# Analysis of the Inhomogeneous Growth of Sputtered Black TiO2 Thin Films

## TL;DR

This paper investigates how sputtered black TiO2 thin films grow unevenly and how this affects their electrochemical performance.

## Contribution

The study reveals that asymmetric bipolar sputtering creates multilayer films with distinct phases that enhance performance.

## Key findings

- Asymmetric bipolar sputtering produces two phases: a transparent crystalline phase and a black amorphous phase.
- The inhomogeneous structure of the films improves their electrical and photoelectrochemical properties.
- The film structure is similar to core–shell nanoparticles of B-TiO2.

## Abstract

Black titanium dioxide (B-TiO2) is a highly
active photoelectrochemical
material compared to pure titanium dioxide due to its increased light
absorption properties. Recently, we presented the deposition of thin-film
B-TiO2 using an asymmetric bipolar reactive magnetron sputter
process. The resulting samples exhibit excellent photoelectrochemical
properties, which can be fine-tuned by varying the process parameters.
In this article, results of morphological, electrical, and photoelectrochemical
measurements are discussed to better understand the surprisingly high
electrochemical activity of the films. In order to study the influence
of the dynamic process on film formation, we use static sputtering
with a fixed substrate covering the entire chamber area in front of
the two targets. This allows the material composition of the sputtered
film to be analyzed depending on its relative position to the targets.
The results lead to the conclusion that the asymmetric bipolar sputtering
mainly produces two phases, a transparent, nonconductive crystalline
phase and a black, conductive amorphous phase. As a consequence, the
dynamically sputtered samples are multilayers of these two materials.
We discuss that the significantly better electrical and photoelectrochemical
properties emerge from the inhomogeneous nature of the laminates,
like also found in core–shell nanoparticles of B-TiO2.