Defect-related optical bandgap narrowing and visible photoluminescence of hydrothermal-derived SnO₂ nanoparticles

Tin oxide (SnO₂) nanoparticles are fabricated by a novel hydrothermal route using H₂O₂ as oxidizer without adding any corrosive acids or bases. The morphology, composition, and microstructure are characterized by transmission electron microscopy, X-ray diffraction (XRD) and Raman spectra. XRD analysis reveals the formation of single phase rutile type tetragonal structure of all samples which is further supported by Raman studies. The average crystallite size is observed to vary from 2.8 to 4.2 nm as the reacting temperature increases from 120 to 180 °C, suggesting the promotion of crystal growth with increasing temperature. Raman spectroscopy measurement presents the existence of the defect modes, of which the mode at about 565 cm⁻¹ is identified to relate to oxygen vacancies as this mode becomes more prominent in the as-grown sample. Unusual optical band gap narrowing and the excitation wavelength dependent visible luminescence are observed, and both are supposed to be induced by the intrinsic defects. The photoluminescence (PL) spectra of all samples consist of two defect-related subbands, i.e., orange and blue luminescence bands. The two subbands are attributed to optical transitions in oxygen vacancies and some intrinsic surface states, respectively. Spectral analysis suggests that the excitation wavelength dependences of the two subbands are due to a distribution of the band gap of the nanoparticles induced by the internal strain, defect concentration as well as the quantum confinement effect. This work provides a feasible route to modulate the band gap and defect emissions and a good understanding of the PL behavior in the present SnO₂ nanoparticles.