Sharper Crystals Turn Sunlight Into a Greener Route to a Key Bio-Based Chemical

Chemists have long dreamed of turning plant waste into the building blocks of modern industry, and a new study brings that dream a step closer to reality. A research team led by Yun Ju of the Nantong Institute of Technology, working with collaborators including Muhammad Tayyab of King Fahd University of Petroleum and Minerals, has shown that simply improving the crystalline quality of a light-absorbing semiconductor can more than double its ability to convert a biomass-derived molecule into a valuable industrial chemical using nothing but visible light. The work, published in Catalysis Letters, demonstrates that crystallinity is not a secondary detail of catalyst design but a powerful, independently tunable lever for controlling photocatalytic performance.

The molecule at the heart of the study is 5-hydroxymethylfurfural, commonly abbreviated as HMF. HMF is a platform chemical derived from lignocellulosic biomass, the woody, fibrous material that plants are largely made of and that is often treated as agricultural waste. When one of HMF’s alcohol groups is selectively oxidized into an aldehyde, the result is 2,5-diformylfuran, or DFF, a compound prized for making polymers, pharmaceuticals, furan-based resins, and a range of fine chemicals. Conventional routes to DFF typically rely on stoichiometric oxidants or energy-intensive thermal processes. Photocatalysis offers an alternative: harvest sunlight, generate energetic charge carriers inside a semiconductor, and use those carriers to drive the oxidation under mild conditions with oxygen as the ultimate oxidant.

The catalyst chosen for this task was cadmium indium sulfide, CdIn2S4, a ternary metal sulfide with a well-earned reputation in the photocatalysis community. Its band gap sits squarely in the visible region of the solar spectrum, meaning it can absorb the abundant visible photons that make up the bulk of sunlight rather than only the small ultraviolet fraction. That property makes it attractive for solar-driven chemistry. But CdIn2S4 has an Achilles heel. When synthesized by conventional routes, it tends to form with poor crystallinity, and the disordered, defect-riddled lattice acts like a trap for the very charge carriers the material is supposed to deliver. Photogenerated electrons and holes recombine rapidly, releasing their energy as heat or light before they can reach the surface and do useful chemistry. The result is a catalyst that absorbs light beautifully but converts little of it into product.

The team’s solution was elegantly simple: crystallinity engineering. Using a hydrothermal synthesis method, in which the material crystallizes from a hot aqueous solution inside a sealed vessel, the researchers tuned the reaction conditions to produce highly crystalline CdIn2S4 with a striking octahedral morphology, which they designated CISH. As a control, they also prepared a low-crystallinity counterpart, CISL, using conditions that yielded the same chemical compound but with a more disordered lattice. Because both materials share the same composition, the comparison isolates crystallinity as the single variable, allowing the researchers to attribute any difference in performance directly to the quality of the crystal lattice rather than to changes in chemistry, composition, or light absorption.

The performance gap was dramatic. Under visible-light irradiation, the highly crystalline octahedral catalyst produced DFF at a rate of 2.56 millimoles per gram of catalyst per hour, approximately 2.3 times the rate achieved by the low-crystallinity material. Equally important, both catalysts maintained selectivity for DFF above 90 percent. Selectivity is the quiet hero of selective oxidation chemistry: HMF contains multiple oxidizable functional groups, and an unselective catalyst will chew through the molecule to produce a mixture of byproducts, including over-oxidized species that destroy the value of the feedstock. Holding selectivity above 90 percent while simultaneously boosting the conversion rate means the improved catalyst is not just faster but genuinely better at the specific transformation the chemists wanted.

To understand why the better crystal wins, the team turned to a battery of photoelectrochemical measurements. Electrochemical impedance spectroscopy revealed that the highly crystalline material has significantly reduced charge-transfer impedance, meaning electrons can move through it and across its surface with less resistance. Linear sweep voltammetry and transient photocurrent measurements showed stronger and more durable photocurrents, consistent with more charge carriers surviving long enough to be collected. Together, the data painted a coherent physical picture: a well-ordered lattice contains fewer defects to act as recombination centers, so photogenerated electrons and holes separate more efficiently, migrate to the surface more quickly, and live longer before they are lost. In photocatalysis, longer-lived carriers mean a greater chance that an electron will reduce an oxygen molecule or that a hole will oxidize an alcohol group before the pair annihilates each other.

Identifying which of those carriers actually does the chemical work was the next question. The researchers performed radical trapping experiments, adding selective scavengers that quench specific reactive species and observing how the reaction responds. When superoxide radicals were scavenged, the reaction slowed sharply; when holes were captured, the same happened. Electron paramagnetic resonance spectroscopy, using a spin-trapping agent, provided direct spectroscopic confirmation that superoxide radical anions are generated on the illuminated catalyst surface. The combined evidence pointed to a two-pronged mechanism in which superoxide radicals, formed when photogenerated electrons reduce dissolved oxygen, and photogenerated holes, left behind in the valence band, act together as the primary reactive species driving the oxidation.

The mechanistic study went a step further and traced the pathway by which HMF becomes DFF. The evidence supports a carbon-centered radical intermediate route: a photogenerated hole abstracts an electron from the alcohol group of HMF, generating a carbon-centered radical on the substrate, which then undergoes further oxidation and deprotonation steps to yield the aldehyde. This level of mechanistic detail matters because it tells future catalyst designers exactly which elementary steps need to be accelerated and which reactive species must be stabilized. A catalyst that excels at producing long-lived holes and efficient oxygen reduction, the study suggests, is precisely the kind of catalyst that will excel at this class of alcohol-to-aldehyde transformations.

Beyond the specific numbers, the study carries a broader design lesson for the field of solar fuels and biomass valorization. Much of the recent effort in photocatalysis has focused on constructing heterojunctions, decorating catalysts with cocatalysts, or introducing vacancies and dopants to tune electronic structure. Those strategies remain powerful, but this work makes the case that crystallinity deserves equal attention as an independent design variable, particularly for ternary metal sulfides, whose synthesis often leaves them defect-rich. The comparison between CISH and CISL shows that two chemically identical materials can differ by more than a factor of two in productivity purely because of how well their atoms are ordered. For a field racing to make solar-driven chemistry competitive with fossil-derived processes, that is a reminder that sometimes the most impactful engineering happens at the level of the crystal itself.

The implications extend to the industrial vision of a biorefinery in which plant waste flows in and high-value chemicals flow out, powered by sunlight. DFF is a gateway to polymers and pharmaceutical intermediates that today are made largely from petrochemical feedstocks, and HMF is available in principle from abundant cellulose and hemicellulose. A photocatalytic route that runs at ambient conditions, uses oxygen as the oxidant, and achieves high rates with high selectivity could dramatically shrink the energy and waste footprint of that chemistry. Challenges remain before such catalysts see the inside of a commercial reactor, including long-term stability, catalyst cost, and scaling hydrothermal synthesis, but the study provides a clear and quantified target: better crystals, longer-lived charges, faster chemistry. In the quiet ordering of atoms within an octahedral particle, the researchers have found a surprisingly potent recipe for making sunlight do useful work on plant-derived molecules.

Subject of Research: Crystallinity engineering of CdIn2S4 octahedral photocatalysts for visible-light selective oxidation of 5-hydroxymethylfurfural to 2,5-diformylfuran

Article Title: Enhancing Visible-Light Photocatalytic Selective Oxidation of 5-Hydroxymethylfurfural to 2,5-Diformylfuran via Crystallinity Engineering of CdIn2S4 Octahedra

Article References: Ju, Y., Wang, F., Chen, T., Ge, C., Wang, Y., & Tayyab, M. (2026). Enhancing Visible-Light Photocatalytic Selective Oxidation of 5-Hydroxymethylfurfural to 2,5-Diformylfuran via Crystallinity Engineering of CdIn2S4 Octahedra. Catalysis Letters, 156(11), Article 304. https://doi.org/10.1007/s10562-026-05546-7

Image Credits: AI Generated

DOI: 10.1007/s10562-026-05546-7

Keywords: photocatalysis, CdIn2S4, crystallinity, 5-hydroxymethylfurfural, 2,5-diformylfuran, biomass valorization, selective oxidation, charge separation, superoxide radicals, visible light, ternary metal sulfides, hydrothermal synthesis