Tungsten Oxide Coated Copper Oxide Nanowire Arrays for Enhanced Activity and Durability with Photo-Electro-Chemical Water Splitting
Authors: Garcia Alejandro Martinez , Sunkara Mahendra Kumar , Sunkara Swathi , Vendra Venkat Kalyan 
Published: February 2020

Abstract: Recently, we showed that the deposition of crystalline WO3 layers on copper oxide nanowire arrays to result in a five-fold improvement of photocurrent density (~ 1.7 mA/cm2 at 0 V 10 vs. RHE) over titania coated copper oxide NW arrays. Most importantly, the deposition of WO3 or CuWO4 reduced CuO phase impurity to Cu2O. Improvement in the phase purity of nanowire arrays, led to the enhancement in photoactivity.1 The origin of degradation in photoelectrochemical performance with time is currently being investigated.

Introduction
: Photoelectrochemical (PEC) water splitting using sunlight is an attractive approach for producing clean hydrogen towards reducing carbon dioxide emissions and meeting the growing global energy demand. The biggest obstacle with PEC water splitting is the unavailability of a suitable semiconductor with proper band gap and band edge energetics and durability. Cu2O is a promising semiconducting material for photo-electro-chemical water splitting due to its band gap (Eg = 2.0 eV) and its band edge 25 positions straddling the water oxidation and reduction reactions. Major drawbacks with Cu2O include its poor stability in aqueous solutions, short carrier diffusion lengths (20-100 nm) and high absorption depth (10 microns). A semiconductor with a 2 eV band gap is expected to yield a photocurrent density of about 15 mA/cm2. The photoactivity performance of Cu2O has been a major concern, in addition to its rapid dissolution in aqueous solution.
One-dimensional nanostructures such as nanowires have the potential to produce high photoactivity due to fast charge transport properties and reduced length scales for minority carrier diffusion expected in single crystalline nanowires. In our previous work, we developed a rapid two-step process for the scalable synthesis of copper oxide nanowire arrays on copper foils.2 This involves a wet chemical oxidation of copper foils followed by oxidation in atmospheric microwave plasma. The nanowire arrays were subsequently coated with titania using atomic layer deposition to improve the aqueous stability. The photocurrents obtained with this method are much lower (<0.4 mA/cm2) but better than the data obtained with polycrystalline thin films to that date. The presence of a mixed phase of Cu2O and CuO has been identified as a possible cause for the low photoactivity. CuO has an indirect band gap of 1.4 eV leading to inefficient photon absorption and also cannot generate sufficient energy required to split water. Hence, phase purity of Cu2O is an important factor that needs to be addressed to improve the photoactivity of the Cu2O NW arrays made through atmospheric plasma oxidation. In this contribution, we investigated the use of n-type crystalline WO3 conformal layers on Cu2O nanowire arrays using hot wire chemical vapor deposition and studied their impact on photoactivity and durability.

Experimental: Copper oxide nanowire arrays were synthesized by wet chemical oxidation followed by short exposure to atmospheric plasma. Copper foils were immersed in an aqueous solution of sodium hydroxide and ammonium persulfate. The copper hydroxide nanowires produced were converted into copper oxide nanowires by means of exposure to an atmospheric plasma discharge for 2 minutes. The details of the process are described elsewhere. The deposition of WO3 or CuWO4 was performed in a HWCVD reactor using oxygen flow over hot tungsten and copper filaments used as the sources for tungsten oxide and copper oxide vapors.

Results and Discussion: The copper oxide nanowires are coated with titania layer using atomic layer deposition and tungsten oxide layers and copper tungstate layers using hot-wire chemical vapor deposition. The PEC characterization studies show 3-4 times enhancement in photoactivity for copper tungstate coated copper oxide nanowire arrays compared to that titania coated copper oxide nanowire arrays. See Figure 1.

Acknowledgements: Financial support from US DOE (DE-FG02-07ER46375) is acknowledged.

References:

A. Martinez-Garcia et al., J. Mater. Chem. A., In Press (2013).

S. Sunkara et al., Catalysis Today, 199, 27-35 (2013)

Fig. 1 PEC performance of copper tungstate coated copper oxide nanowires compared to titania coated copper oxide nanowire arrays in pH 5, 0.5M aqueous solution of Na2SO4.