Abstract
A solution-type TiO2-based photocatalyst for the decomposition of volatile organic compounds was synthesized using a modified sol-gel method under either alcohol (ethanol or isopropanol) or aqueous conditions. Anatase-type TiO2 was successfully synthesized with additional hydrothermal treatment using either type of medium. However, the aqueous condition for TiO2 synthesis was more convenient for the formation of anatase-type TiO2. Based on X-ray diffraction analysis data, the optimal hydrothermal treatment temperature was 80 °C for anatase-type TiO2 formation; at temperatures below 80 °C or above 90 °C, mostly rutile-type TiO2 was formed. The synthesized anatase-type TiO2 solution was applied to the surface of carbonized medium-density fiberboard (c-MDF). The anatase-type TiO2 on c-MDF showed good maintenance of toluene decomposition performance even after repeated use for 14 weeks.
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Synthesis of TiO2 via Modified Sol-gel Method and its Use in Carbonized Medium-density Fiberboard for Toluene Decomposition
Min Lee,a Sang-Bum Park,a and Sung-Phil Mun b,*
A solution-type TiO2-based photocatalyst for the decomposition of volatile organic compounds was synthesized using a modified sol-gel method under either alcohol (ethanol or isopropanol) or aqueous conditions. Anatase-type TiO2 was successfully synthesized with additional hydrothermal treatment using either type of medium. However, the aqueous condition for TiO2 synthesis was more convenient for the formation of anatase-type TiO2. Based on X-ray diffraction analysis data, the optimal hydrothermal treatment temperature was 80 °C for anatase-type TiO2 formation; at temperatures below 80 °C or above 90 °C, mostly rutile-type TiO2 was formed. The synthesized anatase-type TiO2 solution was applied to the surface of carbonized medium-density fiberboard (c-MDF). The anatase-type TiO2 on c-MDF showed good maintenance of toluene decomposition performance even after repeated use for 14 weeks.
Keywords: Toluene; Decomposition; TiO2; Carbonization; Medium-density fiberboard
Contact information: a: Department of Forest Products, National Institute of Forest Science, Seoul, South Korea; b: Department of Wood Science and Technology, Chonbuk National University, Jeonju, South Korea; *Corresponding author: msp@jbnu.ac.kr
INTRODUCTION
As industrialization and urbanization have progressed rapidly since the third industrial revolution, pollution of the urban environment has become increasingly severe due to urban concentration and increases in industrial facilities and traffic volumes (Field 2010). In addition, with economic improvements and changes in living and working styles, modern people usually spend more than 90% of their time indoors (US EPA 2010). Therefore, the effect of indoor environments on human health has increased (CDC 2006). Indoor air quality problems have begun to increase with the closure of buildings and the installation of artificial energy-decomposition devices used to improve energy efficiency in various industries (Kim 1999; Spengler et al. 2006). Recently, populations vulnerable to environmental illnesses, such as the elderly, children, and pregnant women, have increasingly suffered from atopy and asthma (US EPA 2010).
In the last decade, nanostructured materials have received great interest for catalysis and other applications because of their unique texture and structural properties (Molinari et al. 2000; Ao and Lee 2004; Guo et al. 2004; Hashimoto et al. 2005). Many studies have focused on metal oxides such as TiO2, SnO2, VO2, and ZnO (Torimoto et al. 1997; Fujishima et al. 2000). Titanium dioxide has received research attention due to its chemical structure and biocompatibility as well as its physical, optical, and electrical properties (Takeda et al. 1995; Doi et al. 2000; Tokoro and Saka 2001; Varghese et al. 2003; Wen et al. 2005). Photocatalytic reactions are widely used for various environmental purification purposes, such as the decomposition of contaminants in water and air (Andersson et al. 2002; Wahi et al. 2006; Wu and Qi 2007). When TiO2 is used as a photocatalyst, it has three crystal structures, classified as anatase, rutile, and brookite. The low-formation-temperature anatase-type and high-formation-temperature rutile-type are commonly found. Although the two structures have similar tetragonal symmetry, in the rutile crystal structure, the octahedral structures with Ti centers share oxygen atoms at the vertex positions; in the anatase-type, the octahedral structures share edges. Because of these structural differences, the two phases exhibit different physical and chemical properties. To synthesize nanostructured TiO2, the sol-gel process, direct oxidation, chemical vapor deposition, electrodeposition, an ultrasonic chemical method, and a microwave method have been used (Zhang and Gao 2003; Corradi et al. 2005; Wang et al. 2009; Xue et al. 2011; Bazargan et al. 2012; Byranvand et al. 2013; Zhou et al. 2013). Photocatalysts prepared via the sol-gel method are typically applied as coating agents through spin-coating or spraying.
Kercher and Nagle (2002) introduced carbonized boards by pyrolyzing wood-based panels at high temperatures; they were intended for use as fuel cell membranes. The initial carbonized board was prepared at the laboratory scale of approximately 10 cm in size. In 2009, a crack- and twist-free carbonized board was developed by using medium-density fiberboard (MDF), plywood, and wood panels (Park et al. 2009). The physicochemical properties and functionality of carbonized MDF (c-MDF) have been investigated (Zhou et al. 2013; Lee et al. 2014; Lee et al. 2017a,b). The c-MDF can easily remove formaldehyde (a major volatile compound contributing to sick house syndrome) and shows some ability to remove other aromatic volatile organic compounds (VOCs), such as toluene and xylene, from indoor atmospheres (Park et al. 2009).
In this study, to improve the decomposition ability of c-MDF, a nano-sized TiO2 photocatalyst was prepared by using the modified sol-gel synthetic method, and then it was applied to prepared c-MDF to provide photocatalytic function. The anatase-type TiO2 sol-applied c-MDF was then investigated to confirm enhancement of the photocatalytic decomposition ability for toluene.
EXPERIMENTAL
Materials
The MDF (0.64 g/cm2, E1 grade) was purchased from Sunchang Industry (Incheon, Korea).
Titanium tetrapropoxide (Ti-tip) and titanium tetrachloride (TiCl4; DaeJung Chemicals & Metals Co., Ltd., Siheung-si, Gyeonggi-do, Korea) were used as photocatalyst precursors for the synthesis of TiO2. The synthesis was performed in the presence of HCl (1 mol/L, Showa Chemical Industry Co., Ltd., Tokyo, Japan), NH4OH (DaeJung Chemicals & Metals Co., Ltd., Siheung-si, Gyeonggi-do, Korea), NaOH (Duksan Pure Chemical Co., Ltd., Seoul, Korea), isopropyl alcohol (IPA; DaeJung Chemicals & Metals Co., Ltd., Siheung-si, Gyeonggi-do, Korea), and ethyl alcohol (99.5%, EtOH; Duksan Pure Chemical Co., Ltd., Seoul, Korea).
Standard TiO2 (mixture of rutile and anatase types, nanopowder, < 100 nm) was purchased from Sigma-Aldrich (St. Louis, MO, USA).
Methods
Preparation of c-MDF
The MDF, which is a raw material for manufacturing the photocatalytic c-MDF, was cut into samples measuring 130 cm × 260 cm × 1.2 cm (W × L × T) and then wrapped in newspaper and aluminum foil according to Park’s method (2009). The prepared MDF was stacked between graphite plates in a heat resistant box and then placed in an electric furnace. The carbonization program used a heating rate of 50 °C/h and a maximum temperature of 600 °C, which was held for 2 h followed by natural cooling (Park et al. 2009).
Synthesis of TiO2-sol using alcohols
The Ti-tip was used as a photocatalyst precursor to synthesize the sol-type photocatalyst using alcohol (either EtOH or IPA) as the solvent (Fig. 1). In an Erlenmeyer flask (1 L), 340 g of the selected alcohol and 20 g of Ti-tip were mixed to prepare Solution 1. To prepare Solution 2, distilled and deionized (DI) water (80 g) and concentrated hydrochloric acid (5 g) were mixed in 340 g of alcohol (EtOH or IPA) and placed in a separatory funnel. Solution 2 from the separatory funnel was added dropwise at 60 drops/min to Solution 1 while it was stirred in an ice bath. The mixing of Solution 1 and Solution 2 continued for 3 h with an additional 1 h of stirring. To replace the alcohols with DI water, centrifugation was performed at 6,000 rpm (652 G-force) for 20 min at 10 °C. After decantation, 400 mL of DI water was added to the precipitate, followed by ultrasonic treatment (10 min) for dispersion. The TiO2 dispersed in the DI water was refluxed at 80 °C for 1 h.
Fig. 1. Flow chart of sol-type TiO2 preparation using alcohols
Synthesis of sol-type TiO2 using water
The TiCl4 was used as a photocatalyst precursor to synthesize a sol-type photocatalyst under an aqueous condition. Approximately 19 g of TiCl4 was placed in an Erlenmeyer flask (1 L) and cooled in iced water. The DI water (200 mL) was slowly added dropwise to the cooled TiCl4 solution to prepare a 0.5 M TiOCl2 aqueous solution. A 5 M NH4OH aqueous solution was added dropwise for approximately 10 min to the prepared 0.5 M TiOCl2 aqueous solution to obtain the TiOH precipitate as a reaction product. To remove Cl ions from the TiOH precipitate, it was washed three times with DI water and the supernatant was removed via centrifugation. The conditions for centrifugation were 6,000 rpm (652 G-force) for 20 min at 10 °C. The washed TiOH was dispersed via sonication for 10 min in 200 mL of 5 M NaOH solution. The aqueous solution of NaOH in which TiOH was dispersed was thermally treated at reaction temperatures ranging from 50 °C to 100 °C for 1 h, as well as at room temperature (25 °C). The supernatant was removed through centrifugation. The resulting precipitate was treated in 200 mL of 0.5 M HCl solution at 60 °C for 24 h with occasional shaking. After treatment, the suspension was centrifuged under the same conditions mentioned above. The resulting TiO2 was subsequently repeatedly washed with DI water until the pH reached 7.0. The washed TiO2 was dispersed in 200 mL of DI water through ultrasonication for 10 min (Fig. 2).
Fig. 2. Flow chart of sol-type TiO2 preparation using water
Crystallinity of TiO2
X-ray diffraction (XRD; X’pert Powder; Malvern Panalytical Ltd., Malvern, United Kingdom) was used to confirm the crystal form of TiO2. The synthesized sol-type TiO2 was dried and finely ground in an agate mortar and then placed in a sample holder. The holder was mounted, and the sample was measured at 2θ values from 15° to 80° at 40 kV and 30 mA. All spectra obtained were normalized and then used.
Microscopic analysis of sol-type TiO2/c-MDF
After the synthesis of sol-type TiO2-treated c-MDF (sol-type TiO2/c-MDF), a scanning electron microscope (SEM, JSM 6400; JEOL Ltd., Tokyo, Japan) and energy-dispersive spectroscopy (EDS, JED-2300; JEOL Ltd., Tokyo, Japan) were performed to determine the Ti distribution on the surface of the c-MDF. To analyze the particle size of the synthesized TiO2, a spherical aberration-corrected transmission electron microscopy (TEM, JEM-ARM 200F; JEOL Ltd., Tokyo, Japan) was used. The sample grid was immersed in a 1/10 diluted TiO2-sol in IPA. The grid was then dried on a hot plate to remove the solvent. Before analysis, the dried grid was pretreated by an ion cleaner (EC-52000IC, JEOL Ltd., Tokyo, Japan) for 10 min. The acceleration voltage used was 200 kV and the lattice resolution was 0.2 nm.
Toluene decomposition performance
A sol-type TiO2/c-MDF (300 cm2) was placed in a 20-L stainless steel chamber that was filled with 20-ppm toluene standard gas three times to evaluate the toluene decomposition performance. A Tedlar bag (25-L, TP-25; TRS Environment™, Chesterfield, MI, USA) filled with nitrogen was connected to one of the inlets at the bottom of the chamber. The concentration of toluene in the chamber was monitored at 0 h, 1 h, 3 h, 5 h, 7 h, and 9 h after the chamber was irradiated with ultraviolet (UV) radiation with wavelengths of 360 nm. A 1.0 L sample of air was collected in a Tenax-TA adsorption tube (Supelco Inc., Bellefonte, PA, USA) at a flow rate of 167 mL/min and then analyzed with a gas chromatograph-mass spectrometer (GC-MS, QP2010; Shimadzu, Kyoto, Japan). The analysis conditions are shown in Table 1.
Table 1. GC-MS and TD Analysis Conditions
RESULTS AND DISCUSSION
Sol-type TiO2 Synthesized Using Alcohols
Sol-type TiO2 synthesis
The final concentrations of sol-type TiO2 synthesized using lower alcohols (IPA and EtOH) were 0.71% and 0.72%, respectively. After the preparation of sol-type TiO2 by IPA or EtOH, TiO2 appeared to be well dispersed. However, after standing overnight, precipitates were observed in the TiO2 synthesized using IPA as the solvent, which would impede spraying. The TiO2 synthesized in EtOH remained well dispersed after standing overnight (Fig. 3).
Crystallinity of sol-type TiO2 synthesized in alcohols
The X-ray diffraction analysis of TiO2 synthesized using alcohols as solvents showed anatase-type TiO2 in both the solvents. However, the peak width and intensity in each sample were wide and low, respectively (Fig. 4). Therefore, the crystallinity of TiO2 may be very low in both sol-type photocatalysts. As previously mentioned in the TEM results, TiO2 that was synthesized under these alcohol conditions was mostly amorphous in phase.
Fig. 3. Photos of sol-type TiO2 synthesized in IPA and EtOH after 1 day
Fig. 4. XRD patterns of sol-type TiO2 synthesized in IPA and EtOH
Crystallinity of hydrothermally treated sol-type TiO2 synthesized in alcohols
The two types of alcohols, IPA and EtOH, used in the reaction were replaced with DI water, and then thermal treatment was conducted under refluxing conditions. As shown in Fig. 5, XRD data indicated that the amorphous form of TiO2 was transformed into crystalline TiO2. However, the peak width remained greater than that of the standard TiO2 used as the standard sample. Therefore, low photocatalyst activity was expected. In addition, the synthesis of a sol-type photocatalyst using alcohols may be less economically efficient because additional processes of water substitution and thermal treatment are necessary.
Fig. 5. XRD patterns of sol-type TiO2 synthesized in EtOH by thermal treatment at 80 °C
Sol-type TiO2 Synthesized with Water
Sol-type TiO2 synthesis
The final concentration of the sol-type TiO2 synthesized in water was 3.11%. In the syntheses using IPA and EtOH, phase separation was observed after TiO2 preparation, followed by gelation as time passed. However, the sol-type TiO2 synthesized using water as a solvent showed good dispersity, although some precipitates were observed 10 days after preparation (Fig. 6.).
Fig. 6. Photos of synthesized sol-type TiO2 (Left: IPA, right: DI water)
Microscopic observation of sol-type TiO2 synthesized with water
Figure 7 shows TEM images of TiO2 synthesized in water. The TiO2 prepared by hydrothermal treatment at 80 °C showed spherical crystals; TiO2 that was hydrothermally treated at 100 °C showed acicular crystals. In general, anatase TiO2 was spherical and rutile TiO2 showed rod or needle forms (Fujishima et al. 2000; Hashimoto et al. 2005). The anatase-type TiO2 was formed by hydrothermal treatment at 80 °C. The TiO2 treated at 80 °C had an average length of 35 ± 11 nm and a width of 25 ± 12 nm. The TiO2 treated at 100 °C had an average length of 72 ± 14 nm and a width of 14 ± 5 nm.
Fig. 7. TEM images of sol-type TiO2 synthesized in water by hydrothermal treatment
Crystallinity of sol-type TiO2 synthesized in water
During the alkali treatment that followed the ammonia water treatment, the effect of various temperatures on the crystal form changes of the sol-type TiO2 was investigated. Particles of the sol-type photocatalyst alkali that was treated at room temperature appeared amorphous, similar to those of the photocatalysts prepared using alcohols. From Fig. 8, the amorphous phase of TiO2 was transformed into a crystalline one with heating applied during the alkali treatment.
Fig. 8. XRD pattern of sol-type TiO2 prepared at 80 °C and 100 °C
During treatment at 80 °C, all peaks related to rutile crystals in the XRD data disappeared, with only anatase crystal peaks remaining. With treatment at 90 °C or higher, the anatase crystal peaks disappeared again and rutile crystals peaks appeared. These results indicated that 80 °C hydrothermal treatment was the optimum condition for synthesizing anatase-type TiO2, which is known to have higher photocatalytic activities. Therefore, the hydrothermal conditions were critical in synthesizing the anatase form of the TiO2 photocatalyst in water. In addition, XRD pattern of TiO2 treated under 70 °C showed similar pattern to 100 °C treated TiO2 (data not shown). Therefore, a mixture of anatase and rutile crystals of TiO2 was formed with treatment at 50 °C to 70 °C. A spherical aberration-corrected TEM was used to observe the crystal lattice of TiO2 obtained from the photocatalyst precursors Ti-tip and TiCl4. The TiO2 was shown to have a hexagonal close-packed structure, which is expected to have a face-centered cubic (FCC) lattice with 001 zone axis. Figure 9 shows the crystal lattice of sol-type anatase TiO2 particles treated at 80 °C. The TiO2 lattice dimension was 0.344 nm to the first point and 0.262 nm to the second point, confirming that the axis was 001 in an FCC lattice. These values were compared with Joint Committee on Powder Diffraction Standards (JCPDS) card values. The synthesized TiO2 from water conditions with 80 °C hydrothermal treatment was thus confirmed as anatase-type through a spherical aberration-corrected TEM observation.