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Defect Induced Yellow Color in Nb-doped TiO2 and Its Impact on Visible-Light Photocatalysis Lina Kong, Changhua Wang, Han Zheng, Xintong Zhang, and Yichun Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b03448 • Publication Date (Web): 03 Jul 2015 Downloaded from http://pubs.acs.org on July 6, 2015

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The Journal of Physical Chemistry

Defect Induced Yellow Color in Nb-doped TiO2 and Its Impact on Visible-Light Photocatalysis Lina Kong, Changhua Wang, Han Zheng, Xintong Zhang*, Yichun Liu Center for Advanced Optoelectronic Functional Materials Research, and Key Laboratory for UV-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, 5268 Renmin Street, Changchun 130024, China. *E-mail: [email protected]. Fax: +86 431 85099772. Tel.: +86 431 85099772.

ABSTRACT. Doping TiO2 photocatalysts with foreign ions has been deemed an effective method to enhance visible light absorption, and thus increase their photocatalytic performance. Herein, we report that Nb-doped TiO2 porous microspheres prepared by ultrasonic spray pyrolysis of peroxide precursor solution are yellow colored, and the yellow coloration becomes increasingly conspicuous with increasing Nb dopant concentration. Comprehensive spectral analyses show that both surface peroxo species and bulk Ti3+ are introduced into TiO2 microsphere samples together by charge compensation with Nb5+ dopant, and are responsible for the coloration of TiO2. The Nb-doped microspheres show higher photocatalytic rates than undoped TiO2 for the degradation of gaseous acetaldehyde under visible irradiation, but slower rates under ultraviolet light. Moreover, the photocatalytic mineralization rates of acetaldehyde to CO2 are lowered with Nb-doping under both visible and UV irradiation. Correlation between the results of surface photovoltage spectroscopic (SPS) characterizations and photocatalytic tests suggests that surface peroxo states are

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relevant to the visible light stimulated charge separation and photocatalytic reactions, albeit holes trapped in these states have lower reactivity than in valence band. On the other hand, the enhanced photoluminescence in the near-infrared region, reduced SPS response in the UV region as well as photochromic phenomena during photocatalytic process indicated that Ti3+ defects serve as charge carrier recombination centers and display adverse effect to photocatalytic activity of Nb-doped TiO2, especially under UV irradiation.

KEYWORDS. Nb-doped TiO2, coloration, visible light photocatalysis, defects, surface peroxo states

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1. Introduction

TiO2 has been extensively studied as a photocatalytic material for the degradation of organic pollutants due to its high chemical stability, low toxicity, and suitable energy band positions1, 2. However, TiO2 is a typical ultraviolet (UV) active photocatalyst (wherein anatase has a bandgap Eg = 3.2 eV)3, which indicates that the material can utilize only 5% of the available solar spectrum.4 Therefore, many efforts, such as doping,5-13 hetero-junction formation,14 and co-catalyst modification,15 have been devoted to developing visible-light responsive TiO2 photocatalysts. To this end, doping with cations (Fe, Cr, Nb, Mo, etc.)

5-9

or

anions (C, S, N, B, etc.) 10-13 has been found to work well for extending the light absorption of TiO2 from UV to the visible spectrum. Alternatively, self-doping, such as the creation of Ti3+ defects, oxygen vacancies,16 or O interstitials in TiO2,17 is a promising means for the photocatalytic activity enhancement of TiO2 under visible light. Recent studies have shown that Nb-doped TiO2 is a promising material for transparent conducting oxides, 18 photo-electrochemical systems, 19-20 dye-sensitized solar cells, 21 sensors, 22

and photocatalysts,

23-25

etc. In this material, Nb substitution on the Ti site creates a Nb5+

defect state located upon the conduction band minimum, which contributes electrons to the unoccupied Ti 3d orbital without introducing impurity states in the bandgap.26-29 Different from the doping with transition metal cations that have electrons in d orbitals (Fe30, Co31, V32, Cr33, etc), it seems difficult to make TiO2 respond to the visible light by sole Nb doping. However, substituted doping of Nb5+ to Ti4+ is usually accompanied by the incorporation of other defects in order to satisfy the charge compensation mechanism. On the one hand, Ti3+ defects which lie below the CB of TiO2 could coexist with Nb5+ and lead to red/near-infrared absorption, giving rise to feeble blue colored samples.34 Analogously, Nb5+ doped into TiO2 might introduce O interstitial defects,35 and the interstitial oxygen states lie upon the VB of

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TiO2, giving rise to yellow colored samples.17 Notably, the synergistic effect of Nb doping and accompanying defects for obtaining visible light absorption and the corresponding photocatalytic activity is highly dependent on the doping method.36 Mattsson et al.37 reported a pale yellow-colored Nb-doped TiO2 made by a one-step sol-gel method. This is, to the best of our knowledge, the sole reported preparation of a yellow-colored Nb-doped TiO2 in conjunction with a systematic analysis of the visible light response induced by the defects species. However, the material exhibited an inferior photocatalytic activity. Herein we report the preparation of yellow-colored Nb-doped TiO2 porous microspheres by means of ultrasonic spray pyrolysis of aqueous peroxide precursors solution. Significant yellow coloration and visible light absorption of TiO2 are observed after Nb doping. Comprehensive spectral analyses demonstrates that the increased visible light absorption and the appeared yellow color of Nb-doped TiO2 are closely related to the surface peroxo species and bulk Ti3+ defects, which are incorporated into TiO2 samples together with Nb dopant by charge compensation. The photocatalytic activity of the Nb-doped TiO2 samples is investigated in terms of the photodegradation and mineralization of gaseous acetaldehyde under visible light and UV light, respectively. The impact of surface peroxo states and bulk Ti3+ to photocatalytic process is analyzed on the basis of surface and spectral characterizations results, and some implication to the visible light responsive TiO2 photocatalysts is discussed.

2. Experimental Section

2.1 Preparation of TiO2 and Nb-doped TiO2 microspheres Liquid Titanium tetrachloride (99.0%, Beijing Chemical Co.) 7.9 mL was carefully dissolved in 450 mL ice water by slow addition at room temperature, forming a transparent

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colorless aqueous solution. 50 mL of ammonia solution (25% w/w, Beijing Chemical Co.) was added to the TiCl4 solution dropwise to obtain a white precipitate. The precipitate was washed with distilled water exhaustively to remove Cl-, and then diluted with 600 mL distilled water. 100 mL of hydrogen peroxide (30%, Shanghai Chemical Co.) was added into the mixed solution under vigorous stirring until a transparent orange solution was obtained. Niobium oxalate (99.95%, Shanghai Chemical Co.) was dissolved in the orange transparent solution in stoichiometric proportions. The undoped sample was prepared using the same procedure minus the niobium oxalate. The transparent orange solution was atomized by an ultrasonic nebulizer, and the formed mist was passed through a quartz tube heated at 1073 K. Powders were obtained by pyrolysis of the precursor solution and collected using an electrostatic collecting device connected to the end of the quartz tube. Samples of 0, 2, and 10 mol% Nb-doped TiO2 were obtained. Finally, the as-prepared samples were annealed at 500 oC for 1 hour under ambient conditions. 2.2 Characterization Size and morphology of TiO2 samples were measured with a field emission scanning electron microscope (FESEM, FEI Quanta250), and a transmission electron microscope (TEM, JEOL JEM-2100) at an acceleration voltage of 200 kV, respectively. The crystal structure and phase identification of the samples were determined by X-ray diffraction (XRD) using a Rigaku, D/max-2500 X-ray diffractometer. X-Ray photoelectron spectroscopy (XPS) was performed using a VGESCALAB MKII instrument with a Mg Kα ADES source at a residual gas pressure below 10-8 Pa. Fourier transform infrared (FTIR) spectra were recorded using a Nicolet Magna 560 FTIR spectrometer. Electron paramagnetic resonance (EPR) spectra were obtained using a CW-EPR Bruker ElEXSYS spectrometer in the X band (9.38 5



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GHz) at 10 K equipped with a cylindrical cavity operating at a 100 kHz field modulation. The UV-visible (UV-vis) diffuse reflectance (DR) spectra of the samples were collected using a PerkinElmer UV WinLab spectrophotometer. Surface photovoltage spectroscopy (SPS) measurements were carried out using a lab-made instrument.38 Photoluminescence (PL) spectra were recorded using a JY HR-800 Lab Ram infinity spectrophotometer excited with a He-Cd laser at 325 nm. 2.3 Photocatalytic experiments Photocatalytic properties of the Nb-doped TiO2 microspheres were investigated by measuring the photodegradation of gaseous acetaldehyde under different light sources. All photocatalytic degradation experiments were carried out in a 500 ml Pyrex glass vessel at room temperature. Visible light (420-700 nm) and simulated solar light (350-700 nm) were obtained from the Xenon lamp (Hayashi LA-410) with and without a 420 nm long-pass filter. UV light (350-400 nm) was provided by the Hayashi LA-410UV Xenon lamp directly. A 100 mg sample was uniformly spread over the 2 cm 2 irradiation area in the glass vessel, then humidified synthetic air (relative humidity = 33 %) was filled into the vessel, followed by the injection of 4.5 μmol (200 ppmv) of the acetaldehyde gas. The glass vessel with photocatalyst was kept in the dark for 30 min, and then was placed under light source to start photocatalytic reactions. The concentrations of the acetaldehyde gas and the produced CO2 were monitored using a gas chromatograph (model GC-2014C, Shimadzu Co., Ltd.) equipped with a porapak Q column and a flame ionization detector, using N2 as the carrier gas.

3. Results and Discussion

3.1 Morphology and structure characterization The morphology of Nb-doped TiO2 samples were characterized by SEM and TEM 6


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measurements. As shown in Figure S1, all the Nb doped TiO2 samples exhibit microspheric morphology and have a size distribution centering from 200 to 1000 nm, as evidenced by the size distribution analysis in Figure S1 inset. The size distribution of the microspheres shows no correlation with the concentration of the Nb dopant. Figure 1(a), (b), and (c) show the high magnification surface morphology SEM images of 0, 2 and 10 mol% Nb-doped TiO2 microspheres. The surfaces of all the microspheres are not smooth, indicating a polycrystalline structure of each microsphere. TEM is used to investigate the microstructure of the Nb-doped TiO2 microspheres in more details. Figure 1(d), (e), and (f) shows the TEM image of 0, 2 and 10 mol% Nb-doped TiO2 microspheres, respectively. From the TEM images, it can be seen that the TiO2 microspheres are porous and are stacked by numerous small TiO2 nanoparticles compactly, which is consistent with the SEM observation.

Figure 1. (a. b, and c) SEM micrographs of 0, 2, and 10 mol% Nb-doped TiO2 microspheres, respectively. (d. e, and f) TEM images of 0, 2, and 10 mol% Nb-doped TiO2 microspheres, respectively.

Figure 2 shows the XRD patterns of the undoped microspheres and those doped with 2 and 10 mol% Nb concentrations. The composition of the phases were calculated using the following empirical formula: 39

 IR   IR    0.312  %Rutile  0.679  I  I I  I  R A  R A

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(1)


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where %Rutile is the content percentage of rutile, IA is the intensity of the main anatase reflection peak (101) (2θ = 25.51°), and IR is the intensity of the main rutile reflection peak (110) (2θ = 27.69°). According to the XRD spectrum, the content percentage of rutile was calculated by Eq. (1) to be 6.8% for the undoped TiO2 microspheres. No rutile phase was detected in the Nb-doped TiO2 microspheres. It was well known that the anatase phase is metastable and irreversibly converted to the stable rutile phase in the temperature range 600-700 oC, and During phase transformation, the oxygen vacancies placed in anatase planes act as nucleation sites for the anatase-to-rutile phase transformation. 40, 41 When niobium ions substitute for Ti in TiO2, the charge of the Nb5+ ions is typically compensated by a decreased concentration of oxygen vacancies. Thus, Nb-doping could result in the decrease of oxygen vacancies and hence inhibit anatase-to-rutile transformation.42

Figure 2. (a) XRD patterns of as-prepared samples: undoped TiO2 microspheres; 2 mol% Nb-doped; 10 mol% Nb-doped. (b and c) Magniﬁed XRD patterns of those 2θ regions from 22°to 30°and from 45°to 53°, respectively.

Furthermore, as can be observed in Figure 2(b) and (c), the peaks are slightly shifted to lower diffraction angles with increasing Nb dopant concentration, which is related to the lattice expansion induced by the doping of Nb into the TiO2 crystal obtained due to the slightly larger radius of Nb5+ (0.64 Å) than that of Ti4+ (0.605 Å), suggesting that the lattice constants of a Nb-doped TiO2 crystal will be larger than those of pure TiO2. The lattice parameters of the undoped and Nb-doped TiO2 microspheres were calculated, and are listed

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in Table 1, the findings of which agree with the above observation. Table 1. Crystal parameters and BET surface area of undoped and Nb-doped TiO2. Samples TiO2 Nb 2 mol% Nb 10 mol%

a (Å) 3.7709 3.7726 3.7882

c (Å) 9.4594 9.4641 9.4651

BET (m2/g) 23.6026 36.6950 79.6275

In addition, the characteristic XRD peaks become slightly broadened after Nb doping. Using Scherrer Equation, the grain sizes are estimated to 19.4, 16.7 and 11.4 nm from the (101) peak of anatase phase for the undoped and Nb-doped TiO2 samples, respectively. These grains are the stacking units of the microspheres. Owing to its slightly larger radius, the introduction of Nb5+ ions induces a slight stress in the TiO2 lattice, which may hinder the growth of TiO2 grains, as reported by Sharma and Bhatnagar.43 BET surface area of the samples increases with increasing Nb dopant concentration , as shown in Table 1, which is consistent with the declining trend of grain size. 3.2 XPS, FTIR and EPR characterizations The TiO2 samples were further characterized by XPS. In Figure 3a, Ti and O elements are observed in all samples, and Nb element is only detected in the Nb-doped TiO2 samples. To investigate the chemical states of the individual elements in the undoped and Nb-doped TiO2 samples, high resolution XPS peaks for the Nb 3d, O 1s, and Ti 2p states are provided in Figs. 3b, 3c, and 3d, respectively. For the Nb-doped TiO2 samples (Figure 3b), peaks at 206.80 eV and 209.66 eV are consistent with the standard Nb 3d5/2 and Nb 3d3/2 binding energy for niobium in the pentavalent oxidation state.44, 45 No feature of tetravalent niobium with lower binding energy was observed. In addition, the atomic ratios of Nb/(Ti + Nb) were determined as 3.58% and 9



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12.36% for 2 mol% and 10 mol% Nb-doped samples, respectively, which is close to the stoichiometric ratios of the starting materials. Figure 3c shows the high-resolution O 1s spectra of the undoped and Nb-doped TiO2 samples. The figure shows that O 1s peak of the Nb-doped TiO2 samples can be fitted by three dominant peaks at 529.44, 531.40, and 532.70 eV, which are defined as O-Ti, O-H (surface hydroxyl), and O-O (surface peroxo), respectively.46 The relative contents of oxygen in the form of surface hydroxyl (O-H) and surface peroxo (O-O) significantly increase with Nb doping into TiO2 as shown in table 2. The ratios of O(O-H)/total O are estimated to 16%, 23%, and 35% for TiO2, Nb 2 mol%, and Nb 10 mol% doped samples, respectively, indicating an increasing amount of adsorbed H2O on the surface of the samples with increasing Nb dopant concentration. Meanwhile, the ratios of O(O-O)/total O are estimated to 12%, 20%, and 32% for TiO2, Nb 2 mol%, and Nb 10 mol% doped samples, respectively, which again demonstrates that the content of surface peroxo species progressively increases with increasing Nb dopant concentration. Figure 3d shows two pairs of peaks belonging to Ti 2p3/2 and 2p1/2 appearing on the 0, 2, and 10 mol% Nb-doped TiO2, respectively, which implies two different chemical bonding of Ti4+ exist on the surface of TiO2. For undoped TiO2, the peaks located at binding energies 458.17 eV and 463.86 eV are assigned to Ti 2p3/2 and 2p1/2, respectively, which is attributed to surface O-Ti-O linkages of TiO2. Notably, another pair of peaks of Ti 2p3/2 and 2p1/2 appeared at 459.06 and 464.74 eV, which can be assigned to the surface Ti-peroxo species of TiO2.46 The binding energy of Ti 2p (belonging to the surface O-Ti-O and Ti-peroxo species) increase slightly in Nb 2 mol% and 10 mol% doped TiO2 compared with undoped TiO2, as shown in Table 2, which is believed to be induced by the presence of Ti-peroxo species on the surface of TiO2. The formation of peroxo species can cause partial electron transfer from the

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neighboring Ti to peroxo groups and a decrease of the electron density on Ti, resulting in an increase of binding energy.47 Accordingly, the ratios of Ti-peroxo/total Ti are estimated to 9%, 14%, and 36% for TiO2, Nb 2 mol%, and Nb 10 mol% doped samples, respectively, indicating an increased concentration of surface Ti-peroxo species after Nb doping.

Figure 3. (a) XPS survey spectra of undoped and Nb-doped TiO2 samples. (b, c, and d) Nb 3d, O 1s, and Ti 2p spectra, respectively, of the three samples. The dot lines are the experimental data, whereas the thin lines are the fitting results.

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Table 2. XPS results for undoped and Nb-doped TiO2. Samples

a, b

Nb 2 mol % 458.17 458.23 463.86 463.92

Nb 10 mol % 458.28 464.02

459.04

459.20

464.74

464.85

529.48 531.40 532.70 206.80 209.66 23 % 20 %

529.99 531.40 532.70 206.80 209.66 35 % 32 %

14 %

36 %

TiO2

TiTi-O 2p3/2 (eV) TiTi-O 2p1/2 (eV) TiTi-peroxo 2p3/2 459.06 (eV) TiTi-peroxo 2p1/2 464.74 (eV) OO-Ti 1s (eV) 529.44 OO-H 1s (eV) 531.40 OO-O 1s (eV) 532.70 Nb 3d5/2 (eV) -Nb 3d3/2 (eV) -O-H/Total O a 16 % b O-O/Total O 12 % Ti-peroxo/Total 9% Ti c Nb content d 0% The calculated O-H (surface hydroxyl band)

3.58 % 12.36 % and O-O (surface peroxo band) oxygen

concentration ratios relative to the total O contained in the samples.

c

The calculated

Ti-peroxo species concentration ratios relative to the total Ti contained in the samples. d The Nb content corresponds to the atomic ratio of Nb/(Ti+Nb) calculated from the XPS spectra. FTIR spectra were collected and shown in Figure 4(a) which can further confirm the presence of surface peroxo species in Nb-doped TiO2 samples. Relative to the undoped TiO2 spectrum, a new peak appears at around 944 cm-1 in the Nb 2 mol% and Nb 10 mol% doped spectra, which may originate from the metal peroxide group bands at the TiO2 surface. As has been well reported, bands in the range of 700-1000 cm-1 are ascribed to different Ti-peroxo vibrational modes. For example, Muhlebach et al.48 reported that the vibrational modes of the metal peroxide group fall between 800-1000 cm-1. Miksztal et al. ascribed the band at 890 cm-1 to the Ti-peroxo stretching vibration.49 Ohno et al. observed that H2O2-treated anatase TiO2 exhibited infrared (IR) absorption in the range 940-820 cm-1, which was attributed to a stretching vibration of the -O-O- bonds of Ti-η2-peroxide.50 Nakamura et al. confirmed that an IR band at 943 cm-1 was attributable to surface peroxo species in TiO2.51 Accordingly, it 12


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can be inferred from the characteristic FTIR peak at 944 cm-1 in the Nb-doped TiO2 samples that the concentration of Ti-peroxo species dramatic increase in the Nb-doped TiO2 photocatalysts, which is consistent with the XPS results above. To demonstrate that Nb doping plays an important role in the existence of Ti-peroxo species, we prepared a Nb-Al co-doping TiO2 sample by the same method as that employed for preparation of Nb-doped TiO2 samples. The as-obtained Nb-Al-co-doped TiO2 sample was white in color. Additional FTIR (see supporting information Figure S2) investigation found no sign of absorbance at 944 cm-1, which illustrates that no surface peroxo species exist when the charge imbalance induced by Nb5+ doping is compensated by Al3+ doping. Simultaneously, an Nb-doped TiO2 sample was prepared by hydrolyzation of titanium isopropoxide rather than aqueous peroxo-titania as a precursor solution. Subsequent FTIR investigation revealed the absence of surface superoxo species. Therefore, it is safe to conclude by a combination of XPS, FTIR analysis, and comparative experiments that the existence of surface peroxo species in the Nb-doped TiO2 samples depends on the TiO2 precursor used, and is predominantly induced by Nb doping to compensate for the resulting charge imbalance. Additionally, Ti3+ defects are also detected in Nb-doped TiO2 samples by EPR measurement. As shown in Figure 4(b), there are two types of EPR active centers (denoted as A and B). Signal A is observed in all cases with little variation in concentration. We speculate that signal A was attributed to the NO species adsorbed on the surfaces of all three samples, as an oxidation products of residual ammonia in the preparation process. It is notable that signal B, assigned to Ti3+ species52-55, was observed in the region of g⊥=1.975 and g//=1.945. The small Ti3+ signal observed in the undoped TiO2 probably results from the high temperature treatment. After the introduction of Nb5+, the concentration of Ti3+ defects

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appears to increase, which has also been detected in other cases of pentavalent-cation-doped TiO2.56, 57 This increased concentration of Ti3+ defects is the result of charge compensation after substitution of Ti4+ by Nb5+. However, no Ti3+ feature was defected in the XPS spectra. Instead, peroxo species present on the surface of TiO2 which could restrict the formation of Ti3+ defects. Thus, these Ti3+ defects detected by EPR spectra mostly exist inside TiO2 microspheres as bulk defects.

Figure 4. (a) FTIR spectra of undoped and Nb-doped TiO2 samples. The bands at 3421 cm-1 and 1627 cm-1 are assigned to the stretching and flexion vibrations of the O-H group, respectively, which are mainly ascribed to the absorbed water.51, 58 The main peaks at around 574 cm-1 result from the absorption band of the Ti-O and O-Ti-O flexion vibrations.59, 60 (b) EPR spectra of undoped and Nb-doped TiO2 samples at 10 K. The increased concentration of surface peroxo species and bulk Ti3+ defects was induced by substituted dopant of Ti4+ by Nb5+ which could also be interpreted by the Kröger-Vink theory. 61 Under oxygen excess conditions, the extra charge of Nb5+ in compare with Ti4+ may produce lattice defects to form charge balance structure. The lattice defect can be either Ti vacancy or oxygen interstitial. No apparent titanium vacancies were observed in our samples. Meanwhile, the oxygen interstitial mechanism appears less reliable owing to the close-packed oxygen sub-lattice. Thus, the peroxo species appeared at the surface of the crystal could maintain charge balance, similar as the O interstitial. Under anoxic conditions, Nb5+ induced

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the generation of Ti3+ defects as electronic defects. Ti3+ defects appears to exist inside the grain of Nb-doped TiO2 samples since such large concentration of peroxo species existed on the surface of Nb-doped TiO2 samples. On the basis of the above spectral characterizations and analyses, we could speculate the types of defects involved in Nb-doped TiO2, which are illustrated in Scheme 1. These include the couple of Nb5+ -Ti3+ in the bulk of TiO2 named as species A, Nb5+ on the surface of TiO2 (species B), surface peroxo defects named as species C or D, respectively. Muhlebach et al.48 believed that a stable crystal structure of peroxo defects might follow a structure whereby two Ti atoms are bonded by a peroxo group that apparently occupies only a single oxygen coordination site as species C. In contrast, Miksztal et al.49 and Nakamura et al.51 considered that the peroxo ligand is bound to Ti atom in a triangular fashion like species D.

Scheme 1. Schematic defect structure of Nb-doped TiO2: A.Ti3+ and Nb5+ defects couple; B. surface Nb5+ defects; C and D. surface peroxide defects. 3.3 Optical properties of TiO2 and Nb-doped TiO2 Figure 5(a) shows the UV-vis diffused reflectance spectra of the three samples. The spectra were converted from reflectance to absorbance using the Kubelka-Munk method, as shown by the inset spectra of Figure 5(a).62, 63 The optical absorption edges (indicative of the

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band gap Eg) are determined to be 2.93, 2.85, and 2.75 eV for TiO2, Nb 2 mol%, and Nb 10 mol% doped samples, respectively. The narrower band gap of the undoped TiO2 sample (2.93 eV) than that of anatase TiO2 (3.2 eV) could be attributed to two reasons: (1) the existence of 6.8% rutile TiO2, which has an Eg = 3.0 eV; (2) the existence of surface peroxo states. Clearly, with increasing Nb concentration, the optical band gap becomes increasingly narrow, and distinct shoulders appear in the longer wavelength region. The inset of Figure 5(a) shows images of undoped and Nb-doped TiO2 powders. It is apparently that samples become increasingly yellow colored as the concentration of Nb dopant increases. Etacheri et al.17 defined Ti-peroxo species as oxygen interstitials that introduce peroxo defect states above the valence band of TiO2. Meanwhile, the state of Ti3+ defects is reported to lie below the CB 0.75-1.18 eV below the conduction band of TiO2.64 Therefore, the red shift in the UV-vis spectrum and the resulting yellow color of the Nb-doped TiO2 are related to surface peroxo states and bulk Ti3+ defects. These defect states have been further analyzed according to PL spectra, as shown in Figure 5(c). The PL spectra exhibit a visible emission (green-yellow) and a NIR emission. The visible emission is attributed to the transition between defect states to band,65 and the obviously stronger NIR emission intensity of the Nb-doped samples is attributed to the radiative recombination of electrons in Ti3+ states with holes in peroxo states.

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Figure 5. (a) UV-vis diffuse reflectance spectra of the undoped and Nb-doped TiO2 samples, where the inset at top provides a photograph of the different samples and the inset at bottom illustrates conversion from reflectance to absorbance using the Kubelka-Munk method; (b) the optical absorption edges (eV) of the samples; (c) photoluminance spectra of the samples for 325 nm laser excitation; and (d) surface photovoltage spectroscopy of the undoped and Nb-doped TiO2 samples. SPS is a useful technique for detecting the photo-induced charge separation in semiconducting materials.66 Figure 5(d) shows that (i) the SPS response decreases as the Nb dopant concentration increases in the UV light region, and (ii) the SPS response increases as 17



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the Nb dopant concentration increases in the region between 400-500 nm. The declined trend of SPS response indicates that charge separation in TiO2 samples is reduced with Nb-doping under UV excitation. It is reasonable if we consider bulk Ti3+ defects as recombination centers, and is also consistent with the enhanced NIR photoluminescence of Nb-doped TiO2. The increased SPS response in the 400-500 nm spectral region, however, indicates that effective charge separation occurs in Nb-doped TiO2 under the excitation. This SPS response is related to the electron transition from surface peroxo states to conduction band (CB). Increasing the amount of surface peroxo states with Nb-doping therefore give rise to greater absorption and stronger SPS response in the spectral region. We overlay SPS, PL, and diffusion reflectance (DR) results of Nb 10 mol% doped TiO2, as shown in Scheme 2, to obtain the relationship between the spectral response and the defects. In Scheme 2, there are four spectral regions identified by photon energy: I. super-bandgap absorption region (E > 3.29 eV); II. surface peroxo states to CB absorption region (2.44 eV < E < 3.29 eV); III. valence band (VB) to inner Ti3+ states absorption region (2.06 eV < E < 2.44 eV); and IV. No absorption region (E < 2.06 eV). Scheme 2(b) shows the dominant mechanisms of super-bandgap (region I) and sub-bandgap (region II) illumination, which result in the SPS response as reported by Kronik et al.67 In region I, the SPS response in the UV region is assigned to band-band transition. The decrease of SPS response after Nb doping in region I can be attributed to the incorporation of Ti3+ defects. In region II, a stronger SPS response appears after Nb doping, which originates from excitation by photons of energy Eg > hν > Eg-Esurf.O-O that may produce electron transitions from the surface peroxo states to the CB. The surface peroxo states can thus be estimated as 0.85 eV above the VB. In region III, electron transition related to bulk Ti3+ defects cannot produce considerable SPS response, because of the competition of charge recombination at the Ti3+ defect sites. Both in region II and III, the visible light absorption is generated from the electron transitions from 18


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the VB to the Ti3+ states given as the arrow A as well as the surface peroxo states to the CB given as the arrow B in scheme 2c. Meanwhile, the visible emission is attributed to the radiative recombination of carriers from band states to defects states, given as the arrow C and arrow D in Scheme 2b. In region IV, NIR emission can only be observed in the Nb-doped samples, and is attributed to the recombination from the bulk Ti3+ states to the surface peroxo states, given as arrow E in Scheme 2c. The Ti3+ defect states can be calculated as 1.09 eV below the CB, which is close to the values determined by experiment16 and calculations68 by other researchers.

Scheme 2. (a) Surface photovoltage spectroscopy (SPS), diffuse reflectance (DR), and photoluminescence (PL) spectra of Nb 10 mol% doped samples in energy coordinates. (b) Schematic band diagrams of the SPS measurements at a semiconductor surface under various illumination conditions: I. super-band gap illumination with charge carrier separation; II. sub-bandgap illumination with excitation of trapped holes. (c) Mechanism scheme of band gap narrowing after niobium doping. 19



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3.4 Photocatalytic activity of Nb-doped TiO2 Photocatalytic activity of the undoped and Nb-doped TiO2 was assessed by monitoring the degradation of gaseous acetaldehyde under visible light (420-700 nm), UV light (350-400 nm) and solar light (350-700 nm) irradiation. As reported in literatures, the photodegradation of acetaldehyde proceeds as follows: 69 CH3CHO + H2O + 2h+ → CH3COOH + 2H+ CH3CHO + 3H2O+ 10h+ →2CO2+ 10H+

(2) (3)

Since acetic acid, one of the intermediates, is more difficult to oxidize than acetaldehyde, the generation of CO2 was more difficult than that of acetaldehyde degradation. Figure 6 traces the photodegradation of acetaldehyde and the evolution of CO2 with TiO2, Nb (2 mol%)-, and Nb (10 mol%)- doped samples. The photocatalytic results can be summarized as following: (1) Under visible light irradiation, the photocatalytic activities on acetaldehyde degradation follow the order of Nb 10 mol% > Nb 2 mol% > TiO2. The apparent rate constants (k) of acetaldehyde degradation are calculated from Figure 6b via assuming a first-order kinetics and the data are listed in Table 3. It should be noted that the curves of Nb-doped TiO2 are actually somewhat nonlinear, i.e. deviated from first-order kinetics with the proceeding of photodegradation. Moreover, CO2 generation rate follows a different order: TiO2 > Nb 10 mol% > Nb 2 mol%, as shown in Figure 6c, which indicates a decreased mineralization ability of the Nb-doped TiO2 in comparison with the undoped TiO2. (2) Under UV light irradiation , Nb (10 mol%)-doped TiO2 exhibited inferior photocatalytic activity to that of undoped TiO2, which is reflected by both lower decomposition rate of acetaldehyde and generation rate of CO2, as shown in Figure 6 (d, e 20


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and f). Similar trend is also observed under solar light (350-700 nm) irradiation, which is shown in Figure S3. Notably, the photocatalytic performance of the Nb (2 mol%)-doped TiO2 is similar to the undoped TiO2 under UV light irradiation in terms of both acetaldehyde degradation and CO2 generation rates.

Figure 6. Photocatalytic degradation curves, degradation kinetics curves and generation of CO2 during the photocatalytic degradation of acetaldehyde for undoped and Nb-doped TiO2. (a, b and c) are for visible light (420-700 nm) irradiation and (d, e and f) are for UV light (350-400 nm) irradiation. Table 3. Apparent rate constants (k) of acetaldehyde degradation under visible light and UV light for the undoped and Nb-doped TiO2. Samples TiO2 TiO2-Nb 2 mol% TiO2-Nb 10 mol%

k / min-1 Visible light 0.0033 0.0040 0.0050

k / min-1 UV light 0.0563 0.0512 0.0139

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Next, let us to analyze the influencing factors about the photocatalytic activity of Nb-doped TiO2, especially under visible light irradiation. As mentioned above, the yellow color of Nb-doped TiO2 is related to defects like surface peroxo species and bulk Ti3+. The former is effective in the charge generation and separation, as evidenced by the SPS measurements, and hence should play a critical role in the photocatalytic process under visible light. The latter, however, serves as recombination centers, as evidenced by the PL and SPS measurements, and hence has adverse influence on photocatalysis. Both XPS and FTIR characterizations have shown that Nb-doping drastically increases the amount of surface peroxo species on TiO2 porous microspheres. The ratios of O(O-O)/total monotonically increase from 12% for the undoped TiO2 to 32% for 10 mol% Nb-doped TiO2. Moreover, the BET surface area also increases from 23.6 m2 g-1 for the undoped TiO2 to 79.6 m2 g-1, for 10 mol% Nb-doped TiO2. Hence, the amount of surface peroxo species might be 2.6 and 8.9 times, respectively for the 2 mol% Nb and the 10 mol% Nb-doped TiO2, higher than the undoped TiO2. Thus, it is reasonable to observe the monotonically increased photocatalytic activity of Nb-doped TiO2 with Nb-doping concentration, since holes generated in surface peroxo species participate in the oxidative degradation of acetaldehyde directly. However, the apparent rate of the acetaldehyde decomposition only increased for 1.2 and 1.5 times, respectively for the 2 mol% and 10 mol% Nb-doped TiO2 under visible light irradiation than the undoped TiO2. In other word, the improvement on photocatalytic activity is not as great as expected after Nb-doping, in particular the activity of 10 mol% is by far less than expectation. The most probable reason for the loss of activity enhancement is attributed to bulk Ti 3+ defects. The amount of Ti3+ defects also increases with the increase of Nb doping. We

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actually observed that Nb-doped TiO2 samples gradually turned from yellow to a light blue color during the photocatalyst degradation of acetaldehyde (Figure S4). However undoped TiO2 retained its white color during the entire photocatalytic process. To better understand the phenomenon, we designed an accelerated photochromic experiments via irradiating Nb-doped TiO2 film in high-concentration ethanol atmosphere. Detailed experimental method and results were listed in the Supporting Information. From the results of the photochromic test we could conclude that the color change of the Nb-doped sample was due to the accumulation of the photogenerated electrons on Ti3+ defects. The electron accumulation competes with the reaction of photogenerated electrons with adsorbed dioxygen molecules, which brought an adverse influence to the photocatalytic degradation of acetaldehyde on the Nb-doped TiO2 samples, and should also lead to the nonlinear behavior shown in Figure 6(b). It should be noted that the CO2 generation rate, in other word the mineraliztion rate, with Nb-doped TiO2 was lower than the undoped TiO2 sample under visible light irradiation, albeit the photocatalytic degradation of acetaldehyde is monotonically increased with Nb-doping. Possible reasons include: (a) The adverse effect of bulk Ti3+ defects becomes more prominent in the complete mineralization of acetaldehyde to CO2, since the mineralization process is proceeded step by step and at least 10 holes should be involved in mineralizing one acetaldehyde molecule to two CO2 molecules, according to equation (2) and (3). By contrast, only two holes are needed to degrade one acetaldehyde molecule. (b) The surface peroxo states lay ca. 0.85 eV upon the VB as discussed above, holes generated in these states exhibit lower oxidation activity than in the VB. (c) The reaction intermediates, for example acetic acid, are often more difficult to oxidize than acetaldehyde. As for the photocatalytic activity under UV light, the Nb (10 mol%)-doped TiO2 sample displays drastically lower activity than the undoped TiO2, which is in accordance with the

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reduced SPS response under UV excitation. The main reason should be due to the involvement of large amount of Ti3+ defects as charge recombination centers. However, the Nb (2 mol%)-doped sample interestingly displays a comparable activity to the undoped TiO2 in terms of both acetaldehyde degradation and CO2 generation, despite it contains larger amount of Ti3+ defects and its SPS response is actually lower. One possible reason is the Nb-doped TiO2 contains larger amount of surface hydroxyl groups, as evidenced by XPS characterizations, which act as acceptors of photogenerated holes so as to promote charge separation and photocatalytic reactions. The ratios of O(O-H)/total O increase from 16% for the undoped TiO2 to 23% for 2 mol% Nb-doped TiO2. Moreover, the BET surface area also increases from 23.6 m2 g-1 for the undoped TiO2 to 36.7 m2 g-1, for 2 mol% Nb-doped TiO2. Hence, the amount of surface hydroxyls on 2 mol% Nb-doped TiO2 might be 2.2 times higher than the undoped TiO2. The balance of the effect of Ti3+ defects and surface hydroxyls renders the Nb (2 mol%)-doped TiO2 display considerably high photocatalytic activity under UV irradiation. Lastly, one may ask the necessity of Nb-doping for obtaining colored TiO2 with high photocatalytic activity? Our work has shown that the critical role of Nb-doping includes (1) inhibit the particle growth to obtain high surface area TiO2, (2) drastically increase the amount of surface peroxo species to obtain considerable coloration, (3) increase the amount of surface hydroxyls that are good for photocatalytic process. However, Nb-doping also leads to the incorporation of bulk Ti3+ recombination centers. Therefore, searching good preparation conditions or optimizing the doping concentration might lead to the development of Nb-doped TiO2 photocatalysts with good activity. Alternatively one may try to dope Nb5+ only on the surface of TiO2, thereby can help the incorporation of surface peroxo species without the unwanted Ti3+ defects.

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4. Conclusions

In summary, yellow-colored Nb-doped TiO2 porous microspheres were obtained by ultrasonic spray pyrolysis of aqueous peroxoide precursor solution. Comprehensive spectral analyses showed that both surface peroxo species and bulk Ti3+ were introduced into TiO2 microsphere samples together with Nb5+ dopants, and were responsible for the coloration of TiO2. Nb-doping reduces the grain size of TiO2, drastically increase the amount of surface peroxo species that are relevant to visible light stimulated charge generation and separation, and increase the amount of surface hydroxyls that are good for photocatalytic process. However, Nb-doping also leads to the incorporation of bulk Ti3+ recombination centers. As a result, Nb-doped TiO2 microspheres exhibit higher photocatalytic rates than undoped TiO2 for the degradation of gaseous acetaldehyde under visible irradiation, but slower rates under ultraviolet light. Moreover, the photocatalytic mineralization rates of acetaldehyde to CO2 are lowered with Nb-doping under both visible and UV irradiation. Our study thus concludes that careful control of the types and density of defects in Nb-doped TiO2 systems is the key issue for obtaining both enhanced light harvesting and photocatalytic activity under visible light irradiation.

Supporting Information

Low magnified SEM micrographs and diametre distribution of the microspheres, FTIR spectrum of Nb-Al codoped TiO2 and Nb-doped TiO2 prepared by hydrolyzation of titanium isopropoxide, photocatalytic degradation curves of acetaldehyde under solar light irradiation, experimental condition of

photochromic test and corresponding discussion, diffuse

reflectance spectra and photoluminescence spectra of the photochromic recovering process for the sampes after UV light irradiation. This information is available free of charge via the

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Internet at http://pubs.acs.org.

Acknowledgements

This work was supported by the Natural Science Foundation of China (Grant No. 51072032, 91233204, 51372036, 51102001, and 21301041), the Key Project of Chinese Ministry of Education (No 113020A), the Specialized Research Fund for the Doctoral Program of Higher Education (20120043110002), the National Basic Research Program (2012CB933703), and the International Science & Technology Cooperation Program of China (2013DFG50150).

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TOC GRAPHICS

Doping of Nb into TiO2 by ultrasonic spray pyrolysis of Ti-peroxide precursor successfully changes the white color of TiO2 to yellow. A comprehensive analysis indicate that the yellow color originate from presence of surface-peroxo species and bulk Ti3+ defects. Correlation between defects and photocatalytic activity is systematically discussed in this paper.

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Defect-Induced Yellow Color in Nb-Doped TiO2 ... - ACS Publications

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