Boron-doped diamond semiconductor electrodes: Efficient photoelectrochemical CO₂ reduction through surface modification

Abstract

Competitive hydrogen evolution and multiple proton-coupled electron transfer reactions limit photoelectrochemical CO₂ reduction in aqueous electrolyte. Here, oxygen-terminated lightly boron-doped diamond (BDD_L) thin films were synthesized as a semiconductor electron source to accelerate CO₂ reduction. However, BDD_L alone could not stabilize the intermediates of CO₂ reduction, yielding a negligible amount of reduction products. Silver nanoparticles were then deposited on BDD_L because of their selective electrochemical CO₂ reduction ability. Excellent selectivity (estimated CO:H₂ mass ratio of 318:1) and recyclability (stable for five cycles of 3 h each) for photoelectrochemical CO₂ reduction were obtained for the optimum silver nanoparticle-modified BDD_L electrode at −1.1 V vs. RHE under 222-nm irradiation. The high efficiency and stability of this catalyst are ascribed to the in situ photoactivation of the BDD_L surface during the photoelectrochemical reaction. The present work reveals the potential of BDD_L as a high-energy electron source for use with co-catalysts in photochemical conversion.

Introduction

Artificial photosynthesis is an emerging process for sustainable conversion of solar energy to chemical energy, and is important because of the increasing atmospheric CO₂ concentration and global energy demand^1,2,3. Artificial photosynthesis generally uses a semiconductor that absorbs solar radiation and generates electron/hole pairs^1,4,5. These electron/hole pairs produce active radical species on the semiconductor surface prior to the adsorption of water or reacting molecules, or electron/hole pairs directly transfer to guest molecules through space^6,7. The high energy of these active radicals and electron/hole pairs means they readily take part in certain chemical conversions under suitable conditions. For example, TiO₂ is a well-known photocatalyst for water splitting and pollutant degradation^6,7. TiO₂ splits water into hydrogen and/or oxygen in the presence of a Pt counter electrode or hole scavenger using a photon energy greater than its bandgap. Other chemical conversions have also been achieved through semiconductor/molecular photocatalysis^8,9,10,11. However, in multistep chemical conversion, which involve many intermediates to obtain a desired product, each intermediate has the possibility to divert the reaction course, resulting in poor product selectivity^12,13,14. Although product selectivity in multistep reactions can be increased by choosing a suitable photocatalyst or controlling the reaction conditions, the reliability and efficiency of such reactions are still far from those required for commercial use¹³. Therefore, efficient conversion through multistep or multi-intermediate photochemical reaction remains challenging. Chemists typically attempt to divert such conversions into new mechanistic pathways with the fewest possible intermediates.

Photocatalytic or photoelectrochemical CO₂ reduction in aqueous solution is one example where product selectivity is limited by both thermodynamics and kinetics. Commercial conversion of CO₂ into high-value chemicals is generally inefficient, laborious, and expensive^12,13,14. Efficient photocatalytic or photoelectrochemical CO₂ reduction with excellent product selectivity is a research goal in the field of sustainable energy conversion, and represents an alternative to natural photosynthesis.

The atmospheric concentration of CO₂ has increased tremendously over the last 50 years. Selective, efficient conversion of CO₂ into a valuable chemical fuel is not only important for sustainability but also economically favourable^15,16,17. Natural photosynthesis is an indispensable tool to capture CO₂, but gross destruction of green plants and rapid civilization have forced researchers to search for alternatives to natural photosynthesis to efficiently and selectively convert CO₂ into high-value chemicals. The selectivity of chlorophyll in natural photosynthesis is unprecedented. In artificial photocatalytic or photoelectrochemical CO₂ reduction, conversion of CO₂ into useful chemicals requires suitable catalysts^2,4. Photocatalytic or photoelectrochemical CO₂ reduction generally involves multiple proton-coupled electron transfer reactions in aqueous media. These reactions provide numerous intermediates, and thereby produce multiple CO₂ reduction products⁴. Electrochemical or photoelectrochemical CO₂ reduction has been explored using various catalysts with the aim of identifying mechanistic pathways that finally could lead to efficient conversion of CO₂ into a particular product^{12,18,19,20,21,22}.

Halmann et al.²². first reported photoelectrochemical CO₂ reduction to produce formic acid, methanol and formaldehyde using a p-type GaP semiconductor in 1978. Since then, many semiconductor photocatalysts have been used for CO₂ reduction^23,24,25,26. Metal nanoparticles deposition on the semiconductors and fabrication of metal-semiconductor junction have successfully studied for improvement of photocatalytic CO₂ reduction activity^27,28,29. Recently, Hamers and co-workers found that hydrogen-terminated boron-doped diamond (H-BDD) can produce solvated electrons through injection of photoexcited electrons into aqueous electrolytes³⁰. The solvated electrons produced by H-BDD can reduce CO₂ into carbon monoxide (CO) alone via a one-electron transfer reduction mechanism under sufficiently high pressure. However, the extremely high pressure used in this approach is undesirable. In addition, H-BDD is easily oxidized under illumination^30,31.

In this work, we study an alternative solution to overcome the challenges of one-electron transfer under normal pressure and temperature by using oxygen-terminated lightly boron-doped (1,000 ppm) diamond (BDD_L) incorporated with co-catalysts. The surface of oxygen-terminated BDD_L is reduced during photoelectrochemical reaction so it acts as a semiconducting high-energy electron source^30,31. Silver (Ag) co-catalyst deposited on BDD_L stabilizes the intermediates of CO₂ reduction reactions under normal pressure to realize selective photoelectrochemical reduction. The semiconducting nature of BDD_L is investigated by changing the light sources used in photoelectrochemical studies.

Results

Surface morphology and photoelectrochemical study

Field-emission scanning electron microscopy (FESEM) and elemental analysis were used to investigate the deposition of Ag nanoparticles on BDD_L. First, Ag nanoparticles were deposited on BDD_L by a chronoamperometric method in 0.1 M AgNO₃ at −0.5 V for 60 s to produce a sample denoted 0.1 Ag-BDD_L. Low-magnification FESEM analysis of this sample revealed that Ag nanoparticles with a diameter of ~300 nm were deposited on BDD_L (Fig. 1a). However, a high-magnification FESEM image indicated that primarily smaller Ag nanoparticles (average size ~20 nm) were deposited on BDD_L (Fig. 1b). The size distributions of smaller Ag nanoparticles are shown in Supplementary Fig. S1 by analysing several high magnified FESEM images. Average size of the smaller Ag nanoparticles was found to be around ~20 nm (Supplementary Fig. S1) with the 0.1 Ag-BDD_L. The 0.1 Ag-BDD_L sample was then used for electrochemical/photoelectrochemical CO₂ reduction. Figure 1c displays cyclic voltammograms (CVs) of a 0.1 Ag-BDD_L electrode in 25 mM Na₂SO₄. In N₂, no cathodic peak was obtained up to −1.0 V vs. RHE, suggesting no reduction of Ag-BDD_L or the electrolyte. The low cathodic current density (onset potential −1.0 V vs. RHE) may indicate a poor hydrogen evolution reaction³². Indeed, under CO₂-saturated conditions, 0.1 Ag-BDD_L displayed a strong cathodic peak at −1.1 V vs. RHE in 25 mM Na₂SO₄, clearly indicating the cathodic electrochemical reduction of CO₂. The cathodic current density gradually decreased with increasing cycle number (Supplementary Fig. S2a). Careful observation revealed that the difference of cathodic current density between two consecutive cycles decreased as cycling progressed. After several cycles, the cathodic current density remained almost constant at −1.1 V (Supplementary Fig. S2a). The opposite behaviour was observed under irradiation at 222 nm with an excimer lamp (7 W). Under irradiation, the cathodic current density increased slowly and reached its maximum value after several cycles (Supplementary Fig. S2b). The increase in cathodic current density is attributed to the photoexcitation and photoactivation of 0.1 Ag-BDD_L by high-energy photons at −1.1 V vs. RHE. Therefore, under irradiation and applied bias potential, charge transport and cathodic CO₂ reduction are facilitated, increasing cathodic current density. The final cycles in both the dark and light-irradiation conditions show quite different cathodic current densities (Fig. 1c). This large difference clearly reveals the excellent photoelectrochemical activity of the 0.1 Ag-BDD_L electrode for CO₂ reduction.

Figure 1

Surface morphology and photoelectrochemical CO₂ reduction activity of Ag-BDD.

(a) Low-magnification FESEM image of 0.1 Ag-BDD_L (0.1 Ag indicates that the AgNO₃ concentration during deposition was 0.1 M; other parameters including potential and time were fixed at −0.5 V and 60 s, respectively), clearly indicating its smooth surface. Brighter Ag nanoparticles with a diameter of ~300 nm are indicated by arrows. (b) High-magnification FESEM image showing smaller Ag nanoparticles (~20 nm) as confirmed by elemental mapping (inset). (c) CVs of 0.1 Ag-BDD_L in 25 mM Na₂SO₄ aqueous electrolyte. The cathodic peak current at −1.1 V vs. RHE under CO₂-saturated conditions indicates the cathodic reduction of CO₂ on the 0.1 Ag-BDD_L electrode. (d) Chronoamperometric current–time curves of the photocurrent generated by the 0.1 Ag-BDD_L electrode in the dark and under irradiation (222 nm). Photocurrent increased in the first hour and then became almost constant.

To further confirm the photoelectrochemical behaviour of the 0.1 Ag-BDD_L electrode, partial current densities were measured in both the dark and light in 25 mM Na₂SO₄ at −1.1 V vs. RHE. In the first few minutes in the dark, the partial current density changed sharply from −0.3 to −0.2 V and then remained constant (Fig. 1d). This clearly agrees with the CV characteristics of Ag-BDD_L under CO₂-saturated conditions. However, under photoexcitation at 222 nm, the partial current density at −1.1 V began to increase over time (Fig. 1d). This increase in partial current density is caused by the photoexcitation and photoactivation of 0.1 Ag-BDD_L and the subsequent participation of photoexcited electrons in the cathodic CO₂ reduction reaction on the Ag surface, as revealed by CV analysis.

Product analysis

CV and chronoamperometric analysis clearly suggest the electrochemical and photoelectrochemical reduction of CO₂ by the 0.1 Ag-BDD_L electrode at −1.1 V vs. RHE in 25 mM Na₂SO₄. Therefore, the CO₂ reduction products obtained under similar conditions at −1.1 V vs. RHE with different amounts of Ag loaded on the BDD_L electrodes were detected. Gas chromatography (GC) and ion chromatography (IC) were used to investigate the reduction products. Figure 2 shows the amount of CO and H₂ obtained using Ag-BDD_L electrodes with different Ag loading during irradiation (222 nm). The concentration of AgNO₃ used during deposition was slowly increased from 0 to 0.1 M (0 indicates a bare BDD_L electrode, while 0.025 Ag-BDD_L, 0.05 Ag-BDD_L and 0.1 Ag-BDD_L denote deposition of Ag from 0.025, 0.05 and 0.1 M AgNO₃ aqueous solutions on BDD_L at −0.5 V for 60 s, respectively). The detailed of surface morphology with 0.025, and 0.05 Ag-BDD_L is shown in Supplement Fig. S1. Average sizes of the smaller Ag nanoparticles were found to be ~10 and ~15 nm with the samples 0.025, and 0.05 Ag-BDD_L electrodes, respectively. The photoelectrochemical CO₂ reduction products obtained over these electrodes were analysed (Fig. 2a). The bare electrode (0 Ag-BDD_L) produced a negligible amount of CO. The amount of CO produced increased markedly with Ag loading on BDD_L. Although similar amounts of CO were obtained over 0.1 Ag-BDD_L (590 μg in 3 h) and 0.05 Ag-BDD_L (577 μg in 3 h), less H₂ was formed by 0.1 Ag-BDD_L (1.85 μg in 3 h) than 0.05 Ag-BDD_L (2.59 μg in 3 h). Minor amount of H₂ could be produced from competitive proton reduction in aqueous electrolyte. The weight ratios of CO to H₂ were 29:1, 222:1 and 318:1 over 0.025 Ag-BDD_L, 0.05 Ag-BDD_L and 0.1 Ag-BDD_L, respectively. Therefore, the 0.1 Ag-BDD_L electrode produces CO with minimal H₂ at −1.1 V vs. RHE under 222 nm irradiation.

Figure 2

CO₂ reduction product analysis.

(a) Amount of CO produced during irradiation (222 nm) over different Ag-BDD_L electrodes in 25 mM Na₂SO₄ aqueous electrolyte at −1.1 V vs. RHE. At a particular duration, the amount of CO increases with the Ag concentration used during deposition on the BDD_L substrate. (b) Production of H₂ over the electrodes. At a particular irradiation time, the amount of H₂ decreased with increasing AgNO₃ concentration during electrode synthesis. (c) Photoelectrochemical effect of 0.1 Ag-BDD_L under different conditions in 25 mM Na₂SO₄ at −1.1 V vs. RHE. The amount of CO produced under irradiation (222 nm) is higher than that produced in the dark, revealing that the very high amount of CO produced originates from the photoelectrochemical effect of 0.1 Ag-BDD_L. Negligible CO was produced over 0.1 Ag-BDD_L under N₂-saturated conditions. (d) Amount of H₂ produced over the 0.1 Ag-BDD_L electrode under different conditions at −1.1 V, indicating that H₂ is mostly produced through electrochemical reactions.

The role of photoelectrochemical CO₂ reduction at 0.1 Ag-BDD_L is emphasized by comparing the products obtained with and without light irradiation and under N₂-saturated conditions at −1.1 V. GC analysis reveals that in the dark, the amount of CO produced (88 μg in 3 h) decreased considerably compared with that under irradiation, while the amount of H₂ (4.35 μg in 3 h) remained similar (Fig. 2c,d). Similarly, under N₂-saturated conditions, a negligible amount of CO (15 μg in 3 h) was detected, while the amount of H₂ remained similar. Therefore, the greater amount of CO produced under CO₂-saturated conditions and 222 nm irradiation results from the photoelectrochemical effect of 0.1 Ag-BDD_L.

Ag-BDD_L could also produce liquid products because CO₂ reduction often follows a multiple proton-coupled electron transfer reaction mechanism in aqueous electrolyte, providing numerous products. Therefore, IC was used to detect possible liquid products including formate, acetate and oxalate. No liquid products were detected from the Ag-BDD_L electrodes under CO₂-saturated conditions in the light (222 nm) or dark at −1.1 V.

Selectivity of different Ag-modified carbon electrodes

We found that 0.1 Ag-BDD_L displayed more efficient and selective photoelectrochemical CO₂ reduction than 0.05 and 0.025 Ag-BDD_L electrodes. To confirm the superior photoelectrochemical activity and selectivity of 0.1 Ag-BDD_L, Ag nanoparticles were deposited on heavily boron-doped (10,000 ppm) diamond (denoted BDD_H) and glassy carbon electrodes (GCEs) at −0.5 V for 60 s in 0.1 M AgNO₃. The photoelectrochemical performance of the resulting electrodes was studied. Figure 3 shows the total Faradaic efficiency of these electrodes in 25 mM Na₂SO₄ at −1.1 V vs. RHE after 3 h of photoelectrolysis under 222 nm irradiation. The total Faradaic efficiency of 0.1 Ag-BDD_L was 81.5%. The Faradaic efficiency for CO with this electrode was 72.5%, and only 9.08% for H₂, thereby providing very high selectivity for CO. Conversely, the total Faradaic efficiency of 0.1 Ag-BDD_H was 75.1%. Although this value is close to that of 0.1 Ag-BDD_L, the Faradaic efficiency for CO of the 0.1 Ag-BDD_H electrode is only 36.8% and the CO:H ratio is ~1:1 indicating facile proton reduction at 0.1 Ag-BDD_H. Meanwhile, 0.1 Ag-GCE exhibited a very poor total Faradaic efficiency (39.8%) with a Faradaic efficiency for CO of just 8.7%. Therefore, Ag-BDD_L displays increased conversion and selectivity in the photoelectrochemical reduction of CO₂ in 25 mM Na₂SO₄ at −1.1 V vs. RHE compared with those of 0.1 Ag-BDD_H and 0.1 Ag-GCE.

Figure 3

Selectivity of different carbon electrodes.

Ag was deposited on BDD_L, BDD_H and GCE in 0.1 M AgNO₃ at −0.5 V for 60 s. Total Faradaic efficiency was measured after 3 h of photoelectrolysis at −1.1 V in 25 mM Na₂SO₄ under an excimer lamp (222 nm, 7 W).

Stability and recyclability of the optimal photocathode

Electrode stability and recyclability are important factors that must be considered for their reliable use in photoelectrochemical solar energy conversion devices. Therefore, we studied the stability and recyclability of the optimized 0.1 Ag-BDD_L photocathode by performing an isotopic experiment because very high energy photons (222 nm, ~5 eV) could degrade the BDD_L surface or the resin used in the electrode contacts. The isotopic experiment was carried out by purging the 25 mM Na₂SO₄ electrolyte for 10 min with ¹³CO₂ at a flow rate of 3 mL/min. Photoelectrochemical reduction was then carried out at −1.1 V vs. RHE under 222 nm irradiation and GC–mass spectrometry (GC-MS) was used to analyse the isotopic yield. Figure 4a shows the amounts of isotopic ¹³CO and normal ¹²CO after different periods. The amount of ¹³CO produced (267.56 μg after 5 h of illumination at −1.1 V vs. RHE) is lower than that of ¹²CO obtained previously (590 μg after 3 h; see Fig. 2c). This is because CO₂ purging was carried out for 1 h in the previous experiment, so more CO₂ was dissolved in the electrolyte compared with that in the case of isotopic ¹³CO₂ (Fig. 1d, Supplementary Fig. S3). After illumination for 1 and 5 h, the amounts of isotopic ¹³CO were 86.42% and 94.17%, respectively. The increase in ¹³CO content is because only a little non-isotopic ¹²CO (16.8 μg after 5 h) could enter the system from the resin used to fabricate the electrode contact, which might be degraded by high-energy photons. The amount of ¹²CO produced by resin degradation is very small compared with that of ¹³CO produced through photoelectrochemical reduction. Therefore, the relative amount of isotopic ¹³CO increased over time. This isotopic experiment clearly reveals that CO originates from the reduction of CO₂, not from degradation of resin, other organics or the BDD surface by the high-energy irradiation. Even after 5 h of 222 nm irradiation, 0.1 Ag-BDD_L was stable and produced ¹³CO with an excellent yield.

Figure 4

Stability and recyclability of the optimal photoelectrode.

(a) Amount of isotopic ¹³CO and normal ¹²CO produced over time by the 0.1 Ag-BDD_L electrode in 25 mM Na₂SO₄ at −1.1 V vs. RHE. The amount of isotopic ¹³CO increased with irradiation time and reached 94.17% after 5 h. (b) XPS analysis of the 0.1 Ag-BDD_L electrode before and after photoelectrolysis at −1.1 V vs. RHE for 5 h in 25 mM Na₂SO₄ under 222 nm irradiation. The Ag 3d photoelectron peaks suggest the metallic state of Ag is not changed during the photoelectrochemical reaction. (c) Recyclability of the 0.1 Ag-BDD_L electrode in CO₂-purged 25 mM Na₂SO₄ at −1.1 V. In each run, the electrolyte was purged with N₂ and then CO₂ for 1 h. The amount of CO produced decreased as the number of runs increased. (d) The amount of hydrogen produced in the consecutive runs indicates that hydrogen evolution increased with run number.

X-ray photoelectron spectroscopy (XPS) was performed before and after photoelectrocatalysis to investigate the elemental states of the 0.1 Ag-BDD_L electrode. Figure 4b displays the metallic Ag 3d photoelectron peaks^33,34. The relative amount of Ag and nature of Ag 3d photoelectron peaks remained similar before and after photoelectrolysis at −1.1 V under 222 nm irradiation, indicating the oxidation states of the Ag nanoparticles did not change during photoelectrochemical operation; i.e., the metallic state of Ag is stable.

The recyclability of the 0.1 Ag-BDD_L electrode was examined by performing five runs for 3 h at −1.1 V in 25 mM Na₂SO₄. After each run, the cathodic peak position shifted to lower overpotential with slightly increased hydrogen evolution current compared with that at higher overpotential. After five runs, the cathodic peak for CO₂ reduction moved to lower overpotential by 0.15 V with a current density of 0.2 mA/cm² (Supplementary Fig. S4). The amounts of CO and H₂ produced were measured (Fig. 4c,d). After five runs, the amount of CO produced in 3 h was 260 μg (44%), comparatively lower than that obtained for the first cycle (590 μg in 3 h). Conversely, the amount of H₂ produced in the fifth cycle (10.51 μg) was greater than that obtained in the first (1.85 μg). The decrease in CO and increase in H₂ produced with cycle number are attributed to the shift of the onset potential of CO₂ reduction to lower overpotential as photoelectrolysis was conducted at −1.1 V (Supplementary Fig. S4).

Discussion

Efficient photoelectrochemical CO₂ reduction is very important for use of CO₂ as a chemical feed stock and from an environmental viewpoint. Diamond is a highly stable material with a very wide bandgap (5 eV), possessing a wide electrochemical potential window, mechanical stability and biocompatibility^19,30,35. Another advantage of diamond is that its conduction band (CB) position is very high, and hydrogen termination can shift its CB above the vacuum level, providing negative electron affinity towards photoexcited electrons³¹. Despite the very high energy of the photoexcited electrons of diamond, it is difficult to use under normal conditions. This is because pure diamond has very poor active sites for adsorption of foreign molecules. At sufficiently high pressure, the photoexcited electrons of hydrogen-terminated diamond or H-BDD can be used for photoelectrochemical conversion^30,31. The photoexcited electrons in hydrogen-terminated diamond or H-BDD are capable of reducing CO₂ via a one-electron transfer mechanism in 0.1 M Na₂SO₄ at sufficiently high pressure (2.5 MPa)³⁰. Both the requirements of high pressure and hydrogen termination of the catalyst limit the application of diamond as a catalyst. Therefore, here we modified oxygen-terminated BDD_L with Ag nanoparticles to realize photoelectrochemical CO₂ reduction under normal pressure and temperature. Ag nanoparticles were deposited on BDD_L because Ag is known for its selective electrochemical CO₂ reduction^36,37. The selectivity of Ag originates from its ability to form strong chemical bonds with CO₂ under a certain applied potential, which is very important for product selectivity^36,37,38. Strong chemical bonding of a catalyst surface with CO₂ reduction intermediates can terminate the reduction reaction at a particular point without further propagation, unlike Cu surfaces, which are known to further propagate reduction reactions to produce multiple CO₂ reduction products^12,36.

Photoelectrochemical studies and product analysis revealed that the optimized 0.1 Ag-BDD_L electrode exhibited high efficiency and selectivity in the formation of CO in aqueous electrolyte at −1.1 V vs. RHE with a Faradaic efficiency of 72.5% after 3 h under 222 nm irradiation (Fig. 3). Photoelectrochemical studies using this optimized 0.1 Ag-BDD_L electrode were also carried out under similar conditions with 172- and 308-nm irradiation. Irradiation at 308 nm induced a small increase of CO production (235 μg in 3 h). Conversely, 172-nm irradiation yielded less CO (97 μg in 3 h), similar to -electrochemical CO production alone over the same electrode (Fig. 2c). The marked photoelectrochemical effect of 222 nm irradiation on the performance of the optimized 0.1 Ag-BDD_L electrode reveals the role of excitation of valence band (VB) electrons to the CB (Figs 1c,d and 2, Supplementary Fig. S5) in CO production. This is because the band gap of BDD_L (~5 eV) matches well with a 222 nm light source.

Bare BDD_L irradiated with 222 nm light did not efficiently produce CO₂ reduction products (~19 μg CO that originated from the degradation of impurities; see Fig. 4a) under similar experimental conditions, as shown in Fig. 2a and Supplementary Fig. S6. This is consistent with the finding of Hamers et al.³⁰. that photoexcited electrons produced in a BDD semiconductor could not reduce CO₂ into CO under normal pressure (Supplementary Fig. S6). Thus, the electrochemical CO₂ reduction activity of Ag-BDD_L is related to the presence of Ag nanoparticles³⁶. However, electrochemical reduction of CO₂ by Ag-BDD_L in aqueous electrolyte at −1.1 V vs. RHE decreased markedly after a few minutes and then became poor (Fig. 1d). This behaviour is also evident from the consecutive CV runs conducted under CO₂-saturated conditions in the dark (Supplementary Fig. S2a). In the dark, the cathodic peak current obtained at −1.1 V vs. RHE in 25 mM Na₂SO₄ decreased until it reached a constant value (Supplementary Fig. S2a). Conversely, the cathodic peak current increased until it reached its maximum value after a certain irradiation time under 222 nm irradiation at −1.1 V vs. RHE. This increase in photocurrent (Fig. 1d, Supplementary Fig. S3) is thought to be caused by two factors: first, photoreduction of the BDD_L surface produces photoactive sites at high negative applied potential, and second, simultaneous enhanced photoelectrochemical reduction of CO₂ on the Ag surface. This situation was confirmed by XPS analysis. The C 1 s peaks of 0.1 Ag-BDD_L before and after photoelectrolysis at −1.1 V vs. RHE in 25 mM Na₂SO₄ for 5 h are presented in Supplementary Figs S7 and S8. A broader C 1 s peak [full width at half maximum (FWHM) of 2.73 eV] is obtained before photoelectrochemical reaction, and becomes narrower (FWHM=1.51 eV) after photoelectrolysis for 5 h at −1.1 V in the presence of 222 nm irradiation (Supplementary Fig. S8). This is because BDD_L is oxygen terminated, so a small amount of C-O-C and C=O/C-OH bonds are present in addition to C-C bonds at its surface. The C=O/C-OH bonds might be reduced at −1.1 V under 222 nm irradiation. Therefore, the C 1 s peak displays greater C-C sp³ character after photoelectrolysis because of the decreased amount of C=O/C-OH bonds, causing it to narrow (Supplementary Fig. S8). Note that electrochemical bias potential alone cannot reduce the diamond surface; 222 nm irradiation is needed for BDD surface reduction, as evident from XPS, CV, and chronoamperometric analyses (Fig. 1c,d; Supplementary Figs S7 and S8). Our results reveal that 0.1 Ag-BDD_L is activated under 222 nm irradiation at −1.1 V vs. RHE to provide very high photoelectrochemical CO₂ reduction activity.

There are three plausible formation pathways of CO from CO₂ electrochemical or photoelectrochemical reduction:^30,38,39

In aqueous electrolyte, CO is mostly formed from CO₂ via reaction (1), where 2 H⁺ and 2e⁻ participate. In organic solvent/aprotic electrolyte, reaction (2) is favoured at sufficiently high overpotential or high pressure^39,40. Generally, reaction (3) is rare for photoelectrochemical CO₂ reduction because reduction is usually carried out at negative bias potential, so such photodissociation on the electrode surface is difficult³⁰. Note that in aqueous media, hydrogen is often produced through proton reduction⁴¹, e.g., 2H⁺ + 2H⁺ + 2e⁻ = H₂2e⁻ in addition to CO₂ reduction at the electrode surface. A negligible amount of hydrogen found with the Ag-BDD_L electrode could produce via proton reduction. However, the experimental results reveal that this smaller amount of hydrogen is produced through electrochemical reduction of proton at the BDD surface (Figs 2 and 3). Hori et al.³⁶. reported that Ag, Au and Zn can electrochemically reduce CO₂ to CO with high selectivity. This is possibly because of the formation of strong chemical bonds between and the catalyst surface. Formation of the anion radical involves one-electron transfer (), and this anion species is very unstable. The high photoelectrochemical activity of 0.1 Ag-BDD_L in aqueous electrolyte at −1.1 V vs. RHE is attributed to the transfer of very high energy photoexcited electrons in the CB of BDD_L to the Ag nanoparticle surfaces to facilitate CO₂ reduction. The electrochemical cathodic peak of the 0.1 Ag-BDD_L electrode under CO₂-saturated conditions suggests that CO₂ reduction occurs on the Ag surface. Therefore, the intermediates of CO₂ reduction must be adsorbed on the Ag surface³⁷. It has been reported that Ag surfaces could stabilize anion radicals at certain negative potential depending on the experimental conditions, electrolyte and pH (standard redox potential for is −1.9 V)^12,36,37,42. BDD can produce the anion radical in 0.1 M Na₂SO₄ and this radical can be stabilized as an intermediate product under suitable experimental conditions (high pressure)³⁰. Therefore, the enhanced photoelectrochemical CO₂ reduction of 0.1 Ag-BDD_L is thought to be related to the formation of anion radicals by the transfer of photoexcited electrons to the Ag nanoparticles. The produced anion radicals are then immediately stabilized by a proton-coupled electron transfer mechanism at the Ag surface (Fig. 5) to produce the more stable products CO and H₂O³⁷.

Figure 5

Schematic diagram of half-cell charge transfer with an Ag-BDD_L photocathode in 25 mM Na₂SO₄ for CO₂ reduction.

Photoexcited electrons are produced in BDD_L under 222 nm irradiation. The photoexcited electrons are easily transferred to the Ag nanoparticles deposited on BDD_L. These photoexcited electrons could be transferred to the CO₂ molecules adsorbed on the Ag surface to form anion radicals. The highly energetic anion radicals are readily stabilized by the proton-coupled electron transfer mechanism to produce CO at −1.1 V vs. RHE. For clarity, donor levels near the VB of BDD_L originating from the boron impurities are not shown.

Conclusion

In summary, the optimized Ag-BDD_L electrode exhibits enhanced photoelectrochemical CO₂ reduction activity for CO formation with minimal H₂ production under 222 nm irradiation. This Ag-BDD_L electrode exhibits superior activity to that of highly doped BDD_H and GCE modified with Ag nanoparticles. The optimized Ag-BDD_L electrode is stable after five cycles, retaining 44% of its initial activity for CO production. A high isotopic yield (94.17% after 5 h) revealed that the electrode is not corroded under 222 nm irradiation at −1.1 V. The content of surface oxygenated species decreases slightly during the photoelectrochemical reaction, which might increase the number of active sites for CO₂ reduction. Therefore, the photocurrent increases during the first hour of photoelectrochemical reaction with the 0.1 Ag-BDD_L electrode. Such surface photoreduction was not observed after 1 h of photoelectrochemical reaction. The enhanced activity of Ag-BDD_L is ascribed to the synergistic interaction of the Ag nanoparticles with semiconducting BDD_L. Ag is known for its selective CO₂ reduction, while the very high energy of the CB of BDD_L enhanced the formation of anion radicals on the Ag surface in aqueous electrolyte. The anion radicals were stabilized on the Ag surface at −1.1 V via a proton-coupled electron transfer mechanism to produce CO. The very high energy of the CB electrons of BDD_L could be coupled with other co-catalysts to potentially improve the yield of several kinetically and thermodynamically limited photochemical conversions.

Methods

Deposition of Ag nanoparticles

BDD thin films were fabricated on silicon substrates using a reported microwave plasma chemical vapour deposition method³⁵. Ag nanoparticles were electrodeposited on BDD_L (effective area 1 × 1 cm) in a two-electrode system at −0.5 V with respect to a Pt counter electrode in aqueous AgNO₃ (0.025–0.1 M) for 60 s. After Ag nanoparticle deposition, the electrodes were cleaned by sonication in deionized water (18.2 MΩ) for 15 min. After washing, electrodes were dried under a N₂ flow at room temperature.

Photoelectrochemical studies

Photoelectrochemical CO₂ reduction activity was investigated in an H-type electrochemical cell. Working and counter electrodes were separated by a proton-exchange Nafion membrane, and an aqueous solution of Na₂SO₄ was used as the supporting electrolyte. Ag/AgCl saturated with KCl was used as a reference electrode and Pt mesh served as a counter electrode. The scan rate was 100 mV/s. Excimer lamps (7 W) with wavelengths of 172, 222 and 308 nm were used in the photoelectrochemical studies. Light sources were positioned 2 cm away from the electrode and the reaction temperature was held at 25 °C throughout the experiments.

Gas chromatography analysis

Gaseous products were analysed by a gas chromatograph (GC-2014, Shimadzu, Japan) equipped with a thermal conductivity detector and flame ionization detector. Isotopic ¹³CO was measured by GC-MS (GC-2010 Plus, Shimadzu) and the relative amount was estimated by comparison with the amount of ¹²CO produced.

Ion chromatography analysis

An IC system (Dionex ICS-1000, Thermo Scientific) equipped with DionexIonPac AS12A and DionexIonPac CS12A columns was used to identify any detectable CO₂ reduction liquid products obtained with the Ag-modified BDD_L electrodes.

Sample characterization

An FESEM (JEOL, JEM-3100F) was used to analyse the surface morphology of the Ag-BDD_L electrodes. Energy-dispersive X-ray spectroscopy was conducted for elemental mapping of the Ag nanoparticle-modified BDD_L substrates. Elemental states of Ag-BDD_L electrodes before and after photoelectrochemical reaction at −1.1 V for 5 h were determined with an X-ray photoelectron spectrometer (ESCA-3400, Shimadzu) using a monochromatic Mg Kα source.