Solar Photocatalytic Degradation of Typical Indoor Air Pollutants Using TiO 2 Thin Film Codoped with Iron ( III ) and Nitrogen

A type of iron and nitrogen codoped titania thin filmwas prepared by sol-gel method to degrade three typical indoor air pollutants: formaldehyde (HCHO), ammonia (NH 3 ), and benzene (C 6 H 6 ) under solar light. X-ray diffraction (XRD), UV-Vis spectroscopy, and energy dispersive spectra (EDS) were employed to characterize the photocatalysts. The results showed that the Fe/N codoped TiO 2 had a stronger absorption in the visible region than pure, Fe-doped, and N-doped TiO 2 and exhibited excellent photocatalytic ability for the degradation of indoor HCHO, NH 3 , and C 6 H 6 . When the three pollutants existed in indoor air at the same time, the removal percentages of HCHO, NH 3 , or C 6 H 6 after 6 h photocatalytic reaction under solar light reached 48.8%, 50.6%, and 32.0%. The degradation reaction of the three pollutants followed the pseudo-first-order kinetics with the reaction rate constants in the order of 0.110 h for ammonia, 0.109 h for formaldehyde, and 0.060 h for benzene. The reaction rate constant decreased with the increase of initial reactant concentration, which reflected that there was oxidation competition between the substrate and its intermediate during the photocatalytic process.


Introduction
Indoor air quality within buildings has been paid great attention, since many metropolitans generally spend more than 80% of time in indoor environment [1].Indoor air pollutants mainly include carbonyl compounds, volatile organic compounds, ammonia, particulate matter, and so forth.Among them, formaldehyde, ammonia, and benzene are the most representative, which come from the furnishings and decorating materials, causing nausea, chest tightness, wheezing, skin rashes, allergic reaction, and chronic poisoning [2].
Photocatalysis is an emerging and promising technology for the indoor air purification [3], and titania has been identified as the most effective and useful photocatalyst because of its outstanding physical and chemical properties [4][5][6][7][8].However, with a wide band gap energy of 3.0-3.2eV, titania cannot be activated to generate photoexcited electrons and holes to promote redox reaction unless it is irradiated by ultraviolet.But the ultraviolet light energy only accounts for 4-6% of solar energy reaching the ground, so the solar energy cannot be utilized efficiently in the photocatalytic process, which hinders the application of TiO 2 as a photocatalyst with response to solar light [6,9].
In the enhancement of the photoresponse of TiO 2 from ultraviolet to the visible range without decreasing photocatalytic activity, the modification to titania by doping with metal or nonmetal has been considered as one of the most promising methods [10][11][12][13][14][15].Choi et al. conducted a systematic study of metal ions doping into TiO 2 for 21 metal ions [16].Among various transition metal ions, Fe 3+ was considered to be a successful doping element due to its half-filled electronic configuration [17].Nitrogen and carbon doping have received attention due to low costs and the demonstration of band-gap narrowing, with significant improvement in visible light absorption capability [18].Recent research results showed that the modification to titania by codoping may be a more effective method to improve the photocatalytic activity under visible light [19,20].Zhao et al. prepared the B-Ni codoped photocatalyst using the modified sol-gel method.They pointed out that incorporation of B into TiO 2 could extend the spectral response to the visible region and that Ni doping could increase greatly the photocatalytic activity [21].
In this paper, a new solar photocatalyst, TiO 2 thin film codoped with iron(III) and nitrogen, was developed with flat glass as carrier to degrade three typical indoor air pollutants, formaldehyde, ammonia, and benzene, under solar light.The research finding provides a feasible way for enhancement of indoor air quality.

Experimental Procedure
2.1.Photocatalyst Preparation.Pure and doped TiO 2 thin films were prepared by sol-gel technique.The schematic diagram of preparing procedures of gels was described in Figure 1.
Then the plate glass was inserted vertically into the gel.After keeping for 30 seconds, it was pulled out of the gel with the speed of 12 cm/min.Thus, the gel film was formed on the glass surface.Then the glass coated with the gel was dried for 30 min at 100 ∘ C. At last, the glass was calcined for 4 h at 450 ∘ C (20 ∘ C/min).

Photocatalyst Characterization.
The crystal structures of pure and doped TiO 2 were determined by a D-max-2500/PC X-ray diffractometer (XRD) equipped with Cu-Ka radiation.The ultraviolet-visible (UV-Vis) absorption spectra of pure and doped TiO 2 thin films were recorded on a Shimadzu (Japan) UV-2550 spectrophotometer.The energy dispersive spectra of the Fe/N codoped TiO 2 thin film were recorded on JEM-2010 transmission electron microscopy (EDS).

Photodegradation of Gaseous Indoor
Pollutants.Photocatalytic experiments with the prepared photocatalysts were carried out in a self-designed, cuboid, airtight, glass reactor (60 × 60 × 20 cm) with two holes, which were connected to sampling tubes of KC-6D air sampler.An electric fan was installed on the bracket of the airtight reactor in order to circulate the mixture of gaseous pollutants and air.Eight pieces of coated glass were placed vertically and evenly in the reactor.A predetermined amount of pollutant was injected in vessel under dark.The sunlight illumination was started following a dark period of 2 h which was sufficient to attain adsorption equilibrium with the pollutants.The residual concentration of pollutant was periodically measured every 60 min.

UV-Vis Spectra.
Figure 3 shows the ultraviolet-visible (UV-Vis) absorption spectra of the undoped TiO 2 thin film, Fe-doped TiO 2 thin film (the doping mole ratio is 1.00%), N-doped TiO 2 thin film (the doping mole ratio is 25.0%), and Fe/N codoped TiO 2 thin film (the doping mole ratios of iron and nitrogen are 1.00% and 25%, resp.).According to Figure 3, all the doped TiO 2 have stronger absorption than the undoped TiO 2 in the visible region.Among the photocatalysts, Fe/N codoped TiO 2 thin film has the strongest absorption in the visible region.

EDS Spectra.
Figure 4 shows the energy dispersive spectra of the Fe/N codoped TiO 2 with the doping mole ratio of iron and nitrogen are 1.00 and 25.0%, respectively.The result showed that the iron and nitrogen were doped in the TiO 2 photocatalyst successfully.The mole ratio of Fe and Ti was very close to 1%, and the mole ratio of N and Ti was very close to 25%.

The Optimal Doping Amount of Iron and Nitrogen.
The solar photocatalytic activity of doped TiO 2 thin films was determined by testing the degradation ratio of formaldehyde after 6 h photocatalytic reaction.Figure 5 shows the effect of Fe(III) doping amount on the photocatalytic activity of Fedoped TiO 2 thin films, and Figure 6 shows the effect of N doping amount on the photocatalytic activity of N-doped TiO 2 thin films.From Figures 5 and 6, we could see that the solar photocatalytic performance of doped TiO 2 thin films exceeded that of pure TiO 2 thin film, indicating that both of Fe and N dopant could increase the solar photocatalytic activity.Among the Fe-doped TiO 2 thin films, the film with the doping mole ratios of iron was 1.00% and owned the best solar photocatalytic activity.And the solar photocatalytic activity of N-doped TiO 2 thin film with the doping mole ratio of nitrogen was 25.0% and was the best among the N-doped   TiO 2 thin films.So the Fe/N codoped TiO 2 thin film was prepared with the doping mole ratio of iron and nitrogen that was 1.00 and 25.0%, respectively.

Degradation of Formaldehyde, Ammonia, and Benzene.
Formaldehyde, ammonia, and TVOC exist in the indoor air at  the same time usually.So, we not only studied the photocatalytic degradation of formaldehyde, ammonia, and benzene when they exist in indoor air separately but also studied the photocatalytic degradation of the mixed formaldehyde, ammonia, and benzene in the indoor air. Figure 7 shows the degradation ratios of separate formaldehyde, ammonia, and benzene and the mixture of three pollutants after 6 h of photocatalytic reaction under solar light.As shown in Figure 7, appropriate amount of Fe 3+ and N codoped into the TiO 2 thin film was helpful to further improve photocatalytic activity.The removal percentage of separate HCHO in the presence of the Fe/N codoped TiO 2 thin film was 63.1%, which was much greater than that in the presence of Fe-doped TiO 2 thin film (49.8%) and Ndoped TiO 2 thin film (44.0%).The cooperation of Fe 3+ and nitrogen could induce the formation of new energy levels close to the conduction band and valence band, respectively, leading to the much narrowing band gap.Besides, Fe 3+ could trap the photogenerated electrons, while nitrogen could trap the photogenerated holes; thus, the codoping of Fe 3+ and nitrogen could further restrain the recombination of the photogenerated electron and hole and enhance photocatalytic activity [22].Whether the three pollutants, formaldehyde, ammonia, and benzene, existed separately or mixed, they could be removed effectively using the Fe/N codoped TiO 2 thin film.But the removal percentage of formaldehyde, ammonia, and benzene in gas mixture (48.8%, 50.6%, and 32.0%) after 6 h photocatalytic reaction under solar light was lower than those of formaldehyde, ammonia, and benzene existing in air separately (63.1%, 65.8%, and 36.0%).This is due to gas molecules occupying fully the active sites of the photocatalyst, different molecules compete with each other, and the intermediate product of different pollutant accumulating on the photocatalyst surface prevents the effective contact of gas molecules and photocatalyst.

Kinetics of Photocatalytic
Reaction.Formaldehyde, ammonia, and benzene exist in the indoor air at the same time usually, so the kinetics of photocatalytic reaction was studied with the experimental data of mixed pollutants over the Fe/N codoped TiO 2 thin film.Langmuir-Hinshelwood (L-H) equation has been widely used to describe the process of photocatalytic reaction.At low reactant concentration, which is a reasonable assumption for most indoor air pollution problems, the L-H model is simplified to a pseudo-first-order expression ln where the reactant residue (/ 0 ) is a ratio of the instantaneous concentration of gaseous pollutant () to the initial concentration ( 0 ),  is the reaction rate constant, and  is the irradiation time.The experimental data were fitted by plotting − ln(/ 0 ) against irradiation time.The fitting result was shown in Figure 8.As shown in Figure 8, almost good linear relationships with the correlation coefficient  2 > 0.98 were observed, indicating that the degradation reaction of formaldehyde, ammonia, and benzene followed the pseudo-first-order kinetics.The reaction rate constants of three pollutants were in the following order: ammonia (0.110 h −1 ), formaldehyde (0.109 h −1 ), and benzene (0.060 h −1 ).This shows that the photocatalytic reaction is controlled by surface chemical reaction and reaction rate is controlled by reactant concentration.

Effect of Initial Reactant Concentration and Amount of
Catalyst on the Reaction Rate.Since the photocatalytic reaction rate is controlled by reactant concentration, the photocatalytic degradation experiments with different initial concentration of HCHO (3.02 mg/m 3 , 3.88 mg/m 3 , 5.19 mg/m 3 , 6.15 mg/m 3 , and 7.00 mg/m 3 ) as degradation objects were conducted in order to understand controlling mechanism, and the results were shown in Figure 9. From Figure 9, we could see that there was good linear relationship between the rate constant and initial reactant concentration.The relation equation was  = 0.207 − 0.0189 0 ,  2 = 0.956.The rate constant decreased with the increase of initial reactant concentration.This reflected that there was oxidation competition between substrate concentration and its intermediate.
In addition, the amount of the photocatalyst is also the important influencing factor of photocatalytic reaction.Figure 10 shows the effect of the amount of Fe/N codoped TiO 2 (0.01 g, 0.02 g, 0.03 g, 0.04 g, and 0.05 g) on the photocatalytic degradation rate of formaldehyde.The results showed that the photocatalytic reaction rate constant increased with the increase of the amount of photocatalyst.The relation equation was  = 3.16 − 0.0088 with the correlation coefficient  2 = 0.964, and  was the amount of catalyst.

Conclusions
The Fe/N codoped TiO 2 thin film was prepared by means of sol-gel method.The Fe/N codoped TiO 2 exhibited much higher photocatalytic activities than the pure one and those doped with Fe(III) or nitrogen alone under solar light.Indoor