Developing an efficient material as a counter electrode (CE) with excellent catalytic activity, intrinsic stability, and low cost is essential for the commercial application of dye-sensitized solar cells (DSSCs). Transition metal phosphides have been demonstrated as outstanding multifunctional catalysts in a broad range of energy conversion technologies. Here, we exploited different phases of iron phosphide as CEs in DSSCs with an I^–/I₃^–-based electrolyte. Solvothermal synthesis using a triphenylphosphine precursor as a phosphorus source allows to grow a Fe₂P phase at 300 °C and a FeP phase at 350 °C. The obtained iron phosphide catalysts were coated on fluorine-doped tin oxide substrates and heat-treated at 450 °C under an inert gas atmosphere. The solar-to-current conversion efficiency of the solar cells assembled with the Fe₂P material reached 3.96 ± 0.06%, which is comparable to the device assembled with a platinum (Pt) CE. DFT calculations support the experimental observations and explain the fundamental origin behind the improved performance of Fe₂P compared to FeP. These results indicate that the Fe₂P catalyst exhibits excellent performance along with desired stability to be deployed as an efficient Pt-free alternative in DSSCs.

In this study, we report a facile one-step chemical method to synthesize reduced titanium dioxide (TiO₂) nanotube arrays (NTAs) with point defects. Treatment with NaBH₄ introduces oxygen vacancies (OVs) in the TiO₂ lattice. Chemical analysis and optical studies indicate that the OV density can be significantly increased by changing reduction time treatment, leading to higher optical transmission of the TiO₂ NTAs and retarded carrier recombination in the photoelectrochemical process. A cathodoluminescence (CL) study of reduced TiO₂ (TiO_2-x) NTAs revealed that OVs contribute significantly to the emission bands in the visible range. It was found that the TiO₂ NTAs reduced for a longer duration exhibited a higher concentration of OVs. A typical CL spectrum of TiO₂ was deconvoluted to four Gaussian components, assigned to F, F⁺, and Ti³⁺ centers. X-ray photoelectron spectroscopy measurements were used to support the change in the surface chemical bonding and electronic valence band position in TiO₂. Electron paramagnetic resonance spectra confirmed the presence of OVs in the TiO_2-x sample. The prepared TiO_2-x NTAs show an enhanced photocurrent for water splitting due to pronounced light absorption in the visible region, enhanced electrical conductivity, and improved charge transportation.

Oxygen vacancies (OVs) are one of the most critical factors that enhance the electrical and catalytic characteristics of metal oxide-based photoelectrodes. In this work, a simple procedure was applied to prepare reduced TiO₂ nanotube arrays (NTAs) (TiO_2-x) via a one-step reduction method using NaBH₄. A series of characterization techniques were used to study the structural, optical, and electronic properties of TiO_2-x NTAs. X-ray photoelectron spectroscopy confirmed the presence of defects in TiO_2-x NTAs. Photoacoustic measurements were used to estimate the electron-trap density in the NTAs. Photoelectrochemical studies show that the photocurrent density of TiO_2-x NTAs was nearly 3 times higher than that of pristine TiO₂. It was found that increasing OVs in TiO₂ affects the surface recombination centers, enhances electrical conductivity, and improves charge transport. For the first time, a TiO_2-x photoanode was used in the photoelectrochemical (PEC) degradation of a textile dye (basic blue 41, B41) and ibuprofen (IBF) pharmaceutical using in situ generated reactive chlorine species (RCS). Liquid chromatography coupled with mass spectrometry was used to study the mechanisms for the degradation of B41 and IBF. Phytotoxicity tests of B41 and IBF solutions were performed using Lepidium sativum L. to evaluate the potential acute toxicity before and after the PEC treatment. The present work provides efficient PEC degradation of the B41 dye and IBF in the presence of RCS without generating harmful products.