- Email: [email protected]
Synthesis amorphous TiO2 with oxygen vacancy as carriers transport channels for enhancing photocatalytic activity
Materials Letters 265 (2020) 127465
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Synthesis amorphous TiO2 with oxygen vacancy as carriers transport channels for enhancing photocatalytic activity Tao Jia a,1, Jing Zhang a,1, Jiang Wu a,b, Daolei Wang a,⇑, Qizhen Liu c, Yongfeng Qi d, Bin Hu e,⇑, Ping He a, Weiguo Pan a, Xuemei Qi a a
College of Energy and Mechanical Engineering, Shanghai University of Electric Power, Shanghai 200090, China Shanghai Institute of Pollution Control and Ecological Security, Shanghai, China Shanghai Environment Monitoring Center, Shanghai 200030, China d School of Hydraulic Energy and Power Engineering, Yangzhou University, Yangzhou 225127, China e Ninth People’s Hospital Affiliated to Shanghai Jiao Tong University, Shanghai 200011, China b c
a r t i c l e
i n f o
Article history: Received 15 November 2019 Received in revised form 27 December 2019 Accepted 4 February 2020 Available online 5 February 2020 Keywords: Amorphous TiO2 Specific surface area Defects Oxygen vacancy Carriers transport channels Oxidation
a b s t r a c t An amorphous phase TiO2 was fabricated via a fast and simple hydrolysis method and it was characterized with XRD, TEM, BET, UV–Vis, XPS and EPR. The as-prepared amorphous TiO2 has a specific surface area record-breaking 100 times that of crystal TiO2. The results show that the as-samples had better photocatalytic property, which was evaluated for gas-phase Hg0 oxidation under LED light, and photocatalytic efficiency reached up to 40%, which is 1.4 times that of commercial P25. It can be mainly attributed to the large specific surface area of amorphous TiO2 providing more reactive sites and its surface oxygen vacancies as carriers transport channels. Ó 2020 Elsevier B.V. All rights reserved.
1. Introduction Since 1972, Fujishima and Honda managed to decompose water by using titanium dioxide (TiO2) [1], plenty of scientific works have been paid to explore the photocatalytic mechanism of photocatalysts, expecting to develop their solar conversion and wide application. TiO2 has been regarded as the most common photocatalyst due to its environmental friendly characteristics, such as low cost, high chemical stability [2]. As is known to all, TiO2 exists in nature as amorphous or polycrystalline state, such as anatase, rutile and titanite. Amorphous TiO2 is always thought to possess relatively poor photocatalytic activity for its high concentration of atomic defects. However, compared with the crystalline TiO2, amorphous TiO2 had more advantages, including milder synthetic conditions, larger surface area and allowing more chemicals to be embedded, which can lead to high absorptivity [2]. Oxygen vacancies are considered to be one of the most effective ways to tune the surface microstructure, electronic structure, and
charge separation of photocatalysts, which could easily create and form in metal oxide [2]. As an intrinsic defect in photocatalysts surface, oxygen vacancies have significant effects on the electronic structure and photoelectric characteristic of photocatalysts, which can capture electrons and block the recombination of photogenerated electron pairs [3]. Therefore, oxygen vacancies engineering is an effective way to enhance visible light absorption and photocatalytic activity. Herein, we successfully synthesized amorphous phase TiO2 with oxygen vacancies via hydrolysis method, which could not only improve a record-breaking adsorption performance by increasing the specific surface area, but also could provide carriers transport channels by oxygen vacancies. The properties of amorphous TiO2 were evaluated by characterization and photocatalytic mercury removal experiments. It provides a new way for more active sites on the catalyst surface and high separation efficiency of charge carriers. 2. Materials and methods
⇑ Corresponding authors at: NO. 2103 Pingliang Road, Shanghai 200090, China (D. Wang). No. 639 Zhizaoju Road, Shanghai 200011, China (B. Hu). E-mail addresses: [email protected] (D. Wang), [email protected] (B. Hu). 1 T. Jia and J. Zhang contributed equally. https://doi.org/10.1016/j.matlet.2020.127465 0167-577X/Ó 2020 Elsevier B.V. All rights reserved.
2.1. Preparation of different phases of TiO2 Amorphous TiO2: Under the condition of strong ultrasound, tetra-n-butyl titanate (5 ml) was slowly dropped into ultrapure
2
T. Jia et al. / Materials Letters 265 (2020) 127465
water (50 ml) to dissolve. After ultrasonic for 6 h, the white precipitates were collected and washed with ethanol and pure water for three times. The white wet gel was then dried for 10 h in oven at 70 °C. The sample was labeled as Am-TiO2. Crystalline TiO2: Another sample was also exactly prepared by the above fabrication method, and then it was calcined at 500 for 2 h. The sample was labeled as C-TiO2. 2.2. Photocatalytic oxidation Hg0 test The experimental system was comprised following three parts, simulated flue gas, photocatalytic reactor and mercury analyzer [4]. The flow rates of the two branches of the simulated flue gas were controlled by two mass flow meters (MFC, CS200 type). One of them was used to introduce Hg0 vapor from mercury generating unit at a flow rate of 0.2 L/min, and the other was used to dilute the Hg0 vapor concentration at a flow rate of 1 L/min. The mixed flow introduced into photocatalytic reactor, and then entered into the on-line mercury analyzer (RA-915M, Lumex, Russia) to measure the concentration of Hg0. Finally, exhaust gas was discharged into the atmosphere through KMnO4 solution. In the photocatalytic test, 50 mg photocatalyst was evenly coated on quartz glass under 24 W LED light with the filter (k < 400 nm). The removal efficiency of Hg0 under light irradiation condition was defined as following:
g¼
Hg0in Hg0out Hg0in
100%
ð1Þ
where Hg0in and Hg0out represented Hg0 (lg/m3) concentration at the inlet and outlet of the reactor respectively.
3. Results and discussions The phase and crystal of the samples were obtained by XRD analysis. As shown in Fig. 1a, Am-TiO2 has no obvious diffraction peak, indicating that the product of the hydrolysis method is amorphous phase TiO2 [5]. On the contrary, the C-TiO2 possesses sharp diffraction peaks, demonstrating that after calcination at 500 °C for 2 h, the amorphous TiO2 was converted into crystalline TiO2. The XRD peaks of C-TiO2 are located at 2h = 25.28°, 36.94°, 37.80° and 38.57°, which perfectly match with the JCPDS data (PDFNo.04-0477), indicating that there is anatase phase in C-TiO2. The morphology of the Am-TiO2 was analyzed by TEM. TEM and SAED of Am-TiO2 and C-TiO2 in Fig. S1 further confirm that AmTiO2 is amorphous. The calculated particle sizes of Am-TiO2 and C-TiO2 (Fig. S1) are mainly distributed at 10–20 nm. Fig. 1b shows that only a small portion of the area appears lattice fringe, and the rest does not appear in the lattice, which and SAED (Fig. S1) indicate that Am-TiO2 appears in amorphous phase, which is consistent with the results of XRD. In order to obtain the optical properties, UV–vis DRS of Am-TiO2 and C-TiO2 was conducted in the range of 200–900 nm. From Fig. 1c, it can be seen that the absorption boundaries of Am-TiO2 and C-TiO2 are 390 and 415 nm respectively, indicating that AmTiO2 exhibits better light absorption properties. The band gap (Eg) followed the equation:
aðhmÞ ¼ Aðhm EgÞn
ð2Þ
where a, hm and A represented the absorption coefficient, the photon energy and the absorption edge width parameter respectively. TiO2 was indirect semiconductor, the values of n was 2 [6]. As
Fig. 1. (a) XRD patterns of Am-TiO2 and C-TiO2; (b) HRTEM image of Am-TiO2; (c) UV–vis DRS of Am-TiO2 and C-TiO2; (d) The plots of (ahv)1/2 vs. energy (hv); (e) SAED image of Am-TiO2.
3
T. Jia et al. / Materials Letters 265 (2020) 127465
shown in Fig. 1d, the band gap of Am-TiO2 and C-TiO2 are 3.0 eV and 3.2 eV respectively. The specific surface area (SBET) of Am-TiO2 and C-TiO2 is shown in Fig. S2, and all properties data are calculated in Table 1. SBET and pore volume of Am-TiO2 were 508.82 cm2 g1 and 0.290 cm3 g1 respectively, which is almost 100 times and 20 times that of CTiO2 and results in strong adsorption ability. The X-ray photoelectron spectroscopy (XPS) was conducted to further explore the chemical state of corresponding elements in Am-TiO2. As shown in Fig. 2a, the survey spectra confirm the characteristic peaks of Ti, O and C of the as-prepared Am-TiO2. The Fig. 2b shows that C1s spectra of Am-TiO2 can be fitted into two peaks with binding energy of 284.6 eV and 288.1 eV, which are contributed to elemental carbon and carboxylate respectively [7]. The Fig. 2c shows that Ti2p spectra of Am-TiO2 can be fitted into two main peaks of 458.3 eV and 463.9 eV, which can be assigned to Ti2p3/2 and Ti2p1/2 respectively [7]. The spacing between Ti2p3/2 and Ti2p1/2 was 5.7 eV, demonstrating that the standardized state of Ti4+ forms in Am-TiO2. The presence of Ti2p signal was at 455.4 eV, indicating that there was few Ti3+ [8]. Fig. 2d shows that O1s spectra of Am-TiO2 exists three oxygen peaks. The peak located at 530.2 eV is attributed to Ti-O, the peak located at 531.6 eV is attributed to the presence of oxygen vacancy, and the peak located at 532.7 eV is assigned to the surface hydroxyl [7]. Moreover, the O1s spectra of C-TiO2 (Fig. S3) shows only two peaks
belong to Ti-O and surface hydroxyl, suggesting that there are no peaks for oxygen vacancies, which proves that no oxygen vacancy existed for C-TiO2. The results confirm Am-TiO2 was successfully synthesized. It is well known that oxygen vacancy can act as the carriers transport channel to reduce the recombination of hole-electron pairs [9]. Electron paramagnetic resonance (EPR) of Am-TiO2 was performed to further explore oxygen vacancy levels. The signal at g = 2.003 shown in Fig. 3a belongs to the characteristics of oxygen vacancies, which matches above XPS analysis. The disordered arrangement of atoms in amorphous TiO2 can reduce the formation energy of oxygen vacancy, which may also lead to generate oxygen vacancy [3]. The removal efficiency of Hg0 was used to evaluate the photocatalytic properties of Am-TiO2, C-TiO2 and P25 under LED light. From Fig. 3b, the photocatalytic mercury removal efficiencies of Am-TiO2, C-TiO2 and P25 are 40%, 2% and 28% respectively, indicating that Am-TiO2 has the best photocatalytic performance, which is 1.4 times more active than P25. Furthermore, the photocatalytic efficiency of Am-TiO2 increased to 54% under UV irradiation. In order to clarify the enhancement on removal efficiency of AmTiO2, EPR was conducted in Fig. 3c, after UV light irradiation, the intensity of signal at g = 2.003 were obviously tripled, indicating that the level of oxygen vacancy has a significant effect on photocatalytic reaction.
Table 1 Physicochemical properties of Am-TiO2, C-TiO2. Sample
BET surface area (cm2 g1)
Pore volume (cm3 g1)
Aborbing boundary (nm)
Bandgap (eV)
Hg0 removal efficiency g (%)
Am-TiO2 C-TiO2
508.82 4.61
0.290 0.014
415 390
3.0 3.2
40% 2%
.
Fig. 2. XPS survey spectrum of Am-TiO2 (a); and high-resolution XPS spectra of C1s (b); Ti2p (c); O1s (d).
4
T. Jia et al. / Materials Letters 265 (2020) 127465
Fig. 3. (a) EPR spectrum of Am-TiO2; (b) Hg0 removal efficiency under visible light; (c) EPR spectra of Am-TiO2 before irradiation and after irradiation; (d) Schematic mechanism illustration for enhanced photogenerated electron transfer processes induced by oxygen vacancies.
The schematic mechanism of photocatalytic removal mercury under visible light is shown in Fig. 3d. Oxygen vacancy as catalytic centers can capture and shed electrons to enhance the transfer and inhibit the recombination of photogenerated charge carriers [9]. As photo-excited holes and electrons transferred to the surface of AmTiO2, which could capture and react with O2 and OH to form O 2 and OH with strong oxidation ability, which could oxidize Hg0 [7]. Photo-induced electrons can also be transferred from the trapped state of oxygen vacancies to oxygen adsorbed on the surface, thereby producing reactive species such as O 2 [9]. The reaction process is described in Eqs. (3)–(9).
talline forms of TiO2 were evaluated by Hg0 removal. The results show that amorphous TiO2 exhibits the best photocatalytic performance, which is beneficial to the larger specific surface area and the effective separation and transfer of charge carriers due to surface oxygen vacancies. This work provides a new strategy for developing highly efficient photocatalysts for pollutant control.
CRediT authorship contribution statement
hv þ TiO2 ! TiO2 h þ e
ð3Þ
H2 O $ Hþ þ OH
ð4Þ
þ
ð5Þ
Tao Jia: Writing - original draft, Formal analysis, Software, Data curation. Jing Zhang: Writing - original draft, Formal analysis, Software, Data curation. Jiang Wu: Conceptualization. Daolei Wang: Writing - review & editing, Project administration. Qizhen Liu: Supervision, Resources. Yongfeng Qi: Supervision, Resources. Bin Hu: Supervision, Resources. Ping He: Supervision, Resources. Weiguo Pan: Investigation. Xuemei Qi: Investigation.
H2 Oad þ h ! OHad þ Hþ
ð6Þ
Declaration of Competing Interest
O2ad þ e ! O2ad
ð7Þ
OH þ h ! OHad þ
3Hg0ad
þ2
O2ad
þ
þ 2H ! 3HgOad þ H2 O
Hg0ad þ 2 OHad ! HgOad þ H2 O
ð8Þ ð9Þ
4. Conclusions In this work, amorphous TiO2 was successfully prepared via hydrolysis method. The photocatalytic properties of different crys-
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments This work was partially sponsored by NSFC (National Natural Science Foundation of China, 50806041, 51606115, 51106133), Natural Science Foundation of Shanghai (18ZR1416200, 16ZR1413500).
T. Jia et al. / Materials Letters 265 (2020) 127465
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.matlet.2020.127465. References [1] A. Fujishima, K. Honda, Nature 238 (1972) 37.
[2] [3] [4] [5] [6] [7] [8] [9]
H.H. Pham, L.W. Wang, Phys. Chem. Chem. Phys. 17 (2015) 541–550. H.Z. Zhang, B. Chen, J.F. Banfield, Phys. Rev. B 78 (2008) 214106. T. Jia et al., J. Colloid Interface Sci. 562 (2020) 429–443. X.J. Wang, W.Y. Yang, F.T. Li, et al., J. Hazard. Mater. 292 (2015) 126–136. P.H. Wang, L.G. Yang, L.Z. Wang, J.L. Zhang, Mater. Lett. 164 (2016) 405–408. X. Zhou et al., J. Catal. 355 (2017) 26–39. S. Hashimoto, A. Tanaka, Surf. Interface Anal. 34 (2002) 262–265. Y. Lu et al., Appl. Catal. B-Environ. 231 (2018) 357–367.
5
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Synthesis amorphous TiO2 with oxygen vacancy as carriers transport channels for enhancing photocatalytic activity Tao Jia a,1, Jing Zhang a,1, Jiang Wu a,b, Daolei Wang a,⇑, Qizhen Liu c, Yongfeng Qi d, Bin Hu e,⇑, Ping He a, Weiguo Pan a, Xuemei Qi a a
College of Energy and Mechanical Engineering, Shanghai University of Electric Power, Shanghai 200090, China Shanghai Institute of Pollution Control and Ecological Security, Shanghai, China Shanghai Environment Monitoring Center, Shanghai 200030, China d School of Hydraulic Energy and Power Engineering, Yangzhou University, Yangzhou 225127, China e Ninth People’s Hospital Affiliated to Shanghai Jiao Tong University, Shanghai 200011, China b c
a r t i c l e
i n f o
Article history: Received 15 November 2019 Received in revised form 27 December 2019 Accepted 4 February 2020 Available online 5 February 2020 Keywords: Amorphous TiO2 Specific surface area Defects Oxygen vacancy Carriers transport channels Oxidation
a b s t r a c t An amorphous phase TiO2 was fabricated via a fast and simple hydrolysis method and it was characterized with XRD, TEM, BET, UV–Vis, XPS and EPR. The as-prepared amorphous TiO2 has a specific surface area record-breaking 100 times that of crystal TiO2. The results show that the as-samples had better photocatalytic property, which was evaluated for gas-phase Hg0 oxidation under LED light, and photocatalytic efficiency reached up to 40%, which is 1.4 times that of commercial P25. It can be mainly attributed to the large specific surface area of amorphous TiO2 providing more reactive sites and its surface oxygen vacancies as carriers transport channels. Ó 2020 Elsevier B.V. All rights reserved.
1. Introduction Since 1972, Fujishima and Honda managed to decompose water by using titanium dioxide (TiO2) [1], plenty of scientific works have been paid to explore the photocatalytic mechanism of photocatalysts, expecting to develop their solar conversion and wide application. TiO2 has been regarded as the most common photocatalyst due to its environmental friendly characteristics, such as low cost, high chemical stability [2]. As is known to all, TiO2 exists in nature as amorphous or polycrystalline state, such as anatase, rutile and titanite. Amorphous TiO2 is always thought to possess relatively poor photocatalytic activity for its high concentration of atomic defects. However, compared with the crystalline TiO2, amorphous TiO2 had more advantages, including milder synthetic conditions, larger surface area and allowing more chemicals to be embedded, which can lead to high absorptivity [2]. Oxygen vacancies are considered to be one of the most effective ways to tune the surface microstructure, electronic structure, and
charge separation of photocatalysts, which could easily create and form in metal oxide [2]. As an intrinsic defect in photocatalysts surface, oxygen vacancies have significant effects on the electronic structure and photoelectric characteristic of photocatalysts, which can capture electrons and block the recombination of photogenerated electron pairs [3]. Therefore, oxygen vacancies engineering is an effective way to enhance visible light absorption and photocatalytic activity. Herein, we successfully synthesized amorphous phase TiO2 with oxygen vacancies via hydrolysis method, which could not only improve a record-breaking adsorption performance by increasing the specific surface area, but also could provide carriers transport channels by oxygen vacancies. The properties of amorphous TiO2 were evaluated by characterization and photocatalytic mercury removal experiments. It provides a new way for more active sites on the catalyst surface and high separation efficiency of charge carriers. 2. Materials and methods
⇑ Corresponding authors at: NO. 2103 Pingliang Road, Shanghai 200090, China (D. Wang). No. 639 Zhizaoju Road, Shanghai 200011, China (B. Hu). E-mail addresses: [email protected] (D. Wang), [email protected] (B. Hu). 1 T. Jia and J. Zhang contributed equally. https://doi.org/10.1016/j.matlet.2020.127465 0167-577X/Ó 2020 Elsevier B.V. All rights reserved.
2.1. Preparation of different phases of TiO2 Amorphous TiO2: Under the condition of strong ultrasound, tetra-n-butyl titanate (5 ml) was slowly dropped into ultrapure
2
T. Jia et al. / Materials Letters 265 (2020) 127465
water (50 ml) to dissolve. After ultrasonic for 6 h, the white precipitates were collected and washed with ethanol and pure water for three times. The white wet gel was then dried for 10 h in oven at 70 °C. The sample was labeled as Am-TiO2. Crystalline TiO2: Another sample was also exactly prepared by the above fabrication method, and then it was calcined at 500 for 2 h. The sample was labeled as C-TiO2. 2.2. Photocatalytic oxidation Hg0 test The experimental system was comprised following three parts, simulated flue gas, photocatalytic reactor and mercury analyzer [4]. The flow rates of the two branches of the simulated flue gas were controlled by two mass flow meters (MFC, CS200 type). One of them was used to introduce Hg0 vapor from mercury generating unit at a flow rate of 0.2 L/min, and the other was used to dilute the Hg0 vapor concentration at a flow rate of 1 L/min. The mixed flow introduced into photocatalytic reactor, and then entered into the on-line mercury analyzer (RA-915M, Lumex, Russia) to measure the concentration of Hg0. Finally, exhaust gas was discharged into the atmosphere through KMnO4 solution. In the photocatalytic test, 50 mg photocatalyst was evenly coated on quartz glass under 24 W LED light with the filter (k < 400 nm). The removal efficiency of Hg0 under light irradiation condition was defined as following:
g¼
Hg0in Hg0out Hg0in
100%
ð1Þ
where Hg0in and Hg0out represented Hg0 (lg/m3) concentration at the inlet and outlet of the reactor respectively.
3. Results and discussions The phase and crystal of the samples were obtained by XRD analysis. As shown in Fig. 1a, Am-TiO2 has no obvious diffraction peak, indicating that the product of the hydrolysis method is amorphous phase TiO2 [5]. On the contrary, the C-TiO2 possesses sharp diffraction peaks, demonstrating that after calcination at 500 °C for 2 h, the amorphous TiO2 was converted into crystalline TiO2. The XRD peaks of C-TiO2 are located at 2h = 25.28°, 36.94°, 37.80° and 38.57°, which perfectly match with the JCPDS data (PDFNo.04-0477), indicating that there is anatase phase in C-TiO2. The morphology of the Am-TiO2 was analyzed by TEM. TEM and SAED of Am-TiO2 and C-TiO2 in Fig. S1 further confirm that AmTiO2 is amorphous. The calculated particle sizes of Am-TiO2 and C-TiO2 (Fig. S1) are mainly distributed at 10–20 nm. Fig. 1b shows that only a small portion of the area appears lattice fringe, and the rest does not appear in the lattice, which and SAED (Fig. S1) indicate that Am-TiO2 appears in amorphous phase, which is consistent with the results of XRD. In order to obtain the optical properties, UV–vis DRS of Am-TiO2 and C-TiO2 was conducted in the range of 200–900 nm. From Fig. 1c, it can be seen that the absorption boundaries of Am-TiO2 and C-TiO2 are 390 and 415 nm respectively, indicating that AmTiO2 exhibits better light absorption properties. The band gap (Eg) followed the equation:
aðhmÞ ¼ Aðhm EgÞn
ð2Þ
where a, hm and A represented the absorption coefficient, the photon energy and the absorption edge width parameter respectively. TiO2 was indirect semiconductor, the values of n was 2 [6]. As
Fig. 1. (a) XRD patterns of Am-TiO2 and C-TiO2; (b) HRTEM image of Am-TiO2; (c) UV–vis DRS of Am-TiO2 and C-TiO2; (d) The plots of (ahv)1/2 vs. energy (hv); (e) SAED image of Am-TiO2.
3
T. Jia et al. / Materials Letters 265 (2020) 127465
shown in Fig. 1d, the band gap of Am-TiO2 and C-TiO2 are 3.0 eV and 3.2 eV respectively. The specific surface area (SBET) of Am-TiO2 and C-TiO2 is shown in Fig. S2, and all properties data are calculated in Table 1. SBET and pore volume of Am-TiO2 were 508.82 cm2 g1 and 0.290 cm3 g1 respectively, which is almost 100 times and 20 times that of CTiO2 and results in strong adsorption ability. The X-ray photoelectron spectroscopy (XPS) was conducted to further explore the chemical state of corresponding elements in Am-TiO2. As shown in Fig. 2a, the survey spectra confirm the characteristic peaks of Ti, O and C of the as-prepared Am-TiO2. The Fig. 2b shows that C1s spectra of Am-TiO2 can be fitted into two peaks with binding energy of 284.6 eV and 288.1 eV, which are contributed to elemental carbon and carboxylate respectively [7]. The Fig. 2c shows that Ti2p spectra of Am-TiO2 can be fitted into two main peaks of 458.3 eV and 463.9 eV, which can be assigned to Ti2p3/2 and Ti2p1/2 respectively [7]. The spacing between Ti2p3/2 and Ti2p1/2 was 5.7 eV, demonstrating that the standardized state of Ti4+ forms in Am-TiO2. The presence of Ti2p signal was at 455.4 eV, indicating that there was few Ti3+ [8]. Fig. 2d shows that O1s spectra of Am-TiO2 exists three oxygen peaks. The peak located at 530.2 eV is attributed to Ti-O, the peak located at 531.6 eV is attributed to the presence of oxygen vacancy, and the peak located at 532.7 eV is assigned to the surface hydroxyl [7]. Moreover, the O1s spectra of C-TiO2 (Fig. S3) shows only two peaks
belong to Ti-O and surface hydroxyl, suggesting that there are no peaks for oxygen vacancies, which proves that no oxygen vacancy existed for C-TiO2. The results confirm Am-TiO2 was successfully synthesized. It is well known that oxygen vacancy can act as the carriers transport channel to reduce the recombination of hole-electron pairs [9]. Electron paramagnetic resonance (EPR) of Am-TiO2 was performed to further explore oxygen vacancy levels. The signal at g = 2.003 shown in Fig. 3a belongs to the characteristics of oxygen vacancies, which matches above XPS analysis. The disordered arrangement of atoms in amorphous TiO2 can reduce the formation energy of oxygen vacancy, which may also lead to generate oxygen vacancy [3]. The removal efficiency of Hg0 was used to evaluate the photocatalytic properties of Am-TiO2, C-TiO2 and P25 under LED light. From Fig. 3b, the photocatalytic mercury removal efficiencies of Am-TiO2, C-TiO2 and P25 are 40%, 2% and 28% respectively, indicating that Am-TiO2 has the best photocatalytic performance, which is 1.4 times more active than P25. Furthermore, the photocatalytic efficiency of Am-TiO2 increased to 54% under UV irradiation. In order to clarify the enhancement on removal efficiency of AmTiO2, EPR was conducted in Fig. 3c, after UV light irradiation, the intensity of signal at g = 2.003 were obviously tripled, indicating that the level of oxygen vacancy has a significant effect on photocatalytic reaction.
Table 1 Physicochemical properties of Am-TiO2, C-TiO2. Sample
BET surface area (cm2 g1)
Pore volume (cm3 g1)
Aborbing boundary (nm)
Bandgap (eV)
Hg0 removal efficiency g (%)
Am-TiO2 C-TiO2
508.82 4.61
0.290 0.014
415 390
3.0 3.2
40% 2%
.
Fig. 2. XPS survey spectrum of Am-TiO2 (a); and high-resolution XPS spectra of C1s (b); Ti2p (c); O1s (d).
4
T. Jia et al. / Materials Letters 265 (2020) 127465
Fig. 3. (a) EPR spectrum of Am-TiO2; (b) Hg0 removal efficiency under visible light; (c) EPR spectra of Am-TiO2 before irradiation and after irradiation; (d) Schematic mechanism illustration for enhanced photogenerated electron transfer processes induced by oxygen vacancies.
The schematic mechanism of photocatalytic removal mercury under visible light is shown in Fig. 3d. Oxygen vacancy as catalytic centers can capture and shed electrons to enhance the transfer and inhibit the recombination of photogenerated charge carriers [9]. As photo-excited holes and electrons transferred to the surface of AmTiO2, which could capture and react with O2 and OH to form O 2 and OH with strong oxidation ability, which could oxidize Hg0 [7]. Photo-induced electrons can also be transferred from the trapped state of oxygen vacancies to oxygen adsorbed on the surface, thereby producing reactive species such as O 2 [9]. The reaction process is described in Eqs. (3)–(9).
talline forms of TiO2 were evaluated by Hg0 removal. The results show that amorphous TiO2 exhibits the best photocatalytic performance, which is beneficial to the larger specific surface area and the effective separation and transfer of charge carriers due to surface oxygen vacancies. This work provides a new strategy for developing highly efficient photocatalysts for pollutant control.
CRediT authorship contribution statement
hv þ TiO2 ! TiO2 h þ e
ð3Þ
H2 O $ Hþ þ OH
ð4Þ
þ
ð5Þ
Tao Jia: Writing - original draft, Formal analysis, Software, Data curation. Jing Zhang: Writing - original draft, Formal analysis, Software, Data curation. Jiang Wu: Conceptualization. Daolei Wang: Writing - review & editing, Project administration. Qizhen Liu: Supervision, Resources. Yongfeng Qi: Supervision, Resources. Bin Hu: Supervision, Resources. Ping He: Supervision, Resources. Weiguo Pan: Investigation. Xuemei Qi: Investigation.
H2 Oad þ h ! OHad þ Hþ
ð6Þ
Declaration of Competing Interest
O2ad þ e ! O2ad
ð7Þ
OH þ h ! OHad þ
3Hg0ad
þ2
O2ad
þ
þ 2H ! 3HgOad þ H2 O
Hg0ad þ 2 OHad ! HgOad þ H2 O
ð8Þ ð9Þ
4. Conclusions In this work, amorphous TiO2 was successfully prepared via hydrolysis method. The photocatalytic properties of different crys-
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments This work was partially sponsored by NSFC (National Natural Science Foundation of China, 50806041, 51606115, 51106133), Natural Science Foundation of Shanghai (18ZR1416200, 16ZR1413500).
T. Jia et al. / Materials Letters 265 (2020) 127465
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.matlet.2020.127465. References [1] A. Fujishima, K. Honda, Nature 238 (1972) 37.
[2] [3] [4] [5] [6] [7] [8] [9]
H.H. Pham, L.W. Wang, Phys. Chem. Chem. Phys. 17 (2015) 541–550. H.Z. Zhang, B. Chen, J.F. Banfield, Phys. Rev. B 78 (2008) 214106. T. Jia et al., J. Colloid Interface Sci. 562 (2020) 429–443. X.J. Wang, W.Y. Yang, F.T. Li, et al., J. Hazard. Mater. 292 (2015) 126–136. P.H. Wang, L.G. Yang, L.Z. Wang, J.L. Zhang, Mater. Lett. 164 (2016) 405–408. X. Zhou et al., J. Catal. 355 (2017) 26–39. S. Hashimoto, A. Tanaka, Surf. Interface Anal. 34 (2002) 262–265. Y. Lu et al., Appl. Catal. B-Environ. 231 (2018) 357–367.
5