Surface Functionalization of Boron/Nitrogen Doped Diamond for Electrocatalytic Nitrate Reduction

Numerous diamond materials have been emerging as a critical material for various applications, due to their unique chemical, physical, and mechanical properties. Among them, conductive diamond films, in most cases doped diamond films (Dia) have been widely employed as electrodes for advanced electrochemical applications. Doping of diamond films refers to the introduction of foreign atoms such as boron, nitrogen, or phosphorus into the diamond lattice, leading to their altered electronic and structural characteristics. Of special interest, a co-doping process involves simultaneous incorporation of two or more dopants (e.g., boron and nitrogen), thus synergistically optimizing the diamond’s conductivity, electrochemical activity, and stability. These co-doped diamond films enhance charge transport, create additional active sites, and improve catalytic efficiency, enabling their broader applications in fields such as energy storage, pollutant degradation, and catalysis.
Different from (co-)doped diamond films, diamond composites (DCs) - comprise diamond and other components, thus offering unique opportunities to further leverage these advancements. They effectively combine the outstanding properties of diamond with those of its components, leading to extensive exploration and testing in mechanical, physical, chemical, and biomedical applications. This "1+1 > 2" strategy has proven to be highly efficient for designing and developing advanced diamond-based functional materials. Although DCs offer exceptional chemical and physical properties, the poor electrocatalytic performance of diamond-based electrodes, mainly due to limited active sites, presents a significant challenge.
In this context, this thesis addresses above issues by harnessing boron and nitrogen co-doped diamond (BNDD) composites, designing various interlayers and further adding surface modifications – the formation of DCs - to gain enhanced electrocatalytic activity and stability of these diamond materials, and exploring their electrochemical applicaitons in the fields of electrochemical energy storage (here supercapacitors and zinc batteries) and conversion (here electrocatalytic nitrate reduction reaction). This thesis firstly outlines the experimental techniques for synthesizing and characterizing these materials, including magnetron sputtering, electrodeposition, and microwave plasma-enhanced chemical vapor deposition. Its focus then shifts to the preparation and characterization of BDDD films and their composites, including BNDD/Metal/Si, Cu2O@CNF-BNDD (CNF = carbon nano fibers), and partially covered BNDD/Ti/Si. Detailed methods for analyzing the morphology, surface chemistry, and electrochemical performance of these films are discussed, along with their electrochemical applications such as supercapacitor formation, catalytic nitrate reduction and its integration with electrochemical methyl orange (MO) degradation using BNDD films, and Zn-nitrate batteries. The details of these contents are described as follows.
In the first experimental system, the BNDD films were grown on a Si substrate with/without Ti and Ta interlayers, resulting in the fabrication of BNDD/Si, BNDD/Ti/Si, and BNDD/Ta/Ti/Si electrodes. After they were characterized using microscopies, spectroscopies, electrochemical techniques, and density functional theory (DFT) simulations, the relationship between their composition, interfacial structure, charge transport, and electrochemical properties was investigated. The BNDD/Ta/Ti/Si electrodes demonstrated faster electron transfer rates and lower resistances for redox probes compared to other diamond electrodes. The formation of TaC facilitated carrier tunneling and increased the concentration of electrically active defects, enhancing its electrochemical performance. As a case study, the BNDD/Ta/Ti/Si electrode was employed as a capacitor electrode to construct a redox-electrolyte-based supercapacitor. This section highlights the importance of interlayer composition and its impact on charge transfer and relevant electrochemical performance of co-doped diamond films, in other worlds how a tailored interlayer design can unlock new capabilities of co-doped diamond (e.g., BNDD) films.
In the second experimental system, the BNDD surface was functionalized with carbon nanofibers (CNF) and further with Cu2O. The resultant Cu2O@CNF-BNDD composite was introduced as an efficient electrocatalyst for the electrochemical nitrate reduction reaction (NO3RR). This composite has unique properties of BNDD films, CNFs and Cu2O. For example, the BNDD films have a wider electrochemical potential window that suppresses the competing hydrogen evolution reaction (HER), resulting in a high NO3RR faraday efficiency. The CNFs grown on the BNDD surface regulate the sp2 content, enhancing electrical conductivity and electrochemical activity. Furthermore, the Cu2O catalyst donates electrons to the lowest unoccupied π* orbital of NO3-, making it highly effective for NO3RR. As revealed in DFT calculations, the d-band center of Cu 3d orbitals of reconstructed Cu NPs is shifted to −2.47 eV, balancing the adsorption of nitrogen-contained intermediates, and reducing the overpotential of NO3RR from 1.11 eV to 0.15 eV. Taking the advantage of a eight-electron nitrate-to-ammonia reaction and the exceptional activity of Cu2O@CNF-BNDD, a Zn-nitrate battery system was further proposed, where dual functionality was offered by using NO3RR-generated electrons for both electricity production and ammonia generation. This strategy simultaneously degrades nitrate pollutants and produces valuable chemicals.
In the third experimental system, the sluggish oxygen evolution reaction (OER) was proposed to be replaced with a fast photo-electrocatalytic oxidation reaction of methyl orange (MO) with an aim to address the slow kinetics of the eight-electron NO3RR. In such an integrated system, the anodic MO degradation reaction occurring on a BNDD/Ti/Si electrode provides electrons, while the cathodic reaction performed on a Cu2O/CNF-BNDD cathode reduces nitrate to ammonia. Comprehensive characterization of these composite electrodes demonstrates their impact on electron transfer and electrochemically active surface area for both processes. This "three birds with one stone" strategy not only degrades nitrate pollutants and produces valuable chemicals, but also degrades dye molecules, providing a framework for viable NO3RR and waste degradation processes.
In summary, this thesis provides new insights into the design and optimization of BNDD films and their composites for some potential electrochemical energy storage and conversion applications. By enhancing active sites and modifying surface structure of BNDD films, this thesis proves the great appeal of co-doped diamond films and their composite in the aspects of electrochemical energy storage, pollutant degradation, and electrocatalysis, boosting future developments in the synthesis, characterization and electrochemical applications of various diamond materials and related devices.