Adriano C. Rabelo

Structural refinement, growth process, photoluminescence and photocatalytic properties of (Ba1-xPr2x/3)WO4 crystals synthesized by the coprecipitation method

Barium praseodymium tungstate (Ba1-xPr2x/3)WO4 crystals with (x = 0, 0.01, and 0.02) were prepared by the coprecipitation method. These crystals were structurally characterized by X-ray diffraction (XRD), Rietveld refinements, Fourier-transform Raman (FT-Raman) and Fourier-transform infrared (FT-IR) spectroscopies. The shape and size of these crystals were observed by field emission scanning electron microcopy (FE-SEM). Their optical properties were investigated by ultraviolet visible (UV-vis) absorption and photoluminescence (PL) measurements. Moreover, we have studied the photocatalytic (PC) activity of crystals for degradation of rhodamine B (RhB) dye. XRD patterns, Rietveld refinements data, FT-Raman and FT-IR spectroscopies indicate that all crystals exhibit a tetragonal structure without deleterious phases. FT-Raman spectra exhibited 13 Raman-active modes in a range from 50 to 1000 cm−1, while FT-IR spectra have 8 infrared active modes in a range from 200 to 1050 cm−1. FE-SEM images showed different shapes (bonbon-, spindle-, rice- and flake-like) as well as a reduction in the crystal size with an increase in Pr3+ ions. A possible growth process was proposed for these crystals. UV-vis absorption measurements revealed a decrease in optical band gap values with an increase of Pr3+ into the matrix. An intense green PL emission was noted for (Ba1-xPr2x/3)WO4 crystals (x = 0), while crystals with (x = 0.01 and 0.02) produced a reduction in the wide band PL emission and the narrow band PL emission which is related to f–f transitions from Pr3+ ions. High photocatalytic efficiency was verified for the bonbon-like BaWO4 crystals as a catalyst in the degradation of the RhB dye after 25 min under UV-light. Finally, we discuss possible mechanisms for PL and PC properties of these crystals.

Revisiting SrTiO3 as a photoanode for water splitting: development of thin films with enhanced charge separation under standard solar irradiation

Strontium titanate (SrTiO3) is an n-type semiconductor with high chemical and photochemical stability. This wide band gap oxide has a band gap energy of about 3.2 eV as well as a favorable energy for photocatalysis. In this study, we demonstrate an alternative and superior method to produce Nb-doped and undoped SrTiO3 photoanode thin films based on a colloidal deposition process which possess good activity under standard solar illumination conditions. Methanol was used as “hole scavenger,” and the results showed that the semiconductor–liquid junction (SCLJ) charge accumulation is not an important mechanism to control the photocurrent density and overpotential. In addition, experimental results suggest that the dominance of photocurrent density is controlled by the potential at the surface space charge layer for the Nb-doped SrTiO3 and by recombination at the depletion layer for the undoped oxide.

Optical Properties of Semiconductor Nanocrystals into the Glass and Colloidal Environments for New Technological Applications

Semiconductor quantum dots (QDs) became in the last decade an important class of materials by present continuous tunability of electronic and optical properties by changing size and shape [1–4]. These new physical properties presented by QDs have demonstrated potential applications in different technologic areas as light-emitting devices [5–8], low-threshold lasers [9], optical amplifiers [10], photovoltaic devices [11–14], biological labels [15, 16], antibacterial control [17], and cancer therapy [18]. In the visible and violet spectral range, CdS- and CdSe-based QDs become a prototypical among QDs, for above applications, due to highly reproducible and controllable emission from violet to red. The ability to synthesize stable QDs via colloidal aqueous solutions is extremely important for same application areas. For medical and biotechnological applications, for example, water-soluble QDs are needed [19, 20], and appropriate functionalizations will define its specific applications [21–23]. Thus, surface passivation and functionalization of QD systems are important methods that can improve their well-defined physical and chemical properties [24, 25]. Although QDs or QDs doped with impurities (metal or magnetic) are currently being synthesized by colloidal chemistry techniques [26, 27] or molecular beam epitaxy (MBE) [28], some possible applications of interest, technological, in particular, require nanoparticles to be embedded in robust and transparent host materials. In this context, the melting-nucleation approach appears as an appropriate synthesis technique since it allows the growth of diluted magnetic semiconductor NCs embedded in different glass matrices, which can avoid undesirable effects on the nanostructures, such as corrosion and humidity [29–31].

Antiferromagnetism induced by oxygen vacancies in V2O5 polycrystals synthesized by the Pechini method

Optical and magnetic properties of V2O5 polycrystals were investigated through photoluminescence (PL) and magnetization measurements. The PL spectra comprise two main bands attributed to oxygen vacancies and to band-edge-related transitions. The magnetization measurements showed a predominant paramagnetic behavior in the temperature range studied (5 K–300 K) with a superimposed peak around 80 K associated with an antiferromagnetic phase. The temperature dependence of the magnetic results and the relative PL intensity presented a remarkable correlation while in the range of 80–200 K the intensity associated with oxygen vacancies became considerably more intense than band-to-band emissions, the same range in which the antiferromagnetic transition was observed. The observation of an antiferromagnetic phase in V2O5 was predicted in the literature although this result had not previously been measured experimentally. Raman spectroscopy was used as a complementary technique in order to exclude the presence of spurious vanadium oxide phases in the sample.