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Remarkably enhanced photocatalytic activity by sulfur-doped titanium dioxide in nanohybrids with carbon nanotubes
Author’s Accepted Manuscript Remarkably enhanced photocatalytic activity by sulfur-doped titanium dioxide in nanohybrids with carbon nanotubes Nguyen Cao Khang, Duong Quoc Van, Nguyen Minh Thuy, Nguyen Van Minh, Phan Ngoc Minh www.elsevier.com/locate/jpcs
PII: DOI: Reference:
S0022-3697(16)30184-6 http://dx.doi.org/10.1016/j.jpcs.2016.06.011 PCS7795
To appear in: Journal of Physical and Chemistry of Solids Received date: 17 December 2015 Revised date: 1 April 2016 Accepted date: 21 June 2016 Cite this article as: Nguyen Cao Khang, Duong Quoc Van, Nguyen Minh Thuy, Nguyen Van Minh and Phan Ngoc Minh, Remarkably enhanced photocatalytic activity by sulfur-doped titanium dioxide in nanohybrids with carbon nanotubes, Journal of Physical and Chemistry of Solids, http://dx.doi.org/10.1016/j.jpcs.2016.06.011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Remarkably enhanced photocatalytic activity by sulfur-doped titanium dioxide in nanohybrids with carbon nanotubes Nguyen Cao Khanga1, Duong Quoc Vana, Nguyen Minh Thuya, Nguyen Van Minha, Phan Ngoc Minhb a
Center for Nano Science and Technology, Hanoi National University of Education, 136 Xuan Thuy,
Hanoi, Vietnam b
Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Hanoi, Vietnam
[email protected] Abstract TiO2 doped S nanohybrids with carbon nanotubes (CNTs) were synthesized with CNTs, thiourea and TiO2 nanoparticles. The result indicated that the TiO2 nanoparticles with about 8 nm in size are attached on the sidewall of CNTs. The nanohybrids material can absorb at longer wavelength and the absorption even covers the whole range of visible region than that only TiO 2 nanoparticles. Application of the catalysts to photocatalytic degradation of methylene blue (MB) was tested under visible light irradiation. The result suggests that a high MB degradation activity of S-TiO2/CNTs due to a reduce band gap of TiO2 when S is doped, and the decrease in the possibility of electron-hole recombination by CNTs. In addition, the density functional-theory (DFT) calculations of the electronic band structures and density of states (DOS) to understand the bonding states between TiO2 and CNTs, proved that the TiO2/CNTs system is stable. Keywords: TiO2 doped S; CNTs; nanohybrids; photocatalytic.
1
Phone: +84-912-988-186 1
I. INTRODUCTION Semiconductors have been extensively researched and widely applied in many applications such as thin-film optical devices, biomaterials, catalyst in many reactions [1-6]. Among all the semiconductors, titanium dioxide (TiO2) has been the focus of researchers because of its chemical and physical durability, high activity, nontoxicity, and low cost. However, the TiO2 semiconducting has a large band gap (3.0-3.2 eV). Without modification, TiO2 can only harvest the UV part, which constitutes only about 4% of the total solar spectrum. Therefore, it is an important and challenging issue to develop new TiO2 systems with enhanced activities under both UV and visible light irradiation compared with bare TiO2, thereby improving the utilization of solar energy. Many efforts have been made to modify TiO2 with nonmetals or metals to efficiently extend photoresponse from the UV to the visible light region [7-14]. Furthermore, theoretical calculations also suggest that metal or nonmetal doping has considerably impacted on the band gap of TiO2. However, the high concentration of dopants is difficult to obtain and the synthesized materials are usually unstable to photo-corrosion. Recently, Karran et al. and Masakazu et al. [15, 16] indicated that the photocatalytic activity of TiO2 can be improved by making nanohybrids with CNTs due to the fact that the CNTs could not only provide a large surface area to support the catalyst but also decrease the ability of recombination of the electron-hole pairs. Their outstanding charge transfer abilities can favour the excited electron in the conduction band of nanocrystal semiconductor to migrate into the CNTs, and increase photocatalytic activity under visible light [17, 18]. Therefore, combination with reduction band gap of modified-TiO2 with CNTs will possible channel to increase the photocatlytic performance. This article highlights the literature on the S-doped TiO2, the composition of S-doped TiO2 with CNTs and discussion of the mechanism of enhancing photocatalytic activity of this material. We proposed that simultaneously reduce band gap and decrease the ability of the recombination of the electron-hole pairs of TiO2 will origin significantly to increase their photocatalytic properties. 2
II. EXPERIMENTS CNTs were firstly modified according to following procedure: 0.5 g CNTs were suspended in 40 mL solution of nitric acid and then refluxed for 12 hours at 120 oC. The resulting solution after this oxidization process was filtered and washed with deionized water, dried at 80 oC in air for 24 hours before being transferred to the 2-propanol solution. For nanocoating on CNTs, titanium tetraisopropoxide was added dropwise to the prepared CNTs dispersion, maintained at pH = 4 by HNO3 acid. To introduce sulfur dopant into the titania nanoparticles, thiourea is added to the colloidal nanoparticle solution. The prepared particles were dried at 80 °C for one day and heat treated at 450 °C for the crystallization. The samples S1, S2, S4, and S8 are synthesized with [TiO2]/[thiourea] mole ratio of 1/1, 1/2, 1/4, and 1/8, respectively. The [TiO2]/[CNTs] mole ratio is 10/1 for all samples. The structure of TiO2 samples were determined by X-ray diffractometer D5005 (Siemen) with CuKα radiation (λ = 1.5406 Å). The concentration of S remain in TiO2 were characterized by using energy dispersive X-ray (EDX). The morphologies and the elemental components of the samples were observed using a Hitachi S-4800 scanning electron microscope (SEM), and Tecnai F30 machine operated under 300 kV (HR-TEM). Optical absorption spectra were measured by Jasco V-670 spectrophotometer. The density functional theory (DFT) was used to calculate the electronic structures and PDOS of the nanohybrids TiO2/CNTs. For the photocatalytic activity, 25 mg of photocatalyst is mixed with 50 ml of 10 ppm methylene blue (MB) solution. The light source used was a 150 W high-pressure Xenon lamp with a cut-off filter of 420 nm. Before turning on the light, the suspension containing MB and photocatalyst was magnetically stirred continuously in dark until no change in the absorbance of the solution is observed. The purpose of this process is to make sure that the physical adsorption plays no role in reducing the MB concentration. The reduction in the concentration of the MB was determined from the absorbance value at the maximum of the 665 nm absorption spectrum for every MB by monitoring UV-Vis spectra.
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III. RESULTS AND DISCUSSIONS The crystalline morphology, particle size, and lattice spacing of the products were firstly investigated using SEM and HR-TEM, as shown in Fig. 1. SEM images of nanohybrids in Fig. 1a, 1b, and 1c show that the TiO2 nanoparticles with an average diameter of around 8 nm are attached on the CNTs surfaces, which is consistent with XRD results (see Fig.2). No aggregation of TiO2 particles were observed. The TiO2 decorated CNTs were further confirmed by the HR-TEM image in Fig. 1c. It can be seen that the diameters of carbon structures range from 40 nm to 60 nm. The HR-TEM image in Fig. 1d shows that the TiO2/CNTs is highly crystalline. The spacing of carbon nanotube tube is of 3.33 Å , while the <101> direction of TiO2 is determined to be 3.56 Å. Fig. 2a shows the powder XRD of the S-TiO2/CNTs. The XRD shows typical peaks that can be well assigned to the anatase TiO2 as well as characteristic peaks of CNTs, indicating the successful decoration of the anatase TiO2 nanoparticles on CNTs. The peaks at 2θ of 26.0 and 43.0o were associated with the <002> and <100> diffractions of the hexagonal graphite structure, similar to pristine CNTs, and the peaks at 2θ of 25.5, 37.8, 48.1, 54.0, and 62.8o can be perfectly attributed to the crystal planes of the <101>, <004>, <200>, <105>, and <204> of the anatase TiO2. The peak at 26.0o of CNTs does not appear. However, the broadening and asymmetry of the 25.5o peak of TiO2 in the samples shows the effect of CNTs on XRD spectrum of TiO2. The average crystalline size of TiO2-S powder was estimated from the full width at half maximum (FWHM) of the (101) XRD peak using Scherrer’s equation: d=k/βcos, where d is the average crystalline size, k the constant, the X-ray wavelength, β the full width at half maximum of the diffraction line, and the angle of diffraction. For the elemental analysis in the nanohybrids S-TiO2/CNTs, the EDX was used. The results confirm the existence of Ti, O, C, and S as well as other impure elements. The results of EDX elemental microanalysis, the average size, and the cell constant of TiO2 nanoparticles of the nanohybrids STiO2/CNTs are listed in Table 1.
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Raman spectra was performed to study the crystallographic structure of the samples. Four Raman peaks at 145, 399, 520, and 644 cm−1 were assigned to Eg, B1g, A1g, and Eg, correspond to the TiO2 anatase crystal respectively, as suggested in previous reports [19]. The positions and intensity of the four Raman active modes are also in good agreement with the reference values previously determined for the TiO2 anatase structure. Both the XRD and Raman results indicate that the samples are crystallized in the anatase phase without any impurity. To understand the loss of organic residues on the surface of the S-TiO2/CNTs under sintering, the thermogravimetric analysis was used to study the weight loss behaviors of the S-TiO2/CNTs. The TGA results of the S1 sample in Fig. 3 show that there is tiny weight loss of 1.8% from room temperature to 100 °C, which is mainly due to the evaporation of physically adsorbed or embedded water on the surface of the S-TiO2/CNTs. The weight loss of 2.1% from 150 to 480 °C is related to the loss of surface and deeper seated organics. Thermogravimetry and differential thermal analyses in Fig. 3 show that carbon nanotube starts to decompose at temperatures above 480 °C. A weight loss of 9.7% was obtained from 480 to 700 oC, corresponding to thermo-decomposition of CNTs. However, an exothermic peak at 612 °C indicates that the decomposition of CNTs mainly occurs at 612 °C. Finally, there is a very slow weight loss from 700 to 1000 oC, and TiO2 becomes the major component in the product. The absorption spectra of the samples are shifted to the visible light region as shown in Fig.4. According to previous studies, Augugliaro et al. [20] reveal that the sulfur dopants are indeed incorporated into the TiO2, thus changing the electronic structures. Umebayashi et al. [21] conclude that when S replaced into the O site of the TiO2 lattice, the mixing of the sulfur 3p states with the valence band was found to contribute to the increased width of the valence band, leading to the narrowing of the band gap. Dong et al. [22] reported that the substitution of Ti4+ by S6+ was the cause narrowed the band gap. Both anionic and cationic doping was accordingly proposed through the substitution of S2− or S4+/S6+ for O or Ti ions in the TiO2, respectively, leading to the effective 5
narrowing of the TiO2 band gap [23, 24]. So, the optical absorption was found to shift to lower energies as show in Fig. 4 show that S doped into TiO2 lattice. Furthermore, due to the presence of CNTs, the nanohybrids materials can absorb light of longer wavelengths and the absorption even covers the whole range of the measured UV-Vis region. Different from the pure TiO2, where most of these electron-hole pairs quickly recombine, in the TiO2/CNTs, the photogenerated electrons in the conduction band of anatase flow through the conductive CNTs and the holes in the valence band of anatase can generate hydroxyl radicals on the surface [4]. The relative position of the CNTs conduction band edge permits the transfer of electrons from the anatase surface to CNTs, allowing charge separation, stabilization, and hindering of recombination. The electrons can be shuttled to be nearly free. On the other, in the TiO2/CNT, the photogenerated electrons are injected into the conduction band of the TiO2 and can react with O2, triggering the formation of the very reactive superoxide radical ion (O2.-). At the same time, holes (h+) might be formed with electrons transferred from valence bond in TiO2 to CNTs [25]. The positive charged hole (h+) may react with the OH− derived from H2O, triggering the formation of hydroxyl radical (HO•). Consequently, these radical groups are responsible for the decomposition of the organic compounds. For the photocatalytic activity of the samples, visible-light was used to test the photodecomposition of MB. Fig. 5 displays the photodegradation behaviours of MB, using TiO2 nanoparticles, CNTs, and nanohybrids S-TiO2/CNTs. Pure CNTs exhibits almost no photocatalytic activity and the sample with TiO2 nanoparticles removes only 7% of MB after 4 hours irradiation. However, the percentage of MB removed by the samples S-TiO2/CNTS are about 6 to 8 times more than that of pure TiO2. The best photocatalytic MB degradation result is observed for the S4 sample ([TiO2]/[thiourea] = 1/4), which destroys up to 59% of MB molecules within 4 hours irradiation. Nevertheless, the best ratio of STiO2/CNTs for photocatalytic application depends on the experimental conditions and the substances used. Reducing the band gap and enhancing the absorption of radiation in the visible range are two main factors that help increase the photocatalytic efficiency of TiO2. The increase in photocatalytic 6
performance is also due to a large surface area of the nanohybrids S-TiO2/CNTs. Moreover, since CNTs have a high adsorption capacity for MB molecules, the concentration of MB in the vicinity of CNTs is higher than those in other places in the reaction system. When some MB molecules on the surface of TiO2 are degraded, the other MB molecules adsorbed on the CNTs can transfer to the residual vacancies through slip-induced surface diffusion, which may be a faster process than the free diffusion in solution. There are many reasons to increase the photocatalytic of S-TiO2/CNTs, but these reduce band gap of TiO2 due to the S doped, and higher absorb visible light due to the CNTs are the main reasons. High performance of S-TiO2/CNTs to the decomposition of MB was compared with the previous research on the photodecomposition of MB as listed in Table 2. Theoretical calculations were performed to investigate the interfacial electronic structures and electron transfer of CNT interfaced with Ti4O8 clusters. The CNT (10, 0) tube is used to represent typical 1 nm semiconducting CNT. The supercell is (a, b, c) = (30.00, 30.00, 17.04) Å. To optimize the structure of Ti4O8, the Task Geometry Optimization in DMol3 with the generalized gradient approximation (GGA) [34-36] was used. For an optimize the structure of Ti4O8 cluster, the results shown that in the six possible configurations, the configuration as shown in Fig. 6(i) with a least total energy (-77.7 eV) is the most sustainable. The total energy, the bonding energy, the Fermi energy and the relative volume of space of the configuration in Fig. 6(i) are -24137.1 eV, 77.7 eV, -5.6 eV and 82.10-30 m3, respectively. The ionization potential (IP) and electron affinity (EA) are a useful quantity for providing fundamental insight of electronic properties of Ti4O8. IP is the energy to remove an electron, and EA is the energy released when adding an electron to a neutral atom. The Dmol3 was used to calculations of IP and EA of Ti4O8. The results shown that the IP and EA of Ti4O8 are 8.94 eV and 2.85 eV, respectively. The charge analysis confirms that it is easier to add an electron to Ti4O8 cluster than to remove it. This result is consistent with the result of Fig. 6(b). As shown in Fig. 6b, in the Ti4O8/CNT systems, the density of electrons is increased in the Ti4O8 spatial domain, while the density of electrons 7
in the CNT is reduced. This trend is further confirmed by the result of DOS analysis in Fig. 6(a), in which a contribution of the d orbital in the total DOS of C atom can be clearly seen. The adsorption energy (Eads), defined as the reversible energy required to separate an adsorption system (Eads.sys) into a CNT and an adsorbed Ti4O8 clusters was calculated. ΔEads can be calculated by subtracting the sum of the total energy of optimized Ti4O8 clusters (ECluster) and CNT (ECNT) from the total energy of the adsorption system (Eads): ΔEads = Eads.sys – (ECNT – Ecluster) = -1.95 eV. A negative ΔEads indicates that the molecule adsorption is exothermic and thus the adsorption system is energetically stable.
III. CONCLUSION The S-TiO2/CNTs photocatalysts containing CNTs were prepared using a hydrolysis method. The nanohybrids materials can absorb longer wavelength light and the absorption even covers the whole range of visible region. The photocatalytic degradation of MB was observed from 6 to 8 times more efficient in the presence of nanohybrids TiO2/CNTs catalysts than neat TiO2. The percent of S depends on the ratio between TiO2 and thiourea. The highest decomposition rate of MB on the catalysts was observed for samples with [TiO2]/[thiourea] = 1/4. The HR-TEM image, the XRD, the DFT result, as well as a negative ΔEads indicate that the TiO2 attachment on the CNTs surface is energetically stable. Band gap reduction due to S doping and lower electron-hole recombination probability due to CNTs nanohybrids with TiO2 are two main reasons to increase S-TiO2/CNTs photocatalytic properties.
Acknowledgement This work was supported by the Ministry of Education and Training Grant No. B2014-17-46.
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[24] T. Ohno, M. Akiyohsi, T. Umebayashi, K. Asai, T. Mitsui, M. Matsumura, Appl. Catal. A 265 (2004) 115. [25] W. Wang, P. Serp, P. Kalck, J.L. Faria, J. Mol. Catal. A-Chem. 235 (2005) 194. [26] Y. Zhao, X. Qiu, C. Burda, Chem. Mater. 20 (2008) 2629. [27] T. Peng, A.J. Law, Y.F. Han, Catal. Lett. 139 (2010) 77. [28] H.L. Wang, D.Y. Zhao, W.F. Jiang, Desalination Water Treat. 51 (2013) 2826. [29] M.L. Chen, W.C. Oh, Fresen. Environ. Bull. 19 (2010) 2938. [30] S.W. Kim, R. Khan, T.J. Kim, W.J. Kim, Bull. Korean Chem. Soc. 29 (2008) 1217. [31] Z. Li, F.J. Zhang, W.C. Oh, Mater. Sci. 36 (2013) 293. [32] F.J. Zhang, M.L. Chen, W.C. Oh, J. Mater. Res. 18 (2008) 583. [33] M.L. Chen, F.J. Zhang, W.C. Oh, J. Kor. Ceram. Soc. 45 (2008) 651. [34] J.P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 77 (1996) 3865. [35] D. Vanderbilt, Phys. Rev. 41(1990) 7892. [36] M. Ding, D.C.Sorescu, A. Star, J. Amer. Chem. Soc. 13 (2013) 9015.
Figure Captions. Fig. 1. SEM and HR-TEM images of the samples with (a) [TiO2]/[urea] = 1/1, (b) [TiO2]/[CNT] = 1/2, (c) [TiO2]/[CNT] = 1/8, and (d) [TiO2]/[CNT] = 4/1. Fig. 2. (a) The X-ray diffraction, and (b) Raman spectra of the samples. Fig. 3. Thermogravimetric and differential thermal analyses (TGA/DTA) curves of S1 sample from room temperature to 1000 °C. 10
Fig. 4. Absorption spectra of TiO2, CNTs, TiO2 doped S, and S-TiO2/CNTs nanohybrids materials. Fig. 5. Photodecomposition of MB catalyzed by the catalysts under visible-light illumination. Fig. 6. The structure of Ti4O8, the PDOS, and charge density difference in the Ti4O8 cluster adsorbed on CNT.
Table 1. Particlesize, cell constant, and EDX elemental (wt. %) of S-TiO2/CNTs. Elements (wt. %) Sample
Cell constant (Å)
Ti
O
C
S
S1
8.1 nm
16.1 nm
a=3.75
c=9.51
27.74
62.80
8.80
0.66
S2
8.2 nm
9.6 nm
a=3.75
c=9.52
27.62
65.24
6.50
0.64
S4
8.2 nm
6.5 nm
a=3.75
c=9.52
30,72
64.93
3.55
0.80
S8
8.2 nm
4.1 nm
a=3.75
c=9.52
28.41
64.34
6.48
0.77
Table 2. Photodecomposition of MB with the previous research. Samples
Light source
Time
MB left
Ref.
Our samples
150 W Xenon lamp, cut-off
4h
60%
3h
30% to 60%
Zhao et al. [25]
4h
45%
Peng et al. [26]
4h
55%
Wang et al. [27]
filter of 420 nm, N-doped TiO2
150 W Xenon lamp, cut-off filter of 400 nm
S-doped TiO2
300 W Xenon lamp, cut-off filter of 420 nm
PoPD/TiO2
500 W Xenon lamp, cut-off 11
filter of 420 nm
Cr/CNT/TiO2
Visible light irradiation
1h
15 to 25%
Chen et al. [28]
Zr-TiO2-S
Visible light Irradiation
4h
55% to 75%
Kim et al. [29]
Fe/CNT–TiO2,
Visible light irradiation
2h
38%
Li et al. [30]
TiO2/CNT
UV irradiation
1h
27 to 48%
Zhang et al. [31]
TiO2/CNT
UV irradiation
1h
34%
Chen et al. [32]
Highlights The nanohybrids S-doped TiO2/CNTs materials were synthesized by hydrolysis method. The percent of dyes decomposed highest at the sample with [TiO2]/[thiourea] = 1/4. ΔEads are −1.95 eV suggest that the adsorption processes are spontaneous. High photocatalytic due to bandgap reduction by S-doped, lower electron-hole recombination by CNTs.
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13
14
15
16
17
Graphical abstract
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PII: DOI: Reference:
S0022-3697(16)30184-6 http://dx.doi.org/10.1016/j.jpcs.2016.06.011 PCS7795
To appear in: Journal of Physical and Chemistry of Solids Received date: 17 December 2015 Revised date: 1 April 2016 Accepted date: 21 June 2016 Cite this article as: Nguyen Cao Khang, Duong Quoc Van, Nguyen Minh Thuy, Nguyen Van Minh and Phan Ngoc Minh, Remarkably enhanced photocatalytic activity by sulfur-doped titanium dioxide in nanohybrids with carbon nanotubes, Journal of Physical and Chemistry of Solids, http://dx.doi.org/10.1016/j.jpcs.2016.06.011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Remarkably enhanced photocatalytic activity by sulfur-doped titanium dioxide in nanohybrids with carbon nanotubes Nguyen Cao Khanga1, Duong Quoc Vana, Nguyen Minh Thuya, Nguyen Van Minha, Phan Ngoc Minhb a
Center for Nano Science and Technology, Hanoi National University of Education, 136 Xuan Thuy,
Hanoi, Vietnam b
Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Hanoi, Vietnam
[email protected] Abstract TiO2 doped S nanohybrids with carbon nanotubes (CNTs) were synthesized with CNTs, thiourea and TiO2 nanoparticles. The result indicated that the TiO2 nanoparticles with about 8 nm in size are attached on the sidewall of CNTs. The nanohybrids material can absorb at longer wavelength and the absorption even covers the whole range of visible region than that only TiO 2 nanoparticles. Application of the catalysts to photocatalytic degradation of methylene blue (MB) was tested under visible light irradiation. The result suggests that a high MB degradation activity of S-TiO2/CNTs due to a reduce band gap of TiO2 when S is doped, and the decrease in the possibility of electron-hole recombination by CNTs. In addition, the density functional-theory (DFT) calculations of the electronic band structures and density of states (DOS) to understand the bonding states between TiO2 and CNTs, proved that the TiO2/CNTs system is stable. Keywords: TiO2 doped S; CNTs; nanohybrids; photocatalytic.
1
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I. INTRODUCTION Semiconductors have been extensively researched and widely applied in many applications such as thin-film optical devices, biomaterials, catalyst in many reactions [1-6]. Among all the semiconductors, titanium dioxide (TiO2) has been the focus of researchers because of its chemical and physical durability, high activity, nontoxicity, and low cost. However, the TiO2 semiconducting has a large band gap (3.0-3.2 eV). Without modification, TiO2 can only harvest the UV part, which constitutes only about 4% of the total solar spectrum. Therefore, it is an important and challenging issue to develop new TiO2 systems with enhanced activities under both UV and visible light irradiation compared with bare TiO2, thereby improving the utilization of solar energy. Many efforts have been made to modify TiO2 with nonmetals or metals to efficiently extend photoresponse from the UV to the visible light region [7-14]. Furthermore, theoretical calculations also suggest that metal or nonmetal doping has considerably impacted on the band gap of TiO2. However, the high concentration of dopants is difficult to obtain and the synthesized materials are usually unstable to photo-corrosion. Recently, Karran et al. and Masakazu et al. [15, 16] indicated that the photocatalytic activity of TiO2 can be improved by making nanohybrids with CNTs due to the fact that the CNTs could not only provide a large surface area to support the catalyst but also decrease the ability of recombination of the electron-hole pairs. Their outstanding charge transfer abilities can favour the excited electron in the conduction band of nanocrystal semiconductor to migrate into the CNTs, and increase photocatalytic activity under visible light [17, 18]. Therefore, combination with reduction band gap of modified-TiO2 with CNTs will possible channel to increase the photocatlytic performance. This article highlights the literature on the S-doped TiO2, the composition of S-doped TiO2 with CNTs and discussion of the mechanism of enhancing photocatalytic activity of this material. We proposed that simultaneously reduce band gap and decrease the ability of the recombination of the electron-hole pairs of TiO2 will origin significantly to increase their photocatalytic properties. 2
II. EXPERIMENTS CNTs were firstly modified according to following procedure: 0.5 g CNTs were suspended in 40 mL solution of nitric acid and then refluxed for 12 hours at 120 oC. The resulting solution after this oxidization process was filtered and washed with deionized water, dried at 80 oC in air for 24 hours before being transferred to the 2-propanol solution. For nanocoating on CNTs, titanium tetraisopropoxide was added dropwise to the prepared CNTs dispersion, maintained at pH = 4 by HNO3 acid. To introduce sulfur dopant into the titania nanoparticles, thiourea is added to the colloidal nanoparticle solution. The prepared particles were dried at 80 °C for one day and heat treated at 450 °C for the crystallization. The samples S1, S2, S4, and S8 are synthesized with [TiO2]/[thiourea] mole ratio of 1/1, 1/2, 1/4, and 1/8, respectively. The [TiO2]/[CNTs] mole ratio is 10/1 for all samples. The structure of TiO2 samples were determined by X-ray diffractometer D5005 (Siemen) with CuKα radiation (λ = 1.5406 Å). The concentration of S remain in TiO2 were characterized by using energy dispersive X-ray (EDX). The morphologies and the elemental components of the samples were observed using a Hitachi S-4800 scanning electron microscope (SEM), and Tecnai F30 machine operated under 300 kV (HR-TEM). Optical absorption spectra were measured by Jasco V-670 spectrophotometer. The density functional theory (DFT) was used to calculate the electronic structures and PDOS of the nanohybrids TiO2/CNTs. For the photocatalytic activity, 25 mg of photocatalyst is mixed with 50 ml of 10 ppm methylene blue (MB) solution. The light source used was a 150 W high-pressure Xenon lamp with a cut-off filter of 420 nm. Before turning on the light, the suspension containing MB and photocatalyst was magnetically stirred continuously in dark until no change in the absorbance of the solution is observed. The purpose of this process is to make sure that the physical adsorption plays no role in reducing the MB concentration. The reduction in the concentration of the MB was determined from the absorbance value at the maximum of the 665 nm absorption spectrum for every MB by monitoring UV-Vis spectra.
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III. RESULTS AND DISCUSSIONS The crystalline morphology, particle size, and lattice spacing of the products were firstly investigated using SEM and HR-TEM, as shown in Fig. 1. SEM images of nanohybrids in Fig. 1a, 1b, and 1c show that the TiO2 nanoparticles with an average diameter of around 8 nm are attached on the CNTs surfaces, which is consistent with XRD results (see Fig.2). No aggregation of TiO2 particles were observed. The TiO2 decorated CNTs were further confirmed by the HR-TEM image in Fig. 1c. It can be seen that the diameters of carbon structures range from 40 nm to 60 nm. The HR-TEM image in Fig. 1d shows that the TiO2/CNTs is highly crystalline. The spacing of carbon nanotube tube is of 3.33 Å , while the <101> direction of TiO2 is determined to be 3.56 Å. Fig. 2a shows the powder XRD of the S-TiO2/CNTs. The XRD shows typical peaks that can be well assigned to the anatase TiO2 as well as characteristic peaks of CNTs, indicating the successful decoration of the anatase TiO2 nanoparticles on CNTs. The peaks at 2θ of 26.0 and 43.0o were associated with the <002> and <100> diffractions of the hexagonal graphite structure, similar to pristine CNTs, and the peaks at 2θ of 25.5, 37.8, 48.1, 54.0, and 62.8o can be perfectly attributed to the crystal planes of the <101>, <004>, <200>, <105>, and <204> of the anatase TiO2. The peak at 26.0o of CNTs does not appear. However, the broadening and asymmetry of the 25.5o peak of TiO2 in the samples shows the effect of CNTs on XRD spectrum of TiO2. The average crystalline size of TiO2-S powder was estimated from the full width at half maximum (FWHM) of the (101) XRD peak using Scherrer’s equation: d=k/βcos, where d is the average crystalline size, k the constant, the X-ray wavelength, β the full width at half maximum of the diffraction line, and the angle of diffraction. For the elemental analysis in the nanohybrids S-TiO2/CNTs, the EDX was used. The results confirm the existence of Ti, O, C, and S as well as other impure elements. The results of EDX elemental microanalysis, the average size, and the cell constant of TiO2 nanoparticles of the nanohybrids STiO2/CNTs are listed in Table 1.
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Raman spectra was performed to study the crystallographic structure of the samples. Four Raman peaks at 145, 399, 520, and 644 cm−1 were assigned to Eg, B1g, A1g, and Eg, correspond to the TiO2 anatase crystal respectively, as suggested in previous reports [19]. The positions and intensity of the four Raman active modes are also in good agreement with the reference values previously determined for the TiO2 anatase structure. Both the XRD and Raman results indicate that the samples are crystallized in the anatase phase without any impurity. To understand the loss of organic residues on the surface of the S-TiO2/CNTs under sintering, the thermogravimetric analysis was used to study the weight loss behaviors of the S-TiO2/CNTs. The TGA results of the S1 sample in Fig. 3 show that there is tiny weight loss of 1.8% from room temperature to 100 °C, which is mainly due to the evaporation of physically adsorbed or embedded water on the surface of the S-TiO2/CNTs. The weight loss of 2.1% from 150 to 480 °C is related to the loss of surface and deeper seated organics. Thermogravimetry and differential thermal analyses in Fig. 3 show that carbon nanotube starts to decompose at temperatures above 480 °C. A weight loss of 9.7% was obtained from 480 to 700 oC, corresponding to thermo-decomposition of CNTs. However, an exothermic peak at 612 °C indicates that the decomposition of CNTs mainly occurs at 612 °C. Finally, there is a very slow weight loss from 700 to 1000 oC, and TiO2 becomes the major component in the product. The absorption spectra of the samples are shifted to the visible light region as shown in Fig.4. According to previous studies, Augugliaro et al. [20] reveal that the sulfur dopants are indeed incorporated into the TiO2, thus changing the electronic structures. Umebayashi et al. [21] conclude that when S replaced into the O site of the TiO2 lattice, the mixing of the sulfur 3p states with the valence band was found to contribute to the increased width of the valence band, leading to the narrowing of the band gap. Dong et al. [22] reported that the substitution of Ti4+ by S6+ was the cause narrowed the band gap. Both anionic and cationic doping was accordingly proposed through the substitution of S2− or S4+/S6+ for O or Ti ions in the TiO2, respectively, leading to the effective 5
narrowing of the TiO2 band gap [23, 24]. So, the optical absorption was found to shift to lower energies as show in Fig. 4 show that S doped into TiO2 lattice. Furthermore, due to the presence of CNTs, the nanohybrids materials can absorb light of longer wavelengths and the absorption even covers the whole range of the measured UV-Vis region. Different from the pure TiO2, where most of these electron-hole pairs quickly recombine, in the TiO2/CNTs, the photogenerated electrons in the conduction band of anatase flow through the conductive CNTs and the holes in the valence band of anatase can generate hydroxyl radicals on the surface [4]. The relative position of the CNTs conduction band edge permits the transfer of electrons from the anatase surface to CNTs, allowing charge separation, stabilization, and hindering of recombination. The electrons can be shuttled to be nearly free. On the other, in the TiO2/CNT, the photogenerated electrons are injected into the conduction band of the TiO2 and can react with O2, triggering the formation of the very reactive superoxide radical ion (O2.-). At the same time, holes (h+) might be formed with electrons transferred from valence bond in TiO2 to CNTs [25]. The positive charged hole (h+) may react with the OH− derived from H2O, triggering the formation of hydroxyl radical (HO•). Consequently, these radical groups are responsible for the decomposition of the organic compounds. For the photocatalytic activity of the samples, visible-light was used to test the photodecomposition of MB. Fig. 5 displays the photodegradation behaviours of MB, using TiO2 nanoparticles, CNTs, and nanohybrids S-TiO2/CNTs. Pure CNTs exhibits almost no photocatalytic activity and the sample with TiO2 nanoparticles removes only 7% of MB after 4 hours irradiation. However, the percentage of MB removed by the samples S-TiO2/CNTS are about 6 to 8 times more than that of pure TiO2. The best photocatalytic MB degradation result is observed for the S4 sample ([TiO2]/[thiourea] = 1/4), which destroys up to 59% of MB molecules within 4 hours irradiation. Nevertheless, the best ratio of STiO2/CNTs for photocatalytic application depends on the experimental conditions and the substances used. Reducing the band gap and enhancing the absorption of radiation in the visible range are two main factors that help increase the photocatalytic efficiency of TiO2. The increase in photocatalytic 6
performance is also due to a large surface area of the nanohybrids S-TiO2/CNTs. Moreover, since CNTs have a high adsorption capacity for MB molecules, the concentration of MB in the vicinity of CNTs is higher than those in other places in the reaction system. When some MB molecules on the surface of TiO2 are degraded, the other MB molecules adsorbed on the CNTs can transfer to the residual vacancies through slip-induced surface diffusion, which may be a faster process than the free diffusion in solution. There are many reasons to increase the photocatalytic of S-TiO2/CNTs, but these reduce band gap of TiO2 due to the S doped, and higher absorb visible light due to the CNTs are the main reasons. High performance of S-TiO2/CNTs to the decomposition of MB was compared with the previous research on the photodecomposition of MB as listed in Table 2. Theoretical calculations were performed to investigate the interfacial electronic structures and electron transfer of CNT interfaced with Ti4O8 clusters. The CNT (10, 0) tube is used to represent typical 1 nm semiconducting CNT. The supercell is (a, b, c) = (30.00, 30.00, 17.04) Å. To optimize the structure of Ti4O8, the Task Geometry Optimization in DMol3 with the generalized gradient approximation (GGA) [34-36] was used. For an optimize the structure of Ti4O8 cluster, the results shown that in the six possible configurations, the configuration as shown in Fig. 6(i) with a least total energy (-77.7 eV) is the most sustainable. The total energy, the bonding energy, the Fermi energy and the relative volume of space of the configuration in Fig. 6(i) are -24137.1 eV, 77.7 eV, -5.6 eV and 82.10-30 m3, respectively. The ionization potential (IP) and electron affinity (EA) are a useful quantity for providing fundamental insight of electronic properties of Ti4O8. IP is the energy to remove an electron, and EA is the energy released when adding an electron to a neutral atom. The Dmol3 was used to calculations of IP and EA of Ti4O8. The results shown that the IP and EA of Ti4O8 are 8.94 eV and 2.85 eV, respectively. The charge analysis confirms that it is easier to add an electron to Ti4O8 cluster than to remove it. This result is consistent with the result of Fig. 6(b). As shown in Fig. 6b, in the Ti4O8/CNT systems, the density of electrons is increased in the Ti4O8 spatial domain, while the density of electrons 7
in the CNT is reduced. This trend is further confirmed by the result of DOS analysis in Fig. 6(a), in which a contribution of the d orbital in the total DOS of C atom can be clearly seen. The adsorption energy (Eads), defined as the reversible energy required to separate an adsorption system (Eads.sys) into a CNT and an adsorbed Ti4O8 clusters was calculated. ΔEads can be calculated by subtracting the sum of the total energy of optimized Ti4O8 clusters (ECluster) and CNT (ECNT) from the total energy of the adsorption system (Eads): ΔEads = Eads.sys – (ECNT – Ecluster) = -1.95 eV. A negative ΔEads indicates that the molecule adsorption is exothermic and thus the adsorption system is energetically stable.
III. CONCLUSION The S-TiO2/CNTs photocatalysts containing CNTs were prepared using a hydrolysis method. The nanohybrids materials can absorb longer wavelength light and the absorption even covers the whole range of visible region. The photocatalytic degradation of MB was observed from 6 to 8 times more efficient in the presence of nanohybrids TiO2/CNTs catalysts than neat TiO2. The percent of S depends on the ratio between TiO2 and thiourea. The highest decomposition rate of MB on the catalysts was observed for samples with [TiO2]/[thiourea] = 1/4. The HR-TEM image, the XRD, the DFT result, as well as a negative ΔEads indicate that the TiO2 attachment on the CNTs surface is energetically stable. Band gap reduction due to S doping and lower electron-hole recombination probability due to CNTs nanohybrids with TiO2 are two main reasons to increase S-TiO2/CNTs photocatalytic properties.
Acknowledgement This work was supported by the Ministry of Education and Training Grant No. B2014-17-46.
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Figure Captions. Fig. 1. SEM and HR-TEM images of the samples with (a) [TiO2]/[urea] = 1/1, (b) [TiO2]/[CNT] = 1/2, (c) [TiO2]/[CNT] = 1/8, and (d) [TiO2]/[CNT] = 4/1. Fig. 2. (a) The X-ray diffraction, and (b) Raman spectra of the samples. Fig. 3. Thermogravimetric and differential thermal analyses (TGA/DTA) curves of S1 sample from room temperature to 1000 °C. 10
Fig. 4. Absorption spectra of TiO2, CNTs, TiO2 doped S, and S-TiO2/CNTs nanohybrids materials. Fig. 5. Photodecomposition of MB catalyzed by the catalysts under visible-light illumination. Fig. 6. The structure of Ti4O8, the PDOS, and charge density difference in the Ti4O8 cluster adsorbed on CNT.
Table 1. Particlesize, cell constant, and EDX elemental (wt. %) of S-TiO2/CNTs. Elements (wt. %) Sample
Cell constant (Å)
Ti
O
C
S
S1
8.1 nm
16.1 nm
a=3.75
c=9.51
27.74
62.80
8.80
0.66
S2
8.2 nm
9.6 nm
a=3.75
c=9.52
27.62
65.24
6.50
0.64
S4
8.2 nm
6.5 nm
a=3.75
c=9.52
30,72
64.93
3.55
0.80
S8
8.2 nm
4.1 nm
a=3.75
c=9.52
28.41
64.34
6.48
0.77
Table 2. Photodecomposition of MB with the previous research. Samples
Light source
Time
MB left
Ref.
Our samples
150 W Xenon lamp, cut-off
4h
60%
3h
30% to 60%
Zhao et al. [25]
4h
45%
Peng et al. [26]
4h
55%
Wang et al. [27]
filter of 420 nm, N-doped TiO2
150 W Xenon lamp, cut-off filter of 400 nm
S-doped TiO2
300 W Xenon lamp, cut-off filter of 420 nm
PoPD/TiO2
500 W Xenon lamp, cut-off 11
filter of 420 nm
Cr/CNT/TiO2
Visible light irradiation
1h
15 to 25%
Chen et al. [28]
Zr-TiO2-S
Visible light Irradiation
4h
55% to 75%
Kim et al. [29]
Fe/CNT–TiO2,
Visible light irradiation
2h
38%
Li et al. [30]
TiO2/CNT
UV irradiation
1h
27 to 48%
Zhang et al. [31]
TiO2/CNT
UV irradiation
1h
34%
Chen et al. [32]
Highlights The nanohybrids S-doped TiO2/CNTs materials were synthesized by hydrolysis method. The percent of dyes decomposed highest at the sample with [TiO2]/[thiourea] = 1/4. ΔEads are −1.95 eV suggest that the adsorption processes are spontaneous. High photocatalytic due to bandgap reduction by S-doped, lower electron-hole recombination by CNTs.
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Graphical abstract
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