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Nickel-doped Excess Oxygen Defects Titanium Dioxide for Efficient Selective Photocatalytic Oxidation of Benzyl Alcohol Houde She, Hua Zhou, Liangshan Li, Lei Wang, Jingwei Huang, and Qizhao Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b02217 • Publication Date (Web): 19 Jul 2018 Downloaded from http://pubs.acs.org on July 22, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Sustainable Chemistry & Engineering

Nickel-doped Dioxide

for

Excess

Oxygen

Efficient

Defects

Selective

Titanium

Photocatalytic

Oxidation of Benzyl Alcohol Houde She,†,‡ Hua Zhou,†,‡ Liangshan Li,†,‡ Lei Wang,†,‡ Jingwei Huang,†,‡ Qizhao Wang†,‡ *

†

College of Chemistry and Chemical Engineering, Northwest Normal University, 967

Anning East Rd., Lanzhou 730070, P.R. China

‡

Key Laboratory of Eco-Environment Related Polymer Materials, Ministry of

Education of China, 967 Anning East Rd., Lanzhou 730070, P.R. China

The corresponding author: E-mail: [email protected]; [email protected]

KEYWORDS: Nickel doping, Oxygen defects, Benzyl alcohol, Photocatalytic oxidation, Synergistic effect

ABSTRACT

In this study, a novel composite (Ni-OTiO2) was prepared by doping nickel and introduction of excess oxygen defect in TiO2. The as-synthesized Ni-OTiO2 particles were characterized by SEM, XRD, TEM, FT-IR, DRS, PL, PEC and XPS. When employing Ni (1%)-OTiO2 as photocatalyst, the conversion of BA was up to 93% by 1 1

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hour irradiation derived from a 300W xenon lamp, which is about 8 times higher than that of using pure TiO2. Moreover, while being irradiated with 300W xenon lamp (using a filter, λ> 420 nm), Ni (1%)-OTiO2 conducted photocatalytic system can give 86% conversion of BA into BAD within 1 h. Speciﬁcally, during photocatalysis, peroxo group and nickel ion acts as the electron carrier, promoting the separation of the electron- hole pair. It is considered that the highly improved photocatalytic ability after modification of pure TiO2 is ascribable to the synergistic effect of excess oxygen defects and nickel doping in TiO2.

Introduction

Aromatic aldehydes, one group of the most important synthetic fine chemicals and pharmaceuticals, are widely used in the fields of medicine, dye, perfume and pesticide1. For example, a typical one, benzaldehyde (BAD), is one of the simplest aromatic aldehyde containing an active carbonyl group and thereby plays a crucial role of organic reaction intermediate2. Industrially, when using hazardous reagents (such as Cl2, Mn7+, ClO4- and Cr6+) as oxidizing agent, BAD could be finely produced by means of liquid phase oxidation of toluene3. Nonetheless, common methods always produce various by-products bringing about severe pollution to environment and also require a large amount of power that can highly intensify the energy shortage4. In this regard, seeking for an environment-friendly route to produce BAD has greatly drawn scientists’ interests. So far, photocatalystic technology is a quite promising way to address this dilemma. Although a huge 2


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variety of photocatalytic materials, such as CdS5, ZnO6, CeO27, BiOCl8, g-C3N49 and BiVO410 have been well studied for oxidation of benzyl alcohol (BA), the requirements are still far beyond the currently achieved merits (activity, selectivity, stability, cost, etc.). For example, CdS is a classical semiconductor with narrow bandgap (Eg= 2.4 eV), but it is vulnerable to light corrosion, leaking toxic Cd2+ and causing damage to environment11. Another commonly used semiconductor, CeO2, due to its wide bandgap energy (Eg=3.2 eV) and poor carrier conductivity, is weak in harvesting solar energy and thus resulting in that its photocatalytic activity generally might not satisfy the oxidation of benzyl alcohol12. TiO2 is a commercially excellent photocatalyst with salient activity, high stability, affordable cost and non-toxicity13, 14. However, prestine TiO2 can only be used under UV irradiation (λ 420 nm) was selected as a light source for photocurrent test. All electrochemical measurements were performed at room temperature. The catalyst was deposited on the FTO (1.0 cm-2) to act as a working electrode. A small amount of Nafion solution (10 µl) was dropped on the conductive glass. A solid sample (10 mg) was dispersed in ethylene glycol and sonicated for 20 minutes, then dropped on FTO conductive glass coated with a Nafion solution. The working electrode was photoelectrochemically tested under a bias voltage of 0.6 V Vs Ag/AgCl. With the illumination on the back of the FTO, the lighting area is approximately 1.0 cm-2.

Photocatalytic activity The selective oxidation activity of as-prepared photocatalysts was carried out in a 50 mL home-made glass reactor system. In general, 80 mg photocatalyst and 0.5 mmol of BA and 5 mL BTF were mixed in a quartz glass bottle (40 mm * 25 mm) placed in the reactor. The mixture was stirred without illumination for 1 h to ensure adsorption-desorption equilibrium between the sample and BA. The suspension was saturated with pure O2 (2 atm) for 5 min with stirring, then illuminated using a 300 W xenon lamp (CEL-HXF300, Beijing Gold) as a light source. The system temperature was controlled by a recycle water cooling system. The catalysts particles were removed from the reaction system by high speed centrifugation before the supernatant was analyzed by a gas chromatograph (GC9600, China) to identify the organic product. For cyclic photoactivity testing, the catalyst was washed repeatedly with absolute ethanol and deionized water, then dried overnight in an oven at 80°C. Different free radical scavengers such as 7



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AgNO3, oxalic acid (OA), isopropyl alcohol (IPA) and benzoquinone (BQ) were used as scavengers for photogenerated electron, photogenerated hole, hydroxyl radical and superoxide radicals, respectively. For the control experiments, all details were the same except that 0.5 mmol of scavenger was used in the reaction system. The alcohol conversion, aldehyde yield and aldehyde selectivity are calculated as follows:

Error! Reference source not found.Conversion % = [(C0-C1)/C0] *100% (1)

Yield % = (C2/C0Error! Reference source not found.) *100%

(2)

Error! Reference source not found.Selectivity % = [C2/ (C0-C1)] *100% (3) Where C0 is the initial amount of substrate alcohol before illumination; C1 is the amount of substrate alcohol after illumination for 1 h; C2 is the amount of the corresponding aldehyde after illumination the reaction.

Results and discussion

Catalyst Characterization XRD analysis was performed to study the crystal structure and gain size of the samples. Figure 1a shows the mixed crystal phases (anatase and brookite phases) of TiO2 catalyst. All high intensity peaks correspond to anatase (JCPDS file No. 21-1272) except for peak at 25.68° and 30.80°, which can be attributed to (111) and 8


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(121) diffraction planes of the brookite phase (JCPDS file No. 29-1360). Ni-TiO2 also shows mixed crystal phases (anatase and brookite phases) similar to prestine TiO2 sample except that the peaks of brookite phase is lower than that of prestine TiO2. It is clear that all diffraction peaks of O2-TiO2 are well indexed to the lattice planes of anatase TiO2 while no peaks of brookite phase can be found. Average crystallite size of the sample is calculated using the Scherrer equation29. The size of O2-TiO2 sample is 46 nm, indicating that the O2-TiO2 sample is not only pure phase, but well crystallized as compared with prestine TiO2. After doping with Ni, the crystal size decreases to 25 nm without appearance of brookite phase. No peaks of metal Ni or NiTiO3 can be found in the diffraction pattern of the Ni (1%)-OTiO2, suggesting there is no secondary phase formed due to cation ions doping. Figure 1b shows a high-resolution transmission electron microscope (HRTEM) image of the Ni (1%)-OTiO2 photocatalyst. The measured lattice fringe is 0.35 nm, which matches with the (101) crystal plane of anatase TiO230. The SEM images show that all the samples have a clear spherical structure, and the sample size distribution is in the range of 20-50 nm (Figure S1). Elemental mappings of Ni (1%)-OTiO2 exhibits the presence of O, Ti, and Ni elements (Figure 1c-e), demonstrating that the composite Ni-OTiO2 doped of Ni (1%) has been successfully prepared.

9



Figure 1. X-ray diffraction (XRD) spectra of different TiO2 samples (a) HRTEM image of Ni (1%)-OTiO2 (b) the corresponding elemental EDS mapping of O (c) the corresponding elemental EDS mapping of Ti (d) and the corresponding elemental EDS mapping of Ni (e).

Figure 2 depicts FTIR spectra of TiO2, Ni (1%)-OTiO2 and Ni (1%)-OTiO2(HF), respectively. All the samples have a wide band at 3400 cm-1 and a sharp band at 1630 cm-1, which is attributable to the tensile and bending vibrations generated by the surface OH groups26. In the FTIR spectrum of Ni-OTiO2, the representative peroxo group (Ti-O-O) showed an absorption signal at 691 cm-131. The band at around 496 cm-1 was considered ascribing to the vibration of the Ti-O bond32. The weak absorption at 891 cm-1 was attributed to the O-O stretching vibration33. HF treatment of Ni-OTiO2 experiments were carried out to study the role of Ti-O-O bonds in the oxidation process. The synthetic details of which were given in supporting information. It is clear that after the treatment, the Ti-O-O

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bonds disappeared and the photocatalytic activity decreased greatly (Figure S2). It is considered the resulting Ti-O-O bonds and O-O bonds corresponding to excess oxygen defects can play a key role in photocatalytic oxidation.

Figure 2. FTIR diffuse reflectance spectra of different samples.

The crystal surface chemical composition and chemical state can be studied by XPS. A wide scan XPS spectrum of the Ni (1%)-OTiO2 composites were collected in Figure 3a, Ti, O, Ni and C elements were observed to be present in the sample. The element of C may come from the atmosphere. Illustration shows the content of elements in the sample and proves that nickel ions are doped in the sample. As shown in Figure 3b, the two TiO2 peaks at 458.3 and 464.1 eV correspond to the spin-orbit split photoelectrons for Ti 2p

3/2

and Ti 2p

1/2,

respectively34. Since the

split between these two bands is 5.7 eV, Ti4+ states is confirmed as primary existence form of titanic element existing in the composite35. As shown in Figure 3c, four peaks can be observed in O 1s pattern. The peaks at 530.1 and 532.1 eV can be assigned to lattice oxygen of TiO2 and peroxo group36. The peaks at 531.0 and 11



532.9 eV can be attributed to surface hydroxyl group and adsorbed surface water of TiO228. The most important signal is that at 532.0 eV, which is ascribed to the presence of peroxo group introduced into TiO2 lattice. XPS measurements also confirmed that Ni was doped into O2-TiO2. As shown in Figure 3d, the peaks of Ni 2p 3/2 and Ni 2p 1/2 (855.6 eV and 873.1 eV) typify Ni2 +, revealing that Ni is doped in TiO2 lattice and bound to O2- 37.

Figure 3. Wide survey XPS spectrum, inset of quantification report (a) high-resolution XPS spectra of Ti 2p (b) O 1s (c) and Ni 2p (d) of Ni (1%)-OTiO2 sample.

UV-visible diffuse reflectance spectra were used to characterize the light absorption properties of synthetic samples. As shown in Figure 4a, when peroxo is introduced into the lattice of TiO2, the resultant spectrum of O2-TiO2 photocatalyst

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is red-shifted to 420 nm. When nickel ions are doped into the TiO2 lattice, the spectrum of Ni-TiO2 photocatalyst is red-shifted to about 460 nm. Compared with the spectra of TiO2, O2-TiO2 and Ni-TiO2, Ni-OTiO2 has a strong absorption at 550 nm range, which can be attributed to the synergistic effect between the doped nickel ion and rich oxygen defects in TiO2 lattice. As can be seen from Figure 4b, all samples show an absorption edge at 550 nm, suggesting that changing annealing temperature did not exert obvious effects on the optical properties. The sample color gradually darkened with the addition of nickel ions and peroxo group (Figure S5). The band gap energy can be estimated by the Kubelka-Munk function (Equation (4))38:

α (hv) = A(hv − Eg ) n / 2

(4)

Among them, α, h, ν, hv, Eg and a typify absorption coefficient, Planck constant, vibration frequency, photon energy, bandgap energy and proportional constant, respectively. The coefficient characterizes the electronic transition during absorption, which is determined by the type of semiconductor optical transition. In this experiment, n = 1/238. As shown in inset of Figure 4a, the estimated Eg value of the Ni-OTiO2 is about 2.25 eV. Therefore, the synergistic effect of oxygen deficiency and doping nickel ion leads to an obviously improved optical performance of as-prepared composite.

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Figure 4. UV–vis diffuse reflectance spectra of different samples: (1) TiO2, (2) O2-TiO2, (3) Ni (1%)-TiO2 and (4) Ni (1%)-OTiO2 (a) (inset is the bandgaps converted from UV−vis absorption spectra) UV-vis diffuse reflectance spectra of different annealed samples (b).

Photoluminescence (PL) spectroscopy provides a convincing indicator of the photoexcitation excitonic recombination rate39. The intensity of the PL spectrum represents the combination rate of photoelectrons and holes: a strong PL spectrum suggests a rapid charge recombination while a weak PL spectrum typifies a low electron-hole recombination efficiency40. Photoluminescence (PL) spectra of TiO2, O2-TiO2, Ni (1%)-TiO2 and Ni (1%)-OTiO2 photocatalysts are reported in Figure 5a. Compared with prestine TiO2, sample O2-TiO2 has a similar but weaker spectrum, indicating that peroxo group modified TiO2 does not show any obvious superiority in terms of promoting charge separation efficiency despite that peroxo goup can capture electro and improve charge separation efficiency28. It can be observed that Ni (1%)-TiO2 shows much lower peak intensity than TiO2 and O2-TiO2, indicating

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that nickel ion-doping can suppress the charge carrier recombination in TiO2 particles. The intensity of Ni (1%)-OTiO2 spectrum is also weaker than that of Ni (1%)-TiO2, indicating the separation efficiency of electron and hole is further improved through peroxo group modification. Actually, via annealing of TiO2, oxygen interstitial might combine with lattice oxygen to form interstitial O2 and leave oxygen vacancy defect at oxygen lattice position, which can act as the recombination center of charge carriers26. We assume there is synergistic effect between the Ni doped and peroxo group, which improves charge separation efficiency. The incident light-induced transient photocurrent response can reflect the number of charge carriers separated on the photocatalyst surface41. The higher photocurrent response, the more carriers are separated42. In order to further elucidate the ability of Ni-OTiO2 to promote photoinduced electron-hole separation efficiency, the transient photocurrent response of LED lamp during ON / OFF cycles was studied. As shown in Figure 5b, when the light was turned on, the photocurrent increased immediately; when the light went out, the photocurrent dropped rapidly. The results show that the charge carrier recombination rate of TiO2 is the highest in as-prepared composites in this work. On the other hand, the measurement results show that the transient photocurrent density of Ni-OTiO2 catalyst is higher than TiO2, O2-TiO2 and Ni-TiO2 catalyst, so that the ability to separate electrons and holes is stronger. Nyquist plots were also used to further determine the advantages of Ni-OTiO2 catalysts over TiO2, O2-TiO2 and Ni-TiO2 to improve carrier transfer. In general, the smaller radius the Nyquist circle has, the 15



smaller charge transfer resistance lays behind43. As can be seen in Figure 5c, Ni-OTiO2 shows smaller semicircles compared to TiO2, indicating faster interface charge transfer to electrons receptor. The results of photocurrent and EIS are consistent with those of PL experiment.

Figure 5. PL spectra of (1) TiO2, (2) O2-TiO2, (3) Ni (1%)-TiO2 and (4) Ni (1%)-OTiO2 (a) the transient photocurrent responses of different electrodes under visible-light irradiation at 0.6 V Vs Ag/AgCl in 0.5 M sodium sulfate electrolyte (b) EIS Nyquist curves of different electrodes measured under visible-light irradiation at 0.6V Vs Ag/AgCl in 0.5 M sodium sulfate electrolyte (c). The dark current data were normalized for comparation.

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Evaluation of photocatalytic activity Figure 6a shows the photocatalytic performance of selectively oxidization of BA to BAD under irradiation with a 300 W xenon lamp for 1 h. When peroxide group is introduced and the concentration of nickel ions increases (below 2wt %), the conversion rate of BA exhibits an increasing tendency. On the other hand, the introduction of excess nickel ions (Ni(3%)-OTiO2) will greatly reduce the conversion of BA, probably because doping excess cationic metal (Ni2+) might cause the formation of secondary phase (NiTiO3), thus downgrading the overall photocatalytic performance. When the Ni (1%)-OTiO2 sample was used as catalyst, the BA engendered the best oxidation performance. The conversion rate was up 93% and the yield and selectivity was 88% and 99%, respectively. Photocatalytic oxidation of BA only with nickel-doped titanium dioxide (Ni-TiO2) is shown in Figure S4. When the Ni (1%)-TiO2 sample was used as a catalyst, the conversion of BA was 43% and the yield of BAD was 40%. It shows that the peroxo group and nickel ion doping have a synergistic effect on the oxidation of BA to BAD. Figure 6b shows the photocatalytic performance of selective oxidation of BA with 300W xenon lamp (using a filter, λ> 420 nm) irradiation for 1 hour. As a result, it is found that when Ni (1%)-OTiO2 was employed as catalyst, catalytic performance insignificantly changed as compared with the case when no filters are used (the conversion rate is 86%, the yield is 85%, and the selectivity is 99%). This shows that the catalyst can provide a high activity under irradiation of spectrum of visible light as well. In order to verify the role of oxygen and light to our experiment, blank 17



tests were also given (Figure S3). In the absence of a light source, the conversion of BA is 7%. While in the absence of O2, the conversion of BA is 6%, demonstrating that the light source and oxygen play an important role in the photocatalytic oxidation of BA to BAD.

Figure 6. Photocatalytic performance under 300W Xenon lamp irradiation for 1 h (a) photocatalytic performance under 300W Xenon lamp (using a filter, λ > 420 nm) irradiation for 1 h (b). (C%, Y% and S% mean Conversion, Yield and Selectivity, respectively)

To investigate the effect of different solvents on the selective oxidation of BA under irradiation of 300W xenon lamp, different solvents (BTF, acetonitrile, carbon tetrachloride and toluene) instead of BTF were used in control experiments. As shown in Figure 7a, the conversion rate of BA in BTF solvent is higher than other solvents (BTF> toluene> acetonitrile> carbon tetrachloride). The selectivity of reaction in BTF, acetonitrile, toluene and carbon tetrachloride solution is 93%, 59%, 70%, and 58% respectively. Figure 7b shows the photocatalytic performance of

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selective oxidation of benzyl alcohol, p-methoxybenzyl alcohol and p-chlorobenzyl alcohol to the corresponding aldehyde under 1 h irradiation of 300W xenon lamp. The results show that the as-prepared catalyst is not sensitive to substrates, as the conversion rate of p-chlorobenzyl alcohol and p-methoxybenzyl are almost the same (90%). It is explained that either the substituent showing electron withdrawing or electron donating, if it can help the alpha-hydrogen activation of benzyl alcohol, the corresponding alcohol will give a good conversion rate44.

Figure 7. Performance of photocatalytic selective oxidation of BA in different solvents (a) performance comparison of photocatalytic selective oxidation of different aromatic alcohols in BTF (b).

The stability of Ni (1%)-OTiO2 was studied by four successive cycles of experiments and XRD patterns of the pre-reaction and post-reaction samples. A cycle experiment was conducted using the Ni (1%)-OTiO2 photocatalyst under the same conditions. After each reaction, the catalyst was centrifuged and washed three times with ethanol and deionized water, and then dried overnight before next cycle

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test. The results are shown in Figure 8a. Four cycles of experiments exhibit that no significant change in the yield, conversion and selectivity, indicating the good stability of our samples under the reaction conditions, which is further proved by the XRD results of the sample after cycling, shown in Figure 8b.

Figure 8. Cyclic experiments of Ni (1%)-OTiO2 microsphere photocatalyst (a) XRD patterns of sample before and after recycling (b).

Photocatalytic mechanism To study the reaction mechanism of the catalyst on the photocatalytic oxidation of BA to BAD, control experiments were conducted with different radical scavengers45. Different free radical scavengers (BQ for •O2-, IPA for •OH, OA for h+ and AgNO3 for e-) were added to the reaction system so as to remove these corresponding active species. As shown in Figure 9, when AgNO3 or IPA was added to the photocatalytic oxidation of BA reaction system, the conversion of BA decreased slightly; whereas, as OA or BQ played the role of scavenger, the conversion of BA rapidly abated. The above results indicate that h+ and •O2- have a

20


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great influence on the photocatalytic reaction, while the effect of •OH radical is negligible39. But it is also clear that introducing BQ has different influence on Ni (1%)-OTiO2 and Ni (1%)-TiO2 catalyzed systems. In Figure 9a, adding BQ as scavenger for superoxide radical in Ni (1%)-OTiO2 conducted system do not lower the reaction conversion to the extent it is expected to be since adding BQ and OA can reduce the conversion rate to the similar lever as shown in Figure 9b. The surface peroxo group could account for this phenomenon. In Ni (1%)-TiO2 conducted photocatalystic reaction, photogenerated electrons reduce O2 to superoxide radicals (•O2-), which then produce •OOH radicals. The •OOH radicals further react with alcohol cations which come from the reaction of alcohols and h+, to give corresponding aromatic aldehydes. Introducing BQ will cut off this reaction route since •O2- is depleted. But in Ni (1%)-OTiO2 conducted photocatalystic reaction, the existence of surface peroxo group could cover the deficit of superoxide radicals. This role of surface peroxo group is verified by our control experiment (Figure S6) conducted on Ni-OTiO2(HF) whose surface peroxo group was removed by treatment of HF solution. When BQ was added to the reaction, the conversion rate of BA generated by Ni-OTiO2 (HF) was found decreasing faster rather than that conducted by Ni-OTiO2, suggesting that, the presence of peroxide radicals can compensate for the lack of superoxide radicals.

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Figure 9. Benzyl alcohol of conversion using Ni (1%)-OTiO2 photocatalyst in the presence of different radical scavengers under irradiation for 1h (a) BA of conversion using Ni (1%)-TiO2 photocatalyst in the presence of different radical scavengers under irradiation for 1h (b).

A possible mechanism for the oxidation of BA into BAD by Ni-OTiO2 photo-catalyst is illustrated in Figure 10. The positions of the CB and VB bands can be obtained from the Mott-schottky curve (Figure S7). The positive slope of the tangent line in the Mott-schottky curve suggests the synthesized material exhibits the characteristics of n-type semiconductor. The reduction ability would become higher when the flat band potential was more negative, whereas the oxidation ability of the material would be greater when the flat band potential was more positive46. The Vfb values of TiO2, O2-TiO2, Ni (1%)-TiO2, and Ni (1%)-OTiO2 are -0.3, -0.28, -0.26, and -0.21, respectively (using a saturated hydrogen electrode (NHE) as a reference, and 0.5 M Na2SO4 as electrolyte solution). For n-type semiconductors, the difference between the flat potential and the conduction band

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of the semiconductor material is negligible47. Therefore, Vfb is approximately equivalent to ECB (V vs NHE). As shown in Figure 10, the valence band position of TiO2 and Ni-OTiO2 were calculated according to the formula (EVB= Eg+ECB), in which Eg is the band gap of semiconductor. After absorbing equivalent or greater energy in comparison with band gap of TiO2, TiO2 nano particles was thus activated to engender holes (h+) in the valence band and electrons (e-) in the conduction band. Due to the difference in Fermi level between the Ni and TiO2 CB edges, photogenerated electrons tend to migrate from TiO2 to nickel ions, while photogenerated holes stuck around the TiO2 particles. Thereby, the integrated nickel ions in TiO2 effectively suppressed the recombination of photogenerated electrons and holes, which is evidently advantageous for the photocatalytic oxidation reaction48. Nickel ion doped TiO2 is particularly conducive for the activation of molecular oxygen, thereby promoting the participation of molecular oxygen in selective photocatalytic oxidation. After the formation of excess oxygen defects on TiO2, not only can the absorption of visible light be further enhanced, but electron captures are facilitated, allowing electrons to transfer to active sites on the surface of the incorporated composite28. During the oxidation, photogenerated electrons react with oxygen to form •O2- radicals and then produce superoxide (•OOH) radicals. Meanwhile, the holes (h +) on the valence band react with BA to form BA cation radical. The radical is vulnerable and would further react with superoxide (•OOH) radicals to produce BAD. In sum, with respect to the selective alcohol oxidation, the introduction of excess oxygen defects and nickel ions creates an 23



interesting synergistic effect, capable of inhibiting the recombination of the photo-generated carriers and promote the selective oxidation of BA.

Figure 10. Schematic diagram of the proposed mechanism for selective oxidation of BA to BAD using Ni-OTiO2 under irradiation for 1 h.

Conclusions

In short, we synthesized Ni-OTiO2 nanocomposite mainly by uniformly doping nickel ions on the surface of defective O2-TiO2. Compared with pure TiO2, the obtained Ni-OTiO2 nanocomposites showed strong activity for the selective oxidation of BA under visible light irradiation. The enhanced photoactivity is mainly due to the synergistic effect of oxygen defects (providing Ti-O-O bonds and O-O bonds) and metal doping. The Ni (1%)-OTiO2 nanoparticle synthesized by us can reach 93% conversion under the irradiation of a 300W xenon lamp, which is 8 24


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times of pure TiO2. At the same time, Ni (1%)-OTiO2 nanoparticles also respond to visible light, and the conversion rate can reach 86% under the irradiation of visible light. In addition, we also found that the photocatalyst thus synthesized still exhibits excellent effects when catalyzing different substrates. Through the analysis of different free radical scavengers, the photocatalytic reaction mechanism of Ni-OTiO2 was proposed.

Associated Content

Supporting Information

Additional data for experiments, electron microscopy characterization of samples, preparation methods for additional experiments, and photographs of samples.

This material is available free of charge via the Internet at http://pubs.acs.org.

Author Information

*Corresponding authors:

Prof. Q. Wang (E-mail: [email protected]; [email protected]);

Notes

The authors declare no competing financial interest.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of 25



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China (21663027, 21261021, 51262028), the Science and Technology Support Project of Gansu Province (1504GKCA027), the Program for the Young Innovative Talents of Long yuan and the Program for Innovative Research Team (NWNU-LKQN-15-2).

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Table of Contents

Preparation

of

nickel-doped

excess

oxygen

defects

titanium

nanomaterials for efficient photocatalytic oxidation of benzyl alcohol.


dioxide

Nickel-doped Excess Oxygen Defects Titanium ... - ACS Publications

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