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 ∼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 <i>I</i>–<i>V</i> 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

Photocatalytic Activity Suppression of CdS Nanoparticle-Decorated Cu₂O Octahedra and Rhombic Dodecahedra

Wurtzite CdS nanoparticles have been lightly deposited on Cu₂O cubes, octahedra, and rhombic dodecahedra to examine facet effects on the interfacial charge transfer in a photocatalytic reaction. Instead of an expected photocatalytic activity enhancement on the basis of a favorable band alignment at the heterojunction, CdS-decorated Cu₂O octahedra and rhombic dodecahedra show drastically reduced photocatalytic activities. Further increasing the CdS deposition amount leads to complete suppression of photocatalytic activity. Cu₂O cubes remain inactive even after CdS deposition. Transmission electron microscopy analysis reveals epitaxial growth of the (101) planes of CdS on the (110) planes of a Cu₂O rhombic dodecahedron, whereas the (110) planes of CdS align parallel to the (111) planes of a Cu₂O octahedron. Because facet-dependent photocatalytic activity can be understood from different degrees of band bending at the crystal surfaces, significantly upward bending for the CdS-contacting planes can explain the observed photocatalytic inactivity. This work demonstrates that strong facet effects tuning the band energies of both semiconductors at the heterojunctions make the predictions of an enhanced photocatalytic activity, simply through bulk band energy alignment analysis, highly unreliable