TiO₂-WO₃ Loaded onto Wood Surface for Photocatalytic Degradation of Formaldehyde

Abstract

In this work, a facile method was adopted to prepare TiO₂-WO₃ loaded onto a wood surface by a two-step hydrothermal method. The as-prepared wood composite material can be used as a photocatalyst under UV irradiation for the photodegradation of formaldehyde. Related tests showed that TiO₂-WO₃ nano-architectonic materials with spherical particles loaded onto the wood substratewere mainly caused by self-photodegradation of formaldehyde. The TiO₂-WO₃ nanostructured material firmly adheres to the wood substrate through electrostatic and hydrogen bonding interactions. Meanwhile, the appearance of the new chemical bond Ti-O-W indicates the successful loading of TiO₂-WO₃ onto the wood surface. The photodegradation rate was measured and it was confirmed that the highest photodegradation performance of the modified wood was achieved at a molar ratio of 5:1 of TiO₂ to WO₃. This work provides a new strategy for the preparing of novel photocatalysts based on wood substrate. Moreover, the wood loaded with TiO₂-WO₃ is a promising candidate for indoor formaldehyde treatment in practical applications.

1. Introduction

Adhesives with formaldehyde as the main component are widely used in producing house decoration materials and furniture. In addition, they will release formaldehyde into the air, affecting human health [1,2]. With the improvement in people’s concept of health and environmental protection, increasingly more people are aware of the harm of formaldehyde. Therefore, the removal of indoor formaldehyde has important practical significance. The commonly used methods for the elimination of formaldehyde mainly include plasma catalytic degradation, electrocatalytic oxidation degradation, physical absorption treatment, plant purification, and photocatalytic degradation [3,4,5,6]. It includes not only the physical method but also the chemical method. The photocatalytic degradation materials developed in recent years have shown great potential in the degradation of free formaldehyde in indoor air [7,8,9]. However, recycling these powdery semiconductor photocatalysts has become an essential issue in the practical application process.

The material most closely related to the human living environment is wood. It has a high strength-to-weight ratio, low density, easy processing, an excellent thermal-electrical insulation, but also outstanding acoustic properties and other advantages [10,11,12,13]. In addition, wood is also a superior biomass carrier and has a naturally porous structure. In interior decoration and furniture-making materials, wood is also an inevitable consumable. Photocatalytic technology only needs to be illuminated in nature or indoors to cause chemical changes, and it is harmless to humans and the environment, which is a “green” process [14]. Therefore, it is feasible to carry out composite modification on wood and apply it to the photocatalytic degradation of formaldehyde. The composite modification of wood and inorganic nanomaterials makes it recyclable and formaldehyde degradable, becoming a research hotspot in this field in recent years [15,16,17].

The TiO₂ semiconductor with a band gap of 3.2 eV is a promising photocatalyst. Because of its non-toxic, harmless, corrosion resistance, environmental protection, strong photocatalytic oxidation ability, and high stability, it is widely used in the photodegradation of water or gaseous toxic organic pollutants [18,19,20]. In the photodegradation of toxic pollutants by TiO₂, the whole process is simple and environmentally friendly. It takes place at normal temperature and pressure, and the reaction products are usually CO₂ and water. Since Fujishima et al. [21] invented photocatalytic decomposition on TiO₂ single crystal electrodes in 1972, they have attracted extensive attention from many researchers. It has attracted scientists from different research fields to carry out significant research on TiO₂ preparation and photocatalytic performance. However, TiO₂ has a large band gap, which makes the solar energy utilization rate low. The photocatalytic performance of TiO₂ can be improved by using methods such as doping metal oxides, doping non-metal oxides, co-doping metal and non-metal oxides, semiconductor compounding, and photocatalyst photosensitization [22,23,24,25,26]. Related studies showed that the photocatalytic efficiency of TiO₂-WO₃ composite nanomaterials prepared by the sol-gel method and temperature-controlled calcination method could reach 94.8% for acid red B [27]. WO₃ has a narrower band gap, a wider light absorption range, and a more efficient use of sunlight [28]. Therefore, compounding TiO₂ and WO₃ to improve the photocatalytic degradation efficiency of formaldehyde has aroused extensive research in the academic community.

As one of the best photocatalysts, WO₃ has a small band gap and a large light absorption range. Making more efficient use of visible light accounts for nearly half of the sun’s radiation [29,30]. However, it is difficult to obtain stable photocatalytic performance of pure WO₃ due to its defects, such as easy photo corrosion. Moreover, TiO₂ is a catalyst with many advantages, but its small absorption range is the main application limitation. WO₃ can improve this limitation, so materials doped with WO₃ have also become a current research hotspot [31,32]. On the other hand, wood is an outstanding carrier, which has a natural porous structure, a large number of capillaries inside, and a large specific surface area. Therefore, it is of great significance to explore a simple and effective method to obtain a composite catalyst with a better catalytic effect on the surface of TiO₂ and WO₃ composite wood.

In this study, wood surfaces loaded with TiO₂-WO₃ composites with different composite proportions were prepared by a two-step low temperature hydrothermal method. Meanwhile, the crystallinity and morphology characteristics were discussed, and its growth on the wood surface was studied. Furthermore, the photocatalytic mechanism of formaldehyde degradation was discussed by studying its photocatalytic performance. The properties of photocatalytic formaldehyde were also studied. The purpose of this study was to obtain wood loaded with TiO₂-WO₃ composites with high photocatalytic performance and to investigate their application to degrade the free formaldehyde released indoors.

2. Materials and Methods

2.1. Materials

Poplar wood samples (Populus sp., 8 × 8 × 2 mm³, length × width × high, moisture content was 10.33%) were supplied by Dehua Bunny Decoration New Materials Co., Ltd. (Huzhou, China), which was sapwood with eight annual rings. Ammonium hexafluorotitanate(IV) ((NH₄)₂TiF₆, ≥99%), boric acid (H₃BO₃), and tungsten(VI) chloride (WCl₆·6H₂O, ≥99.9%) were purchased from Shanghai Boyle Chemical Co., Ltd. (Shanghai, China). Ethanol absolute (C₂H₅OH, ≥99.9%), and hydrochloric acid (HCl, ≥99.9%) were bought from Nanjing Lisheng Chemical Company (Nanjing, China). All reagents are analytical pure grade and can be used directly without secondary purification. Moreover, deionized water was used in all experiments.

2.2. Preparation of TiO₂ Loaded onto Wood

Briefly, (NH₄)₂TiF₆ (0.989 g) and H3BO3 (0.927 g) were mixed with a deionized water solution (600 mL) and stirred for two h at room temperature. After that, adjusted the pH value to 3 with 0.3 mol/L HCl solution. Subsequently, the mixed solution was placed in a reaction kettle, and five wood samples were respectively placed and reacted at 70 °C for 5 h. Then, the wood samples were removed and cleaned with deionized water three times (10 min each time). Finally, the samples were placed in a vacuum oven and dried at 45 °C for 24 h. Thus, the samples of TiO2 loaded onto wood were prepared. In addition, each set of experiments was repeated three times to ensure the accuracy of the experiment.

2.3. Preparation of TiO₂-WO₃ Loaded onto Wood

The schematic illustration of the preparation of TiO₂-WO₃ loaded onto wood is shown in Figure 1. Four of the above wood samples and one unreacted wood sample were obtained and sonicated with ethanol. Then, a certain amount of WCl₆·6H₂O was dissolved in 100 mL of ethanol solution, placed in the wood sample and stirred magnetically for 2 h at room temperature. Subsequently, after standing and impregnating at room temperature for 24 h, the wood samples were removed and placed in a vacuum-drying oven for 24 h to obtain TiO₂-WO₃ loaded onto wood. The changed amounts of WO₃ in TiO₂-WO₃ can be obtained by controlling the molar ratio of WCl₆·6H₂O in the mixed solution. The molar ratios of WO₃ and TiO₂ were set as 1:0, 1:1, 1:3, and 1:5. The as-prepared wood samples with the molar ratio of WO₃ and TiO₂ (1:0, 1:1, 1:3, and 1:5) were labeled as TW1, TW2, TW3, and TW4, respectively (the specific formulation is shown in

Table 1. The molar ratio of WO₃ and TiO₂ in as-prepared wood samples.

	Samples	TW0	TW1	TW2	TW3	TW4
Molar Ratio
WO3:TiO2		0:0	1:0	1:1	1:3	1:5

). Moreover, the unmodified blank control sample was named TW0.

2.4. Characterization

The surface morphology of the wood substrate before and after being loaded by TiO₂-WO₃ was investigated by scanning electron microscopy (SEM, Quant 200, FEI Company, Hillsboro, OR, USA). The crystalline structures of the samples were measured by X-ray diffraction (XRD, D/MAX 2200, Rigaku, Japan). The XRD was conducted with Cu-Kα radiation at 40 kV and 30 mA with a step rate of 4°/min ranging from 10° to 80°. The chemical functional groups change of the samples was carried out using a attenuated total reflection Fourier Transformed Infrared Spectrometer (ATR-FTIR, Magna-IR 560, Nicolet Bankshares, Inc., Green Bay, WI, USA), which was conducted in the range of 400~4000 cm⁻¹ with a resolution of 4 cm⁻¹. All spectra were recorded at room temperature and there was a drying system which could prevent atmospheric moisture from interfering with the spectrum.

2.5. Formaldehyde Degradation Performance Test

The photocatalytic formaldehyde degradation performance of the as-prepared samples was tested at room temperature, and the schematic diagram of the device is shown in Figure 2. Moreover, three samples were made of each type of TW1–TW4 and all samples were studied in the photocatalytic degradation experiment. The average of the three test results was obtained as the final test result. The whole tests were carried out in a cylindrical glass caulk with an expanded volume of 0.1 m³. The LED light (λ max = 458 nm, emission intensity of approximately 3.6 mW/cm²) was fixed at the center of the opening and used to simulate visible light. The desired dose of formaldehyde was added to the tamponade, and the fan was maintained for 30 min to obtain an adsorption and desorption balance. After the adsorption equilibrium, the initial concentration of formaldehyde in the obturator can be determined by Formaldemeter (LB-HD05, China). Subsequently, the LED light was turned on to illuminate the sample, and each set of degradation experiments was continued under the LED light for 6 h. The degradation efficiency of formaldehyde was estimated by the following formula:

$$D(\%)=\frac{C_0-C}{C_0}\times 100\%$$

(1)

where C₀ is the initial concentration of the formaldehyde, and C represents the measured formaldehyde concentration.

3. Results and Discussion

3.1. Morphological Characterizations

Figure 3 displays the SEM images of the as-prepared wood samples loaded with different proportions of TiO₂-WO₃. Figure 3a presents the microtopography of the surface of the TW1 samples; it can be clearly seen from the figure that tiny particles grow on the surface of the modified wood sample and cover the surface. These tiny particles are roughly spherical with many edges and corners. Figure 3b,c are the surface morphologies of TW2 and TW3. As shown in the figures, the surface of the samples is covered with small spherical particles, and some of them are agglomerated together. Moreover, the agglomeration phenomenon on the surface of the sample was more severe than that of the TW1 sample, and the particle size also increased. Figure 3d displays the SEM image of TW4, and the growth conditions of the nanoparticles on the surface are also similar to those in Figure 3a–c, but the particles are denser. However, the particle size is smaller than that in Figure 3a–c. Based on the above analysis, it can be inferred that the average particle size of TiO₂ particles on the wood surface increases gradually with the increase in WO₃ content. Furthermore, the agglomeration effect will also increase, but the addition of WO₃ has little effect on the morphology of the product. The successful loading of TiO₂-WO₃ nanoparticles on the wood sample makes the wood surface form a dense metal oxide film structure, which is beneficial to improve the photocatalytic formaldehyde degradation performance of wood. More importantly, the surface microstructure of the wood did not change significantly after irradiation, suggesting that the surface-loaded nanoparticles only acted as catalysts.

3.2. XRD Analysis

The crystal structure of the wood sample before and after the modification can be analyzed by XRD patterns, as shown in Figure 4. The diffraction peaks present at 16° and 22.5° belong to the (101) and (002) crystal surfaces of the cellulose in the wood [33]. The diffraction peaks of the wood composite loaded with TiO₂-WO₃ at 25.3°, 38.0°, 47.6°, 54.3°, and 64.5° are consistent with the (101), (004), (200), (105), and (204) crystal surfaces in TiO₂ [34,35], respectively. Therefore, the TiO₂ loaded onto the surface of the wood samples corresponds to rutile phase TiO₂ and anatase phase TiO₂. Meanwhile, the results also showed that TiO₂ was successfully loaded onto the wood surface. Moreover, the modified wood samples showed diffraction characteristic peaks at 2 θ of 22.6°, 25.8°, 30°, and 34.5°, which matched with the (001), (110), (200), and (201) crystal planes in WO₃, respectively. It is consistent with the WO₃ triclinic phase (JCPDS No. 32-1295), which indicates that WO₃ particles were successfully loaded onto the surface of the samples [36]. In the XRD spectra of samples TW1, TW2, TW3, and TW4, some obvious TiO₂ and WO₃ characteristic diffraction peaks also appeared. With the increase in the concentration of TiO₂ in TiO₃-WO₃, the characteristic peak of TiO₂ in the composite sample is also enhanced. This indicates that different ratios of TiO₂ and WO₃ can be successfully loaded onto the surface of wood samples.

3.3. ATR-FTIR Analysis

ATR-FTIR is a conventional test method for analyzing the functional groups, chemical structures, and binding mechanisms of samples. Figure 5 shows the spectra of the wood modified by TiO₂-WO₃. It can be clearly seen from Figure 5a that the vibrational absorption peak at 3398 cm⁻¹ is caused by the hydrogen bond of the hydroxyl group (-OH) in wood or the stretching vibration of O-H in water [37]. This peak was enhanced after the modification, which may result from the hydrogen bond generated from the reaction of TiO₂ or WO₃ with the hydroxyl group on the wood surface. The absorption peak at a wavelength of 2927 cm⁻¹ is caused by the asymmetric stretching vibration of the C-H bond in the long-chain group -CH₃ in the wood [38]. The characteristic absorption peaks occurring at 1060 cm⁻¹ and 1629 cm⁻¹ are caused by C-O and C=O stretching vibrations in wood, respectively. The appearance of the above characteristic peaks showed that the wood did not destroy its unique structure of wood during the experiment [39]. Moreover, it can be seen from Figure 5b that TW1, TW2, TW3, and TW4 have W-O stretching vibration peaks at 820 cm⁻¹ and 890 cm⁻¹, which were not found in the TW0 absorption curve of the samples. Meanwhile, the Ti-O vibration peak appeared at the wavenumber of 641 cm⁻¹, but it was not found in the spectrum of TW0. The spectrum indicated that the composite products of TiO₂-WO₃ appeared in TW1, TW2, TW3, and TW4, and were well loaded onto the wood surface. Meanwhile, the spectra of the irradiated wood did not change.

3.4. Photocatalytic Degradation Performance

In order to explore the effect of wood samples modified with different TiO₂-WO₃ molar ratios on the performance of the photocatalytic degradation of formaldehyde, the degradation performance of all samples under UV light was tested, and the results are shown in Figure 6a. The concentration of formaldehyde was set to 2.5 mg/m³ during the test. The photocatalytic degradation efficiency of formaldehyde in untreated wood samples was close to zero, indicating that there was almost no degradation of formaldehyde. The photocatalytic efficiency of the TW1 sample under visible light irradiation was low, which was about 37.03%, while the samples of TW2, TW3 and TW4 showed higher photocatalytic degradation performance, and the photocatalytic efficiency of TW2, TW3, and TW4 was 73.13%, 92.10%, and 97.86%, respectively. The results showed that the photocatalytic properties of the as-prepared wood products improved rapidly with the increasing amount of TiO₂. Compared with other samples, TW4 had the best photocatalytic performance with a degradation rate of about 98%. There may be several reasons for the above phenomenon. Firstly, TiO₂ can deposit the WO₃ band with a higher donor value than the initial value on the wood substrate, which improves the absorption efficiency of visible light irradiation [40]. With increasing TiO₂ content, the distance between the donor level and the valence band becomes greater. Therefore, a large number of electrons are generated under visible light excitation, which is conducive to enhancing photocatalytic degradation. Secondly, when the content of TiO₂ is lower than its proper molar ratio, TiO₂ mainly occurs on the surface area of the sample to hunt and convert electrons and holes, which will inhibit the recombination of photoexcited holes and electrons. The photoexcited electrons and holes of the modified wood samples will photodegrade formaldehyde into CO₂ and H₂O.

In addition, the reusability of TW4 samples as typical representatives was studied, as shown in Figure 6b. In order to evaluate the recyclability of the as-prepared wood sample, 10 recycling tests were carried out on the degradation of formaldehyde. Obviously, the TW4 sample can still maintain excellent photocatalytic performance after 10 photocatalytic cycles, with an efficiency of more than 85%. The decrease in degradation rate was mainly due to the slight leaching of TiO₂-WO₃ nanoparticles in each recovery experiment. Therefore, the wood surface-loaded TiO₂-WO₃ composite material has a certain application potential as a photocatalyst in the removal of indoor formaldehyde pollutants.

3.5. Mechanism of Photocatalytic Degradation of Formaldehyde

Based on the above photocatalytic results, we constructed the potential energy diagram and formaldehyde degradation mechanism diagram of the wood surface loaded with TiO₂-WO₃ composite material, as shown in Figure 7. When the treated wood is irradiated by ultraviolet light, the conduction bands (CB) of TiO₂ and WO₃ both generate excited electrons. Due to the potential difference, the photogenerated electrons in WO₃ can easily move to the CB of TiO₂. Therefore, electron transfer will reduce the chance of recombination with the holes formed in the valence bands (VB) of the TiO₂ and WO₃. The holes migrate directly from TiO₂ to WO₃ and then to the wood substrate interface. Thus, reduced recombination results in enhanced photoactivity. According to the previous references [41,42], Figure 7 and Equations (2)–(8) showed the photocatalytic degradation reaction of formaldehyde, where hv, h⁺, e⁻, •OH, and•O₂⁻ represent light energy, holes, electron, hydroxyl radicals, and superoxide radicals.

$${\mathit{TiO}}_2+\mathit{hv}\to {\mathit{TiO}}_2{(h}^{+}+e^{-})$$

(2)

$$W{O}_3+\mathit{hv}\to W{O}_3(h^{+}+e^{-})$$

(3)

$$h^{+}+H_{{}^2}O\to { }^{•}OH+H^{+}$$

(4)

$$T{i}^{4+}+e^{-}\to T{i}^{3+}$$

(5)

$$T{i}^{3+}+O_2\to T{i}^{4+}+{ }^{•}{O}_2^{-}$$

(6)

$$HCHO+{ }^{•}OH\to { }^{•}CHO+H_{2}O$$

(7)

$${ }^{•}CHO+{ }^{•}OH\to HCOOH$$

(8)

Obviously, the electrons in the VB of TiO₂ are excited to the CB of TiO₂ under UV irradiation. It shifted from the CB of TiO₂ to WO₃ due to the lower CB of WO₃. The holes left in the VB of WO₃ can move to the valence electrons of TiO₂, which is beneficial to the electron/hole separation. Therefore, the photocatalytic degradation efficiency of the sample was improved. The free radicals (•OH and •O₂⁻) produced by catalysis can efficiently degrade formaldehyde to produce carbon dioxide and water.

4. Conclusions

In this work, we have presented TiO₂-WO₃ loaded onto a wood substrate fabricated by a two-step hydrothermal method. The wood served as biomass substrates to prepare the TiO₂-WO₃ photocatalysts with nano-architectonic spherical particles. The wood loaded with TiO₂-WO₃ composite material was characterized via various techniques and the photocatalytic degradation property was tested by degrading formaldehyde. The as-prepared wood sample exhibits higher potential for application as a photocatalyst for the degradation of formaldehyde, and the photocatalytic degradation efficiency can reach 98%. These studies indicated that TiO₂-WO₃ successfully loaded onto the wood surface concern only the surface, which could provide more active sites for photocatalysis. Furthermore, the as-prepared wood can still maintain more than 80% formaldehyde photodegradation efficiency after 10 times of recycling. Additionally, the photocatalytic degradation mechanism of formaldehyde shows that the loaded TiO₂-WO₃ reduces the recombination probability of photoexcited carriers and increases the transport of charges. This work will provide a new strategy for preparing novel wood-based photocatalysts with photocatalytic formaldehyde degradation performance. Moreover, it has practical significance to apply it to the degradation of indoor formaldehyde.