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Photodegradation of methylene blue in the visible spectrum: An efficient W6+ ion doped anatase titania photocatalyst via a solvothermal method
Vacuum 126 (2016) 63e69
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Photodegradation of methylene blue in the visible spectrum: An efficient W6þ ion doped anatase titania photocatalyst via a solvothermal method Yongkun Zou a, c, Yuxuan Gong b, *, Bizhou Lin c, Nathan P. Mellott b a b c
College of Applied Science and Engineering, University of Cincinnati, Cincinnati, OH 45221, USA Kazuo Inamori School of Engineering, Alfred University, Alfred, NY 14802, USA Fujian Key Laboratory of Functional Materials, College of Materials Science and Engineering, Huaqiao University, Xiamen 361021, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 December 2015 Received in revised form 22 January 2016 Accepted 23 January 2016 Available online 26 January 2016
Solvothermal prepared tungsten (VI) doped titania were evaluated as photocatalysts for methylene blue (MB) degradation under the visible spectrum. Incorporation of hexavalent tungsten alters both the surface and bulk properties. Addition of tungsten into the titania lattice changes the lattice parameter, morphology and photocatalytic performance. X-ray photoelectron spectroscopy and x-ray diffraction suggested that W6þ does not change the chemical environment of Ti4þ but contracts the lattice of titania. By adding 5 wt% tungsten, the resulting sample shows high surface area (130.4 m2/g), a contracted lattice and high efficiency for photodegradation (120 min bleaching with a remaining concentration of 21%). However, doping of tungsten beyond 5 wt% creates more recombination center for photogenerated electronehole pairs, which deteriorates the photocatalytic performance. The W6þ ions function as electron receivers and avoid the recombination of electronehole pairs. It was shown that OH. radicals are responsible for the ring opening reaction of aromatic MB. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Photocatalyst Tungsten doped titania X-ray photoelectron spectroscopy
1. Introduction Photocatalysis is a process where the combination of excitation light and a catalyst induces or accelerates a photochemical reaction. Some semiconductors act as a photocatalyst for the light-induced photochemical reactions due to its unique electronic structure: in particular, a filled valence band (VB) and an empty conduction band (CB) [1]. Titania has risen to be one of the most promising photocatalyst for degradation of organic pollutants in both water and air since being reported by Fujishima and Honda in 1972 [2]. The excitation of electrons in the VB of titania, by photons with higher energy than the bandgap, is the primary mechanism underlying its photochemistry and photoelectrochemisty [1,2]. Titania is used and studied in large part because of its nontoxicity, water insolubility, hydrophilicity, cost-efficiency and stability against photocorrosion [1]. Additionally, titania has been deposited to various surfaces including glass [3], fibers [4], metals [5], inorganic materials
* Corresponding author. Inamori School of Engineering, 2 Pine St, Alfred, NY, 14802, USA. E-mail address: [email protected] (Y. Gong). http://dx.doi.org/10.1016/j.vacuum.2016.01.018 0042-207X/© 2016 Elsevier Ltd. All rights reserved.
[2,4e6], sand and porous carbon [6]. However, the large bandgap of titania (~3.2 eV for anatase and brookite, ~3.0 eV for rutile) requires an excitation wavelength that falls into the UV light region. Given that less than ~5% of the solar flux incident on earth lies in the UV regime, it is crucial to manipulate the bandgap in order to increase the photoactivity in the visible spectrum. For the purpose of increasing its visible light photocatalytic activity, titania has been modified using various strategies including: coupling with a narrow bandgap semiconductor [7], metal [8] or non-metal ion doping [9], co-doping [10,11], surface fluorination [12,13] and noble metal deposition [14e16]. The related underlying hypothesis is that the electronic structure change and/or the surface modification of titania can result in higher quantum efficiency [7e16]. Although space charge carrier species during photocatalytic reactions vary with different pollutants and depend on the surface electronic structure of titania, it is widely accepted that the primary reaction responsible for the photocatalysis process is the interfacial redox of CB electron and VB hole [1,2]. Thus, strategies of modifying both surface and bulk properties of titania could result in the use of titania as photocatalyst in visible light range. Incorporation of dopant cations into titania can affect the
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surface and bulk properties [17]. Tungsten (W) ions tend to migrate toward the surface of titania, shifting the isoelectric point to lower values than pristine titania [18]. Bulk properties of titania are also modified by tungsten due to the isomorphic substitution within lattice [17,18]. Tungsten, when in the 6þ valence state, works as an electron trap. Existence of the electron trap enables photochromic materials to have energy storage capacity, which can then be used in the photocatalytic reactions. In principle, presence of W6þ improves the photodegradation ability of titania by two mechanisms: 1) the decrease in the recombination of electronehole pairs and 2) expanding of the useful range of radiation to the visible regime by narrowing the titania bandgap [19e21]. Aromatic dyes such as methylene blue (MB) are discharged into water in developing countries, and hence affect the whole environment cycle by polluting the water sources. Thus, in this study, titania and tungsten doped titania are synthesized using solvothermal method and evaluated as a photocatalyst for the degradation of MB. A facile, one-step synthesis of resultant tungsten doped titania is reported with good photodegradation properties in the visible light spectrum. Phase identification, morphology and valence state of resulting titanias was investigated using x-ray diffraction, scanning electron microscopy and x-ray photoelectron spectroscopy, respectively. It was found that W6þ is the valence state of tungsten ions and doping of tungsten ions of 5 wt% concentration greatly enhanced the photodegradation rate in the visible regime. In the present work, W (VI) ions function as electron traps, decreased the lattice size of titania, and further enhanced the photodegradation property in visible spectra. 2. Materials & methods 2.1. Materials preparation Reagent purity TiSO4 and CO(NH2)2 (Sigma Aldrich) were dissolved in ethanol/DI-water (1:1 volume ratio) under vigorous stirring with a molar ratio of 1 TiSO4:1 CO(NH2)2. Doping of tungsten into titania was achieved by adding Na2WO.42H2O at the desired weight ratio (eg.3 wt%, 5 wt% and 10 wt%). The as-prepared precursor solution was stable and clear at room temperature. For a typical synthesis process, as prepared precursor solution was added to a Teflon-lined stainless steel autoclave. The autoclave was kept at 150 C for 8 h in a resistive electric oven. After reaction, the nanosized titania powder was collected by centrifugation, and then washed in DI-water/ethanol, and dried in a vacuum oven. 2.2. Photocatalytic activity measurement Employed wavelength of photocatalytic activity measurement was determined by measuring the absorbance maxima of methylene blue (MB) using a UV light source with two glass filters (excitation wavelength: 200e770 nm, Philips. Absorption spectrum (eg.Fig. 1) shows an absorbance maximum at 664.5 nm, which corresponds to the MBþ. For all photocatalytic reaction measurements, an excitation wavelength of 664.5 nm was used. Additionally, photocatalytic reaction at UV radiation (~365 nm) was measured in order to evaluate the effect of tungsten dopants on the degradation of MB. Photocatalysts (1 g/L) were suspended in 100 mL of MB solution (concentration: 10 mg/L) by magnetic stirring for all measurements. Prior to exposure of light irradiation, the suspension was stirred in dark room for 30 min to avoid any interaction between incident photons and photocatalysts. Upon radiation of light, 3 mL of solution were taken out every 10 min, and the titania was separated by centrifugation. The remaining solution after centrifugation was used for the measurement of MB concentration.
Fig. 1. Absorbance curve of methylene blue under variable wavelength excitation
2.3. BET surface area BET surface area of the titania and tungsten doped titania powders were measured by nitrogen adsorption at 197 C (Gemini, Micromeretics). Prior to nitrogen adsorption, ~0.1 g of powders were degassed at 150 C for 2 h under a helium atmosphere. Nitrogen adsorption isotherms were collected using relative pressure ranging from 0.05 to 0.99. Triplicates and standardization (alumina reference, Micromeretics) was performed to yield an error better than stated instrumental error. 2.4. UVeVis spectra UVeVis spectra of as-synthesized titania were measured using pressed pellet in a UVeVis spectrometer (UV-2550, Shimadzu). The wavelength of measurements ranged from 200 nm to 800 nm, with BaSO4 used as the reference material. 2.5. Powder diffraction The crystallographic structure of resulting powders was characterized using X-ray powder diffraction (D8 Advance, Bruker). The emission current was set to 40 mA with a CuKa source. A scanning range of 15 to 80 and an accelerating voltage of 40 kV were used in all measurements. MDI Jade 8.0 with ICDD PDF4þ was used for the phase identification. Rietveld refinement was used to obtain the detailed crystallography. Crystallographic construction (version 2.2.4, CrystalMaker Ltd., USA) and crystallographic values of anatase TiO2 is obtained from previously reported data [22]. 2.6. Scanning electron microscopy Morphology of resultant titania photocatalysts was captured by a scanning electron microscope (XL30 ESEM, FEI-Philips) with an accelerating voltage of 10 kV. 2.7. X-ray photoelectron spectroscopy The valence states of the resultant powders were investigated using X-ray photoelectron spectroscopy (5300, PerkineElmer). The XPS spectra were collected with MgKa radiation source (photon energy: 1253.6 eV; beam size: 100 mm) at 20 mA anode current.
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Survey scans were performed with a pass energy of 89.45 eV and a step size of 0.2 eV. Resultant spectra were used to adjust the acquisition width, beam dwelling time and pass energy of high resolution scans (pass energy: 35.75 eV; step size: 0.1 eV; beam dwelling time: ~ 1s/step). All acquired spectra were corrected using hydrocarbon C1s peak located at 284.6 eV. Resolved peaks (eg.Ti3p and W4f) were deconvoluted using CasaXPS analysis software. Constraints such as FWHM, peak position and peak area are shown in Table 1. To isolate Ti3p peak from W4f peaks, peak position and FWHM of the lab prepared pure titania was used as reference. However, the use of pure titania as a reference relies on the unchanging valence state of Ti after doping, which was confirmed by XPS. The relationship between the peak area ratio of W4f5/2 and W4f7/2 is 3:4 given the electron distribution of quantum orbital split in f electrons; also, the peak area of Ti3p (obtained from pure titania) is not constrained due to the peak area or intensity varies as a function of vacuum level, sample morphology as well as other instrumental uncertainties. Valence states of Ti and W from high resolution scans were determined through comparison with the literature. 3. Results and discussion Fig. 2 shows the diffraction pattern of pure TiO2 and tungsten doped TiO2. Characteristic diffraction planes of anatase TiO2 were observed in all measured samples, and anatase TiO2 (tetragonal) is the only identifiable phase (PDF 86-1156). With increasing doping of tungsten into titania, no observable difference in terms of peak shape and position can be identified. Given the similar size between Ti4þ (0.68 Å) and Wnþ (0.41e0.70 Å), it can be assumed that lattice sites of Ti4þ were replaced by Wnþ. Observed peaks at 25.4 , 38.0 , 48.1, 54.1, 54.9 , 62.7 and 75.3 correspond to (101), (004), (200), (105), (211), (204) and (215) diffraction plane of anatase TiO2, respectively. Table 2 shows properties such as lattice parameter (obtained from Rietveld refinement), surface area. As shown in Table 2, doping of tungsten into titania decreases the lattice parameters a & c. Such a decrease of lattice parameters can be well explained by lattice contraction caused by replacing Ti with high valency ions [23,24]. High tungsten doping, as shown by 10 wt%, resulted in a further decrease of lattice parameter. It has been found that increasing substitution of the octahedral site of TiO2 with a smaller size cation (eg.W6þ ions; ionic radius ~0.41 Å) contracts the lattice of anatase titania [25,26]. This suggests that the W in the sample here are likely in the 6 þ state, which is further confirmed by XPS below. Among all the measured samples, the 10 wt% tungsten doped titania exhibits the smallest lattice, and 3 wt% tungsten doped titania has the most intense nitrogen adsorption behavior (SBET ¼ 134.6 m2/g). By applying the solvothermal method to synthesis of titania or doped titania in this study, surface area reported here (113.8e134.6 m2/g) is higher than similar studies using different processing route [17,20]. Measured UVeVis spectra, as shown in Fig. 3, the 5 wt% tungsten doped titania exhibits the largest red shift. Red shifts of higher
Table 1 Applied constraints for deconvoluting resolved Ti3p and W4f peaks. Peak
FWHM(eV)
Peak position
Peak area
Ti3p (from undoped) W4f5/2 W4f7/2
1.9 0 to 2.5 0 to 2.5
37.2 b þ 2.2 b
e a*0.75 a
*Note: a is the assumed value of peak area, and b is the assumed value for peak position.
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valence ion doped titania (in this case, W6 þ doped titania) can be attributed to the charge-transfer transition between the f electrons and TiO2 conduction or valence band, as reported the work of Xu and et al. on rare-earth ion doped titania photocatalysts [27]. As reported in their study [27] as well as other's work [28,29], a larger red shift generally indicates that the sample absorbs more photon and thus the photocatalytic reactivity is enhanced. It has been suggested that the higher degree of lattice distortion (contraction in this case) could induce more carriers (electronehole pairs) under longer wavelengths, resulting in the largest red shift; however, this is not the case here [30]. Excessive doping of tungsten beyond 5% generates more recombination center for the photogenerated carriers [31], and thus decrease the degree of red shift. A small bandgap is more favorable for maximizing photocatalytic activities, given longer wavelengths excitation (eg.visible light, infrared and etc) can be utilized. However, photocatalysis efficiency is also greatly dependent on recombination rate of hþ and e pairs [19e21], which can be affected by materials surface morphology [32], surface area [33] and electronic structure [34]. In pure titania systems, recombination of hþ and e pairs happens in 109 s [35], and hence efficiency of photocatalysis can be greatly deteriorated. To avoid the recombination of electron/hole pairs in titania, higher or lower valence ions can be used as dopant to receive electrons or holes [36,37]. In this study, tungsten was employed as a higher valence ion to follow such mechanism and approach. To verify the valence state of W ion as well as Ti ion, XPS high resolution spectra of W4f and Ti2p were acquired as shown in Fig. 4. As shown from Fig. 4a to d, peak positions of Ti2p1/2 and Ti2p3/2 remain unchanged w/ or w/o doping of tungsten. Summary of peak positions and FWHMs are shown in Table 3. Through the series of samples (undoped/doped) measured, the FWHM of Ti2p1/2 and Ti2p3/2 peaks remains a value of 1.9 eV and 1.0 eV respectively. Also, there is no observable change in terms of the peak position of Ti2p1/ 2 and Ti2p3/2 peaks. This implies that the chemical environmental of Ti ions remains relatively unchanged after doping with tungsten. Ti2p1/2 and Ti2p3/2 peaks located at 464.0 eV and 458.4 eV (standard deviation of peak position: ~0.2 eV) correspond to a valence state of Ti4þ [38e40]. According to selected literature references [41e46], Ti2p3/2 peak of the oxidation states of Ti(0), Ti(II) and Ti(III) is located at 453.9 eV, 455.3 eV and 457.1 eV, respectively. In order to identify the valence state of W ions from a resolved W4f & Ti3p peak, FWHM and peak position of Ti3p peak measured from pure titania (eg.Fig. 4f) was used as a deconvolution constraint in addition to other constraints (eg.Table 1). Given the unchanged valence state of titania after doping with tungsten, and the fact that no quantification (eg. percentage of valence state) was attempted, we believe this approach is valid [46]. After applying the constraints (eg.Table 1), W4f5/2 and W4f7/2 peaks can be isolated from Ti3p peak as shown in Fig. 4f through 4h. Peak positions of W4f5/2 and W4f7/2 at ~37.2 eV and ~35.0 eV corresponds to W6þ [47e50], which follows lattice contraction mechanism as indicated by results of Rietveld refinement (eg.Table 2). Other common oxidation states of tungsten such as W(0) and W(IV) features a W4f7/2 locates at ~31.2 and ~32.9 eV, respectively [51]. The FWHM of W4f5/2 and W4f7/2 peaks decreases with increasing tungsten doping amount. Narrowed distribution of characteristic W photoelectrons (after doping) again confirmed the lattice contraction effect, which was caused by replacing Ti4þ ions with higher valence W6þ ions [52,53]. For O1s peak of undoped and doped titania, within a standard deviation of peak position of ~0.2 eV, no observable trend in terms of peak position and FWHM was found after doping. Resultant morphologies of as-synthesized undoped and doped titania were investigated by scanning electron microscopy, as shown in Fig. 5a through 5d.
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Fig. 2. Powder diffraction pattern of pure titania and tungsten doped titania.
Table 2 Textural properties of pure titania and tungsten doped titania. Sample
a(Å)
c(Å)
SBET(m2/g)
Undoped 3wt%W 5wt%W 10wt%W
3.7874 3.7848 3.7806 3.7769
9.4879 9.4748 9.4730 9.4594
125.3 134.6 130.4 113.8
Fig. 3. UVeVis spectra of pure titania and tungsten doped titania.
As synthesized titania generally have a spherical morphology in the size range of ~500 nme~3 mm. Dumbbell shaped titania particles can be observed for undoped and 3 wt% tungsten doped samples. A further increase in the doping concentration of tungsten to 10 wt% resulted in a less developed spherical particle
morphology with random shaped small particles, as shown in Fig. 5d. However, agglomeration of particles was observed for all samples and can be related to strategies of sample preparation as well as storage. To evaluate the photocatalytic performance of as-synthesized titania and its doped counterparts, remaining MB concentration ratio was studied as a function of reaction time under visible light of 664.5 nm wavelength (eg.Fig. 6a). As shown by Fig. 6a, 5 wt% tungsten doped titania shows the most efficient photocatalysis process with a C/C0 value of 0.21 after 120 min of reaction. Further increase (10 wt% tungsten) in tungsten concentration leads to a deleterious photocatalytic performance, and after 120 min of reaction a C/C0 value of 0.93 is reported. To better compare the photocatalytic efficiency, the apparent first-order rate constant (kapp) was calculated. The kapp of undoped titania, 3 wt%, 5 wt% and 10 wt% tungsten doped titania is 1.3 103, 5.2 103, 13.0 103 and 0.6 103 (min1), respectively. The rate constants in this study are equivalent or better comparing to literature references [29,54] related to the photocatalytic performance of titania under visible light (degradation of MB), while a facile processing is retained and cost of raw materials is low. Additionally, we have measured the photocatalytic performance of undoped and doped titania under UV radiation (eg. Fig. 6b). The results in Fig. 6b show that the doping of tungsten unnoticeably affects the photocatalytic activity of titania with respect to its photocatalytic performance under UV radiation (~365 nm). Thus, the doping of tungsten into titania significantly alters the photocatalytic performance of titania under visible light, while there is an unnoticeable effect of doping on its photocatalytic behavior under UV radiation. Conclusive results of the key factors affecting the photocatalytic performance can be challenging and is beyond the perspective of this paper. However, most efficient photocatalytic performance of 5 wt% tungsten doped titania can be attributed to its contracted lattice (quantum effect), most significant red shift
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Fig. 4. XPS spectra of a) Ti2p of undoped titania, b) Ti2p of 3 wt% tungsten doped titania, c) Ti2p of 5 wt% tungsten doped titania, d) Ti2p of 10 wt% tungsten doped titania, e) Ti3p of undoped titania, f) Ti3p & W4f of 3 wt% tungsten doped titania, g) Ti3p & W4f of 5 wt% tungsten doped titania and h) Ti3p & W4f of 10 wt% tungsten doped titania.
Table 3 Summary of acquired XPS high resolution spectra. Sample
Undoped 3wt%W 5wt%W 10wt%W
Ti2p1/2/Ti2p3/2
O1s
W4f5/2/W4f7/2
Ti3p
Peak
FWHM
Peak
FWHM
Peak
FWHM
Peak
FWHM
464.2/458.6 464.0/458.4 464.0/458.4 464.0/458.4
1.9/1.0 1.9/1.0 1.9/1.0 1.9/1.0
529.8 529.6 529.6 529.7
1.3 1.3 1.3 1.4
NA 37.2/35.0 37.2/35 37.4/35.2
NA 1.7/1.9 1.5/1.9 1.3/1.5
37.2 37.2 37.2 37.2
1.9 1.9 1.9 1.9
among all samples, relatively large surface area (more reaction sites) and well-developed spherical morphology. In terms of its photocatalytic reaction, general mechanisms of degradation of MB by tungsten doped titania can be described as follows [55e57]: ) þ TiO2(hþ) TiO2 þ hn(visible) /TiO2(e cb
(1)
, TiO2(hþ) þ OH / OH þ Hþ
(2)
) þ O2 / O, TiO2(e 2 cb
(3)
TiO2(e ) þ W(VI) / W(V) cb
(4)
þ O, 2 þ TiO2(ecb ) þ 2H / H2O2
(5)
þ O, 2 þ 2H / H2O2 þ O2
(6)
, TiO2(e ) þ H2O2 / OH þ OHcb
(7)
OH. or O, 2 þ MB /// ring opening / CO2þH2O
(8)
In the present study, visible excitation triggers the generation of an electronehole pair in tungsten doped titania (eq (1)). Simultaneously, OH. and superoxide O.2 radicals were generated (eq (2) and eq (3)) by the first reaction. For doped titania, W6þ captures the conduction band electrons (eq (4)), thus avoiding the recombination of electronehole pairs and hinders the O.2 generation. The role of pH as shown by eq (5) and eq (6) suggests that low proton concentraions prevent the superoxide radical transformation into
H2O2 and the subsequent generation of secondary OH. (eq (7)) [55,56]. Since DI-water is used in this study for the photocatalytic reaction, it would be most probable that main mechanism for photocatalytic reaction is the interaction between OH. radicals and MB [57]. It has been well established that OH. radicals react with aromatic rings (MB) leading finally to ring-opening products (eq (8)), which are essentially non-toxic and environmentally friendly. 4. Conclusion A facile, one-step synthesis of nano-sized titania and tungsten doped titania was achieved by a solvothermal method. Such process yields powders with large surface area (in the magnitude of 100' s m2/g) as well as a contracted lattice with doping. It was found that photocatalytic performance can be related to surface area of as synthesized powder, lattice contraction, red shift over longer wavelength and morphology of resultant nano-particles, but mechanisms regarding the dominant factor (surface area, lattice distortion degree, red shift or morphology) remain unveiled. Addition of 5 wt% tungsten into anatase can significantly enhances its photocatalytic performance. By applying contraints to the deconvolution of XPS spectra, it was found that high valence W6þ ion substituting for Ti4þ ions is the main mechanism for change in lattice parameter as well as the red shift. Generally lattice parameters decrease with increasing addition of W6þ ion due to a lattice contraction effect. However, excessive addition of W6þ ions beyond 5 wt% can lead to a decrease in photocatalytic performance due to creation of more recombination centers for photogenerated electronehole pairs.
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Fig. 5. Scanning electron micrograph of a) undoped titania, b) 3 wt% tungsten doped titania, c) 5 wt% tungsten doped titania and d) 10 wt% tungsten doped titania.
Fig. 6. Concentration of methylene blue as a function of photocatalytic reaction time under a) visible radiation, 664.5 nm and b) UV radiation, 365 nm
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Photodegradation of methylene blue in the visible spectrum: An efficient W6þ ion doped anatase titania photocatalyst via a solvothermal method Yongkun Zou a, c, Yuxuan Gong b, *, Bizhou Lin c, Nathan P. Mellott b a b c
College of Applied Science and Engineering, University of Cincinnati, Cincinnati, OH 45221, USA Kazuo Inamori School of Engineering, Alfred University, Alfred, NY 14802, USA Fujian Key Laboratory of Functional Materials, College of Materials Science and Engineering, Huaqiao University, Xiamen 361021, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 December 2015 Received in revised form 22 January 2016 Accepted 23 January 2016 Available online 26 January 2016
Solvothermal prepared tungsten (VI) doped titania were evaluated as photocatalysts for methylene blue (MB) degradation under the visible spectrum. Incorporation of hexavalent tungsten alters both the surface and bulk properties. Addition of tungsten into the titania lattice changes the lattice parameter, morphology and photocatalytic performance. X-ray photoelectron spectroscopy and x-ray diffraction suggested that W6þ does not change the chemical environment of Ti4þ but contracts the lattice of titania. By adding 5 wt% tungsten, the resulting sample shows high surface area (130.4 m2/g), a contracted lattice and high efficiency for photodegradation (120 min bleaching with a remaining concentration of 21%). However, doping of tungsten beyond 5 wt% creates more recombination center for photogenerated electronehole pairs, which deteriorates the photocatalytic performance. The W6þ ions function as electron receivers and avoid the recombination of electronehole pairs. It was shown that OH. radicals are responsible for the ring opening reaction of aromatic MB. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Photocatalyst Tungsten doped titania X-ray photoelectron spectroscopy
1. Introduction Photocatalysis is a process where the combination of excitation light and a catalyst induces or accelerates a photochemical reaction. Some semiconductors act as a photocatalyst for the light-induced photochemical reactions due to its unique electronic structure: in particular, a filled valence band (VB) and an empty conduction band (CB) [1]. Titania has risen to be one of the most promising photocatalyst for degradation of organic pollutants in both water and air since being reported by Fujishima and Honda in 1972 [2]. The excitation of electrons in the VB of titania, by photons with higher energy than the bandgap, is the primary mechanism underlying its photochemistry and photoelectrochemisty [1,2]. Titania is used and studied in large part because of its nontoxicity, water insolubility, hydrophilicity, cost-efficiency and stability against photocorrosion [1]. Additionally, titania has been deposited to various surfaces including glass [3], fibers [4], metals [5], inorganic materials
* Corresponding author. Inamori School of Engineering, 2 Pine St, Alfred, NY, 14802, USA. E-mail address: [email protected] (Y. Gong). http://dx.doi.org/10.1016/j.vacuum.2016.01.018 0042-207X/© 2016 Elsevier Ltd. All rights reserved.
[2,4e6], sand and porous carbon [6]. However, the large bandgap of titania (~3.2 eV for anatase and brookite, ~3.0 eV for rutile) requires an excitation wavelength that falls into the UV light region. Given that less than ~5% of the solar flux incident on earth lies in the UV regime, it is crucial to manipulate the bandgap in order to increase the photoactivity in the visible spectrum. For the purpose of increasing its visible light photocatalytic activity, titania has been modified using various strategies including: coupling with a narrow bandgap semiconductor [7], metal [8] or non-metal ion doping [9], co-doping [10,11], surface fluorination [12,13] and noble metal deposition [14e16]. The related underlying hypothesis is that the electronic structure change and/or the surface modification of titania can result in higher quantum efficiency [7e16]. Although space charge carrier species during photocatalytic reactions vary with different pollutants and depend on the surface electronic structure of titania, it is widely accepted that the primary reaction responsible for the photocatalysis process is the interfacial redox of CB electron and VB hole [1,2]. Thus, strategies of modifying both surface and bulk properties of titania could result in the use of titania as photocatalyst in visible light range. Incorporation of dopant cations into titania can affect the
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surface and bulk properties [17]. Tungsten (W) ions tend to migrate toward the surface of titania, shifting the isoelectric point to lower values than pristine titania [18]. Bulk properties of titania are also modified by tungsten due to the isomorphic substitution within lattice [17,18]. Tungsten, when in the 6þ valence state, works as an electron trap. Existence of the electron trap enables photochromic materials to have energy storage capacity, which can then be used in the photocatalytic reactions. In principle, presence of W6þ improves the photodegradation ability of titania by two mechanisms: 1) the decrease in the recombination of electronehole pairs and 2) expanding of the useful range of radiation to the visible regime by narrowing the titania bandgap [19e21]. Aromatic dyes such as methylene blue (MB) are discharged into water in developing countries, and hence affect the whole environment cycle by polluting the water sources. Thus, in this study, titania and tungsten doped titania are synthesized using solvothermal method and evaluated as a photocatalyst for the degradation of MB. A facile, one-step synthesis of resultant tungsten doped titania is reported with good photodegradation properties in the visible light spectrum. Phase identification, morphology and valence state of resulting titanias was investigated using x-ray diffraction, scanning electron microscopy and x-ray photoelectron spectroscopy, respectively. It was found that W6þ is the valence state of tungsten ions and doping of tungsten ions of 5 wt% concentration greatly enhanced the photodegradation rate in the visible regime. In the present work, W (VI) ions function as electron traps, decreased the lattice size of titania, and further enhanced the photodegradation property in visible spectra. 2. Materials & methods 2.1. Materials preparation Reagent purity TiSO4 and CO(NH2)2 (Sigma Aldrich) were dissolved in ethanol/DI-water (1:1 volume ratio) under vigorous stirring with a molar ratio of 1 TiSO4:1 CO(NH2)2. Doping of tungsten into titania was achieved by adding Na2WO.42H2O at the desired weight ratio (eg.3 wt%, 5 wt% and 10 wt%). The as-prepared precursor solution was stable and clear at room temperature. For a typical synthesis process, as prepared precursor solution was added to a Teflon-lined stainless steel autoclave. The autoclave was kept at 150 C for 8 h in a resistive electric oven. After reaction, the nanosized titania powder was collected by centrifugation, and then washed in DI-water/ethanol, and dried in a vacuum oven. 2.2. Photocatalytic activity measurement Employed wavelength of photocatalytic activity measurement was determined by measuring the absorbance maxima of methylene blue (MB) using a UV light source with two glass filters (excitation wavelength: 200e770 nm, Philips. Absorption spectrum (eg.Fig. 1) shows an absorbance maximum at 664.5 nm, which corresponds to the MBþ. For all photocatalytic reaction measurements, an excitation wavelength of 664.5 nm was used. Additionally, photocatalytic reaction at UV radiation (~365 nm) was measured in order to evaluate the effect of tungsten dopants on the degradation of MB. Photocatalysts (1 g/L) were suspended in 100 mL of MB solution (concentration: 10 mg/L) by magnetic stirring for all measurements. Prior to exposure of light irradiation, the suspension was stirred in dark room for 30 min to avoid any interaction between incident photons and photocatalysts. Upon radiation of light, 3 mL of solution were taken out every 10 min, and the titania was separated by centrifugation. The remaining solution after centrifugation was used for the measurement of MB concentration.
Fig. 1. Absorbance curve of methylene blue under variable wavelength excitation
2.3. BET surface area BET surface area of the titania and tungsten doped titania powders were measured by nitrogen adsorption at 197 C (Gemini, Micromeretics). Prior to nitrogen adsorption, ~0.1 g of powders were degassed at 150 C for 2 h under a helium atmosphere. Nitrogen adsorption isotherms were collected using relative pressure ranging from 0.05 to 0.99. Triplicates and standardization (alumina reference, Micromeretics) was performed to yield an error better than stated instrumental error. 2.4. UVeVis spectra UVeVis spectra of as-synthesized titania were measured using pressed pellet in a UVeVis spectrometer (UV-2550, Shimadzu). The wavelength of measurements ranged from 200 nm to 800 nm, with BaSO4 used as the reference material. 2.5. Powder diffraction The crystallographic structure of resulting powders was characterized using X-ray powder diffraction (D8 Advance, Bruker). The emission current was set to 40 mA with a CuKa source. A scanning range of 15 to 80 and an accelerating voltage of 40 kV were used in all measurements. MDI Jade 8.0 with ICDD PDF4þ was used for the phase identification. Rietveld refinement was used to obtain the detailed crystallography. Crystallographic construction (version 2.2.4, CrystalMaker Ltd., USA) and crystallographic values of anatase TiO2 is obtained from previously reported data [22]. 2.6. Scanning electron microscopy Morphology of resultant titania photocatalysts was captured by a scanning electron microscope (XL30 ESEM, FEI-Philips) with an accelerating voltage of 10 kV. 2.7. X-ray photoelectron spectroscopy The valence states of the resultant powders were investigated using X-ray photoelectron spectroscopy (5300, PerkineElmer). The XPS spectra were collected with MgKa radiation source (photon energy: 1253.6 eV; beam size: 100 mm) at 20 mA anode current.
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Survey scans were performed with a pass energy of 89.45 eV and a step size of 0.2 eV. Resultant spectra were used to adjust the acquisition width, beam dwelling time and pass energy of high resolution scans (pass energy: 35.75 eV; step size: 0.1 eV; beam dwelling time: ~ 1s/step). All acquired spectra were corrected using hydrocarbon C1s peak located at 284.6 eV. Resolved peaks (eg.Ti3p and W4f) were deconvoluted using CasaXPS analysis software. Constraints such as FWHM, peak position and peak area are shown in Table 1. To isolate Ti3p peak from W4f peaks, peak position and FWHM of the lab prepared pure titania was used as reference. However, the use of pure titania as a reference relies on the unchanging valence state of Ti after doping, which was confirmed by XPS. The relationship between the peak area ratio of W4f5/2 and W4f7/2 is 3:4 given the electron distribution of quantum orbital split in f electrons; also, the peak area of Ti3p (obtained from pure titania) is not constrained due to the peak area or intensity varies as a function of vacuum level, sample morphology as well as other instrumental uncertainties. Valence states of Ti and W from high resolution scans were determined through comparison with the literature. 3. Results and discussion Fig. 2 shows the diffraction pattern of pure TiO2 and tungsten doped TiO2. Characteristic diffraction planes of anatase TiO2 were observed in all measured samples, and anatase TiO2 (tetragonal) is the only identifiable phase (PDF 86-1156). With increasing doping of tungsten into titania, no observable difference in terms of peak shape and position can be identified. Given the similar size between Ti4þ (0.68 Å) and Wnþ (0.41e0.70 Å), it can be assumed that lattice sites of Ti4þ were replaced by Wnþ. Observed peaks at 25.4 , 38.0 , 48.1, 54.1, 54.9 , 62.7 and 75.3 correspond to (101), (004), (200), (105), (211), (204) and (215) diffraction plane of anatase TiO2, respectively. Table 2 shows properties such as lattice parameter (obtained from Rietveld refinement), surface area. As shown in Table 2, doping of tungsten into titania decreases the lattice parameters a & c. Such a decrease of lattice parameters can be well explained by lattice contraction caused by replacing Ti with high valency ions [23,24]. High tungsten doping, as shown by 10 wt%, resulted in a further decrease of lattice parameter. It has been found that increasing substitution of the octahedral site of TiO2 with a smaller size cation (eg.W6þ ions; ionic radius ~0.41 Å) contracts the lattice of anatase titania [25,26]. This suggests that the W in the sample here are likely in the 6 þ state, which is further confirmed by XPS below. Among all the measured samples, the 10 wt% tungsten doped titania exhibits the smallest lattice, and 3 wt% tungsten doped titania has the most intense nitrogen adsorption behavior (SBET ¼ 134.6 m2/g). By applying the solvothermal method to synthesis of titania or doped titania in this study, surface area reported here (113.8e134.6 m2/g) is higher than similar studies using different processing route [17,20]. Measured UVeVis spectra, as shown in Fig. 3, the 5 wt% tungsten doped titania exhibits the largest red shift. Red shifts of higher
Table 1 Applied constraints for deconvoluting resolved Ti3p and W4f peaks. Peak
FWHM(eV)
Peak position
Peak area
Ti3p (from undoped) W4f5/2 W4f7/2
1.9 0 to 2.5 0 to 2.5
37.2 b þ 2.2 b
e a*0.75 a
*Note: a is the assumed value of peak area, and b is the assumed value for peak position.
65
valence ion doped titania (in this case, W6 þ doped titania) can be attributed to the charge-transfer transition between the f electrons and TiO2 conduction or valence band, as reported the work of Xu and et al. on rare-earth ion doped titania photocatalysts [27]. As reported in their study [27] as well as other's work [28,29], a larger red shift generally indicates that the sample absorbs more photon and thus the photocatalytic reactivity is enhanced. It has been suggested that the higher degree of lattice distortion (contraction in this case) could induce more carriers (electronehole pairs) under longer wavelengths, resulting in the largest red shift; however, this is not the case here [30]. Excessive doping of tungsten beyond 5% generates more recombination center for the photogenerated carriers [31], and thus decrease the degree of red shift. A small bandgap is more favorable for maximizing photocatalytic activities, given longer wavelengths excitation (eg.visible light, infrared and etc) can be utilized. However, photocatalysis efficiency is also greatly dependent on recombination rate of hþ and e pairs [19e21], which can be affected by materials surface morphology [32], surface area [33] and electronic structure [34]. In pure titania systems, recombination of hþ and e pairs happens in 109 s [35], and hence efficiency of photocatalysis can be greatly deteriorated. To avoid the recombination of electron/hole pairs in titania, higher or lower valence ions can be used as dopant to receive electrons or holes [36,37]. In this study, tungsten was employed as a higher valence ion to follow such mechanism and approach. To verify the valence state of W ion as well as Ti ion, XPS high resolution spectra of W4f and Ti2p were acquired as shown in Fig. 4. As shown from Fig. 4a to d, peak positions of Ti2p1/2 and Ti2p3/2 remain unchanged w/ or w/o doping of tungsten. Summary of peak positions and FWHMs are shown in Table 3. Through the series of samples (undoped/doped) measured, the FWHM of Ti2p1/2 and Ti2p3/2 peaks remains a value of 1.9 eV and 1.0 eV respectively. Also, there is no observable change in terms of the peak position of Ti2p1/ 2 and Ti2p3/2 peaks. This implies that the chemical environmental of Ti ions remains relatively unchanged after doping with tungsten. Ti2p1/2 and Ti2p3/2 peaks located at 464.0 eV and 458.4 eV (standard deviation of peak position: ~0.2 eV) correspond to a valence state of Ti4þ [38e40]. According to selected literature references [41e46], Ti2p3/2 peak of the oxidation states of Ti(0), Ti(II) and Ti(III) is located at 453.9 eV, 455.3 eV and 457.1 eV, respectively. In order to identify the valence state of W ions from a resolved W4f & Ti3p peak, FWHM and peak position of Ti3p peak measured from pure titania (eg.Fig. 4f) was used as a deconvolution constraint in addition to other constraints (eg.Table 1). Given the unchanged valence state of titania after doping with tungsten, and the fact that no quantification (eg. percentage of valence state) was attempted, we believe this approach is valid [46]. After applying the constraints (eg.Table 1), W4f5/2 and W4f7/2 peaks can be isolated from Ti3p peak as shown in Fig. 4f through 4h. Peak positions of W4f5/2 and W4f7/2 at ~37.2 eV and ~35.0 eV corresponds to W6þ [47e50], which follows lattice contraction mechanism as indicated by results of Rietveld refinement (eg.Table 2). Other common oxidation states of tungsten such as W(0) and W(IV) features a W4f7/2 locates at ~31.2 and ~32.9 eV, respectively [51]. The FWHM of W4f5/2 and W4f7/2 peaks decreases with increasing tungsten doping amount. Narrowed distribution of characteristic W photoelectrons (after doping) again confirmed the lattice contraction effect, which was caused by replacing Ti4þ ions with higher valence W6þ ions [52,53]. For O1s peak of undoped and doped titania, within a standard deviation of peak position of ~0.2 eV, no observable trend in terms of peak position and FWHM was found after doping. Resultant morphologies of as-synthesized undoped and doped titania were investigated by scanning electron microscopy, as shown in Fig. 5a through 5d.
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Fig. 2. Powder diffraction pattern of pure titania and tungsten doped titania.
Table 2 Textural properties of pure titania and tungsten doped titania. Sample
a(Å)
c(Å)
SBET(m2/g)
Undoped 3wt%W 5wt%W 10wt%W
3.7874 3.7848 3.7806 3.7769
9.4879 9.4748 9.4730 9.4594
125.3 134.6 130.4 113.8
Fig. 3. UVeVis spectra of pure titania and tungsten doped titania.
As synthesized titania generally have a spherical morphology in the size range of ~500 nme~3 mm. Dumbbell shaped titania particles can be observed for undoped and 3 wt% tungsten doped samples. A further increase in the doping concentration of tungsten to 10 wt% resulted in a less developed spherical particle
morphology with random shaped small particles, as shown in Fig. 5d. However, agglomeration of particles was observed for all samples and can be related to strategies of sample preparation as well as storage. To evaluate the photocatalytic performance of as-synthesized titania and its doped counterparts, remaining MB concentration ratio was studied as a function of reaction time under visible light of 664.5 nm wavelength (eg.Fig. 6a). As shown by Fig. 6a, 5 wt% tungsten doped titania shows the most efficient photocatalysis process with a C/C0 value of 0.21 after 120 min of reaction. Further increase (10 wt% tungsten) in tungsten concentration leads to a deleterious photocatalytic performance, and after 120 min of reaction a C/C0 value of 0.93 is reported. To better compare the photocatalytic efficiency, the apparent first-order rate constant (kapp) was calculated. The kapp of undoped titania, 3 wt%, 5 wt% and 10 wt% tungsten doped titania is 1.3 103, 5.2 103, 13.0 103 and 0.6 103 (min1), respectively. The rate constants in this study are equivalent or better comparing to literature references [29,54] related to the photocatalytic performance of titania under visible light (degradation of MB), while a facile processing is retained and cost of raw materials is low. Additionally, we have measured the photocatalytic performance of undoped and doped titania under UV radiation (eg. Fig. 6b). The results in Fig. 6b show that the doping of tungsten unnoticeably affects the photocatalytic activity of titania with respect to its photocatalytic performance under UV radiation (~365 nm). Thus, the doping of tungsten into titania significantly alters the photocatalytic performance of titania under visible light, while there is an unnoticeable effect of doping on its photocatalytic behavior under UV radiation. Conclusive results of the key factors affecting the photocatalytic performance can be challenging and is beyond the perspective of this paper. However, most efficient photocatalytic performance of 5 wt% tungsten doped titania can be attributed to its contracted lattice (quantum effect), most significant red shift
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Fig. 4. XPS spectra of a) Ti2p of undoped titania, b) Ti2p of 3 wt% tungsten doped titania, c) Ti2p of 5 wt% tungsten doped titania, d) Ti2p of 10 wt% tungsten doped titania, e) Ti3p of undoped titania, f) Ti3p & W4f of 3 wt% tungsten doped titania, g) Ti3p & W4f of 5 wt% tungsten doped titania and h) Ti3p & W4f of 10 wt% tungsten doped titania.
Table 3 Summary of acquired XPS high resolution spectra. Sample
Undoped 3wt%W 5wt%W 10wt%W
Ti2p1/2/Ti2p3/2
O1s
W4f5/2/W4f7/2
Ti3p
Peak
FWHM
Peak
FWHM
Peak
FWHM
Peak
FWHM
464.2/458.6 464.0/458.4 464.0/458.4 464.0/458.4
1.9/1.0 1.9/1.0 1.9/1.0 1.9/1.0
529.8 529.6 529.6 529.7
1.3 1.3 1.3 1.4
NA 37.2/35.0 37.2/35 37.4/35.2
NA 1.7/1.9 1.5/1.9 1.3/1.5
37.2 37.2 37.2 37.2
1.9 1.9 1.9 1.9
among all samples, relatively large surface area (more reaction sites) and well-developed spherical morphology. In terms of its photocatalytic reaction, general mechanisms of degradation of MB by tungsten doped titania can be described as follows [55e57]: ) þ TiO2(hþ) TiO2 þ hn(visible) /TiO2(e cb
(1)
, TiO2(hþ) þ OH / OH þ Hþ
(2)
) þ O2 / O, TiO2(e 2 cb
(3)
TiO2(e ) þ W(VI) / W(V) cb
(4)
þ O, 2 þ TiO2(ecb ) þ 2H / H2O2
(5)
þ O, 2 þ 2H / H2O2 þ O2
(6)
, TiO2(e ) þ H2O2 / OH þ OHcb
(7)
OH. or O, 2 þ MB /// ring opening / CO2þH2O
(8)
In the present study, visible excitation triggers the generation of an electronehole pair in tungsten doped titania (eq (1)). Simultaneously, OH. and superoxide O.2 radicals were generated (eq (2) and eq (3)) by the first reaction. For doped titania, W6þ captures the conduction band electrons (eq (4)), thus avoiding the recombination of electronehole pairs and hinders the O.2 generation. The role of pH as shown by eq (5) and eq (6) suggests that low proton concentraions prevent the superoxide radical transformation into
H2O2 and the subsequent generation of secondary OH. (eq (7)) [55,56]. Since DI-water is used in this study for the photocatalytic reaction, it would be most probable that main mechanism for photocatalytic reaction is the interaction between OH. radicals and MB [57]. It has been well established that OH. radicals react with aromatic rings (MB) leading finally to ring-opening products (eq (8)), which are essentially non-toxic and environmentally friendly. 4. Conclusion A facile, one-step synthesis of nano-sized titania and tungsten doped titania was achieved by a solvothermal method. Such process yields powders with large surface area (in the magnitude of 100' s m2/g) as well as a contracted lattice with doping. It was found that photocatalytic performance can be related to surface area of as synthesized powder, lattice contraction, red shift over longer wavelength and morphology of resultant nano-particles, but mechanisms regarding the dominant factor (surface area, lattice distortion degree, red shift or morphology) remain unveiled. Addition of 5 wt% tungsten into anatase can significantly enhances its photocatalytic performance. By applying contraints to the deconvolution of XPS spectra, it was found that high valence W6þ ion substituting for Ti4þ ions is the main mechanism for change in lattice parameter as well as the red shift. Generally lattice parameters decrease with increasing addition of W6þ ion due to a lattice contraction effect. However, excessive addition of W6þ ions beyond 5 wt% can lead to a decrease in photocatalytic performance due to creation of more recombination centers for photogenerated electronehole pairs.
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Y. Zou et al. / Vacuum 126 (2016) 63e69
Fig. 5. Scanning electron micrograph of a) undoped titania, b) 3 wt% tungsten doped titania, c) 5 wt% tungsten doped titania and d) 10 wt% tungsten doped titania.
Fig. 6. Concentration of methylene blue as a function of photocatalytic reaction time under a) visible radiation, 664.5 nm and b) UV radiation, 365 nm
Y. Zou et al. / Vacuum 126 (2016) 63e69
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