Inﬂuence of Gas Flow Rate for Formation of Aligned Nanorods in ZnO Thin Films for Solar-Driven Hydrogen Production SUDHAKAR SHET, 1,2,3 LE CHEN, 1 HOUWEN TANG, 1 RAVINDRA NUGGEHALLI, 2 HELI WANG, 1 YANFA YAN, 1 JOHN TURNER, 1 and MOWAFAK AL-JASSIM 1 1.—National Renewable Energy Laboratory, Golden, CO 80401, USA. 2.—New Jersey Institute of Technology, Newark, NJ 07102, USA. 3.—e-mail: Sudhakar.Shet@nrel.gov ZnO thin ﬁlms have been deposited in mixed Ar/N 2 gas ambient at substrate temperature of 500°C by radiofrequency sputtering of ZnO targets. We ﬁnd that an optimum N 2 -to-Ar ratio in the deposition ambient promotes the for- mation of well-aligned nanorods. ZnO thin ﬁlms grown in ambient with 25% N 2 gas ﬂow rate promoted nanorods aligned along c-axis and exhibit signiﬁ- cantly enhanced photoelectrochemical (PEC) response, compared with ZnO thin ﬁlms grown in an ambient with different N 2 -to-Ar gas ﬂow ratios. Our results suggest that chamber ambient is critical for the formation of aligned nanostructures, which offer potential advantages for improving the efﬁciency of PEC water splitting for H 2 production. INTRODUCTION Photoelectrochemical (PEC) systems based on transition-metal oxides such as TiO 2 , ZnO, and WO 3 have received extensive attention due to their potential application for H 2 production in aqueous solution by solar energy. 1–9 TiO 2 has been exten- sively studied for such applications. ZnO has a similar bandgap (3.3 eV) and band-edge positions as compared with TiO 2 . Furthermore, ZnO has a direct bandgap and higher electron mobility than TiO 2 . 8 Thus, ZnO can be a potential candidate for PEC splitting of water for H 2 production. 9 To improve the PEC response, a photoelectrode should have a high contact area with the electrolyte to provide more interfacial reaction sites. The morpho- logical features of thin ﬁlms such as grain size, grain shape, and surface area have profound inﬂuence on the performance of thin-ﬁlm electrodes. Thus, elec- trodes with nanostructures have been applied to improve PEC properties. 10–12 However, most of these nanostructures are not single crystals and contain defects. It is known that defects typically act as recombination centers that can kill photon-generated electron–hole pairs before they can reach surfaces for reactions. Therefore, single-crystal nanostructures are highly desirable. For ZnO, various forms of single- crystal nanostructures such as nanowires, nanobelts, and nanorods have been reported. It has been reported that ZnO nanorods were synthesized using catalysts, and recently, catalyst- free ZnO nanorods have been synthesized by various chemical and physical techniques such as metal- organic vapor-phase epitaxy (MOVPE), plasma- enhanced chemical vapor deposition (PECVD), and pulsed laser deposition (PLD). 10–13 In our earlier studies, 14,15 we found that single-crystal ZnO nano- rods aligned along the c-axis can be synthesized by radiofrequency (RF) sputter deposition in mixed Ar and N 2 ambient, and ZnO ﬁlms with aligned nano- rods exhibited improved PEC performance as com- pared with ZnO ﬁlms without nanorods. So far, RF sputtering has not been considered as widely as other methods for growth of ZnO nano- rods. Detailed examination of the deposition condi- tions for ZnO nanorod growth and study of ZnO nanorods for PEC application are needed. In this paper, we report on the inﬂuence of the N 2 - to-Ar gas ﬂow rate ratio on the formation of aligned nanorods in ZnO thin ﬁlms and their PEC perfor- mance. We ﬁnd that deposition in optimum N 2 -to-Ar ratio ambient leads to formation of single-crystal ZnO nanorods aligned along the c-axis. ZnO ﬁlms with aligned nanorods exhibit signiﬁcantly enhanced PEC response compared with ZnO thin ﬁlms deposited in ambient with different N 2 -to-Ar ratios. Our results suggest that the deposition ambient can be used to control the morphology of ZnO thin ﬁlms JOM, Vol. 64, No. 4, 2012 DOI: 10.1007/s11837-012-0299-z Ó 2012 TMS 526 (Published online March 28, 2012)