Ionic-Liquid-Functionalized Copper Oxide Nanorods for Photocatalytic Splitting of Water Rashi Gusain + , [a, b] Nikita Singhal + , [a, b] Raghuvir Singh, [a] Umesh Kumar, [a, b] and Om P. Khatri* [a, b] Introduction Clean energy and environmental protection are emergent technological challenges. In this context, hydrogen plays an important role as a clean and sustainable energy, particularly when it is produced by the photocatalytic splitting of water. The breakthrough finding by Fujishima and Honda to generate hydrogen by using TiO 2 /Pt electrochemical cell system opened up the scope of using various semiconducting materials for water splitting. [1–11] In recent decades, several studies have demonstrated the photocatalytic potential of various semicon- ductors, metal oxides, and organometallic complexes (TiO 2 , CdS, ZnS, ZrO 2 , NaTaO 3 , CdTe, g-C 3 N 4 , WO 3 , Ta 2 O 5 , etc.), along with cocatalysts [Pt, Rh, NiO x , RhO 2 , CoO x , CuS, MoS 2 , reduced graphene oxide (rGO), graphene oxide (GO), CuO, etc.] for water splitting. [2–10] The band-gap energy, surface properties, size, and dimensions of nanomaterials are key features for pho- tocatalytic activity and influence light absorption, band-edge potential, charge separation, and recombination dynamics. Copper-based semiconducting materials are inexpensive, abun- dantly available in the earth’s crust, and are gaining increasing interest for photon-induced reactions. Cu 2 O and CuO were re- vealed as promising nanomaterials for enhanced solar-cell effi- ciency. [12, 13] The Cu TiO 2 , Cu 2 O TiO 2 , and CuO TiO 2 nanocom- posites exhibit excellent light-absorption abilities and show higher hydrogen generation rates than those of noble-metal- loaded TiO 2 . The higher photocatalytic activities of these nano- composites were deduced from the collective effect of reduced structural defects, high surface area, and favorable charge transfer (which inhibits recombination). [14–18] The bare CuO nanomaterials absorb visible light efficiently. However, the fast recombination of electrons and holes, owing to a narrow band gap, inhibits their potential for photocatalytic splitting of water. [19–20] Recently, Barreca et al. revealed the potential of Cu 2 O and CuO nanosystems supported on Si (100) for photoca- talytic hydrogen generation in the absence of TiO 2 . [21] CuO grafted on rGO facilitated the photocatalytic reduction of CO 2 under visible-light irradiation and was attributed to the slow recombination of photogenerated charge carriers and the effi- cient transfer of electrons through the rGO skeleton. [22] Ionic liquids (ILs) have been widely accepted as designer sol- vents and templates for the preparation, stabilization, and shape regulation of various nanomaterials. [23–25] Imidazolium (Im) IL-modified TiO 2 nanoparticles effectively extend the ab- sorption of visible light and exhibit higher photocatalytic activ- ity than that of pristine TiO 2 . [26–28] Recently, Zhang et al. demon- strated that IL-modified bismuth oxyiodide (BiOI) could trap the photoexcited electron in its conduction band and inhibit the recombination of photoinduced electron–hole pairs. [29] The Im ring in 1-butyl-3-methylimidazolium iodide ([BMIm]I)-modi- fied WO 3 was demonstrated to suppress the recombination of Thin films of imidazolium (Im) ionic liquids with bis(salicylato)- borate (BScB) and hexafluorophosphate (PF 6  ) anions were grafted onto copper oxide (CuO) nanorods. Chemical and structural features of ionic-liquid-functionalized CuO (CuO IL) nanorods were examined by X-ray photoelectron spectroscopy, FTIR spectroscopy, XRD, and high-resolution TEM analyses. The CuO IL nanorods were demonstrated to be efficient photoca- talysts for the splitting of water under visible-light irradiation without using any sacrificial agent. The pristine CuO nanorods could not split water, whereas CuO IL nanorods exhibited ex- cellent photocatalytic activities and produced 1827 and 1082 mmol of hydrogen in 2 h with 20 mg of CuO ImBScB and CuO ImPF 6 as photocatalysts, respectively. The photocatalytic activity of the CuO IL nanorods was attributed to the synergis- tic effect of ionic-liquid thin films and CuO nanorods. The trap- ping of photoinduced charge carriers by ionic liquids inhibits the recombination process, and consequently, the CuO nano- rods facilitate the water-splitting reaction. The CuO IL photo- catalysts were efficiently recycled without loss of catalytic ac- tivity, which revealed the stability of the ionic-liquid thin films grafted on the CuO nanorods. [a] R. Gusain, + N. Singhal, + R. Singh, Dr. U. Kumar, Dr. O.P. Khatri Chemical Science Division, CSIR–Indian Institute of Petroleum Mohkampur, Dehradun 248005 (India) E-mail: opkhatri@iip.res.in [b] R. Gusain, + N. Singhal, + Dr. U. Kumar, Dr. O.P. Khatri Academy of Scientific and Innovative Research New Delhi 110025 (India) [ + ] These authors contributed equally to this work. Supporting information for this article can be found under http:// dx.doi.org/10.1002/cplu.201600047. It contains C 1 s XPS and UV/Vis spectra of CuO and CuOIL nanorods; FESEM and elemental mapping of CuOIL nanorods; HRTEM images, XRD pattern, and FTIR spectra of pris- tine and recovered CuOImBScB nanorods after photocatalytic reaction; and the electrochemical potential window of ionic liquids. ChemPlusChem 2016, 81, 489 – 495 # 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 489 Full Papers DOI: 10.1002/cplu.201600047