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Description
Porous materials with a large specific surface area are used in batteries, supercapacitors, sensors, nanogenerators, and drug delivery systems. Porous (foamed) glass and glass-ceramics also attract attention because they can be produced from industrial waste. The mechanical and thermal properties of such systems are well studied. Their optical and electrophysical properties remain largely overlooked, although it is these properties that determine the suitability of a material for optoelectronic and sensing devices. Glass of the P2O5–MoO3–K2O–Bi2O3 system has a low melting temperature and intrinsic photoluminescence (380–850 nm), but porous glass of this composition has not been studied yet.
The base glass 27.18P2O5–36.24MoO3–31.71K2O–4.86Bi2O3 was synthesized by melt quenching. The glass was ground into a powder, mixed with polyvinyl acetate (10:1 by weight), and pressed into pellets under a pressure of 200 bar. The pellets were annealed at 550 °C for 2 h. The samples were studied by optical and scanning electron microscopy, energy-dispersive X-ray spectroscopy (EDX), diffuse reflectance, photoluminescence (λex = 473 nm), and current–voltage characteristics measurements.
Porous glass with open pores ranging from approximately 1 to 100 μm in size (macropores, according to the IUPAC classification) was obtained through simple thermal annealing. No crystalline phases were observed in the SEM images, indicating that the samples are glass rather than glass-ceramics. EDX analysis revealed the presence of a carbon component resulting from the decomposition and combustion of polyvinyl acetate. The following effects are associated with this component:
• a sharp decrease in the diffuse reflectance over the entire spectral range, caused by multiple scattering and by light absorption by carbon micro- and nanoparticles;
• a decrease in the optical band gap: direct band gap from 3.72 to 2.49 eV and indirect band gap from 2.94 to 1.44 eV;
• an approximately tenfold increase in photoluminescence intensity and a larger contribution of the long-wavelength band associated with Bi2+ ions and molybdate groups with oxygen vacancies;
• the appearance of electrical conductivity (the resistance of the base glass exceeds 2 × 10¹¹ Ω, so its current–voltage characteristics could not be measured).
The current–voltage curves of the porous glass depend on the probe position, are asymmetric with respect to the polarity of the bias voltage, and exhibit ohmic behavior only over limited voltage ranges. These findings indicate a nonuniform distribution of carbon inclusions, an incompletely formed percolation network, and the presence of distinct charge-transport channels.
Porous phosphate-molybdate glass has been produced using a simple thermal annealing method involving a mixture of glass powder and polyvinyl acetate. The carbon component formed during this process substantially alters the optical properties of the glass by reducing reflectance and the band gap while enhancing luminescence. It also imparts electrical conductivity. These properties can be controlled by varying the polyvinyl acetate content, annealing temperature, and pressing pressure. Further studies will focus on filling the pores with micro- and nanosized phosphors, in particular the mechanoluminescent material SrAl2O4:Eu,Dy.
| Type of presence | Presence at Taras Shevchenko National University |
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