Thermal decomposition of the wood template oxide
Thermogravimetric analysis (TGA) was carried out to investigate the calcination process of the impregnated wood templates with different size scales. The decomposition occurred at approximately 320°C, and this is mainly attribute to the cellulose decomposition of wood(Ren et al., 2021). Generally, the temperature at which cellulose is decomposed by heat is 290°C; in this case, the hydroxyl group in the cellulose of the wood stencil combines with titanium ions to increase the temperature at which the cellulose decomposes by heat(Jiang et al., 2019). Therefore, in Fig. 1c, 439°C can be understood as the thermal decomposition temperature of lignin and cellulose. Moreover, the lignin content in the lignocellulose and fiber is lower. This is due to the destruction of part of the cell wall and delignification at the early stage processing the wood into wood flour. Clearly, it can be seen from the figure that the components of the wood template have been completely removed from 400–700°C, which is the temperature range of TiO2 formation. The yields of the lignocellulose-, wood flour-, and solid wood-templated TiO2 are 23.0%, 23.3%, and 3.74%, respectively. The yield of solid wood is lower than that of the lignocellulose and wood flour templates. The layered structure of wood starts from the millimeter-level ducts and lumens and continues to the micron-level layered cell wall. The fine processing of wood raw materials exposes the mesoscopic pores existing in the wood cell wall; thus, a large specific surface area that can adsorb more nanoscale particles and flakes is realized(Fu et al., 2017).
Effect of wood template on TiO2 morphology
Figure 2 shows the microscopic surface morphology of TiO2 templated by wood at different size scales. Figure 2(a)(d)(g) are the original images of the lignocellulose, wood flour, and solid wood. Wood has a complex multiscale hierarchical structure. Wood cells can be divided into tracheids, ducts, pores, and other pore structures according to their functions, which play a role in the transmission of water and nutrients for tree growth(Singh et al., 2019). When processing wood into wood flour, mechanical grinding destroys the original macroporous structure; thus, the wood flour mostly has a structure of long, slender strips (Fig. 2d). Wood flour is a typical structure composed of many microtubes with a diameter of approximately 10–20 µm. It is made of intertwined wood fibers that are interwoven to form a layered porous microstructure. Viewed from the longitudinal section of the fir, it is composed of axial tracheids. The shaft tube contains countless stomata, which are channels for the transport of nutrients and water during the growth of the wood. It undergoes delignification when processing the wood flour into lignocellulose; thus, lignocellulose has a kind of interlaced fiber structure (Fig. 2a) that is composed of numerous interlaced microcrystals.
Figure 2(b)(e)(h) shows the TiO2 templated by different scales of wood. It can be clearly observed from the figure that the microscopic morphology of the initial template surface is almost the same, indicating that Ti binds to the hydroxyl groups of the wood cell wall at the beginning of biotemplate impregnation; then, the high temperature further controls the growth and oxidation of crystals(Liu et al., 2020). This proves that the TiO2 generated by the multiscale hierarchical structure of wood faithfully inherits the macro- and microscale structures. Moreover, the TEM images of TiO2 with lignocellulose, wood flour, and solid wood as templates show that the basic morphology of TiO2 prepared by templates at different size scales is consistent (Fig. 2(c)(f)(i)). They are all composed of dispersed irregular nanoparticles with an average particle size of 20 nm.
The retention of the pore structure of the wood template has a significant effect on the catalytic performance of the catalyst. To further study the pore structure and specific surface area of the TiO2 templated with different size scales of wood, N2 adsorption and desorption tests were carried out. The adsorption and desorption curve is shown in Fig. 2(j)(k)(l), and the specific surface area, pore volume, and pore size results are shown in Table 1. Pore structure of wood provides a reaction channel for the effective reaction of reactants(Lu et al., 2013).TiO2 templated with solid wood and wood flour and TiO2 without a template are type III, while TiO2 with a lignocellulose template is type IV. All samples have H3 hysteresis rings, which belong to micro-mesoporous materials, thereby ensuring the pore structure connectivity of the catalyst(Luo et al., 2015). The mesoporous structure facilitates to preventing the aggregation of TiO2 particles and thus maintaining a large specific surface. The specific surface area and pore volume of TiO2 with different wood templates increase with decreasing size of the wood template (lignocellulose > wood flour > solid wood). The collapse of the macroporous structure of the solid wood template during sintering leads to a decrease in its specific surface area, while the specific surface area of the lignocellulose-templated TiO2 is large, which results in excellent photocatalytic performance. In contrast to the pore volume, a large pore diameter (wood flour > lignocellulose > solid wood) correspondingly results in higher adsorption performance.
Table 1
Pore structure parameters of the wood-templated TiO2
Template type | Specific surface area (m2g-1) | Mean pore diameter (nm) | Average pore volume (cm3g-1) |
Lignocellulose | 19.207 | 13.372 | 0.066 |
Wood flour | 9.158 | 15.590 | 0.034 |
Solid wood | 7.693 | 11.252 | 0.013 |
TiO2 | 50.425 | 8.808 | 0.092 |
Characterization of the wood-templated TiO2
The XRD patterns of pure TiO2 and biomorphic TiO2 templated by different size scales of wood are shown in Fig. 3a. All TiO2 samples obtained in this work are a mixture of anatase and rutile. These materials are known to be more effective in photocatalytic processes because the synergy between the two phases reduces the recombination effect(Huang et al., 2015). This result was also confirmed by the Raman spectroscopy results (Fig. 3b). The peaks at 143.9 cm-1, 396.5 cm-1, 514.8 cm-1, and 639.5 cm-1 correspond to four Raman vibration modes of anatase TiO2, while the peak at 196.8 cm-1 belongs to rutile TiO2. Because the lattice vibration of rutile is weaker than that of anatase, the characteristic Raman peak of the rutile phase is covered by that of the anatase phase.
The proportions of anatase and rutile TiO2 were calculated (Table 2). Wood template slightly influenced the formation of nanoparticles and rutile phase content existing in the TiO2. Similar result has been reported by Sun (Sun et al., 2016). Charge separation of mixed-phase TiO2 nanoparticles are significantly better than simple mixtures of single-phase nanoparticles due to the close contact of their heterogeneous junction. Due to the favorable conduction band arrangement, conduction band electrons from the rutile phase are transferred to the anatase phase, which is a method to inhibit charge recombination. In addition, in the mixed anatase/rutile configuration, the crystal lattice is aligned to promote charge separation; thus, mixed-phase TiO2 has a better photocatalytic efficiency than single-phase TiO2(Liu et al., 2016). As the size of the wood template decreases, the spatial domain effect is more significant, so the average grain size decreases accordingly.
Table 2
Characterization results of TiO2 templated by different size scales of wood
Sample | Rutile content (wt%) | Average grain size (nm) | λg (nm) | Eg (eV) | ECB (eV) | EVB (eV) |
Lignocellulose | 16.8 | 17.38 | 450 | 2.75 | -0.075 | 2.675 |
Wood flour | 17.3 | 18.72 | 446 | 2.78 | -0.090 | 2.690 |
Solid wood | 16.4 | 19.04 | 435 | 2.85 | -0.125 | 2.725 |
TiO2 | 15.6 | 19.54 | 398 | 3.11 | -0.250 | 2.860 |
In previous studies, it has been pointed out that the hydrophilicity of TiO2 is closely related to surface hydroxyl groups. Compared with TiO2 without the template, the increase in the number of hydroxyl groups shows that TiO2 prepared by the template method contains hydroxyl groups and good hydrophilicity. The broad peak at 400 ~ 8800 cm− 1 and the peak at approximately 700 cm− 1 are the stretching vibration peaks of Ti-O-Ti (Fig. 3c), and there is no obvious difference between the four catalysts. The absorption peaks of Ti-O-H at 2195 cm− 1 and 1970 cm− 1 should be the embodiment of the two valence states of Ti4+ and Ti3+ in TiO2. The reason for this change is that the spatial confinement of the template on a small scale is more obvious, which affects the change in its electronic action(Ola and Maroto-Valer, 2015). As shown in Fig. 3d, the peaks of the O KILL, Ti 2p, O 1s, Ti 2p, C 1s, and Ti 3p spectra were observed for all samples. The C 1s peaks are caused by carbon impurities in the XPS instrument. The binding energy in the Ti 2p spectrum can be divided into two peaks, Ti4+ 2p3/2 and Ti4+ 2p1/2, and the binding energies are located at 458.4-458.9 eV and 464.1-464.3 eV, respectively. There is no obvious difference in the binding energy of Ti 2p among the four samples. High-resolution O 1s spectra are shown in Fig. 3f. The binding energies of 531.1-531.6 eV and 529.5-529.7 eV represent the lattice oxygen peak and hydroxyl oxygen peak of TiO2, respectively. In terms of peak area, the hydroxyl oxygen peak of the lignocellulose- and wood flour-templated TiO2 is higher than that of TiO2 and the solid wood-templated TiO2, indicating that the hydroxyl oxygen content of wood flour and lignocellulose as a TiO2 catalyst template is higher. It is well known that the hydroxyl group on the surface of TiO2 is a scavenger of photogenerated holes. The hydroxyl groups on the surface of the catalyst will be converted into active · OH radicals after interacting with the photogenerated pores of the catalyst(Wu et al., 2021).
Absorption and photocatalysis performance of the wood-templated TiO2
The adsorption process is very important for photocatalysis because it is the first step to achieve close contact between the photocatalyst and the pollutants. The effect of the pore structure of different wood templates on the adsorption performance of TiO2 was explored.
After stirring for 60 min in the dark, adsorption equilibrium is reached in the reaction system (Fig. 4a). Compared with TiO2 without a template, TiO2 templated by different size scales of wood has good adsorption capacity for MB. TiO2 templated with wood flour exhibited the best adsorption efficiency (Fig. 4b). This is mainly attributed to the rich hierarchical porous structure in the wood flour-templated TiO2. The adsorption efficiency of TiO2 templated with solid wood and TiO2 without the template is approximately the same. During the sintering process, the collapse of the macroporous structure of the fir-templated TiO2 axial tracheid is not conducive to the rapid diffusion and adsorption of pollutants on its surface. Table 3 shows the results of fitting the experimental adsorption equilibrium data according to the adsorption kinetic equation. The correlation coefficients (R2) obtained by Langmuir and Freundlich model fitting are relatively close and relatively high, showing that these models can effectively describe the adsorption behavior of the catalyst in an MB solution. At room temperature, the R2 value of the quasi-first-order kinetic equation is greater than that of the quasi-second-order kinetic equation, indicating that the adsorption process of MB on TiO2 obeys the quasi-first-order kinetic equation.
Table 3
Kinetic parameters of methylene blue dye adsorption on the obtained templated TiO2 samples
Kinetic | TiO2 | Solid wood | Wood flour | Lignocellulose |
Pseudo-first order |
Qe(mg g− 1) | 0.915 | 0.927 | 1.337 | 1.108 |
k1(min− 1) | 0.550 | 0.492 | 0.408 | 0.536 |
R2 | 0.9993 | 0.9981 | 0.9986 | 0.9995 |
Pseudo-second order |
Qe(mg g− 1) | 0.961 | 0.982 | 1.441 | 1.169 |
k2(g mg− 1min− 1) | 1.306 | 1.305 | 0.501 | 0.993 |
R2 | 0.9985 | 0.9949 | 0.9985 | 0.9971 |
The photocatalytic activity of biomorphic wood-templated TiO2 under simulated visible light is shown in Fig. 4b. The TiO2 templated by different scales of wood exhibit excellent photocatalytic performance. Among them, TiO2 templated with lignocellulose presented the best degradation effect, with 98.9% degradation in 60 min. The wood flour- and solid wood-templated TiO2 result in 87.2% and 86.1% degradation within 60 min, respectively, while TiO2 without a template exhibited 55.7% degradation. As shown in Fig. 4c, the photocatalytic degradation process of all wood-templated TiO2 composites is a quasi-first-order kinetic model. The photocatalytic reaction kinetic constants of lignocellulose-, wood flour-, solid wood-templated TiO2 and template-free TiO2 are 7.509×10− 2min− 1, 3.129×10− 2min− 1, 3.088×10− 2min− 1, and 1.125×10− 2min− 1, respectively.
In the reaction process of the wood-templated TiO2 for the adsorption-photocatalytic degradation of MB, wood flour-templated TiO2 has the best adsorption efficiency, while lignocellulose-templated TiO2 has the highest degradation efficiency (Fig. 4d). Lignocellulose is at a smaller size scale, and the synthesized TiO2 has a large specific surface area, thereby increasing its catalytic activity. This result proves the synergistic influence of the microscopic morphology and physical and chemical properties of the template on the photocatalytic ability of TiO2. By designing and constructing hierarchical porous micro/nanostructures with higher light absorption efficiency, the degradation kinetics and degradation amount can be improved. These structures have higher light absorption efficiency, provide additional surface hydroxyl groups and are more prone to have exposed photocatalytic degradation active sites(Reddy et al., 2017; Kumar et al., 2020). In addition, in the mixed anatase/rutile configuration, the crystal lattice is arranged to promote charge separation, thereby increasing the photocatalytic efficiency of mixed-phase TiO2 compared to single-phase TiO2(Ding et al., 2020).
Photocatalytic degradation of phenol by the wood-template TiO2 under visible light irradiation was also performed using a 300W xenon lamp. Similar results were found in phenol adsorption, solid wood-templated TiO2 and lignocellulose-templated TiO2 adsorb less phenol than wood flour-templated TiO2 samples (Fig. 5). The highest surface density for phenol adsorption was obtained on wood flour -template TiO2 (27.78 mg/m2). It was 8 times higher than that for MB adsorption. The ratios between the average pore diameter of the wood flour -templated TiO2 to the kinetic diameter of phenol and the MB are 20.0 and 9.4. It can be suggested that the diffusion of phenol and MB from the bulk of the solution to the pores of the TiO2 is not limited. This indicates that the adsorption of contaminants is dependent not only on the pore structure features but also on the kinetic diameter of pollutant molecules. The photocatalytic activity of three wood templated TiO2 samples observed for the phenol photodegradation were only 67.93%, 72.18% and 84.26% after 1 h irradiation. Despite of the phenol adsorbed on the samples were similar with that for the MB, the k constant obtained for the phenol degradation were lower than the value obtained for MB photodegradation (Table S1). This can be attributed to the fact that phenol is a more refractory molecule than MB.
To further assess the recyclability of wood-templated TiO2 with pollutants, the adsorption and photodegradation cycling experiments were performed, as shown in Fig. 6. The results indicated that the removal efficiency of MB remained above 95% after five cycles. The SEM images of lignocellulose- TiO2 after recycle degradation demonstrated that the material maintains its porous morphology. The XRD spectra of lignocellulose- TiO2 indicated the crystal structure of TiO2 has no change.
Some previous studies have confirmed the adsorption and photodegradation properties of biomass template TiO2 (Table 4). In order to further enhance the photoresponse capability and charge separation efficiency, metal doping and surface heterojunction are often used. However, findings of this study highlight the importance of the association between wood template and TiO2. Decrease the size scale of wood template material caused many beneficial effects, and it helps the precursors to impregnate adequately on the surface of the template material to obtain better surface area and activity of the wood template titanium dioxide material. The existence of biomass structure can not only enrich the pollutant in trace amount, but also cause lattice defects in TiO2 and weaken the recombination of photogenerated electron and holes.
Table 4
Comparison of the photocatalytic activity of photocatalysts based on wood -template
Photocatalyst | Wood materials | Pollutant | Light source | Photodegradation efficiency | Ref. |
TiO2 | Lignocellulose wood flour solid wood | MB Phenol | 300 W xenon lamp | 86.1%-98.9% 67.9%-84.3% | This work |
TiO2 | Birch wood slices | RhB | 100 W UV light | 92.5% | (Sun et al., 2013) |
TiO2 | Poplar, Korean pine, teak wood | Formaldehyde MB | 300W Xenon lamp | 71.8% 97.9% | (Yang et al., 2021) |
TiO2 | Wood flour | MB | 20 W UV light | 97.7% | (Ye et al., 2020) |
Pt doped TiO2/C | Birch wood chips | Gasous formaldehyde | Visible light | 99.5% | (Liu et al., 2019) |
WO3/TiO2 | Poplar wood fibers | RhB MB MO | 500w Ultraviolet lamp | 99.8% 96.6% 96.6% | (Gao et al., 2017) |
K2Ti6O13/TiO2 | Wood ash | MB RhB | blacklight lamp | 95% | (Amornpitoksuk et al., 2020) |
Catalytic mechanism
The photocatalytic degradation of pollutant has three key processes, namely (i) catalyst adsorption of reactants, (ii) absorption of photons to generate electron-hole pairs, and (iii) light-induced separation and migration/recombination of electron-hole pairs, thereby a photocatalytic reaction occurs. To understand the size scale effect of the wood template on the photocatalysis mechanism, TiO2 was characterized by UV–Vis, PL and ESR spectroscopy. It is observed that template-free TiO2 shows a strong ultraviolet absorption edge at a wavelength of approximately 380 nm (Fig. 7a). Compared with template-free TiO2, TiO2 templated by different scales of wood show an obvious redshift of the absorption edge and an obvious enhancement of the visible light capture ability (400–500 nm). The calculated bandgap energies of the four catalysts are 3.11 eV, 2.85 eV, 2.78 eV and 2.75 eV. This indicates that the template is conducive to providing a favorable prerequisite for the photocatalytic degradation of dyes under visible light. Here, the narrowing of the bandgap is mainly due to the formation of a small amount of rutile TiO2. The two crystalline phases have different valence and conduction bands and have different valence states, which can effectively inhibit the recombination of photogenerated carriers(Guo et al., 2019; Andersen et al., 2014). In addition, the size of TiO2 particles also affects the bandgap energy(Jangid et al., 2021).
The peak intensity of PL spectra for wood templated TiO2 is much lower than that of the template-free TiO2 (Fig. 7b). TiO2 with lignocellulose as the template shows more significant fluorescence quenching. This trend may be due to the spatial confinement of the template causing more Ti3+ and oxygen vacancies to be generated on the surface. Thus, photogenerated electrons-holes can be effectively separated, and this is the main reason for the increase in photocatalytic activity. Superoxide radicals and superoxide radicals are involved in the photocatalytic reaction and play a vital role. Notably, the peaks intensity of DMPO-·O2− and DMPO-·OH spectra follows the order of lignocellulose > wood flour > solid wood with visible light irradiation. The formation of a large number of hydroxide radicals and superoxide radicals after light irradiation shows that effective charge separation can be achieved, and photogenerated electron-hole recombination is inhibited. It is evident that wood templated TiO2 has significant advantages regarding the photocatalytic degradation of dyes.
Compared with TiO2 without a template, the size scale effect of the wood template changes the light absorption range of TiO2 and reduces the bandgap (Fig. 8). Photogenerated electrons are more easily excited due to the decreased bandgap during the photocatalytic degradation of dyes. The electrons and holes on the surface undergo a series of redox reactions with the reactants. Positive holes are oxidized with hydroxyl groups or water to form ·OH, electrons combine with adsorbed oxygen to form •O2, and •O2 continues to react with water to form H2O2, which is reduced to form ·OH. Due to the presence of ·OH, the photocatalytic reaction proceeds. Therefore, three key factors corresponding to wood-templated TiO2 to achieve enhanced photocatalytic performance are narrower band gap, lower photocarrier recombination rate, and stronger adsorption capacity for pollutants.