2.2. Experimental Details
2.3.2. Structural Analysis
Structure, phase and crystallinity of the as-grown TiO2 NRs, Ag2O NPs and their HS are studied by the XRD patterns, as shown in Fig. 2.3. Each diffraction peak of pristine TiO2 NRs
corresponds to a pure TiO2(B) phase11 with a monoclinic structure, and that of Ag2O NPs corresponds to the cubic structure.6 The XRD peaks corresponding to the TiO2(B) phase are marked by red diamonds, while those of the cubic Ag2O are marked by black circles (filled). The relatively sharp diffraction peaks of Ag2O NPs clearly imply its higher crystallinity compared to that of TiO2(B). The phases of TiO2(B) and Ag2O co-exist in the Ag2O/TiO2(B) HS sample. Note that the XRD analysis on the HS sample does not show any measurable change in the crystal structure as well as the lattice parameter of the TiO2(B) NRs.
Fig. 2.3. XRD patterns of pristine TiO2(B) NRs, Ag2O NPs and TA2 HS. The curves are vertically shifted for clarity of presentation. The peaks corresponding to TiO2(B) are indicated by red diamonds and the same for Ag2O as black circles (filled).
To investigate the chemical environment, elemental composition and surface defects on the nanostructures, XPS studies were carried out on each of the catalysts. A comparison of the XPS core level O 1s spectra of pristine TiO2(B) NRs and TA2 HS is shown in Fig. 2.4(a), while that for Ti 2p spectra is shown in Fig. 2.4(d). Fig. 2.4(g) shows a comparison of the Ag 3d core level spectra of pristine Ag2O NPs and TA2 HS. O 1s spectrum of TA2 HS is observed to be slightly red shifted than the pristine TiO2(B) NRs. Ti 2p3/2 peak for pristine TiO2(B) NRs has been found at 458.6 eV, which is assigned to Ti4+ valence state, confirming the formation of TiO2.11
However, after the decoration of Ag2O NPs on TiO2(B) NRs, the Ti 2p3/2 peak is slightly red- shifted to a lower binding energy of 458.2 eV (as indicated by a vertical dashed line). This shift in the Ti 2p3/2 peak of TA2 HS may be due to the lattice distortion induced in TiO2(B) due to the strong coupling of TiO2(B) lattice with Ag2O NPs.12This may partly be due to the band bending
at the TiO2/Ag2O interface. Both the samples shown in Fig. 2.4(g) exhibit a single valence state, Ag+in Ag2O with Ag 3d5/2 peak at ~367.9 eV.13
Fig. 2.4. A comparison of the core level XPS spectra of: (a) O 1s for pristine TiO2(B) and TA2 HS, (d) Ti 2p for pristine TiO2(B) and TA2 HS and (g) Ag 3d for Ag2O NPs and TA2 HS. The vertical dashed lines drawn at 530.0 eV in (a), 458.6 eV in (d) and 367.9 eV in (g) indicate the red shift in the binding energy in the HS sample. (b, c) The Gaussian deconvolution of O 1s core level XPS spectra for pristine TiO2(B) and TA2 HS, respectively. (e, f) The fitted Ti 2p XPS spectra for pristine TiO2(B) and TA2 HS, respectively. (h, i) The fitted Ag 3d XPS spectra for Ag2O and TA2, respectively. Each XPS spectrum is fitted with a Shirley baseline. The identity of each peak is denoted with the corresponding charge state in the respective cases.
Note that in TA2 HS, the Ag 3d5/2 peak is slightly shifted to the lower binding energy as compared to that in bare Ag2O NPs, indicating the lattice distortion after HS formation. A vertical dashed line is drawn at 367.9 eV to discern the redshift. Fig. 2.4(b, c) show the O 1s core level XPS spectra of pristine TiO2(B) and TA2 HS, respectively. In both cases, the O 1s spectra exhibit asymmetry in line shape with a shoulder at the higher binding energy side, indicating the presence of surface defect states. The O 1s spectrum can be deconvoluted into two symmetric Gaussian peaks. The intense peak at ~529.9 eV is attributed to the oxygen in the TiO2 crystal
lattice (OL), while the other lower intensity peak detected at ~531.2 eV can be assigned to the oxygen vacancy (OTi3+). The result reveals that the oxygen vacancy (OV) concentration decreases from 22.7% to 16.2% after the formation of HS with Ag2O NPs. This may be due to the defect filling on the TiO2(B) surface after the loading of oxide material (Ag2O). Fig. 2.4(e, f) show the deconvolution of XPS Ti 2p spectra for pristine TiO2(B) and TA2 HS with four symmetric Gaussian peaks in each case. For the pristine TiO2(B), two major characteristic doublets for Ti 2p3/2 and 2p1/2, encompassing a set of two 2p3/2 peaks at 456.6 eV and 458.5 eV, are assigned to the 3+ and 4+ valence states of Ti, respectively. The result shows that the Ti4+ ions in the vicinity of OV cavity accept an electron and transform to Ti3+ ions with an F+ centre. In the case of TA2 HS, deconvolution of 2p3/2 peak shows two Gaussian peaks at 457.2 eV and 458.2 eV, assigned to the 3+ and 4+ valence states of Ti, respectively. It can be noticed that after the formation of HS, the Ti3+ concentration is reduced substantially (~2.8 fold) (shown in Fig. 2.4(e, f)) due to the defect filling, as confirmed from the O 1s spectrum. This is fully consistent with the PL analysis that shows lower vacancy concentration in TA2, as discussed later. Gaussian fittings of the Ag 3d XPS spectra for pristine Ag2O NPs and TA2 HS with appropriate baseline (Shirley) are shown in Fig. 2.4(h, i), respectively. In both the cases, each of the 3d peaks fits to a single peak corresponding to Ag+ state. The Gaussian fitted Ag 3d5/2 peak for Ag2O NPs is detected at 367.9 eV, which is shifted to the lower binding energy after the HS formation and detected at 367.7 eV.
Thus, the redshift in XPS peak (Ti 2p and Ag 3d) after the HS formation could be resulting from the strong interaction of Ag2O with TiO2(B) NRs and may also result from the strain induced in the HS. Note that the atomic ratio of Ag to O found from XPS analysis is 2.1:1 indicating that no other phases of Ag oxides are present in the sample. Thus, XPS results show strong coupling between the Ag2O NPs and TiO2(B) NRs in HS samples, and this plays an important role in the enhanced photocatalytic efficiency.
2.3.2.2. Raman Analysis
For further endorsement of crystallinity and phase of TiO2 NRs and its various HSs, Raman analysis has been performed for all the samples using a 633 nm laser excitation. Fig. 2.5 shows a comparison of the Raman spectra for all the samples. The inset shows a magnified view of the prominent Raman modes in the region 250-1000 cm-1. The peaks corresponding to the B-phase of TiO2 are marked with ‘*’ symbol, while those of the Ag2O are marked with ‘’ symbol.11 The crystalline Ag2O peaks at 323 and 489 cm-1 corresponds the Ag-O stretching/bending modes.14
Fig. 2.5. Raman spectra of pristine TiO2(B) NRs and its HSs. The inset shows the magnified view of the spectra in the range 250-1000 cm-1. Peaks related to Ag2O and TiO2(B) are labelled with ‘’ and ‘’ symbols, respectively.