Diamond-Based Solvated Electron Generators: A Perspective on Applications in NRR, CO₂RR, and Pollutant Degradation

Abstract

The generation of solvated electrons (SEs) from solid-state sources represents a transformative approach to driving challenging reduction reactions under ambient conditions. Diamond, with its almost unique negative electron affinity (NEA) and tunable electronic properties, is emerging as a promising candidate for SE generation in aqueous media. This perspective article reviews the current state of diamond-based SE generators and discusses their potential to catalyze sustainable nitrogen reduction (NRR) to ammonia, carbon dioxide reduction (CO₂RR), and the degradation of persistent environmental pollutants. Emphasis is placed on the fundamental processes enabling SE photoinjection from diamond to water, recent experimental breakthroughs, and the prospects for scalable, green applications.

1. Introduction

The growing need for sustainable and selective catalytic processes has intensified research into alternative materials capable of driving challenging redox reactions under mild conditions. Solvated electrons (SEs) are transient and highly reducing species also known as hydrated electrons in aqueous environments and have long been recognized for their potential in driving challenging reduction reactions. However, generating SEs in a controlled, efficient, and scalable manner remains a critical challenge.

SEs have long been of interest in the transversal and multidisciplinary field, attracting chemists, physicists, and biologists alike [1,2,3,4]. Indeed, SEs, for physicists, represent prototypical quantum systems embedded in a fluctuating environment [1]. Biologists have proved that SEs are emitted by various biological systems in humans [4], and for chemists, they play a fundamental role, being central to the Birch reduction mechanism, which involves the well-known reaction of ammonia with alkali metals or the use of electrides [2,3]. Extensive research has focused on SEs generated in ammonia through the dissolution of alkali metals [5], in water via ultrafast light pulses [6], and through nonthermal plasma sources [7]. Despite these advancements, the development of efficient solid-state sources of SEs remains largely unexplored.

Diamond, particularly when hydrogen-terminated, presents a unique solution. As a wide-bandgap semiconductor with a negative electron affinity (NEA), hydrogenated diamonds enable the direct emission of electrons into vacuum or solution upon ultraviolet (UV) excitation. This property results from its electronic band structure, where the conduction band minimum (CBM) lies above the vacuum level, facilitating nearly barrier-free electron emission. Recent developments in surface science and photoemission spectroscopy have investigated the mechanisms behind this phenomenon [8], revealing opportunities to exploit diamonds as a robust, tunable platform for SE generation [9].

Beyond fundamental interest, this capability has practical implications. Unlike conventional photocatalysts, diamond enables electron-transfer reactions without requiring molecular adsorption of the reactant on the solid surface, allowing homogeneous-phase reactions driven by SEs in bulk solution [10].

It must be pointed out that diamond is not the only solid-state source of SEs. Indeed, other wide-bandgap materials, such as AlN [11] and cubic boron nitride (c-BN) [12,13], could also generate SEs in liquid via the same mechanism as diamond. Recently, electrides, which are crystalline ionic salts in which electrons serve as anions, have been studied for their ability to exchange electrons with other anions [2,3,14]. Interestingly, reduced transition metal oxides have also been found to function as sources of SEs. In these materials, the primary active phase consists of lattice defects that must be preserved to host and emit SEs. Oxygen vacancies, which are crucial for this process, can be introduced via chemical reduction (e.g., with NaBH₄) or high-temperature annealing in a reducing atmosphere. A key difference between diamond and transition metal oxides is that SE generation in the latter is limited by the number of stored electrons; most trapped electrons remain stabilized in the bulk and require only mild additional thermal energy for release, as demonstrated in TiO_2−x [15,16,17]. Recent developments have also shown that SE production is possible with nanostructured oxides, such as Sc₂O₃ [18].

Diamond, however, represents the only well-characterized system that can link well-defined electronic band structure properties with a distinct ability to photoinject SEs into solution. Therefore, this perspective focuses on this material. Recent progress on the use of hydrogen-terminated diamond as an SE generator, its electronic properties, and its emerging role in sustainable photocatalytic systems is presented, along with a list of reactions in which it has been applied, such as nitrogen reduction (NRR), carbon dioxide reduction reaction (CO₂RR), and the degradation of persistent organic pollutants (POPs).

2. Diamond as a Solvated Electron Generator

Diamond is a wide-bandgap semiconductor with an intrinsic bandgap of approximately 5.5 eV [8,9]. When terminated with hydrogen, it facilitates electron emission in vacuum and in solution generating SEs [10]. To understand the enhanced production of SEs from hydrogen-terminated diamond, one has to study its peculiar electronic structure. For this material, the conduction band (CB) lies higher in energy than the work function (WF). The difference between these energy levels is known as electron affinity (EA), which, for hydrogen-terminated diamond, has a negative value, referred to as NEA [8].

Specifically, the boron-doped hydrogen-terminated diamond is well suited for NEA because it exhibits downward band bending due to boron-doping, along a WF reduction caused by surface dipole formation by the hydrogen termination [9,19,20,21]. Furthermore, the outward-oriented C–H dipoles at the surface further facilitate efficient electron emission into the surrounding medium [10]. The surface dipole formation can be explained by the difference in electronegativity between carbon (2.5) and hydrogen (2.1). A key advantage of NEA materials is their significantly enhanced electron emission efficiency compared to much more common materials with positive EA (PEA), where the WF is above the CBM. In NEA materials, electrons excited to the CB can diffuse to the surface and be directly emitted with negligible energy barriers, making these materials an effective solid-state electron source.

Careful investigations of diamond terminated surfaces for electron emission have been carried out typically in vacuum with surface science equipment. Photoemission spectroscopy (PES) experiments are particularly important for these materials but conventionally are limited by measuring states below the Fermi Energy, such as the CB, and are blind to the VB and bandgap, unless complex pump-and-probe setups or time-resolved instrumentation are used. Interestingly, diamond offers the possibility to analyze the entire electronic band structure by conventional PES experiments, which can be performed in-house in a laboratory with continuous sources such as discharge lamps [8]. Indeed, the presence of NEA enables the measurement of all the energy levels of diamond (100), including the bandgap, CBM, valence band maximum (VBM), and the value of the NEA itself. This is possible due to the accumulation of the electrons that are excited above the CBM and subsequently relax to the CBM without reaching the WF level that falls inside the prohibited bandgap region, as schematized in Figure 1A by yellow and green arrows. Moreover, applying a newly developed photoemission technique for secondary electrons, the angle-resolved PES (ARPES) [8,22], it is possible to calculate the effective mass of the electron at the CBM by analyzing its curvature, a measurement that is typically difficult to obtain directly and is usually estimated indirectly. Measuring the electron effective mass in the CB is crucial because it determines the charge carrier mobility, density of states, and overall electronic transport properties of a material, directly impacting its performance in semiconductor and optoelectronic applications and probably also in SE generation.

It should be noted that the direct ionization of electrons from the VB to the surface is also possible. When photons with energy exceeding a threshold, defined by the subtraction of the bandgap and NEA (≈hν > 5.5 − 1.1 = 4.4 eV), are present, the wave functions of VB electrons at or near the surface interact with the surface C–H dipole, resulting in electron emission at energies significantly below the bandgap [10,23,24].

It has been demostrated that in liquid, upon UV illumination, electrons in the VB are excited to the CB and they can be injected into the adjacent aqueous environment as SEs [10,25,26,27]. The almost unique properties of diamond as an electron photoemitter are highlighted as the following:

• High reduction potential: CB electrons exhibit potentials up to −5.5 V versus the NHE, enabling reactions that are inaccessible to conventional semiconductors [27]; see Figure 2.
• SEs emission in sorrounding media: It has been found that photoinjected SEs from diamond can exhibit an estimated diffusion length of up to 700 nm in extremely pure, degassed water. This derives from an estimated SE lifetime of about 100 μs, although the experimental lifetime can be calculated from transient absorption measurements and may be much shorter depending on the solution composition and experimental factors such as excitation wavelength and incident power. For example, from a chemical point of view, at pH 3, the rapid reaction of SEs with H⁺ reduces the lifetime to ~50 ns, corresponding to a diffusion length of ~16 nm. Dissolved gases, such as oxygen, can similarly quench SEs and shorten their diffusion [10,28]. Nonetheless, the photoinjection of electrons into solution gives the opportunity to perform reduction reactions in the bulk of the solution, avoiding the usually limiting reactant’s surface adsorption.
• Bulk chemical robustness: Diamond’s resistance to corrosion in aqueous environments makes it a suitable candidate for continuous operation. However, its termination stability must be further investigated [26].
• Tunability: Beyond the hydrogen termination, defect engineering [27] can introduce sub-bandgap states, allowing excitation with longer-wavelength light and broadening the operational window. Moreover, surface engineering offers a wide range of parameters to explore for tuning its electronic properties [9,19].

These characteristics establish diamond as a promising candidate for developing robust, solid-state SE generators for various photocatalytic applications.

Challenges: Large Bandgap and Surface Degradation

Hydrogen-terminated diamond can be considered the archetypal NEA material and almost the only one that has been applied to generate SEs [10,25,26,27,28,29,30]. Interestingly, diamond is a versatile material that can be deposited as a thin film [30] or in nanoparticle form [25]; even diamond grit, widely used as a polishing material, which can be produced very affordably, has shown good activity in ammonia production [27]. This versatility makes diamond a suitable material for large-scale photochemical applications.

One challenge when using diamond or other NEA materials as photocatalysts is their optical absorption. Diamond has a large bandgap of 5.47 eV [31], requiring UV light excitation with λ < 225 nm to promote electrons from the VBM to the CBM, as reported in Figure 3A, for the yield of ammonia production using SEs generated by diamond. This wide bandgap is a common characteristic of all NEA materials, as their WF must be low enough to fall within the bandgap (see Figure 1, Figure 2 and Figure 3) [11,12,13,26,28,32]. For hydrogen-terminated diamond, longer-wavelength photons, such as those around 280 nm, can also induce direct ionization from the VB to C–H surface states, although with significantly lower yield compared to photons below 225 nm (i.e., above-bandgap radiation) (see Figure 3A) [10,23,26].

Diamond’s reliance on UV light, rather than readily available visible light, may limit its practical application in photochemistry. Ideally, a photocatalyst should operate efficiently under visible-light illumination. To enhance visible-light absorption, doping with impurities or introducing defects that create mid-gap states is a potential solution. For example, substitutional nitrogen introduces a state approximately 1.7 eV below the CB, which can be excited by visible light and subsequently ionized into the CB [32].

Fortunately, the formation of defect states and their interaction with water can generate sub-bandgap states that require much less energy than 5.47 eV to excite electrons into the CB, thereby facilitating electron injection in solution [26,28]. Indeed, recent studies have shown that sub-bandgap excitation can also induce significant electron emission into water [33], while vacuum studies report electron emission using photon energies as low as 4.4 eV (~280 nm) [24,33]. Bulk or surface states lying within the bandgap such as dopant states, sp²-hybridized (“graphitic”) impurity states, or true surface states associated with the surface termination play an important role [33]. Additionally, multi-photon absorption processes with lower-energy photons (~3.05 eV, ~400 nm) are achievable, although SEs production is significantly lower [26]. Electron emission from sub-bandgap sources may occur via direct excitation from the VB to a free-electron state [23,24] or, in water, to the water CB [34]. Notably, surface defects have been recently found to be critical for enhancing photoemission in water, as the water CB interacts with the diamond band structure, potentially revolutionizing findings related to vacuum excitation [26].

A critical issue is the degradation of the surface termination, see Figure 3B. As explained above, the termination is responsible for imparting a dipole on the surface, which allows the material to exhibit NEA. The most studied is hydrogen termination, which provides NEA in ultrahigh vacuum (UHV) and has been very successfully used for SE generation in water; however, it lacks long-term stability [25,26,27]. Very recently, PEA oxygen termination has been demonstrated to enable SE emission in water by creating the abovementioned surface states that align with water energy levels [26]. However, even in studies from the same authors, the mechanism by which hydrogen termination, essential for NEA in vacuum, degrades within hours in solution under UV irradiation remains poorly understood [25,26,27].

Many questions regarding the stability and transformation of the surface termination still need to be addressed. Moreover, new types of termination are being studied and can present NEA. These terminations, formed by metals and oxides on diamond surfaces, represent a significant advancement in the field. One notable example for vacuum emission is the use of lithium on oxygen-terminated diamond surfaces [35]. The application of bilayers composed of oxygen and metals greatly enhances air stability and imparts NEA to the surface, which significantly improves secondary electron photoemission. Metal and oxide terminations have already been experimentally investigated under vacuum conditions using Li [35], Sn [36,37,38,39], Ti [37], Al [31,37], Sc [40,41], and others. In general, while the ejection of electrons from diamond with several terminations into a vacuum has been extensively studied [8,19,20,31,36,37,39,40,41,42,43], their corresponding emission into liquids remains largely confined to H-terminated diamond [10,25,26,27,29,30]. This highlights the considerable potential for further research to enhance SE generation from diamond for addressing several important and challenging redox reactions.

3. Nitrogen Reduction Reaction (NRR)

The development of sustainable ammonia synthesis is critical for many fields including fertilizer production with roughly 80% of the annually produced ammonia used for this purpose [44]. However, it is also important for the hydrogen economy, as ammonia is considered a promising hydrogen storage medium. Its high volumetric hydrogen density, low storage pressure, and long-term stability make it an attractive alternative to compressed or liquefied hydrogen. Additionally, ammonia offers several safety advantages, including a high auto-ignition temperature, low condensation pressure, and lower gas density than air [45]. However, using ammonia as a hydrogen storage medium requires on-site, small-scale production instead of the large Haber–Bosch process, which accounts for over 96% of global ammonia output, relies on fossil-fuel feedstocks, and raises serious sustainability concerns [46]. Indeed, the Haber–Bosch process produces ammonia by the reaction of molecular nitrogen and hydrogen by employing iron-based catalysts at high temperatures (400–500 °C) and pressures (100–200 bar). However, it is both energetically demanding and kinetically complex, especially because it involves the dissociative adsorption of N₂, followed by the reduction of surface nitride by H₂ through a sequence of NH_X intermediates, ultimately leading to the formation and desorption of NH₃ [47]. Furthermore, a significant amount of energy is required for hydrogen production, which is primarily obtained via steam-methane reforming. It is estimated that the Haber–Bosch process accounts for approximately 3–5% of the world’s natural gas consumption [47].

Thus, there is a clear need to explore alternative nitrogen reduction pathways. The photocatalysis approach is one of the most studied techniques to produce sustainable ammonia [48]. Photocatalytic NRR outperforms the Haber–Bosch process in terms of safety, low energy consumption, environmental friendliness, and gentle reaction conditions. These characteristics render photocatalysis an attractive, environmentally friendly option for the sustainable production of ammonia [49].

Studies indicate that SEs can perform NRR via a mechanism called hydrogen atom addition (HAA) in which SEs react with protons (H⁺) to form neutral hydrogen atoms (H•), which then react with dissolved N₂ gas to form N₂H, the key intermediate in ammonia synthesis [29]. This HAA mechanism is similar to electrochemical (EC) nitrogen reduction at electrode surfaces [49]; however, in the EC process, because of the potential involved, it is expected that the direct reduction of protons to H₂ would be a dominant side reaction. In particular, in EC nitrogen reduction, the extremely high local concentration of adsorbed H atoms at the electrode surface, coupled with the small amount of adsorbed N₂, results in exclusive production of H₂ over NH₃ [29]. In contrast, it has been found that photoinjected SEs from diamond have an relatively large diffusion length in liquid [10]. Therefore, the presence of SEs in solution leads to a reduced concentration of H atoms and allows N₂ reduction to compete more favorably with H₂ production [29]. Indeed, the effectiveness of SEs also stems from their major advantage in eliminating the need for the critical surface adsorption of N₂ on a catalyst [10]. The independence from the catalyst surface for NRR has been demonstrated in a catalyst-free, plasma-based electrolytic system, confirming the active role of SEs. However, this process remains inefficient due to the high power consumption required for plasma generation [7].

Notably, photoinjected SEs from hydrogen-terminated diamond demonstrate the ability to produce more ammonia than Ru/TiO₂ [27], one of the most studied photocatalysts for NRR [48,50], This result is particularly significant as it allows the use of inexpensive diamond grit while avoiding the need for critical raw materials such as Ru [10].

4. Carbon Dioxide Reduction Reaction (CO₂RR)

The sustainable conversion of carbon dioxide into useful chemical feedstocks is of paramount importance, not only as a strategy to mitigate rising greenhouse gas levels but also as a route to produce value-added chemicals [51]. CO₂, an abundant and thermodynamically stable molecule, presents significant challenges in activation, particularly due to the need for efficient and selective electron-transfer steps [52]. Conventional photochemical and electrochemical methods, which often involve proton-coupled electron-transfer (PCET) reactions, tend to suffer from low selectivity and energy inefficiencies as a result of competing hydrogen evolution reactions [53].

Recent advances have opened up a promising alternative that circumvents these issues [25,30]. In a study by Zhang et al. [25,30] a novel photoelectrochemical approach was developed in which SEs are generated in water from hydrogen-terminated diamond surfaces under UV illumination. The effectiveness of these procedures was verified by Fourier transform infrared spectroscopy (FTIR). Figure 4 shows FTIR spectra of the gas-phase headspace after CO₂ photoreduction by illumination of a diamond sample. The spectrum from illuminated diamond in CO₂/water shows distinct absorption lines of CO. The features shown here are the P(ΔJ = −1) and R(ΔJ = +1) rotational branches associated with excitation from the lowest (v = 0) to the first excited (v = 1) vibrational state of CO. No significant CO is produced in the absence of UV light or in an argon-purged solution, and only a very small amount of CO is observed when a CO₂-saturated solution is illuminated without the diamond sample due to very low photochemical reactivity of CO₂ at short wavelengths [54].

This CO₂RR method leverages the unique advantage that the electrons are injected directly into the solution, thereby eliminating the need for reactant adsorption on a catalyst surface, a key limitation in many traditional CO₂ reduction systems as well as the abovementioned NRR. Once in solution, SEs react with dissolved CO₂ to form the transient CO₂ radical anion (CO₂^•−). The reaction proceeds with exceptional selectivity because, under the experimental conditions (CO₂ pressures around 2.5 MPa and pH ≈ 3.2), the concentration of CO₂ in solution far exceeds that of protons. Consequently, the rate of electron transfer to CO₂ overwhelmingly outcompetes the reduction of protons to H₂, leading to CO as the dominant product [30].

The works of Zhang et al. demonstrated that by employing H-terminated diamond as a solid-state source of SEs, it is possible to drive the one-electron reduction of CO₂ to CO with remarkable selectivity, and they proved this for diamond slabs as well for nanodiamond in solution [25,30]. This innovative method bypasses the limitations inherent in conventional heterogeneous catalysis and offers a new pathway for the sustainable conversion of CO₂ into valuable chemicals, potentially paving the way for advanced strategies in carbon management and renewable energy technologies.

5. Degradation of Environmental Pollutants

Addressing the challenge of POPs in water and air is critical for safeguarding environmental and human health [55]. SEs in water, also called e_aq⁻, are extremely reducing species that play a crucial role in ultraviolet-advanced reduction processes (UV-ARP) for water treatment. These reactive intermediates are typically produced via the UV-photolysis of chemical sensitizers (e.g., sulfite) and have been widely applied to transform and degrade resistant contaminants. Their high reactivity allows them to break down many persistent pollutants that conventional treatments cannot efficiently remove [56].

In this field, hydrogen-terminated nanodiamonds (HNDs) have been employed to photogenerated SEs in water to achieve efficient pollutant degradation under mild conditions (Figure 5A) [28]. A model is presented in Figure 5C to illustrate the fragmentation of perfluorooctanesulfonic acid (PFOS) following UV irradiation of HNDs, although the positions of the H–F exchanges cannot be determined from the data (those depicted in Figure 5C are arbitrary). However, the time-dependent product evolution shown in Figure 5B suggests possible C–C bond cleavage between C3 and C4 [28].

The authors of this work observed that the surface interaction between HNDs and molecules enhances the efficiency of photoelectron generation. However, it remains unclear whether this interaction directly contributes to or facilitates the degradation of PFOS, once again highlighting the importance of surface termination states in diamond-based SE generators [26,28]. Interestingly, this novel UV-HND advanced reduction process also appears to be less sensitive to changes in solution pH, maintaining its performance down to pH 7 [28], unlike the state-of-the-art UV-sulfite process [57].

6. Heterogeneous Catalysts and Microfluidic Reactor Approaches

Integrating heterostructured catalysts with diamond-based SE generators offers significant potential for enhancing the overall performance of chemical transformations. Beyond exploiting the intrinsic properties of diamond, the innovative combination of these materials can overcome current limitations, as demonstrated by the common practice in heterogeneous catalysis of combining materials to enhance reaction rates. For example, in conventional ammonia synthesis, ruthenium catalysts are often promoted with magnesium and calcium oxides (MgO, CaO) or with barium and alkali metals to improve their catalytic properties [14,58,59].

Recent studies on Ru catalysts for NRR have further advanced this concept by integrating standard materials with solid-state SE generators. One notable approach involves combining Ru with the electride C12A7:e^–, which facilitates N₂ dissociation by transferring electrons from C12A7:e^– (work function, WF = 2.4 eV) to Ru (WF = 4.7 eV). In this system, the SE generator acts as a promoter for the Ru catalyst, mitigating the bottleneck of N₂ dissociation caused by the trapping of electrons and H^– within the electride cages [14]. However, to date, electrides have predominantly functioned as catalyst promoters rather than directly photoinjecting electrons in solution to drive ammonia synthesis.

The combination of SEs generators and other catalysts can be further explored by innovatively designing heterostructures on diamond surfaces, where SEs serve as the primary active species. By employing not only perfectly flat terminations but also creating heterostructures with nanoparticles, researchers can develop stable SE generators that minimize degradation over time. Enhanced SE production could arise from several factors: the WF differences between metal/oxide islands and the diamond surface, the presence of local surface states that improve light absorption, optimized band alignments, and the adsorption of molecules on metal/oxide surfaces, which may increase the concentration of reactants near the catalytic sites.

The distinct advantages of heterogeneous catalysts over homogeneous ones such as easier separation, greater stability, and reusability, make them ideally suited for industrial applications, despite homogeneous catalysts often offering higher activity and selectivity. As stated above, hydrogen-terminated diamond, recognized as the archetypal NEA material, has been successfully employed in NRR [27,29], CO₂RR [25,30], and environment remediation processes [28]. Interestingly, diamond can be deposited as a thin film [30] or in nanoparticle form [25]. Its properties change drastically with modifications to the outermost atomic layers [9], and such atomic terminations can be achieved via physical vapor deposition or liquid-phase methods. For example, lithium oxide terminations can be applied using both approaches [39], providing flexibility without compromising reactor geometry due to the ultra-thin nature of the termination. This versatility positions diamond as a highly promising material for large-scale photochemical applications.

Despite these promising laboratory-scale results, translating SE-driven processes, such as ammonia synthesis, into industrial applications remains a challenge. Indeed, diamond as a heterogenous solid-state SE generator has been only studied as a model catalyst so far [10,26,27,30], and to achieve a large yield, diamond nanoparticles in solution have been used [25,28]. To bridge the gap from model catalyst to scalable photoreactors, a multi-faceted approach is required. Rigorous laboratory-scale testing must evaluate SE generation, reaction efficiencies, and degradation under operating conditions to identify optimal SE sources. Simultaneously, reactor designs must be engineered to withstand harsh conditions (e.g., strongly reducing environments) while preserving the integrity of sensitive surface terminations.

7. Conclusions

Diamond offers a compelling platform for the generation of SEs under UV illumination, merging favorable material properties with exceptional photocatalytic capabilities. When hydrogen-terminated, its NEA and high reduction potential allow for efficient electron injection into aqueous environments, supporting reactions that are typically inaccessible with traditional semiconductors. Applications spanning NRR, CO₂RR, and environmental remediation underscore the material’s versatility and promise.

However, challenges such as the wide bandgap and instability of hydrogen termination in UV-illuminated aqueous media limit its practical deployment. Ongoing efforts in doping, surface and defect engineering, and the development of sub-bandgap excitation strategies are essential to extend diamond’s functionality to the visible spectrum and improve long-term stability. As research advances, diamond stands at the forefront of innovative, sustainable materials for photochemical applications, bridging the gap between fundamental surface science and practical catalysis.