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Article

Tuning Ag Loading and Particle Size in Ag@g-C3N4 Photocatalysts for Selective CO2 Conversion to CO and CH4

by
Shicheng Liu
,
Na Li
and
Qulan Zhou
*
Multiphase Flow in Power Engineering, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China
*
Author to whom correspondence should be addressed.
Nanomaterials 2025, 15(18), 1443; https://doi.org/10.3390/nano15181443
Submission received: 28 July 2025 / Revised: 11 September 2025 / Accepted: 18 September 2025 / Published: 19 September 2025
(This article belongs to the Special Issue Nanostructured Catalysts for Energy Conversion and Environmental Applications)
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Abstract

Elucidating the mechanisms of CO2 photocatalytic conversion systems is crucial for tackling the challenges of carbon neutrality. In this study, a series of Ag@g-C3N4 photocatalysts were constructed with metal particle size modulation as the core strategy to systematically reveal the modulation mechanism of Ag nanoparticles (Ag NPs) size variation on the selectivity of CO2 photoreduction products. Systematic characterizations revealed that increasing Ag size enhanced visible light absorption, promoted charge separation, and improved CH4 selectivity. Photocatalytic tests showed Ag3.0%@CN achieved optimal activity and electron utilization. Energy band analyses indicated that Ag modification preserved favorable conduction band positions while increasing donor capacity. Further density-functional theory (DFT) calculations reveal that Ag NPs size variations significantly affect the adsorption stability and conversion energy barriers of intermediates such as *COOH, CO and CHO, with small-sized Ag7 NPs favoring the CO pathway, while large-sized Ag NPs stabilize the key intermediates and drive the reaction towards the CH4 pathway evolution. The experimental and theoretical results corroborate each other and clarify the dominant role of Ag NPs size in regulating the reaction path between CO and CH4. This study provides mechanistic guidance for the selective regulation of the multi-electron reduction pathway, which is of great significance for the construction of efficient and highly selective CO2 photocatalytic systems.

1. Introduction

The escalating global carbon emissions crisis has made the achievement of carbon neutrality and establishment of sustainable low-carbon energy systems a paramount international priority in recent years [1,2]. CO2 photocatalytic reduction technology is considered to be one of the most promising paths for “artificial photosynthesis” as it can directly utilize solar energy to convert CO2 into high-value-added C1 chemicals (CO2 and CH4) to achieve greenhouse gas emission reductions and at the same time provide sustainable energy [3,4,5]. However, the multielectronic conversion process of CO2 (CO2 → CH4) is usually accompanied by complex intermediate migration and pathway bifurcation. The product distribution is strongly governed by critical catalyst structural parameters, including active component particle size, interfacial properties, and surface microenvironment characteristics [6,7]. Therefore, the precise modulation of catalyst structure, aiming at synergistically enhancing the photogenerated carrier separation efficiency and reactant activation process, has become a research hotspot. Further enhancement of the selective control of target products is one of the core challenges currently facing the field of photocatalytic CO2 reduction [8,9].
Graphite-phase carbon nitride (g-C3N4) is a non-metallic semiconductor with a medium bandgap (~2.7 eV), low synthesis cost and a broad visible-light response, which has been widely used in the photocatalytic reduction reaction of CO2 [10,11,12]. However, pristine g-C3N4 suffers from defects such as fast carrier complexation rate, poor reactive site activity, and weak interfacial electron mobility, which severely limit its photocatalytic performance [13,14]. Currently, researchers have widely used various modification studies such as vacancy engineering [15,16], Heteroatom doping [17,18], and construction of heterojunctions [19,20] to improve their defects. Among them, it was found that the photogenerated charge separation and surface reactivity were improved by introducing metal co-catalysts [21,22,23,24]. Benefiting from their LSPR-enhancing effect, high electron affinity and excellent CO2 adsorption capacity, silver nanoparticles (Ag NPs) show significant potential for applications in photocatalysis. Therefore, it is often employed in the g-C3N4 system to construct metal/semiconductor heterojunction structures to enhance CO2 photoreduction performance [25,26]. Chen et al. constructed plasmonic Ag NPs on nitrogen vacancy-modified g-C3N4 nanotubes (ACNNT) to achieve high photocatalytic conversion of CO2 under visible light. The uniform dispersion of Ag NPs coupled with the spatially oriented charge segregation and migration enabled by the one-dimensional tubular g-C3N4 architecture significantly enhances proton-assisted metal utilization efficiency [27].
Notably, the size variation in Ag NPs critically influences both their physicochemical stability and surface-active site distribution, while simultaneously governing the adsorption behavior and conversion pathways of key reaction intermediates (*CO, CHO). It has been pointed out that reducing the particle size of Ag NPs can enhance their specific surface area and metal-carrier interfacial interaction, thus improving the separation and transfer efficiency of photogenerated electrons. The above advantage makes them more prominent in driving the multi-electron hydrogenation reduction of CO2 and promoting the generation of deep reduction products such as CH4 [28]. Large-size Ag NPs, on the other hand, are more suitable for stabilizing key intermediates in the CO pathway because their electronic structure tends to be in the bulk state, thus enhancing the selective yield of CO [29]. However, current research has predominantly employed Ag NPs merely as auxiliary components. A fundamental knowledge gap persists regarding how particle size governs CO2 photoreduction pathway selectivity, particularly at the mechanistic level. This limited understanding substantially constrains the strategic application of metal particle size engineering in developing high-selectivity photocatalytic systems.
In this study, we constructed a series of g-C3N4 catalysts modified with Ag NPs of different particle sizes (Ag@g-C3N4) and systematically evaluated their effects on the activity and product distribution of the CO2 photo-reduction reaction under visible light. Controlled particle size growth of Ag NPs was achieved by modulating the loading of Ag. Combined with morphological characterization, photophysical property analysis, electronic structure determination and in situ reaction tests, the modulation of photogenerated electron separation efficiency, energy band structure evolution, and product path selection by Ag NPs size variation was systematically revealed. Complemented by density functional theory (DFT) calculations, we systematically compared the adsorption stability of key reaction intermediates and reaction energy barriers between small (Ag7) and large Ag NPs, revealing size-dependent catalytic effects. The linkage between the energy band structure evolution and the intermediate adsorption behaviors under the regulation of Ag NPs size has been further clarified to drive the change in product distribution. This study not only provides theoretical support for the understanding of the microscopic mechanism of Ag NPs size-regulated reaction pathways but also provides feasible strategies and mechanisms for the construction of highly selective CO2 photoreduction systems.

2. Materials and Methods

2.1. Catalyst Preparation

g-C3N4 was prepared by thermal polymerization. Weighing 10 g of urea (Aladdin, 99%), placed in an alumina crucible with a lid, heated to 550 °C in a muffle furnace at a heating rate of 5 °C/min and held for 2 h. After cooling, a light yellow mass was obtained, and g-C3N4 powder was obtained by grinding and sieving [30]. Ag@g-C3N4 catalyst was prepared by chemical reduction method. 100 mg of g-C3N4 was dispersed in 100 mL of deionized water and sonicated for 30 min to obtain a homogeneous dispersion. Subsequently, different proportions of AgNO3 solution (corresponding to Ag loading of 0.5%, 1.0%, 3.0%, and 5.0%) were added, and an appropriate amount of NaBH4 solution was added dropwise as a reducing agent after stirring well. After stirring and reacting the system in the dark for 2 h, Ag@g-C3N4 solid powder was obtained by centrifugation, washing and drying.

2.2. Structural Characterization of Catalysts

The samples were characterized using transmission electron microscopy (TEM; JEM-2100, JEOL Ltd., Tokyo, Japan) and high-resolution TEM (HRTEM) to examine morphology and microstructure. Crystalline features were analyzed through selected-area electron diffraction (SAED) with corresponding fast Fourier transform (FFT) patterns. Elemental distribution mapping was performed using scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy (STEM-EDS). X-ray diffraction (XRD) patterns were collected on a Bruker D8 Advance diffractometer with Cu Kα radiation (λ = 1.5406 Å). Surface area measurements were obtained from N2 adsorption–desorption isotherms (Micromeritics ASAP 2020) using the BET method. Optical properties were evaluated by ultraviolet-visible diffuse reflectance spectroscopy (UV–Vis DRS; UV-2600, Shimadzu Corporation, Kyoto, Japan), while photoluminescence (PL) spectra were recorded on a Horiba Fluorolog-3 spectrofluorometer (excitation at 325 nm). Chemical states were analyzed by X-ray photoelectron spectroscopy (XPS; ESCALAB 250Xi, Thermo Fisher Scientific, Waltham, MA, USA), with valence band (VB) spectra providing energy band structure information.

2.3. Photocatalytic Performance Test

The CO2 photoreduction reaction was carried out in a closed quartz reaction system consisting of 100 mg of catalyst, 20 mL of deionized water and 100 mL of CO2, with the reaction gas pre-filled with the quartz reactor. The light source was a 300 W xenon lamp (Xenon lamp; PLS-SXE300, Perfectlight Technology Co., Ltd., Beijing, China) with an irradiation wavelength range of 320–780 nm and a light intensity of 100 mW-cm−2, simulating solar lighting conditions. The reaction temperature was maintained at 25 °C. Reaction products were sampled hourly and analyzed by gas chromatography (FULI-F80, equipped with FID and TCD detectors) to quantify CO and CH4 yields. All experimental results represent the average of three independent measurements.

2.4. Density Functional Theory Calculations

Density Functional Theory (DFT) calculations are accomplished in the Vienna abinitio simulation package (VASP2.4.6) software, using the projective affixed plus wave (PAW) method for the pseudopotentials, while the exchange-correlation generalization takes the form of the PBE in the Generalized Gradient Approximation (GGA) [31]. The plane-wave cutoff energy was set to 450 eV, with Γ-point sampling of the system momentum space. A convergence threshold of 10−5 eV was employed for all calculations. The simulation models include the structures of pure g-C3N4 sheet layers, loaded Ag7 NPs and Ag large-size NPs, and all models are constructed at vacuum layer thicknesses greater than 15 Å [32,33]. The adsorption and reaction free energies of each key intermediate in the CO2 reduction pathway (*COOH, *CO, *CHO, *CH4) were evaluated by calculating ΔG, with the temperature set to 298 K, using the standard free energy correction method.

3. Results and Discussion

3.1. Structural Characteristics

Figure 1a demonstrates the overall morphology of the pure g-C3N4 sample, which is structured as a typical lamellar body with regular edges, and no obvious particles attached to the surface. Further observation of the local magnification as shown in Figure 1b reveals that the thickness of the lamellae is uniform and the boundaries are well defined. Figure 1c shows the fast Fourier transform (FFT) image of the corresponding region of Figure 1b, which shows only fuzzy amorphous diffraction rings and no clear lattice spots, indicating that g-C3N4 as a whole is amorphous and lacks long-range ordered structure, providing a baseline for the subsequent observation of the crystalline characteristics of Ag NPs [34]. On this basis, the morphological evolution of the samples with different silver loadings was further investigated. As shown in Figure 1d, the Ag0.5%@CN sample maintains the characteristic lamellar morphology of g-C3N4 with smooth surfaces. No distinct Ag NPs were resolved within the observed field of view, likely due to either their ultrasmall size or low electron density contrast. As the silver content rises to 1.0%, a number of low-contrast dark spots appear in Figure 1e, indicating that Ag NPs are beginning to form, although the morphology is still blurred. Figure 1f presents a high-magnification view of the region marked in Figure 1e, clearly revealing well-dispersed Ag NPs with diameters below 5 nm on the g-C3N4 surface. The NPs exhibit well-defined crystallinity and uniform dispersion, confirming the successful incorporation of small-sized silver particles with excellent spatial distribution.
Continuing to increase the percentage of silver loading, the distribution density and size of Ag NPs in the samples changed significantly. As shown in Figure 1g, a large number of contrast-enhanced particles were attached to the surface of the lamellae in the Ag3.0%@CN samples, with a significant increase in the number. At the same time, the particle spacing was significantly reduced, reflecting higher nucleation density. Figure 1h further magnified observation reveals the presence of several large-sized particles in addition to the small-sized Ag NPs, with lattice stripes clearly visible within the particles. The measured crystal plane spacing is about 2.36 Å, which corresponds to the (111) crystal plane of metallic silver, indicating that some of the particles have grown into highly crystalline nanocrystals [35]. Figure 1i displays the EDS elemental mapping of the Ag5.0%@CN sample, showing uniform distribution of C and N across the lamellar framework. The Ag signals exhibit continuous spatial dispersion without apparent aggregation or vacancy regions, demonstrating that structural integrity and nanoparticle dispersion are effectively preserved even at high loading concentrations.
The XRD patterns of g-C3N4 and Ag@g-C3N4 with varying Ag loadings are presented for comparison (Figure 2a). Pure g-C3N4 exhibits a distinct diffraction peak at 2θ = 27.5°, corresponding to the (002) crystal plane, which reflects its stacking structure along the interlayer direction [36]. Following Ag incorporation, while retaining the dominant g-C3N4 diffraction features, new peaks gradually emerged at 38.1°, 44.3°, 64.4°, and 77.4°. These peaks correspond to the (111), (200), (220), and (311) crystallographic planes of metallic Ag (JCPDS No. 99-0094), respectively [37]. The Ag diffraction peak intensities exhibited a loading-dependent enhancement, becoming particularly prominent in Ag3.0%@CN and higher-loading samples. The dominant Ag (111) peak evolution indicates both improved crystallinity and particle growth beyond the XRD detection threshold. In addition, no other impurity phases or relevant features of the oxidized state of silver were seen, suggesting that the introduced Ag is mainly present in the system as a zero-valent metal.
As shown in Figure 2b, all samples showed typical type IV adsorption isotherms, with a significant increase in the P/P0 ≈ 0.9–1.0 interval, indicating that the samples have a mesoporous structure [38]. Pristine g-C3N4 exhibits the highest specific surface area (65.6 m2 g−1) and pore volume (0.222 cm3 g−1), indicative of a well-developed porous structure. Upon the gradual introduction of Ag, the adsorption isotherms shift downward, and the overall adsorption capacity decreases markedly, suggesting that Ag NPs partially occupy or block the pores of g-C3N4, thereby reducing the accessible surface area. The pore size distribution further supports this observation. Pristine g-C3N4 exhibits a clear mesoporous feature centered at ~13 nm, while Ag@CN samples show weakened peaks accompanied by a slight shift toward smaller pore sizes. As summarized in Table 1, further increasing the Ag loading to 3.0–5.0wt% leads to a modest recovery of surface area and pore volume, with the pore size approaching its original value. This trend indicates that at higher Ag loadings, larger Ag particles tend to grow on the outer surface rather than infiltrating the pores, thereby alleviating the blocking effect and allowing the intrinsic mesoporous framework of g-C3N4 to be partially preserved. This trend indicates that at higher Ag loadings, larger Ag NPs preferentially grow on the outer surface rather than infiltrating the pores. As a result, the blocking effect is alleviated, and the intrinsic mesoporous framework of g-C3N4 is partially preserved. These structural changes are consistent with the TEM observations, which reveal particle agglomeration and diminished dispersion at high loadings. While elevated Ag loading enhances reactive site density, it concurrently induces structural pore occlusion and light shielding effects.

3.2. Photocatalytic Performance Evaluation

As shown in Figure 3a, the catalysts with different Ag loadings exhibited significant differences in CO and CH4 yields. Pure g-C3N4 achieved only 4.82 μmol-g−1-h−1 CO and 1.05 μmol-g−1-h−1 CH4 yields, indicating that its limited intrinsic activity. The catalytic activity increased significantly with the introduction of Ag NPs. The Ag3.0%@CN sample showed the best performance, with CO and CH4 production rates of 18.31 and 4.59 μmol-g−1-h−1, respectively. Continuing to increase the Ag loading to 4.0% versus 5.0%, the gas production performance instead decreased, suggesting that a moderate amount of Ag could promote CO2 reduction, whereas excessive loading could cause carrier complexation or masking of light absorption. The electron consumption in the catalytic reaction of each sample also showed a similar trend as shown in Figure 3b. The electron consumption was determined based on the quantified CO and CH4 yields, with 2 electrons consumed per CO molecule and 8 electrons consumed per CH4 molecule. Notably, Ag3.0%@CN consumed the highest total electrons (73.34 μmol-h−1-g−1). The proportion of electrons involved in the CH4 pathway also increased significantly, suggesting that the electron utilization efficiency and reduction depth were both optimal at this loading.
In order to verify the real contribution of Ag loading to the photoreduction performance of CO2, the gas production behaviors under different systems are compared in Figure 3c. Under standard conditions, the Ag1.0%, Ag3.0% and Ag5.0%@CN samples all exhibited higher catalytic ability than the g-C3N4 native, and Ag3.0%@CN again demonstrated the best performance. In the control experiments, gas production was almost zero if light, catalyst, or CO2 was removed, indicating that the reaction was indeed a light-driven CO2 reduction promoted by Ag NPs-modified g-C3N4 catalyst. Figure 3d further examines the stability behavior of the Ag3.0%@CN sample under cyclic reaction conditions. As the reaction time was extended, the CO and CH4 yields showed good reproducibility in seven consecutive rounds of reaction, and the cumulative production increased basically linearly. No obvious activity decay was observed during the reaction, suggesting that the catalyst has excellent structural stability and catalytic durability under long-time light conditions.

3.3. Electronic Structure and Band Modulation

The UV-visible diffuse reflectance spectra (Figure 4a) reveal strong visible-light absorption across all samples, demonstrating excellent photoresponsive characteristics. Ag incorporation further enhanced absorption intensity, likely due to the LSPR effects. Pure g-C3N4 shows a pronounced absorption edge near 400 nm, attributed to its intrinsic π-π* electron jump, corresponding to a band gap of about 2.7 eV [30]. As the Ag loading increased, the catalyst exhibited no appreciable absorption edge redshift but demonstrated progressive enhancement of broadband absorption between 450 and 500 nm, which was most prominent in Ag3.0%@CN and Ag5.0%@CN. This phenomenon can be attributed to the LSPR effect of Ag NPs, which induces new absorption bands that help to enhance the light harvesting capability and extend the light response range for subsequent photogenerated electron generation and transfer.
As shown in Figure 4b, all the samples show obvious broad emission peaks around 470 nm, originating from the radiative complexation process of photogenerated electrons and holes [39]. Of them, the g-C3N4 body has the highest PL intensity, suggesting a serious carrier complexation problem. Whereas, after Ag loading, the PL intensity of the samples decreases gradually with the increase in Ag NPs. This reflects that the non-radiative composite process is enhanced and electrons are more likely to migrate towards Ag, which inhibits the composite behavior. Among all samples, Ag5.0%@CN exhibits the lowest PL intensity. However, its excessive Ag loading induces NPs agglomeration and partial surface coverage, which block active sites and hinder efficient charge utilization. Notably, Ag3.0%@CN exhibits the highest photocatalytic activity, indicating an optimal balance between suppressed carrier recombination and plentiful accessible active sites.
To further confirm the Ag introduction and its valence information, the full XPS survey spectrum of each sample is given in Figure 4c. All samples exhibit distinct C 1s (≈284 eV) and N 1s (≈399 eV) major peaks, constituting the basic skeleton of g-C3N4 [40]. For the Ag-modified samples, Ag 3d double peaks were observed in the region of 368–374 eV, and their peak intensities were enhanced with the increase in loading. This reflects a controlled increase in Ag NPs, showing that Ag was successfully introduced and existed in a surface stable state [30].
Figure 4d illustrates the high-resolution spectrum of C 1s, which mainly contains two fitted components at 288.3 eV (N-C=N) and 284.8 eV (C-C/C=C), representing the presence of C atoms in the triazine ring with a small amount of carbon-rich regions, respectively [41,42]. The position of the main peak was not significantly shifted in each sample, indicating that the introduction of Ag did not disrupt the carbon skeleton structure of g-C3N4.
Figure 4e shows a high-resolution XPS image of N 1s, which can be divided into two peaks: 398.6 eV corresponds to sp2 hybridized nitrogen (N-(C)3), and 400.1 eV corresponds to bridged nitrogen (C-N-C) [27]. The stable peak positions and similar intensities between the different Ag-loaded samples illustrate that the Ag deposition process did not significantly change the chemical environment of elemental nitrogen, with the structural integrity of g-C3N4 being maintained.
To elucidate the chemical valence states of Ag, the corresponding results are displayed in Figure 4f. In all Ag-modified samples, Ag 3d5/2 and Ag 3d3/2 are located at 368.2 eV and 374.2 eV, respectively, with a spacing of about 6.0 eV, which is in accordance with the characteristics of the metallic state Ag0 [43]. No additional peaks representing Ag+ or AgOx in the 373–378 eV region were observed, suggesting that Ag exists predominantly as an unoxidized metal. Combining the LSPR absorption features with the PL bursting behavior in Figure 4a,b, the Ag NPs not only exist stably on the surface, but also act as electron acceptors and participate effectively in the electron transfer process.
The Tauc method was applied to fit the optical bandgaps of g-C3N4 and the Ag-loaded samples (Figure 5a). The band gap of the pure g-C3N4 sample is 2.85 eV, which exhibits typical wide bandgap semiconductor characteristics. Upon Ag incorporation, the samples exhibit no significant absorption edge redshift, maintaining bandgap values between 2.83 and 2.86 eV. This confirms that Ag loading preserves the fundamental π-conjugated framework of g-C3N4 while introducing additional light-harvesting capabilities [44]. The slight narrowing of Ag0.5%@CN to 2.83 eV is related to its formation of surface-isolated excitonic coupling states. Overall, the Ag modification did not break the main framework of light absorption of g-C3N4 and ensured the catalyst’s light-absorbing ability in the visible region.
The XPS valence band spectrum of each sample is shown in Figure 5b, showing the position of its valence band top (VB) relative to the Fermi energy level. The g-C3N4 body has the largest VB of about 2.23 eV, and the position of the valence band is gradually shifted towards lower energies with increasing Ag NPs [45,46]. With the gradual increase in Ag loading from 0.5% to 5.0%, the valence band positions of the samples decreased to 2.09, 1.90, 1.86, 1.81, 1.75 and 1.73 eV in sequence, suggesting that the introduction of Ag effectively modulates the surface energy level structure of the materials while enhancing the electron density. The lower VB position facilitates the enhancement of the migration potential energy of the electrons on the reducing side, which enhances the CO2 molecule activation and its intermediate conversion.
The energy band structure evolution of the Ag@g-C3N4 sample is constructed in Figure 5c based on the data of Figure 5a with Figure 5b and superimposed on the standard potentials of a typical CO2 reduction half-reaction [47,48]. As seen in the figure, the conduction band bottoms (CB) of all the samples are located above the potentials required for the reduction of CO2 to produce CH4 (−0.24 V), CH3OH (−0.38 V), and CO (−0.53 V), showing that they have thermodynamic driving capability. In particular, the energy band structure of Ag3.0%@CN has both a relatively suitable band gap and a high conduction band potential, which combines light absorption and electron reduction, matching the performance of the optimal yield in the aforementioned catalytic experiments. Furthermore, the elevated valence band position exceeds the H2O/O2 oxidation potential (+1.23 V), guaranteeing thermodynamic favorability for the oxidative half-reaction. This optimized band alignment provides comprehensive energetic support for the multi-electron CO2 reduction pathway.

3.4. DFT Calculations

To elucidate the effect of Ag loading, the g-C3N4, Ag1.0%@CN and Ag3.0%@CN samples are considered as theoretical model structures, enabling a comparison of the modulation of reactive center and electronic structure. Figure 6a shows a model of the native g-C3N4 with a regular lamellar structure and exposed N sites on the surface, constituting the basic adsorption and reaction interface. The computational model of Ag7 NPs anchored on g-C3N4, representing the Ag1.0%@CN system with characteristic smaller Ag NPs, is shown in Figure 6b. The NPs adopts a bridging configuration between two adjacent nitrogen sites, forming a stable interfacial architecture while maintaining accessible metal active centers. Figure 6c displays the Ag18 NPs model representing the Ag3.0%@CN system, where increased NP dimensions create additional low-coordination sites. This structural configuration is anticipated to significantly strengthen CO2 adsorption activation and subsequent intermediate stabilization during reduction.
Figure 6d illustrates the Gibbs free energy (ΔG) change curves of each intermediate during CO2 photoreduction for the three model systems, which covers a multi-step reduction path from CO2 adsorption to CH4 desorption. In the g-C3N4 model, COOH formation is the main rate-limiting step (ΔG = 1.68 eV), and the CO state adsorption is less stable and easy to desorb, so that it prefers to generate CO products [49]. After the introduction of Ag7 NPs into the Ag1.0%@CN model, the free energy curves shifted downward overall, especially on the formation of two key intermediates, COOH and CO, ΔG was significantly reduced, with the former dropping to 1.21 eV and the latter stabilized below 0.59 eV. This suggests that small-sized Ag NPs play a positive role in lowering the electron transfer barrier and enhancing the preliminary reduction activity [50,51]. DFT calculations reveal that Ag1.0%@CN lowers the energy barrier for CO2 photoreduction, thereby contributing to its overall enhanced photocatalytic activity compared with g-C3N4. After further increasing the Ag loading to form larger NPs, the Ag3.0%@CN system showed a clear advantage in the reaction path. The ΔG for initial COOH formation decreases significantly to 1.02 eV, while subsequent reaction barriers along the entire CO→CH4 pathway show systematic reduction.
Additionally, Ag3.0%@CN shows a higher CO desorption free energy (+0.47 eV) than Ag1.0%@CN (+0.32 eV), making CO desorption less favorable and thereby facilitating subsequent hydrogenation. Although ΔG (CO desorption) is higher than ΔG (*CHO formation) in Ag3.0%@CN, experimental results show that CO remains the primary product of CO2 photoreduction. Direct *CO desorption represents a shorter pathway, rapidly generating CO. In contrast, although hydrogenation to *CHO is thermodynamically accessible, it requires additional steps to ultimately form CH4, making this process less favorable. As a result, CO remains the main photocatalytic product. In summary, the DFT free energy analysis in Figure 6d clearly reveals the regulatory mechanism of Ag NPs size on reaction path selection. Smaller Ag NPs primarily lower the initial reduction barrier. In contrast, larger Ag NPs (Ag3.0%@CN) create a more electron-rich interface and strengthen CO adsorption on the catalyst surface, directing the reaction toward the deep reduction pathway leading to CH4. This size-dependent conformational relationship provides theoretical support for the experimentally observed trends in the distribution of CO and CH4.

4. Conclusions

This work systematically demonstrates that Ag nanoparticle size plays a key role in modulating the photocatalytic CO2 reduction behavior of g-C3N4. Regulating the Ag loading allows control over the size of Ag NPs on g-C3N4, which in turn governs the photocatalytic activity. Smaller Ag NPs (Ag1.0%@CN) facilitate CO desorption due to weaker intermediate adsorption, favoring CO production. In contrast, larger Ag NPs (Ag3.0%@CN) stabilize CO intermediates and lower the hydrogenation barrier, promoting CH4 formation. Spectroscopic characterizations confirm that Ag enhances visible light absorption via LSPR and improves charge separation efficiency without altering the intrinsic bandgap of g-C3N4. Combined XPS and UV–Vis analyses reveal that Ag modification modulates surface energy levels and improves electron reduction potential, meeting the thermodynamic requirements for multi-electron transfer. DFT calculations further show that small Ag7 NPs reduce CO desorption energy, while large Ag NPs create electron-rich interfaces that stabilize key intermediates and facilitate CH4 evolution. These results establish a structure–function relationship linking Ag size with energy band structure, intermediate binding strength, and product pathway differentiation. The study provides mechanistic insights and structural guidance for designing highly selective CO2 photocatalysts based on g-C3N4 platforms.

Author Contributions

Conceptualization, S.L.; methodology, S.L.; software, S.L.; validation, S.L.; formal analysis, S.L.; investigation, S.L.; resources, S.L. and N.L.; data curation, S.L.; writing—original draft preparation, S.L.; writing—review and editing, N.L. and Q.Z.; visualization, S.L.; supervision, Q.Z.; project administration, Q.Z.; funding acquisition, N.L. and Q.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This work is Supported by National Key Research and Development Program of China (2022YFB4100300).

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Acknowledgments

We thank the Instrumental Analysis Center of Xi’an Jiaotong University.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) HAADF-STEM image and (b) Magnified view of g-C3N4. (c) FFT images of g-C3N4. (d) HAADF-STEM image of Ag0.5%@CN. (e) HAADF-STEM image and (f) Magnified view of Ag1.0%@CN. (g) HAADF-STEM image and (h) Magnified view of Ag3.0%@CN. (i) Elemental maps of Ag5.0%@CN.
Figure 1. (a) HAADF-STEM image and (b) Magnified view of g-C3N4. (c) FFT images of g-C3N4. (d) HAADF-STEM image of Ag0.5%@CN. (e) HAADF-STEM image and (f) Magnified view of Ag1.0%@CN. (g) HAADF-STEM image and (h) Magnified view of Ag3.0%@CN. (i) Elemental maps of Ag5.0%@CN.
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Figure 2. (a) XRD patterns and (b) N2 adsorption–desorption isotherms of g-C3N4 and Ag-loaded g-C3N4 samples.
Figure 2. (a) XRD patterns and (b) N2 adsorption–desorption isotherms of g-C3N4 and Ag-loaded g-C3N4 samples.
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Figure 3. (a) CO and CH4 production rates and (b) electron consumption amounts of different samples. (c) Comparative tests under different control conditions (without light, catalyst, or CO2). (d) Photocatalytic stability of Ag3.0%@CN over 7 consecutive cycles. (Reaction conditions: 100 mg catalyst, 20 mL deionized water, and 100 mL CO2 in a closed quartz reactor; light source: 300 W Xe lamp (320–780 nm, 100 mW cm−2); reaction temperature: 25 °C; products analyzed hourly by gas chromatography).
Figure 3. (a) CO and CH4 production rates and (b) electron consumption amounts of different samples. (c) Comparative tests under different control conditions (without light, catalyst, or CO2). (d) Photocatalytic stability of Ag3.0%@CN over 7 consecutive cycles. (Reaction conditions: 100 mg catalyst, 20 mL deionized water, and 100 mL CO2 in a closed quartz reactor; light source: 300 W Xe lamp (320–780 nm, 100 mW cm−2); reaction temperature: 25 °C; products analyzed hourly by gas chromatography).
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Figure 4. (a) UV–Vis diffuse reflectance spectra of g-C3N4 and Agx%@CN (b) PL spectra of g-C3N4 and Agx%@CN. (c) XPS survey spectra of all samples. High-resolution (d) C 1s and (e) N 1s spectra of all samples. (f) Ag 3d spectra of all samples.
Figure 4. (a) UV–Vis diffuse reflectance spectra of g-C3N4 and Agx%@CN (b) PL spectra of g-C3N4 and Agx%@CN. (c) XPS survey spectra of all samples. High-resolution (d) C 1s and (e) N 1s spectra of all samples. (f) Ag 3d spectra of all samples.
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Figure 5. (a) Tauc’s plots for band gap estimation of g-C3N4 and Agx%@CN. (b) Valence band of g-C3N4 and Agx%@CN. (c) Schematic energy band diagrams of g-C3N4 and Agx%@CN.
Figure 5. (a) Tauc’s plots for band gap estimation of g-C3N4 and Agx%@CN. (b) Valence band of g-C3N4 and Agx%@CN. (c) Schematic energy band diagrams of g-C3N4 and Agx%@CN.
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Figure 6. Optimized structure of (a) g-C3N4, (b) Ag1.0%@CN and (c) Ag3.0%@CN. (d) Free energy profiles of g-C3N4, Ag1.0%@CN and Ag3.0%@CN.
Figure 6. Optimized structure of (a) g-C3N4, (b) Ag1.0%@CN and (c) Ag3.0%@CN. (d) Free energy profiles of g-C3N4, Ag1.0%@CN and Ag3.0%@CN.
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Table 1. Surface area (SBET), Pore Volume (Vpore) and Pore Diameter (Dpore) of pristine g-C3N4 and Ag-loaded g-C3N4 samples.
Table 1. Surface area (SBET), Pore Volume (Vpore) and Pore Diameter (Dpore) of pristine g-C3N4 and Ag-loaded g-C3N4 samples.
CatalystSBET (m2 g−1)Vpore (cm3 g−1)Dpore (nm)
g-C3N465.6290.22213.532
Ag0.5%@CN53.3940.17712.450
Ag1.0%@CN43.4760.13911.596
Ag2.0%@CN47.1530.15612.359
Ag3.0%@CN49.7460.18512.526
Ag4.0%@CN50.7510.19113.311
Ag5.0%@CN52.7440.20413.543
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Liu, S.; Li, N.; Zhou, Q. Tuning Ag Loading and Particle Size in Ag@g-C3N4 Photocatalysts for Selective CO2 Conversion to CO and CH4. Nanomaterials 2025, 15, 1443. https://doi.org/10.3390/nano15181443

AMA Style

Liu S, Li N, Zhou Q. Tuning Ag Loading and Particle Size in Ag@g-C3N4 Photocatalysts for Selective CO2 Conversion to CO and CH4. Nanomaterials. 2025; 15(18):1443. https://doi.org/10.3390/nano15181443

Chicago/Turabian Style

Liu, Shicheng, Na Li, and Qulan Zhou. 2025. "Tuning Ag Loading and Particle Size in Ag@g-C3N4 Photocatalysts for Selective CO2 Conversion to CO and CH4" Nanomaterials 15, no. 18: 1443. https://doi.org/10.3390/nano15181443

APA Style

Liu, S., Li, N., & Zhou, Q. (2025). Tuning Ag Loading and Particle Size in Ag@g-C3N4 Photocatalysts for Selective CO2 Conversion to CO and CH4. Nanomaterials, 15(18), 1443. https://doi.org/10.3390/nano15181443

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