Vertically aligned nanowire/nanorod arrays are promising building blocks for devices such as photodetectors, light emitting devices, and solar cells.1-4 Deposition of functional materials into nano-porous templates is a facile “bottom-up” technique for the fabrication of vertically aligned nanowire/nanorod arrays.5 Due to its simple fabrication process and controllable structures, porous anodic alumina (PAA) membrane is widely used as the template material. 6,7 In a conventional process, PAA templates are first formed on top of substrates and then precursor materials are deposition into the templates to form nanowires/nanorods by liquid-phase deposition such as electrodeposition or vapor-phase deposition such as chemical vapor deposition (CVD) and atomic layer deposition (ALD).8-10 The materials that form the nanowires/nanorods are either from the electrolyte for electrodeposition or from the vapor for CVD and ALD. Alternatively, the metal substrates beneath the PAA templates could be used as the sources to grow metal oxide nanorods in a process called “through-mask anodization”.11,12 This novel anodization method results in nanorods grown directly from the metal substrate having more intimate contact and better mechanical stability, which is more favorable from the practical point of view.
In this protocol, we report the fabrication of barium-doped tantalum nitride (Ba-Ta3N5) following the procedure depicted in the flow diagram in figure 1. Starting with a clean tantalum (Ta) substrate, a thin layer of aluminum (Al) is deposited on the Ta substrate. The Al layer is first anodized thoroughly to form a PAA mask on top of the Ta substrate. Then, though-mask anodization of the Ta substrate is performed after adjusting the pore size of the PAA mask. During the through-mask anodization process, tantalum oxide (Ta2O5) formed by anodization is partially filled into the nanochannels of the PAA mask. Vertically aligned Ta2O5 nanorod array is obtained after selectively etching the PAA mask. After that, bariumTa2O5 precursor is loaded on the surface of the Ta2O5 nanorods. Finally, vertically aligned Ba-Ta3N5 nanorod array is achieved by nitriding the Ba-loaded Ta2O5 nanorod array in ammonia atmosphere. Using the above-described method, vertically aligned Ba-Ta3N5 nanorod array with uniform diameter and length was fabricated on a large scale. As shown in the associated publication, the Ba-Ta3N5 nanorod array modified with a cobalt-phosphate co-catalyst showed high solar energy conversion efficiency when used as a photoanode for photoelectrochemical water splitting. This method can also be applied to other materials (e.g., Nb, Zr, Ti) to fabricate vertically aligned nanorod arrays of metal oxides/nitrides for different applications.