Carbon nitride (CN_x), an inexpensive, non-toxic, noble metal-free polymeric semiconductor, has attracted much recent attention for its use as a sustainable light absorber in artificial photosynthesis and organic photocatalysis. The use of this material as a photoelectrocatalyst and its use in photoelectrochemical cells could result in cheap and sustainable devices capable of storing solar energy in chemical bonds. Although the performances of CN_x suspensions are already approaching practical levels, the photocurrent density reported for carbon nitride photoelectrodes is still limited by some of its intrinsic characteristics (i.e., low conductivity, high recombination rates, low surface area, and weak particle adhesion).^[1] Moreover, the current reports are lacking a basic understanding of the intrinsic processes happening within the CN_x films, preventing the rational design of more competitive photoelectrodes.^[2]
In this poster, we report a facile and reproducible method to synthesize versatile and high-performance cyanamide-functionalized carbon nitride (^NCNCN_x) photoanodes based on a rational design. The co-deposition of ^NCNCN_x with indium tin oxide (ITO) nanoparticles onto a 1.8 Å thick alumina-coated FTO glass substrate led to a record 1.4 ± 0.2 mA cm^–2 at 1.23 V vs RHE with a very low onset of –0.4 V vs RHE, incident photon-to-current efficiency (IPCE) of 60.0 ± 3.6%, and Faradaic efficiencies ≥ 95% for the selective oxidation of 4-methylbenzyl alcohol to the corresponding aldehyde. These performances are new benchmarks in the development of carbon nitride photoelectrodes (c.f., 0.67 mA cm^–2 using a sacrificial hole scavenger as the previous record).^[3] Moreover, we proved the versatility towards the oxidation of various alcohols such as glycerol, the main by-product of biodiesel production, ethylene glycol, co-monomer in polyethylene terephthalate and model molecule for plastic photoreforming; and small simple alcohols accessible from plant biomass, such as methanol and ethanol. Furthermore, we have carried out detailed photoelectrochemical, photo-induced absorption, transient absorption spectroscopy, transient photocurrent, and photoelectrochemical impedance spectroscopy measurements to rationalize and provide a well-supported explanation for the role of each component in achieving this excellent charge separation properties, and high hole-extraction efficiency. More specifically, the ITO nanoparticles act as a conductive binder and improve the extraction of electrons from the carbon nitride, which otherwise remain trapped in the organic semiconductor, while the alumina underlayer increases the electrical contact between the ITO nanoparticles and the FTO-coated electrode.