Effects of Calcination Temperature on Preparation of Boron-Doped TiO 2 by Sol-Gel Method

Boron-doped TiO2 photocatalyst was prepared by a modified sol-gel method. Being calcinated at temperatures from 300◦C to 600◦C, all the 3% B-TiO2 samples presented anatase TiO2 phase, and TiO2 crystallite sizes were calculated to be 7.6, 10.3, 13.6, and 27.3 nm, respectively. The samples were composed of irregular particles with rough surfaces in the size range within 3 μm. Ti atoms were in an octahedron skeleton and existed mainly in the form of Ti4+, while the Ti-O-B structure was the main boron existing form in the 3% B-TiO2 sample. When calcination temperature increased from 300◦C to 600◦C, specific surface area decreased sharply from 205.6 m2/g to 31.8 m2/g. The average pore diameter was 10.53 nm with accumulative pore volume of 0.244 mL/g for the 3% B-TiO2 sample calcinated at 400◦C, which performed optimal photocatalytic degradation activity. After 90 min of UV-light irradiation, degradation rate of methyl orange was 96.7% on the optimized photocatalyst.


Introduction
In recent years, many kinds of metal or nonmetal doped TiO 2 photocatalysts were prepared because semiconductive TiO 2 was considered to be the most attractive photocatalyst due to its properties of chemically stable, nontoxic, high efficient, and relatively inexpensive [1,2].TiO 2 has attracted much attention in view of its practical applications such as self-cleaning surfaces, wastewater and air purification, bacteria inactivation, and CO 2 photoconversion to methane and low hydrocarbons.Many environmental pollutants can be degraded by oxidation and reduction processes on TiO 2 surface [3][4][5].However, the application of TiO 2 is limited by its UV activation requirement because of its large band gap (3.2 eV in the anatase phase), and recombination rate of photogenerated electrons and holes is usually very quickly.
Doping technology is one of the effective means to overcome the disadvantages of TiO 2 .Since Asahi et al. [6] found out that N doped into TiO 2 effectively enhanced the photocatalytic activity of TiO 2 , there has been an explosion of interest in TiO 2 doping with non-metal ions because of high thermal stability and low carrier recombination centers of nonmetals doped TiO 2 nanostructures.A variety of nonmetal ions such as N [7][8][9], C [10], S [11], F [12], P [13], I [14], and B [15,16] has been explored to promote separation of photogenerated charges in TiO 2 .Due to the electron deficiency structure of boron, boron-doped TiO 2 has already attracted much attention, and researches have increasingly focused on the development of boron doped TiO 2 systems in recent years.Zhao et al. [17] reported that doping with B can extend the spectrum response of TiO 2 to visible region and thus can improve its visible light photocatalytic activity.Chen et al. [18] found that B-doped TiO 2 showed higher photocatalytic activity than that of pure TiO 2 in photocatalytic NADH regeneration.They ascribed the improvement of photocatalytic activity to the formation of Ti 3+ , which can facilitate the separation of photoexcited electrons and holes and slow their recombination rate.Yuan et al. [19] prepared B and N codoped TiO 2 photocatalyst via sol-gel method and found that interstitial N and [NOB] species in the TiO 2 crystal lattice narrowed band gap and extended optical absorption of TiO 2 .Xu et al. [20] indicated that low-temperature hydrothermal method could be used to prepare boron-doped TiO 2 , and the photocatalyst showed larger surface area and higher photocatalytic activity than that prepared by sol-gel method.Zaleska et al. [21] used a simple surface impregnation method to prepare International Journal of Photoenergy boron-modified TiO 2 , and boron as a B-O-Ti species existed in the surface of TiO 2 grains.
Despite the work dedicated to the properties of photoactive B-TiO 2 , most work focused on effects of B doping amounts.Very limited published works were related to the effects of calcination temperature on properties of B-doped TiO 2 .In the present work, 3% boron-doped TiO 2 -based photocatalysts were prepared by sol-gel method at different calcination temperatures using tributyl borate as boron precursor, and their characteristics were investigated by XRD, SEM, FT-IR, XPS, surface area (BET), and porosity determination (BJH).Photocatalytic degradation of an organic azodye, methyl orange, was investigated under ultraviolet irradiation.The effects of calcination temperature on structure, surface area, crystallinity, and photocatalytic activity of B-TiO 2 photocatalyst were systematically investigated.

Experimental
2.1.Preparation of B-TiO 2 Photocatalysts.3 wt% borondoped TiO 2 photocatalysts were prepared by a modified solgel approach.The detailed process was described as follows.Tetrabutyl titanate of chemical pure grade was chosen as the Ti precursor and tributyl borate (99.5%) was used as the boron source.Hydrochloric acid (HCl) and anhydrous ethanol were in the analytical reagent grade.8 mL anhydrous ethanol and 0.1 mL hydrochloric acid were mixed in a beaker, and then 2 mL tetrabutyl titanate and desired volume of tributyl borate were dropwisely added to the former solution under constant magnetic stirring to prepare solution 1.Meanwhile, 1 mL of distilled water was mixed with 4 mL anhydrous ethanol to prepare solution 2. After solution 1 was stirred for 30 min, solution 2 was dropwisely added into solution 1.The final mixed solution was continuously stirred until the formation of a gel.After aging for 24 h at room temperature, the gel was dried at 80 • C for 8 h.Subsequently, the obtained solid was grinded and calcinated at different temperatures for 3 h, respectively.The obtained 3% borondoped TiO 2 was ascribed as 3% B-TiO 2 in the following experiments.

Catalyst
Characterization.X-ray diffraction (XRD) patterns were obtained by a Rigaku D/Max-rB diffractometer using Cu Ka radiation.The XRD estimation of crystallite size was based on the Scherrer formula.Scanning electron microscopy (SEM, Hitachi, S-3400N) was used for morphology characterization of B-doped TiO 2 crystal.The samples for SEM imaging were coated with a thin layer of gold film to avoid charging.FT-IR spectra of the samples were obtained using a Fourier transform infrared (FT-IR) spectrometer (WQF-410) with KBr pellets.The samples were analyzed in the wavenumber range of 4000-400 cm −1 .The elemental composition of 3% B-TiO 2 nanocrytals was determined by X-ray photoelectron spectroscopy (XPS, MULTILAB2000).Specific surface area measurements were performed using a surface area and pore size analyzer (F-sorb 3400).The specific surface area was determined by the multipoint BET method using the adsorption data in the relative pressure (P/P 0 ) range of 0.05-0.25.The desorption isotherm was used to determine pore size distribution using the Barrett, Joyner, and Halenda (BJH) method.

Measurement of Photocatalytic Activity.
The effectiveness of B-doped TiO 2 nanocrystal was evaluated by degradation of methyl orange (MO) solution under UV light irradiation.Before photocatalytic experiment, adsorption of MO solution in the dark on the 3% B-TiO 2 photocatalyst was measured in the suspension.50 mL of 10 mg/L methyl orange aqueous solution was mixed with 30 mg photocatalyst in a 250 mL beaker.The suspension was stirred magnetically for 20 min to reach adsorption equilibrium.After that, 5 mL suspension was taken out of the reactor and filtrated through a millipore filter (pore size 0.45 μm) to remove the photocatalyst.Finally, absorbency of the solution was measured using a 721E spectrophotometer at the MO maximum absorption wavelength of 468 nm.Photocatalytic activities of the prepared catalysts were evaluated afterwards.A 20 W ultraviolet lamp was located over the 250 mL beaker with a distance of 11 cm from the lamp to the surface of the solution.The lamp can irradiate UV light at wavelength of 253.7 nm with the intensity of 1100 μW/cm 2 .In prior to turn on the lamp, the solution should ensure adsorption equilibrium according to the above process.Irradiation time in the subsequent experiments was set for 30 min except for the prolonged time reaction.After photocatalytic reaction, 5 mL of the suspension was filtrated through millipore filter to measure the change of MO concentration.

Results and Discussion
3.1.Characterization of B-TiO 2 Photocatalysts.XRD was carried out to investigate phase structure of B-TiO 2 .Figure 1 shows XRD patterns of 3% B-TiO 2 samples that were calcinated at 300, 400, 500, and 600 • C for 3 h, respectively.All the diffraction peaks in the patterns well match the diffraction The small particles among the big ones came from grinding after calcination.The particle size seems quite suitable for suspending in methyl orange solution under magnetic stirring.There was no strong particles aggregation during sol-gel preparation and calcination processes.All of the 3% B-TiO 2 samples can undergo well dispersion without apparent deposition during photocatalytic reaction.
Figure 3 shows FT-IR spectra of 3% B-TiO 2 samples with different calcination temperature.For all samples, the bands at 3384 cm −1 and 1621 cm −1 are assigned to the stretching of hydroxyl groups and the bending vibration of H 2 O adsorbed on the surface of the samples.There are peaks at 469 cm −1 and 670 cm −1 for the stretching of Ti-O bond and the bending vibration of Ti-O bond, respectively.In the IR spectra of the samples, another peak appears at 1397 cm −1 can be ascribed to the vibration of tri-coordinated boron [23].Furthermore, neither absorption peak corresponding to pure B 2 O 3 (1202 cm −1 ) [22] nor peak of incorporated BO 4 (1096 cm −1 ) [24] appears in the spectra.It reveals that boron is introduced into the titania framework in the form of B-O-Ti bond, and this structure is further confirmed by XPS measurements later.However, the peak at 1397 cm −1 [23,25]  boron in Ti-O-B form preferably appears at low temperature calcination.Figure 4(a) shows XPS B1s spectrum of 3% boron-doped TiO 2 photocatalyst prepared by sol-gel method followed by calcination at 400 • C. Normally, B1s electron-binding energy peak situates around 188 eV-194 eV for B-TiO 2 .In the figure, B1s peak can be separated to three independent peaks using XPSPEAK 4.1 software.This means that different chemical forms of B atoms might exist in B-doped TiO 2 nanoparticles.The standard binding energy of B1s in B 2 O 3 or H 3 BO 3 equals to 193.0 eV (B-O bond) and in TiB 2 equals to 187.5 eV (B-Ti bond) [18].In the figure, the first peak at 192.9 eV is related to B-O-B bonds in B 2 O 3 or H 3 BO 3 and the low energy peak at 189.6 eV corresponds to boron incorporated into the TiO 2 lattice through occupying O sites to form O-Ti-B band.Su et al. also pointed out that the peak at 189.6 eV might correspond to B-Ti in TiB 2 [26].The strongest peak at 191.7 eV is related to boron that is probably weaved into the interstitial TiO 2 and exists in the form of Ti-O-B structure [23,27,28].The Ti-O-B structure is the main boron existing form in the 3% B-TiO 2 sample.
Figure 4(b) shows XPS Ti2p spectra of 3% B-TiO 2 .There are two isolated symmetrical peaks in the XPS patterns, showing that Ti atoms are in an octahedron skeleton and existed mainly in the form of Ti 4+ .The binding energies of Ti2p3/2 and Ti2p1/2 for 3% B-TiO 2 sample are at 456.6 eV and 462.2 eV, and distance between Ti2p3/2 and Ti2p1/2 peaks is 5.6 eV (The standard value is 5.6 to 5.7 eV [29]).It was reported that boron doping favored the formation of Ti 3+ on the surface of TiO 2 [30,31], but there is no evidence of Ti 3+ formation in Figure 4(b).
As shown in Figure 4(c), XPS O1s region is composed of three peaks situating at 532.7 eV, 531.7 eV, and 530.2 eV.The first peak at 532.7 eV is related to oxygen in the TiO 2 crystal lattice, and the second peak at 531.7 eV corresponds to the surface hydroxyl groups.The peak at 530.2 eV indicates oxygen in the Ti-O-B bond [21], which is in accordance to the result of XPS B1s region.Therefore, XPS analysis confirms that sol-gel synthesis allows incorporation of boron atoms into TiO 2 matrix.Figure 4(d) shows XPS C1s spectrum of the 3% B-TiO 2 sample.The peak at lower binding energy of 284.8 eV was used for calibration of the XPS results, and the peak at 289.3 eV is attributed to the adsorbed carbon on the sample.
In order to study porous status of the boron-doped TiO 2 materials, Brunauer-Emmett-Teller nitrogen sorption measurements were carried out.N 2 molecules were in single or multiple layers adsorbed on the internal pore surface of the materials.As shown in Figure 5(a), N 2 desorption changed significantly at relative pressure between 0.95 and 0.6 in N 2 desorption process, which was mainly caused by capillary aggregation of N 2 molecules occurring in micropores inside the material [32].Figure 5(b) shows that pore size of 3% B-TiO 2 mainly distributes in the range from 1.5 nm to 18 nm.The average pore diameter of 3% B-TiO 2 calcinated at 400 • C is 10.53 nm, and the accumulative pore volume is 0.244 mL/g for the material.
As shown in Table 1, the specific surface areas of 3% B-TiO 2 samples decreased continuously with increasing calcination temperature.When calcination temperature increased from 300 • C to 600 • C, surface area decreased sharply from 205.6 m 2 /g to 31.8 m 2 /g.The sample calcinated at 300 • C had the largest surface area because organic substances did not burn out totally at low temperature, and the residual carbon contributed to the large BET surface area.Lattice parameters were obtained by using Bragg's law (2d sin θ = λ) and a formula for a tetragonal system, 1/d 2 = (h 2 + k 2 )/a 2 + l 2 /c 2 .As summarized in Table 1, the lattice parameters of all the B-TiO 2 samples changed along with the change of calcination temperature.The cell volumes of 3% B-TiO 2 samples became larger at higher calcination temperature due to accelerated crystal growth at high temperature.It can also be deduced that crystallite sizes of B-TiO 2 could grow up with the increase of calcination temperature, which is confirmed by XRD analysis.activity, as shown in Figure 6.The optimal degradation rate was 31.5% on 3% B-TiO 2 sample calcinated at 400 • C. The highest absorption of the dye was observed on 3% B-TiO 2 calcinated at 300 • C. As previously indicated (see in Table 1), BET surface area of 3% B-TiO 2 calcinated at 300 • C is 205.6 m 2 /g, which is much more than that prepared at higher temperatures.Since 300 • C was a low calcination temperature, some organic substances did not burn out totally, so that carbon residues caused the large BET surface area and high-absorption capacity.Low calcination temperature can be also responsible for insufficient formation of anatase TiO 2 crystals as the reason of low photocatalytic activity.

Photocatalytic Activity of 3% B-TiO
As can be seen from Figure 6, the adsorption of methyl orange changed slightly on the samples when calcination temperature varied from 400 • C to 600 • C, indicating thoroughly burning of organic substances at high temperatures.The sample that was calcinated at 400 • C presented the optimal photocatalytic degradation activity.As described before, BET surface areas of the samples calcinated at 500 • C and 600 • C are smaller than that of the sample calcinated at 400 • C.Meanwhile, FT-IR results show that the formation of Ti-O-B is weakened with the increase of calcination temperature.It can be deduced that the sample calcinated at 400 • C represents the optimal physic-chemical and structural characteristics that are suitable for photocatalytic degradation of methyl orange.
Figure 7 presents the adsorption and photocatalytic activities of 3% B-TiO 2 calcinated at 400 • C with prolonged irradiation time.Adsorption rate did not change noticeably after the dye reached its adsorption equilibrium.After 90 min of UV-light irradiation, degradation rate of methyl orange was 96.7% on 3% B-TiO 2 powder.It indicates that 3% B-TiO 2 has satisfactory photocatalytic activity.
Figure 8 shows absorption spectra of methyl orange aqueous solution in presence of 3% B-TiO 2 calcinated at 400 • C. The maximum MO absorption peak at 468 nm gradually decreased during irradiation process.After 90 min of irradiation, the dye was completely decomposed according to disappearance of the main absorption peak, due to breaking up of methyl orange molecules into small parts under photocatalytic degradation.

Conclusion
Boron-doped TiO 2 photocatalyst was prepared by a modified sol-gel method.Crystallite sizes of boron-doped TiO 2 increased gradually, and BET surface areas of the 3% B-TiO 2 samples decreased sharply with increasing calcination temperature.Calcination temperature had no apparent impact on surface morphology of B-TiO 2 .XPS and FT-IR results proved that the B-O-Ti structure in 3% B-TiO 2 reduced at high calcination temperature.The sample with the optimal photocatalytic activity was obtained after being calcinated at 400 • C. Degradation rate of methyl orange was 96.7% after 90 minutes of UV irradiation.

Table 1 :
Lattice parameters and BET surface areas of 3% B-TiO 2 samples calcinated at different temperatures.
2. Photocatalytic activity of B-doped TiO 2 was evaluated by photocatalytic degradation of methyl orange under UV light irradiation.3% B-TiO 2 prepared at different calcination temperature performed obviously different photocatalytic degradation