed into a medical syringe, filtered through a filter membrane (13 mm, 0.22 µm), and the resulting solution was transferred to a cuvette.The absorbance of RhB solution at the maximum spectral absorption peak (554 nm) was measured using a UV-vis spectrometer (Shanghai Jingke 722N UV-Vis Spectrometer, Shanghai, China) to assess the dye degradation process.The degradation process of TC-HCl was similar to that of RhB, except that the concentration of TC-HCl was 20 mg L −1 and the amount of photocatalyst was 0.05 g.The absorbance value of TC-HCl at 357 nm was measured and recorded by a double-beam UV spectrophotometer (Shanghai Jingke L5S, Shanghai, China) to evaluate the degradation rate of TC-HCl.The procedure for the reactive group burst experiment was similar to the photocatalytic experiment described above, except that 0.1 mM of a specific scavenger was added to the solution before the xenon lamp was turned on.The identification of h + , •OH and •O 2 − was performed with triethanolamine (TEOA), tert-butanol (TBA) and p-benzoquinone (BQ), respectively.
Photocatalytic H 2 evolution coupled with FFA oxidation experiments were conducted in a reactor with a quartz glass lid and a circulating water-cooling jacket.A total of 20 mL of 5 µmol FFA-acetonitrile solution and 3 mg of photocatalyst were introduced into the reactor.The reactor was then degassed under vacuum, energized with argon, and finally placed under a 500 W Xe lamp for 1 h.At the end of the reaction, the resulting solution was centrifuged and the supernatant was filtered through a syringe fitted with a 0.22 µm filter.The samples of H 2 and furfural (FAL) were injected into gas chromatograph (GC9790 Plus, Fuli Analytical Instruments Corp, Hangzhou, China) equipped with flame ion detection.

Conclusions
In summary, a novel T-BC-BWO with 3D/3D rod/flower spherical morphology photocatalyst was synthesized by hydrothermal method.The removal efficiency of RhB and TC-HCl were 98.5% and 77.1%, and H 2 evolution rate (2833 µmol•g −1 •h −1 ) coupled with FFA oxidation (3097 µmol•g −1 •h −1 ) by 210BC-BWO photocatalyst was conducted under visible light irradiation.The forbidden band width of the composite photocatalyst was reduced from 2.50 eV (BWO) to 1.89 eV (210BC-BWO), and the suitable VB and CB positions led to a substantial improvement in the catalytic performance.This study provides a bifunctional photocatalyst for the degradation of organic pollutants or the catalytic conversion of biomass FFA to H 2 and FAL with high value-added chemicals and proposes a green and sustainable strategy for solving environmental pollution and energy shortages.

Scheme 1 .
Figure1ashows the XRD spectrum for three T-BC samples.As we know, cellulose has characteristic peaks at 16.3°, 22.3°, and 35°, corresponding to the crystalline structures of type II as (101), (002) and (040)[27].The presence of characteristic peaks of cellulose in the diffraction pattern of 170BC indicated that the bamboo powder was not completely charred at 170 °C.The disappearance of the characteristic peaks of 210BC and 250BC indicate that the hydrothermal carbonization reaction destroys the crystalline structure of cellulose to form amorphous carbon at a higher temperature, and generates biomass carbon materials that possess high specific surface area, endowing the carbon material with high adsorption.The XRD pattern of the T-BC-BWO composite sample is shown in Figure1b.T-BC has no effect on the crystal structure of the composite photocatalysts, with peaks located at 28.2°, 32.7°, 47.1°, 55.8°, 58.6°, and 69.0°, corresponding to the (131), (200), (260), (133), (262), and (400) crystallographic planes of the orthorhombic BWOs[28].For the following two reasons, the diffraction peaks of T-BC are not observed in all composites.Firstly, T-BC belongs to an amorphous structure and there is no obvious crystal diffraction information.Secondly, the diffraction peak intensity of T-BC is distinctly low compared with BWO.

Figure 1 .
Figure 1.XRD patterns of (a) T-BC, (b) T-BC-BWO composite photocatalysts, and reused photocatalysts.The XRD pattern of the T-BC-BWO composite sample is shown in Figure 1b.T-BC has no effect on the crystal structure of the composite photocatalysts, with peaks located

Figure 3 displays
Figure3displays Field Emission Scanning Electron Microscope (FESEM) images for the photocatalytic composites (170BC-BWO, 210BC-BWO, and 250BC-BWO).BWO composites show a 3D flower-and sphere-like structure with a diameter of approximately 4 μm (Figure3a).The carbonization temperature has a large influence on the morphology of the T-BC-BWO composite photocatalyst.The morphology of 3D/3D 170BC-BWO consists of 3D rod-shaped 170BC and 3D flower-spherical BWO (Figure3b).The 3D rodshaped carbon material can improve the dispersion of catalysts in this 3D/3D structure.The morphology of 3D/3D 210BC-BWO showing the BWO flower ball is firmly fixed on the surface of 210BC, and the morphology and structure of the BWO flower bulbs were not changed by the incorporation of bamboo charcoal, but the size of the flower bulb decreased to 2 μm (Figure3c).In contrast, 250BC-BWO is a composite of blob-shaped BWO and crumbled 250BC, the rod structure is crumbled to form a 3D/3D blob/blob-shaped 250BC-BWO (Figure3d), which provides more uniform dispersion in treating organic pollutants.Therefore, the optimal hydrothermal temperature can be locked at 210 °C for

Figure 3 displays
Figure3displays Field Emission Scanning Electron Microscope (FESEM) images for the photocatalytic composites (170BC-BWO, 210BC-BWO, and 250BC-BWO).BWO composites show a 3D flower-and sphere-like structure with a diameter of approximately 4 µm (Figure3a).The carbonization temperature has a large influence on the morphology of the T-BC-BWO composite photocatalyst.The morphology of 3D/3D 170BC-BWO consists of 3D rod-shaped 170BC and 3D flower-spherical BWO (Figure3b).The 3D rod-shaped carbon material can improve the dispersion of catalysts in this 3D/3D structure.The morphology of 3D/3D 210BC-BWO showing the BWO flower ball is firmly fixed on the surface of 210BC, and the morphology and structure of the BWO flower bulbs were not changed by the incorporation of bamboo charcoal, but the size of the flower bulb decreased to 2 µm (Figure3c).In contrast, 250BC-BWO is a composite of blob-shaped BWO and crumbled 250BC, the rod structure is crumbled to form a 3D/3D blob/blob-shaped 250BC-BWO (Figure3d), which provides more uniform dispersion in treating organic pollutants.Therefore, the optimal hydrothermal temperature can be locked at 210 • C for bamboo charcoal composites, taking into account that the stabilized heterogeneous structure could exhibit excellent photocatalytic performance.In addition, as shown in the elemental mapping results C, O, Bi, and W elements are uniformly distributed in the hybrids, indicating the close coupling between BC and BWO, and the successful formation of 210BC-BWO composites (FigureS2).

Molecules 2024 ,
29,  x FOR PEER REVIEW 5 of 18 bamboo charcoal composites, taking into account that the stabilized heterogeneous structure could exhibit excellent photocatalytic performance.In addition, as shown in the elemental mapping results C, O, Bi, and W elements are uniformly distributed in the hybrids, indicating the close coupling between BC and BWO, and the successful formation of 210BC-BWO composites (FigureS2).
) in Supporting Information.The flat band potentials (V fb ) of the 210BC-BWO photocatalyst are −0.458eV (vs.SCE) and −0.217 eV (vs.NHE).Molecules 2024, 29, x FOR PEER REVIEW 8 of 18 of introducing T-BC on the electrochemical properties.The 210-BC-BWO photocatalyst show positive slopes, thus this photocatalyst could be ascribed to an n-type semiconductor [36].All flat band potentials of raw materials and composites were determined through analysis of the M-S curves combined in Equation (3) (Supporting Information).The flat band potentials (Vfb) of the 210BC-BWO photocatalyst are −0.458eV (vs.SCE) and −0.217 eV (vs.NHE).

Figure 8 .
Figure 8. Photocatalytic degradation curves of (a) absorbance and (b) concentration for TC-HCl by T-BC-BWO photocatalyst under visible light conditions; (c) active species trapping experiment for the RhB degradation; (d) EPR spectra of 210BC-BWO with DMPO under visible light irradiation.

Figure 8 .
Figure 8. Photocatalytic degradation curves of (a) absorbance and (b) concentration for TC-HCl by T-BC-BWO photocatalyst under visible light conditions; (c) active species trapping experiment for the RhB degradation; (d) EPR spectra of 210BC-BWO with DMPO under visible light irradiation.

Scheme 2 .
Scheme 2. Diagrammatic depiction of the plausible photocatalytic mechanism involving the 210BC-BWO photocatalyst during the degradation of RhB/TC-HCL and H2 evolution coupled with FFA oxidation under visible light irradiation.

Scheme 2 .
Scheme 2. Diagrammatic depiction of the plausible photocatalytic mechanism involving the 210BC-BWO photocatalyst during the degradation of RhB/TC-HCL and H 2 evolution coupled with FFA oxidation under visible light irradiation.

Table 1 .
Pore properties of T-BC and T-BC-BWO samples.

Table 2 .
The photocatalytic H 2 production and aldehyde generation rates (including other composite photocatalysts and sacrificial agents).