Conformal ZnO coatings on high surface area silica gel using atomic layer deposition

Silica gel is a mesoporous form of silica with a high specific surface area that is widely used as a support material for heterogeneous catalysis. In this study, we use atomic layer deposition (ALD) methods to deposit ZnO layers onto silica gel supports. ZnO ALD was performed using alternating exposures of diethyl zinc (DEZ) and water to coat 1 g quantities of silica gel in a conventional viscous flow reactor. The coated materials were analyzed using weight gain, X-ray fluorescence, X-ray diffraction, energy dispersive X-ray analysis, and scanning and transmission electron microscopy. These measurements revealed that the silica gel support could be conformally coated using reactant exposure times of ∼ 90 s. The ALD ZnO was amorphous for films deposited using < 5 ALD cycles, but became hexagonal, nanocrystalline ZnO for films deposited using > 5 ALD cycles. In addition to the ZnO films, we also discovered that metallic Zn was deposited in the silica gel using very large DEZ exposures and deposition temperatures > 150 °C. The metallic Zn phase begins to form as soon as the ZnO ALD surface reactions have saturated, indicating that the Zn growth is strongly dependent on the availability of excess DEZ precursor.