The central message of the review is that heteroatom functionalization is not a single trick but a family of strategies, each producing a different chemical landscape. Oxygen, nitrogen and sulphur atoms bonded to carbon alter surface polarity, acidity and basicity, hydrophilicity and electronic structure. Oxygen groups tend to enhance adsorption through hydrogen bonding, electrostatic interactions and metal-oxygen complexation. Nitrogen functionalities improve electron transfer, catalytic activity and electrochemical behaviour. Sulphur groups modify polarity and acidity and confer a distinctive affinity for certain pollutants and metal ions. Crucially, the type, distribution and stability of these groups, not merely their quantity, determine whether a material excels at capturing carbon dioxide, catalysing the oxygen reduction reaction or storing charge.

Oxygen is the most familiar of the three heteroatoms, and the review devotes detailed attention to the so-called surface oxides. Hydroxyl, carbonyl, carboxyl, lactone, quinone and ether groups can all decorate carbon surfaces, and each carries its own acidity, dipole moment and thermal fingerprint. The authors emphasize that oxidation is a progressive, multi-step process rather than a one-shot event. Under mild conditions, oxygen first appears as hydroxyl and phenolic groups at edge sites and defects. As oxidation intensifies, these species transform into carbonyls and quinones, and further oxidation of adjacent carbon atoms yields lactones, anhydrides and carboxylic acids. In other words, less oxidized groups act as intermediaries on the road to more highly oxidized functionalities, so the final surface composition reflects both the initial treatment and everything that happens afterwards.

The choice of oxidant matters enormously. Wet oxidation with strong acids such as nitric acid packs the surface with acidic carboxylic, lactonic and phenolic groups, but can corrode pore structures and introduce impurities if pushed too hard. Gentler reagents like hydrogen peroxide introduce oxygen with less structural damage, favouring hydroxyl and peroxide-related species. Potassium permanganate consumes carbon as a reductant and deposits manganese dioxide, while ammonium persulfate slowly builds carboxyl and ether groups over time. Gas-phase treatments with oxygen, ozone, carbon dioxide or steam depend strongly on temperature and exposure, generally producing more stable carbonyl and quinone species. Ozonation is comparatively surface-selective at low temperature, and plasma oxidation uses highly reactive oxygen radicals to graft functionality while preserving the bulk texture better than liquid methods. Thermal oxidation between 300 and 700 degrees Celsius adds hydroxyl, carboxyl and carbonyl groups, whereas annealing in inert nitrogen or argon strips oxygen away and shifts the surface toward basicity.

Stability is where the review delivers some of its most practical numbers, drawn from temperature-programmed desorption studies. Carboxylic groups are the fragile ones, decomposing and releasing carbon dioxide at roughly 200 to 400 degrees Celsius. Lactones and anhydrides persist to intermediate temperatures around 400 to 600 degrees, while phenols, carbonyls and quinones survive until 600 to 900 degrees, evolving carbon monoxide as they finally break down. This hierarchy means that heating a carbon material progressively rewrites its surface chemistry, removing the acidic groups first. For engineers regenerating adsorbents or cycling battery electrodes, that thermal choreography can make the difference between a material that keeps working and one that quietly loses its edge.

Nitrogen tells a different story because, unlike oxygen, it almost never appears on carbon spontaneously. It must be introduced deliberately, through nitrogen-rich precursors such as melamine, amino acids or chitosan, or by post-treatment with ammonia, urea or nitrogen plasma. The review distinguishes several bonding configurations, each with its own personality. Pyridinic nitrogen sits at the edges of aromatic structures and is associated with basicity and catalytic activity, particularly in the oxygen reduction reaction that underpins fuel cells and metal-air batteries. Pyrrolic nitrogen, embedded in five-membered rings, contributes to redox behaviour but is thermally fragile, declining between roughly 400 and 600 degrees. Graphitic or quaternary nitrogen, incorporated directly into the carbon lattice, is the most stable configuration, surviving above 800 degrees, and it enhances electrical conductivity and charge transfer. Amine and amide groups add basicity and hydrogen-bonding capacity under milder conditions.

The formation route largely dictates which nitrogen species dominate. Low-temperature approaches such as wet impregnation and hydrothermal synthesis preserve labile amine and pyrrolic species, and hydrothermal routes using natural amino acids with sugars offer a comparatively sustainable path to nitrogen-doped carbons. High-temperature ammonia treatment drives dehydrogenation and condensation reactions that favour stable pyridinic and graphitic nitrogen within the framework. Ammoxidation targets reactive edge and defect sites, while dielectric-barrier discharge plasma can weave nitrogen into aromatic surface structures without disturbing the bulk. The authors stress that performance should be interpreted through the lens of specific nitrogen configurations rather than total nitrogen content, a distinction that many earlier studies glossed over.

Sulphur, the least studied of the trio, is classified into three oxidation-state families. Reduced groups, including thiols, sulphides and disulphides, bind metals strongly and show nucleophilic and redox activity, though they oxidize readily. Aromatic thiophenic structures gain stability from delocalization and survive thermal treatment within the carbon framework. Oxidized species, from sulfoxides and sulfones to sulfonic acid groups, modulate polarity and acidity, with sulfonic acid groups standing out as strong Brønsted acid sites for catalysing esterification, alkylation and dehydration. Direct sulfonation with concentrated sulphuric acid installs these acid groups, while thermal treatment with elemental sulphur or thiourea builds more reduced thiophenic and sulphide structures. Sulfonated carbons have even found fame as solid acid catalysts for biodiesel production from sugars. In environmental contexts, sulphur-functionalized biochars show a striking affinity for mercury, cadmium and other heavy metals in water and soil, and sulphur doping can raise capacitance and cycling stability in supercapacitor electrodes.

Perhaps the most forward-looking section of the review concerns multi-heteroatom co-doping, where combinations such as nitrogen-sulphur, nitrogen-oxygen and oxygen-sulphur may generate synergistic effects unavailable to single dopants. Nitrogen-sulphur co-doping has repeatedly been linked to enhanced electrocatalytic activity for the oxygen reduction reaction, possibly because the two elements create active sites with more favourable charge distribution. Nitrogen-oxygen systems pair basic nitrogen sites with acidic oxygen functionalities to promote polarity and adsorption, while oxygen-sulphur combinations merge redox-active and strongly acidic characteristics useful for biomass conversion and metal-ion capture. Sulphur induces charge polarization and structural distortion that nitrogen alone cannot provide, hinting at electronic effects that researchers are only beginning to map systematically.

The review closes with a candid assessment of what the field still does not know. Reported stability trends vary across studies because precursor materials, treatment conditions and characterization protocols differ, making direct comparison difficult. X-ray photoelectron spectroscopy remains the workhorse for identifying O 1s, N 1s and S 2p species, complemented by X-ray absorption near-edge structure spectroscopy, infrared analysis and temperature-programmed desorption, but no single technique is definitive and peak fitting introduces subjectivity. The behaviour of functional groups under realistic operating conditions, including aqueous environments, elevated temperatures and repeated regeneration cycles, remains poorly quantified. Perveen and Slovák argue that standardized evaluation of functional group stability, combined with greener and more selective functionalization methods, will be essential for predictive material design. If the field gets this right, the humble carbon surface could be engineered as precisely as any semiconductor, with heteroatoms acting as the dopants of a new era in environmental and energy technology.

Subject of Research: Chemical functionalization of carbon surfaces with oxygen, nitrogen and sulphur groups for adsorption, catalysis and electrochemical applications

Article Title: Chemical modification of carbon surfaces with oxygen, nitrogen, and sulphur groups

Article References: Perveen, S., & Slovák, V. (2026). Chemical modification of carbon surfaces with oxygen, nitrogen, and sulphur groups. Journal of Saudi Chemical Society, 30(3), Article 38. https://doi.org/10.1007/s44442-026-00091-9

Image Credits: AI Generated

DOI: 10.1007/s44442-026-00091-9

Keywords: carbon materials, surface functionalization, oxygen functional groups, nitrogen doping, sulphur doping, heteroatoms, adsorption, catalysis, supercapacitors, temperature-programmed desorption, XPS, environmental remediation