Photoluminescence of oxygen vacancies and hydroxyl group surface functionalized SnO₂ nanoparticles

Abstract

We report, for the first time, the luminescence property of the hydroxyl group surface functionalized quantum dots (QDs) and nanoparticles (NPs) of SnO2 using low energy excitations of 2.54 eV (488 nm) and 2.42 eV (514.5 nm). This luminescence is in addition to generally observed luminescence from 'O' defects. The as-prepared SnO2 QDs are annealed at different temperatures under ambient conditions to create NPs with varying sizes. Subsequently, the average size of the NPs is calculated from the acoustic vibrations observed at low frequencies in the Raman spectra and by the transmission electron microscopy measurements. Detailed photoluminescence studies with 3.815 eV (325 nm) excitation reveal the nature of in-plane and bridging 'O' vacancies as well as adsorption and desorption occurring at different annealing temperatures. X-ray photoelectron spectroscopy studies also support this observation. The defect level related to the surface -OH functional groups shows a broad luminescence peak at around 1.96 eV in SnO2 NPs which is elaborated using temperature dependent studies.

Invoking forbidden modes in SnO2 nanoparticles using tip enhanced Raman spectroscopy

“…Symmetry allowed A 1g at 632 cm −1 and defect, D mode at 570 cm −1 were observed in the Raman spectrum without TERS tip. The D mode at 570 cm −1 appears only in small size (diameter around 9 nm and below) SnO 2 nanostructures . Optically inactive A 2g mode was reported to appear at a value of 575 cm −1 following Matossi force constant model that considered modified bond length, space symmetry reduction and lattice distortion because of ‘O’ vacancies and local lattice disorders in SnO 2 NPs .…”

“…In our earlier report we have demonstrated the photoluminescence related to -OH groups (which forms a defect energy state) in different size SnO 2 NPs including the 25-nm NPs using an excitation of 514.5 nm. [12] Xie et al [20] reported that the optical absorption of the laser because of defects in the metal oxides will cause the reduction in the Raman signal intensity. In the present case the defect energy state formed because of -OH groups might be absorbing some percentage of the laser (514.5 nm).…”

“…The D mode at 570 cm À1 appears only in small size (diameter around 9 nm and below) SnO 2 nanostructures. [8,12] Optically inactive A 2g mode was reported to appear at a value of 575 cm À1 following Matossi force constant model that considered modified bond length, space symmetry reduction and lattice distortion because of 'O' vacancies and local lattice disorders in SnO 2 NPs. [12] Along with a newly appeared weak symmetry allowed B 2g mode at 775 cm À1 , three times enhancement of Raman spectral intensity was observed for 4-nm NPs in the TERS measurement with respect to the spectrum recorded without tip ( Fig.…”

“…It is not always easy to identify presence of all these Raman forbidden modes and surface defect‐related vibrational features by using normal Raman spectroscopy because of their weak intensity. Particularly in quantum and quasi‐quantum sized NPs, these features are overlapped with the ‘O’ vacancy‐related broad defect band at 570 cm −1 …”

Electrochemical supercapacitor performance of SnO2 quantum dots

“…Fu et al [36] reported that the ZnO QDs remained stable in water due to surface hydroxyl (OH) groups. It is noteworthy that the presence of high density of OH¯ on the surface of the SnO 2 QDs were disseminated by using FTIR and Raman spectroscopic studies [23]. The 25 nm NPs obtained after annealing at high temperature did not show such widely covered presence of OH¯ groups on the surfaces [23].…”