Synthesis of Ag3PO4 Crystals with Tunable Shapes for Facet-Dependent Optical Property, Photocatalytic Activity, and Electrical Conductivity Examinations

This work has developed conditions for the synthesis of Ag3PO4 cubes, rhombic dodecahedra, {100}-truncated rhombic dodecahedra, tetrahedra, and tetrapods by tuning the amount of NH4NO3, NaOH, AgNO3, and K2HPO4 solutions added. Use of a minimal amount of AgNO3 solution can form much smaller rhombic dodecahedra and tetrahedra. Submicrometer-sized Ag3PO4 cubes and rhombic dodecahedra with sizes larger than 300 nm do not exhibit the optical size effect, but similar to 290 nm rhombic dodecahedra show a smaller band gap value than larger cubes, and tetrahedra show the most blue-shifted absorption edge. The optical facet effect is present in Ag3PO4 crystals. Ag3PO4 cubes are more photocatalytically active than rhombic dodecahedra toward photodegradation of methyl orange, but tetrahedra are inactive, showing clear presence of photocatalytic facet effects. Electron paramagnetic resonance results confirm much higher production of hydroxyl radicals from photoirradiated Ag3PO4 cubes than from rhombic dodecahedra, while tetrahedra yield essentially no radicals. A modified band diagram showing different degrees of band edge bending can explain these observations. All these Ag3PO4 crystals show poor electrical conductivity properties, but the {110} faces are slightly more conductive than the {100} faces. As a result, current rectifying curves have been obtained, demonstrating that facet-dependent electrical properties are broadly observable in many semiconductor materials. This work reveals again that facet-dependent optical, photocatalytic, and electrical conductivity properties are intrinsic semiconductor properties.