Chapter 3: Controlled Growth of TiO 2 Nanostructures
4.5. Optical absorption and Photoluminescence studies
U V -V is -N I R ab s orp tion s tu die s : Light absorption characteristics of the solvothermally synthesized TiO2 nanostructures are shown in F ig . 4.5(a ) a nd (b ). A ll the as-synthesized samples exhibit a red shift of the absorption edge and considerable absorption in the visible region (> 4 4 7 nm) compared to precursor TiO2 (3 8 0 nm). The absorption spectra for B5 0 0 , B7 0 0 , B9 0 0 and precursor TiO2 (PTiO2) NPs are shown in F ig . 4.5(a ). Note that with higher calcinations temperature, the absorption edge extends up to yellow region of the visible spectrum. In F ig . 4.5(a ), three steps of absorptions (i.e., 3 8 0 −4 20 , 4 20 −5 8 0 , and 660 −8 4 0 nm) can be distinguished. The absorption in the 3 8 0 −4 20 nm range (violet region) arises due to the self-trapped states and shallow trap states centers, while absorption in the range 4 20 −5 8 0 nm (blue, green, and yellow region) arises due to the deep trap states centers associated with the oxygen vacancies. However, the absorption in the range 660 −8 4 0 nm (red region and extended up to NIR region) is prominent for samples calcined at higher temperature and it may be originated from the defects that migrate to the near surface region during calcination. Note that B5 0 0 , B7 0 0 and B9 0 0 grown at higher reaction temperature show systematic red shift in the band gap to pure visible region, enabling strong
visible absorption in the undoped TiO2 and this is expected to exhibit strong visible light photocatalytic activity for hydrogen generation. The presence of regular lattice trap center, center, and interstitial are confirmed from our electron spin resonance (E S R) measurement discussed later.
F ig . 4.5(b ) shows the absorption spectra of the samples grown with different reaction durations (i.e., 1 6 and 24 h) and calcined at 9 0 0 ° C . W ith an increase in reaction duration, visible absorption is clearly increased and this is caused by higher concentration of defects.
The band gap is calculated from the linear fit to the linear portion of (h)1 /2 v e r sus h plot and the data are presented in T a b le 4.1. Note that the band gaps of pure anatase phase of TiO2 NRbs (C 5 0 0 and B5 0 0 ) are relatively large (2.7 7 and 2.64 eV , respectively) compared to the other as-grown samples. However, we noticed that within the same phase, the band gap of B5 0 0 is narrower than that of C 5 0 0 grown at lower temperature. This indicates that samples grown at higher temperature followed by calcination at high temperature possess higher concentration of a particular defect that is largely responsible for reduction in the band gap. This is consistent with the theoretical predictions made by various groups.18, 19 , 31
Interestingly, experimental evidence of band gap narrowing due to defects is reported by W endt et al.6 M organ et al.31 reported a larger red-shift of absorption edge due to defects as compared to that due to defects.
Theoretical calculations suggested that a high vacancy concentration could induce a band of electronic states just below the conduction band.32 Z uo et al.33 reported the presence of a mini-band closely below the conduction band minimum, which is related to the oxygen vacancy, associated with Ti3 + and is responsible for the band gap narrowing in TiO2. S imilar observation related to oxygen vacancy induced band gap narrowing has been reported in Z nO system.34 M ore recently, Liu et al.3 reported the band gap narrowing of undoped TiO2 due to oxygen vacancies and Ti3 + species which showed enhanced visible light-driven photocatalytic oxidation on methylene blue and water splitting. F inazzi et al.19 reported that the presence of both and species resulted in new states in the band gap (about 1 −1 .5 eV below the conduction band) of TiO2 materials. It will be shown that our results are more consistent with the Ti interstitial mediated red-shift of the band gap to visible region.27 Note that the absorption edge of precursor TiO2 powder is at ~ 3 8 0 nm (3 .26 eV ), although PL studies show high concentration of oxygen vacancies present in it. Thus, oxygen
vacancies alone do not give rise to visible absorption. On the other hand, high temperature calcined NRbs shows considerable decrease in band gap from 2.7 7 to 1 .9 8 eV (shown in T a b le 4.1) under different growth conditions as compared to precursor TiO2 powder (3 .2 eV ). Our PL data presented below is consistent with the fact that these samples contain very low vacancy concentration. Thus, the control of Ti interstitials in TiO2 nanostructures hold the key to enhanced visible absorption and enhanced photocatalytic performance without introducing any external doping/impurities.
F ig . 4.5. U V -visible-NIR absorption spectra: (a) B5 0 0 , B7 0 0 , and B9 0 0 grown at 23 5 ° C and different calcinations along with precursor TiO2 NPs (PTiO2); (b) A 9 0 0 and D9 0 0 grown at different reaction duration and 9 0 0 ° C calcinations; The insets in each case show the (h)1 /2 v s h plot, indicating the indirect band gap for the corresponding absorption spectrum of each sample. Band gap energy is calculated from the extrapolated line (dashed) fitted to respective linear portions. Room temperature PL spectra for (c) A 5 0 0 , A 7 0 0 , A 9 0 0 , and A 5 0 0 V ; inset is the comparison of A 5 0 0 and D5 0 0 ; S pectrum for A 5 0 0 V is vertically shifted for clarity. (d) B5 0 0 , B7 0 0 , and C 5 0 0 ; inset shows the magnified view of the visible PL in A 5 0 0 V with G aussian peak fitting.
R oom te m p e ratu re p h otolu m ine s ce nce s tu die s : To enable a more precise understanding on the nature of defects and related trap states within the band gap in TiO2 NRbs, PL studies are
performed on different samples. F ig . 4.5(c) shows the room temperature and atmospheric pressure PL spectra of the samples grown at 1 8 0 ° C , 1 6 h. A ll the samples show broadly two peaks, one peak in the NIR range at ~ 1 .4 eV and the other peak in the visible range at ~ 2.2 eV . The broad visible PL is usually ascribed to oxygen vacancies in TiO2.21, 23 The vacuum annealed sample A 5 0 0 V shows highly enhanced visible PL emission as compared to as- synthesized A 5 0 0 , indicating that the concentration of oxygen vacancies are dramatically increased after vacuum annealing, as expected. However, as calcination temperature is increased from 5 0 0 to 9 0 0 ° C , the intensity of NIR PL is clearly increased and the peak is blue-shifted. S imilar features are observed for the samples grown at 23 5 ° C (F ig . 4.5(d)), where the PL intensity is about one order of magnitude higher. On the other hand, the intensity of visible PL is very low and does not change significantly with calcination temperatures. F urther, comparison of PL intensity for B5 0 0 and C 5 0 0 shows that NIR PL intensity is higher for growth at higher temperature (F ig . 4.5(d)). Thus, it is evident that higher the growth temperature and/ or higher the calcinations temperature, higher the NIR PL intensity. F urther, no correlation is found between the visible PL and NIR PL intensity in each sample, which indicates that the origin of these PL emissions is associated with distinctly different defects species (e.g., and ). The inset in F ig . 4.5(c) shows the comparison of PL spectra for A 5 0 0 and D5 0 0 that are grown/ calcined at low temperature. It is clear that at low growth temperature, NIR PL intensity is very low compared to higher growth temperature and both NIR and visible PL intensity increase with reaction durations.
The broad visible PL could be fitted properly with four G aussian bands centered at 2.0 1 , 2.29 , 2.60 , and 2.7 7 eV for A 5 0 0 V (inset of F ig . 4.5(d)). Note that the higher energy tail of the PL spectra is truncated due to the use of a 4 3 5 nm filter during the PL measurement. Peak 1 is ascribed to self-trapped excitons located at TiO6 octahedra, while peaks 2 and 3 are ascribed to oxygen vacancy related trap states.21 The peak 4 is attributed to the presence of hydroxyl (OH) species which may form an acceptor level just above the valence band.23 M oreover, upon the loss of an O atom in the TiO2 lattice, the electron pair that remains trapped in the vacancy cavity give rise to an F center and one of the electrons in the F center tends to occupy the neighboring Ti4 + ion and yield center and center states within the band gap of the material. M ore details of the formation mechanism will be discussed in
C ha p te r 5. Here, we focus our attention to understand the evolution of visible and NIR PL emission and their origin.