The urgency behind this research is easy to grasp. Rising energy demand and climate change have intensified the search for clean, renewable alternatives to fossil fuels, and solar energy stands out as the most universally accessible option. The catch is storage and transport: sunlight cannot be piped or bottled, so researchers want to convert it directly into chemical energy, producing carbon-free or low-carbon fuels and even value-added chemicals such as medicines, polymers and specialty products. That conversion hinges on high-performance photocatalysts, yet most solar-to-chemical processes remain stubbornly inefficient. The review’s authors contend that single-metal-atom oxides, or SMAOs, offer a way forward because their isolated active sites are evenly dispersed across a support, promoting improved electron migration through a fully accessible, well-defined coordination environment.

The field traces its modern origins to 2011, when Zhang and colleagues synthesized a landmark catalyst featuring isolated platinum atoms distributed over iron oxide. That demonstration ignited a wave of work extending single-atom catalysts into hydrogenation and biomass conversion. A pivotal moment came when Yang and coworkers incorporated platinum atoms into titanium dioxide, examining a series of Pt-loaded catalysts and achieving a peak hydrogen production of 169.6 micromoles per hour with the optimally loaded TiO2. Since then, the vocabulary of the field has grown more precise. The review distinguishes atomically distributed base materials, single-atom catalysts, single-site heterogeneous catalysts, in which one or more atoms act as a single site with minimal interaction among themselves, and site-isolated heterogeneous catalysts, where complexes are kept apart on the support by ligands. When all individual atoms in a single-atom catalyst behave identically, it can be described as a single-site heterogeneous catalyst, and these categories differ in structure and therefore in catalytic activity.

Size matters in surprising ways. As particles shrink from the nanoscale to individual atoms, interatomic strain increases, which in turn alters the electronic and geometrical properties of the species. Those changes ripple outward, modifying how the catalyst absorbs light and how charges migrate, properties that are tightly linked to the local environment. This is why the choice of support is not a detail but a design principle: the coordination environment governs the chemical potential difference that drives charge migration between the support and the single atom. Isolated atoms typically carry a charge, a feature that can be confirmed through spectral measurements and theoretical calculations. Dopants can dramatically improve photocatalytic activity under optimal conditions, but efficiency falls off beyond optimal concentrations as surface area declines, which is precisely why researchers are working to convert metallic catalysts into single atoms rather than simply loading more metal.

The review highlights concrete synthetic routes that avoid some of the traditional pitfalls. Magnetite, Fe3O4, is prized as a catalyst and rectifying agent because its mixed valence makes it an n-type material that is non-toxic, inexpensive and easy to tune. The problem is that most syntheses of Fe3O4 from Fe2O3 photocatalysts require 150 to 500 degrees Celsius in a hydrogen atmosphere, conditions that demand high temperatures and potentially explosive gas. The team established a method for producing Fe3O4 at 180 degrees Celsius, a significant practical improvement. Their approach also draws on polyoxometalates such as the Keggin-type heteropoly acid H4[PVW11O40], which are powerful advanced oxidation agents, strong Brønsted acids and effective inorganic ion exchangers. Because heteropoly acids dissolve in water into ionic states, they can adsorb onto support surfaces and deliver single atoms to form catalysts, while reduced graphene oxide, with its high surface area and excellent conductivity, serves as an ideal scaffold.

One flagship synthesis combines TiO2 nanorods, reduced graphene oxide and the heteropoly acid with ethylene glycol and ethylenediamine. After five hours of stirring at room temperature, the mixture is sealed in a hydrothermal autoclave and heated at 180 degrees Celsius for 12 hours, then recovered by centrifugation, washed with distilled water and ethanol, and dried at 60 degrees Celsius. The resulting composite, labeled TiO2-SCu/WAO-rGO, carries single copper and tungsten atom oxides on the TiO2-rGO support. A parallel route produced an SMAO-MrGO-ED-Fe3O4 nanocomposite. In earlier work, the authors used a simple sonication technique to immobilize single-metal-atom oxides on CeO2-rGO, achieving oxygen evolution with a minimal overpotential of 283 millivolts, a result they attribute to the improved electron transport conferred by the graphene support.

Proving that atoms are truly isolated is the central analytical challenge, and the review showcases two complementary techniques. High-angle annular dark-field scanning transmission electron microscopy, or HAADF-STEM, produces contrast that scales with the square of the atomic number, so heavier elements glow brighter. In the SMAO-MrGO-ED compounds, tungsten, with atomic number 74, appears as the brightest dots, and numerous distinctly contrasting single particles were anchored on the Fe3O4 surface. Crucially, when the heteropoly acid was absent, no bright single-atom dots appeared, showing that the dispersed tungsten and vanadium originate from the dissolution of the H4[PVW11O40] precursor during the phase transition from Fe2O3 to Fe3O4. Similar imaging of TiO2-Bi2MoO6-CoW-ED-rGO revealed bismuth with the highest contrast and numerous lower-contrast dots attributed to tungsten, suggesting tungsten atoms replace Bi5+ sites, while energy-dispersive X-ray spectroscopy confirmed the incorporation of cobalt and tungsten.

X-ray absorption spectroscopy sealed the case. In extended X-ray absorption fine structure, or EXAFS, spectra, the tungsten-tungsten bond in metallic tungsten foil appears at 2.59 angstroms and the W-O bond in tungsten trioxide at 1.36 angstroms. The SMAO-MrGO-ED sample showed only a W-O peak, at 1.40 angstroms, and no W-W peak, proving there are no tungsten clusters. X-ray absorption near-edge structure spectroscopy confirmed that tungsten coordination in the sample closely resembles that in WO3, with a broad white line at 10,210 electronvolts corresponding to electron transitions from W 2p3/2 to unoccupied W 5d-O 2p orbitals. The vanadium V-O bond appeared at 1.62 angstroms, close to the 1.52-angstrom V-O distance in V2O5, with matching XANES features at 5,470 electronvolts, confirming single vanadium atom oxides as well.

Do these atomically engineered materials actually perform? The degradation data are striking. For the antibiotic ciprofloxacin, removal efficiencies ranged from 47 to 88 percent after 90 minutes across Fe2O3, Fe3O4, MrGO-ED and SMAO-MrGO-ED, with the single-atom composite leading the field. For ibuprofen, efficiencies spanned 44 to 83.12 percent after 120 minutes, again with the SMAO composite on top. Kinetic analysis gave rate constants of 0.0454 per minute for ciprofloxacin and 0.0312 per minute for ibuprofen, roughly 6.5, 4.5 and 3.0 times greater than the Fe2O3, Fe3O4 and MrGO-ED catalysts respectively. Catalyst loading matters: performance improved up to 50 milligrams per 100 milliliters, then declined as excess catalyst blocked light penetration. The pH optimum was neutral, at 7, where ciprofloxacin and ibuprofen degradation reached 98.43 and 98 percent. In acidic conditions, superoxide radicals react with protons to form hydroperoxide radicals with weaker oxidizing power, while in alkaline conditions hydroxide groups occupy active sites on Fe3O4 and hinder adsorption of organic molecules.

Related composites built on MnO2-rGO with tungsten, copper and cobalt single-atom oxides degraded sulfanilamide and methyl orange at rates of 97.38 and 98.76 percent with 30 milligrams of catalyst, and retained most of their activity over six reuse cycles, losing only 10 and 18 percent respectively. Liquid chromatography-mass spectrometry traced the sulfanilamide degradation pathway through intermediates including 2-aminobenzene-1,4-diol, benzenesulfonamide and p-benzoquinone, ultimately mineralizing the pollutant to carbon dioxide and water. On the energy side, the TiO2-SCu/WAO-rGO catalyst produced hydrogen at 14.32 millimoles per gram per hour under solar illumination, compared with 8.51 for TiO2-rGO and just 0.63 for bare TiO2, with a quantum efficiency of 33.76 percent and no loss of activity after six cycles. The review also cites a 1T-WS2 co-catalyst tripling hydrogen output of P25-TiO2 to 2,570 micromoles per gram per hour, and a Z-scheme 2H-WS2/WO3 heterostructure reaching 680 micromoles per hour per gram. The authors argue that challenges remain, including poorly understood reaction mechanisms, the need for standardized synthesis and purification, and the demand for operando studies to identify active sites. Still, with tunable optoelectronic properties, exceptional charge transport and compatibility with diverse semiconductors, single-metal-atom oxides are positioning themselves as the atom-thin frontier of clean water and solar fuel technology.

Subject of Research: Single-metal-atom oxide photocatalysts for organic pollutant degradation and photocatalytic hydrogen evolution

Article Title: Single metal atom oxides as photocatalysts: synthesis, characterization, and their role in degradation and hydrogen evolution – a mini-review

Article References: Raja, A., Lee, Y.-A., Kang, M., Selvakumar, K., & Swaminathan, M. (2025). Single metal atom oxides as photocatalysts: synthesis, characterization, and their role in degradation and hydrogen evolution – a mini-review. Advances in Industrial and Engineering Chemistry, 1(1), Article 6. https://doi.org/10.1007/s44405-025-00005-0

Image Credits: AI Generated

DOI: 10.1007/s44405-025-00005-0

Keywords: single-atom catalysts, photocatalysis, metal oxides, hydrogen evolution, water purification, reduced graphene oxide, TiO2, HAADF-STEM, EXAFS, heterogeneous catalysis, solar fuels, pollutant degradation