Photocatalytic Activity Enhancement of Anatase TiO 2 by Using TiO

We employed high-energy ball-milling technique to fabricate TiO/TiO 2 heterogeneous nanostructures. XRD proved the existence of TiO/TiO 2 heterogeneous structures. SEM and HRTEM investigation evidenced that the mean particle size and mean grain size of the as-prepared samples are 23 nm and 13 nm, respectively. UV-Vis spectra exhibited that TiO has enhanced the visible light absorption of TiO 2 and has changed the E g of TiO 2 . UPS examination indicated that the electron work function (EWF) of TiO is higher than that of TiO 2 . Photocatalytic degradation experiments revealed that an appropriate TiO content can enhance the photocatalytic activity of pure anatase TiO 2 . The best photocatalytic activity of TiO/TiO 2 heterogeneous nanostructures is even better than that of Au-deposited TiO 2 by keeping high degradation efficiency of 93%. The internal electrical field producing in TiO/TiO 2 heterogeneous nanostructures was considered to be dominantly responsible for the enhanced photocatalytic activity. Therefore, the substitution of TiO with noble metal in TiO 2 will be widely used in the future due to its low cost. This study also provides a clear direction of enhancing photocatalytic activity of TiO 2 : incorporating a guest compound into TiO 2 with an appropriate content if the compound has much higher electron work function than that of TiO 2 .


Introduction
Anatase titanium dioxide (TiO 2 ) has been proved to be the most effective and suitable photocatalyst for the degradation of environment pollutants due to its low cost, nontoxicity, availability, relatively high efficiency, structural stability, and so forth [1].However, its low photocatalytic activity limits the application.Therefore, it is a critical issue to enhance its photocatalytic activity to realize its photocatalytic application.From many works, it can be found that the photocatalytic activity of the photocatalysts is strongly dependent on their structures [2][3][4].And it was also reported that nanosized heterogeneous structures can enhance the quantum yield and therefore can enhance the photocatalytic activity of anatase TiO 2 [5,6].The reason is that the heterogeneous structures in the interface produce internal fields and the recombination of photogenerated carriers can be suppressed [5,6].The most effective approach to suppress charge-carrier recombination in the photocatalysis process is to incorporate noble metal (Au, Ag, or Pt) into TiO 2 [7][8][9][10].In these systems, the electron work functions (EWF) of the noble metals are much higher than that of TiO 2 .Therefore, larger internal fields can be produced in these systems.In addition, the photogenerated carriers tend to be transported from TiO 2 to the noble metals and easily transfer to the sites needed for the deoxidized reactions [11].As a result, the photocatalytic activity of TiO 2 can be improved by deposited noble metals.From the above analysis, one can find a route to increase the photocatalytic activity of TiO 2 : incorporating a second compound which has higher EWF than TiO 2 and has high electrical conductivity.
In this work, we added TiO, rather than the noble metals, into commercial anatase TiO 2 to improve the photocatalytic activity of TiO 2 to lower the cost of photocatalyst.TiO has low cost, safety, and metallic properties [12,13].It is expected to have higher EWF than TiO 2 like most metals.If anatase TiO 2 is incorporated with TiO, it would have high photocatalytic activity of photocatalysis due to TiO/TiO 2 heterogeneous structures.In order to mix TiO with TiO 2 particle evenly,

Photocatalyst Characterization.
The X-ray diffraction (XRD, X'Pert Pro, Panalytical) was employed to investigate the phases of as-ball-milled powders with slow scanning speed (2 ∘ min −1 ).The scanning electron microscopy (SEM, Zeiss Ultra 55, Carl Zeiss) was used to examine the micrographs of the particles of as-prepared powders.The high resolution electron microscopy (HRTEM, JEM-2100HR, JEOL) was utilized to observe the grain size.UV-Vis diffuse reflection spectra (DRS) were obtained using a spectrophotometer Shimadzu 2550PC and the reflection data were converted to absorbance through the standard Kubelka-Munk method.The X-ray photoelectron spectroscopy/ESCA (Esclab 250, Thermo Fisher Scientific) was obtained by using UPS mode and He (I) irradiation to examine the EWF and valence band maximum (VBM) of as-ball-milled powders.We also measured the electrical properties (ZEM-3, Ulvac-Riko) of as-ball-milled powders by testing the pressed powders (at room temperature, 200 MPa).

Photocatalytic Experiments.
The photocatalytic activity of the samples was evaluated by measuring the degradation rates of methylene orange (MO) solution (10 mg L −1 ) under ultraviolet-visible (UV-Vis) light irradiation [15].The MO concentration was monitored by a UV-Vis spectrometer.The photocatalytic reaction was performed in a quartz reactor (XQ350W, Shanghai Lansheng Company).A 350 W xenonarc lamp, with similar emitting spectrum to the sun, was used as light resource, which was located at a distance of 10 cm from the quartz reactor.A stirring bar, MO aqueous solution, and TiO 2 dispersion solution were placed in a quartz cuvette.Before photogradation, the solution was stirred in dark for 40 mins to achieve adsorTiOn/desorTiOn equilibrium.The irradiation time ranged from 0 to 120 min.At given time intervals (15 mins), about 3.5 mL of MO solution was taken out and centrifuged for 15 min.After removing the nanoparticles by filtration, the MO solution was measured by the UV-Vis spectrophotometer.The concentration of the MO solution was analyzed by checking the absoption peak around 464 nm, which is attributed to the Azo functional group (-N=N-) of MO [15].According to the Beer-Lambert law, the absorbance of MO solution is proportional to its concentration.The change of MO concentration can thus be evaluated by the intensity change of the absoption peak around 464 nm.
The degradation rate (  ) of MO is expressed in the following equation [16,17]: where  0 is the concentration of MO after absoption equilibrium in the dark,  0 is the corresponding absorbance intensity of MO after absoption equilibrium in the dark,  is the concentration of MO at reaction time  (min), and  is the corresponding absorbance intensity of MO at reaction time  (min).

Results and Discussion
3.1.Phases, Grain Size, and Particle Size. Figure 1 showed the XRD patterns of the as-prepared samples.It shows that there is only anatase phase (JCPDS file number 21-1272) in PTiO 2 while in TiO-incorporated TiO 2 the majority is anatase TiO 2 and the minority is the incorporated TiO (JCPDS file number 08-0117).And the peak positions of anatase TiO 2 have not been changed by the incorporated TiO.The diffraction peaks at (2) 25.4 ∘ , 37 ∘ , 37.9 ∘ , 38.7 ∘ , 47.9 ∘ , 53.9 ∘ , 54.9 The grain size of the as-prepared samples (Table 1) was estimated according to the Debye-Scherrer equation  = / cos() [18].From Table 1, it can be seen that there is no effect of the incorporated TiO on the grain size of anatase TiO 2 .All the grain sizes of the as-prepared samples are 13∼ 14 nm.The grain size observed under HRTEM (Figure 2) is also about 13 nm.The two data agree very well.SEM investigation (Figure 3) shows that the mean particle size of the as-prepared samples is 23 nm.And all the particle   sizes of the as-prepared samples are very close.Therefore, the incorporated TiO has no effects on the particle size of anatase TiO 2 .

Energy Band Characterization: UV-Vis Absoption, Energy
Band Gap (  ), EWF, and Electrical Conductivity () 3.2.1.UV-Vis Spectrum and   .Figure 4 shows the UV-Vis absoption spectra of the as-prepared samples in the range of 220∼800 nm obtained by calculating the absoption coefficient  from the obtained DRS according to [19] where (∞) is the absolute reflection ratio (%).In our work, we substitute (∞) with   which is the relative reflection ratio by comparing the diffuse reflection ratios (DR) of the as-prepared samples with that of pure BaSO 4 ; that is, Figure 4 indicates that the A-s of as-prepared samples in visible light (VL) spectrum increase with increasing TiO and increase with increasing wavelength of VL.The increasing absoption with increasing wavelength is characteristic of absorbing properties of free carriers because TiO has metallic properties.But generally, free carriers can strongly absorb infrared light (intraband free-carrier absoption (FCA)) not visible light and are characteristic of increasing absoption with increasing wavelength [20].Apparently, the intraband FCA has been blue-shifted to the VL range.This implies a great extension of conduction band in the TiO-incorporated TiO 2 .The blue shift of intraband FCA could be related to the internal fields which exist in TiO/TiO 2 heterogeneous nanostructures.Detailed discussion will be performed in the next text.The absoption of the as-prepared samples in VL suggests that the incorporated TiO can extend the light absoption region of TiO 2 to VL and improve the UV-Vis absoption efficiency of the samples.
Because there is no strong energy dependence of optical absoption coefficient of the as-prepared near absoption edge in Figure 4, the as-prepared samples should have direct band gap [21,22].In other people's works, the nanosized anatase TiO 2 was also found to have direct band gap [23].Furthermore, anatase TiO 2 is an N-type semiconductor [24,25].The optical energy band gap (  , eV) can be estimated by using the Kubelka-Munk function (()), as expressed by We substituted  with   .The band gap (  , eV) was the crossing point between the line extrapolated from the linear part and the -axis of the plot of [( ∞ )ℎ]] 2 as a function of energy of light (ℎV, eV), that is, [( ∞ )ℎ]] 1/2 ∼ ℎ] curve (Figure 5) [23].Table 2 is the   -s of the as-prepared samples (excluding PTiO since it is metallic conduction).It can be seen that all of the   -s of the as-prepared samples are larger than 3.4 eV.The maximum   is that of TT10, 3.504 eV.The   -s of the as-prepared samples would increase with increasing TiO at first and then decrease with increasing TiO.We have not found the exact reason for the   -s variation of the as-prepared samples with TiO content.But we guess that the probable reason is that the internal fields from the nanosized TiO/TiO 2 heterogeneous structures have bent the energy band and energy gap [11].It is known from the previous work that the optical   of TiO 2 is related to the measuring method (absoption spectrum/reflection spectrum), doping, processing technique, material form and particle size, and so forth.Therefore, the reported optical   -s of TiO 2 range widely, for example, from 2.99 eV to 3.6 eV [26][27][28][29].But the most reported optical   is 3.0-3.2eV [26,27].The large   in this work mainly results from the small grain size of ∼13 nm in the as-prepared samples, which has strong size effects [29,30].But the variation of   with the content of TiO cannot be ascribed to size effect because the grain size and particle size of all the samples are very close.And the processing procedure and parameters are identical among all the samples.The only different factor is the content of the incorporated TiO.Therefore, the internal fields generated due to heterogeneous structures TiO/TiO 2 should be responsible for it as discussed above.3 gives the  and the Seebeck coefficients () of the as-prepared samples.Because some samples could not be detected by ZEM-3 due to too low electricity, their -s and  are denoted by "N."   From Table 3, it can be seen that only the  of PTiO can be detected by ZEM-3 due to loose contact between particles in the as-prepared samples since the bulk samples for measuring electrical properties are pressed at room temperature with pressure of 200 MPa.The tested  of PTiO is 86.5 S m −1 and the  is close to 0 with negative sign.From the  sign of PTiO, it can be seen that PTiO is a weak N-type semiconductor, suggesting a little bit oxidization in PTiO during high-energy ball milling.On the other hand, previous work presents the fact that pure anatase PTiO 2 is a natural N-type semiconductor [24,25].Figure 6 is the UPS spectra for PTiO and TT20.The spectra have been referenced to the Fermi level (  ) of the gold reference sample.During UPS experiment, a bias voltage of −5 V has been added.The EWF, which is the minimum energy required to move electrons at the Fermi energy level from inside a metal or semiconductor to its surface with zerokinetic energy, equals to the difference between the onset of the measured UPS spectra and 5 eV.While the valence band maximum ( VBM ) equals 26.22 eV, the cutoff on the high energy side of UPS spectra.Figure 6(a) combines the UPS spectra of PTiO and those of TT20.Figures 6(b) and 6(c) show the five fitted Gaussian peaks of UPS spectra for PTiO and TT20, respectively.The fitted peaks were based on the expected contributions from the molecular orbitals that comprise the valence bonding states, respectively [31].Figure 6(a) indicates that valence band of PTiO has been apparently changed after being ball-milled with TiO 2 .This is another proof that the energy band of TiO 2 has been changed by the incorporated TiO.The EWF of PTiO is 3.01 eV, while that of TT20 is 1.54 eV.The  VBM of PTiO is 2.67 eV, while that of TT50 is 3.08 eV.Apparently, the EWF of TiO 2 is different from that obtained in other works [31].It can be attributed to the band bending arising from nanosized TiO/TiO 2 heterogeneous structures [11,32].We guess that the EWF of TiO 2 is less than that of PTiO.In the nanosized heterogeneous TiO/TiO 2 structures, the   is aligned to the level which is between the EWF of PTiO and that of TiO 2 .Therefore, the EWF of TT20 is less than that of TiO and larger than that of TiO 2 .The above-mentioned two proofs that there are internal fields are powerful support for the assumTiOn about the EWF of TiO and TiO 2 .Actually, the presence of a large number of space charge regions has made the crystal field and the EWF of free electrons, of TT20 different from that of pure TiO and that of pure TiO 2 [32].7 is the photocatalytic degradation of MO solution (10 mg L −1 ) by the as-prepared samples under UV-Vis irradiation.It can be seen from Figure 7(a) that, at the beginning irradiation for 70 mins, the degradation efficiencies (DE) reduce as the order of TT20, TT10, TT50, and TT1 (very close), TiO 2 , and PTiO.Among the six samples, the PTiO possesses far lower DE than the others.However, the maximum DE of all samples except PTiO reach an identical value ∼93% after 75 mins.Therefore, we can conclude that the presence of the incorporated TiO has not enhanced the final DE of nanosized TiO 2 but has improved the photocatalytic activity of the nanosized TiO 2 samples.It can be clearly seen that, before 38.5 mins, the PDRs of MO solution by the as-prepared samples reduce as the same order as that of DE, that is, TT20, TT10, TT50, and TT1 (very close), TT100, TiO 2 , and PTiO.But after the irradiation for 38 mins, the order of PDR is turned over as TiO 2 , TT100, TT50, and TT1, and TT20 (PTiO is an exceTiOn and always is close to 0).This phenomenon can be understood from the view of point that the PDR is related to the MO concentration: when the MO solution is low, the larger the MO concentration, the larger the PDR [33,34].After 38 mins, the residual MO concentration photocatalyzed by the as-prepared samples (excluding PTiO) reduces as the order of TiO 2 , TT100, TT50, and TT1, and TT20, the same order as that of PDR.

Photoactivity. Figure
We here explain why TT20 has a maximum DE and PDR before 38 mins.We think there are three dominate mechanisms playing on the DE of TiO 2 .One mechanism is that larger   would result in reducing photogenerated carriers while less   would increase the recombination possibility of photogenerated electron-hole pairs.The second mechanism is the generated internal electrical fields in the nanosized heterogeneous TiO/TiO 2 structures [31].The internal electrical fields are favors in separating photogenerated electronhole pairs and prolonging the lifetime of photogenerated carriers [31].But too much TiO might add the recombination sites of photogenerated electron-hole pairs [35].The third mechanism is that the more the incorporated TiO is, the larger the Vis absoption is.The combined action of the three mechanisms has resulted in that the DE and PDR of TT20 are the maximum among the as-prepared samples.
Although [36] mentioned that nitrogen-(N-) doped TiO with ∼8 nm has efficient photoactivity under UV and visible irradiation, the nanosized TiO with 13 nm in our work shows no considerable photoactivity under UV-Vis irradiation.The considerable response to UV-Vis in [36] probably results from the dopant N. It was reported that the dopant N can add additional energy levels in energy band gap and improve the photocatalytic properties of TiO 2 [37].Here we think that similar additional levels appear in N-doped TiO and lead to the UV-Vis response.
TiO has much higher EWF than that of TiO 2 .Thus, it introduces an easy route for carriers to transfer from the inside to the outside of TiO 2 particles to participate in photocatalytic activity.Consequently, an appropriate content of the incorporated TiO can increase the photocatalytic activity of anatase TiO 2 .Furthermore, TiO can enhance the Vis absoption.TiO-incorporated TiO 2 has maximum degradation efficiency of 93%.However this value is less than that in noble metal incorporated TiO 2 , for example, 97% in Ag-incorporated TiO 2 system and 100% in Auincorporated TiO 2 [7,9,38].But the photocatalytic activity of TiO-incorporated TiO 2 is much higher than that of Agincorporated TiO 2 .In [7,38], Ag-incorporated TiO 2 took 2 hrs to reach DE of 97% and Au-incorporated TiO 2 took 50 mins to reach DE of 100%.While in this work, TT20 just took 38 mins to reach DE of 93%.Therefore, the TiOincorporated TiO 2 has much application value.

Conclusion
In this work, we employed high-energy ball-milling method to fabricate TiO/TiO 2 heterogeneous nanostructures.XRD proved the existence of TiO/TiO 2 heterogeneous structures.SEM and HRTEM investigation evidenced that the mean particle size and mean grain size of the as-prepared samples are 23 nm and 13 nm, respectively.UV-Vis exhibited that the incorporated TiO has enhanced the Vis absoption of TiO 2 and has changed the   of TiO 2 .UPS examination found that the electron work function of TiO is higher than that of TiO 2 .Photocatalytic experiment has revealed that an appropriate TiO content can lift the photocatalytic activity of pure TiO 2 to a level better than that of Au-incorporated TiO 2 by keeping high degradation efficiency of 93%.The internal electrical field produced by TiO/TiO 2 heterogeneous nanostructures was thought to be dominantly responsible for the enhanced photocatalytic activity.Therefore, the substitution of the noble metal with TiO to enhance the photocatalytic activity of TiO 2 has much application value.This work also gives a clear direction of enhancing photocatalytic activity of TiO 2 by incorporating a second compound into TiO 2 particle with an appropriate content if the second compound has much higher electron work function than that of TiO 2 .

Figure 1 :
Figure 1: XRD of the as-prepared samples.

Figure 2 :
Figure 2: Representative HRTEM image of the as-prepared samples, clearly showing mean grain size around 13 nm.

Table 2 :Figure 3 :Figure 4 :
Figure 3: Typical SEM images of (a) TiO 2 , (b) TT1, (c) TT20, and (d) PTiO, clearly showing that the partice size of the as-prepared samples is very close and around 23 nm.

Table 3 :
The  and S of the as-prepared samples measured by ZEM-3.Because some samples could not be detected by ZEM-3 due to too low , their -s and S are denoted by "N."

Figure 6 :
Figure 6: (a) UPS spectra for PTiO and TT20 and (b) and (c) five fitted peaks (labeled by numbers I, II, III, IV, and V) of UPS spectra of PTiO and TiO 2 , respectively.The five peaks were fitted based on the expected contributions from the molecular orbitals that comprise the valence bonding states, respectively.

Figure 7 :
Figure 7: (a) Degradation and (b) degradation rate of the as-prepared samples.
It can be found that the more the TiO contained in TiO-incorporated TiO 2 is, the higher the diffraction peak at 43.2 ∘ representing TiO is.Clearly, the incorporated TiO has not changed the peak positions of anatase TiO 2 .TiO and TiO 2 just mixed with each other and formed a large number of heterogeneous structures TiO/TiO 2 in host TiO 2 powders after the high-energy ball milling.

Table 1 :
[18] grain size of the as-prepared samples calculated from XRD by the Debye-Scherrer equation[18].