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Sun, M., and Song, K. (2018). "Low temperature hydrothermal fabrication of tungsten trioxide on the surface of wood with photochromic and superhydrophobic properties," BioRes. 13(1), 1075-1087.


Tungsten trioxide (WO3), which is a semiconductor, was hydrothermally synthesized onto the surface of wood. After the in-situ synthesis of WO3 nanoparticles on the wood surface, the wood exhibited photochromic and superhydrophobic properties. The WO3 nanostructures were fabricated on wood surface through a two-step hydrothermal process at 90 °C or 120 °C for 6 h. Chemical composition, crystalline structures, and morphologies of the WO3-coated wood were characterized. The results indicated that the amount of WO3 nanostructures on the surface of the wood substrate was 12.89 wt.%. Meanwhile, the WO3 nanostructures were composed of fine nanoparticles and highly crystallized by SEM and XRD analysis. When the sample was irradiated under ultraviolet (UV) light (365 nm), there was an obvious color change after 10 min (ΔE). The water contact angle measurements demonstrated that the fluorosilane modified WO3-coated wood surfaces possessed a superhydrophobic behavior with a contact angle of 152°. The sliding angle was less than 10°. Photochromic and superhydrophobic properties were achieved by a facile process, which could contribute to the development of functional wood with an aesthetic coloring.

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Low Temperature Hydrothermal Fabrication of Tungsten Trioxide on the Surface of Wood with Photochromic and Superhydrophobic Properties

Minglei Sun,a,b and Kuiyan Song a,*

Tungsten trioxide (WO3), which is a semiconductor, was hydrothermally synthesized onto the surface of wood. After the in-situ synthesis of WO3 nanoparticles on the wood surface, the wood exhibited photochromic and superhydrophobic properties. The WO3 nanostructures were fabricated on wood surface through a two-step hydrothermal process at 90 °C or 120 °C for 6 h. Chemical composition, crystalline structures, and morphologies of the WO3-coated wood were characterized. The results indicated that the amount of WO3 nanostructures on the surface of the wood substrate was 12.89 wt.%. Meanwhile, the WO3 nanostructures were composed of fine nanoparticles and highly crystallized by SEM and XRD analysis. When the sample was irradiated under ultraviolet (UV) light (365 nm), there was an obvious color change after 10 min (ΔE). The water contact angle measurements demonstrated that the fluorosilane modified WO3-coated wood surfaces possessed a superhydrophobic behavior with a contact angle of 152°. The sliding angle was less than 10°. Photochromic and superhydrophobic properties were achieved by a facile process, which could contribute to the development of functional wood with an aesthetic coloring.

Keywords: Tungsten trioxide; Hydrothermal; Wood; Photochromic property; Superhydrophobicity

Contact information: a: Key Laboratory of Bio-Based Material Science and Technology of the Ministry of Education, Northeast Forestry University, Harbin 150040, PR China; b: School of Design & Art, Shenyang Aerospace University, Shenyang 110136, PR China; *Corresponding author:


Wood is composed of a special supramolecular architecture that consists of cellulose, hemicellulose, and lignin in nature (Brostow et al. 2010). Due to its unique color, texture, and special aesthetic characteristics, it is popular and widely used in the furniture and decoration industries, amongst other plastic or metal materials (Janin et al. 2001; Mahraz 2011). In order to meet the increasing demands of wood, fast-growing wood species play an important role in the wood market. While fast-growing wood is generally characterized with light color and high brightness, historically there has been great preference to use wood with dark color in furniture manufacture and decoration. Therefore, different dyeing techniques had been used to visually improve the quality of wood.

The color and texture of wood are very important elements from the view of aesthetic appeal. Dyeing and coating are effective traditional methods that have been used to change the color of wood, but sometimes, by-products such as waste liquor and irritant gases are generated during the processes; in addition, organic dyes are somewhat unstable during sun exposure (Anex and Lund 1999; Hernandez and Cool 2008; Zhao et al. 2014). Therefore, there is still a motivation to develop green and stable methods to change the color of wood. Many recent studies focused on changing the color of wood via the formation of inorganic photochromic nanomaterials on its surface by hydrothermal treatment, which has been regarded as a green process (Miyafuji et al. 2004; Wang et al. 2006; Salla et al. 2012; Sun et al. 2012a,b; Rao et al. 2016). Wang et al. (2006) reported that titanium dioxide coatings grown on the surface of wood could effectively improve the photostability of the substrate. Salla et al. (2012) reported that zinc oxide (ZnO) nanoparticles dispersed on the surface of wood can improve the color stability of the wood. Rao et al. (2016) also revealed that after layer-by-layer assembly of titanium dioxide (TiO2) nanoparticles on the surface of wood, the brightness of wood was improved after UV irradiation. Therefore, the fabrication of inorganic photochromic materials on the surface of wood could be achieved by a simple process and could effectively change the color of the wood according to the consumers’ demands.

Among those inorganic nanomaterials, tungsten dioxide (WO3) films are interesting candidates due to their applicability as electrochromic devices and new signal amplifier devices, high sensitivity for gas sensor development, and field emission properties (Li et al. 2006; Huang et al. 2008; Khoo et al. 2010; Salla et al. 2012; Rao 2013). The WO3 film can be synthesized using the sol-gel process, chemical vapor deposition, anodic oxidation, hydrothermal synthetization, or laser ablation (Djaoued et al. 2003; Lee et al. 2006; Wang et al. 2008; Meng et al. 2009). The various fabrication techniques for WO3 nanostructures often used high temperatures. For example, Djaoued et al. (2003) reported the formation of WO3 films with sol-gel methods and heated in the range 400 to 450 oC. Meng et al. (2009) reported the formation of WO3nanoparticles by annealing at 400-800 oC for 1 h, etc. Hui et al. (2015) reported on the hydrothermal synthetization of WO3 nanosheets on the surface of wood. They also reported that WO3 nanosheets were fabricated on the wood surface at 90 oC for 12 h, as shown in Table 1. Therefore, in our opinion, the fabrication of high quality WO3 nanostructures with shorter time is still important from the view of both wood functionalization and energy conservation.

In this study, WO3 nanoparticles were grown on wood surfaces through a low-temperature hydrothermal method at a short time. The chemical composition of the WO3 layer was characterized by energy disperse spectroscopy (EDS), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). The photochromic property of WO3-coated wood was tested by evaluating the color change after being exposed to UV light. A superhydrophobic surface could be easily obtained by simply modification of inorganic nanomaterials coated wood. (Sun et al. 2011, Fu et al. 2012, Gan et al. 2015, etc.) Therefore, superhydrophobic wood was obtained after a simple fluoroalkyl silane modification to the WO3-coated wood. A possible mechanism of the wood with photochromic and superhydrophobic properties is proposed. After the fabrication of WO3 on the surface of the wood, the color of the wood was more similar to the color of valuable wood, having a yellowish and reddish saturated color. This work provides a further understanding of WO3-coated wood and also offers promising market applications for fast-growing wood.



Polar wood slices of size 20 mm × 20 mm × 10 mm were obtained from a plywood mill in Harbin, China. All of the specimens were ultrasonically rinsed with distilled water and ethanol for 30 min and subsequently dried in a vacuum oven at 60 °C for 48 h. Tungstate chloride (WCl6) (99.5%), anhydrous ethanol (99.7%), and acetone (99.5%) were purchased from Tianjin Kermel Chemical Reagent Co., Ltd. (Tianjin, China). The trimethoxy (1H,1H,2H,2H-heptadecafluorodecyl) silane used for surface hydrophobic modification was purchased from Shanghai Aladdin Chemical Co., Ltd. (Shanghai, China). All the chemicals were supplied without purification.


First, 0.06 M WCl6 was dissolved in anhydrous ethanol for 30 min to obtain a clear liquid. The prepared wood samples were added to the above solution in an 80% filled Teflon stainless steel autoclave (Zhengzhou Keda Machinery and Instrument Equipment Co., Ltd., Zhengzhou, China). The autoclave was placed in an oven at 90 °C for 4 h and then cooled to room temperature.

Then, 0.025 M sodium dodecyl sulfonate (SDS) solution was added into the above solution with a volume ratio of 5%. The mixture was then heated to either 90 °C or 120 °C for 6 h. After cooling to room temperature, the samples were rinsed with ethanol and water several times and dried at 60 °C for 24 h. The obtained samples were labeled as S-temperature, e.g. S-90 or S-120, corresponding to the samples at 90 °C or 120 °C, respectively.


The morphology and chemical compositions of the WO3-coated wood surfaces were determined by a field emission scanning electron microscope (FE-SEM; JSM-7500, JEOL, Tokyo, Japan) at an operating voltage of 15 kV and an X-ray EDS (X-MaxN, Oxford Instruments, Oxford, UK). Transmission electron microscope (TEM) observation of the WO3 nanostructure was performed using a JEM-2100 microscope (JEOL, Tokyo, Japan) at an acceleration voltage of 200 kV. Crystalline structures of the WO3 nanostructures were identified by XRD (Rigaku D/max2200 diffractometer, Rigaku, Tokyo, Japan) using a CuKa radiation generator operated at a voltage of 40 kV and 30 mA. The FTIR spectroscopy (Nicolet, Thermo Fisher Scientific, Waltham, USA) was also utilized for characterizing the obtained samples using the attenuated total reflectance (ATR) mode.

The photochromic property of the WO3-coated wood samples was tested using a color difference meter and digital photographs of the wood under the same light environment. The color change (∆E) of the coated samples was characterized after exposure to a UV irradiation (365 nm) device (Dongguan Guangkai Instrument Co., Ltd., Guangzhou, China). The irradiated samples were also put in the dark for 24 h to test the recovery of the color. The Commission International de’Eclairage Lab (CIELAB) uniform color space for the color sheet system was used to calculate the color change using Eq. 1,


where L*, a*, and b* are the three-dimensional rectangular coordinates, L* refers to the lightness index, it varies from 100 (white) to 0 (black); while a* and b* are the chromaticity indices, +a* is the red direction, −a* is green; +b* is yellow, and −b* is blue, where L*a*, and b* are the differences of initial and final values of a*b*, and L*.

The water contact angle was tested using an OCA20 contact angle tester (Dataphysics, Stuttgart, Germany). Before testing, the samples were modified by immersion into 50 mL ethanol solution with 5% trimethoxy (1H,1H,2H,2H-heptadecafluorodecyl) silane (v/v) at room temperature under continuous stirring for 24 h and then dried at 70 °C in a vacuum oven.


Table 1 shows the conditions used for WO3 synthesis in the present work, in comparison to earlier studies (Hui et al. 2015, 2016).

Table 1. Treatment Conditions Employed in Studies of Hydrothermally Synthesizes WO3 on Wood Surfaces

The elemental composition and distribution on the surface of the wood was characterized using an X-ray EDS, as shown in Fig. 1. In Fig. 1a, there was a noticeable peak at approximately 1.8 keV, which corresponds to the tungsten (W) element. The mass percentage of W was 12.89%. It can be seen that the mapping images from Fig. 1b to 1d, W, carbon (C) and oxygen (O) had a uniform distribution on the surface of the wood. The EDS analysis revealed that the compounds containing W might have grown uniformly on the surface of wood.

Fig. 1. EDS analysis of S-120 sample (a) and the mapping images of C elements (b), O elements (c), and W elements (d)

The samples that had been grown with tungsten compounds were further identified by XRD and FTIR analysis. As can be seen in Fig. 2, the XRD pattern highlights the crystalline structure of wood samples S-90 and S-120. As shown, the wood had two main peaks at around 18° and 23°, which corresponds to the typical cellulose peaks in wood (indicated by the green arrows). After growing tungsten compounds on the wood surface, some new peaks around 25°, 34°, 49°, and 56° appeared. The diffraction peaks were assigned to the (110), (201), (220), and (221) diffraction planes of hexagonal WO3 structures (hexagonal WO3, JCDScardno.75-2187, as indicated by the red arrows). For the high temperature sample (S-120), the peak intensity increased when compared to the lower temperature sample (S-90). This revealed the high crystallinity of WO3structures at high temperatures.

Fig. 2. XRD patterns of wood and WO3-coated wood samples S-90 and S-120

The functional groups on the surface of wood were also characterized by FTIR, as shown in Fig. 3. The FTIR band at 3400 cm-1 was assigned to the O-H stretching mode of hydrogen-bonded hydroxyl groups or absorbed water, which is clearly reduced in the treated wood compared to that of the untreated wood (Fu et al. 2012). The FTIR band at 1735 cm-1 is attributed to C=O stretching in the non-conjugated aryl ketone of lignin carbonyl groups. The band at 1250 cm-1 is caused by C-O stretching in the phenolic hydroxyl group of lignin. These two bands were found in the spectrum of the original wood, but in the treated wood the two bands were weaker. This weakening was because wood was partially hydrolyzed during the hydrothermal process. The FTIR bands at 1460 cm-1 and 1506 cm-1 are assigned to the functional groups of lignin, and 1620 cm-1 is assigned to hemicellulose. The core components of wood were not changed in the hydrothermal process. However, when comparing the untreated and treated wood, there was a remarkable peak at 815 cm-1 in the curves of both the S-90 and S-120 samples that was not found in the untreated sample (Daniel et al. 1987; Guery et al. 1997). This peak is typically described as O–W–O stretching and bending modes of the bridging oxygen atoms in WO(Hui et al. 2015). From the XRD and FTIR analyses, WO3 was successfully fabricated on the surface of wood at both treatment temperatures (90 °C and 120 °C).

Fig. 3. FTIR spectra of untreated wood and WO3-coated wood samples S-90 and S-120

The morphology of the WO3 on the surface of the wood was observed by scanning electron microscopy (SEM). Figures 4a and 4b show that the surface of the untreated wood was smooth.

Fig. 4. SEM images of wood, S-90 and S-120 at low magnification (a, c, and e) and high magnification (b, d, and f) Insert: TEM images of the WO3 nanoparticles, the scale bar is 5 nm

The wood cell and pit structures can be seen clearly. However, after growing WO3 on the surface of the wood at 90 °C, the surface was covered with a layer, and the microstructure of the wood cannot be seen (Fig. 4c). At high magnification (Fig. 4d), it can be seen that the WO3 layer was composed of many nanoparticles and was distributed quite evenly. The WO3 layer became thicker after increasing the temperature to 120 °C, as can be seen in Figs. 4e and 4f. The density of the WO3 nanoparticles increased with increased temperature. This can possibly be attributed to the fact that WO3 grows better at relatively high temperatures. The structure of the WO3 on wood surface was also characterized by TEM. We can see in the insert TEM images that at 120 oC, the WO3 nanoparticles were with higher crystallinity structures. This result agreed with that obtained from the XRD analysis. The results showed that crystalline WO3 nanoparticles were hydrothermally grown on wood surface at a low temperature of 90 °C; meanwhile, the reaction time was comparable as compared with the previous literature (Gavrilyuk 1999; Hui et al. 2015). The WO3 nanoparticles were successfully grown on the wood surface through facile and relatively mild hydrothermal conditions.

The results of the color change of untreated and treated wood (samples S-90 and S-120) before and after UV irradiation are shown in Figs. 5 and 6. The brightness (L) of S-90 and S-120 decreased 32.8% and 47.2% compared to the untreated sample, respectively. This revealed that after the coating with WO3, the wood became darker. This is obvious in the photographs in Fig. 5.

Fig. 5. Photographs of untreated wood, S-90, and S-120 before and after 6 h UV irradiation

However, as shown in Fig. 6, the a* value of the S-90 sample group decreased remarkably (70.8%) after the coating process, while the S-120 sample group remained stable. This result demonstrated that at a low temperature, the coating had a tendency to take on a greener color, while at a high temperature the color remained unchanged when compared to the untreated wood. The b* value changed very little for both the S-90 and S-120 samples. It can be concluded from the above results that the WO3 coating had a remarkable dyeing effect on wood at a low temperature. At a high temperature, the color values remained stable but the brightness changed.

The photochromic property of the WO3-coated wood was tested after 6 h of UV irradiation. The brightness of the untreated wood and the S-90- and S-120-coated samples changed minimally in Fig. 6a. However, for the S-90 sample, the a* value improved after UV radiation, increasing 600% in Fig. 6b. The b* value also increased for the S-90 sample. For the S-120 sample, the a*value increased while the b* value decreased after UV radiation. The results revealed that the color of the S-90-coated wood sample became darker, redder, and more yellow after 6 h of UV radiation, which was similar to that of traditional high-grade wood. The S-120-coated wood sample became redder and bluer. After the growth of WO3 nanoparticles on the surface of the wood at 90 °C, the value of ∆E was lowest compared to the untreated wood and S-120. This color change was attributed to the photochromic property of the WO3 layer on the surface of the wood. The color change was more obvious in the higher-temperature sample, S-120, compared to that of the lower-temperature S-90 sample in Fig. 6d. This difference in color change based on temperature was attributed to the thicker growth of the WO3 layer at a higher temperature. Therefore, the WO3 layer on wood surface was sensitive to UV light, though this result was not permanent.

Fig. 6. Color parameters (L, a*, b*, and ∆E) of untreated wood and treated samples S-90 and S-120 before and after 6 h of UV irradiation

After 24 h in a dark environment, the color was recovered. The results were similar to the literature (Hui et al. 2016). This will be considered with further experiments in the future. The results of photochromic properties of WO3coated wood was due to the following scheme: after UV radiation, the WO3 layer on wood surface was excited after an electron injection process and a further reaction, and then a pair of electrons and holes formed, as shown in Fig. 7a. The WO3layer turned blue due to the inter-valence charge transfer from the valence band of W5to the conduction band of W6+, as reported in previous literature (Su et al. 2010; Chai et al. 2014).

Fig. 7. Schematic diagram of WO3 coated wood surface with photochromic and superhydrophobic property

The modification of hydrophobicity in the S-90 and S-120 samples compared to the untreated wood is shown in Fig. 8. The untreated wood had an initial contact angle of 80°, and it dropped to 30° after 60 s (Fig. 8a). However, the S-90 sample had an initial contact angle of 146° that remained stable at 145° after 60 s (Fig. 8b). The initial contact angle of the S-120 sample was 152° and remained stable after 60 s. Superhydrophobicity of the WO3-coated wood was achieved, as shown in Figs. 8c and 8d. The S-120 sample had a sliding contact angle of less than 10°. To further understand the wetting behavior of the wood samples, the Wenzel and the Cassie and Baxter state models were applied (Boussu et al. 2005; Erbil and Cansoy 2009; Shen et al. 2015). The Wenzel model assumes that a liquid completely fills the grooves where it contacts the substrate.

Fig. 8. Water contact angle (WCA) of untreated wood (a), S-90 (b), and S-120 (c) within 60 s, digital images of water droplets on the S-120 sample surface (d), and sliding angle images of modified S-120 (e)

For the Cassie and Baxter state model, an air pocket is trapped in the groove of the textured surface and a liquid “sits” on the surface (Liu et al. 2013; Wang et al. 2014; Guo et al. 2017). In this study, wood as a hydrophilic material (Fig. 7b), allowed water droplets to fill the grooves on its surface, which resulted in a low contact angle and after some time the water was absorbed into the wood. However, after the growth of WO3 nanoparticles on the surface of the wood, the wood surface became hydrophobic and rough; the Cassie and Baxter model is formed and the water droplet stays stable on the surface. For the S-120 sample, the WO3 layer was thicker and rougher compared to S-90, and a Cassie-Baxter state model was formed, which resulted in a superhydrophobic surface that allowed the water droplets to slide on the surface (Fig. 8e).

In summary, the photochromic and superhydrophobic properties were obtained by a simple hydrothermal method. For the photochromic property, it has potential applications in information storage and light sensor. After a simple modification with low surface energy materials, it also has the self-cleaning property. We also hope that the WO3-coated wood can combine with paints for outdoor application or combine with wax, which is also widely used on furniture for indoor application. In addition, the thermal and mechanical properties are also important for application. These aspects will be considered in further experimental studies.


  1. Wood with photochromic and superhydrophobic properties was achieved by hydrothermally growing WO3 nanoparticles on the surface of the wood at a low temperature. The SEM images proved that temperature played a key role in the thickness and roughness of the WO3 layer.
  2. The WO3-coated wood was sensitive to UV light and was more sensitive at a high temperature of 120 °C.
  3. Self-cleaning character was obtained after a simple fluorosilane modification of the WO3-coated wood.
  4. This study provides a simple and mild method for enhancing the visual quality of fast-growing wood. Changing the reaction conditions and photo stimulation controlled the color.


The authors are grateful for the support of the Natural Science Foundation of China (31770592).


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Article submitted: July 27, 2017; Peer review completed: October 29, 2017; Revised version received and accepted: December 6, 2017; Published: December 14, 2017.

DOI: 10.15376/biores.13.1.1075-1087