Low-Dimensional Nanostructured Catalysts of Copper Oxides and Nickel Oxides Supported on Graphene and Their Applications in Biosensing and Photoelectrochemical Hydrogen Evolution

Abstract

Cuprous oxide (Cu2O) nanomaterials provide a versatile platform for building non-enzymatic glucose sensors. In particular, Cu2O nanocubes with controllable sizes and distributions can be deposited electrochemically on a conductive graphene strip as a soft substrate under different conditions, including overpotential, temperature, copper ion electrolyte concentration, and deposition time. The graphene substrate provides a promising condition for sensing because of its high conductivity, high specific surface area, and unique thermal and mechanical properties. A more negative overpotential is found to produce smaller nanocubes with a large number density, while the deposition temperature could affect the morphology of nanocubes. The size of the nanocubes increases with increasing copper ion concentration and deposition time. Using an optimal condition of –1.0 V vs Ag/AgCl, 1 mM [Cu2+], and 100 s deposition time at room temperature, we obtain a near-homogeneous monolayer of Cu2O-shell Cu-core nanocubes, ~50 nm in size, on a graphene strip substrate. The Cu2O nanocubes/graphene is used as a high-performance sensor with a wide detection range of 0.002-17.1 mM and a high sensitivity appropriate for saliva-range glucose sensing. It is also used to test saliva glucose in the real sample with 95% accuracy. This non-enzymatic glucose sensor is considerably better in performance than other non-enzymatic sensors, including those based on bare graphene, and graphene sputter-coated with a Cu film, and conventional enzymatic sensors such as glucose oxidase immobilized on graphene. For the glucose oxidase/graphene sensor, the addition of the enzyme increases the resistance of the graphene substrate, which leads to poorer performance. Even with an added Nafion film, the glucose oxidase/Nafion/graphene sensor only has a slightly increased detection range. In addition to being an excellent catalyst, Cu2O nanocubes have a large specific surface area and a large number of active sites. These nanomaterial properties, along with the use of a high-conductivity substrate like graphene, make the Cu2O nanocubes/graphene sensor among the best saliva-range glucose sensors reported to date. Photoelectrochemical hydrogen evolution (HER), a half reaction of water splitting, is crucial to the low-cost, environmentally friendly production of clean H2 fuel as part of the solution for transitioning away from a fossil fuel economy. Electrodeposition of a controllable Cu film on graphene followed by thermal oxidation at 200-400 °C has been used to produce copper oxide (CuxO, x=1,2) nanowires. The relative compositions of CuO and Cu2O layers in CuxO-Cu/graphene system form a heterojunction structure enabling high efficiency for electron-hole separation and a fast charge transfer rate, where the CuO layer with a proper thickness enhances light absorption, improves the charge separation, and serves as a protective layer for Cu2O photocorrosion while graphene serves as a flexible high conductive substrate. A high-performance dual Z-scheme heterojunction photocatalyst to greatly improve charge carrier separation, increase carrier density, and reduce electron-hole recombination is obtained by decorating this CuxO-Cu/graphene system with an efficient co-catalyst based on Cu-based ternary CuFe2O4 nanoparticles, obtained by a solvothermal method. The addition of CuFe2O4 nanoparticles on the best optimized CuxO-Cu/graphene is found to nearly double the photocurrent from –2.64 mA cm-2 to –4.91 mA cm-2, making this dual heterojunction catalyst the best catalyst system for HER reported to date. Electrodeposition of nickel nanoparticles, achieved through potentiostatic amperometry, followed by thermal annealing at 200-400 °C has produced nickel oxide nanoparticles on flexible graphene substrates. Using transmission electron microscopy and depth-profiling X-ray photoelectron spectroscopy, we show that the resulting NiO¬x nanoparticles exhibit several Ni oxidation states and a core-shell heterostructure, with a metallic Ni crystalline core and a NiO crystalline shell with a mixed crystalline-amorphous NiOOH/Ni(OH)2 skin. The composition of NiOOH/Ni(OH)2 redox couple relative to NiO is found to vary with the annealing temperature and annealing time. Besides, the high conductive graphene substrate enhances the electron transfer. The NiOx nanoparticle samples obtained with selected annealing temperature-time combinations are used for lactate detection, with the best sample showing an excellent linear range of 0.02-65.1 mM, a high sensitivity of 80.0 μA mM-1 cm-2, and an impressive limit of detection of 0.00015 mM. NiOx nanoparticle sample is also tested for lactate sensing in an artificial sweat electrolyte, and it exhibits a reduced linear range of 0.02-53.1 mM and a lower limit of detection of 0.00013 mM, but the same high sensitivity of 80.0 μA mM-1 cm-2. This sensing performance can be optimized by controlling the NiOOH + Ni(OH)2 relative composition to NiO, with the sample obtained with a higher relative content at a lower annealing temperature found to provide more reactive sites for sweat-range lactate detection and therefore higher sensitivity.