Influence of a NiO Hole Transport Layer on Charge Separation in FTO|WO₃|BiVO₄ Photoanodes for TEMPO-Mediated Oxidation

Favor Igwilo, Texas Christian University, Class of 2026
Laboratory of Dr. Benjamin Sherman, PhD;
Department of Chemistry and Biochemistry

Efficient hole transport is critical for driving oxidative transformations in photoelectrochemical systems. In this study, we investigate multilayer FTO|WO₃|BiVO₄|NiO photoanodes for application in TEMPO-mediated oxidation of benzyl alcohol to benzaldehyde, an important chemical reaction used in industrial processes. The stable radical 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) enables selective alcohol oxidation under mild conditions and represents a more sustainable alternative to conventional stoichiometric oxidants that generate hazardous waste. Enhancing interfacial charge transport is essential to improve the viability of this photoelectrosynthetic process.
We hypothesize that nickel oxide (NiO), a p-type semiconductor, can function as an effective hole transport layer due to its favorable valence band alignment, hole mobility, abundance, and low cost relative to traditional materials such as titanium dioxide (TiO₂). A liquid phase deposition protocol was developed to fabricate uniform NiO thin films on fluorine-doped tin oxide substrates, which were subsequently integrated into FTO|WO₃|BiVO₄ substrates. The resulting multilayer photoanodes were evaluated to determine whether NiO enhances charge separation and hole extraction under various conditions.
Electrochemical characterization was performed using cyclic voltammetry to probe redox behavior and assess catalytic onset potentials, chronoamperometry to quantify steady-state photocurrent and operational stability, and electrochemical impedance spectroscopy to evaluate interfacial charge transfer resistance. Measurements were conducted under both dark and illuminated conditions, with and without TEMPO, in Tetrabutylammonium hexafluorophosphate (TBAPF6)in acetonitrile (ACN) solution. We anticipate that incorporation of NiO will reduce interfacial charge transfer resistance, increase photocurrent density in the presence of TEMPO, and improve kinetic parameters associated with benzyl alcohol oxidation.
Photocurrent densities of FTO–WO₃–BiVO₄ and FTO–WO₃–BiVO₄–NiO photoanodes were directly compared to quantify the effect of the NiO interlayer. Additionally, heterogeneous electron transfer rate constants (k₀) were determined under TEMPO-containing conditions to assess how the multilayer structure influences interfacial electron transfer kinetics.
This work establishes a working protocol for NiO liquid phase deposition and clarifies the role of NiO in enhancing TEMPO-mediated photoelectrosynthetic oxidation. These findings can later inform the design of cost-effective photoelectrode architectures for sustainable organic reactions.

By combining the supportive structure of alginate hydrogels, the semiconductive nature of porous silicon (pSi) membranes, and the biodegradability of both of these materials, a unique, non-invasive biosensor can ideally be created for the chemical analysis of health-relevant analytes.
Hydrogels are water-infused, biodegradable polymer networks that are easily able to interface with human skin. Alginate polymer hydrogels are particularly useful due to being derived from brown algae, making them environmentally abundant and inexpensive. The polymer is modified with acrylamide segments to add stability and shelf-life to the hydrogel material. Ultimately, these characteristics make hydrogels ideal for supporting the pSi membranes while assimilating them to a variety of tissues.
Porous silicon (pSi) is a highly porous form of the widely used elemental semiconductor and is used to conduct and measure electrical signals throughout the hydrogel matrix. When established in a diode form, these membranes exhibit measurable current values as a function of voltage, which can be used to detect bioelectrical stimuli such as the concentration of physiologically relevant ionic species such as Na+, K+, and Ca2+.
Recent experiments focus on integrating pSi membranes in Acrylamide/alginate co-polymer hydrogels to test how variations in ion concentration affect the flow of current measured as a function of applied voltage. Porous silicon membranes ~110 μm thick and 79% porosity, are fabricated from the anodization of low resistivity (100) Si in methanolic HF at an applied bias of 100 mA/cm2 for 30 minutes. Membranes pieces ~2 mm by 2 mm are heated for one hour at 650°C. They are then affixed to Cu wire using Ag epoxy and annealed for 15 minutes at 95°C. The wires are then fashioned to form the membrane diodes and clear nail polish is used to coat the backs of the membranes, the Cu wire connection, and the wire itself to prevent current flow from the back of the membrane or bubble formation. The electrochemical cell is created by placing two pSi membranes parallel to each other ~2 mm apart, vertically, in a fixed electrolyte composition. The current is measured as function of applied voltage (typically from 0-5 V) for systems with different concentrations of NaCl in the nM to mM range. The NaCl solutions are injected directly into the hydrogel in between the two pSi membranes in 2 μL units.
This presentation will focus on the fabrication protocol, as well as results from experiments with varying NaCl concentrations. Previous experiments have determined linearity of the current and applied voltage function in the region of 0.25 mM to 1 mM concentration ranges of pure NaCl solution. Future experiments will seek to repeat these findings within the alginate hydrogel matrix.

Impedimetric Sensing of PFOA in Drinking Water
Qamar Hayat Khan,1 Ramachandra Legundapati,2 Gyu Leem,2 and Benjamin D. Sherman1,
1Department of Chemistry & Biochemistry, TCU, TX 76129, 2 Department of Chemistry & Biochemistry, TCU, TX 76129; 2 Department of Chemistry, SUNY, Syracuse, New York 13210, United States
Abstract
Per- and polyfluoroalkyl substances (PFAS) are persistent environmental contaminants that pose significant risks to human health and ecosystems.1 This poster is focused on the development of a label-free impedimetric sensor2 for the detection of PFAS in aqueous systems. The sensing platform is based on fluorine-doped tin oxide (FTO) electrodes functionalized with perfluorinated self-assembled monolayers (SAMs) to promote fluorophilic interactions with target PFAS molecules, particularly perfluorooctanoic acid (PFOA).
FTO electrodes were modified using trichloro(1H,1H,2H,2H-perfluorooctyl) silane (TCPFOS) to form hydrophobic surface coatings. Successful formation of the SAM layer was confirmed through water contact drop experiment. Surface coverage of the monolayer was evaluated using cyclic voltammetry (CV) with the ferri/ferrocyanide redox couple, where cathodic peak current reduction indicates effective surface blocking by the SAM layer.
Impedance measurements were subsequently performed in 0.1 M NaCl electrolyte at controlled pH (4.5) while exposing the functionalized electrodes to varying concentrations of PFOA. The impedance data were qualitatively by plotting Cole–Cole capacitance plots to evaluate changes in effective interfacial capacitance and quantitatively by circuit fitting.3 These capacitance variations were correlated with PFAS concentration to assess sensor sensitivity and response behavior.
The results demonstrate that the TCPFOS-modified FTO surfaces produce measurable and reproducible capacitance changes in response to PFOA exposure, indicating the potential of fluorophilic surface chemistry combined with impedance spectroscopy for PFAS detection. This work contributes toward the development of a simple, label-free electrochemical sensing platform for monitoring PFAS contamination in water.
References
(1) Evich, M. G.; Davis, M. J.; McCord, J. P.; Acrey, B.; Awkerman, J. A.; Knappe, D. R.; Lindstrom, A. B.; Speth, T. F.; Tebes-Stevens, C.; Strynar, M. J. Per-and polyfluoroalkyl substances in the environment. Science 2022, 375 (6580), eabg9065.
(2) Zhang, M.; Zhao, Y.; Bui, B.; Tang, L.; Xue, J.; Chen, M.; Chen, W. The latest sensor detection methods for per-and polyfluoroalkyl substances. Crit. Rev. Anal. Chem. 2025, 55 (3), 542–558.
(3) Gabriunaite, I.; Valiūnienė, A.; Sabirovas, T.; Valincius, G. Mixed Silane‐based Self‐assembled Monolayers Deposited on Fluorine Doped Tin Oxide as Model System for Development of Biosensors for Toxin Detection. Electroanalysis 2021, 33 (5), 1315–1324.

Alzheimer’s Disease (AD) presents a significant personal and economic burden, yet therapeutic strategies targeting its progression have largely been unsuccessful. Key pathological features of AD include oxidative stress, dysregulation of metal ions, and the aggregation of amyloid-beta (Aβ) peptides into plaques. Previous work in the Green Lab has focused on the development of macrocyclic compounds capable of chelating transition metals such as copper and iron—both of which contribute to oxidative stress and Aβ plaque formation. These macrocycles also incorporate aromatic rings that mitigate oxidative damage by scavenging free radicals. However, while effective in addressing metal ion misregulation and oxidative stress, these compounds do not prevent Aβ aggregation. To address this limitation, we have incorporated the KLVFF peptide—known for its ability to bind Aβ and inhibit its aggregation—into our macrocyclic framework using solid-phase peptide synthesis. The resulting trifunctional molecule is designed to simultaneously chelate metal ions, reduce oxidative stress, and inhibit Aβ plaque formation. This multifunctional approach offers a promising therapeutic strategy for slowing or preventing the progression of AD into its more debilitating stages.

Over seven million people are currently living with Alzheimer’s disease (AD) in the United States today, with that number set to increase due to extended life expectancy. Studies have shown that amyloid-beta (Aβ) plaque accumulation, tau tangles in the brain, metal-ion dysregulation, and oxidative stress are etiological hallmarks of AD. Various treatment methods have been employed to reduce the effects of Alzheimer’s disease, but these treatments aim to reduce Aβ plaque aggregates after they’ve formed, though this strategy focuses on symptom mediation as opposed to prevention. A different approach focuses on preventative treatment of AD to provide an antioxidant that can minimize the effects of oxidative stress through scavenging reactive oxygen species, which are known to lead to oxidative stress. Using this approach, a class of pyridinophanes has been synthesized as antioxidants and metal ion chelators to minimize the effects of oxidative stress through biomimicry of enzymes such as superoxide dismutase. The Green Group has presented multiple pyridinophanes that function as these biomimics, including OH-PyN3. Continued improvement of the synthesis of this small molecule remains a focus, with the intent of a more cost-effective synthesis to facilitate clinical translation. Here we present an improved synthetic scheme, with optimizations to the chelidamic acid esterification and protection of the chelidamic acid and diethylenetriamine moieties. Through this synthetic scheme, the total chemical yield and reduce cost were doubled to 45% and decreased by 81%, respectively.