One-Pot Mixed-Phase Indium Sulfide Outperforms Single-Phase Catalysts in Dye Breakdown

A team of chemists in Kerala, India, has shown that a deliberately imperfect semiconductor can beat its more orderly cousins at cleaning contaminated water. In a study published in Applied Nanoscience, Nisha Chandran, Rani Abraham and R. Jayakrishnan report a simple, low-temperature wet chemical route that produces mixed-phase indium sulfide nanopowder containing two distinct crystalline compounds at once: tetragonal indium(III) sulfide, In2S3, and monoclinic In6S7. When tested against structurally related organic dyes, this dual-phase material degraded more of the pollutants than single-phase indium sulfide samples prepared at lower bath temperatures, suggesting that controlled phase mixing may be an underexplored lever in the design of photocatalysts for environmental remediation.

The synthesis itself is strikingly economical. The researchers combined a minimal set of reagents in a single pot at just 100 degrees Celsius, avoiding the high-temperature furnaces, vacuum systems and template chemicals that often accompany the production of engineered chalcogenide nanomaterials. The team describes the route as green, and the modest thermal budget matters for more than convenience: the temperature at which the reaction bath is held appears to determine which crystalline phases nucleate, and therefore how the final powder behaves under illumination. By tuning that single parameter, the group could steer the material between single-phase and mixed-phase outcomes.

Characterization confirmed that the one-pot product was genuinely a composite of two sulfide phases rather than a contaminated single phase. X-ray diffraction revealed the coexistence of tetragonal In2S3 and monoclinic In6S7, with an average crystallite size of 70.98 nanometers. Scanning electron microscopy showed a microcrystalline morphology, while X-ray photoelectron spectroscopy ruled out the presence of indium-oxygen bonds, excluding In(OH)3 impurities that can complicate the interpretation of photocatalytic results in sulfide systems. That spectroscopic check is important because hydroxide contamination is a common artifact in aqueous synthesis and can mask or mimic catalytic activity.

Perhaps the most consequential structural finding came from gas adsorption measurements. Brunauer-Emmett-Teller analysis showed that the mixed-phase powder is mesoporous, with a mean pore size of 3.39 nanometers and a specific surface area of 45.59 square meters per gram. Porosity at this scale gives dye molecules in solution access to a large internal reaction surface, and the authors link this mesoporous architecture, together with an optimal optical band gap, to the material’s strong performance as a photocatalyst. In photocatalysis, surface area and light absorption must work in tandem; a material that absorbs light well but offers little accessible surface, or vice versa, wastes part of its potential.

To test performance, the team chose two structurally related xanthene dyes, Rhodamine B and Sulforhodamine B, both of which serve as common model pollutants and, in the case of Sulforhodamine B, as a compound of environmental concern. Comparative experiments against single-phase indium sulfide samples synthesized at bath temperatures of 30 and 70 degrees Celsius produced a clear hierarchy. The lowest-temperature sample managed only about 14.3 percent degradation, the 70-degree sample reached 63.1 percent, and the mixed-phase material prepared at 100 degrees achieved 66.5 percent. The trend suggests that both crystallite quality and phase composition improve as the synthesis temperature rises, with the mixed-phase sample capturing the best of both.

Understanding why the mixed-phase material wins required probing the chemistry happening on its surface under light. When a photocatalyst absorbs a photon, it generates electron-hole pairs that can react with oxygen and water to produce reactive oxygen species, the chemical agents that actually tear dye molecules apart. Chandran and colleagues ran scavenging experiments, adding chemicals that selectively neutralize particular radical species, to determine which agents dominate the degradation of each dye. The results revealed that the two dyes, despite their structural similarity, are broken down by different mechanisms depending on the light source used.

Under halogen irradiation, the degradation of Sulforhodamine B was governed primarily by superoxide radical anions, O2 radical anions, which the scavenging data identified as controlling the rate-determining step of the reaction. Under natural sunlight, the degradation of Rhodamine B drew on a broader toolkit: both superoxide radical anions and hydroxyl radicals contributed significantly to the breakdown process. This mechanistic split is more than an academic curiosity. It indicates that the mixed-phase catalyst can operate through different oxidative pathways depending on the illumination available, which is directly relevant to real-world deployment where sunlight, not laboratory lamps, is the practical light source.

The work builds on a line of earlier research from the same group, which had previously examined how pore size influences the photocatalytic breakdown of Sulforhodamine B in mesoporous In2S3 and developed template-free aqueous routes to microporous indium sulfide for water purification. It also fits into a broader international effort to exploit indium sulfide nanostructures, from sulfur-vacancy nanotubes for nitrogen fixation to beta-In2S3 quantum dots and nanoflowers for antibiotic and dye degradation. What distinguishes the new study is its focus on phase coexistence as a design variable rather than a defect to be eliminated, and its demonstration that a single, inexpensive synthesis step can deliver that coexistence reproducibly.

The practical implications are considerable. Textile and dyeing industries discharge large volumes of colored, chemically persistent wastewater, and conventional treatment plants are often poorly equipped to destroy these molecules rather than simply transfer them to a sludge phase. Photocatalysts that can run on sunlight offer a route to genuine mineralization of such pollutants. A material that can be synthesized at 100 degrees Celsius with minimal reagents, no templates and no oxygen contamination lowers the cost barrier substantially compared with routes that demand high-temperature annealing or elaborate precursor chemistry. The Kerala team’s work was supported by the Directorate of Environment and Climate Change, Government of Kerala, reflecting regional interest in applied water-treatment technology.

There remain, of course, the usual caveats that separate a laboratory result from a deployed technology. The reported efficiencies, while clearly superior to the single-phase controls, were measured on model dye solutions under defined illumination, and questions of catalyst recovery, recyclability over many cycles, and performance in real effluents will need attention before mixed-phase indium sulfide powders see industrial use. Indium is also a relatively scarce element, which argues for efficient material use and eventual recovery strategies. Nevertheless, the study makes a persuasive case that in semiconductor photocatalysis, the phase diagram itself is a tool. By holding a reaction bath at the right temperature, the researchers coaxed two sulfide phases into coexisting within one powder, and that coexistence, paired with mesoporosity and a favorable band gap, translated into measurably better pollutant destruction. For a field searching for greener, cheaper catalysts, the message is that sometimes the most productive thing to do with a pure phase is to share the crystal lattice with another one.

Subject of Research: Green synthesis of mixed-phase indium sulfide nanostructures for photocatalytic dye degradation

Article Title: Mixed-phase indium sulfide via one-pot synthesis: superior dye degradation over single-phase

Article References: Chandran, N., Abraham, R., & Jayakrishnan, R. (2026). Mixed-phase indium sulfide via one-pot synthesis: superior dye degradation over single-phase. Applied Nanoscience, 16(5), Article 56. https://doi.org/10.1007/s13204-026-03190-6

Image Credits: AI Generated

DOI: 10.1007/s13204-026-03190-6

Keywords: indium sulfide, mixed-phase nanomaterials, photocatalysis, dye degradation, mesoporous materials, superoxide radical, hydroxyl radicals, green synthesis, water purification, In2S3, In6S7, reactive oxygen species