Photocatalytic and Gas Sensitive Multiwalled Carbon Nanotube/TiO₂-ZnO and ZnO-TiO₂ Composites Prepared by Atomic Layer Deposition

TiO₂ and ZnO single and multilayers were deposited on hydroxyl functionalized multi-walled carbon nanotubes using atomic layer deposition. The bare carbon nanotubes and the resulting heterostructures were characterized by TG/DTA, Raman, XRD, SEM-EDX, XPS, TEM-EELS-SAED and low temperature nitrogen adsorption techniques, and their photocatalytic and gas sensing activities were also studied. The carbon nanotubes (CNTs) were uniformly covered with anatase TiO₂ and wurtzite ZnO layers and with their combinations. In the photocatalytic degradation of methyl orange, the most beneficial structures are those where ZnO is the external layer, both in the case of single and double oxide layer covered CNTs (CNT-ZnO and CNT-TiO₂-ZnO). The samples with multilayer oxides (CNT-ZnO-TiO₂ and CNT-TiO₂-ZnO) have lower catalytic activity due to their larger average densities, and consequently lower surface areas, compared to single oxide layer coated CNTs (CNT-ZnO and CNT-TiO₂). In contrast, in gas sensing it is advantageous to have TiO₂ as the outer layer. Since ZnO has higher conductivity, its gas sensing signals are lower when reacting with NH₃ gas. The double oxide layer samples have higher resistivity, and hence a larger gas sensing response than their single oxide layer counterparts.