Carbon dots prepared from waste wood and residual adhesive and their use as catalysts for hydrogen production

Wu, J., Zhao, L., and Song, G. (2024). “Carbon dots prepared from waste wood and residual adhesive and their use as catalysts for hydrogen production,” BioResources 19(2), 2417-2435.

Abstract

Large quantities of waste wood with residual resin adhesives are not recycled efficiently. To address this issue, waste wood with residual resin adhesives was synthesized into carbon dots (CDs) via a facile hydrothermal self-assembly method to enhance the H₂ evolution performance of graphite C₃N₄ (g-C₃N₄), a metal-free photocatalyst. Among all samples, the most significant enhanced sample was MCN-UF-3.5, which has an H₂ evolution of 22.1 mmol·g^-1·h^-1, which is 3.91 times that of unmodified g-C₃N₄. The band gap and recombination of photogenerated charges were both improved by the doping of CDs. Meanwhile, the DFT calculation showed that adding CDs, especially with the -NH₂ group, can significantly deform the structure and destroy the symmetry. This consequence implies an enhancement in the activity of the hydrogen evolution reaction, confirming the feasibility of the modification.

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Carbon Dots Prepared from Waste Wood and Residual Adhesive and Their Use as Catalysts for Hydrogen Production

Ji Wu,^a Liang Zhao,^a,* and Guohui Song ^b

DOI: 10.15376/biores.19.2.2417-2435

Keywords: Waste wood; Residual adhesive; Carbon dots; g-C₃N₄; Photocatalytic; DFT

Contact information: a: Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, College of Materials Science and Engineering, Nanjing Forestry University, Nanjing, 210037, China; b: School of Energy and Power Engineering, Nanjing Institute of Technology, Nanjing, 211167, China;

* Corresponding author: liangzhao@njfu.edu.cn; zl_06@126.com

GRAPHICAL ABSTRACT

INTRODUCTION

The proliferation of waste wood-based panels is a problem, but these materials could be used as valuable resources (Farjana et al. 2023). Due to the chemical pollutants and resin adhesives, the traditional disposal methods, such as landfilling and burning, are low-efficiency and high cost (Girods et al. 2008; Kim and Song 2014). These residual resins cannot be eliminated easily (Zhong et al. 2017). The residual resins hinder the modulus of rupture, modulus of elasticity, and internal bonding strength of remanufactured particleboards (Czarnecki et al. 2003). The residual resin and other compounds that are not conducive for recycling are difficult to handle efficiently and cost-effectively in the current common treatments. Therefore, there is an urgent need for a better way to utilize waste wood and its residual adhesive compounds.

Carbon dots (CDs) derived from biomass materials are emerging carbon nanomaterials with the advantages of advances in synthesis (Bian et al. 2016), hydrophilicity (Ehtesabi and Massah 2021), tunability (Ma et al. 2016), and optical properties (Liu et al. 2013), as well as a wide variety of raw materials, especially biomass (Li et al. 2020), whose sources are extensive and low-cost. CDs are commonly used in ion detection (Yang et al. 2022b), fluorescence imaging (Liu et al. 2017), photocatalysis (Sbacchi et al. 2023), and other applications. The main structure consists of a core composed mainly of sp² hybridized regions, and a shell composed of sp³ hybridized regions (Zhu et al. 2015). The shell composed of sp³ endows CDs with the sites to bind functional groups such as -OH, -C=O, -NH₂, etc., which makes them well tunable (Xia et al. 2019). The sp²-enriched nucleus of CDs is essential for photocatalytic reactions because of its large specific area and outstanding electron conductivity (Li et al. 2018).

Photocatalytic reactions include photocatalytic degradation and hydrogen evolution. The main principle of the photocatalytic hydrogen evolution reaction (Jafari et al. 2016) is the use of a catalyst to absorb photons to produce electron-hole pairs. Subsequently, H⁺ formed at the catalytic site of the catalyst reacts with the electrons to produce hydrogen. Compared to metal catalysts, which are expensive and prone to secondary pollution, the two-dimensional material with rich sp² hybridization g-C₃N₄ is a more accessible and environmentally advantageous option. The g-C₃N₄ has the advantages of suitable band gap in visible light region, non-toxic feature, high chemical stability, and good elastic and tensile strength (Han et al. 2020), which means that it is suitable for photocatalytic hydrogen evolution (Zhao et al. 2015). However, the photocatalytic efficiency of the pristine g-C₃N₄ is unsatisfactory due to small specific surface area and easy complexation of photogenerated electrons (Mamba and Mishra 2016). At present, the main improvement directions for photocatalysts include (Fajrina and Tahir 2019) doping with heteroatoms, which can change the band gap structure, to enhance the absorption capacity in various wavenumber regions. Another option is to utilize the microstructure of nanoparticles with good mechanical properties and compatibility (Jia et al. 2023), such as SiC, as skeleton loading photocatalysts reduces the recombination of photogenerated electron-hole pairs (Wang et al. 2017).

Wood is made up of cellulose, semi-cellulose, and lignin. Cellulose was studied to be a good carbon source to prepare CDs (Souza et al. 2018). Lignin can offer O atoms and functional groups to enhance CDs performance. In addition, CDs prepared from waste wood can reduce preprocessing steps. So, the work in this study focused on providing a new route for the reuse of waste wood with residual adhesives. Carbon dots, which were prepared from waste wood, was used to modify g-C₃N₄ in order to enhance its photocatalytic H₂ evolution performance. Meanwhile, adding different kinds and ratio of adhesives to adjust photocatalysts performance, including phenolic (PF) resins, urea-formaldehyde (UF) resins, and melamine-formaldehyde (MF) resins. First principle calculation is used to emulate surface of photocatalysts to verify the mechanistic feasibility of the modification in this study.

EXPERIMENTAL

Materials

Melamine (≥ 99.6%, CP) was purchased by Lingfeng Chemical Reagent Co. Ltd. (Shanghai, China). Triethanolamine (TEOA, ≥ 99.0%, AR) was purchased by Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Chloroplatinic acid hexahydrate (≥ 98.0%, AR) was purchased by Rhawn Reagent Co. Ltd. (Shanghai, China). Waste poplar was purchased by Maoyou Timber Co. Ltd. (Jiangsu, China). Urea-formaldehyde resin (UF), phenolic resin (PF) and melamine-formaldehyde resin (MF) were purchased by Zhongcheng Plastic Co. Ltd. (Zhejiang, China).

Synthesis Methods

The preparation method of CDs followed and was adapted from the hydrothermal method in previous studies (Wareing et al. 2021). The holistic synthesis process of CDs, g-C₃N₄ and photocatalyst is shown in Fig. 1(A). Specifically, 80 mesh poplar wood powder was mixed with different types of resins to form a mixture with a total weight of 2.5 g. The types of resins used in this study were urea-formaldehyde (UF), phenol-formaldehyde (PF), and melamine-formaldehyde (MF). The weight ratio of the resins in the mixture was 2.5%, 3.5%, and 4.5%, respectively. The mixture was dispersed in 50 mL deionized water. Then, after thorough stirring, the solution was transferred to a 100 mL Teflon-lined autoclave and heated at 180 °C for 8 h. Moreover, the suspension was centrifuged at 10000 rpm for 15 min to remove the bulk particles, leaving only the supernatant. Subsequently, the supernatant was dialyzed for 24 h using a dialysis bag with a molecular weight cut-off of 1000. Finally, the solution was lyophilized for 48 h and the obtained powder was collected and stored in a drying dish.

A total of 10 g of melamine was placed in a crucible of alumina and calcined in a tube furnace at 550 °C by 10 °C·min^-1 heat rate for 4 h and annealed at 520 °C for 2 h. The samples were allowed to cool naturally to room temperature, and ground into 150-mesh powder.

For the preparation of photocatalysts obtained by functionalization of g-C₃N₄ coupled CDs, 100 mg of g-C₃N₄ was mixed with 10 mg of different kinds of CDs powder in 50 mL deionized water, which was then performed 2 h ultrasonic treatment and magnetically stirred for 2 h. Thereafter, the solution was transferred to a Teflon-lined autoclave and heated at 180 °C for 2 h. Following, the suspension was centrifuged at 10000 rpm for 2 min to obtain the lower precipitate after solid-liquid separation. Collected precipitate was lyophilized for 12 h. The final obtained powder is the photocatalyst denoted as MCN-x-y (modified g-C₃N₄). The x represents the kinds of resins added in preparation of CDs, and y represents the ratios of the CDs system. Samples without the CDs powder are named as g-C₃N₄, and ones with added unmixed CDs powder are named as MCN-PP (poplar powder-derived CDs).

Instruments

SEM (Reguluss8100, Hitachi), BET (BSD-PS(M), BEISHIDE), Raman spectrum (DXR532, Themor), FT-IR spectroscopy (VERTEX 80V, Bruker) and XPS (AXIS UltraDLD, Shimadzu) were used to obtain the morphology, microstructure, and surface elements chemical states of the samples. The optical and photoelectrochemical properties were characterized by a UV-vis diffuse reflectance spectra (Lambda 950, PE) and a photoluminescence (PL) emission spectra (FluoroMax-4, HORIBA). Hydrogen production was analyzed using a gas chromatograph (GC9790 Plus, Fuli).

Photocatalytic Hydrogen Evolution Experiment

For the photocatalytic H₂ evolution experiments, a 350 W cold xenon lamp with a light intensity set to 1100 W·m^-2 was used as a light source to simulate solar irradiation. The reaction solution consisted of deionized water, 10 mL of TEOA, and H₂PtCl₆ at 3wt% to make up a total of 100 mL of the system. Next, 100 mg of catalyst was added to the solution, followed by purging with high-purity argon gas at a flow rate of 50 mL·min^-1 and magnetic stirring for 60 min in a dark environment. After turning on the light source, the reaction gas products were collected using an aluminum foil gas bag, which was changed every 10 min. The gas products were passed into the GC for analysis.

DFT Calculation Details

The Cambridge Serial Total Energy Package (CASTEP) modules in BIOVIA Material Studios were used to emulate photocatalyst structures (Segall et al. 2002). All theoretical simulation calculations adopted Perdew-Burke-Ernzerhof (PBE) (Perdew et al. 1996a) functional within the generalized gradient approximation (GGA) (Perdew et al. 1996b). The ultra-soft pseudopotential and the norm-conserving pseudopotential were chosen to complete different parts of the calculations. The former has looser convergence conditions and was used for geometry optimization. The latter has stricter convergence and can obtain more accurate results. The properties of photocatalyst structures, such as band structure, the density of states (DOS), and potential, were calculated by the norm-conserving pseudopotential. Considering the Van der Waals interactions, the TS custom method was used for DFT+D to correct (Tkatchenko and Scheffler 2009). The cut-off energy was set with 400 eV for geometry optimization and 500 eV for properties (Yang et al. 2022a). Kohn-Sham wave functions were taken as a relativistic treatment. The parameters of convergence tolerance (Yang et al. 2022a): energy, max force, and max displacement were set as 1×10^-5 eV·atom^-1, 3×10^-2 eV·A^-1, and 1×10^-3 A respectively, and SCF tolerance was set as 1×10^-6 eV·atom^-1. In addition, given that hybrid functions have better performance in the band structure and DOS, HSE06 (Heyd et al. 2006), a hybrid function, was separately adopted in corresponding calculations.

RESULTS AND DISCUSSION

Morphology and Structure Characterization

SEM is commonly used to detect the microstructure and morphology of micrometer-sized catalysts. As displayed in Fig. 1(B), the major component of MCN-UF-3.5 photocatalyst was bulk g-C₃N₄, and the diameter was 15 to 20 μm.

Fig. 1. (A) Synthesis of CDs, g-C₃N₄ and photocatalyst; (B, C) SEM images of MCN-UF-3.5

Numerous microparticle CDs were dispersed and attached to the surface of bulk g-C₃N₄. As shown in Fig. 1(C), the major component of the photocatalyst had lamellar features, composed of multiple ultrathin paper-fold sheets, which is the typical structure of g-C₃N₄. The morphology of the synthesized photocatalysts coincided with that reported by Cui et al. (2020). The morphology of CDs distributed on the surface was predominantly sphere-like or granular, matching the features in Singh’s study (Singh et al. 2023). To better demonstrate the features of CDs, TEM, and other microscopy analysis will be carried out in the coming research. By measuring the particles in Fig. 1(C), it was known that their diameter was concentrated in the range of 1.4 to 0.2 μm or even smaller. These results suggested that the photocatalyst had the fundamental g-C₃N₄ structure, which could absorb radiation and emit electrons (Zhang et al. 2021), and the combination of CDs modified the surface.

The BET specific surface area of g-C₃N₄ and its modified samples were characterized by N₂ adsorption-desorption experiments. The isothermal curves are plotted in Fig. 2(A). As depicted in Fig. 2(A), both g-C₃N₄ and CDs doped samples exhibited type IV isothermal curves that did not reach the saturation plateau (Brunauer et al. 1938), indicating the presence of mesoporous architecture. The data revealed that MCN-UF-3.5, with the most significant specific surface area of 38.6 m²·g^-1. The surface area of MCN-UF-3.5 was larger than that of g-C₃N₄ (10.3 m²·g^-1) by 3.74 times. The previous studies reported that increased specific surface area is helpful to enhance photocatalytic performance (Alshammari et al. 2023). Therefore, the doping of CDs increased photocatalysts’ specific surface area, making the 2-dimensional structure turn into the 3-dimensional structure (Zhu et al. 2017). This means that adding CDs improved the efficiency of charges transferring across the interface, positively affecting photocatalytic H₂ evolution.

Raman scattering is often used to obtain vibrational information specific to the chemical bonds in molecules. The Raman spectra in the work were excited at 780 nm using a dual laser system. Figure 2(B) shows Raman spectra of g-C₃N₄ and MCN-x-3.5 (x=PP, UF, MF, PF). MCN-UF-3.5 outperformed all other samples in terms of holistic intensity. The most substantial peak of samples in Fig. 2(B) was the peak located at 700 cm^-1. It is generally noted by the prior studies that this 700 cm^-1 peak is a typical one caused by triazine ring vibrations with bent surface (Li et al. 2015). There was a weaker peak at 750 cm^-1, which was not universally presented in g-C₃N₄. Zinin et al. (2009) found that the 750 cm^-1 peak mainly relates to the properties of raw materials used to synthesize. According to the report (Zinin et al. 2009), one typical peak of melamine, the raw material of the catalyst, was located around 750 cm^-1, which fits the regularity. Besides, the peak at 975 cm^-1 had discrepancies with the simulation report results (Gonze et al., 2002) but was identical with Zinin’s report (Zinin et al. 2009). As has been stated (Deifallah et al. 2008; Kroke et al. 2002), it was ascribed to the s-triazine breathing mode or polymorphs. As for the polymorphs, it was probably formed due to the destruction of the original structure of g-C₃N₄ by the ultrasonic treatment during synthesis. The peak with high intensity near 1340 cm^-1 was a typical D band because of defects on the catalyst surface (Saraswat and Yadav 2020). These defects were caused by the surface modification by CDs, enhancing the sp³ hybridization of the region. The cause of the peak formation at 1410 cm^-1 was similar to the peak at 750 cm^-1. It was attributed to breathing and stretching of the ring (Meier et al. 1995). Furthermore, the undulating peak at 1590 cm^-1 was the G band, mainly related to the sp² hybridization of C atoms (Grimm 2008). The prominent intensity of the peak at 1590 cm^-1 of MCN-UF-3.5 clarified the higher electron conductivity than others (Luo et al. 2019). It may be deduced from the phenomenon that MCN-UF-3.5 possessed a better capacity for H₂ evolution. The curve of g-C₃N₄ and MCN-PP had a more apparent G peak at 1410 cm^-1 than other samples doped CDs. This phenomenon can be interpreted as related to ultrasonic treatment and doping of CDs. They did not destroy the structure of the triazine ring and s-triazine ring but accounted for the low intensity of the G peak.

Fig. 2. (A) Nitrogen adsorption-desorption isotherms of samples; (B) Raman spectra (785 nm) of photocatalysts after background and offset corrections; (C) FT-IR spectra of g-C₃N₄ and different doped samples; (D) XPS survey spectra of samples; (E, F) High-resolution N1s and C1s spectra of photocatalysts

Analysis of information on molecule vibrations in FT-IR spectra referred to identifying the material molecules and functional groups on the surface. The FT-IR spectra of g-C₃N₄ and other samples doped with CDs are exhibited in Fig. 2(C). The key messages in Fig. 2(C) were concentrated in three sections, which were a section located in the range of 3255 to 3050 cm^-1, a section ranging from 1650 to 1200 cm^-1, and a section within 900 to 800 cm^-1 wavenumber range (Li et al. 2021). The first consisted of three scattered weak peaks: 3255, 3165, and 3082 cm^-1. Spectra in this area were analyzed to illustrate the information of the stretching vibrational characteristic of the substance. The peaks of three samples doped with diverse CDs had a higher intensity than the others. In particular, the peak at 3255 cm^-1, thought to be caused by the stretching of the O-H group (Cui et al. 2020; Li et al. 2021), was too weak to recognize in the pristine g-C₃N₄ but was notable in doped samples. Adding CDs synthesized by poplar powder and resins brought more O atoms into the photocatalyst, making the peak visible. The peak at 3165 cm^-1 was regarded as the result of N-H stretching vibrations in uncondensed amino groups (Majdoub et al. 2020). There was an apparent disparity in peak intensity between MCN-UF-3.5 and the pristine g-C₃N₄. The stretching vibration of C-H bonds generally forms the peak located around 3040 cm^-1 (Coates 2000). However, when the C atoms were located at the end of the molecular structure, C-H₂ bonds, whose stretching vibrational peak was found at 3085 cm^-1 nearby (Ogita et al. 2004), would be formed. It can be inferred that C atoms at the edge positions in the structure destroyed by ultrasonic treatment reformed C-H₂ bonds during hydrothermal synthesis. The section, which was brought about by C-N heterocyclic vibrations ranging from 1650 to 1200 cm^-1, was subdivided into six peaks located at 1637, 1562, 1463, 1402, 1325, and 1236 cm^-1. The peak at 1637 cm^-1 corresponded to the C=C bond of the aromatic ring skeleton, and the peak at 1469 cm^-1 referred to the deformation of -CH₂ or the antisymmetric deformation of -CH₃ (Li et al. 2021). The last section, covering 900 to 800 cm^-1 and composed of two peaks, represented the typical breath mode of triazine and s-triazine ring units, the skeleton of g-C₃N₄. The above analysis elaborated that the doping of CDs maintained the skeleton of g-C₃N₄. On this foundation, the treatment inducted more O atoms and reinforced the sp³ hybridization of the surface.

The surface elemental chemical states of g-C₃N₄ and samples doped with CDs were detected using XPS. Figure 2(D) shows a survey scan spectra of five samples, which could be divided into three elemental solid components: O1s, N1s, and C1s. Among them, the intensity of O1s peak on g-C₃N₄ was weaker than others, and its peak area only occupied 4.6% of the total peak area of g-C₃N₄. The doping of CDs significantly increased the intensity of O1s, especially MCN-PF-3.5. The O1s peak area of MCN-PF-3.5 was up to 18.7% of the total peak area, owing to rich oxygenated functional groups on phenolic resin. The characteristics of the N1s spectra of four CDs doped samples were consistent. As presented in Fig. 2(E), five peaks located at 398.2, 398.9, 399.6, 400.8, and 404.2 eV were associated with sp² hybridization in C-N=C, pyridine N, N-(C)₃, N-H and charge effects triggered by stacking of π-bonds, respectively (Cheng et al. 2017; Fang et al. 2017). For the unmodified g-C₃N₄, the lower O content than the others blurred the boundary between the pyridine N peak and the N-(C)₃ peak, resulting in a distinct peak at 399.3 eV. This distinction meant that there was a weak modification of CDs adjusting the structure of g-C₃N₄, thus promoting photocatalytic performance. The C1s peaks of g-C₃N₄ and MCN-x-3.5 were divided into four and five sections, individually. The shared segments were located at binding energies of 284.2 and 285.0 eV, on behalf of the sp² hybridization part of carbon and the sp³ hybridization part of C-C bonds (Chernyak et al. 2020). The trend demonstrated in Fig. 2(F) could be explained by the fact that these two peaks of CDs doped samples had higher intensity than the pristine g-C₃N₄. Oxygenated functional groups were attached to the surface broadened these peaks, increasing the difficulty of separating peaks. The peak located at 286.4 eV was not present in the unmodified g-C₃N₄ but in modified samples, indicating the C-O bonds (Li et al. 2021). There were two peaks at 287.9 and 288.2 eV. The former was related to the C=O bond, and the latter meant the sp² hybridization of N-C=N (Wang et al. 2018). There was a noticeable difference in the area of the peak located at 287.9 eV between samples of modified CDs with adhesives and others. O atoms generally were present in cured resins mixed with wood powder in the configuration of C-O-C, C-O-N, and O-H bonds. Although the solution was not in acidic conditions, the hydrothermal synthesis of waste wood with residual adhesives was reacted at higher temperatures and pressures than in the study of Liu et al. (2018). Due to the hydrolysis of cured resins, the original chemical bonds were broken, and C=O bonds were formed in such conditions. As described by Lukowsky (2002), ether bridges (HNCH₂-O-CH₂NH) in MF resins are more stable than methylene bridges (NHCH₂-NH). It was hard for O atoms to be restricted to the ether bridges to form C=O bonds. This concept was able to account for the lower peak intensity at 287.9 eV of MCN-MF-3.5 than samples modified by UF and PF. The peak unique to the unmodified g-C₃N₄ and MCN-PP had higher separation than others located at 287.6 eV, denoting the existence of the C=N combination. It could be concluded that the analysis of XPS spectra for each sample coincided with the results of FT-IR spectra. They both elaborated that the doping of CDs had modified the combination of chemical bonds on the surface without destroying the classical structure of the pristine g-C₃N₄, which enhanced the hydrogen evolution capacity.

Optical and Photoelectrochemical Properties

The photocatalyst, which occurred photon-excitation, caused the transition to excited states. Then, the emission of light or luminescence formed by the energy or photons released through the relaxation process is photoluminescence (PL). Thus the PL emission spectra were studied to understand the separation of photogenerated electron-hole pairs and recombination to study the optical properties of photocatalysts further. As shown in Fig. 3(A), the intensity of the unmodified g-C₃N₄ was much higher than that of others. The higher PL intensity of the photocatalyst represented more energy released through the recombination of electron-hole pairs than modified samples. This component energy was wasted and not transmitted to the photocatalyst surface for the hydrogen evolution reaction. The PL intensity of the unmodified g-C₃N₄ indicated the lower hydrogen evolution performance than others. Remarkable results relative to the problem of recombination of photogenerated electron-hole achieved by doping CDs could be deduced from Fig. 3(A). The data of time-resolved transient PL decay measured the lifetime of charge carriers. A shorter lifetime of charge carriers meant a faster rate of photogenerated electrons transferred to the photocatalyst surface.

Equation 1 calculates the final average lifetime of PL (Zhang et al. 2022),

(1)

where τ₁ and τ₂ are the first-order fitting and the second-order fitting lifetime, and B₁ and B₂ correspond respectively to their normalized amplitudes (Chauhan et al. 2016). As shown by data listed in Table 1, modified samples obtained a shorter average PL lifetime than the unmodified g-C₃N₄, especially MCN-PF-3.5. Comparing MCN-PF-3.5 with the unmodified g-C₃N₄, the reduction of the PL lifetime was up to 31.2%. This result manifested that the time for the recombination of charge carriers became much less, and due to the increase in the utilization of photogenerated electrons, the number of unrecombined electron-hole pairs increased.

Table 1. Average Lifetime of Samples after Second-Order Fitting

As a standard method to analyze the optical properties, UV-Vis DRS was detected to further investigate the electronic transition between energy levels g-C₃N₄ and samples doped with CDs. On the basis of the intensity that emerged in Fig. 3(B), it can be inferred that the doping of CDs obviously raised the absorbance of related samples in the vicinity of 275 nm and the visible light region. A recognizable peak appeared at 375 nm of all samples, adding CDs with residual resins. The peak was located in the near-ultraviolet region, symbolizing the transition of n-π*, which was contributed by chromophores, e.g., C=O, C=N (Fang et al. 2017; Selmi et al. 2023). The feature of UV-vis DRS spectra of samples containing adhesives conformed to that of C1s spectra. In addition, the presence of chromophores made red shift occur on the highest characteristic peak due to the promotion of the conjugation effect. Consistent with Shibayama et al. (2019), the better absorbance performance of the photocatalyst in the visible light region can drive photocatalytic evolution better because of the more efficient utilization of irradiation. The curves of Tauc plots (Davis and Mott 1970), whose data was solved by Eq. 2 (Chen et al. 2016) for the g-C₃N₄ direct transition photocatalysts, are plotted in Fig. 3(C),

(2)

where E_g is band gap energy, h is Planck’s constant, ν represents light frequency, and A means a constant. As seen in Fig. 3(C), the band gap energy of several samples were 2.79, 2.74, 2.73, 2.65, and 2.78 eV, corresponding to the unmodified g-C₃N₄, MCN-PP, MCN-MF-3.5, MCN-UF-3.5, and MCN-PF-3.5, respectively. It was easier for the photocatalyst with a narrower band gap to absorb the charge with lower energy and react. Based on the results, the doping of CDs, particularly with UF resins, significantly narrowed the band gap and made photocatalytic evolution react more efficiently.

Fig. 3. (A) PL spectra with an excitation (ex) wavelength of 300 nm; (B) UV-Vis DRS spectra; (C) Tauc-plots and bandgap information of g-C₃N₄ and MCN-x-3.5 (x=PP, MF, UF, PF)

Photocatalytic H₂ Evolution

All experiments, including photocatalytic H₂ evolution and stability, were conducted under visible light irradiation (≥420 nm). Before the start of irradiation, 3 wt% Pt was added into the system for attaching the surface of photocatalysts and providing sites for the adsorption of H⁺ and H₂. Moreover, 10 mL of TEOA was added to the reaction system as a sacrificial agent. As shown in Fig. 4(A, B), the doping of CDs conspicuously promoted the H₂ evolution of photocatalysts. The average H₂ evolution rate of the unmodified g-C₃N₄ was 5.7 mmol·g^-1·h^-1, and that of MCN-UF-3.5 was 3.91 times as large as the former’s (22.1 mmol·g^-1·h^-1). Despite the samples being doped with CDs, there were some differences between them. The H₂ evolution of the MCN-UF-3.5, the optimum performance photocatalyst, was 17.0% better than that of MCN-PP. It can be concluded that adding CDs containing UF to photocatalysts was more capable of promoting H₂ evolution than CDs without resins. The samples containing MF were similar to those containing PF, following the same trend: as the amount of doping increased, the hydrogen yield also increased. However, MCN-PF-y had more tremendous advantages than MCN-MF-y. MCN-PF-4.5 had a rate of 21.7 mmol·g^-1·h^-1, which was 18.3% higher than MCN-MF-4.5. The structure of MF resin resembled that of melamine, which was used to synthesize g-C₃N₄. That is why adding MF resin was challenging to generate a variation in the unmodified photocatalyst. Fewer structural changes eventually led to the lower H₂ evolution. In the case of PF resins, the augmentation of H₂ evolution brought in by increased usage retarded when the blended amount was up to 4.5%. Because of the insolubility of PF resin, as pointed out by Shibayama et al. (2019), excess usage may hinder the adsorption of water molecules by photocatalysts. Also, the H₂ evolution of MCN-UF-4.5 was reduced by 4.1% compared to MCN-UF-3.5, elucidating that the superfluous addition of UF degraded the H₂ evolution as well. According to Liu et al. (2018), excessive addition of UF may increase in alkaline concentration to hinder the further decomposition during the hydrolysis process.

Based on the H₂ evolution performance, MCN-UF-3.5 was selected as the object for the photocatalytic stability experiment. Four cyclic reactions lasting three hours with identical reaction conditions, as described in Section 2.2, constituted the photocatalyst stability experiment with a total duration of 12 h. Before each loop began, 10 mL TEOA was extra replenished into the reaction system. As shown in Fig. 4(C), the evolution of each cycle was close and demonstrated a slight downward trend from a holistic view. The H₂ evolution of the initial and final cycles were 65.5 and 62.1 mmol·g^-1, separately. The reduction between the initial and the final was 5.2%, suggesting the high stability of MCN-UF-3.5 against photocorrosion.

Combined analysis of characterization and experimental results, it was reasonable to expect that the residual resin adhesives and groups isolated from them had modified the photocatalyst surface. To sum up, the modification of CDs with adhesives to photocatalysts covered three aspects. Firstly, corresponding to the Raman spectra, the higher degree of sp² hybridization in samples that had CDs added with adhesives enhanced the electron conductivity. So, the activity of the hydrogen evolution reaction got higher. Secondly, the optical properties detected by PL and UV-Vis DRS spectra were embellished by adding CDs. The better absorption of visible radiation and the narrower band gap can generate more electrons for the H₂ evolution reaction. Finally, as displayed in Table 1, the shorter lifetime restrained the possibility of the electron-hole pairs’ recombination, lifting the utilization of photogenerated electrons. Due to these modifications, the amounts of H₂ evolution of samples doped with CDs were improved and distinguished from the pristine g-C₃N₄ and even MCN-PP.

Fig. 4. (A) Photocatalytic H₂ evolution of different samples; (B) Photocatalytic H₂ evolution rate; (C) Stability experiment of H₂ evolution on MCN-UF-3.5 under visible light (≥420 nm)

DFT Calculation and Discussion on Photocatalytic Mechanism

The evidence from the characterization and experimental work confirmed the effective modification of photocatalysts by CDs. However, there were still distinctions between photocatalysts doped with CDs mixed with different residual resins. For the purpose of investigating the influences of the addition of CDs, DFT was taken as the tool. Before modeling the photocatalyst modified by CDs, the band structure, partial density of states (PDOS), and electrostatic potential were calculated to certify the coincidence of the realistic structure and the build. As plotted in Fig. 5(A), the band gap of g-C₃N₄ was 2.79 eV, coinciding with that of the Tauc-plot curve in Fig. 3(C). The electrostatic potential determined the inclination of the photocatalyst to be excited. The potential of the pristine g-C₃N₄ model displayed in Fig. 5(B) was 5.83 eV, which was too high to be excited due to the absence of surface defects. Figures 5(C) and 5(D) demonstrated the PDOS spectra of C and N atoms. Linking and analyzing Fig. 5(A) and Fig. 5(C, D), for the unmodified g-C₃N₄, the valence band maximum was mainly dominated by N-2p orbitals. The conduction band maximum was mainly occupied by C-2p and N-2p orbitals. Compared with the Fermi level in Fig. 5(B), the valence band of the unmodified g-C₃N₄ was far from the Fermi level, implying the lower mobile carrier density and poor charge transfer ability. This result agreed with the data of the PL lifetime. Regarding Mulliken charges, all C atoms in g-C₃N₄ carried a positive charge of 0.54 eV. Most N atoms had a negative charge of 0.41 eV, and the joint N had a negative charge of 0.32 eV. The electronegativity center was deduced to sit in the region enclosed by triazine rings.

The structures of samples doped with CDs were built on the unmodified g-C₃N₄. The internal portion of CDs was composed of sp² hybridization C, and the external was constructed by sp³ hybridization C, which can connect with functional groups. As shown in Fig. 5(C, D), CDs replaced one triazine ring to adjust the surface. The surface structure was nearly unchanged because of the resemblance of the partial double bonds of triazine rings and CDs. Studies by Norskov et al. (2004) and Yang et al. (2022) pointed out that the process of optical H₂ evolution reaction can be detached into four one-electron steps. Based on the reaction steps, *OH is the crucial step in the optical H₂ evolution reaction, which exists in both transformation paths. So, the -OH group was selected to adhere to the edge of CDs to stimulate the absorption of the -OH group on the surface. Furthermore, for the sake of figuring out the superiority of the doping of the UF, the -NH₂ group, the typical functional group of UF, was attached to the CDs to compare with the resin-free model. The top and front views of all structures are displayed in Fig. 5(E-L). For ease of exposition and understanding, the C atom on the leftmost side of CD connected to the N atom in the s-triazine ring formed an internal angle labeled as . In the same way, the right one was marked as .

The resin-free photocatalyst, as shown in Fig. 5(E), was the most stable structure, whose surface was as flat as the pristine g-C₃N₄. The C-N bond length of CD connecting with s-triazine rings was 1.40 Å, close to that of the s-triazine rings’ interconnection. Moreover, CDs structure possessed symmetry, and was 110.31°. These results showed a tight combination between CD and s-triazine rings, and the structure of CD was stretched somewhat in their respective directions by multiple s-triazine rings. The original leveled structure was destroyed when the -OH group was adsorbed on CDs, as shown in Fig. 5(G). The geometric structure of free water molecules in the liquid phase can be described, as the bond length of O-H was 0.96 Å, and the angle of H-O-H was 109.50°. Compared to free water molecules, 0.98 Å in Fig. 5(G) was longer, and 108.95° was narrower. It could be inferred that *OH had a higher activity than free water molecules. Also, the structure of the CD section became asymmetrical. in Fig. 5(I) was 102.80°, and was 115.69°, which was more similar to the original. The bond on the left and right was 1.42 Å and 1.45 Å, respectively. The movement of bonds and angles proved that the right structure was more stable but not as tightly bound to the s-triazine ring as the left. Moreover, the binding energy of C-O was -0.70 eV.

For the CDs with the -NH₂ group, as depicted in Fig. 5(I), there was a radical deformation from the top view compared to the structure in Fig. 5(E). The degree of and was 97.59° and 123.88°, respectively, illustrating that the deformation got greater than that of the structure in Fig. 5(E), and the activity rose. The length of the C-N bond between CD and the -NH₂ group was 1.35 Å, which was consistent with that in s-triazine rings. When the -OH group was adsorbed on -NH₂ of CD, was 103.70°. The length of the bond linking CDs and H-N-OH was 1.37 Å, and the length of O-H was 0.99 Å. These bonds were more extended than that without the adsorption of the -OH group. The binding energy of the O atom reduces to -0.39 eV, explaining that it was easier for CDs with the -NH₂ group than CDs to react. As mentioned in the study of Yin et al. (2018), it was neither too tightly adsorbed for such binding energy, making it challenging to desorb, nor too low in binding energy, making adsorption difficult. The site on -NH₂ was suitable for extracting -OH from water molecules and dismantling H⁺ and electrons for the subsequent reaction.

All structures obtained by calculations revealed that -NH₂ groups on CDs can pull CDs out from the plane and deform the surface. Regarding mechanistic feasibility, the above essential calculation results explained that adding CDs with UF resin altered the surface structure and made it more active for adsorption and reaction.

Besides, though methylene bridges are unstable, more NH₂ groups can be produced by the rupture of methylene bridges. In all samples modified by CDs with adhesives, the H₂ evolution performance of MF was the worst. The primary cause of this appearance was ascribed to the units of MF intended for paper impregnation. To maintain suitable viscosity and penetration, the prepolymers of MF, made up of three to five oligomers of melamine units, possessed low molecule weight (Henriques et al. 2017). Therefore, many MF prepolymer patterns were typically formed by chain and isotropic growth (Al-Moameri et al. 2021). For the situation caused by chain growth, many H-N-OH or -NH₂ groups were exposed to the outside of the structure, contributing to improving the H₂ evolution. However, when the prepolymers were formed by isotropic growth, more free functional groups were formed ether bridges by the literation of water molecules. These ether bridges were stable and were not sp² hybridized, which means that they cannot enhance the reaction activity in the same way as the structure of PF.

Fig. 5. (A) Band Structure and bandgap; (B) Electrostatic potential of g-C₃N₄ optimized; (C, D) Calculated density of states of different elements in g-C₃N₄; (E-L) Top and across view of optimized geometries of g-C₃N₄ doped CDs with different groups, including (E, F) resin-free CDs; (G, H) CDs with -OH; (I, J) CDs with -NH₂; (K, L) CDs with H-N-OH (Color scheme: C, grey; N, blue; O, red; H, white)

CONCLUSIONS

In this research, g-C₃N₄ modified by CDs exhibited superiority over the unmodified in photocatalysis-related indicators, for example, band gap and PL lifetime. Under visible light irradiation, the average evolution of pristine g-C₃N₄, MCN-PP, MCN-MF-4.5, MCN-UF-3.5, and MCN-PF-4.5 were 5.7, 18.9, 18.3, 22.1, and 21.7 mmol·g^-1·h^-1, respectively. MCN-UF-3.5, the best-optimized sample, was up to 391% higher than the unmodified g-C₃N₄and 117% higher than MCN-PP. Such results elucidated the finding that the environmentally friendly CDs prepared from waste wood had a positive enhancement effect on the unmodified g-C₃N₄. Also, the addition of UF and PF resin in moderation became a factor to further promote the efficiency of photocatalytic H₂ evolution performance.
This research put forward a convenient hydrothermal self-assembly method to drive waste wood with residual resin adhesives into CDs to enhance the performance of g-C₃N₄.
The surface morphology of photocatalysts obtained by DFT calculations certified that doping groups on resin could adjust surface patterns and increase the activity of photocatalysts, corresponding to the experimental results.
In summary, this study provided a new reusage opportunity and method for utilizing waste wood with residual resin adhesives to design and synthesize green, efficient metal-free photocatalysts with g-C₃N₄, which can be extended in many applications such as solar energy reserves, photovoltaic equipment, and water pollution treatment.

ACKNOWLEDGMENTS

This study was supported by the Key Research and Development Program of Jiangsu Province (BE2023361). The authors acknowledge the support of the Advanced Analysis and Testing Center of Nanjing Forestry University.

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Article submitted: January 16, 2024; Peer review completed: February 3, 2024; Revised version received and accepted: February 20, 2024; Published: February 27, 2024.

DOI: 10.15376/biores.19.2.2417-2435