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Great enhancement in photocatalytic performance of (001)-TiO2 through N-doping via the vapor-thermal method
Journal of Photochemistry & Photobiology A: Chemistry 386 (2020) 112127
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Great enhancement in photocatalytic performance of (001)-TiO2 through Ndoping via the vapor-thermal method
T
N.X. Qianb, X. Zhangb,c, M. Wangb, , X. Sunb, X.Y. Sunb, C. Liub, R. Raob, Y.Q. Maa,b, ⁎⁎
⁎
a
Institute of Physical Science and Information Technology, Anhui University, Hefei 230039, China Anhui Key Laboratory of Information Materials and Devices, School of Physics and Materials Science, Anhui University, Hefei 230039, China c School of Electronic Engineering, Huainan Normal University, Huainan 232038, China b
ARTICLE INFO
ABSTRACT
Keywords: Vapor-thermal Solvothermal (001)-TiO2 N-doping Photocatalytic properties
Using urea as the N source, (001) face exposed anatase TiO2, denoted as (001)-TiO2, is doped with N element via the vapor-thermal method at 180 ℃. The N/Ti molar ratio (RN/Ti) is 0, 0.5, 1, 1.5, 2, 2.5 and 3. The photocatalytic degradation ability of all samples was evaluated using methylene blue (MB) as a target contaminant. The lattice of (001)-TiO2, ratio of exposed (001) face and particle morphology are affected by the N doping. The N element exists both in the interstitial sites of lattice and at the surface of particles. The ratio of oxygen vacancies (Ov) and Ti3+, BET specific surface area (SBET) and pore volume are increased due to the N doping. The photocatalytic degradation experiments show that the adsorption efficiency of (001)-TiO2 to MB increases from 0.1% to 51.3% and the degradation rate Kapp increases from 2.8 × 10−2 min−1 to 11.5 × 10−2 min−1 before and after the N doping. The enhancement of photocatalytic performance due to the N doping is attributed to the competing effects of SBET, the ratio of Ti3+ and Ov, the recombination probability of photo-generated carriers and the ratio of exposed (001) face.
1. Introduction Anatase TiO2, a semiconductor with excellent photocatalytic performance, possesses the advantages of non-toxicity, low-cost and abundant reserves, as a result, it has been widely used in the fields of pollutant treatment, hydrogen generation via photocatalytic water splitting and self-cleaning [1–5]. As is well known, the photocatalytic properties of anatase TiO2 are sensitive to the exposed crystal face because the different face has the different atomic occupation, electronic structure and surface energy. Numerous efforts have still been devoted to synthesize the anatase TiO2 with high ratio of the exposed (001) crystal face because the (001) face possesses the high reactivity. However, the surface energy of (001) face (0.90 J m−2) is much higher than that of (101) face (0.44 J m−2) [6], so that the TiO2 crystal, synthesized via the hydro-/solvo-thermal methods, tends to grow along the [001] direction based on the principle of minimizing the surface energy, as a result, the ratio of exposed (001) face is reduced. Yang et al. [7] reported a pioneering work on the synthesis of (001)-TiO2. Specifically, HF was added into the precursor solution as a capping agent and 47% (001)-TiO2 nano-sheets was successfully synthesized, because the
F− ions are easily adhered to (001) face, reducing the surface energy of (001) face and inhibiting crystal growth along [001] direction. Since then, the proportion of exposed (001) face was gradually increased to 56% [8], 89% [9] and 100% [10]. Recently, many researches attempted to improve the photocatalytic performance of (001)-TiO2 and to unveil the underlying mechanism. Wang et al. reported that the perfect (001) face of anatase TiO2 is inert to both water and methanol, while the activity of the (001) face can be improved via the reduction or reoxidization [11]. Roy et al. pointed out that a small amount of exposed (101) face is beneficial to improve photocatalytic activity [12]. Theoretical and experimental studies by Yu et al. revealed that photo-generated holes easily migrate to the (001) face, while photo-generated electrons easily transfer to the (101) face, as a result, a "surface heterojunction" is formed between the (001) and (101) faces, promoting the separation of photo-generated carriers and consequently improving the photocatalytic performance [13,14]. Besides, some studies combined (001)-TiO2 with a second phase to improve photocatalytic performance, wherein the second phase includes nano-carbonaceous materials [15], g-C3N4 [16], MoS2 [17] and Au nanoparticles [18,19].
Corresponding author at: Anhui Key Laboratory of Information Materials and Devices, School of Physics and Materials Science, Anhui University, Hefei 230039, China. ⁎⁎ Corresponding author. E-mail addresses: [email protected] (M. Wang), [email protected] (Y.Q. Ma). ⁎
https://doi.org/10.1016/j.jphotochem.2019.112127 Received 16 May 2019; Received in revised form 11 September 2019; Accepted 30 September 2019 Available online 03 October 2019 1010-6030/ © 2019 Elsevier B.V. All rights reserved.
Journal of Photochemistry & Photobiology A: Chemistry 386 (2020) 112127
N.X. Qian, et al.
In addition to the surface treatment, the control of exposed crystal faces and the combination with the second phase, the substitution of the O element in (001)-TiO2 by other non-metallic elements, such as N [20–22], C [23], N/S [24,25], C/F [26], N/P [27], was carried out to improve the photocatalytic performance. The synthetic methods of doped (001)-TiO2 are as follows: (1) the source of dopant were added into the precursor solution of TiO2 [25]; (2) the raw material containing elements of both dopant and Ti, such as TiC or TiN, was treated at a high-temperature [21]; (3) First, (001)-TiO2 was prepared, and then calcined at a high temperature together with the source of dopant such as thiourea [24] or ethylenediamine [22]. The doping via the solid-state reaction at high temperature inevitably causes the aggregation of nanoparticles, resulting the decrease both in the specific surface area and in the proportion of exposed (001) face. Therefore, it is necessary to develop a new wet-chemistry method to synthesize the doped (001)TiO2. Furthermore, what kind of chemical state of the doped element exists in (001)-TiO2? How does the doped element affect the chemical states of Ti and O in TiO2, the crystal structure and electronic structure of TiO2? These issues deserve further investigation. In this work, the 15 ∼ 20 nm (001)-TiO2 particles were first synthesized, and then the N doping was carried out in the water vapor environment at 180 ℃ which was named as the vapor-thermal method, for the purpose of heating the precursor solution rapidly and uniformly. Sometimes, the vaporthermal method can effectively controls the reaction rate [28,29]. For the target pollutant, methylene blue (MB), the degradation rate of the N-doped (001)-TiO2 was over four times higher than that of (001)-TiO2 without experiencing the N doping.
precipitation was washed with deionized water for 2–3 times and then placed in a vacuum drying oven at 60 ℃ for 12 h. According to the N/Ti molar ratio (RN/Ti), the obtained samples were named as NV0, NV0.5, NV1, NV1.5, NV2, NV2.5 and NV3, respectively. 2.3. Characterization The crystal structure of the samples was investigated by X-ray diffraction (XRD) using an X-ray diffractometer (SmartLab 9 kW, Rigaku Industrial Corporation, Osaka, Japan) with Cu Kα radiation (λ =1.5406 Å) in the scanning range 10–80° and with a step size of 0.02°. Samples morphologies were observed by transmission electron microscopy (TEM; JEM-2100, JEOL, Tokyo, Japan). The structural information for samples was measured using Fourier-transform infrared spectroscopy (FT-IR; Vertex80/Hyperion2000, Bruker, Germany). The ultraviolet-visible diffuse reflectance spectra (UV–vis DRS) of samples were tested on a Shimadazu U-4100 spectrometer (U-4100, Shimadazu Corporation, Tokyo, Japan). The photoluminescence (PL) was measured on an FL fluorescence spectrophotometer (F-4500, Hitachi, Japan). X-ray photoelectron spectroscopy (XPS) was performed using a Thermo Scientific ESCALAB 250Xi (Thermo Scientific Inc., USA). The Brunauer–Emmett–Teller (BET) specific surface areas were calculated based on N2 adsorption and desorption isotherms measured at 77 K using the gas adsorption apparatus (Autosorb-iQ, Quantachrome Instruments, U.S.). The electron spin resonance (ESR) measurement was carried out on Bruker EMX plus 10/12 (equipped with Oxford ESR910 Liquid Helium cryostat). 2.5 mg TiO2 was mixed with 5 mL aqueous solution or methanol solution, and sonicated for 30 min, respectively. Two parts of each mixed solution were prepared: The mixed aqueous solution and the mixed methanol solution was added with 10 μL trapping agent of DMPO (C6H11NO, > 97%) under the dark condition, respectively. The other mixed aqueous solution and the mixed methanol solution was also added with 10 mL trapping agent of DMPO, then irradiated for 5 min under xenon lamp (15 A). Then the sample was collected by pipette sampling and then tested. The photocatalytic activity of the prepared samples was evaluated by the degradation of MB, which was exposed under xenon lamp. The experimental process is as follows: 100 mL MB aqueous solution with the concentration of 10 mg/L was mixed with 50 mg TiO2 catalysts in a vessel. Before the irradiation, the mixed solution was stirred in a dark condition for 30 min until an adsorption-desorption equilibrium was established. Samples of solution were extracted every 10 min from the reactor and the concentration of MB was analyzed by an UV–vis spectrometer (UV-3200S, MAPADA, Shanghai, China) and calculated by a calibration curve.
2. Experimental procedure 2.1. Synthesis of (001)-TiO2 25 mL TBT solution was placed into a 100 mL Teflon-lined autoclave. 5 mL distilled water and 5 mL HF solution were added into the autoclave and kept at 180 ℃ for 8 h. After the solution naturally cooled to room temperature, the precipitation was separated by centrifugation, and washed with deionized water for several times. After drying at 60 ℃ for 12 h, (001)-TiO2 (denoted as N0) was obtained. 2.2. Synthesis of N doped (001)-TiO2 via the vapor-thermal method 240 mg prepared (001)-TiO2 was added into 100 mL deionized water and ultrasonicated for 10 min. A certain amount of urea was added into the mixed solution according to the N/Ti molar ratio (RN/ Ti = 0, 0.5, 1, 1.5, 2, 2.5, 3), and then ultrasoniced for 20 min. The resulting mixture was transferred to a quartz-goblet. 150 mL deionized water was transferred to a 500 mL Hastelloy autoclave. Finally, the quartz-goblet was placed in the autoclave. The experiments were carried out in the following conditions: The reaction was performed at 180 ℃ for 12 h. After the solution naturally cooled to room temperature, the solid-liquid mixture was separated by centrifugation. The obtained
3. Results and discussion 3.1. Structure and morphology The crystal structure of all samples was characterized by XRD, and Fig. 1. Experimental (○) and calculated (red line) X-ray powder diffraction patterns of NV0 (a) and NV1.5 (b). Peak positions are shown as small markers (|). The lower trace represents the difference between the calculated and experimental data.
2
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absorption peak around 817 cm−1 for the NV0 sample is attributed to the vibration from Ti-O or Ti-O-Ti. Comparing with the NV0 sample, the N-doped NV1.5 and NV3 samples exhibit the different spectral features: (1) the absorption peak of Ti-O-Ti shifts to shorter wavelength, maybe resulting from the rearrangement of atoms at the surface of particles. (2) The absorption peaks at 922 cm−1 and 1061 cm−1 are attributed to the N-Ti-O stretch vibration. (3) The absorption peak at 1400 cm−1 is contributed by the surface-adsorbed NOx groups, which quantity increases with RN/Ti. All samples exhibit vibration modes at 1634 and 3383 cm−1, attributable to the OH-stretching and HOH-bending vibration. Weak vibration modes appeared in the range of 2308 ∼ 2385 cm−1 and 2850 ∼ 2977 cm−1 respectively derive from the CO2 stretching vibration and the CeH stretching vibration.
Table 1 Crystal lattice parameters a and c obtained from the Retica fitting. Samples
NV0
NV0.5
NV1
NV1.5
NV2
NV2.5
NV3
a (Å) c (Å)
3.792 9.521
3.792 9.517
3.791 9.515
3.792 9.515
3.790 9.515
3.789 9.516
3.787 9.514
the experimental data of XRD was fitted using the Rietveld refinement software Retica. XRD spectra of all samples have similar spectral characteristics. Therefore, Fig. 1 representatively shows the results of NV0 (a) and NV 1.5 (b). The experimental curve matches well with the calculated one. The agreement factors Rp, Rwp and χ2 are also shown in Fig. 1. The fitted results indicate that all samples crystallize as the single-phase anatase structure with the tetragonal symmetry and the space group of I41/amd (141). The crystal lattice parameters a (=b) and c obtained by fitting are listed in Table 1. The lattice parameters a and c of the undoped sample NV0 are largest among all samples, maybe because the N-doping increases the ratio of oxygen vacancies in (001)TiO2 as shown by the XPS results in Fig. 4, resulting the decrease in the lattice parameters. The high ratio of exposed (001) face is achieved when the growth of the TiO2 crystal in the [001] direction is suppressed, causing the decrease in the relative intensity of XRD peak from the (004) faces. Table 2 lists the diffraction intensity ratios between the (004) and (200) faces diffraction peaks, i.e. I(004)/I(200), of all samples. I(004)/I(200) of NV0 with RN/Ti = 0 is the smallest. With increasing RN/Ti, I(004)/I (200) increases first, reaching the maximum for NV2 with RN/Ti = 2, and then gradually decreases. This fact indicates that (001)-TiO2 sample recrystallizes in the process of the N doping. Specifically, urea added in the precursor solution may change the amount of F− ion or the species (such as –OH) absorbed on (001) faces, consequently changing the surface energy of (001) faces, as a result, atoms at the surface of (001)-TiO2 particles may migrate or rearrange between different exposed crystal faces [7]. According to the half width of the (101) faces diffraction peak, the crystallite size calculated by software MDI Jade 5.0 is also listed in Table 2. Thus it can be seen that the crystallite size of TiO2 becomes larger due to the N doping. Fig. 2 shows TEM and HR-TEM images of NV0, NV1.5 and NV3. As shown in Fig. 2(a), the TiO2 particles in NV0 exhibit a regular square sheet-like morphology with the side length of 20 ∼30 nm and thickness of about 10 nm, which is the typical morphology of (001) face exposed TiO2 particles [12,13,30,31] with the top and bottom surfaces being the (001) faces. Fig. 2(d) shows the HRTEM image of the side face, i.e. cross-sectional HRTEM image of (001)-TiO2 particles in the NV0 sample. The lattice fringes with the distance of 0.24 nm correspond to the (004) face of TiO2, indicating that the top and bottom surfaces of TiO2 particles are (001) faces [32]. For the N-doped samples NV1.5 and NV3, as shown in Fig. 2(b) and (c), the edges and corners of TiO2 particles become blurred, maybe result from the rearrangement of atoms at the surface of particles in the process of the N doping. Furthermore, the N doping increases the degree of particle aggregation. The lattice fringes with the distance of 0.35 nm in Fig. 2(e) and (f) correspond to the (101) face of TiO2. The Fourier transform infrared (FT-IR) spectra of all samples were measured and Fig. 3 representatively gives the results for three samples of NV0, NV1.5 and NV3. The wavelength corresponding to the vibration absorption peaks and the vibration modes are listed in Table 3. The
3.2. The chemical state analysis of elements in NV0, NV1.5 and NV3 Fig. 4(a) shows the XPS survey spectrum of NV0, NV1.5 and NV3. The strong signals from Ti and O elements and weak signals from N and F elements appear in all samples. Fig. 4(b–m) show the core energy levels XPS spectra of F 1s (b,c,d), N 1s (e,f,g), O 1s (h,i,j) and Ti 2p (k, l, m) in NV0, NV1.5 and NV3 samples. The chemical state (CS) of each element and its corresponding binding energy (BE) and ratio are listed in Table 4. The F element has two chemical states, Ti–F and Ti-F-Ti, and its corresponding binding energies (BE) are 684.7 eV and 688.1 eV [40,41], respectively. The chemical states of the N element include N in the interstitial sites of lattice and N species adsorbed at the surface of particles, and its corresponding binding energies are 399.9 ∼ 400.1 eV and 401.7 ∼ 401.8 eV [20], respectively. The O 1s XPS spectra can be fitted well by three peaks with binding energy at 530.1∼530.2 eV, 531.0 eV and 532.3 eV, respectively, corresponding to the three chemical states: Ti-O-Ti, -OH and oxygen vacancies (Ov) [42,43]. For the Ti element, it has two chemical states, TiO2 and TiOx, and its corresponding binding energies are 458.8 ∼ 458.9 eV and 460.2 eV [44,45], respectively. Changes in the chemical state of elements induced by N-doping are as follows: (1) N-doping induces the change of the chemical state of F element. F− acts as a capping agent to inhibit the growth of TiO2 along the [001] direction. Therefore, such the change consequentially alters the surface energy of particles, leading to the rearrangement of atoms at the surface of particles and causing the variation of particle morphology, which is consistent with the TEM results. (2) The weak N signal of undoped NV0 sample is attributed to the nitrogen absorbed at the surface of TiO2 particles. The interstitial and surface-adsorbed N elements are observed in NV1.5 and NV3, but the substituent N element with binding energy ∼ 397.8 eV [46] is not observed. It was reported that the existence of F− is beneficial to the N-doping [47], but it is not case herein. The possible reasons are as follows. The N atom tends to enter the interstitial sites of TiO2 lattice in the oxygen-rich condition, while it tends to replace O atom in TiO2 lattice in the reducing or anaerobic conditions [48]. As RN/Ti increases, the ratio of interstitial N atoms decreases while the N element adsorbed at the surface increases. (3) With RN/Ti increasing, the Ov ratio increases from 4.5% for the undoped NV0 sample to 13.5% for the NV3 sample. The N doping can greatly reduce the formation energies of oxygen vacancies from 4.2 eV to 0.6 eV [49]; this is the possible reason for the increase in the Ov ratio. Simultaneously, the ratio of TiOx also increases because the oxygen vacancies always coupling with Ti3+ to maintain the valence balance [50].
Table 2 Ratios of diffraction intensity between (004) and (200) crystal faces, i.e., I (004)/I(200) of all samples. Samples
NV0
NV0.5
NV1
NV1.5
NV2
NV2.5
NV3
I(004)/I(200) Crystallite size (nm)
0.457 18.1
0.486 22.1
0.507 22.3
0.535 22.5
0.549 22.4
0.514 22.5
0.468 22.6
It has been reported that the N doping makes the valence band maximum of TiO2 extend towards the band gap due to hybridization of 3
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Fig. 2. TEM (a, b, c) and HRTEM (c, d, f) images of NV0, NV1.5 and NV3.
The NV0 sample has a “tail-like” absorption. Such a “tail-like” feature was also observed in the (C, N, S)-doped TiO2 and TiO2 through hydrogenation [51,52], respectively attributed to the absorption associated with the impurity (C, N, S) levels introduced by the dopants in the forbidden band and the absorption of disordered oxygen vacancies at the surface. As seen in Table 4, the un-doped NV0 sample contains many defects such as Ti-F, Ti-F-Ti, -OH, Ov and TiOx. The “tail-like” absorption of NV0 may be mediated by these defects. The N-doped NV0.5 ∼ NV3 samples exhibit the similar spectral features that the absorption above 460 nm is slightly enhanced, which might attributes to successfully doping of N into the interstitial sites. Photoluminescence (PL) spectroscopy is an effective way to detect the recombination probability of photo-generated electron-hole pairs, because the PL results from the recombination of photo-generated carriers. The higher the recombination probability, the stronger the PL. Fig. 7 shows the PL results for NV0, NV1.5 and NV3 samples. The emission spectra of all samples showed three emission peaks at 420 nm, 474 nm and 543 nm. The emission peak at 420 nm is caused by the radiation recombination of conduction band electrons and valence band holes in TiO2 crystal [55,56]. The emission peaks at 474 nm and 543 nm are mainly contributed by the self-bound excitons, oxygen vacancies and surface states [41,57]. The lowest PL intensity for NV0 sample indicates the minor recombination probability. In contrast, the N doping increases the recombination probability.
Fig. 3. FT-IR spectra of NV0, NV1.5 and NV3.
N 2p orbital and O 2p orbital. XPS not only can detect the chemical state of elements, but also is an effective means for measuring the valence band density of states (DOS) [51,52]. Fig. 5 shows the valence band DOS of NV0, NV1.5 and NV3. The valence band maximum of all samples locates near 2.2 eV, so the interstitial N and the adsorbed N do not cause the shift of valence band maximum. The intensity of DOS decreases slightly. The valence band of TiO2 is composed of O 2p orbitals. Thus the increasing ratio of oxygen vacancy is a possible reason for the decrease in the intensity of DOS.
3.4. BET specific surface area The BET specific surface area (SBET) of the NV0, NV1.5 and NV3 samples is characterized by nitrogen adsorption-desorption isotherms, as shown in Fig. 8. The Barret–Joyner–Halenda (BJH) desorption pore volume (Vp) and distribution (Dp) analysis results are shown in the inset of Fig. 8. The SBET, Vp and Dp values of all samples are listed in Table 5. The SBET, Vp and Dp values almost increase after N-doping. The larger specific surface area of catalyst promotes the adsorption and degradation of the pollutants. Additionally, the larger pore volume facilitates effective transport of reactants and products during photocatalysis [24,34].
3.3. The optical absorption and photoluminescence properties Fig. 6 shows the UV–vis results of all samples. All samples have a steep absorption edge below 387 nm, attributable to the intrinsic absorption of TiO2, caused by the transition of the electrons from top of the valence band to bottom of the conduction band. A slight blue shift was found for the N doped samples. For anatase TiO2, the blue shift cannot be attributed to the N doping because it has been reported that the N doping with any concentration red-shifts the absorption band edge [53]. Liu et al. found that the band gap of anatase TiO2 increases monotonically from 3.173 eV to 3.239 eV when the crystallite size increases from 17 nm to 29 nm [54]. The XRD result in Table 2 shows that the crystallite size of NV0 is 18.1 nm and it increases to ∼ 22 nm for the N doped samples; the increase in the crystallite size due to the N doping is the possible reason for the blue shift. The inset shows an enlarged image of UV–vis in the visible region.
3.5. The performance of photocatalytic degradation The MB solution was chosen as the target pollutant to test the photocatalytic performance of N-doped photocatalysts. Fig. 9(a) representatively shows the absorbance spectra of MB in the presence of NV1.5 measured at different time (t). In the process of the 4
Journal of Photochemistry & Photobiology A: Chemistry 386 (2020) 112127
N.X. Qian, et al.
Fig. 4. XPS survey spectrum of NV0, NV1.5 and NV3 (a). Core level XPS spectra of F 1s (b, c, d), N 1s (e, f, g), O 1s (h, i, j) and Ti 2p (k, l, m) for NV0, NV1.5 and NV3. The solid line is the experimental curve, the dashed line is the fitted curve, and the open circles are the sum of the fitted curves.
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Table 3 Wavenumber at the absorbance peak and the corresponding vibration mode. Wavenumber (cm–1)
Vibration modes
References
400 ∼ 800 922, 1061 1400 1633, 3150 ∼ 3450
TieO or TieOeTi NeTieO stretching vibration eNOx stretching vibration The flexural vibrations of OeH in free water molecules CO2 stretching vibration The CeH stretching vibration
[33] [34] [34,35,36,37] [38]
2308 ∼ 2385 2850 ∼ 2977
[39] [39]
30-minute adsorption (t < 0), the peak position (λ =662 nm) keeps constant, but the intensity of peak gradually decreases. In the process of photodegradation (t > 0), the peak position shifts from 662 nm (t = 0 min) to 606 nm (t = 30 min) and the intensity decreases monotonically. Fig. 9(b) shows that the variation of MB concentration C/C0 with time t in the presence of NV0 ∼ NV3 under xenon lamp. In the range of −30 min ≤ t ≤ 0 min, the xenon lamp switches off and the change of MB concentration is caused by the catalyst adsorption. In the range of t ≥ 0, the MB concentration change is caused by photocatalytic degradation under the xenon lamp irradiation. The concentration of MB at t = −30 min is denoted as the initial concentration C0 and the concentration of MB at t = 0 is recorded as Ce. So the adsorption coefficient Ae can be calculated by the formula (C0−Ce)/C0 and the degradation coefficient De can be calculated by (Ce−C)/Ce, where C is the concentration of MB solution at any time t. Degradation curves were fitted by pseudo first-order kinetic model ln [Ce/C] = Kappt, where Kapp (min−1) is the apparent reaction rate constant. The plot of ln [Ce/C] versus time is shown in Fig. 9(c). The Ae, De and Kapp values for all samples are listed in Table 6. Un-doped sample NV0 has hardly any absorption to MB. For Ndoped samples, the adsorption coefficient Ae increases monotonically from 12.6% for NV0.5 to 51.3% for NV3 as RN/Ti increases. The higher adsorption coefficient can be attributed to the increase of specific surface area (SBET) and pore volume (Vp). Microscopically, the adsorption ability of TiO2 to water or organic pollutant molecules is related to the quantity and distance of oxygen vacancies (Ti3+) at the surface of TiO2 particles [58]. In the presence of each catalyst, the MB pollutant was almost completely degraded MB within 120 min with the degradation coefficient De changing from 98.3% to 99.9%. After the 60 min degradation, the De value of the un-doped NV0 is 83.0%. However, De is significantly increased for N-doped samples, and its value varies from 93.9% to 99.9%. The apparent reaction rate constant Kapp of NV0 is 2.8 × 10−2 min−1. For N-doped samples, Kapp increases first and then decreases with increasing RN/Ti, Kapp is the largest for NV1.5, reaching 11.5 × 10−2 min−1, which is nearly 4 times higher than the Kapp value of NV0. The possible reasons for the significant increase in the Kapp values for N-doped samples are as follows: the increase in the specific surface area and the ratio of oxygen vacancy (Ti3+) which provide more active sites for the photocatalytic degradation.
Fig. 5. Valence band density of states for NV0, NV1.5 and NV3.
Fig. 6. The UV–vis DRS of all samples.
Fig. 7. The photoluminescence spectra with the excitation wavelength λex =260 nm for NV0, NV1.5 and NV3.
Table 4 The chemical state (CS) of F, N, O and Ti elements and its corresponding binding energy (BE) and the ratio for NV0, NV1.5 and NV3. Core level
N 1s
CS
Intersitial
Surface
Ti-O-Ti
-OH
Ov
TiO2
TiOx
Ti-F
Ti-F-Ti
\ \ 399.1 59.8 400.0 54.3
\ \ 401.7 40.2 401.8 45.7
530.1 83.5 530.2 72.6 530.2 72.1
531.0 12.0 531.0 17.2 531.0 14.4
532.3 4.5 523.3 10.2 532.3 13.5
458.8 96.7 458.9 96.5 458.9 95.8
460.2 3.3 460.2 3.5 460.2 4.2
684.7 84.0 684.7 100 684.7 100
688.1 16.0 \ \ \ \
NV0 NV1.5 NV3
BE (eV) Ratio BE (eV) Ratio BE (eV) Ratio
O 1s
Ti 2p3/2
6
F 1s
Journal of Photochemistry & Photobiology A: Chemistry 386 (2020) 112127
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Table 6 The values of adsorption efficiency (Ae), degradation efficiency (De) and apparent reaction rate constant Kapp of all samples.
Table 5 Textural parameters of all samples. SBET (m2/g)
Pore volume Vp (cm3/g)
Average pore size Dp (nm)
NV0 NV1.5 NV3
95.83 124.44 123.52
0.38 0.48 0.53
15.81 15.52 17.09
NV 0
NV 0.5
NV 1
NV 1.5
NV 2
NV 2.5
NV 3
Kapp (10–2 min–1) Ae (%) De (%) (t = 120 min) De (%) (t = 60 min)
2.8 0.1 99.5 83.0
4.1 12.6 99.5 93.9
8.4 20.8 99.9 99.9
11.5 25.9 99.5 99.5
9.7 39.7 99.7 99.7
8.1 43.0 98.3 97.7
9.3 51.3 99.2 99.2
could be unequivocally assigned to a paramagnetic Ti3+ center in a distorted oxygen ligand field caused by the surface oxygen vacancies [59]. The signal at g = 2.002 arises from Ov [60]. Signals from Ti3+ and Ov increase with N doping, further confirming that N doping increases the ratio Ti3+ and Ov, beneficial for enhancing the photocatalytic performance. Combining the above results, it can be concluded that the enhanced photocatalytic performance can be assigned to the increase in the SBET and the ratios of Ti3+ and Ov. However, the enhancement of PL due to the N doping implies the increase in the recombination probability of photo-generated carries. Additionally, the N doping increases I(004)/I (200) (see Table 2), indicative of the decrease in the ratio of exposed (001) faces. These are the detrimental factors for the photocatalytic performance. Therefore, the enhanced photocatalytic performance due to the N doping may be attributable to the competing effects of advantageous and disadvantageous factors.
Fig. 8. Nitrogen adsorption-desorption isotherms for NV0, NV1.5 and NV3. The insets show the pore size distribution calculated by using the BJH method.
Samples
Samples
3.6. ESR
4. Conclusions
NV0, NV1.5 and NV3 samples were selected for ESR testing to further reveal the mechanism of photocatalytic degradation. The results are shown in Fig. 10. For all samples, no significant hydroxyl radical (%OH) and superoxide radical (%O2−) signals were observed without illumination. After illumination, the ESR spectrum exhibits a typical four-line spectrum for % OH with relative intensities of 1:2:2:1, due to the reaction of photogenerated holes (h+) with water (H2O) or hydroxyl groups (-OH), i.e. h+ + H2O → %OH + H. NV1.5 and NV3 have the same height (h) of % OH signal peak at 3340 G (G), higher than that of un-doped NV0 sample. After illumination, a significant %O2− signal also appeared in the ESR spectra due to the reaction of O2 with photo-generated electrons, i.e. e− + O2 → %O2−. The height h of %O2− signal peak located at 3322 G is also indicated in the figure. The %O2− signal of the N-doped samples is stronger than that of un-doped sample. In summary, both % OH and %O2− signals are enhanced for the N-doped samples compared with NV0. These results suggest that more O2, H2O or -OH in N-doped samples can be activated by photo-generated carriers to be high-reactive %O2− and %OH, consequently enhancing the degradation of MB, i.e. %OH or %O2− + MB → CO2 + H2O. Besides, the %OH signal is much stronger than %O2− for all samples, indicating a greater contribution of % OH during photo-degradation. From the ESR result in Fig. 10, the g value can be obtained for judging the variation of the Ov and Ti3+ ratios with the N doping, and the results are shown in Fig. 11. The signal at g = 1.992 for all samples
The (001) face exposed anatase TiO2 nanoparticles were firstly synthesized by the hydrothermal method at 180 ℃, and then the N doping was performed via the vapor thermal method at 180 ℃. The N/ Ti molar ratio (RN/Ti) is 0, 0.5, 1, 1.5, 2, 2.5 and 3. All samples were characterized by XRD, TEM, FT-IR, XPS, UV–vis DRS, PL, BET, ESR; using MB as the target pollutant, the photocatalytic degradation ability of all samples was tested. The main results are as follows: (1) The N doping slightly reduces the lattice parameters of TiO2, changes the ratio of exposed (001) face and the morphology of TiO2 particles. (2) Some of N atoms exist in the interstitial sites of lattice and others are adsorbed at the surface of TiO2 particles. Meanwhile, the N doping increases the ratio of oxygen vacancies in TiO2 crystal, accompanying with the increase of Ti3+. (3) The specific surface area and pore volume of the samples are increased by the N doping. (4) The N doping increases the adsorption coefficient of TiO2 to MB from 0.1% to 51.3%; The apparent reaction rate constant Kapp increases from 2.8 × 10−2 min−1 to 11.5 × 10−2 min−1, which increased by nearly 4 times. The ESR results show that the number of % OH and %O2− in the N-doped samples is significantly enhanced, beneficial for the improvement of the photocatalytic performance.
Fig. 9. The absorbance spectra of MB in the presence of NV1.5 measured at different time (t) (a). Variation of methylene blue concentration C/C0 with time in the presence of NV0 ∼ NV3 under xenon lamp (b). Linearly fitting of the degradation process according to the pseudo first-order kinetic model (solid line) (c). 7
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Fig. 10. ESR spectra of radical adducts trapped by DMPO in NV0, NV1.5 and NV3 dispersions: (a, b, c) DMPO-OH% formed in irradiated aqueous dispersions. (d, e, f) DMPO-O2−% formed in irradiated methanol dispersion. [4] Z.C. Lian, W.C. Wang, G.S. Li, F.H. Tian, K.S. Schanze, H.X. Li, Pt-enhanced mesoporous Ti3+/TiO2 with rapid bulk to surface Electron transfer for photocatalytic hydrogen evolution, ACS Appl. Mater. Interfaces 9 (2017) 16960–16967. [5] R. Katal, M. Salehi, M.H.D.A. Farahano, S. Masudy-Panah, S.L. Ong, J.Y. Hu, Preparation of a new type of black TiO2 under a vacuum atmosphere for sunlight photocatalysis, ACS Appl. Mater. Interfaces 10 (2018) 35316–35326. [6] G. Liu, H.G. Yang, J. Pan, Y.Q. Yang, G.Q. Lu, H.M. Cheng, Titanium dioxide crystals with tailored facets, Chem. Rev. 114 (2014) 9559–9612. [7] H.G. Yang, C.H. Sun, S.Z. Qiao, J. Zou, G. Liu, S.C. Smith, H.M. Cheng, G.Q. Lu, Anatase TiO2 single crystals with a large percentage of reactive facets, Nature 453 (2008) 638–641. [8] G. Liu, H.G. Yang, X.W. Wang, L.N. Cheng, H.F. Lu, L.Z. Wang, G.Q. Lu, H.M. Cheng, Enhanced photoactivity of oxygen-deficient anatase TiO2 sheets with dominant (001) facets, J. Phys. Chem. C 113 (2009) 21784–21788. [9] X.G. Han, Q. Kuang, M.S. Jin, Z.X. Xie, L.S. Zheng, Synthesis of titania nanosheets with a high percentage of exposed (001) facets and related photocatalytic properties, J. Am. Chem. Soc. 131 (2009) 3152–3153. [10] J.S. Chen, Y.L. Tan, C.M. Li, Y.L. Cheah, D. Luan, S. Madhavi, F.Y.C. Boey, L.A. Archer, X.W. Lou, Constructing hierarchical spheres from large ultrathin anatase TiO2 nanosheets with nearly 100% exposed (001) facets for fast reversible Lithium storage, J. Am. Chem. Soc. 132 (2010) 6124–6130. [11] Y. Wang, H.J. Sun, S.J. Tan, H. Feng, Z.W. Cheng, J. Zhao, A.D. Zhao, B. Wang, Y. Luo, J.L. Yang, J.G. Hou, Role of point defects on the reactivity of reconstructed anatase titanium dioxide (001) surface, Nat. Commun. 4 (2013) 2214. [12] N. Roy, Y. Sohn, D. Pradhan, Synergy of low-energy {101} and high-energy {001} TiO2 crystal facets for enhanced photocatalysis, ACS Nano 7 (2013) 2532–2540. [13] J.G. Yu, J.X. Low, W. Xiao, P. Zhou, M. Jaroniec, Enhanced photocatalytic CO2− reduction activity of anatase TiO2 by coexposed {001} and {101} facets, J. Am. Chem. Soc. 136 (2014) 8839–8842. [14] X.G. Liu, G.J. Dong, S.P. Li, G.X. Lu, Y.P. Bi, Direct observation of charge separation on anatase TiO2 crystals with selectively etched {001} facets, J. Am. Chem. Soc. 138 (2016) 2917–2920. [15] B.B. Li, Z.B. Zhao, F. Gao, X.Z. Wang, J.S. Qiu, Mesoporous microspheres composed of carbon-coated TiO2 nanocrystals with exposed {001} facets for improved visible light photocatalytic activity, Appl. Catal. B: Environ. 147 (2014) 958–964. [16] L.Y. Chen, X.S. Zhou, B. Jin, J. Luo, X.Y. Xu, L.L. Zhang, Y.P. Hong, Heterojunctions in g-C3N4/B-TiO2 nanosheets with exposed {001} plane and enhanced visible-light photocatalytic activities, Int. J. Hydrogen Energy 41 (2016) 7292–7300. [17] L. Cao, R. Wang, D.X. Wang, L.C. Xu, X.Y. Li, Enhanced visible light photocatalytic activity for the hybrid MoS2/anatase TiO2(001) nanocomposite: a first-principles study, Chem. Phys. Lett. 612 (2014) 285–288. [18] H.M. Shi, S. Zhang, X.P. Zhu, Y. Liu, T. Wang, T. Jiang, G.H. Zhang, H.G. Duan, Uniform gold-nanoparticle-decorated {001}-Faceted anatase TiO2 nanosheets for enhanced solar-light photocatalytic reactions, ACS Appl. Mater. Interfaces 9 (2017) 36907–36916. [19] J.Q. Yan, G.J. Wu, W.L. Dai, N.J. Guan, L.D. Li, Synthetic design of gold nanoparticles on anatase TiO2 {001} for enhanced visible light harvesting, ACS Sustain. Chem. Eng. 2 (2014) 1940–1946. [20] G. Liu, H.G. Yang, X. Wang, L. Cheng, J. Pan, G.Q. Lu, H.M. Cheng, Visible light responsive nitrogen doped anatase TiO2 sheets with dominant {001} facets derived from TiN, J. Am. Chem. Soc. 131 (2009) 12868–12869. [21] S.A. Bakar, C. Ribeiro, Nitrogen-doped titanium dioxide: an overview of material design and dimensionality effect over modern applications, J. Photochem. Photobiol. C 27 (2016) 1–29. [22] D. Li, F. Chen, D.L. Jiang, W.D. Shi, W.J. Zheng, Enhanced photocatalytic activity of
Fig. 11. The g value obtained from ESR spectra of NV0, NV1.5 and NV3.
Acknowledgments A portion of this work was performed on the Steady High Magnetic Field Facilities, High Magnetic Field Laboratory, CAS. This work was supported by the open fund for Discipline Construction, Institute of Physical Science and Information Technology, Anhui University; National Natural Science Foundation of China (Grant No. 51471001). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jphotochem.2019. 112127. References [1] A. Abdelhaleem, W. Chu, X.L. Liang, Diphenamid degradation via sulfite activation under visible LED using Fe (III) impregnated N-doped TiO2 photocatalyst, Appl. Catal. B: Environ. 244 (2019) 823–835. [2] Y.Q. Yang, L.C. Yin, Y. Gong, P. Niu, J.Q. Wang, L. Gu, X.Q. Chen, G. Liu, L.Z. Wang, H.M. Cheng, An unusual strong visible-light absorption band in red anatase TiO2 photocatalyst induced by atomic hydrogen-occupied oxygen vacancies, Adv. Mater. 30 (2018) 1704479. [3] S.A. Bakar, C. Ribeiro, A comparative run for visible-light-driven photocatalytic activity of anionic and cationic S-doped TiO2 photocatalysts: a case study of possible sulfur doping through chemical protocol, J. Mol. Catal. A Chem. 421 (2016) 1–15.
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Great enhancement in photocatalytic performance of (001)-TiO2 through Ndoping via the vapor-thermal method
T
N.X. Qianb, X. Zhangb,c, M. Wangb, , X. Sunb, X.Y. Sunb, C. Liub, R. Raob, Y.Q. Maa,b, ⁎⁎
⁎
a
Institute of Physical Science and Information Technology, Anhui University, Hefei 230039, China Anhui Key Laboratory of Information Materials and Devices, School of Physics and Materials Science, Anhui University, Hefei 230039, China c School of Electronic Engineering, Huainan Normal University, Huainan 232038, China b
ARTICLE INFO
ABSTRACT
Keywords: Vapor-thermal Solvothermal (001)-TiO2 N-doping Photocatalytic properties
Using urea as the N source, (001) face exposed anatase TiO2, denoted as (001)-TiO2, is doped with N element via the vapor-thermal method at 180 ℃. The N/Ti molar ratio (RN/Ti) is 0, 0.5, 1, 1.5, 2, 2.5 and 3. The photocatalytic degradation ability of all samples was evaluated using methylene blue (MB) as a target contaminant. The lattice of (001)-TiO2, ratio of exposed (001) face and particle morphology are affected by the N doping. The N element exists both in the interstitial sites of lattice and at the surface of particles. The ratio of oxygen vacancies (Ov) and Ti3+, BET specific surface area (SBET) and pore volume are increased due to the N doping. The photocatalytic degradation experiments show that the adsorption efficiency of (001)-TiO2 to MB increases from 0.1% to 51.3% and the degradation rate Kapp increases from 2.8 × 10−2 min−1 to 11.5 × 10−2 min−1 before and after the N doping. The enhancement of photocatalytic performance due to the N doping is attributed to the competing effects of SBET, the ratio of Ti3+ and Ov, the recombination probability of photo-generated carriers and the ratio of exposed (001) face.
1. Introduction Anatase TiO2, a semiconductor with excellent photocatalytic performance, possesses the advantages of non-toxicity, low-cost and abundant reserves, as a result, it has been widely used in the fields of pollutant treatment, hydrogen generation via photocatalytic water splitting and self-cleaning [1–5]. As is well known, the photocatalytic properties of anatase TiO2 are sensitive to the exposed crystal face because the different face has the different atomic occupation, electronic structure and surface energy. Numerous efforts have still been devoted to synthesize the anatase TiO2 with high ratio of the exposed (001) crystal face because the (001) face possesses the high reactivity. However, the surface energy of (001) face (0.90 J m−2) is much higher than that of (101) face (0.44 J m−2) [6], so that the TiO2 crystal, synthesized via the hydro-/solvo-thermal methods, tends to grow along the [001] direction based on the principle of minimizing the surface energy, as a result, the ratio of exposed (001) face is reduced. Yang et al. [7] reported a pioneering work on the synthesis of (001)-TiO2. Specifically, HF was added into the precursor solution as a capping agent and 47% (001)-TiO2 nano-sheets was successfully synthesized, because the
F− ions are easily adhered to (001) face, reducing the surface energy of (001) face and inhibiting crystal growth along [001] direction. Since then, the proportion of exposed (001) face was gradually increased to 56% [8], 89% [9] and 100% [10]. Recently, many researches attempted to improve the photocatalytic performance of (001)-TiO2 and to unveil the underlying mechanism. Wang et al. reported that the perfect (001) face of anatase TiO2 is inert to both water and methanol, while the activity of the (001) face can be improved via the reduction or reoxidization [11]. Roy et al. pointed out that a small amount of exposed (101) face is beneficial to improve photocatalytic activity [12]. Theoretical and experimental studies by Yu et al. revealed that photo-generated holes easily migrate to the (001) face, while photo-generated electrons easily transfer to the (101) face, as a result, a "surface heterojunction" is formed between the (001) and (101) faces, promoting the separation of photo-generated carriers and consequently improving the photocatalytic performance [13,14]. Besides, some studies combined (001)-TiO2 with a second phase to improve photocatalytic performance, wherein the second phase includes nano-carbonaceous materials [15], g-C3N4 [16], MoS2 [17] and Au nanoparticles [18,19].
Corresponding author at: Anhui Key Laboratory of Information Materials and Devices, School of Physics and Materials Science, Anhui University, Hefei 230039, China. ⁎⁎ Corresponding author. E-mail addresses: [email protected] (M. Wang), [email protected] (Y.Q. Ma). ⁎
https://doi.org/10.1016/j.jphotochem.2019.112127 Received 16 May 2019; Received in revised form 11 September 2019; Accepted 30 September 2019 Available online 03 October 2019 1010-6030/ © 2019 Elsevier B.V. All rights reserved.
Journal of Photochemistry & Photobiology A: Chemistry 386 (2020) 112127
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In addition to the surface treatment, the control of exposed crystal faces and the combination with the second phase, the substitution of the O element in (001)-TiO2 by other non-metallic elements, such as N [20–22], C [23], N/S [24,25], C/F [26], N/P [27], was carried out to improve the photocatalytic performance. The synthetic methods of doped (001)-TiO2 are as follows: (1) the source of dopant were added into the precursor solution of TiO2 [25]; (2) the raw material containing elements of both dopant and Ti, such as TiC or TiN, was treated at a high-temperature [21]; (3) First, (001)-TiO2 was prepared, and then calcined at a high temperature together with the source of dopant such as thiourea [24] or ethylenediamine [22]. The doping via the solid-state reaction at high temperature inevitably causes the aggregation of nanoparticles, resulting the decrease both in the specific surface area and in the proportion of exposed (001) face. Therefore, it is necessary to develop a new wet-chemistry method to synthesize the doped (001)TiO2. Furthermore, what kind of chemical state of the doped element exists in (001)-TiO2? How does the doped element affect the chemical states of Ti and O in TiO2, the crystal structure and electronic structure of TiO2? These issues deserve further investigation. In this work, the 15 ∼ 20 nm (001)-TiO2 particles were first synthesized, and then the N doping was carried out in the water vapor environment at 180 ℃ which was named as the vapor-thermal method, for the purpose of heating the precursor solution rapidly and uniformly. Sometimes, the vaporthermal method can effectively controls the reaction rate [28,29]. For the target pollutant, methylene blue (MB), the degradation rate of the N-doped (001)-TiO2 was over four times higher than that of (001)-TiO2 without experiencing the N doping.
precipitation was washed with deionized water for 2–3 times and then placed in a vacuum drying oven at 60 ℃ for 12 h. According to the N/Ti molar ratio (RN/Ti), the obtained samples were named as NV0, NV0.5, NV1, NV1.5, NV2, NV2.5 and NV3, respectively. 2.3. Characterization The crystal structure of the samples was investigated by X-ray diffraction (XRD) using an X-ray diffractometer (SmartLab 9 kW, Rigaku Industrial Corporation, Osaka, Japan) with Cu Kα radiation (λ =1.5406 Å) in the scanning range 10–80° and with a step size of 0.02°. Samples morphologies were observed by transmission electron microscopy (TEM; JEM-2100, JEOL, Tokyo, Japan). The structural information for samples was measured using Fourier-transform infrared spectroscopy (FT-IR; Vertex80/Hyperion2000, Bruker, Germany). The ultraviolet-visible diffuse reflectance spectra (UV–vis DRS) of samples were tested on a Shimadazu U-4100 spectrometer (U-4100, Shimadazu Corporation, Tokyo, Japan). The photoluminescence (PL) was measured on an FL fluorescence spectrophotometer (F-4500, Hitachi, Japan). X-ray photoelectron spectroscopy (XPS) was performed using a Thermo Scientific ESCALAB 250Xi (Thermo Scientific Inc., USA). The Brunauer–Emmett–Teller (BET) specific surface areas were calculated based on N2 adsorption and desorption isotherms measured at 77 K using the gas adsorption apparatus (Autosorb-iQ, Quantachrome Instruments, U.S.). The electron spin resonance (ESR) measurement was carried out on Bruker EMX plus 10/12 (equipped with Oxford ESR910 Liquid Helium cryostat). 2.5 mg TiO2 was mixed with 5 mL aqueous solution or methanol solution, and sonicated for 30 min, respectively. Two parts of each mixed solution were prepared: The mixed aqueous solution and the mixed methanol solution was added with 10 μL trapping agent of DMPO (C6H11NO, > 97%) under the dark condition, respectively. The other mixed aqueous solution and the mixed methanol solution was also added with 10 mL trapping agent of DMPO, then irradiated for 5 min under xenon lamp (15 A). Then the sample was collected by pipette sampling and then tested. The photocatalytic activity of the prepared samples was evaluated by the degradation of MB, which was exposed under xenon lamp. The experimental process is as follows: 100 mL MB aqueous solution with the concentration of 10 mg/L was mixed with 50 mg TiO2 catalysts in a vessel. Before the irradiation, the mixed solution was stirred in a dark condition for 30 min until an adsorption-desorption equilibrium was established. Samples of solution were extracted every 10 min from the reactor and the concentration of MB was analyzed by an UV–vis spectrometer (UV-3200S, MAPADA, Shanghai, China) and calculated by a calibration curve.
2. Experimental procedure 2.1. Synthesis of (001)-TiO2 25 mL TBT solution was placed into a 100 mL Teflon-lined autoclave. 5 mL distilled water and 5 mL HF solution were added into the autoclave and kept at 180 ℃ for 8 h. After the solution naturally cooled to room temperature, the precipitation was separated by centrifugation, and washed with deionized water for several times. After drying at 60 ℃ for 12 h, (001)-TiO2 (denoted as N0) was obtained. 2.2. Synthesis of N doped (001)-TiO2 via the vapor-thermal method 240 mg prepared (001)-TiO2 was added into 100 mL deionized water and ultrasonicated for 10 min. A certain amount of urea was added into the mixed solution according to the N/Ti molar ratio (RN/ Ti = 0, 0.5, 1, 1.5, 2, 2.5, 3), and then ultrasoniced for 20 min. The resulting mixture was transferred to a quartz-goblet. 150 mL deionized water was transferred to a 500 mL Hastelloy autoclave. Finally, the quartz-goblet was placed in the autoclave. The experiments were carried out in the following conditions: The reaction was performed at 180 ℃ for 12 h. After the solution naturally cooled to room temperature, the solid-liquid mixture was separated by centrifugation. The obtained
3. Results and discussion 3.1. Structure and morphology The crystal structure of all samples was characterized by XRD, and Fig. 1. Experimental (○) and calculated (red line) X-ray powder diffraction patterns of NV0 (a) and NV1.5 (b). Peak positions are shown as small markers (|). The lower trace represents the difference between the calculated and experimental data.
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absorption peak around 817 cm−1 for the NV0 sample is attributed to the vibration from Ti-O or Ti-O-Ti. Comparing with the NV0 sample, the N-doped NV1.5 and NV3 samples exhibit the different spectral features: (1) the absorption peak of Ti-O-Ti shifts to shorter wavelength, maybe resulting from the rearrangement of atoms at the surface of particles. (2) The absorption peaks at 922 cm−1 and 1061 cm−1 are attributed to the N-Ti-O stretch vibration. (3) The absorption peak at 1400 cm−1 is contributed by the surface-adsorbed NOx groups, which quantity increases with RN/Ti. All samples exhibit vibration modes at 1634 and 3383 cm−1, attributable to the OH-stretching and HOH-bending vibration. Weak vibration modes appeared in the range of 2308 ∼ 2385 cm−1 and 2850 ∼ 2977 cm−1 respectively derive from the CO2 stretching vibration and the CeH stretching vibration.
Table 1 Crystal lattice parameters a and c obtained from the Retica fitting. Samples
NV0
NV0.5
NV1
NV1.5
NV2
NV2.5
NV3
a (Å) c (Å)
3.792 9.521
3.792 9.517
3.791 9.515
3.792 9.515
3.790 9.515
3.789 9.516
3.787 9.514
the experimental data of XRD was fitted using the Rietveld refinement software Retica. XRD spectra of all samples have similar spectral characteristics. Therefore, Fig. 1 representatively shows the results of NV0 (a) and NV 1.5 (b). The experimental curve matches well with the calculated one. The agreement factors Rp, Rwp and χ2 are also shown in Fig. 1. The fitted results indicate that all samples crystallize as the single-phase anatase structure with the tetragonal symmetry and the space group of I41/amd (141). The crystal lattice parameters a (=b) and c obtained by fitting are listed in Table 1. The lattice parameters a and c of the undoped sample NV0 are largest among all samples, maybe because the N-doping increases the ratio of oxygen vacancies in (001)TiO2 as shown by the XPS results in Fig. 4, resulting the decrease in the lattice parameters. The high ratio of exposed (001) face is achieved when the growth of the TiO2 crystal in the [001] direction is suppressed, causing the decrease in the relative intensity of XRD peak from the (004) faces. Table 2 lists the diffraction intensity ratios between the (004) and (200) faces diffraction peaks, i.e. I(004)/I(200), of all samples. I(004)/I(200) of NV0 with RN/Ti = 0 is the smallest. With increasing RN/Ti, I(004)/I (200) increases first, reaching the maximum for NV2 with RN/Ti = 2, and then gradually decreases. This fact indicates that (001)-TiO2 sample recrystallizes in the process of the N doping. Specifically, urea added in the precursor solution may change the amount of F− ion or the species (such as –OH) absorbed on (001) faces, consequently changing the surface energy of (001) faces, as a result, atoms at the surface of (001)-TiO2 particles may migrate or rearrange between different exposed crystal faces [7]. According to the half width of the (101) faces diffraction peak, the crystallite size calculated by software MDI Jade 5.0 is also listed in Table 2. Thus it can be seen that the crystallite size of TiO2 becomes larger due to the N doping. Fig. 2 shows TEM and HR-TEM images of NV0, NV1.5 and NV3. As shown in Fig. 2(a), the TiO2 particles in NV0 exhibit a regular square sheet-like morphology with the side length of 20 ∼30 nm and thickness of about 10 nm, which is the typical morphology of (001) face exposed TiO2 particles [12,13,30,31] with the top and bottom surfaces being the (001) faces. Fig. 2(d) shows the HRTEM image of the side face, i.e. cross-sectional HRTEM image of (001)-TiO2 particles in the NV0 sample. The lattice fringes with the distance of 0.24 nm correspond to the (004) face of TiO2, indicating that the top and bottom surfaces of TiO2 particles are (001) faces [32]. For the N-doped samples NV1.5 and NV3, as shown in Fig. 2(b) and (c), the edges and corners of TiO2 particles become blurred, maybe result from the rearrangement of atoms at the surface of particles in the process of the N doping. Furthermore, the N doping increases the degree of particle aggregation. The lattice fringes with the distance of 0.35 nm in Fig. 2(e) and (f) correspond to the (101) face of TiO2. The Fourier transform infrared (FT-IR) spectra of all samples were measured and Fig. 3 representatively gives the results for three samples of NV0, NV1.5 and NV3. The wavelength corresponding to the vibration absorption peaks and the vibration modes are listed in Table 3. The
3.2. The chemical state analysis of elements in NV0, NV1.5 and NV3 Fig. 4(a) shows the XPS survey spectrum of NV0, NV1.5 and NV3. The strong signals from Ti and O elements and weak signals from N and F elements appear in all samples. Fig. 4(b–m) show the core energy levels XPS spectra of F 1s (b,c,d), N 1s (e,f,g), O 1s (h,i,j) and Ti 2p (k, l, m) in NV0, NV1.5 and NV3 samples. The chemical state (CS) of each element and its corresponding binding energy (BE) and ratio are listed in Table 4. The F element has two chemical states, Ti–F and Ti-F-Ti, and its corresponding binding energies (BE) are 684.7 eV and 688.1 eV [40,41], respectively. The chemical states of the N element include N in the interstitial sites of lattice and N species adsorbed at the surface of particles, and its corresponding binding energies are 399.9 ∼ 400.1 eV and 401.7 ∼ 401.8 eV [20], respectively. The O 1s XPS spectra can be fitted well by three peaks with binding energy at 530.1∼530.2 eV, 531.0 eV and 532.3 eV, respectively, corresponding to the three chemical states: Ti-O-Ti, -OH and oxygen vacancies (Ov) [42,43]. For the Ti element, it has two chemical states, TiO2 and TiOx, and its corresponding binding energies are 458.8 ∼ 458.9 eV and 460.2 eV [44,45], respectively. Changes in the chemical state of elements induced by N-doping are as follows: (1) N-doping induces the change of the chemical state of F element. F− acts as a capping agent to inhibit the growth of TiO2 along the [001] direction. Therefore, such the change consequentially alters the surface energy of particles, leading to the rearrangement of atoms at the surface of particles and causing the variation of particle morphology, which is consistent with the TEM results. (2) The weak N signal of undoped NV0 sample is attributed to the nitrogen absorbed at the surface of TiO2 particles. The interstitial and surface-adsorbed N elements are observed in NV1.5 and NV3, but the substituent N element with binding energy ∼ 397.8 eV [46] is not observed. It was reported that the existence of F− is beneficial to the N-doping [47], but it is not case herein. The possible reasons are as follows. The N atom tends to enter the interstitial sites of TiO2 lattice in the oxygen-rich condition, while it tends to replace O atom in TiO2 lattice in the reducing or anaerobic conditions [48]. As RN/Ti increases, the ratio of interstitial N atoms decreases while the N element adsorbed at the surface increases. (3) With RN/Ti increasing, the Ov ratio increases from 4.5% for the undoped NV0 sample to 13.5% for the NV3 sample. The N doping can greatly reduce the formation energies of oxygen vacancies from 4.2 eV to 0.6 eV [49]; this is the possible reason for the increase in the Ov ratio. Simultaneously, the ratio of TiOx also increases because the oxygen vacancies always coupling with Ti3+ to maintain the valence balance [50].
Table 2 Ratios of diffraction intensity between (004) and (200) crystal faces, i.e., I (004)/I(200) of all samples. Samples
NV0
NV0.5
NV1
NV1.5
NV2
NV2.5
NV3
I(004)/I(200) Crystallite size (nm)
0.457 18.1
0.486 22.1
0.507 22.3
0.535 22.5
0.549 22.4
0.514 22.5
0.468 22.6
It has been reported that the N doping makes the valence band maximum of TiO2 extend towards the band gap due to hybridization of 3
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Fig. 2. TEM (a, b, c) and HRTEM (c, d, f) images of NV0, NV1.5 and NV3.
The NV0 sample has a “tail-like” absorption. Such a “tail-like” feature was also observed in the (C, N, S)-doped TiO2 and TiO2 through hydrogenation [51,52], respectively attributed to the absorption associated with the impurity (C, N, S) levels introduced by the dopants in the forbidden band and the absorption of disordered oxygen vacancies at the surface. As seen in Table 4, the un-doped NV0 sample contains many defects such as Ti-F, Ti-F-Ti, -OH, Ov and TiOx. The “tail-like” absorption of NV0 may be mediated by these defects. The N-doped NV0.5 ∼ NV3 samples exhibit the similar spectral features that the absorption above 460 nm is slightly enhanced, which might attributes to successfully doping of N into the interstitial sites. Photoluminescence (PL) spectroscopy is an effective way to detect the recombination probability of photo-generated electron-hole pairs, because the PL results from the recombination of photo-generated carriers. The higher the recombination probability, the stronger the PL. Fig. 7 shows the PL results for NV0, NV1.5 and NV3 samples. The emission spectra of all samples showed three emission peaks at 420 nm, 474 nm and 543 nm. The emission peak at 420 nm is caused by the radiation recombination of conduction band electrons and valence band holes in TiO2 crystal [55,56]. The emission peaks at 474 nm and 543 nm are mainly contributed by the self-bound excitons, oxygen vacancies and surface states [41,57]. The lowest PL intensity for NV0 sample indicates the minor recombination probability. In contrast, the N doping increases the recombination probability.
Fig. 3. FT-IR spectra of NV0, NV1.5 and NV3.
N 2p orbital and O 2p orbital. XPS not only can detect the chemical state of elements, but also is an effective means for measuring the valence band density of states (DOS) [51,52]. Fig. 5 shows the valence band DOS of NV0, NV1.5 and NV3. The valence band maximum of all samples locates near 2.2 eV, so the interstitial N and the adsorbed N do not cause the shift of valence band maximum. The intensity of DOS decreases slightly. The valence band of TiO2 is composed of O 2p orbitals. Thus the increasing ratio of oxygen vacancy is a possible reason for the decrease in the intensity of DOS.
3.4. BET specific surface area The BET specific surface area (SBET) of the NV0, NV1.5 and NV3 samples is characterized by nitrogen adsorption-desorption isotherms, as shown in Fig. 8. The Barret–Joyner–Halenda (BJH) desorption pore volume (Vp) and distribution (Dp) analysis results are shown in the inset of Fig. 8. The SBET, Vp and Dp values of all samples are listed in Table 5. The SBET, Vp and Dp values almost increase after N-doping. The larger specific surface area of catalyst promotes the adsorption and degradation of the pollutants. Additionally, the larger pore volume facilitates effective transport of reactants and products during photocatalysis [24,34].
3.3. The optical absorption and photoluminescence properties Fig. 6 shows the UV–vis results of all samples. All samples have a steep absorption edge below 387 nm, attributable to the intrinsic absorption of TiO2, caused by the transition of the electrons from top of the valence band to bottom of the conduction band. A slight blue shift was found for the N doped samples. For anatase TiO2, the blue shift cannot be attributed to the N doping because it has been reported that the N doping with any concentration red-shifts the absorption band edge [53]. Liu et al. found that the band gap of anatase TiO2 increases monotonically from 3.173 eV to 3.239 eV when the crystallite size increases from 17 nm to 29 nm [54]. The XRD result in Table 2 shows that the crystallite size of NV0 is 18.1 nm and it increases to ∼ 22 nm for the N doped samples; the increase in the crystallite size due to the N doping is the possible reason for the blue shift. The inset shows an enlarged image of UV–vis in the visible region.
3.5. The performance of photocatalytic degradation The MB solution was chosen as the target pollutant to test the photocatalytic performance of N-doped photocatalysts. Fig. 9(a) representatively shows the absorbance spectra of MB in the presence of NV1.5 measured at different time (t). In the process of the 4
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Fig. 4. XPS survey spectrum of NV0, NV1.5 and NV3 (a). Core level XPS spectra of F 1s (b, c, d), N 1s (e, f, g), O 1s (h, i, j) and Ti 2p (k, l, m) for NV0, NV1.5 and NV3. The solid line is the experimental curve, the dashed line is the fitted curve, and the open circles are the sum of the fitted curves.
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Table 3 Wavenumber at the absorbance peak and the corresponding vibration mode. Wavenumber (cm–1)
Vibration modes
References
400 ∼ 800 922, 1061 1400 1633, 3150 ∼ 3450
TieO or TieOeTi NeTieO stretching vibration eNOx stretching vibration The flexural vibrations of OeH in free water molecules CO2 stretching vibration The CeH stretching vibration
[33] [34] [34,35,36,37] [38]
2308 ∼ 2385 2850 ∼ 2977
[39] [39]
30-minute adsorption (t < 0), the peak position (λ =662 nm) keeps constant, but the intensity of peak gradually decreases. In the process of photodegradation (t > 0), the peak position shifts from 662 nm (t = 0 min) to 606 nm (t = 30 min) and the intensity decreases monotonically. Fig. 9(b) shows that the variation of MB concentration C/C0 with time t in the presence of NV0 ∼ NV3 under xenon lamp. In the range of −30 min ≤ t ≤ 0 min, the xenon lamp switches off and the change of MB concentration is caused by the catalyst adsorption. In the range of t ≥ 0, the MB concentration change is caused by photocatalytic degradation under the xenon lamp irradiation. The concentration of MB at t = −30 min is denoted as the initial concentration C0 and the concentration of MB at t = 0 is recorded as Ce. So the adsorption coefficient Ae can be calculated by the formula (C0−Ce)/C0 and the degradation coefficient De can be calculated by (Ce−C)/Ce, where C is the concentration of MB solution at any time t. Degradation curves were fitted by pseudo first-order kinetic model ln [Ce/C] = Kappt, where Kapp (min−1) is the apparent reaction rate constant. The plot of ln [Ce/C] versus time is shown in Fig. 9(c). The Ae, De and Kapp values for all samples are listed in Table 6. Un-doped sample NV0 has hardly any absorption to MB. For Ndoped samples, the adsorption coefficient Ae increases monotonically from 12.6% for NV0.5 to 51.3% for NV3 as RN/Ti increases. The higher adsorption coefficient can be attributed to the increase of specific surface area (SBET) and pore volume (Vp). Microscopically, the adsorption ability of TiO2 to water or organic pollutant molecules is related to the quantity and distance of oxygen vacancies (Ti3+) at the surface of TiO2 particles [58]. In the presence of each catalyst, the MB pollutant was almost completely degraded MB within 120 min with the degradation coefficient De changing from 98.3% to 99.9%. After the 60 min degradation, the De value of the un-doped NV0 is 83.0%. However, De is significantly increased for N-doped samples, and its value varies from 93.9% to 99.9%. The apparent reaction rate constant Kapp of NV0 is 2.8 × 10−2 min−1. For N-doped samples, Kapp increases first and then decreases with increasing RN/Ti, Kapp is the largest for NV1.5, reaching 11.5 × 10−2 min−1, which is nearly 4 times higher than the Kapp value of NV0. The possible reasons for the significant increase in the Kapp values for N-doped samples are as follows: the increase in the specific surface area and the ratio of oxygen vacancy (Ti3+) which provide more active sites for the photocatalytic degradation.
Fig. 5. Valence band density of states for NV0, NV1.5 and NV3.
Fig. 6. The UV–vis DRS of all samples.
Fig. 7. The photoluminescence spectra with the excitation wavelength λex =260 nm for NV0, NV1.5 and NV3.
Table 4 The chemical state (CS) of F, N, O and Ti elements and its corresponding binding energy (BE) and the ratio for NV0, NV1.5 and NV3. Core level
N 1s
CS
Intersitial
Surface
Ti-O-Ti
-OH
Ov
TiO2
TiOx
Ti-F
Ti-F-Ti
\ \ 399.1 59.8 400.0 54.3
\ \ 401.7 40.2 401.8 45.7
530.1 83.5 530.2 72.6 530.2 72.1
531.0 12.0 531.0 17.2 531.0 14.4
532.3 4.5 523.3 10.2 532.3 13.5
458.8 96.7 458.9 96.5 458.9 95.8
460.2 3.3 460.2 3.5 460.2 4.2
684.7 84.0 684.7 100 684.7 100
688.1 16.0 \ \ \ \
NV0 NV1.5 NV3
BE (eV) Ratio BE (eV) Ratio BE (eV) Ratio
O 1s
Ti 2p3/2
6
F 1s
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Table 6 The values of adsorption efficiency (Ae), degradation efficiency (De) and apparent reaction rate constant Kapp of all samples.
Table 5 Textural parameters of all samples. SBET (m2/g)
Pore volume Vp (cm3/g)
Average pore size Dp (nm)
NV0 NV1.5 NV3
95.83 124.44 123.52
0.38 0.48 0.53
15.81 15.52 17.09
NV 0
NV 0.5
NV 1
NV 1.5
NV 2
NV 2.5
NV 3
Kapp (10–2 min–1) Ae (%) De (%) (t = 120 min) De (%) (t = 60 min)
2.8 0.1 99.5 83.0
4.1 12.6 99.5 93.9
8.4 20.8 99.9 99.9
11.5 25.9 99.5 99.5
9.7 39.7 99.7 99.7
8.1 43.0 98.3 97.7
9.3 51.3 99.2 99.2
could be unequivocally assigned to a paramagnetic Ti3+ center in a distorted oxygen ligand field caused by the surface oxygen vacancies [59]. The signal at g = 2.002 arises from Ov [60]. Signals from Ti3+ and Ov increase with N doping, further confirming that N doping increases the ratio Ti3+ and Ov, beneficial for enhancing the photocatalytic performance. Combining the above results, it can be concluded that the enhanced photocatalytic performance can be assigned to the increase in the SBET and the ratios of Ti3+ and Ov. However, the enhancement of PL due to the N doping implies the increase in the recombination probability of photo-generated carries. Additionally, the N doping increases I(004)/I (200) (see Table 2), indicative of the decrease in the ratio of exposed (001) faces. These are the detrimental factors for the photocatalytic performance. Therefore, the enhanced photocatalytic performance due to the N doping may be attributable to the competing effects of advantageous and disadvantageous factors.
Fig. 8. Nitrogen adsorption-desorption isotherms for NV0, NV1.5 and NV3. The insets show the pore size distribution calculated by using the BJH method.
Samples
Samples
3.6. ESR
4. Conclusions
NV0, NV1.5 and NV3 samples were selected for ESR testing to further reveal the mechanism of photocatalytic degradation. The results are shown in Fig. 10. For all samples, no significant hydroxyl radical (%OH) and superoxide radical (%O2−) signals were observed without illumination. After illumination, the ESR spectrum exhibits a typical four-line spectrum for % OH with relative intensities of 1:2:2:1, due to the reaction of photogenerated holes (h+) with water (H2O) or hydroxyl groups (-OH), i.e. h+ + H2O → %OH + H. NV1.5 and NV3 have the same height (h) of % OH signal peak at 3340 G (G), higher than that of un-doped NV0 sample. After illumination, a significant %O2− signal also appeared in the ESR spectra due to the reaction of O2 with photo-generated electrons, i.e. e− + O2 → %O2−. The height h of %O2− signal peak located at 3322 G is also indicated in the figure. The %O2− signal of the N-doped samples is stronger than that of un-doped sample. In summary, both % OH and %O2− signals are enhanced for the N-doped samples compared with NV0. These results suggest that more O2, H2O or -OH in N-doped samples can be activated by photo-generated carriers to be high-reactive %O2− and %OH, consequently enhancing the degradation of MB, i.e. %OH or %O2− + MB → CO2 + H2O. Besides, the %OH signal is much stronger than %O2− for all samples, indicating a greater contribution of % OH during photo-degradation. From the ESR result in Fig. 10, the g value can be obtained for judging the variation of the Ov and Ti3+ ratios with the N doping, and the results are shown in Fig. 11. The signal at g = 1.992 for all samples
The (001) face exposed anatase TiO2 nanoparticles were firstly synthesized by the hydrothermal method at 180 ℃, and then the N doping was performed via the vapor thermal method at 180 ℃. The N/ Ti molar ratio (RN/Ti) is 0, 0.5, 1, 1.5, 2, 2.5 and 3. All samples were characterized by XRD, TEM, FT-IR, XPS, UV–vis DRS, PL, BET, ESR; using MB as the target pollutant, the photocatalytic degradation ability of all samples was tested. The main results are as follows: (1) The N doping slightly reduces the lattice parameters of TiO2, changes the ratio of exposed (001) face and the morphology of TiO2 particles. (2) Some of N atoms exist in the interstitial sites of lattice and others are adsorbed at the surface of TiO2 particles. Meanwhile, the N doping increases the ratio of oxygen vacancies in TiO2 crystal, accompanying with the increase of Ti3+. (3) The specific surface area and pore volume of the samples are increased by the N doping. (4) The N doping increases the adsorption coefficient of TiO2 to MB from 0.1% to 51.3%; The apparent reaction rate constant Kapp increases from 2.8 × 10−2 min−1 to 11.5 × 10−2 min−1, which increased by nearly 4 times. The ESR results show that the number of % OH and %O2− in the N-doped samples is significantly enhanced, beneficial for the improvement of the photocatalytic performance.
Fig. 9. The absorbance spectra of MB in the presence of NV1.5 measured at different time (t) (a). Variation of methylene blue concentration C/C0 with time in the presence of NV0 ∼ NV3 under xenon lamp (b). Linearly fitting of the degradation process according to the pseudo first-order kinetic model (solid line) (c). 7
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Fig. 11. The g value obtained from ESR spectra of NV0, NV1.5 and NV3.
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