Epitaxial Integration of Carbon-Rich Carbon Nitride on TiO₂ Nanorods for Efficient Solar Water Splitting

The ability to harness sunlight—a virtually limitless resource—to split water molecules into hydrogen and oxygen represents a pivotal technology in the transition away from fossil fuels. Central to this endeavor is the discovery and engineering of photocatalytic materials that can efficiently absorb sunlight, generate charge carriers, and drive chemical reactions without succumbing to degradation. Although semiconductor materials such as titanium dioxide (TiO₂) have long served as the bedrock of PEC systems, their large band gaps restrict them primarily to the ultraviolet region, capturing only a fraction of the solar spectrum. Polymeric graphitic carbon nitride (g-C₃N₄) emerged over the past decade as a tantalizing candidate to overcome these limitations. Its earth-abundant composition, photochemical stability, and moderate band gap have drawn considerable attention. Yet, the promise of g-C₃N₄ remains only partially fulfilled. Its relatively wide band gap (~2.7 eV) still precludes absorption of a significant portion of visible light, severely limiting its practical solar-to-hydrogen conversion efficiency. Beyond spectral limitations, conventional g-C₃N₄ suffers from poor charge separation, leading to rapid recombination of photogenerated electrons and holes, thereby suppressing photocatalytic performance. Furthermore, efforts to fabricate uniform, thin films of carbon nitride have encountered persistent difficulties. Traditional deposition techniques often produce discontinuous, mechanically fragile films with poor adhesion to substrates, undermining device stability and electron transport across interfaces.

New research paper published in Journal of the American Chemical Society and led by Professor Peter Müller-Buschbaum from the Technical University of Munich and  Professor Karthik Shankar from the University of Alberta, researchers developed a new in situ heteroepitaxial growth method—carefully engineering carbon-rich carbon nitride films directly on rutile TiO₂ nanorod arrays—they sought to forge a new paradigm. Their strategy aimed not only to narrow the band gap and extend visible light absorption but also to fundamentally enhance interfacial quality, promoting efficient charge separation and mechanical stability. To transform their vision into reality, the researchers first grew dense, vertically aligned rutile TiO₂ nanorod arrays on conductive substrates through hydrothermal synthesis, providing an ordered and crystalline scaffold. They then introduced a specially designed carbon-rich precursor solution directly onto these nanorods, performing a controlled in situ polymerization at high temperatures. Unlike conventional methods that often yield patchy and fragile films, their process produced a remarkably uniform, conformal coating of narrow band gap carbon-rich carbon nitride (NBG-CRCN), just 9 nanometers thick, intimately hugging the nanorod surfaces without cracks or pinholes.

High-resolution transmission electron microscopy (HRTEM) vividly confirmed that the carbon nitride layer grew epitaxially, its (002) planes neatly aligning with the (110) planes of rutile TiO₂. This crystalline coherence suggested strong chemical bonding and minimal interfacial defects, a critical factor for efficient charge transport. Further support came from grazing-incidence wide-angle X-ray scattering (GIWAXS) and selected area electron diffraction (SAED) analyses, both of which revealed clear signatures of heteroepitaxial growth, reinforcing the success of the structural engineering strategy. Chemical composition was another frontier the team rigorously explored. CHNS elemental analysis revealed a substantial increase in carbon content in the NBG-CRCN films compared to conventional g-C₃N₄, while solid-state ¹³C and ¹⁵N NMR spectra confirmed the preservation of the essential heptazine framework despite the extra carbon incorporation. X-ray photoelectron spectroscopy unveiled subtle shifts in binding energies, signaling robust interfacial interactions between carbon nitride and titania, possibly mediated by minor oxygen or carbon intermixing at the boundary.

Optoelectronic characterization painted an equally compelling picture. Diffuse reflectance UV-Vis spectroscopy showed that the NBG-CRCN@TNR hybrids extended light absorption well into the visible range, reaching wavelengths up to 700 nm. Photoluminescence measurements, both steady-state and time-resolved, indicated significant quenching and reduced carrier lifetimes compared to unmodified structures, clear evidence of more efficient charge separation and suppressed recombination.

Finally, when tested under simulated sunlight for photoelectrochemical water splitting, the NBG-CRCN@TNR system delivered outstanding performance. It generated photocurrent densities as high as 4.3 mA cm⁻² at 0.6 V bias, nearly doubling in the presence of methanol as a hole scavenger, and achieved a hydrogen evolution rate of 26.51 μmol h⁻¹ with impressive Faradaic efficiency.

The significance of the research work of Professor Peter Müller-Buschbaum and  Professor Karthik Shankar demonstrated true heteroepitaxial growth of a narrow band gap, carbon-rich carbon nitride onto rutile TiO₂ nanorods, the researchers have shown that it is possible to overcome the persistent challenges that have plagued carbon nitride systems for over a decade. This delicate interfacial engineering ensures that photogenerated electrons and holes no longer face insurmountable recombination barriers, but instead are swiftly and efficiently separated, dramatically enhancing solar-to-hydrogen conversion. More importantly, the work proves that low-cost, scalable techniques can still achieve precise nanoscale control — a combination that makes practical deployment of PEC water-splitting technologies much more plausible. It shifts the focus from merely adjusting bulk properties to meticulously crafting material interfaces, a conceptual leap that will ripple through the broader fields of photocatalysis, photovoltaics, and artificial photosynthesis. The implications extend well beyond the confines of this specific TiO₂-carbon nitride system. The methodology they developed — in situ polymerization coupled with lattice matching — can serve as a blueprint for constructing other hybrid semiconductor architectures where charge separation and mechanical stability are paramount. It opens the door to designing new generations of heterostructures where organic and inorganic materials cooperate at the atomic level rather than coexist uneasily. In the urgent global pursuit of sustainable hydrogen production, such advances offer a pathway that is not only scientifically elegant but also economically viable. This study reminds the research community that solving large-scale energy problems demands more than marginal improvements; it requires rethinking the very fabric of material systems, from the ground up.