Optimization of Boron Doped TiO2 as an Efficient Visible Light-Driven Photocatalyst for Organic Dye Degradation With High Reusability.

No visible light activity is the bottle neck for wide application of TiO2, and Boron doping is one of the effective way to broaden the adsorption edge of TiO2. In this study, several Boron doped TiO2 materials were prepared via a facile co-precipitation and calcination process. The B doping amounts were optimized by the degradation of rhodamine B (Rh B) under visible light irradiation, which indicated that when the mass fraction of boron is 6% (denoted as 6B-TiO2), the boron doped TiO2 materials exhibited the highest activity. In order to investigate the enhanced mechanism, the difference between B-doped TiO2 and bare TiO2 including visible light harvesting abilities, separation efficiencies of photo-generated electron-hole pairs, photo-induced electrons generation abilities, photo-induced charges transferring speed were studied and compared in details. h+ and · O 2 - were determined to be the two main responsible active species in the photocatalytic oxidation process. Besides the high degradation efficiency, 6B-TiO2 also exhibited high reusability in the photocatalysis, which could be reused at least 5 cycles with almost no active reduction. The results indicate that 6B-TiO2 has high photocatalytic degradation ability toward organic dye of rhodamine B under visible light irradiation, which is a highly potential photocatalyst to cope with organic pollution.

Introduction

Environmental problems are global issues and effect all of n class="Species">human kind. These issues and pressures increase in severity as society continues to develop at a very fast pace (Samanta et al., 2002; Shao et al., 2017; Chen et al., 2018; Chowdhary et al., 2018; Tian et al., 2018; Hong et al., 2020). Water pollution is one of the most serious environmental problems and attracts much attention (Wang and Yang, 2016; Jiang et al., 2019b; Kapelewska et al., 2019; Quesad et al., 2019; Wu et al., 2019; Zhao et al., 2019b). Organic dyes have been synthesized on a large scale and are widely applied in our daily lives, resulting in tons of dyes being discharged into the aqueous environment every year, causing many serious environmental problems (Sohni et al., 2019; Tu et al., 2019; Zhan et al., 2019; Zhou X. et al., 2019). Rhodamine B is a toxic alkaline cationic dye, which was used as a food additive, but has been forbidden due to its high carcinogenic potential (Wu et al., 2018; Lops et al., 2019; Tian et al., 2019; Guo et al., 2020). Furthermore, it can also cause other serious diseases such as visceral disease and red skin staining (Alcocer et al., 2018; Liu et al., 2019; Maria Magdalane et al., 2019). It is very difficult to degrade rhodamine B under natural conditions. Methods for the effective removal of rhodamine B are therefore of great importance.

In recent decades, photocatalysis has exhibited its high potential in waste n class="Chemical">water treatment, due to its inpan>herent merits inpan>cludinpan>g low costs, renewability, beinpan>g environment-friendly, and its high efficiency. n class="Chemical">TiO2 is a widely used photocatalyst because of its chemical stability, high redox reactivity, easy preparation, and low cost. However, it can not adsorb and use visible light because of its wide energy band gap. So only ~4% solar energy of ultraviolet light can be used by TiO2, and 45% solar energy of visible light can not be used (Jiang et al., 2018a, 2019a; Yin et al., 2018; Ahadi et al., 2019; Zhou F. et al., 2019; Komtchoua et al., 2020). In order to broaden the light adsorption of TiO2 to visible light, many efforts have been conducted (Zhao et al., 2019a; Zhang et al., 2020). Doing has attracted increasing interest in recent years (Jiang et al., 2018a; Kamaludin et al., 2019; Lu et al., 2019; Xiu et al., 2019; Yan et al., 2019).

In this study, n class="Chemical">boron was used to dope into n class="Chemical">TiO2 to prepare the photocatalyst of B-TiO2. B-TiO2 shows high photocatalytic degradation ability toward rhodamine B under visible light. The preparation conditions were optimized, and the structure and photocatalytic performance of B-TiO2 were carefully investigated. Based on the experimental results, the photocatalytic degradation mechanism was discussed. This study indicates that B-TiO2 has the potential to treat dye pollution through visible light irradiation.

Experiment

Materials

All the chemicals used in this study were of analytical pure grade. n class="Chemical">Boric acid and aqueous n class="Chemical">ammonia were purchased from Xilong Science Co., Ltd, China. Rhodamine B and tetrabutyl titanate were bought from the Aladdin reagent company, China. Other chemicals are all commercial. Deionized water (DI water) was used throughout the study. The materials were directly used without any treatment.

Synthesis of B-TiO2

0.1 mol n class="Chemical">tetrabutyl titanate was dissolved into 100 ml absolute n class="Chemical">alcohol to form a clean solution. Boric acid was dissolved into a solution containing 2 ml nitric acid, 50 ml absolute alcohol and 50 ml DI water. After that, the tetrabutyl titanate solution described above was dripped into the boric acid solution and vigorously stirred. At the same time, aqueous ammonia was dripped into the mixture described above to adjust the pH value to 7. The formation of precipitation was found in the process. After being aged for 5 days, the precipitation was separated and dried at 110°C. Finally, it was calcinated at 500°C to obtain the B doped TiO2 denoted as B-TiO2. The feeding amounts of boric acid were changed to obtain B-TiO2 with a B mass ratio of 0, 3, 6, 9, 12, and 15%, and are denoted as TiO2, 3B-TiO2, 6B-TiO2, 9B-TiO2, 12B-TiO2, and 15 B-TiO2, respectively. The methodology is shown in Scheme 1.

Scheme 1

Synthesis scheme of B-TiO2.

Synthesis scheme of n class="Chemical">B-TiO2.

Photocatalytic Degradation

Ten milligrams of photocatalyst was added into 25 ml n class="Chemical">rhodamine B solution at a concentration of 5 mg/L. The mixture was stirred inpan> the dark for 30 minpan> to obtainpan> adsorption equilibrium. After that, the mixture was irradiated under visible light by a 500 W Xe lamp (Pern class="Chemical">fectlight, Beijing, China) with λ ≦ 400 nm cutoff, and sampled at determined intervals to examine the rhodamine B concentration in the suspension, which was determined by the adsorption at 552 nm.

Characterization

The morphology of the sample was investigated by scanning electron microscopy (SEM) (Hitachi-4800, Japan) and a transmission electron microscope (TEM) (JEM-2100, Japan). An X-ray powder diffractometer (XRD, Rigaku III/B max, n class="Chemical">Cu Ka) was used to analyze the samples. The pH values of solutions were determinpan>ed by a JENCO 6175 pH meter (Renshi electronics Co. Ltd. USA). Electrochemical impedance spectroscopy (EIS) and photon class="Chemical">current response analysis were performed by CHI660C electrochemical workstation (Shanghai Chenhua, China). Photoluminescence (PL) spectra were recorded on a F-7000 fluorescence spectrophotometer (Hitachi, Japan). UV-vis diffuse-reflectance spectra (DRS) of samples were obtained on a UV-vis-NIR spectrometer (Lambda 900).

Results and Discussion

Optimization of B Doping Amount

The bare n class="Chemical">TiO2 and B dopinpan>g n class="Chemical">TiO2 were investigated by XRD analysis, and the results are shown in Figure 1. It can be seen that the samples show similar XRD characteristics. However, it is notable that there is a new peak at ~25.5°, which is attributable to rutile TiO2 (Wang et al., 2015; Warkhade et al., 2017), appears in the spectra of B doping TiO2, but is not the case in the spectrum of bare TiO2. This phenomenon indicates that under the experimental conditions of this study, the doping of B, no matter how much the doping amount is, can induce the pure anatase TiO2 to transform to mixed crystal phases of anatase and rutile (Cui et al., 2017). These results indicate that B has been successfully doped into the crystal lattice of TiO2.

Figure 1

XRD spectrum of B-TiO2 and bare TiO2.

XRD spectrum of n class="Chemical">B-TiO2 and bare n class="Chemical">TiO2.

All the samples, including the bare n class="Chemical">TiO2 and the B dopinpan>g n class="Chemical">TiO2, were used to degrade rhodamine B under visible light irradiation. As shown in Figure 2, the degradation ability first increases with the B doping amount, and when the B doping amount reaches 6%, the degradation ability decreases with the rise of B doping amount. This phenomenon indicates that 6% of the B mass ratio is the optimal value. Thereafter, 6B-TiO2 was determined as the optimal sample, and 6% was determined as the optimal feeding amount of B in the preparation.

Figure 2

The photocatalytic degradation toward rhodamine B over B-TiO2 with different B doping amount.

The photocatalytic degradation toward n class="Chemical">rhodamine B over n class="Chemical">B-TiO2 with different B doping amount.

Characterization of B Doping TiO2 Samples

In order to investigate the mechanism of the enhanced photocatalytic performance of n class="Chemical">6B-TiO2, the samples of bare n class="Chemical">TiO2 and B doping TiO2 were investigated by photocurrent response, EIS and PL spectrum, and the results are shown in Figure 3. It can be seen in Figure 3A that 6B-TiO2 shows highest photocurrent response, indicating that more electrons can be generated in 6B-TiO2 by visible light irradiation (Hu et al., 2019; Murali et al., 2019; Wang et al., 2019). In the EIS analysis (Figure 3B), 6B-TiO2 exhibits a semicircle of the EIS Nyquist plot with the smallest radius, indicating the smallest interfacial charge transference impedance as compared to those of bare TiO2 and other B doping TiO2 (Dai et al., 2015; Zou et al., 2016; Manwar et al., 2019). As shown in Figure 3C, 6B-TiO2 indicates the lowest photoluminescence intensity, which suggests that 6B-TiO2 has the lowest recombination rate of electron-hole pairs (Cai et al., 2019; Huang et al., 2019; Yuan et al., 2019). It can be found from the above analysis, that the most electrons can be generated by visible light in 6B-TiO2, and the charge carriers can transfer in 6B-TiO2 with the lowest impedance and recombination rate. All of these characteristic can favor the subsequent photocatalytic reaction, so there is no doubt that 6B-TiO2 exhibits the best photocatalytic performance as compared to bare TiO2 and other B doping TiO2.

Figure 3

(A) Photocurrent response, (B) EIS, and (C) PL spectrum of bare TiO2 and B doping TiO2.

(A) Photon class="Chemical">current response, (B) EIS, and (C) PL spectrum of bare n class="Chemical">TiO2 and B doping TiO2.

The morphology of n class="Chemical">6B-TiO2 was investigated by SEM and TEM. As shown in Figures 4a,bn class="Chemical">, 6B-TiO2 exhibits nano spheral morphology with a litter aggregation. Elemental mapping indicates that elements of B, O, and Ti homogeneously distribute on the surface of 6B-TiO2, confirming the successful doping of B (Figures 4c–f).

Figure 4

(a) SEM and (b) TEM imagines of 6B-TiO2; (c–f) the elemental mapping of 6B-TiO2.

(a) SEM and (b) TEM imagines of n class="Chemical">6B-TiO2; (c–f) the elemental mapping of n class="Chemical">6B-TiO2.

The bare n class="Chemical">TiO2 and n class="Chemical">6B-TiO2 were investigated by DRS, and the results are shown in Figure 5A. It can be seen that the light adsorption edge of bare TiO2 is about 383 nm, indicating no visible light adsorption activity. As B was doped to form 6B-TiO2, the light adsorption edge significantly red shift to about 411 nm, which indicates that 6B-TiO2 can adsorb visible light. The band gap energies of the two samples were calculated by Tauc plot according to the DRS results by Jiang et al. (2018b); Kato et al. (2019); Khan et al. (2019), and are shown in Figure 5B. As one can see, bare TiO2 has a wide band gap of 3.35 eV, which is too high to be excited by visible light. However, after B doping, the band gap was narrowed to 2.85 eV, and can be excited to produce electrons by visible light. The results clearly show that B doping significantly narrows the band gap of TiO2, and broadens the light adsorption edge to a visible light range.

Figure 5

(A) Uv-vis DRS and (B) Tauc plot of bare TiO2 and 6B-TiO2.

(A) Uv-vis DRS and (B) Tauc plot of bare n class="Chemical">TiO2 and n class="Chemical">6B-TiO2.

Investigation of the Active Species in the Photocatalytic Degradation

In order to study the photocatalytic mechanism, the potential active species in the photocatalytic degradation course were investigated. There are usually four active species involved in the photocatalytic degradation course. They are ·, ·OH, e−, and h+. 0.05 mmol difn class="Chemical">ferent scavengers were added, respectively, inpan> the photocatalytic degradation system, and other conditions were the same as described inpan> “2.3 Photocatalytic degradation.” The scavengers are n class="Chemical">i-propanol (·OH scavenger), triethanolamine (h+ scavenger), 1, 4-Benzoquinone (·), and AgNO3 (e− scavenger) (Asmus et al., 1967; Zhang et al., 2013; Huyen et al., 2018). It can be seen in Figure 6 that h+ and · play the most important roles in the degradation of rhodamine B, because the addition of the scavengers for these two active species can dramatically decrease the degradation capacity. One can also find that ·OH takes part in the degradation, because the addition of i-propanol can impress the degradation too. h+, · and ·OH are three highly oxidative species. The results indicate that their high oxidative activities may be used to degrade rhodamine B in this photocatalysis course. It is notable that the addition of AgNO3 can promote the degradation. The reason is that AgNO3 can consume e− as an e− scavenger, which depress the recombination of e−-h+ pairs, and enhance the photocatalytic ability.

Figure 6

Photocatalytic degradation of rhodamine B over 6B-TiO2 in the presence of different scavengers.

Photocatalytic degradation of n class="Chemical">rhodamine B over n class="Chemical">6B-TiO2 in the presence of different scavengers.

Investigation of Reusability

In order to evaluate of the reusability of n class="Chemical">6B-TiO2, the used n class="Chemical">6B-TiO2 was collected and washed wth DI water. After being dried, it was used in the photocatalytic degradation of rhodamine B again, and the experimental conditions are the same as descried in “2.3 Photocatalytic degradation” with little modification of sampling time. As one can see in Figure 7, 6B-TiO2 can be continuously used to effectively degrade rhodamine B at least in five cycles.

Figure 7

The reusability of 6B-TiO2.

The reusability of n class="Chemical">6B-TiO2.

Conclusion

B doping n class="Chemical">TiO2 were successfully prepared inpan> this study, and n class="Chemical">6B-TiO2 was determined as the optimal B doping amount. 6B-TiO2 shows the best photocurrent response ability, fastest charge transference speed and lowest recombination rate of e−-h+ pairs, which significantly enhances its photocatalytic performance. B doping significantly narrows the band gap of TiO2, and therefore broaden the light adsorption edge to visible light range. h+ and · are the most important active species in the photocatalytic degradation, and ·OH is also involved in the degradation. 6B-TiO2 shows high reusability, which can be effectively used in at least five degradation cycles. 6B-TiO2 is a potential photocatalyst with visible light responsible ability, which can be used to effectively treat dye pollution.

Data Availability Statement

All datasets generated for this study are included in the article/supplementary material.

Author Contributions

Pn class="Chemical">N did the experiments. PC and HJ designed the experiments. GW, HZ, and QC were devoted to the disn class="Chemical">cussion and analysis.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.