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Article

Photothermal Effect of Carbon-Doped Carbon Nitride Synergized with Localized Surface Plasmon Resonance of Ag Nanoparticles for Efficient CO2 Photoreduction

by
Xianghai Song
1,2,*,
Sheng Xu
1,
Fulin Yang
3,
Xiang Liu
4,
Mei Wang
3,
Xin Liu
1,
Weiqiang Zhou
1,
Jisheng Zhang
1,
Yangyang Yang
1 and
Pengwei Huo
1,*
1
Advanced Chemical Engineering Laboratory of Green Materials and Energy of Jiangsu Province, Institute of Green Chemistry and Chemical Technology, School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China
2
International Innovation Center for Forest Chemicals and Materials of Nanjing Forestry, Nanjing Forestry University, Nanjing 210037, China
3
School of Agricultural Engineering, Jiangsu University, Zhenjiang 212013, China
4
Jiangsu Higher Vocational College Engineering Research Center of Green Energy and Low Carbon Materials, Zhenjiang College, Zhenjiang 212028, China
*
Authors to whom correspondence should be addressed.
Catalysts 2025, 15(4), 369; https://doi.org/10.3390/catal15040369
Submission received: 17 March 2025 / Revised: 2 April 2025 / Accepted: 9 April 2025 / Published: 10 April 2025
(This article belongs to the Special Issue Recent Advances in Photocatalytic CO2 Reduction)
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Abstract

:
Converting carbon dioxide (CO2) into high-value fuels through the photothermal effect offers an effective approach to enhancing the carbon cycle and reducing the greenhouse effect. In this study, we developed Ag/C-TCN-x, a carbon nitride-based photocatalyst that integrates both photothermal and localized surface plasmon resonance (LSPR) effects. This material was synthesized through a three-step process involving hydrothermal treatment, calcination, and photo-deposition. Real-time infrared thermography monitoring revealed that Ag/C-TCN-2 reached a surface stabilization temperature of approximately 176 °C, which was 1.5 times higher than C-TCN and 2.2 times higher than g-C3N4. Under the same experimental conditions, Ag/C-TCN demonstrated a carbon monoxide (CO) release rate 3.3 times greater than that of pure g-C3N4. The composite sample Ag/C-TCN-2 maintained good photocatalytic activity in five cycling tests. The structural stability of the sample after the cycling tests was confirmed by X-ray diffraction (XRD) test. The unique tubular structure of Ag/C-TCN increased its specific surface area, facilitating enhanced CO2 adsorption. Carbon doping not only triggered the photothermal effect but also accelerated the conversion of carriers. Additionally, the LSPR effect of Ag nanoparticles, combined with carbon doping, optimized charge carrier dynamics and promoted efficient CO2 photoreduction. The CO2 reduction mechanism over Ag/C-TCN was further examined using in situ Fourier Transform Infrared (FT-IR) spectroscopy. This research offers valuable insights into how photothermal and LSPR effects can be harnessed to enhance the efficiency of CO2 photoreduction.

1. Introduction

In recent years, photocatalytic technology has attracted significant attention, driving extensive research aimed at clean energy production and environmental pollution control [1]. Photocatalytic CO2 reduction is widely recognized as a promising method for mitigating the greenhouse effect and supporting the carbon cycle [2]. Numerous studies have focused on enhancing the efficiency of semiconductor-based artificial photosynthesis for CO2 photoreduction [3]. However, the development of high-performance catalysts that can effectively capture and activate CO2 molecules remains a considerable challenge. This difficulty primarily arises from the thermodynamic stability and kinetic barriers of CO2, along with the complex multi-proton coupled electron transfer processes that occur during the reaction [4].
Two key challenges in the photocatalytic conversion process are the extension of the photo-response spectrum and the suppression of electron–hole pair recombination induced by light [5]. For photocatalysts to be effective, they must exhibit high redox potentials, an ample number of active surface sites, and efficient charge carrier transport [6]. Among the materials used in photocatalysis, graphitic carbon nitride (g-C3N4) has gained significant attention due to its promising potential in CO2 reduction, thanks to its favorable electronic structure [7]. However, the large particle size of g-C3N4 often results in lower activity, as it leads to a high recombination rate of the photo-generated carriers [8]. The structure of g-C3N4 is characterized by a high density of melon groups, which offer effective sites for modifying its electronic structure and enhancing its properties through elemental doping [9]. Elemental doping is a straightforward and effective strategy to modify g-C3N4, enabling a wider light absorption range, the fine-tuning of its electronic properties, improved light absorption, and the better separation of photo-generated electrons and holes [10]. The potential difference formed between electron-rich donor and electron-deficient acceptor sites creates an internal electric field that facilitates the efficient dissociation of excitons [11]. For instance, Che et al. demonstrated that introducing aromatic carbocycles into the g-C3N4 framework using glucose as a carbon precursor notably improved carrier separation, extended carrier lifetime, and enhanced photocatalytic efficiency [12]. Additionally, carbon incorporation enhances the photothermal conversion efficiency of catalysts. Sun et al. further enhanced the photothermal properties of g-C3N4, resulting in higher photocatalytic hydrogen production activity by integrating carbon dots [13]. Moreover, carbon-based materials, such as carbon dots and carbon nanotubes, serve as effective adsorbents when coupled with solar energy, increasing the absorption across a broad spectrum of solar radiation and improving photothermal conversion efficiency.
Similarly to semiconductor photocatalysis, photocatalysis driven by localized surface plasmon resonance (LSPR) has garnered significant attention in the field of solar energy conversion [14,15]. Compared to other modification techniques, the development of hybrid materials that combine metals and semiconductors—by incorporating small amounts of noble metal nanoparticles exhibiting strong LSPR effects—has emerged as a reliable and promising method to enhance the efficiency of photocatalytic CO2 reduction [16,17]. Hot electron transfer mechanisms are widely recognized as the primary pathway for reactions in plasmonic photocatalysis [18]. LSPR is characterized by intense light absorption resulting from the collective oscillation of charge density at the surface of a metal, which produces high-energy hot electrons and increases visible light absorption, thus improving light energy utilization [19,20]. Combining plasmonic materials, such as noble metal nanoparticles, with specific non-metallic semiconductors and conventional semiconductors offers a powerful strategy for capturing and absorbing near-infrared (NIR) light [21].
The impact of plasma arises from the synergistic combination of three primary factors: the generation of hot carriers, localized heating effects, and the enhancement of optical near-fields [22,23]. Silver (Ag) is considered the most cost-effective material, offering superior surface plasmon resonance effects and greater sensitivity compared to other precious metals [24]. Previous research has shown that Ag nanoparticles (NPs) hold significant promise in converting CO2 into CO. For instance, Wang et al. achieved the in situ photothermal enhancement of photocatalytic reactions by incorporating Ag NPs onto BiOCl nanosheets that featured oxygen vacancies, thereby utilizing the LSPR effect to generate localized heating [25]. Similarly, Ding et al. improved CO2 adsorption and electron transfer efficiency by modifying carbon fibers with Ag NPs, leading to enhanced photocatalytic performance for CO2 reduction [26]. These findings suggest that Ag NPs facilitate CO2 adsorption and electron transfer processes by converting light energy into thermal energy, which induces localized heating during photocatalysis and ultimately improves photocatalytic efficiency [25,26,27].
In this study, we successfully synthesized carbon-doped tubular carbon nitride (C-TCN) using glucose and melamine as precursors. We then incorporated plasmonic Ag NPs onto the C-TCN via photo-deposition, forming Ag/C-TCN-x composite photocatalysts (Figure 1). Infrared thermography monitoring revealed a noticeable photothermal effect resulting from both C doping and the presence of Ag NPs. Specifically, C-TCN exhibited a much higher stabilization temperature of 125.0 °C under illumination, compared to g-C3N4, which stabilized at 73.1 °C. Furthermore, the Ag/C-TCN-2 variant showed an even higher stabilization temperature of 176.5 °C, which is associated with improved photocatalytic CO2 reduction performance. Additional characterization results demonstrate that the Ag/C-TCN-x composite photocatalysts, with their dual photothermal effects, also exhibit enhanced visible light absorption. The impact of these dual photothermal effects on electron activation and transfer during photocatalysis was further investigated using in situ Fourier Transform Infrared (FT-IR) spectroscopy.

2. Results and Discussions

2.1. Structure Characterization

A series of characterizations were conducted to examine the structural properties of the synthesized materials. First, the crystal structure of the catalysts was analyzed using X-ray diffraction (XRD), as shown in Figure 2a. The primary diffraction peaks observed at 12.7° and 27.4° in all samples correspond to the (100) and (002) planes of g-C3N4, respectively [28]. These peaks indicate the periodic conjugation and stacking of the aromatic interlayer in the C-N heterocyclic symmetry unit [29,30]. The reduced intensity of the (002) peak in C-TCN, compared to g-C3N4, can be attributed to the size dependence associated with the tubular structure of C-TCN [31]. Moreover, the carbon incorporation during calcination likely leads to a decrease in crystallinity [12]. In the XRD pattern of Ag/C-TCN, four additional peaks at 37.9°, 44.1°, 64.2°, and 77.2° appear, with their intensities increasing as the Ag concentration rises, confirming the successful loading of Ag NPs onto the C-TCN catalyst. Further structural details were examined using FT-IR spectroscopy, as shown in Figure 2b. Notably, there is no significant difference between the absorption peaks of C-TCN and g-C3N4, as all peaks are consistent with the intrinsic crystal framework of g-C3N4 [32].
To assess the surface area and pore characteristics of the synthesized samples, nitrogen (N2) adsorption–desorption isotherms were utilized. The isotherm patterns displayed typical type IV behavior along with H3-type hysteresis loops, indicating a mesoporous structure for the catalyst [33,34]. In comparison to g-C3N4, the hysteresis loops in C-TCN and Ag/C-TCN are noticeably more pronounced, suggesting a higher density of pore structures. This increased porosity contributes to enhanced light scattering [35]. Moreover, C-TCN and Ag/C-TCN-2 exhibit a larger specific surface area, which is attributed to the tubular structure of C-TCN. A larger surface area provides more active sites, and the combination of a high surface area with a well-developed pore system improves light absorption, enhances photothermal conversion, and ultimately increases the photocatalytic efficiency [36].
The morphology of the samples was analyzed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As seen in Figure 3a,b, C-TCN exhibits a well-defined three-dimensional tubular structure, which provides a significantly larger specific surface area compared to pure g-C3N4. This structural enhancement increases the exposure of active sites, thereby improving CO2 adsorption and activation [37]. TEM images of Ag/C-TCN-2, shown in Figure 3c,d, further confirm the distinct tubular structure. Additionally, Figure 3d and Figure S2 suggested that Ag NPs existed in the heptazine unit of the C3N4, with an average size of approximately 10 nm. Elemental mapping in Figure 3e–h clearly demonstrates the presence of carbon (C), nitrogen (N), and silver (Ag), confirming the successful incorporation of Ag NPs.
As shown in Figure 4, X-ray photoelectron spectroscopy (XPS) was used to analyze the surface characteristics and chemical states of the synthesized samples. The XPS results reveal that positive shifts in binding energy correspond to a decrease in electron density, while negative shifts indicate an increase in electron density. These shifts provide valuable insights into the electron transfer mechanisms within the materials under study [38]. The XPS full spectra in Figure 4a show that the synthesized samples consist of carbon (C), nitrogen (N), and oxygen (O). The presence of carbon and nitrogen is attributed to the g-C3N4 structure, while the oxygen element is likely due to the adsorption of H2O or CO2 molecules on the sample surfaces. The incorporation of Ag NPs is confirmed by the presence of the Ag element in the XPS spectrum of the Ag/C-TCN sample. The high-resolution C 1s spectrum reveals three distinct peaks at binding energies of 288.2, 286.1, and 284.8 eV, which correspond to the g-C3N4 structures of N=C-N, C-NH, and C-C/C=C, respectively [39]. Compared to g-C3N4, the C-NH peak for C-TCN and Ag/C-TCN-2 shifts slightly from 286.1 eV to 286.2 eV, while the N=C-N peak shifts from 288.2 eV to 288.1 eV. The high-resolution N 1s spectrum of g-C3N4 shows three peaks at 401.3, 400.1, and 398.6 eV. The peak at 398.6 eV is attributed to the sp2 hybridized N atom in the triazine group (C=N-C), the peak at 400.1 eV corresponds to the tertiary nitrogen group (N-(C)3), and the weaker peak at 401.3 eV is linked to the incompletely condensed amino group (C-NH/NH2) in g-C3N4. In contrast to g-C3N4, the binding energy of N-(C)3 in C-TCN shifts to higher binding energies, suggesting an increase in electron density on the carbon atom involved [40]. These shifts in the C 1s and N 1s spectra indicate the formation of C-active centers in C-TCN due to carbon doping. Figure 3d shows two distinct peaks corresponding to elemental Ag, identified as Ag 3d3/2 and Ag 3d5/2, respectively [41]. The data in Table 1 confirm that the carbon content in C-TCN exceeds that of traditional carbon nitride, confirming successful carbon doping. Additionally, Ag was clearly identified in Ag/C-TCN-2, further validating the successful incorporation of Ag NPs (Table 2).

2.2. Optical Absorption and Band Structure

In Figure 5a, the light absorption properties of the synthesized catalysts were evaluated using UV–visible diffuse reflectance spectroscopy (DRS). Pure g-C3N4 exhibits limited light absorption in the visible range. In contrast, C-TCN demonstrates considerable shoulder–tail absorption between 400 and 1000 nm, indicating its enhanced capacity to capture light at lower energy wavelengths [42,43]. This improvement is attributed to the donor–acceptor (D-A) structure formed by carbon doping [44]. Ag/C-TCN-2 shows strong visible light absorption around 470 nm, a feature linked to the LSPR effect of Ag NPs [45]. Moreover, plasmonic interactions enhance the near-infrared light response of Ag NPs and promote photothermal synergy [46]. The band gaps of g-C3N4, C-TCN, and Ag/C-TCN-2 were determined using the Tauc method, as shown in Figure 5b. The calculated band gaps for g-C3N4, C-TCN, and Ag/C-TCN-2 are 2.81 eV, 2.71 eV, and 2.60 eV, respectively. The carbon doping facilitates the formation of the D-A structure, leading to a narrowed band gap in C-TCN. XPS valence band spectra reveal that the valence band positions for g-C3N4 and C-TCN are 1.76 eV and 1.88 eV, respectively. Using the Tauc plot data and the equation EVB = ECB + Eg, the conduction band levels for g-C3N4 and C-TCN were calculated to be −1.05 eV and −0.83 eV, respectively. Notably, the conduction band of C-TCN (−0.83 eV) is more negative than the reduction potential of CO2/CO (−0.53 eV), while its valence band (1.88 eV) is more positive than the oxidation potential of H2O (0.83 eV). This suggests that C-TCN can effectively promote CO2 reduction to CO in the presence of H2O. Furthermore, the carbon doping significantly influences the band gap and improves electron transfer at the interface [47].

2.3. Photoelectrochemical Properties

The excitation and separation of photogenerated carriers in semiconductor materials play a crucial role in determining the catalytic efficiency of the catalysts [48]. The charge transfer dynamics of the synthesized catalysts were examined through photoluminescence (PL) and time-resolved transient decay spectroscopy. As shown in Figure 6b, the PL emission intensity of C-TCN is significantly lower than that of g-C3N4. This decrease in steady-state PL intensity for C-TCN indicates a faster rate of photogenerated carrier transfer compared to g-C3N4 [49,50]. This effect is likely due to the carbon doping, which alters the binding states of atoms in g-C3N4, leading to a redistribution of the electron cloud and energy levels. Additionally, the incorporation of Ag NPs interferes with surface charge reorganization, and the Schottky junctions formed at the interfaces between Ag NPs and C-TCN facilitate efficient electron transfer [51,52]. In Figure 6d, the time-resolved photoluminescence (TRPL) decay curve, fitted with a double exponential function, shows that the average lifetime of C-TCN is longer than that of g-C3N4. However, the introduction of Ag NPs shortens the average lifetime of C-TCN from 8.78 ns to 8.20 ns, suggesting the emergence of additional non-radiative decay pathways. This reduction in lifetime for Ag/C-TCN indicates that photogenerated electrons in C-TCN can quickly migrate to the Ag NPs, enhancing the conversion of light energy into heat [53]. The PL and TRPL analysis reveals that Ag/C-TCN-2 exhibits a low electron–hole excited state, and compared to g-C3N4, the additional non-radiative decay channels and prolonged fluorescence lifetimes improve the efficient migration of charge carriers to the catalytic active sites, thereby facilitating the photoreduction of CO2.
The photoconversion efficiency and carrier transport capacity of the synthesized catalysts were evaluated using transient photocurrent response (TPR) and electrochemical impedance spectroscopy (EIS). The results show that g-C3N4 exhibits the lowest photocurrent intensity, while Ag/C-TCN-2 generates the highest photocurrent. Additionally, C-TCN shows higher photocurrent density compared to g-C3N4, indicating that the introduction of carbon enhances charge transfer and carrier separation [54]. The EIS curves for the samples, as shown in Figure S3, reveal that the charge transfer resistance at the interface between the semiconductor surface and the electrolyte is represented by the radius of the high-frequency semicircle [55]. C-TCN has a smaller arc diameter than g-C3N4, and the incorporation of Ag NPs further reduces the arc diameter, signifying improved interfacial charge transfer efficiency. Among all samples, Ag/C-TCN-2 exhibits the smallest arc diameter and the lowest interfacial resistance. This enhanced photogenerated carrier transfer in Ag/C-TCN-2 is attributed to the synergistic effects of the D-A system that promotes electron transfer, along with the built-in electric field generated by the LSPR effect of the Ag NPs. Figure 6c presents the Linear Scanning Voltammetry (LSV) curves for the synthesized samples. The overpotential for C-TCN is lower than that for g-C3N4, which can be attributed to the D-A system created by the carbon doping, facilitating charge transfer and separation. Moreover, the overpotential of Ag/C-TCN is lower than both C-TCN and g-C3N4, likely due to the enhanced electrical conductivity provided by the Ag NPs. Furthermore, Figure S4 shows that the low overpotential of Ag/C-TCN in a CO2 environment suggests that surface reaction kinetics related to CO2 reduction are crucial to the photocatalytic process [56]. Finally, in the competitive reaction, CO2 is preferentially reduced over H+.

2.4. Photocatalytic CO2 Reduction Performance

In this study, we investigated the photocatalytic reduction of CO2 in a gas–solid system. Aligned with the principles of green chemistry and sustainability, the experiments were conducted without the use of co-catalysts, photosensitizers, or sacrificial agents. Gas chromatography was employed to analyze the reaction products, and the results confirmed that carbon monoxide (CO) was the sole product of the CO2 reduction reaction. As shown in Figure S1, the CO production rate from g-C3N4 was measured at 2.44 μmol g−1 h−1 and the CO production rate of g-C3N4-1 was 2.49 μmol g−1 h−1 under simulated sunlight exposure. In contrast, the CO production from C-TCN under the same experimental conditions was significantly higher, at 4.02 μmol g−1 h−1, indicating an enhanced catalytic performance. Furthermore, the photocatalytic activity of Ag/C-TCN-x was found to depend on the amount of Ag NPs used. Notably, Ag/C-TCN-2 exhibited the highest photocatalytic performance, achieving a CO production rate of 8.18 μmol g−1 h−1, nearly 3.4 times greater than that of g-C3N4.
The durability and stability of Ag/C-TCN-2 were also assessed through a series of tests. In a nitrogen atmosphere, no CO was detected when all other experimental parameters were maintained, confirming that the CO originated from the reduction of CO2 rather than from the catalyst itself. Additionally, no CO was produced under dark conditions or in the absence of the catalyst, further supporting that CO generation was solely driven by the photoreduction of CO2. Over five consecutive cycles, Ag/C-TCN-2 exhibited consistent CO yields, underscoring its stability. Furthermore, the XRD pattern of the catalyst recovered after five cycles showed no change in its crystal structure (Figure 7).
To assess the photothermal behavior of the synthesized catalysts, we used an infrared thermography camera to monitor their surface temperature under light exposure. As illustrated in Figure 8a, all samples exhibited a rapid rise in surface temperature within the first 0–2 min of illumination, followed by a slower increase until they reached a stable temperature. After 30 min of light exposure, when the light source was turned off, the surface temperature of the samples rapidly decreased and ultimately stabilized at approximately room temperature.
As depicted in Figure 8b–d, the stabilization temperature of C-TCN was notably higher than that of g-C3N4. Additionally, the introduction of Ag NPs further elevated the stabilization temperature, with the highest recorded temperature observed in Ag/C-TCN-2. This observation is consistent with the results of the photocatalytic CO2 reduction tests. The presence of elemental carbon enhances the catalyst’s ability to absorb near-infrared light, thereby improving the photothermal conversion efficiency of C-TCN. Moreover, the LSPR effect of Ag NPs strengthens the localized electric field, which promotes the generation of hot electrons, contributing to a dual photothermal effect that ultimately enhances photocatalytic performance [57].

2.5. Reaction Process and Mechanism

In situ FT-IR spectroscopy was employed to investigate the reaction mechanism of catalysts involved in the photocatalytic reduction of CO2, as well as to identify the reaction intermediates. Figure 9a,b display the interaction between CO2 and water vapor at the gas–solid interface under dark conditions. The peaks observed between 3640 and 3550 cm−1 are attributed to the O-H bending vibration in water molecules, with these signals growing more pronounced as the exposure time increased, suggesting the accumulation of H2O on the catalyst surface. Furthermore, peaks in the range of 3750–3680 cm−1 correspond to the weak overtone of CO2 molecules [58]. In contrast, negative peaks at 2340 and 2350 cm−1 are associated with the antisymmetric stretching vibration of O=C=O in CO2, indicating a reduction in CO2 concentration during the dark reaction. This decrease is likely a result of CO2 interacting with water vapor absorbed on the Ag/C-TCN-2 surface, leading to the formation of carbonate and bicarbonate species, which aid in trapping and converting gaseous CO2.
The intermediates formed from the interaction of CO2 and H2O with the catalyst during the photocatalytic process are shown in Figure 9c, which also highlights the formation of various new species during CO2 adsorption and activation. As illustrated in Figure S5, the vibrational absorption peak at 3500 cm−1 suggests the involvement of -OH groups in CO2 reduction. In Figure 9c, absorption bands at 1191 and 1240 cm−1 are attributed to bicarbonate (HCO3−), while the bands at 1336 and 1658 cm−1 are linked to bidentate carbonate (b-CO32−). Additionally, the bands observed at 1430, 1542, and 1698 cm−1 correspond to monodentate carbonate (m-CO32−), and those at 1525 and 1580 cm−1 are associated with HCOO− [59]. In the CO2 reduction process to CO, HCOO− serves as a crucial intermediate, decomposing into H+, CO, and electrons [60]. The proposed pathway for CO production may be CO2 → CO2* → HCOO- → CO* → CO. The absorption bands at 1951 and 2015 cm−1, attributed to CO*, provide further support for this mechanism (Figure 10).

3. Materials and Methods

3.1. Materials

Detailed specifications of the experimental materials are provided in the Supplementary Materials.

3.2. Preparation of Samples

3.2.1. Synthesis of C-TCN

C-TCN was produced following the procedure outlined in the existing literature [7]. The detailed steps are as follows: 3 g of melamine and 0.03 g of glucose were combined with 60 mL of deionized water and stirred vigorously for 1 h. This mixture was subsequently placed in a 100 mL Teflon-lined autoclave and held at 200 °C for a duration of 12 h. Afterward, the resulting substance underwent three washings with deionized water and was dried at 60 °C overnight. Following this, the material was subjected to a heating process at 550 °C for four hours in a nitrogen environment, with a temperature ramp rate of 2 °C/min. The end product was referred to as C-TCN.

3.2.2. Synthesis of Ag/C-TCN-x

A total of 0.1 g of C-TCN was introduced into a reaction vial that held 30 mL of deionized water along with 2.3 mL of isopropanol, and the mixture was then stirred vigorously for 1 h. After that, a measured amount of 0.025 mol/L AgNO3 solution was added gradually. The reaction vials were subsequently flushed with nitrogen for a duration of 30 min and exposed to a 300 W xenon lamp for a period of 2 h. In the final steps, the obtained product underwent washing three times with ethanol and deionized water, respectively, before being dried under vacuum at 60 °C overnight. The resultant C-TCN, now loaded with Ag NPs, was labeled as Ag/C-TCN-x, where ‘x’ represents the added volume of AgNO3 solution.
Meanwhile, the preparation of g-C3N4 was conducted using a previously described method [61]. A total of 10 g of melamine was uniformly ground and placed into a 50 mL crucible. This crucible was subsequently covered with tinfoil and subjected to heating in a muffle furnace at a rate of 2.5 °C/min until it attained a temperature of 550 °C, which was held for 4 h. The final product was designated as g-C3N4. For comparison, 3 g of melamine calcined at 550 °C for 4 h under N2 atmosphere with a temperature increase rate of 2 °C/min to obtain the product g-C3N4-1.

4. Conclusions

In this research, C-doped tubular carbon nitride (C-TCN) was synthesized using a two-step hydrothermal-calcination process, followed by the successful deposition of Ag NPs on its surface via photo-deposition, resulting in the composite photocatalyst Ag/C-TCN. The findings revealed that the CO yield of Ag/C-TCN-2 reached 31.8 μmol g−1 in 4 h, which was 3.3 times higher than that of pure g-C3N4, achieved without the need for co-catalysts or sacrificial agents. The charge transfer and separation were enhanced by the D-A system formed through C doping, while the LSPR effect intensified the local electric field, generating additional hot electrons. These effects work together to effectively promote the photocatalytic CO2 reduction process. This study highlights the potential of combining elemental doping and the LSPR effect to design highly efficient photocatalysts for CO2 conversion.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15040369/s1, Table S1 Catalytic performance of the compared with related CN-based photocatalysts. Figure S1 Catalytic activity of g-C3N4, g-C3N4-1 and C-TCN. Figure S2. TEM image of the Ag nanoparticles of Ag/C-TCN-2. Figure S3. Nyquist plots of the prepared catalysts. Figure S4. Linear sweep voltammogram (LSV) curves of Ag/C-TCN-2 under different conditions. Figure S5. In-situ FTIR spectra of Ag/C-TCN-2 during CO2 reduction under 420 nm light irradiation for 15 min (3 W, LED) [62].

Author Contributions

X.S.: resources, conceptualization, writing—review and editing, visualization, supervision. S.X.: conceptualization, methodology, validation, investigation, writing—original draft, visualization. F.Y.: investigation, formal analysis. X.L. (Xiang Liu): conceptualization, validation. M.W.: investigation, formal analysis. X.L. (Xin Liu): conceptualization, formal analysis. W.Z.: methodology, investigation, supervision. J.Z.: methodology, investigation. Y.Y.: investigation, supervision. P.H.: conceptualization, supervision. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the National Natural Science Foundation of China (Grant no. 22108102, 22078131) and the Science and Technology Planning Social Development Project of Zhenjiang City (SH2023102).

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Catalysts 15 00369 g001
Figure 1. The synthesis process of Ag/C-TCN-x.
Figure 1. The synthesis process of Ag/C-TCN-x.
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Figure 2. (a) XRD patterns of samples; (b) FT-IR spectra of g-C3N4 and C-TCN; (c) N2 adsorption–desorption isotherms; and (d) pore size distribution of the prepared samples.
Figure 2. (a) XRD patterns of samples; (b) FT-IR spectra of g-C3N4 and C-TCN; (c) N2 adsorption–desorption isotherms; and (d) pore size distribution of the prepared samples.
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Figure 3. (a,b) SEM images of C-TCN; (c,d) TEM images of Ag/C-TCN-2; (e–h) elemental mapping of Ag/C-TCN.
Figure 3. (a,b) SEM images of C-TCN; (c,d) TEM images of Ag/C-TCN-2; (e–h) elemental mapping of Ag/C-TCN.
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Catalysts 15 00369 g004
Figure 4. (a) XPS full spectra of samples; (b) N 1s, (c) C 1s, and (d) Ag 3d X-ray photoelectron spectroscopy.
Figure 4. (a) XPS full spectra of samples; (b) N 1s, (c) C 1s, and (d) Ag 3d X-ray photoelectron spectroscopy.
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Catalysts 15 00369 g005
Figure 5. (a) UV–Vis DRS of the prepared catalysts; (b) Tauc plots; (c) XPS valence band spectra; and (d) band structure of g-C3N4 and C-TCN.
Figure 5. (a) UV–Vis DRS of the prepared catalysts; (b) Tauc plots; (c) XPS valence band spectra; and (d) band structure of g-C3N4 and C-TCN.
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Figure 6. The obtained (a) transient photocurrent response spectroscopy; (b) PL spectra; (c) linear sweep voltammogram (LSV) curves o; and (d) time-resolved PL spectra of g-C3N4, C-TCN, and Ag/C-TCN-2.
Figure 6. The obtained (a) transient photocurrent response spectroscopy; (b) PL spectra; (c) linear sweep voltammogram (LSV) curves o; and (d) time-resolved PL spectra of g-C3N4, C-TCN, and Ag/C-TCN-2.
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Figure 7. (a) Catalytic activity of g-C3N4, C-TCN, Ag/C-TCN-x (x represents the volume of the added AgNO3 solution); (b) catalytic stability of Ag/C-TCN-2; (c) the activity of Ag/C-TCN-2 under different conditions; (d) XRD patterns of Ag/C-TCN-2 before and after five cycles.
Figure 7. (a) Catalytic activity of g-C3N4, C-TCN, Ag/C-TCN-x (x represents the volume of the added AgNO3 solution); (b) catalytic stability of Ag/C-TCN-2; (c) the activity of Ag/C-TCN-2 under different conditions; (d) XRD patterns of Ag/C-TCN-2 before and after five cycles.
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Figure 8. (a) Temperature response curves of the catalysts corresponding to illumination time (light on → off); (b–d) the stable temperature (20 mg catalyst, gas–solid phase) at 30 min corresponding to the photocatalyst monitored by the thermal imager.
Figure 8. (a) Temperature response curves of the catalysts corresponding to illumination time (light on → off); (b–d) the stable temperature (20 mg catalyst, gas–solid phase) at 30 min corresponding to the photocatalyst monitored by the thermal imager.
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Figure 9. (a,b) In situ FT-IR adsorption curve of Ag/C-TCN-2 (CO2 + H2O), DRIFTS, and dark conditions; (c) in situ FT-IR spectra of Ag/C-TCN-2.
Figure 9. (a,b) In situ FT-IR adsorption curve of Ag/C-TCN-2 (CO2 + H2O), DRIFTS, and dark conditions; (c) in situ FT-IR spectra of Ag/C-TCN-2.
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Figure 10. Possible photocatalytic CO2 reduction mechanism of Ag/C-TCN-2.
Figure 10. Possible photocatalytic CO2 reduction mechanism of Ag/C-TCN-2.
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Table 1. Structural parameters of the prepared catalysts.
Table 1. Structural parameters of the prepared catalysts.
SampleSpecific Surface Area (m2/g)Pore Volume (cm3/g)Average Pore Diameter (nm)
g-C3N413.5800.10430.610
C-TCN26.4150.20531.035
Ag/C-TCN-225.9950.19129.418
Table 2. Surface elemental composition of prepared samples based on XPS analysis.
Table 2. Surface elemental composition of prepared samples based on XPS analysis.
CatalystC (Atom%)N (Atom%)Ag (Atom%)
g-C3N444.3255.680
C-TCN47.7952.210
Ag/C-TCN-248.6449.741.59
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Song, X.; Xu, S.; Yang, F.; Liu, X.; Wang, M.; Liu, X.; Zhou, W.; Zhang, J.; Yang, Y.; Huo, P. Photothermal Effect of Carbon-Doped Carbon Nitride Synergized with Localized Surface Plasmon Resonance of Ag Nanoparticles for Efficient CO2 Photoreduction. Catalysts 2025, 15, 369. https://doi.org/10.3390/catal15040369

AMA Style

Song X, Xu S, Yang F, Liu X, Wang M, Liu X, Zhou W, Zhang J, Yang Y, Huo P. Photothermal Effect of Carbon-Doped Carbon Nitride Synergized with Localized Surface Plasmon Resonance of Ag Nanoparticles for Efficient CO2 Photoreduction. Catalysts. 2025; 15(4):369. https://doi.org/10.3390/catal15040369

Chicago/Turabian Style

Song, Xianghai, Sheng Xu, Fulin Yang, Xiang Liu, Mei Wang, Xin Liu, Weiqiang Zhou, Jisheng Zhang, Yangyang Yang, and Pengwei Huo. 2025. "Photothermal Effect of Carbon-Doped Carbon Nitride Synergized with Localized Surface Plasmon Resonance of Ag Nanoparticles for Efficient CO2 Photoreduction" Catalysts 15, no. 4: 369. https://doi.org/10.3390/catal15040369

APA Style

Song, X., Xu, S., Yang, F., Liu, X., Wang, M., Liu, X., Zhou, W., Zhang, J., Yang, Y., & Huo, P. (2025). Photothermal Effect of Carbon-Doped Carbon Nitride Synergized with Localized Surface Plasmon Resonance of Ag Nanoparticles for Efficient CO2 Photoreduction. Catalysts, 15(4), 369. https://doi.org/10.3390/catal15040369

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