WO₃ photocatalyzes the conversion of CO₂ to CH₄, but the conversion requires light of ≤ 312 nm. Particle growth, quantum size effects, offers the potential of shifting the photoaction spectrum to longer wavelengths. Tungsten hexacarbonyl adsorbed on porous Vycor glass, photoactivated by ultraviolet light, was heated in air to produce WO₃ nanoparticles in the surface layers of glass substrate. SIMS depth profiling shows that tungsten species are uniform to a depth of 800 nm. No measurable particles could be detected after photolysis. However, heating of the samples resulted initially in the appearance of WO₃ absorption bands and prolonged heating leads to a small but noticeable redshift of the initial 460-nm band of the WO₃ particles. X-ray diffraction indicates the particles are both monoclinic and orthorhombic structures, the latter more apparent with longer heating times. Analysis of the XRD linewidth via the Scherrer equation indicates a particle size ≤ 10 nm. The calculation is consistent with TEM imaging of the doped glasses which indicate a WO₃ particle size of ≤ 0.3 nm.