RSC Applied Interfaces publishes interdisciplinary work with an applied focus, which can be read for free here. To celebrate the excellent articles that have been published so far in our journal, we asked some of our authors to discuss their work in more detail.
In this post, we hear from Soumantika Jana, Debika Gogoi, Neha Patel, Bo-Yan Feng, Jeffrey Chi-Sheng Wu and Rakesh Kumar Sharma as they discuss their recently published article entitled ‘Bifunctional NaGdF4:Yb3+/Er3+ upconversion nanoparticles for efficient overall water splitting‘.
Insights from the authors
Background & Motivation
The global push for clean, sustainable energy has intensified research into green hydrogen production. Water splitting, the decomposition of water into hydrogen and oxygen, is a leading pathway for green hydrogen production. However, the oxygen evolution reaction (OER) component involves a complex four-electron transfer mechanism and sluggish kinetics, making it the primary bottleneck. Additionally, the near-infrared (NIR) region accounts for approximately 43% of the solar spectrum yet remains severely underutilised by most conventional photocatalysts.
Upconversion nanoparticles (UCNPs) offer a compelling solution: they absorb multiple low-energy NIR photons and re-emit higher-energy visible or UV photons, effectively unlocking this underutilised solar resource. Among UCNP host materials, β-NaGdF4 stands out for its robust photochemical stability and low phonon energy, ideal properties for co-doping with sensitiser-activator lanthanide pairs such as Yb3+/Er3+. While NaGdF4-based UCNPs have been explored in hybrid photocatalytic systems for dye degradation and selective photoreduction, their dual role as both electrocatalyst and photoelectrocatalyst for complete water splitting has not been demonstrated. This work addresses that gap.
Synthesis
The β-NaGdF4:Yb3+/Er3+ nanoparticles were prepared using a modified single-step solvothermal method. Gadolinium, ytterbium, and erbium nitrate precursors were dissolved in deionised water and combined with a citric acid solution as a capping agent, followed by the addition of sodium fluoride. The mixture was subjected to solvothermal treatment at 200°C for 2 hours in a Teflon-lined autoclave, yielding phase-pure, highly crystalline hexagonal nanocrystals after centrifugation, washing, and drying.
Materials Characterisation
Crystal Structure: XRD confirmed exclusive formation of the hexagonal β-phase NaGdF4 (JCPDS 27-0699), with no traces of cubic α-phase. HRTEM revealed clear lattice fringes with d-spacings of 0.203 nm (201), 0.228 nm (111), and 0.284 nm (101), consistent with XRD data.
Morphology: FESEM and TEM imaging showed uniform hexagonal nanoparticles with well-defined geometry and sharp edges. The mean particle size was 42.5 nm (from 70 measurements). The SAED pattern confirmed the polycrystalline nature of the sample.
Elemental Composition: XPS confirmed the presence of Na, Gd, F, Yb, and Er in their expected oxidation states. EDS mapping corroborated these findings. Raman spectroscopy identified five characteristic peaks at 245, 300, 353, 486, and 609 cm-1 attributable to lattice vibrational modes and Na-F bond vibrations of β-NaGdF4.
Photoluminescence: Under 980 nm NIR excitation, the UCNPs produced characteristic green (485 nm, 498 nm) and red (652 nm) upconversion emissions from Er3+, corresponding to the 4H11/2 → 4I15/2, 4S3/2 → 4I15/2, and 4F9/2 → 4I15/2 transitions. The Yb3+ ions serve as sensitisers, absorbing 980 nm photons and transferring energy non-radiatively to neighbouring Er3+ activator ions via a two-photon excitation pathway.
Electrochemical & Photoelectrochemical Performance
All measurements were conducted in a standard three-electrode configuration using 1 M KOH electrolyte. Ni foam served as the working electrode substrate. Ag/AgCl as reference and Pt wire as counter electrode were used. Photoelectrochemical tests were run under AM 1.5G simulated solar irradiation.
Key performance metrics
Active Surface Area & Charge-Transfer Properties
The UCNP electrode demonstrated excellent electrochemical accessibility, reflecting an abundance of active catalytic sites:
- HER: ECSA = 4 cm2 (Cdl = 0.16 mF cm-2); Roughness Factor = 16
- OER: ECSA = 17.5 cm2 (Cdl = 0.7 mF cm-2); Roughness Factor = 70
Upon illumination, charge-transfer resistance dropped further to 2.12 Ω (HER) and 9.2 Ω (OER), confirming that light-induced carrier generation enhances both charge mobility and ion transport.
Role of Upconversion in Enhanced PEC Activity
The pronounced improvement in photoelectrocatalytic performance is directly linked to the photon upconversion capability of the Yb3+/Er3+ dopant pair. Under NIR-inclusive solar irradiation, Yb3+ sensitiser ions absorb 980 nm photons and transfer energy non-radiatively to Er3+ activator ions, which emit in the green and red visible range. This upconverted emission effectively raises the photon energy available at the catalyst surface, promoting charge carrier excitation and separation.
A comparative study with undoped NaGdF4, synthesised under identical conditions, confirmed that the undoped sample showed substantially lower electrochemical and photoelectrochemical activity. This validates that the enhanced performance arises from the combined effects of NIR-assisted light harvesting and the redox-active sites introduced by the Yb3+/Er3+ ionic pair, rather than from the fluoride host lattice alone.
Mechanistic Insight: Successive light ON/OFF cycling experiments confirmed stable and reproducible photocurrent generation, consistent with efficient and sustained charge carrier formation and separation under illuminated conditions.
Scientific Significance
This work represents the first demonstration of NaGdF4:Yb3+/Er3+ UCNPs functioning as a bifunctional catalyst for both electrochemical and photoelectrochemical overall water splitting. Key contributions include:
- Establishes compositionally simple lanthanide fluoride UCNPs as a viable dual-function platform, distinct from conventional semiconductor-coupled hybrid systems
- Demonstrates full-spectrum solar energy utilisation by harnessing the underexplored NIR region (≃43% of solar energy) through photon upconversion
- Provides a cost-effective, noble-metal-free alternative that matches performance benchmarks reported for state-of-the-art electrolysers in literature
- Opens new design pathways for multifunctional UCNP-based catalysts in solar-driven hydrogen production
Novel Claim: To the best of the authors’ knowledge, this is the first report of NaGdF4:Yb3+/Er3+ UCNPs serving as a bifunctional catalyst for complete electrochemical and photoelectrochemical water splitting.
Future Directions
Future research will focus on coupling these UCNPs with complementary visible-light-active semiconductors to form heterojunction architectures, and on surface chemistry optimisation, including defect engineering and co-catalyst deposition, to further maximise solar energy conversion efficiency and hydrogen generation rates.
Affiliations & Funding
Sustainable Materials and Catalysis Research Lab, IIT Jodhpur, Rajasthan, India; Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan
Funding: INDO-TAIWAN Research Grant, India (Grant No. 2024/IN-TW/04)
Meet the authors
 Soumantika Jana |
Soumantika Jana is currently pursuing her Master’s degree in the Department of Materials Engineering, Indian Institute of Technology Jodhpur. She completed her M.Sc. degree in Chemistry from the Indian Institute of Technology Jodhpur. She completed her bachelor’s degree in Chemistry from Vidyasagar University, West Bengal. Her research work focuses on the development of upconversion nanomaterials for sustainable hydrogen production and advanced energy applications. |
 Debika Gogoi |
Debika Gogoi is currently working as a Postdoctoral Research Associate in the Department of Chemistry at the Indian Institute of Technology Jodhpur under the Indo-Taiwan collaborative research project. She completed her Master’s degree in Chemistry from Assam University in 2018 and obtained her Ph.D. in Chemistry from Birla Institute of Technology and Science, Pilani – Goa Campus in 2023. Her research focuses on the development of functional nanomaterials for sustainable energy applications, particularly in electrocatalytic oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and electrochemical energy storage systems. |
 Neha Patel |
Neha Patel is currently pursuing her Ph.D. degree in the Department of Chemistry at the Indian Institute of Technology Jodhpur. She completed her Master’s degree in Chemistry from Sarojini Naidu Government Post-graduation College, Bhopal, in 2020. Her work involves the design and fabrication of advanced materials for sustainable and efficient electrochemical energy technologies. |
 Bo-Yan Feng |
Bo-Yan Feng received his bachelor’s degree in Chemical Engineering from National Taiwan University. He is currently pursuing his master’s degree in the Department of Chemical Engineering at National Taiwan University. His research focuses on photocatalysis for dye degradation and environmental remediation applications. |
 Jeffrey Chi-Sheng Wu |
Jeffrey Chi-Sheng Wu is a distinguished professor and former chairman of the Chemical Engineering Department at National Taiwan University. He received his PhD. degree in Chemical Engineering from the University of Pittsburgh, USA. Professor Jeffrey C.S. Wu has long-term research activity in solar energy conversion. His research interests include (a) photoreduction of CO2 to fuel, (b) photocatalytic water splitting for H2, and (c) photocatalytic oxidation of air pollutants. (d) Catalytic hydrogenation of CO2. He is a fellow of the Taiwan Institute of Chemical Engineers since 2022. Prof. Wu received many awards including, the FutureTech Award of the National Science and Technology Council, Taiwan in 2024 and 2025; the Qin-Lang Chair Professor of NTU Engineering College in 2022, Outstanding Paper of Taiwan Catalysis Society in 2020, Yang Bing Yan Chair Professor of NTU Engineering College in 2018, Outstanding Engineering Professors Award of the Chinese Institute of Engineers Taiwan in 2016; Outstanding Cross-Sector Collaboration Award of 2nd National Industrial Innovation in 2012; “Lai Tzai-Der award” of Taiwan Institute of Chemical Engineers in 2009, “Chemical Technology Award” of Taiwan Institute of Chemical Engineers in 2006, and “Silver medal of National Invention,” Taiwan in 2004. He also serves as a member of the editorial boards in several SCI journals. He is the author and co-author of over 160 SCI journal papers. His h-index is 57 (Jan. 2026) with a total of citations > 11000. |
 Rakesh K Sharma |
Rakesh K Sharma, FRSC, is a Professor at the Department of Chemistry at IIT Jodhpur, India. He received his BSc and MSc from the University of Rajasthan, Jaipur, and Ph.D. from the Indian Institute of Science, Bangalore, in 2008. He worked as a postdoctoral researcher at the Ohio State University in Columbus, USA, from 2007 to 2010. He has 13 patents and has transferred 6 technologies in biofuels, energy storage, environmental technologies, and automotive applications. He has published over 140 articles in peer-reviewed journals. He has also published fifteen books/book chapters. His research interests include catalysis for biofuels and fine chemicals, asymmetric catalysis and catalysts for environmental remediation, and advanced materials for energy generation and storage. |
Bifunctional NaGdF4:Yb3+/Er3+ upconversion nanoparticles for efficient overall water splitting
Soumantika Jana, Debika Gogoi, Neha Patel, Bo-Yan Feng, Jeffrey C. S. Wu and Rakesh K. Sharma
RSC Appl. Interfaces, 2026, 3, 799-806. DOI: 10.1039/D5LF00409H
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RSC Applied Interfaces is a dedicated, interdisciplinary reference journal for cutting-edge research on the applications of surfaces and interfaces. In addition to the applied focus, work considered for publication in RSC Applied Interfaces is expected to be highly original and of top quality. The journal seeks to report major scientific advances beyond the state of the art, at the cutting edge of this interdisciplinary field. |