Carbon nitride (CN) materials are promising, cheap, and benign semiconductors for photoelectrochemical (PEC) cells due to their stability and suitable energy band edges for water splitting. However, their PEC performance is limited by moderate light harvesting, poor charge separation, and a limited variety of growth methods and monomers for continuous CN layers with intimate substrate contact. Existing methods, such as doctor-blade, thermal vapor condensation, solvothermal, micro-contact printing, and liquid-based methods, often restrict monomer choice or have scalability issues. This lack of versatile growth methods hinders the development of CN materials with enhanced optical and electronic properties crucial for improved PEC performance. Most CN materials show strong light response only up to ~420 nm due to their wide bandgap, limiting solar spectrum harvesting. Therefore, new methods are needed to synthesize CN films with extended visible light absorption and improved charge separation to enhance PEC activity.

The literature extensively documents the challenges and progress in developing efficient carbon nitride photoelectrodes for water splitting. Studies have explored various synthetic routes, including doctor blading, vapor deposition, and solvothermal methods, each with its advantages and limitations in terms of film uniformity, substrate compatibility, and scalability. Several papers highlight the need for improved light absorption in the visible region and efficient charge separation to enhance photoelectrochemical performance. The authors reference works demonstrating the use of different monomers and surface modifications to improve the performance of CN-based PEC cells. The review emphasizes the lack of a universally applicable method for growing high-quality, uniform CN films on various substrates, motivating the present research.

This study introduces a simple and general method to grow porous CN films with extended optical absorption and improved charge separation. The method involves the direct and fast growth of monomers from saturated solutions onto various substrates, followed by high-temperature calcination. Melamine vapor is introduced during growth to enhance charge separation and substrate contact. Thiourea is used as the initial precursor to synthesize the CN_TM film. The process begins by immersing a clean FTO-coated glass substrate into a hot (70 °C) saturated thiourea aqueous solution for 1 second. This is repeated to adjust the film thickness. The resulting thiourea film is then calcined at 500 °C for 2 hours under nitrogen. The resulting CN_TM film is characterized using various techniques including scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, UV-Vis spectroscopy, photoluminescence (PL) spectroscopy, and X-ray photoelectron spectroscopy (XPS). The PEC performance is evaluated in a three-electrode configuration under one-sun illumination. The effects of film thickness and calcination temperature are investigated. The hole extraction efficiency is determined by adding triethanolamine (TEOA), a hole scavenger, to the electrolyte. The method's generality is demonstrated by using other monomers such as urea, dicyandiamide (DCDA), ammonium thiocyanate (NH₄SCN), and guanidine carbonate. For comparison, CN films are also synthesized using other methods including direct calcination of thiourea powder, drop casting, and doctor blading. Finally, melamine powder is introduced during calcination to modify the surface of the CN film (CN_TM), further enhancing its PEC performance. The resulting CN_TM film is characterized using the same techniques as before, along with gas chromatography and electrochemical impedance spectroscopy (EIS). Adhesion tests using ultrasonication and adhesive tape are performed to assess the film's adhesion to the substrate. The overall PEC performance is then compared to literature values and the charge transfer efficiency is estimated.

The new synthesis method produces highly uniform and porous CN films on various substrates, including FTO, carbon paper, TiO₂-coated electrodes, and glass slides. The film thickness is easily controlled by adjusting the number of dip-dry cycles. The CN_TM film (using thiourea as a precursor) shows a benchmark photocurrent density of 353 µA cm⁻² at 1.23 V vs. RHE in 0.1 M KOH under one-sun illumination. This represents a significant improvement over previous results. The onset potential is low at 0.32 V vs. RHE, indicating high efficiency. Gas evolution rates of 1.88 µmol h⁻¹ cm⁻² for H₂ and 0.91 µmol h⁻¹ cm⁻² for O₂ are detected, with a near 2:1 molar ratio indicating efficient water splitting. The IPCE value is 18% at 400 nm and above 12% at 450 nm, extending the photoresponse to 600 nm. The hole extraction efficiency is as high as 62% with the addition of TEOA. Using different precursors, such as urea and dicyandiamide, also produces uniform CN films with good PEC performance. The method is simple, scalable, and versatile. Surface modification with melamine further improves the PEC performance, likely due to improved charge separation and better contact with the substrate. Adhesion tests confirm the strong adhesion of the CN_TM films to the substrate. The CN_TM photoanode demonstrates good stability, retaining approximately 50% of its initial photocurrent after 1 hour of continuous operation in 0.1 M KOH. The performance is also good in neutral conditions (0.1 M Na₂SO₄), with higher gas evolution rates and better stability than in alkaline conditions.

The results demonstrate a significant advancement in the synthesis of carbon nitride photoanodes for water splitting. The simple, scalable, and versatile synthesis method allows for the production of high-quality films with enhanced optical and electronic properties. The improved light absorption and charge separation lead to superior PEC performance compared to previously reported CN materials. The incorporation of melamine during the synthesis enhances the surface properties and charge transfer efficiency, contributing to the observed improvement. The high photocurrent density, low onset potential, and extended visible light response make this material promising for practical applications in solar-to-fuel conversion. Future research could focus on further optimization of the synthesis parameters, exploring different monomers and surface modifications to improve stability and efficiency. Investigating the long-term stability under various conditions and scaling up the synthesis for larger-area devices are also important next steps.

This study presents a novel and efficient method for synthesizing uniform carbon nitride layers with enhanced optical absorption and charge separation, leading to benchmark performance as photoanodes in water-splitting PEC cells. The simple, scalable, and versatile method, combined with the melamine surface modification, produces high-performance materials with significant potential for practical applications in sustainable energy technologies. Future work should focus on long-term stability enhancement and large-scale production.

While the presented method significantly advances CN photoanode synthesis, some limitations exist. The long-term stability in alkaline conditions could be improved. Further investigation into the mechanism of the melamine-induced surface modification is needed for complete understanding. Although the method shows good scalability potential, further studies are required to fully optimize the process for large-scale industrial production. The study primarily focused on alkaline conditions; further exploration of performance in neutral pH is warranted for broader applicability.