Concentrated solar CO₂ reduction in H₂O vapour with >1% energy conversion efficiency

Abstract

H₂O dissociation plays a crucial role in solar-driven catalytic CO₂ methanation, demanding high temperature even for solar-to-chemical conversion efficiencies <1% with modest product selectivity. Herein, we report an oxygen-vacancy (V_o) rich CeO₂ catalyst with single-atom Ni anchored around its surface V_o sites by replacing Ce atoms to promote H₂O dissociation and achieve effective photothermal CO₂ reduction under concentrated light irradiation. The high photon flux reduces the apparent activation energy for CH₄ production and prevents V_o from depletion. The defects coordinated with single-atom Ni, significantly promote the capture of charges and local phonons at the Ni d-impurity orbitals, thereby inducing more effective H₂O activation. The catalyst presents a CH₄ yield of 192.75 µmol/cm²/h, with a solar-to-chemical efficiency of 1.14% and a selectivity ~100%. The mechanistic insights uncovered in this study should help further the development of H₂O-activating catalysts for CO₂ reduction and thereby expedite the practical utilization of solar-to-chemical technologies.

Introduction

CO₂ reduction involves multiple proton-coupled electron transfers, where the final product is determined based on the kinetic and thermodynamic parameters of the reduction pathway^1,2,3,4. CH₄ formation is thermodynamically more favourable than CO₂ reduction to CO, but it is kinetically more challenging^5,6,7. Although using H₂O is the most cost-effective proton source, H₂O dissociation is the primary rate-limiting step in the photo-thermal catalytic CH₄ production from CO₂ and H₂O^8,9,10, requiring high temperatures for very modest (<1%) energy conversion efficiencies¹¹. To circumvent these issues, efficient activation of H₂O molecules, separation of photo-catalysis and thermo-catalysis on a microscopic scale, and generation of active sites that can capture localised phonons as well as photo-generated charge carriers are key research goals in catalyst development.

Single-atom Ni is widely used as a catalyst for the oxygen evolution reaction to promote the dissociation of H₂O and production of reactive oxygen species while enhancing charge trapping and transfer^12,13,14,15. In addition, single-atom Ni can generate hot spots with strong electron–phonon coupling, which overlap with the captured photo-generated charge carriers^16,17,18, thereby enhancing the molecular dissociation at the catalyst surface and improving the solar-to-chemical (STC) energy conversion efficiency. Accordingly, we set out to investigate a single-atom Ni-based catalyst approach for the efficient photo-thermal reduction of CO₂ with H₂O to CH₄ under concentrated solar irradiation conditions.

The catalyst, in the form of single-atom Ni-loaded oxygen-vacancy (V_o) rich CeO₂, presented a CH₄ yield of 192.75 µmol/cm²/h with the selectivity of ~100%. Significantly, the STC energy conversion efficiency in this system reaches 1.14%. In-depth mechanism investigations are performed using both experimental and theoretical methods. Our findings demonstrate that V_o regenerated on the catalyst surface ensures a steady supply of active sites for CO₂ reduction. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations confirm the crucial roles of Ni and V_o in enhancing carrier kinetics and reactant activation. Furthermore, ab initio molecular dynamics (AIMD) and TDDFT simulations demonstrate that the H₂O dissociation on the catalyst surface proceed via thermally assisted photo-catalysis. In this work, we offer an effective photo-thermal approach to achieve high STC energy conversion by integrating concentrating solar irradiation and elucidates how the surface atomic structures impact the catalytic efficiency.

Results

Structural characterization of catalysts

CeO₂ exhibits a strong response to the full spectrum, high catalytic stability under high temperatures and pressures^19,20,21, and abundant V_o and lattice defects on its surface^22,23,24. Based on these characteristics, we initially fabricated CeO₂ non-porous nanorods using the hydrothermal method with strict temperature control and chemical reduction, which allowed for creating multiple V_o and anchoring single-atom Ni centres on their surface (Supplementary Fig. 1). The X-ray diffraction (XRD) pattern of the obtained products (Supplementary Fig. 2) can be accurately indexed to the standard fluorite structure of CeO₂ (PDF card no. 43-1002). No notable diffraction peaks for Ni were observed, probably because of the low Ni content. Transmission electron microscopy (TEM) images revealed that NF@0.1%Ni@CeO₂-V_o and NF@0.1%Ni@CeO₂ exhibited a porous nanorod morphology (Fig. 1a and Supplementary Figs. 3a, b), which increases the catalyst-specific surface area, enhancing the CO₂ adsorption and providing more active sites (Supplementary Fig. 4; Supplementary Table 1). In-situ CO-DRIFT images failed to identify whether Ni is a single atom due to the poor chemisorption of CO on the non-precious metal Ni (Supplementary Figs. 5, 6). HAADF-STEM images of NF@0.1%Ni@CeO₂-V_o show lattice stripes with a crystal plane spacing of 0.31 nm, corresponding to the (111) crystal plane of CeO₂ on the primarily exposed planes (Fig. 1b). Besides, the presence of Ni single atoms was observed (Fig. 1c).

Fig. 1: Morphological characterizations of NF@0.1%Ni@CeO₂-V_o.

a TEM image of NF@0.1%Ni@CeO₂-V_o. b, c HAADF-STEM image of NF@0.1%Ni@CeO₂-V_o. d–g HDAAF (d) EDX elemental mapping images of Ce (red) (e), O (green) (f), and Ni (yellow) (g) in NF@0.1%Ni@CeO₂-V_o. h, i Normalized X-ray absorption near-edge spectra at the Ni K-edge (h) and k²-weighted Fourier transform extended X-ray absorption fine-structure spectra (EXAFS) in r-space (i) for NF@0.1%Ni@CeO₂ (blue curve) and NF@0.1%Ni@CeO₂-V_o (red curve) catalysts. j, k Ni–O coordination environment in NF@0.1%Ni@CeO₂ (j) and NF@0.1%Ni@CeO₂-V_o (k) catalysts. l–p Wavelet Transformation for the k²-weighted EXAFS signal of the Ni foil (l), NiO (m), Ni₂O₃ (n), NF@0.1%Ni@CeO₂ (o), and NF@0.1%Ni@CeO₂-V_o (p) catalysts.

The energy-dispersive X-ray spectroscopy elemental mapping images (Fig. 1d–g) showed that Ce, O and Ni were evenly distributed. The electronic and coordinative structures of NF@0.1%Ni@CeO₂-V_o and NF@0.1%Ni@CeO₂ were supported by X-ray absorption spectroscopy (XAS) results. According to the X-ray absorption near-edge spectra at the Ni K-edge of NF@0.1%Ni@CeO₂-V_o and NF@0.1%Ni@CeO₂ (Fig. 1h), the E₀ of NF@0.1%Ni@CeO₂-V_o is similar to that of NiO, suggesting that Ni atoms carry a +2 charge, while the E₀ of NF@0.1%Ni@CeO₂ is similar to that of Ni₂O₃, indicating an oxidation state of +3 for the Ni atoms. This can be attributed to the presence of V_o, which enables the transfer of electrons from O^2- on the catalyst surface to Ni, reducing Ni³⁺ to the +2 valence state; conversely, Ni remains in the +3 valence state in the absence of V_o (Supplementary Fig. 7).

The coordination environment of single-atom Ni was determined using extended X-ray absorption fine-structure (EXAFS) spectroscopy. The Fourier-transformed k²-weighted EXAFS spectra at the Ni K-edge of NF@0.1%Ni@CeO₂-V_o and NF@0.1%Ni@CeO₂ (Fig. 1i) show a prominent peak at the position of the Ni–O shell (1.5 Å), which is absent in the spectra of the reference samples of Ni foil and NiO. The EXAFS data fitting results (Supplementary Table 2) indicate that the coordination number of Ni–O in NF@0.1%Ni@CeO₂-V_o is 4.3, whereas that of Ni–O in NF@0.1%Ni@CeO₂ is 5.1 (Supplementary Fig. 8a–d). These results suggest that the introduction of V_o causes surface reconstruction, with Ni replacing Ce and anchored around the surface V_o sites, decreasing the H₂O dissociation energy The coordination environment of Ni–O in NF@0.1%Ni@CeO₂-V_o and NF@0.1%Ni@CeO₂ was modelled using density functional theory (DFT) (Fig. 1j, k), revealing that the single-atom Ni loaded on the (111) face replaced Ce, forming two structures with coordination numbers and bond lengths that align with the XAS results (Supplementary Table 3). The Ni foil exhibits a wave flap associated with Ni–Ni coordination at (7.2 Å, 2.2 Å⁻¹) (Fig. 1l), whereas NiO and Ni₂O₃ present an additional wave flap at (5.5 Å, 1.5 Å⁻¹), which is attributed to Ni–O coordination (Fig. 1m, n). In the case of NF@0.1%Ni@CeO₂-V_o and NF@0.1%Ni@CeO₂, the absence of wave flaps at high k values (Fig. 1o, p) indicates that the central Ni is not bound to another heavy atom; in contrast, wave flaps appear at low k values (5.5 Å, 1.5 Å⁻¹), which can be attributed to Ni–O coordination. Meanwhile, XAS and EPR analyses confirmed the electronic structure, coordination structure of the Ce, and V_o content of the catalysts (Supplementary Figs. 9, 10; Supplementary Table 4).

Photo-thermal catalytic performance

The presence of V_o can exert diverse effects on the local coordination environments of Ni (i.e., NiO₄ and NiO₅). To explore the impact of different coordination environments on the surface temperature and photo-thermal catalytic performance, we utilised the reaction between CO₂ and H₂O to form CH₄ under concentrated solar irradiation as a probe. The infrared (IR) images presented in Supplementary Fig. 11 indicate the surface temperatures of NF@0.1%Ni@CeO₂-V_o and NF@0.1%Ni@CeO₂ under concentrated solar irradiation (those recorded under non-concentrated solar irradiation conditions are presented in Supplementary Fig. 12). Meanwhile, light source spectra are provided in Supplementary Fig. 13. NF@0.1%Ni@CeO₂-V_o stabilised at 362.1 °C in 7.5 s, while NF@0.1%Ni@CeO₂ stabilised at 306.5 °C in 12.6 s (Fig. 2a). This difference in surface temperature can be attributed to Ni forming a local octahedral coordination with lattice O atoms in the absence of V_o, resulting in the formation of non-occupied mid-gap states. These states can trap hot electrons and, in combination with holes, can locally exhibit a hot-spot effect (Fig. 2b). In contrast, in the presence of V_o, Ni forms a planar quadrilateral coordination closer to the valence-band maximum (VBM), which can trap holes that subsequently recombine with electrons at the conduction-band minimum (CBM), generating a hot-spot effect (Fig. 2c). However, the impurity state of the d orbital is located at the top of the VBM, which implies that the VBM hole capture is equivalent to a relaxation process with an energy level that is similar to the bandgap value between the CBM and VBM, increasing the possibility of capturing the photo-generated holes (Detailed description in Supplementary Fig. 14). The wave function distribution of the corresponding mid-gap states confirms this hypothesis (Supplementary Fig. 15). Moreover, the electron–phonon interactions in NF@0.1%Ni@CeO₂ (Fig. 2b) include two steps of scattering to form two phonons of low frequencies. However, in NF@0.1%Ni@CeO₂-V_o (Fig. 2c), only one phonon of a high frequency is formed, resulting in a higher average surface temperature. Besides, the scattering illustrated in Fig. 2b is a multi-stage process, resulting in a lower scattering probability.

Fig. 2: Impact of Ni coordination environments on photo-thermal catalytic performance.

a Surface temperature of NF@0.1%Ni@CeO₂ and NF@0.1%Ni@CeO₂-V_o catalysts under concentrated solar irradiation (4200 mW/cm²). b, c Density of states for NF@0.1%Ni@CeO₂ (b) and NF@0.1%Ni@CeO₂-V_o (c). d Thermocatalytic yields of CH₄ from the NF@0.1%Ni@CeO₂-V_o catalyst in the dark and with the addition of 1200 mW/cm² solar light. e Photo-thermal catalytic CH₄ yields from NF@0.1%Ni@CeO₂ and NF@0.1%Ni@CeO₂-V_o catalysts under different conditions (non-concentrated solar irradiation represents 420 mW/cm², and concentrated solar irradiation represents 4200 mW/cm²). f Comparison of energy efficiency of a series of literature-reported catalysts under similar conditions. g–i In-situ XRD pattern (g), in-situ EPR pattern (h), and apparent activation energy of NF@0.1%Ni@CeO₂-V_o (i) under concentrated solar irradiation and non-concentrated solar irradiation.

To gain insight into the catalytic CO₂ reduction process under concentrated solar irradiation conditions, we evaluated the thermo-catalysed CO₂ reduction using NF@0.1%Ni@CeO₂-V_o under dark conditions. Upon increasing the temperature from 200 °C to 400 °C, no CH₄ was produced (Fig. 2d). Conversely, under simulated solar light irradiation of 1200 mW/cm², the CH₄ yield substantially increased. In addition, catalytic experiments were conducted with cooled circulating water or under irradiation of variable wavelengths (Supplementary Figs. 16, 17, 18). The catalytic reaction is facilitated by raising the temperature and photogenerated electrons play a key role. The above results suggest that the CO₂ photo-thermal catalysis is proceeded in a manner of thermally assisted photo-catalysis. Then, we used plastic Fresnel lenses as cost-effective solar concentrators to perform experiments under dark, non-concentrated solar irradiation and concentrated solar irradiation conditions (Fig. 2e). The concentrated solar irradiation reactor and specific experimental parameters are described in detail in Supplementary Fig. 19a, b. Meanwhile, with conventional lenses, Fresnel lenses can focus parallel light rays to a closer common focal length and reduce the absorption of light owing to their thin thickness (Supplementary Fig. 19c)²⁵. NF@0.1%Ni@CeO₂-V_o afforded a CH₄ yield of 192.75 µmol/cm²/h under concentrated solar irradiation conditions, which was 78 times higher than that achieved under non-concentrated solar irradiation (2.47 µmol/cm²/h) and 42 times higher than that obtained with NF@0.1%Ni@CeO₂ under concentrated solar irradiation conditions (4.62 µmol/cm²/h). Additionally, the optimal catalyst preparation conditions were screened (Supplementary Figs. 20, 21). The concentration of V_o can be regulated by the hydrothermal temperature (Supplementary Fig. 22). Under identical conditions, catalytic activities of 0.1%Ni@CeO₂-V_o catalysts loaded on nickel foam did not present notable difference from those of 0.1%Ni@CeO₂-V_o powder catalysts (Supplementary Fig. 23a, b). This result demonstrates that nickel foam does not actively participate in the reaction. Therefore, it is rational to consider nickel foam as a support material for the catalyst. To identify the carbon source of the reduced products, isotope tracing experiments were carried out using reactant ¹³CO₂. The main signal at m/z = 17 is assigned to ¹³CH₄ and the other signal at m/z = 29 can be assigned to ¹³CO. This result indicates that the CO and CH₄ generation originates from the CO₂ reactant, thereby ruling out catalyst decomposition or carbon pollution as contributing factors (Supplementary Fig. 24).

The energy conversion efficiency is a crucial indicator for evaluating photocatalytic activity. Three benchmark photo-thermal catalysts were synthesized for comparison with the present catalyst and existing studies (WO₃, ZrO₂, and TiO₂, see Supplementary Figs. 25, 26 for the structure/phase information). The direct STC energy conversion of the present catalyst exhibits high efficiency (1.14%; Fig. 2f; Supplementary Fig. 27; Supplementary Table 5), with a CH₄ selectivity of approximately 100% (Supplementary Fig. 28; Supplementary Table 6). Moreover, the differential thermal analysis and thermogravimetric analysis results indicate that NF@0.1%Ni@CeO₂-V_o has high-temperature resistance (Supplementary Fig. 29a, b). According to the XAS of NF@0.1%Ni@CeO₂-V_o after the CO₂ reduction reaction under concentrated solar irradiation conditions (Supplementary Figs. 30, 31; Supplementary Tables 7, 8), the valence state and coordination environment of Ni in the catalyst did not clearly change, confirming the strong thermal stability of the catalyst. The catalytic performance did not decline significantly after six cycles and storage in an Ar atmosphere for 30 days (Supplementary Fig. 32), indicating high catalytic stability. To elucidate the reason for the high catalytic activity under concentrated solar irradiation conditions, we subjected NF@0.1%Ni@CeO₂ and NF@0.1%Ni@CeO₂-V_o to UV–vis diffuse reflectance spectroscopy (DRS). An enhanced full-spectrum absorption response was observed for NF@0.1%Ni@CeO₂-V_o, as evidenced by a convex peak attributable to the V_o absorption near 1400 and 1800 nm (Supplementary Fig. 33)²⁶. This suggests that the V_o may absorb a portion of the energy from the IR light, thereby enhancing the thermal effect in the photo-thermal coupling process. Furthermore, we conducted in-situ XRD tests, finding that the catalyst diffraction peak shifted towards the small-angle direction under concentrated solar irradiation. This phenomenon can be attributed to two reasons: (1) thermal expansion and (2) high flux photon density. The variable-temperature XRD patterns exhibit no shift of the diffraction peaks with increasing temperature. In fact, thermal expansion induces uniform lattice distortions and the distance changes between each crystal plane. However, the changes in lattice constants induced by thermal effects are limited, so the shifts in the positions of the diffraction peaks are weak (Supplementary Fig. 34). On the other hand, high flux photon densities can excite electrons in the catalyst, causing the lattice atoms to leap to the defect sites. These leaps can lead to the formation of defects, which can alter the distances between atoms in the crystal, resulting in a change in the lattice constant and shift of the diffraction peak (Fig. 2g).

In situ, electron paramagnetic resonance (EPR) also provided compelling evidence for the increase in V_o in the catalysts under concentrated solar irradiation conditions (Fig. 2h), which enhances the recombination between electrons and holes. Photogenerated carriers can be captured by Ni single atoms and release high-frequency phonons via recombination. Meanwhile, Ni single atoms and V_o form atomic-scale active sites. These active sites, combined with the hot-spot effect induced by carriers’ recombination, promote the CO₂ reduction reaction. The surface temperature elevation with the introduction of Ni single atoms and V_o is shown in Supplementary Fig. 35a. Besides, V_o indeed possesses the capability to capture photo-generated charges (Supplementary Fig. 35b, c). On the other hand, the in-situ XPS spectrum demonstrates the valence state change of Ni was undetectable due to its low content of 0.1% (Detailed description in Supplementary Fig. 36a). Conversely, the content of Ce³⁺ increased with light irradiation. This result also suggests the increased V_o content under concentrated solar irradiation (Supplementary Fig. 36b).

Additionally, we determined the apparent activation energy for the CO₂ reduction to CH₄ under concentrated and non-concentrated solar irradiation conditions, respectively. The relation between ln(r_CH4) and 1/T is plotted in Fig. 2i and Supplementary Table 9, where the measured points are fitted as straight lines using the Arrhenius equation^27,28:

$${{\mbox{lnk}}}={{{{\mathrm{ln}}}}}{{\mbox{A}}}-{{{\mbox{E}}}}_{{{\mbox{a}}}}/{{\mbox{RT}}}$$

(1)

where k, R, T, E_a and A denote the rate constant, molar gas constant, thermodynamic temperature, apparent activation energy, and frequency factor, respectively. A smaller slope represents a lower E_a. The results suggest that the CH₄ production increases owing to a decrease in E_a from 19.54 kJ/mol under non-concentrated solar irradiation to 5.49 kJ/mol under concentrated solar irradiation conditions. The added heat increases the probability of reactant molecules overcoming the activation energy barrier during CO₂ reduction reactions. The equation for the Boltzmann distribution is as follows²⁷:

$${{{\mbox{E}}}}_{{{\mbox{t}}}}=\frac{3}{2}{{{\mbox{k}}}}_{{{\mbox{B}}}}{{\mbox{T}}}$$

(2)

where E_t, k_B and T denote the molecular average kinetic energy, Boltzmann constant and temperature, respectively. E_t increased according to this equation with increasing temperature. This was confirmed by determining the CH₄ yield at different temperatures under a constant concentrated solar irradiation intensity (Supplementary Figs. 37, 38; Supplementary Tables 10, 11). The CH₄ yield increased with increasing temperature, demonstrating the auxiliary role of heat in the catalysis. Photo-electrons generated by concentrated solar irradiation can further reduce the E_a of the product, and heat can increase the E_t (Supplementary Fig. 39a–d).

Mechanism of light and thermal effects over the Ni single atom

The photo-thermal catalytic CO₂ reduction unavoidably involves both thermal and optical effects on the catalyst. Therefore, we comprehensively examined the light and thermal effects on the catalytic mechanism separately. When photo-excitation occurs from the valence band (VB) to the conduction band (CB), a hole relaxes to the VBM and an electron relaxes to the CBM (Fig. 3a). In the presence of a mid-gap state, the electron and hole may directly combine through an intermediate impurity state that captures both charge carriers, forming a thermally dissociated molecule while the photo-generated electron–hole pair eventually become phonons (i.e., heat). However, if only a hole or an electron is captured by the intermediate impurity state, it is rapidly transferred to the adsorbed molecule through the d orbital.

Fig. 3: Light and thermal effects on the catalytic mechanism.

a Schematic diagram of two paths of photon energy conversion. b–e Steady-state PL spectra at different temperatures. b TRPL at different temperatures c transient surface photovoltage at different irradiation intensities d transient surface photovoltage at a different irradiation intensity, obtained by taking the logarithm of the abscissa of 3d (e) for NF@0.1%Ni@CeO₂-V_o.

Subsequently, we investigated in detail the migration of carriers induced by the thermal and light effects in NF@0.1%Ni@CeO₂-V_o. Variable-temperature PL showed that the fluorescence intensity progressively decreases with increasing temperature (Fig. 3b), suggesting that the thermal movement of the carriers increases, thus increasing carrier mobility and separation. Moreover, the fluorescence lifetime gradually decreases with increasing temperature (Fig. 3c) because heat enhances the carrier recombination rate. However, because the migration rate is greater than the complexation rate, heat is favourable for the reaction. OCP tests at variable temperatures also validate this mechanism (Supplementary Fig. 40). Meanwhile, the photo-response of NF@0.1%Ni@CeO₂-V_o was the most substantial under concentrated solar irradiation conditions (Supplementary Figs. 41, 42), suggesting that the high photoelectron flux density drives single-atom Ni and V_o to further enhance the conductivity and carrier separation efficiency. Furthermore, the photo-generated electrons under concentrated solar irradiation not only increase in quantity but also exhibit enhanced quality, which can be attributed to the increase in the Fermi energy level in NF@0.1%Ni@CeO₂-V_o (Supplementary Fig. 43). The elevated Fermi energy level indicates an increased carrier density in the CB, which is conducive to the photo-thermal reduction of CO₂.

Furthermore, we evaluated the TPV curves for different irradiation intensities (Fig. 3d). The electron lifetime decreases with increasing irradiation intensity, which can be attributed to the Auger loss under the high density of excited electrons²⁷. For NF@0.1%Ni@CeO₂-V_o, numerous electrons are generated after being excited by concentrated solar irradiation, some of which migrate and participate in the reduction reaction. Others undergo Auger recombination, releasing the rest of the energy, which is transferred to a third charge carrier that thermalises to the edge of the CB by emitting phonon vibrations, raising the catalyst and facilitating the photo-thermal conversion. We applied logarithmic processing to the tails of the TPV curves; the resulting combined shaded area at different irradiation intensities suggests that the charge extraction efficiency improved with increasing irradiation intensity (Fig. 3e).

Charge carrier dynamics

To investigate the enhancement of the electron transfer in single-atom Ni under light irradiation, we prepared a NF@CeO₂-V_o catalyst without Ni. Its physical phases and morphology are presented in Supplementary Fig. 44. Regardless of the presence of V_o, the surface temperature of the catalyst without Ni was lower than that of the Ni-based catalyst (Supplementary Fig. 45). This can be attributed to the localised property of the d orbital of Ni, which enables the capture of photo-generated charge carriers and their efficient conversion into phonons. Furthermore, this finding provides additional evidence for the local hot-spot effect of single-atom Ni. Then, we evaluated the catalytic performance of NF@CeO₂-V_o under dark, non-concentrated and concentrated light conditions (Fig. 4a), finding that under concentrated solar irradiation conditions, NF@CeO₂-V_o afforded a CH₄ yield of 3.88 µmol/cm²/h, which was 50 times lower than that of NF@0.1%Ni@CeO₂-V_o. This remarkable enhancement suggests that single-atom Ni promotes the separation and transfer of photo-generated carriers. The energy level positions of NF@CeO₂-V_o, NF@0.1%Ni@CeO₂ and NF@0.1%Ni@CeO₂-V_o were calculated via UV–vis DRS and valence-band X-ray photoelectron spectroscopy (XPS) using the Kubelka–Munk function (Supplementary Figs. 46, 47), revealing the consistency of the band gaps with the potential for the reduction of CO₂ to CH₄.

Fig. 4: Catalytic performance of the Ni-based catalysts and charge carrier dynamics.

a Photo-thermal catalytic CO/CH₄ yields from NF@0.1%Ni@CeO₂-V_o and NF@CeO₂-V_o catalysts under different conditions. b–f Surface photovoltage (b), transient surface photovoltage (c), plot of the rate constant of charge recombination vs. potential (d), plot of the rate constant charge transfer rate vs. potential (e), and plot of the average lifetime of photoinduced electrons (τ_d) vs. potential (f) for NF@CeO₂-V_o, NF@0.1%Ni@CeO₂, and NF@0.1%Ni@CeO₂-V_o.

The results suggest the occurrence of considerable synergistic effects between V_o and single-atom Ni. The CO₂ temperature-programmed desorption spectra shown in Supplementary Fig. 48 confirm that V_o and Ni can provide surface basic sites to facilitate the CO₂ adsorption and decomposition. To investigate the recombination of photo-generated carriers, steady-state photoluminescence (PL) tests were conducted using an excitation wavelength of 255 nm (Supplementary Fig. 49a). Both NF@CeO₂-V_o and NF@0.1%Ni@CeO₂-V_o produced a sharp convex peak at approximately 400 nm, which can be attributed to defective-state luminescence²⁹. Moreover, the photo-generated carriers in NF@0.1%Ni@CeO₂-V_o are highly resistant to recombination, resulting in more electrons available for the CH₄ production reaction. Supplementary Fig. 49b demonstrates that among all samples, NF@0.1%Ni@CeO₂-V_o showed the longest emission lifetime (114.42 µs), as measured using time-resolved PL spectroscopy. The values of τ₁, τ₂ and average emission lifetime (τ) were obtained through data fitting (Supplementary Table 13). The synergistic effect of Ni and V_o was found to enhance the electron transport efficiency at the catalyst surface, prolonging the excited state of the fluorescent molecules and thereby substantially reducing the carrier recombination. The synergistic effect of Ni and V_o was found to enhance the electron transport efficiency at the catalyst surface, prolonging the excited state of the fluorescent molecules and thereby substantially reducing the carrier recombination. This phenomenon arises due to the presence of a protective charge, preventing the emission of fluorescence when carriers are captured by the trap state. Therefore, while the overall fluorescence intensity decreases, carrier recombination resulting in fluorescence forms a longer temporal tail. Additionally, this protective charge increases the carrier lifetime by localizing around the trap state and reducing spatial overlap with other charge carriers. The above analysis explains why V_o acting as a trap state leads to weaker fluorescence intensity yet longer carrier lifetime.

Subsequently, to obtain information about the charge separation direction, we investigated the photo-electrical properties of the catalysts by measuring the surface photo-voltage (SPV) signal generated by the spatial separation of photo-generated charge carriers. NF@0.1%Ni@CeO₂-V_o exhibited a strong response signal in the range of 300–1000 nm compared with other catalysts (Fig. 4b) because Ni and V_o can accept photo-generated charge carriers from CeO₂ through interface charge transfer, improving the separation efficiency. Transient surface photo-voltage (TPV) spectroscopy measures the SPV signal as a function of the photon energy and relaxation time³⁰. For the CO₂ reduction reaction, the photo-generated electrons must migrate towards the outer surface, generating SPV signal peaks in the downward direction. The TPV curves of the catalysts (Fig. 4c) revealed that NF@0.1%Ni@CeO₂-V_o exhibits the largest photo-voltage and shifts towards a long-time scale (Supplementary Table 14). Furthermore, we logarithmically processed the tails of the TPV curves and observed a general rightward shift in the curves (Supplementary Fig. 50), further confirming that the co-existence of Ni and V_o alters the charge separation channel and enhances the separation of photo-induced electron and hole pairs. This was also supported by linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) measurements (Supplementary Fig. 49c, d).

Investigating the behaviour of photo-induced carriers on the Helmholtz layer is also essential to reveal the charge transfer and complexation dynamics in NF@0.1%Ni@CeO₂-V_o. Therefore, we used intensity-modulated photocurrent spectroscopy (IMPS) to evaluate the pseudo-first-order rate constants for surface complexation (k_rec) and hole transfer (k_tran)³¹. Supplementary Fig. 51 displays the typical IMPS responses of different catalysts in the complex plane. The k_rec and k_tran values were calculated using the IMPS spectra obtained at different applied bias voltages. NF@0.1%Ni@CeO₂-V_o showed lower k_rec and higher k_tran values than NF@CeO₂-V_o and NF@0.1%Ni@CeO₂, indicating that the Ni-and-V_o–containing catalyst exhibited improved charge separation/transfer kinetics (Fig. 4d, e). Specifically, the lack of a substantial increase in the k_tran values of NF@CeO₂-V_o and NF@0.1%Ni@CeO₂ suggests that single-atom Ni and V_o are the main contributors to the charge transfer process. The average photo-induced electron transfer time (τ_d) estimated using the lowest imaginary frequency was shorter for NF@0.1%Ni@CeO₂-V_o (Fig. 4f), demonstrating its superior charge transfer kinetics. Furthermore, we conducted IMPS measurements on NF@0.1%Ni@CeO₂-V_o under light sources of different wavelengths (Supplementary Fig. 52a). The τ_d value was minimised after UV–visible irradiation (Supplementary Fig. 52b), suggesting that photo-thermal coupling conditions promoted the charge separation and transfer on NF@0.1%Ni@CeO₂-V_o.

CO₂ reduction mechanism under concentrated solar irradiation

In the photo-thermally coupled catalytic reaction of CO₂ and H₂O, the activation of H₂O is the primary rate-limiting step (Supplementary Fig. 53a). According to the thermodynamic state, H₂O decomposition requires a high energy barrier to proceed with the CO₂ reduction reaction (Supplementary Fig. 53b, c). The Gibbs free energy for H₂ and O₂ production from H₂O decomposition is 273 KJ mol⁻¹, indicating that the reaction cannot proceed spontaneously and requires additional energy to occur. Therefore, to identify the active site responsible for H₂O activation, we employed ab initio molecular dynamics (AIMD) simulations to investigate the desorption of CO₂ on a single-atom Ni active site (Fig. 5a). The results showed that the C in CO₂ cannot coordinate to the Ni sites of the catalyst surface because they are positively charged. Therefore, CO₂ is not stably adsorbed to the surface by bonding single-atom Ni. The CO₂ adsorption energy confirms this view (Detailed description in Supplementary Figs. 54, 55). Furthermore, when CO₂ molecules adsorb to the Ni active site, desorption occurs within approximately 100 fs, indicating that Ni is not an effective active site for CO₂ adsorption.

Fig. 5: Investigation into CO₂ reduction mechanism by NF@0.1%Ni@CeO₂-V_o under concentration solar irradiation.

a AIMD simulation of CO₂ desorbing from Ni active site. b In-situ DRIFTS spectra of NF@0.1%Ni@CeO₂-V_o under CO₂/H₂O conditions and 1000-2200 cm⁻¹ spectra. c Density of states for NF@0.1%Ni@CeO₂-V_o with H₂O adsorption. d AIMD simulation of heat-induced H₂O dissociation. e, f TDDFT simulation of laser-induced H₂O dissociation. g Cycling stability profile after H₂ exposure (each cycle is 12 h). h Schematic illustration of the mechanism of NF@0.1%Ni@CeO₂-V_o for CO₂ reduction upon concentrated solar irradiation.

In-situ DRIFTS spectroscopy was used to identify the critical reaction intermediates generated during the catalytic reaction (Fig. 5b). The remarkably strong m-CO₃²⁻ peaks indicate that CO₂ is adsorbed on the lattice O of CeO₂ instead of on Ni. The production of –CH₂O, –CH₃O and –CH₃ intermediates confirms the migration of multiple photo-generated electrons and, notably, the production of CH₄^32,33. The adsorption bands at 3725, 3677, 3624 and 3594 cm⁻¹ are attributed to physical adsorption via hydrogen-bound OH groups and H₂O stretching vibrations (Supplementary Fig. 56)³⁴. Upon adsorption of a H₂O molecule to single-atom Ni, the structure undergoes relaxation, resulting in a deformed tetrahedral structure. Hydrogen bonding occurs between the two H atoms of the H₂O molecule and the two lattice O atoms originally coordinated to Ni. Near the VBM, the molecular orbital of the H₂O molecule hybridises with the d orbital of Ni, forming a bridge to transfer the photo-generated charge carriers. This hybridised state is an occupied state; the photo-generated holes on the VBM are subsequently transferred to the H₂O molecule through Ni (Fig. 5c).

If only thermal dissociation (600 K) is considered after the adsorption of H₂O molecules on the catalyst surface, the H₂O on the CeO₂ surface would exist in a dissociated state and the released O would fill the V_o around single-atom Ni directly after the dissociation of H₂O molecules. However, H would still be trapped near the lattice O and not transfer to CO₂, resulting in low reactivity (Fig. 5d). In the absence of light excitation and at a temperature of only 10 K, H₂O molecules are stably adsorbed (Fig. 5e). However, H₂O molecules are dissociated upon applying an intense laser field. Owing to the presence of captured photo-generated holes on the H₂O molecules, the released O species are rapidly desorbed from the Ni sites (Fig. 5f). H species are then transferred to CO₂ and captured by an O atom, leading to CO₂ reduction and completing the methanation process. The adsorption energies of H₂O on V_o and Ni single atoms are −0.67 eV and −1.47 eV, respectively (Supplementary Fig. 57). It is worth noting that on CeO₂-V_o, H₂O is adsorbed by the unsaturated Ce atoms instead of V_o sites. Meanwhile, on Ni@CeO₂, H₂O is adsorbed by the Ni single atoms instead of Ce atoms (Supplementary Fig. 58). Additionally, H₂O-TPD demonstrates that the incorporation of Ni single atoms and V_o can significantly enhance the dissociation capacity of H₂O (Supplementary Fig. 59). Any remaining O species may form O₂ molecules with other O species or fill the V_o, leading to Ni deactivation, which could shift from the planar quadrilateral coordination structure to an octahedral one. To confirm the mechanism, we assessed the concentration of oxygen vacancies before and after the reaction. The findings revealed an increase in the concentration of oxygen vacancies after the reaction, thereby implying that the oxygen vacancies were not occupied by O₂ (Supplementary Fig. 60). Moreover, the O₂ yield reached 762.5 µmol/cm² of concentrated solar irradiation (Supplementary Fig. 61). A slight decrease in activity was observed after six catalytic cycles. The microstructural changes upon H₂O dissociation during the thermally induced and laser-induced reactions are illustrated in Supplementary Fig. 62.

Additionally, we conducted time-dependent DFT (TDDFT) simulations to observe changes in the absence of a catalyst. The application of a laser field resulted in no dissociation of small molecules, indicating that the dissociation of H₂O molecules is due to the electron–hole excitation in the catalyst and not the direct action of the laser field (Supplementary Figs. 63, 64). We also performed three cycles of photo-thermal coupled CO₂ reduction experiments. Although a considerable decrease in the CH₄ yield was observed after the first cycle, a substantial increase occurred in the second and third cycles upon the introduction of hydrogen (Fig. 5g). This phenomenon may be attributed to the regenerative function of V_o under concentrated light conditions, providing a continuous supply of active sites for CO₂ reduction. This hypothesis was verified by subjecting NF@CeO₂-V_o and NF@0.1%Ni@CeO₂-V_o to valence-band XPS under different conditions (Supplementary Figs. 65, 66 and Supplementary Table 15), as discussed in detail in Supplementary Fig. 67.

A plausible mechanism for the photo-thermal reduction of CO₂ catalysed by NF@0.1%Ni@CeO₂-V_o under concentrated solar irradiation conditions is depicted in Fig. 5h. Initially, CO₂ is adsorbed to the surface lattice O of CeO₂, while H₂O molecules are adsorbed on single-atom Ni. The thermally assisted photo-dissociation of H₂O by Ni generates H, which converts CO₂ to CH₄ and O to O₂, with the concomitant release of the products from the catalyst surface. This can be illustrated in detail using a Feynman diagram, where W represents the photons that excite the CB and VB, releasing electrons and holes, respectively. Some holes are responsible for the photo-dissociation of H₂O molecules, while other holes combine with electrons through the intermediate impurity state of Ni, forming phonon Y, which facilitates H₂O dissociation through heating.

Discussion

To conclude, we synthesized a high-performance NF@0.1%Ni@CeO₂-V_o catalyst with single-atom Ni anchored to CeO₂ porous nanorods containing V_o. This catalyst demonstrated CH₄ production with 192.75 µmol/cm²/h yield, ~100% selectivity, and 1.14% efficiency in direct STC energy conversion under concentrated solar irradiation. This irradiation triggers V_o regeneration on the catalyst surface, ensuring a continuous supply of active sites for CO₂ reduction. The hybridized molecular orbitals of H₂O and single-atom Ni, along with structural changes induced by V_o, enhance charge carrier trapping and activation of reactant molecules, while thermal- and photo-dissociation of H₂O on the catalyst surface was confirmed through computational and spectroscopic studies. This research provides insights into the development of highly efficient systems for direct STC energy conversion via the CO₂/H₂O reaction and the design of catalysts for CO₂ conversion under concentrated solar irradiation.

Methods

Materials and reagents

All chemicals utilized in this study, including cerium nitrate hexahydrate (Ce(NO₃)₃·6H₂O, analytical reagent grade (AR), 99.95%), sodium hydroxide (NaOH, AR, 95%), nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O, AR, 98%), sodium borohydride (NaBH₄, 98%), sodium tungstate dihydrate (Na₂WO₄·2H₂O, 99.5%), hydrazine monohydrate (N₂H₄·H₂O, >98%), hydrochloric acid (HCl, 37wt.% in H₂O, 99.999%), zirconyl chloride hydrate (ZrOCl₂·nH₂O, 99.99%), hydrofluoric acid (HF, 48wt.% in H₂O, 99.99%), tetraisobutyl titanate (C₁₆H₃₆O₄Ti, 99.5%) were purchased from Aladdin and directly employed without any additional purification steps. The Ni foam substrate was procured from Kun Shan Electronic Material Co., Ltd. (China). A mixed gas mixture composed of carbon dioxide and nitrogen (CO₂: N₂ = 1:99, v:v) was obtained from the Nanjing Shangyuan industrial gas plant, while all aqueous solutions used during the experimental procedures were prepared using ultrapure water.

Synthesis

Preparation of the monolithic NF@NP-CeO₂

In this study, we synthesized CeO₂ non-porous nanorods using an improved method³⁵, which involved the use of nickel foam as a loading substrate for monolithic catalysts. The nickel foam used had a thickness of 2 mm and dimensions of 1 cm × 1 cm. Before use, it was sonicated in hydrochloric acid solution (10 mL, 0.1 M) for 30 min, followed by sonication in deionized water (10 mL) for another 30 min to eliminate any organic carbon impurities that may have been present on the surface of the nickel foam. It was then dried in a vacuum oven at 70 °C for 1 h. To prepare the CeO₂ precursor solution, Ce(NO₃)₃·6H₂O (1.375 g) was dissolved in deionized water (10 mL) and agitated at 600 r/min for 30 min. Separately, NaOH (9.5 g) was dissolved in deionized water (70 mL) and agitated at 600 r/min for 30 min. The Ce(NO₃)₃·6H₂O solution was slowly added drop by drop into the NaOH solution while being vigorously stirred. The treated nickel foam was then added to the solution, and the mixture was agitated at 600 r/min for 2 h. The nickel foam mixture solution containing the precipitate was transferred to a Teflon-lined reactor, sealed, and heated to 100 °C for 24 h. The resulting non-porous cerium oxide grown on nickel foam and non-porous cerium oxide powder were each washed three times each with ethanol and water, respectively. They were then dried in a vacuum oven at 70 °C for 10 h. The monolithic catalysts obtained were named NF@NP-CeO₂.

Preparation of the monolithic NF@0.1%Ni@CeO₂-V_o, NF@0.1%Ni@CeO₂ and NF@CeO₂-V_o

To prepare the NF@0.1%Ni@CeO₂-V_o catalyst, nickel nitrate hexahydrate (9.91 mg) and a certain amount of sodium borohydride were dissolved in 10 mL of deionized water and sonicated for 1 h. Next, non-porous cerium oxide powder (2 g) and a certain amount of monolithic catalyst NF@NP-CeO₂ were added to the mixed solution and sonicated for 2 h. The resulting mixture solution was transferred to a Teflon-lined reactor, sealed, and heated at 160 °C for 14 h. Finally, the porous cerium oxide nanorods grown on nickel foam were washed three times each with ethanol and water, respectively, and dried in a vacuum oven at 70 °C for 8 h. The sample obtained was named NF@0.1%Ni@CeO₂-V_o. The preparation recipe for NF@0.1%Ni@CeO₂ catalyst is similar to that for NF@0.1%Ni@CeO₂-V_o catalyst, except that hydrothermal conditions (110 °C, 14 h) were used. The NF@CeO₂-V_o catalyst was prepared similarly to the NF@0.1%Ni@CeO₂-V_o catalyst, but without adding Ni(NO₃)₂·6H₂O. Both NF@0.1%Ni@CeO₂-V_o and NF@0.1%Ni@CeO₂ had 0.1% Ni content measured by inductively coupled plasma mass spectrometry (ICP-MS) (Supplementary Table 15).

Preparations of WO₃

1 mmol Na₂WO₄·2H₂O along with nickel foam (1 cm × 1 cm × 2 mm) were dissolved in 20 mL H₂O and the solution was then stirred for 1 h. Next, 0.5 mmol N₂H₄·H₂O was added to the solution and stirred for 1 h. The pH of the mixture solution was then adjusted to 3 using concentrated hydrochloric acid and the solution was stirred for an additional 0.5 h. Afterwards, the mixture solution was heated at 180 °C for 12 h. Finally, the mixture solution was centrifuged, washed, and dried, which was labelled as WO₃.

Preparations of ZrO₂

We dissolved 8 g of ZrOCl₂·nH₂O and nickel foam (1 cm × 1 cm × 2 mm) in 70 mL of deionized water and stirred the solution for 30 min at 600 r/min. Next, we transferred the solution to a Teflon autoclave, sealed it, and heated it to 150 °C for 6 h. After cooling the samples to room temperature, we separated the product by centrifugation, washed it three times with deionized water, and dried it in a vacuum oven at 70 °C for 10 h. This sample was labeled as ZrO₂.

Preparations of TiO₂

A mixture of 1.5 mL of HF solution and 15 mL of C₁₆H₃₆O₄Ti solution was stirred for 1 h. Subsequently, a solvothermal reaction was carried out at 200 °C for 24 h in a 25 mL Teflon autoclave. After natural cooling to room temperature, the precipitants were collected and fully washed with distilled deionized H₂O, then dried in a vacuum oven at 70 °C for 10 h. Then, the powder and nickel foam (1 cm × 1 cm × 2 mm) were stirred in deionized water for 1 h and then aged for 2 h. Finally, the TiO₂ nanosheets were annealed for 3 h in air atmosphere at 450 °C with the temperature elevation rate of 5 °C/min.

Characterizations

High-resolution transmission electron microscopy (HRTEM) images and the corresponding energy-dispersive spectroscopy mapping analyses were obtained using a Talos F200X system. HAADF-STEM images and the images with atomic resolution of samples were performed using a Titan Cubed Themis 60-300 operated at 300 kV, equipped with a probe spherical aberration corrector. X-ray diffraction (Bruker D8-Discover diffractometer, Cu-Kα radiation (λ = 0.1542 nm)) and in situ, X-ray diffraction (Nippon Rigaku Ultimate IV) was used to investigate the crystal phase structure, scanned in the range of 2θ = 10–90 with a scanning rate of 5° min⁻¹, specific test temperature range: 25–400 °C in an N₂ atmosphere. X-ray photoelectron spectroscopy (XPS) spectra were collected using an ESCALab MKII X-ray photoelectron spectrometer, and all binding energies were calibrated using the contaminant carbon (C1s = 284.6 eV) as a reference. X-ray absorption spectra (XAS) including X-ray absorption near-edge structure (XANES) at Ni K-edge and Ce L₃-edge of the samples and extended X-ray absorption fine-structure (EXAFS) at Ni K-edge and Ce L₃-edge of the samples were measured at the beamline 14 W of Shanghai Synchrotron Radiation Facility (SSRF) in China. The output beam was selected by Si (311) monochromator, and the energy was calibrated by Ni foil, NiO, Ni₂O₃, and CeO₂. The data were collected at room temperature under transmission mode. Athena software package was employed to process the XAS data. The specific surface area was calculated using a Brunauer-Emmett-Teller (BET) method. Electron paramagnetic resonance (EPR, Bruker EMXPLUS-6/1) spectroscopy and in situ, electron paramagnetic resonance (EPR, Bruker EMXPLUS-6/1) were used to detect the oxygen-vacancy signal. UV–Vis diffuse reflectance spectra (UV-2600, Shimadzu) were used to study the optical properties of the different catalysts at 300–2500 nm. Catalyst surface temperature and environment temperatures were determined by thermocouple and infrared thermal imager (FLIR). Simultaneous thermal analysis TG-DSC (TA) tests were used to investigate the thermal stability of the catalysts in an N₂ atmosphere at a ramp-up rate of 10 °C/min from room temperature to 600 °C. The CO₂ temperature-programmed desorption measurement (CO₂-TPD, AUTO CHEM 2920) was used to confirm the active adsorption sites of the catalysts. In-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS, Bruker TENSOR II) measurements were also performed to investigate the intermediate during the CO₂ reduction reaction. Considering the limited experimental conditions, the catalysts were carefully extracted from the nickel foam substrate and tested by XAS, XPS, HRTEM, BET, TG-DSC, surface photovoltage (SPV), and transient surface photovoltage (TPV), PL, TRPL, EPR, in-situ DRIFTS, and CO₂-TPD.

Details of the photocatalytic CO₂ reduction

CO₂ photocatalytic reduction experiments were carried out in a 100 mL laterally irradiated heat-resistant closed concentrated solar reactor. Light irradiation was provided by a PLS-SXE300/300UV Xe lamp (Beijing China Education Au lamp) with a standard AM 1.5 G filter and simulated sunlight using an AM 1.5 G filter with an output optical density of approximately 4200 mW/cm². Two Fresnel lenses (diameter: 30 mm) were used to focus the sunlight. First, the prepared monolithic catalysts were fixed with high-temperature-resistant PTFE clips. Then, water (1 mL) was added as the reactant. In addition, the concentrated solar reactor was purged with N₂ for 30 min to ensure a carbon-free atmosphere, and then CO₂ (1% CO₂ and 99% N₂) was passed through for 5 min. Finally, the sealed reactor was left at room temperature, and the gas was analyzed every 30 min using a gas chromatograph (GC-9860-5C). The reaction time for the entire experiment was 2 h.

Photoelectrochemical measurements

Electrochemical impedance spectroscopy (EIS), instantaneous photocurrent measurements (It), linear scanning voltammetry (LSV), and open circuit voltage (OCP) analyses were performed in a standard three-electrode configuration using an electrochemical workstation (CHI660E). To prepare the working electrode, the catalyst (10 mg) was sonicated and dispersed in a mixture of ethanol (1 mL) and Nafion (15 µL), and subsequently applied to the conductive side of FTO glass (1 × 1 cm²) before being vacuum-dried at 70 °C for 3 h. Platinum foil was used as the counter electrode, Ag/AgCl was used as the reference electrode, and the electrolyte was an aqueous Na₂SO₄ solution of 0.1 M. Lighting for the experiments was provided via a high-powered 300 W Xe lamp.

IMPS measurements

The intensity-modulated photocurrent spectroscopy (IMPS) measurements were conducted in a 1 M KOH solution, utilizing a constant potential meter (Zahner PP211, Zahner, Germany) and a three-electrode setup while varying the bias potentials. For the control IMPS test, high-intensity light-emitting diodes (LED: LSW-2) were utilized as the light source across different wavelengths, including 325 nm in the UV region, 560 nm in the visible region, and 780 nm in the NIR region, with an average intensity of 100 mW cm⁻². During the IMPS test, the small ac disturbance light was set to 10% of the dc bias light, while the experiments were performed over a frequency range from 10 kHz to 100 MHz under illumination conditions. Furthermore, impedance data were analyzed via a Randles equivalent circuit using Zview software (Scribner Associates).

In a typical IMPS response, the frequency of the maximum imaginary part matches the sum of the rate constants for charge transfer (k_tran) and recombination (k_rec) as expressed³¹,

$${{{\mbox{k}}}}_{{{\mbox{tran}}}}+{{{\mbox{k}}}}_{{{\mbox{rec}}}}={2\pi {{\mbox{f}}}}_{\max }$$

(3)

Additionally, the ratio of the steady-state photocurrent (j_ss) to the instantaneous photocurrent (j_hole) could be used to calculate the hole transfer efficiency (η_tran) at the semiconductor/electrolyte interface. The hole transfer efficiency could also be expressed by the ratio of k_rec and k_tran, assuming that both hole transfer and recombination were pseudo-first-order in the surface hole concentration.

$${{{{{{\rm{\eta }}}}}}}_{{{{{{\rm{tran}}}}}}}=\frac{{{{\mbox{j}}}}_{{{{{{\rm{ss}}}}}}}}{{{{\mbox{j}}}}_{{{{{{\rm{hole}}}}}}}}=\frac{{{{\mbox{k}}}}_{{{{{{\rm{tran}}}}}}}}{{{{\mbox{k}}}}_{{{{{{\rm{tran}}}}}}}+{{{\mbox{k}}}}_{{{{{{\rm{rec}}}}}}}}$$

(4)

The average photogenerated electron transfer time (τ_d) could be estimated from the frequency at the minimum imaginary part,

$${{{{{{\rm{\tau }}}}}}}_{{{\mbox{d}}}}=\frac{1}{{2{{{{{\rm{\pi }}}}}}{{\mbox{f}}}}_{\min }}$$

(5)

Solar-to-chemical energy conversion efficiency

The solar-to-chemical energy conversion efficiency could be expressed by the input vs. output energy, which can be calculated by the following formula²⁷:

$${{{{{\rm{\eta }}}}}}_{{{\mbox{STF}}}}=\frac{{{\mbox{qm}}}}{{{\mbox{IAt}}}}{{\times }}100\%\left(\frac{{{\mbox{J}}}\cdot {{\mbox{mo}}}{{{\mbox{l}}}}^{{{{{{\rm{-}}}}}}1}\cdot {{\mbox{mol}}}}{{{\mbox{W}}}\cdot {{\mbox{c}}}{{{\mbox{m}}}}^{{{{{{\rm{-}}}}}}2}\cdot {{\mbox{c}}}{{{\mbox{m}}}}^{2}\cdot {{\mbox{s}}}}=\frac{{{\mbox{J}}}}{{{\mbox{W}}}\cdot {{\mbox{s}}}}=1\right)$$

(6)

Where q is the combustion heat value of CH₄ (${{{\mbox{q}}}}_{{{\mbox{C}}}{{{\mbox{H}}}}_{4}}=890.31{{{{{\rm{\times }}}}}}{10}^{3}{{\mbox{J}}}{{{{{\rm{\cdot }}}}}}{{{\mbox{mol}}}}^{{{{{{\rm{-}}}}}}1}$), m denotes the mole of produced CH₄ (mol), I denotes the light intensity (W cm^-2), A is the irradiation area (cm²), and t denotes the reaction time (s). Thus, the solar-to-chemical energy conversion efficiency for the monolithic NF@0.1%Ni@CeO₂-V_o catalyst was calculated to be 1.14%.

For 1 cm² NF@0.1%Ni@CeO₂-V_o catalyst reacting under the concentrated solar (light) reactor for 2 h and producing 385.5 μmol/cm² CH₄:

$${{{{{{\rm{\eta }}}}}}}_{{{\mbox{STF}}}}=\frac{890.8{{\times }}{10}^{3}{{\times }}385.5{{\times }}{10}^{{{{{{\rm{-}}}}}}6}}{4200{{\times }}{10}^{{{{{{\rm{-}}}}}}3}{{\times }}1{{\times }}7200}{{\times }}100\%=1.14\%$$

(7)

SPV measurements

The SPV spectra were obtained based on a lock-in amplifier. The measurement system consisted of a lock-in amplifier (SR830, Stanford Research Systems), a monochromator (190–2000 nm), a light chopper (SR540, Stanford Research Systems), and a sample chamber³⁶. Monochromatic light was generated by a 500 W Xe lamp (CHF-XM-500 W, Global Xenon Power) passing through a monochromator (Omni-3007, no. 16047, Zolix).

Details of PL, TRPL, SPV, and TPV measurements

The excitation wavelength for the PL tests was 255 nm, and the emission lifetime was obtained by fitting the fluorescence attenuation curve according to the following formula:

$${{\mbox{y}}}={{{\mbox{y}}}}_{0}+{{{\mbox{A}}}}_{1}{{{\mbox{e}}}}^{{{{{{\rm{-}}}}}}{{\mbox{x}}}/{{{\mbox{t}}}}_{1}}+{{{\mbox{A}}}}_{2}{{{\mbox{e}}}}^{{{{{{\rm{-}}}}}}{{\mbox{x}}}/{{{\mbox{t}}}}_{2}}$$

(8)

Moreover, all photovoltage measurements were strictly conducted under the same experimental conditions to ensure the precision and reliability of the experimental outcomes.

Calculation details

A (3 × 3) supercell of the 111 surfaces of CeO₂ was adopted, containing four layers and 108 atoms; the bottom two layers were fixed to maintain the bulk properties. A vacuum layer of 15 Å along the z direction, perpendicular to the surface, was adopted. The Ni atom replaced one Ce atom to simulate the Ni single-atom catalyst.

Ground-state density functional theory (DFT) and ab initio molecular dynamics (AIMD) calculations were conducted with the Vienna Ab initio Simulation Package^37,38, using a projector-augmented wave method and Perdew-Burke-Ernzerhof (PBE) functional³⁹. The energy cutoff was set to 500 eV. For Ni and Ce, Ueff = 4.5 and 5.0 eV were employed on the 3d and 4 f orbitals, respectively^40,41,42,43. The structures were relaxed until forces were less than 0.001 eV/Å. The total energy convergence criterion for electronic self-consistent iteration was set at 10–8 eV. A (3 × 3 × 1). Monkhorst-Pack grid k-point mesh was used for sampling the Brillouin zone during relaxation. A (9 × 9 × 1) k-point mesh was used for calculating the electronic structure. The van der Waals interaction was corrected with the DFT-D3 method⁴⁴. During the AIMD simulation, we used the NVT ensemble implemented in VASP^45,46,47,48, and only the gamma point was used to sample the Brillouin zone.

Nonadiabatic molecular dynamics (NAMD) with a real-time time-dimensional DFT (TDDFT) method^49,50 was performed to simulate the laser-induced molecular dissociation on the surface of the catalyst. The norm-conserving pseudopotential^51,52 and numerical atomic orbital basis sets⁵³ with an energy cutoff of 100 Ry (about 1360.5 eV) and PBE functional were employed for NAMD simulations. Only the gamma point sampled the Brillouin zone. The atoms in catalysts were all fixed during the NAMD simulation to avoid heat-induced molecular dissociation. The initial velocity of adsorbed molecules was initialized using a temperature of 10 K by running an AIMD simulation with the NVT ensemble. Then the NAMD simulation adopted the NVE ensemble and excited the photon-induced chemical reaction by applying an external laser field perpendicular to the surface of the catalyst. The external laser field was shaped by a Gaussian envelope, shown in Supplementary Fig. 63.

$${{\mbox{E}}}\left({{{{{\rm{\omega }}}}}},\, {{\mbox{t}}}\right)={{{\mbox{E}}}}_{0}\exp \left[{{{{{\rm{-}}}}}}\frac{{({{\mbox{t}}}{{{{{\rm{-}}}}}}{{{\mbox{t}}}}_{0})}^{2}}{{2{{{{{\rm{\tau }}}}}}}^{2}}\right]\cos ({{{{{\rm{\omega }}}}}}{{\mbox{t}}}{{{{{\rm{-}}}}}}{{{{{\rm{\omega }}}}}}{{{\mbox{t}}}}_{0})$$

(9)

where the τ was set to 10 fs. The electric field reached the maximum of 1.25 V/Å (E₀) at t₀ = 45 fs. The frequency (ω) of the laser field was set so that the photon energy hω was equal to 4 eV. The real-time TDDFT in the NVE ensemble evolved to 80 fs with a time step of 10 as. The external laser field was applied from t = 30 fs to 60 fs.

Reporting summary

Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.