Photocatalytic degradation of formaldehyde by nanostructured TiO₂ composite films

Abstract

Interest in the photocatalytic oxidation of formaldehyde from contaminated wastewater is growing rapidly. The photocatalytic activity of the nanocrystalline Fe³⁺/F⁻ co-doped TiO₂–SiO₂ composite film for the degradation of formaldehyde solution under visible light was discussed in this study. The films were characterised by field emission scanning electron microscopy (FE-SEM) equipped with energy-dispersive spectroscopy, X-ray diffraction (XRD), BET surface area, UV–Vis absorption spectroscopy, and photoluminescence spectroscopy. The FE-SEM results revealed that the Fe³⁺/F⁻ co-doped TiO₂–SiO₂ film was composed of uniform round-like nanoparticles or aggregates with the size range of 5–10 nm. The XRD results indicated that only the anatase phase was observed in the film. Compared with a pure TiO₂ film and a singly modified TiO₂ film, the Fe³⁺/F⁻ co-doped TiO₂–SiO₂ composite film showed the best photocatalytic properties due to its strong visible light adsorption and diminished electrons-holes recombination.

Keywords:

• TiO₂–SiO₂ composite
• Fe³⁺/F⁻ doping
• formaldehyde photodegradation

1. Introduction

Environmental pollution caused by volatile organic compounds, such as formaldehyde, is becoming a serious problem all over the world. However, it is difficult to remove them by traditional biological and physical treatment methods. Heterogeneous photocatalyst has attracted a great interest in the field of the degradation of toxic pollutants.[1,2] TiO₂ is the most suitable material for heterogeneous photocatalysts owing to its long-term stability, high photocatalytic activity, non-toxicity, chemical and biological inertness, controllable structure and its low cost.[3,4] Recently, many studies have pointed towards easily recycled TiO₂ films because conventional TiO₂ powders are difficult to recover in wastewater treatment processes.[5,6]

TiO₂ is a UV-absorbing material with a gap energy in the 3.0–3.4 eV range.[7] Usually the band gap is equal to or greater than 3.2 eV for anatase, which means only under the irradiation of ultraviolet light can the reaction occur to create electrons and holes. Another problem is the high recombination rate of photogenerated electrons and holes. Many methods have been proposed to reduce the recombination of photogenerated electron-hole pairs and widen the light response region of TiO₂. Doping TiO₂ with foreign ions is one of the most promising strategies. Easy availability and low cost have given iron broad applications in the modification of TiO₂. Wu et al. reported that Fe doping could enhance the intensity of absorption in the visible light region and make a red shift in the band gap transition of Fe-doped TiO₂ samples.[8] Transition metal doping can enhance the visible light response, but the problems of TiO₂ with metal ion doping are only a small red shift, poor thermal stability, and surface aggregation rather than substitution. Doping with non-metal ions into TiO₂ can provide a favourable visible light response and facilitate efficient charge carrier transfer processes. Yu et al. reported that F-doped TiO₂ powders exhibited stronger absorption in the UV–visible range with a red shift in the band gap transition.[9] Metal dopant energy levels are usually below the conduction band edge, while non-metal dopant energy levels are just above the top of the valence band of TiO₂.[10] Therefore, co-doping with metal and non-metal ions can effectively narrow the band gap of TiO₂ and thus enhancing the photocatalytic activity of TiO₂.[11] Composite semiconductors are also a good method to improve the photocatalytic activity of TiO₂. Kim et al.[12] reported that the high photocatalytic activity of a Zr/TiO₂/SiO₂ photocatalyst could be attributed to the large specific surface area and strong absorption in the visible light region. With a high specific surface area, SiO₂ can provide more effective adsorption sites in the SiO₂–TiO₂ composite system.

Considering the green environmental protection, we prepared the nanocrystalline Fe³⁺/F⁻ co-doped TiO₂–SiO₂ composite film using the sol–gel method in this work. It is expected that the film may exhibit better photocatalytic properties for the photocatalytic degradation of formaldehyde in an aqueous solution. The improvement mechanism by Fe³⁺/F⁻ co-doping and SiO₂ composite was also discussed.

2. Experimental

TiO₂ or SiO₂ sol was prepared by a sol--gel method using tetrabutyl titanate [TiO(C₄H₉O)₄] or tetraethylorthosilicate (TEOS) as the precursor, as previously reported.[13,14] One mL Ti(OC₄H₉)₄ was added to 20 mL absolute C₂H₅OH. Then, 20 mL HNO₃ (0.2M) was added to the above solution with vigorous stirring. One mL TEOS was added to 30 mL absolute ethanol. Then, 0.5 mL ammonium hydroxide was added to the above solution with vigorous stirring. TiO₂–SiO₂ composite sol was prepared by mixing TiO₂ and SiO₂ colloids in a 2/1 volume ratio with vigorous stirring. A controllable dip-coating device was used to prepare pure TiO₂ or TiO₂–SiO₂ composite films on the surface of a glass substrate open to the atmosphere. Fe(NO₃)₃ or NH₄F aqueous solutions were chosen as the iron or fluorine sources. The Fe³⁺, F⁻_, or Fe³⁺/F⁻ ions were doped onto the surface layer by coating a thin Fe³⁺, F⁻_, or Fe³⁺/F⁻ doped TiO₂ sol onto a dried TiO₂–SiO₂ composite film. Finally, the films were calcined at 450 °C in air for 1 h in a horizontal furnace. The thickness of the film was 0.3–0.6 μm in our preparations.

The surface morphology of the sample was studied by a high-resolution field emission scanning electron microscopy (FE-SEM, Nova NanoSEM 450) equipped with an energy-dispersive spectroscopy analysis (EDS, X-Max 50) to determine the composition of the samples. The X-ray diffraction (XRD) profiles of the samples were recorded by XRD with a diffractometer (Bruker D8 FOCUS) employing Cu Kα radiation at a scan rate of 0.05° s⁻¹ to characterise the phases and crystal size of the samples. The specific surface area of the sample was analysed using a nitrogen adsorption/desorption apparatus (ASAP2020). The UV–Vis diffuse reflectance spectra of the sample was recorded on a UV–Vis spectrophotometer (TU-1901) with an integrating sphere accessory (IS 19-1) using a blank glass plate as reference. The recombination rate of photogenerated electrons and holes in the sample was studied by photoluminescence (PL) emission spectra, recorded at room temperature using a spectrometer (FLS 920).

The photodegradation of formaldehyde (20 g L⁻¹) in aqueous solutions was used to evaluate the photocatalytic activities of the samples. In each photocatalytic experiment a catalyst sample was placed into 5 mL aqueous solution of a target pollutant in a quartz beaker. A tungsten halogen lamp equipped with a UV cut-off filter (λ > 400 nm) was used as a visible light source whose average light intensity was 40 mW·cm⁻². The distance between the beaker and the light source was kept at about 15 cm. The formaldehyde concentration was determined by an acetylacetone spectrophotometry. Figure 1 shows the adsorption curve of the samples in formaldehyde solutions. It is clear that the adsorption–desorption equilibrium has been reached after stirring in the dark for 10 min. in our case.

Figure 1. Adsorption curve of the samples in formaldehyde solutions.

3. Results and discussion

3.1. Photocatalytic activity

Volatile organic compounds, such as formaldehyde, may cause a serious threat to groundwater, indoor environments, and even human health. Liquid effluents with high formaldehyde concentrations are difficult to treat using conventional processes.[15] Interest in the photocatalytic oxidation of formaldehyde from contaminated wastewater is growing rapidly. Figure 2 shows the decomposition kinetics of formaldehyde solutions using pure TiO₂, TiO₂–SiO₂, and Fe³⁺/F⁻ co-doped TiO₂–SiO₂ films on glass substrates under visible light irradiation. From Figure 2, it can be seen that the photocatalytic activity of the Fe³⁺/F⁻ co-doped TiO₂–SiO₂ film is highest in three samples. The degradation rate of the formaldehyde solution using the Fe³⁺/F⁻ co-doped TiO₂–SiO₂ film is about 57% under visible light irradiation for 200 min compared to pure TiO₂ film 8% and TiO₂–SiO₂ film 18%. The photocatalytic activity of the Fe³⁺/F⁻ co-doped TiO₂–SiO₂ film may be attributed to enhance visible light absorption efficiency and reduce the recombination rate of the photogenerated charges.

Figure 2. Decomposition kinetics of formaldehyde solutions using pure TiO₂, TiO₂–SiO₂, and Fe³⁺/F⁻ co-doped TiO₂–SiO₂ films on glass substrates under visible light irradiation for 200 min.

3.2. Photocatalyst characterisation

The UV–Vis absorption spectra (a) and plots of (αhν)^1/2 versus energy (hν) (b) for pure TiO₂, TiO₂–SiO₂, and Fe³⁺/F⁻ co-doped TiO₂–SiO₂ films are presented in Figure 3. From Figure 3(a), the spectrum of the Fe³⁺/F⁻ co-doped TiO₂–SiO₂ film shows a significant enhancement of light absorption at wavelengths of 400–700 nm compared to the pure TiO₂ film and the TiO₂–SiO₂ film. Fe³⁺/F⁻ co-doping can result from the formation of the dopant energy level within the band gap of TiO₂. The levels introduced by F merge with the valence band of TiO₂ whereas the states introduced by Fe are placed near the middle of the band gap of TiO₂. Thus, a low concentration co-doping of cations and anions can effectively narrow the band gap thus enhancing the visible light absorption efficiency.[11] The optical band gap can be deduced from the equation for a crystalline semiconductor, αhν = A(hν - E_g)^n/2, where α, ν, E_g, and A are the absorption coefficient, light frequency, band gap, and a constant, respectively. The value of n is determined by the type of optical transition of a semiconductor. Therefore, the band gap energy of TiO₂ can be determined from a plot of (αhν)^1/2 versus energy (hν),[16] as is shown in Figure 3(b). The band gap of pure TiO₂, TiO₂–SiO, and Fe³⁺/F⁻ co-doped TiO₂–SiO film is 3.25, 3.15, and 2.85 eV, respectively. These data suggest that Fe³⁺/F⁻ co-doping can result in an obviously narrowing of the band gap of TiO₂. The lower band gap has a positive effect on the photocatalytic activity of TiO₂.

Figure 3. (a) UV–Vis absorption spectra and (b) plots of (αhν)^1/2 versus energy (hν) for pure TiO₂, TiO₂–SiO₂, and Fe³⁺/F⁻ co-doped TiO₂–SiO₂ films.

PL emission is related to the formation of photo-induced charge carriers and their recombination kinetics. Figure 4 shows the PL emission spectra of pure TiO₂, TiO₂–SiO₂, and Fe³⁺/F⁻ co-doped TiO₂–SiO₂ films. The PL intensity of the Fe³⁺/F⁻ co-doped TiO₂–SiO₂ film is the lowest among the three samples indicating that the recombination of electrons and holes was effectively prohibited. In the Fe³⁺/F⁻ co-doped TiO₂–SiO₂ composite system, Fe³⁺ can trap electrons to separate e⁻/h⁺ pairs whereas F⁻ can adsorb holes to separate e⁻/h⁺ pairs. Especially, SiO₂ can provide more effective adsorption sites in the SiO₂–TiO₂ composite system, so the photogenerated holes can reach them before recombination with electrons thus enhancing the photocatalytic activity of TiO₂.[17] This indicates that the synergistic effects of Fe³⁺, F⁻, and SiO₂ can greatly reduce the recombination of photo-induced electrons and holes in TiO₂.

Figure 4. PL spectra of pure TiO₂, TiO₂–SiO₂, and Fe³⁺/F⁻ co-doped TiO₂–SiO₂ films.

Surface area is one of the important factors in the photocatalytic activity of TiO₂ because the reaction takes places initially by the adsorption of O₂ and H₂O on the surface of the photocatalyst to generate oxidising agents by electrons and holes, respectively. Figure 5 shows the nitrogen adsorption/desorption isotherms of TiO₂, TiO₂–SiO₂, and Fe³⁺/F⁻ co-doped TiO₂–SiO₂ powders calcined at 450 °C in the air for 1 h. The specific surface areas of the samples are calculated according to the BET method. In our case, the specific surface area of pure TiO₂, TiO₂–SiO₂, and Fe³⁺/F⁻ co-doped TiO₂–SiO₂ are 95.9, 294.3, and 397.0 m² g⁻¹, respectively. The incorporation of SiO₂ and Fe³⁺/F⁻ into the TiO₂ catalysts can reduce the particle size thus increasing the specific surface area, while the large surface area can effectively adsorb more H₂O, O₂, and pollutants onto the reaction sites of the catalysts.

Figure 5. N₂ adsorption/ desorption isotherms of pure TiO₂, TiO₂–SiO₂, and Fe³⁺/F ⁻ co-doped TiO₂–SiO₂ powders.

Figure 6 shows the XRD spectra of pure TiO₂, Fe³⁺/F⁻ co-doped TiO₂–SiO₂ samples calcined at 450 °C in air for 1 h. Pure TiO₂ exhibit a mixed crystallinity composed of anatase (25.43°, 37.94°, and 48.08°), rutile (27.47° and 36.26°), and brookite (30.77°). However, only the anatase phase is observed in the Fe³⁺/F⁻ co-doped TiO₂–SiO₂ sample. The introduction of SiO₂ and the addition of Fe³⁺/F⁻ can effectively suppress the anatase-to-rutile phase transformation and prevent the formation of brookite phases. This is because SiO₂ is well dispersed in TiO₂,[12,18] or atoms are well-inserted into the crystal lattice of TiO₂ as interstitial atoms.[19] However, the overall peak intensities of the anatase TiO₂ decrease with the insertion of SiO₂ and Fe³⁺/F⁻, indicating that the addition of SiO₂ and Fe³⁺/F⁻ can also effectively suppress the growth of TiO₂ crystals. No SiO₂ diffraction peaks are observed because the SiO₂ is an amorphous phase at this temperature.[20] No obvious Fe- and F-related compound peaks are observed indicating that the small amounts of Fe and F dopants are uniformly distributed on TiO₂. It is well documented that the anatase phase has a higher photoactivity compared to rutile or brookite phases.[21]

Figure 6. XRD patterns of pure TiO₂, and Fe³⁺/F⁻ co-doped TiO₂–SiO₂ samples.

It is accepted that the photocatalytic activity of TiO₂ is related to its surface morphology. The surface morphology of the Fe³⁺/F⁻ co-doped TiO₂–SiO₂ film is very different from that of pure TiO₂ film calcined at 450 °C in air for 1 h, as shown in Figure 7. The surface of the Fe³⁺/F⁻ co-doped TiO₂–SiO₂ film is smoother without cracks compared to that of a pure TiO₂ film. Moreover, the multi-modified TiO₂ film is composed of smaller spherical nanoparticles with a lower level of agglomeration compared to a pure TiO₂ film. The amorphous silica framework segregates at the anatase TiO₂ grain boundary and retards TiO₂ species from aggregating together.[22] EDS indicates that the weight per cents of Ti, O, Si, Fe, and F in the Fe³⁺/F⁻ co-doped TiO₂–SiO₂ film are 23.24%, 55.38%, 17.31%, 1.02%, and 3.05%, respectively, as shown in Figure 8.

Figure 7. FE-SEM images of (a) pure TiO₂ and (b) Fe³⁺/F⁻ co-doped TiO₂–SiO₂ composite films calcined at 450 °C in air for 1 h.

Figure 8. EDS images of the Fe³⁺/F⁻ co-doped TiO₂–SiO₂ composite film calcined at 450 °C in air for 1 h.

In a word, the Fe³⁺/F⁻ co-doped TiO₂–SiO₂ film exhibit excellent photocatalytic activity in the degradation of formaldehyde under visible light irradiation. The enhanced visible-light activity may originate from the synergistic effects of Fe³⁺, F⁻, and SiO₂. First, Fe³⁺/F⁻ co-doping can decrease the band gap of the TiO₂ from 3.25 to 2.85 eV, which may facilitate its absorption of visible light. Second, the synergistic effects of Fe³⁺, F⁻, and SiO₂ can greatly reduce the recombination of photo-induced electrons and holes in TiO₂ thus enhancing the photocatalytic activity of TiO₂. Third, the large specific surface areas caused by Fe³⁺/F⁻ co-doping and SiO₂ compositing can improve the adsorption of the pollutants. These factors can result in excellent visible-light photocatalytic activity. The major steps in the mechanism may be summarised by Equations (1(1) )–(4(4) ).[1,23–25] (1) (2) (3) (4)

4. Conclusions

A highly photoactive nanocrystalline Fe³⁺/F⁻ co-doped TiO₂–SiO₂ film has been successfully synthesised using the sol–gel method. The multi-modified TiO₂ film shows strong visible light adsorption compared to a pure TiO₂ film and a singly modified film. Experimental results show that the Fe³⁺/F⁻ co-doped TiO₂–SiO₂ film is effective in the degradation of formaldehyde under visible light irradiation. The enhanced photocatalytic activity of the multi-modified TiO₂ film can be understood on the basis of increasing photo generated species and reducing the recombination rate of electrons and holes.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was financially supported partially by the National Natural Science Foundation of China [grant number 51102114]; Shandong Provincial Natural Science Foundation, China [ZR2012BM009]; the Ningxia Science and Technology Support Program, China [2013ZYG035].