Tae Hwa Jeon

Cobalt–phosphate complexes catalyze the photoelectrochemical water oxidation of BiVO4 electrodes

BiVO(4) semiconductor electrodes were coupled with cobalt-phosphate complexes (CoPi) to enhance the photoelectrochemical (PEC) performance for water oxidation reaction. CoPi was deposited on a 550 nm-thick BiVO(4) film via electrodeposition (ED) and photodeposition (PD) methods for comparison of their effects. The CoPi on BiVO(4) exhibited Co : P atomic ratios of approximately 1 : 7 for the electrodeposited sample and approximately 1 : 18 for the photodeposited sample, and Co(2+) and Co(3+) co-existed in both samples. Optimized CoPi ED resulted in a CoPi overlayer of approximately 850 nm thick, which showed an electrochromic-like behavior that was likely due to limited access of phosphate into BiVO(4) across the CoPi layer. Optimized CoPi PD, however, had very thin and rather uniform CoPi dispersion and did not show electrochromic-like behavior. Despite the lesser amount of CoPi, the PEC performance of BiVO(4)/CoPi (PD) was comparable to that of BiVO(4)/CoPi (ED). Real-time measurements of the headspace molecular oxygen that evolved from water oxidation indicated that CoPi enhances O(2) production and photocurrent generation at BiVO(4) by a factor of around 15 and a maximum of 20, respectively, at 0.576 V(SCE) (equivalent to 1.23 V(RHE)) under air mass 1.5 irradiation (400 mW cm(-2)). Prolonged irradiation of BiVO(4)/CoPi (ED) resulted in a reduced Co : P ratio to 1 : 1.77 without changing the mixed valency of Co(II/III). This finding indicates that incorporation of phosphate into the CoPi was kinetically slower than water oxidation. The primary role of CoPi has been suggested as a hole-conducting electrocatalyst making the photogenerated electrons more mobile and, consequently, increasing conductivity and boosting the PEC water oxidation performance of BiVO(4).

Iron Oxide Photoelectrode with Multidimensional Architecture for Highly Efficient Photoelectrochemical Water Splitting

Nanostructured metal oxide semiconductors have shown outstanding performances in photoelectrochemical (PEC) water splitting, but limitations in light harvesting and charge collection have necessitated further advances in photoelectrode design. Herein, we propose anodized Fe foams (AFFs) with multidimensional nano/micro-architectures as a highly efficient photoelectrode for PEC water splitting. Fe foams fabricated by freeze-casting and sintering were electrochemically anodized and directly used as photoanodes. We verified the superiority of our design concept by achieving an unprecedented photocurrent density in PEC water splitting over 5 mA cm-2 before the dark current onset, which originated from the large surface area and low electrical resistance of the AFFs. A photocurrent of over 6.8 mA cm-2 and an accordingly high incident photon-to-current efficiency of over 50 % at 400 nm were achieved with incorporation of Co oxygen evolution catalysts. In addition, research opportunities for further advances by structual and compositional modifications are discussed, which can resolve the low fill factoring behavior and improve the overall performance.

Photoelectrochemical Water Oxidation Using ZnO Nanorods Coupled with Cobalt-Based Catalysts

ABSTRACT Photoelectrochemical performances of ZnO electrodes are enhanced by coupling with cobalt-basedcatalyst (CoPi) in phosphate electrolyte (pH 7). For this study, hexagonal pillar-shaped ZnO nanorodsare grown on ZnO electrodes through a chemical bath deposition, onto which CoPi is depositedwith different photodeposition times (10-30 min). A scanning electron microscopic study indicatesthat CoPi deposition does not induce any change of ZnO morphology and an energy-dispersive X-rayspectroscopic analysis shows that inorganic phosphate ions (Pi) exist on ZnO surface. Bare ZnOelectrodes generate the current of ca. 0.36 mA/cm 2 at a bias potential of 0.5 V vs. SCE, whereasZnO/CoPi (deposited for 10 min) has ca. 50%-enhanced current (0.54 mW/cm 2 ) under irradiation ofAM 1.5G-light (400 mW/cm 2 ). The excess loading of CoPi on ZnO results in decrease of photo-currents as compared to bare ZnO likely due to limited electrolyte access to ZnO and/or CoPi-mediatedrecombination of photogenerated charge carriers. The primary role of CoPi is speculated to trap thephotogenerated holes and thereby oxidize water into molecular oxygen via an intervalency cycleamong Co(II), Co(III), and Co(IV).Key words: Artificial photosynthesis, Solar hydrogen, Electrocatalyst, Water splitting, Semicon-ductors

Distorted Carbon Nitride Structure with Substituted Benzene Moieties for Enhanced Visible Light Photocatalytic Activities

Carbon nitride (CN) is being intensively investigated as a low-cost visible light active photocatalyst, but its practical applications are limited because of the fast charge pair recombination and low visible light absorption. Here, we introduce a new strategy for enhancing its visible light photocatalytic activity by designing the CN structure in which the nitrogen of tertiary amine is substituted with a benzene molecule connected by three heptazine rings. The intramolecular benzene doping induced the structural changes from planar symmetric structure to distorted geometry, which could be predicted by density functional theory calculation. This structural distortion facilitated the spatial separation of photogenerated charge pairs and retarded charge recombination via exciton dissociation. Such unique properties of the benzene-incorporated CN were confirmed by the photoluminescence (PL) and photoelectrochemical analyses. The optimal loading of benzene doping reduced the PL of the conjugated ring system (π → π* transition) but enhanced the PL of the forbidden n → π* transition at the nitrogen atoms with lone pair electrons due to the distortion from the planar geometry. The photoelectrode of benzene-doped CN exhibited higher photocurrent and lower charge transfer resistance than bare CN electrode, indicating that the photogenerated charge pairs are more efficiently separated. As a result, the benzene-doped CN markedly increased the photocatalytic activity for the degradation of various organic pollutants and that for H2O2 production (via O2 reduction). This study proposes a simple strategy for chemical structural modification of carbon nitride to boost the visible light photocatalytic activity.

Two-Dimensional Materials in Semiconductor Photoelectrocatalytic Systems for Water Splitting

Hydrogen (H2) production via solar water splitting is one of the most ideal strategies for providing sustainable fuel because this requires only water and sunlight. In achieving high-yield production of hydrogen as a recyclable energy carrier, the nanoscale design of semiconductor (SC) materials plays a pivotal role in both photoelectrochemical (PEC) and photocatalytic (PC) water splitting reactions. In this context, the advent of two-dimensional (2D) materials with remarkable electronic and optical characteristics has attracted great attention for their application to PEC/PC systems. The elaborate design of combined 2D layered materials interfaced with other SCs can markedly enhance the PEC/PC efficiencies via bandgap alteration and heterojunction formation. Three classes of 2D materials including graphene, transition metal dichalcogenides (TMDs), and graphitic carbon nitride (g-C3N4), and their main roles in the photoelectrocatalytic production of H2, are discussed in detail herein. We highlight the various roles of these 2D materials, such as enhanced light harvesting, suitable band edge alignment, facilitated charge separation, and stability during the water splitting reaction, in various SC/2D photoelectrode and photocatalytic systems. The roles of emerging 2D nanomaterials, such as 2D metal oxyhalides, 2D metal oxides, and layered double hydroxides, in PEC H2 production are also discussed.