Next Article in Journal
Electrochemical Generation of Reactive Chlorine Species via Chloride Oxidation on –COOH-Modified Graphite Electrode to Attain Dye Degradation
Previous Article in Journal
Carbon Nitride-Based Catalysts for Photocatalytic NO Removal
Previous Article in Special Issue
Hybrid PK-P/Fe3O4 Catalyst Derived from Pumpkin Peel (Bio-Waste) for Synozol Red KHL Dye Oxidation Under Photo-Fenton Reaction
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Catalysts
Volume 15
Issue 11
10.3390/catal15111045
catalysts-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Promotional Effect of Semiconductor-Supported Plasmonic Copper Nanoparticles in Visible-Light-Driven Photocatalytic Oxidative Homocoupling of Alkynes

by
Nan Deng
†,
Yaqi Wu
†,
Yi Sun
and
Peng Liu
*
Key Laboratory of Material Chemistry for Energy Conversion and Storage (Ministry of Education), Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Catalysts 2025, 15(11), 1045; https://doi.org/10.3390/catal15111045
Submission received: 10 September 2025 / Revised: 21 October 2025 / Accepted: 29 October 2025 / Published: 3 November 2025
(This article belongs to the Special Issue Environmentally Friendly Catalysis for Green Future)
Browse Figures
Versions Notes

Abstract

Enhancing the oxidation resistance of copper nanoparticles (CuNPs) is a crucial objective in plasmonic photocatalytic reactions. In this study, a series of Cu/X catalysts was synthesized using semiconductor nanomaterials (X = TiO2, ZnO, BN, TiN, SiC, and C3N4) as supports for CuNPs. These catalysts were systematically evaluated in visible-light-driven photocatalytic oxidative homocoupling of phenylacetylene (OHA). Comprehensive characterization revealed distinct metal-support interactions and nanostructure evolution during repeated catalytic cycles. The photocatalytic performance, copper leaching, and structural stability of the catalysts were compared. Cu/TiO2 achieved the highest 1,3-diyne yield (up to 93%) in the first two cycles. In contrast, Cu/ZnO showed minimal copper leaching and excellent recyclability, retaining high activity over three consecutive cycles without the need for reduction pretreatment. Comparative studies revealed that the combination of localized surface plasmon resonance (LSPR) and efficient electron transfer within the Cu0-Cu2O-CuO composite was a key factor in enhancing the photocatalytic activity and stability. These findings provide new insights into the rational design of durable CuNP-based photocatalysts for visible-light-driven organic transformations.

1. Introduction

Sunlight-driven photocatalytic reactions offer significant potential for carbon-neutralization and for addressing environmental concerns [1,2,3]. Plasmonic metal nanomaterials are particularly promising due to their strong visible-light absorption via the localized surface plasmon resonance (LSPR) effect, making them effective photocatalysts for diverse chemical transformations [4,5]. Plasmonic catalysis enables enhanced catalytic activity at lower reaction temperatures, overcoming the equilibrium limits typically encountered in thermocatalytic processes [6]. Among the different plasmonic metals, inexpensive copper stands out as a viable alternative to noble metals like silver and gold. Copper nanoparticle (CuNP)-based plasmonic catalysts have been successfully utilized in applications such as CO2 reduction, N2 photofixation, pollutant degradation, and selective organic synthesis [7,8]. However, a significant challenge in developing oxidation-stable CuNP-based plasmonic catalysts for oxidation reactions is the susceptibility of metallic Cu to oxidation in air.
Copper-catalyzed oxidative homocoupling of alkynes (OHA) is a crucial organic transformation for producing conjugated 1,3-diynes [9], which are valuable building blocks in the synthesis of natural products, pharmaceuticals, optoelectronic materials, and polymers [10]. While various homogeneous and heterogeneous Cu-based catalytic systems have been developed for the OHA reaction [11,12,13,14,15,16,17,18,19], many of these systems raise concerns about corrosion and environmental pollution due to the use of harmful solvents and base additives at elevated temperatures. Therefore, there is a strong demand for synthesizing 1,3-diynes at room temperature (RT) under green and base additive-free conditions. Hwang’s group reported a photocatalytic OHA reaction at RT using CuCl as a homogeneous catalyst in CH3CN, induced by blue LED irradiation [20]. Our group demonstrated that partially reduced copper ferrite (Cu/Fe3O4-CuFe2O4) exhibited comparable catalytic efficiency in photocatalytic OHA at RT and thermocatalytic OHA at 120 °C [21]. Despite advancements in CuNPs-mediated photocatalytic OHA, CuNPs-based plasmonic catalysts exhibit low LSPR stability and poor activity due to the oxidation of metallic Cu in oxidative environments [21,22,23,24,25]. Consequently, developing efficient and recyclable plasmonic photocatalytic OHA systems with improved stability remains a significant challenge.
Semiconductor-supported CuNPs have emerged as promising candidates for visible-light-driven catalysis, utilizing the LSPR effect to enhance light absorption and facilitate the efficient separation and transfer of photoinduced charge carriers at the metal-semiconductor interface [26,27,28,29,30,31,32,33,34]. For example, Cu@TiO2 [26] and Cu/SrTiO3 [34] have demonstrated effective performance in CO2 photoreduction. Additionally, Cu/TiO2 has shown high activity in both the photocatalytic degradation of Sarin [27] and the generation of H2 from water [28,29]. Cu/TiN can selectively catalyze the epoxidation of alkenes with O2 under visible light irradiation [30], while Cu/ZnO exhibits enhanced activity in photocatalytic H2 evolution [31] and uranium photoreduction [32]. However, to our knowledge, simple semiconductor-supported CuNPs have not yet been explored in plasmonic photocatalytic OHA, leaving their potential advantages unverified. While fundamental aspects such as Mie-resonance tuning, dielectric screening, and photothermal effects have been discussed in other copper-based systems [7,8], the structure–performance relationships governing photocatalytic OHA activity are still unclear. In this work, representative semiconductors (X = TiO2, ZnO, BN, TiN, SiC, and C3N4) were employed as supports for CuNPs to construct a series of Cu/X catalysts. These catalysts were applied for the first time in visible-light-driven photocatalytic OHA reactions, with the aim of leveraging the synergistic effects between plasmonic excitation and semiconductor-mediated charge transfer to prevent the complete oxidation of metallic Cu. The influence of the support and the structure-performance relationship of the Cu/X catalysts was systematically investigated. Our results underscore the critical role of metal-support interactions and identify Cu/ZnO as a promising catalyst for achieving stable plasmonic photocatalytic OHA reactions.

2. Results and Discussion

2.1. Characterization of Cu/X Composite Photocatalysts

Various Cu/X catalysts were prepared using a deposition-reduction procedure, with a target Cu loading of 5 wt% (see Section 3 for details). Except for the Cu/ZnO and Cu/C3N4 catalysts, which were reduced with NaBH4 in aqueous solution, the other Cu/X catalysts were reduced with H2 at 250 °C. The structural, textural, and chemical properties of the Cu/X catalysts were thoroughly characterized using X-ray diffraction (XRD), ultraviolet-visible spectroscopy (UV-vis), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), nitrogen physisorption, atomic absorption spectrometer (AAS), Fourier transform infrared (FT-IR), and X-ray photoelectron spectroscopy (XPS). Figure 1 displays the XRD results. The XRD patterns of the bare semiconductors and the SEM images of the Cu/X catalysts are presented in Figures S1 and S2. Notably, the copper-loading procedure did not alter the structure and morphology of the semiconductors. While Cu/TiO2 and Cu/SiC exhibit highly porous nanostructures, the other Cu/X catalysts are composed of irregular nanoaggregates.
The diffraction peak corresponding to the metallic Cu (111) plane at 2θ = 43.3° is weak in all Cu/X catalysts (Figure 1B), indicating a well-dispersed distribution of CuNPs. This observation is further supported by HRTEM images shown in Figure 2, which reveal that the CuNPs are small, with sizes ranging from 2 to 6 nm. Characteristic lattice fringes corresponding to the (111) and (200) planes of metallic Cu0, with spacings of 0.21 nm and 0.18 nm, respectively [23,28], are observed in all Cu/X catalysts. For Cu/C3N4, which possesses a larger CuNP size (>5 nm), the (111) plane of Cu2O with a lattice fringe of 0.25 nm is found in close proximity to the planes of metallic Cu0 [22,25]. In contrast, for other Cu/X catalysts with smaller CuNP sizes (<3 nm), the edge lattices of the metallic CuNPs appear disordered, indicating partial oxidation of the CuNPs. This observation aligns with previous reports that an inherent surface Cu2O layer rapidly forms on metallic CuNPs upon exposure to ambient conditions [8]. Additionally, HRTEM images clearly reveal the formation of heterojunctions at the metal-semiconductor interfaces, which can facilitate charge carrier transfer between the CuNPs and the semiconductor supports [7,8].
The average CuNP sizes, copper loadings, and specific surface areas of the copper catalysts are summarized in Table 1. Although the surface area increases significantly from 2.3 m2/g for Cu/TiN to 46.8 m2/g for Cu/TiO2, the copper loadings are close to the targeted value of 5.0 wt%. The relatively lower copper loadings observed for Cu/ZnO (4.3 wt%) and Cu/C3N4 (3.7 wt%) may be attributed to the liquid-phase reduction method, which tends to cause leaching of copper. Notably, the surface area of the semiconductor supports does not directly affect the copper loadings or CuNP size. The larger average CuNP sizes observed for Cu/SiC (4.7 nm) and Cu/C3N4 (5.5 nm) are likely due to weaker metal-support interactions between CuNPs and the SiC or C3N4 supports [35,36]. In contrast, the smallest average CuNP size, found in Cu/ZnO (2.0 nm), is consistent with the XRD results and suggests a stronger metal-support interaction.
XPS analysis was conducted to study the surface chemical states of copper and oxygen species in the Cu/X catalysts. In the Cu 2p3/2 spectra (Figure 3A), the main peak at a binding energy (BE) of approximately 932.2 eV is attributed to either Cu0 or Cu+ species [37,38,39]. Notably, except for Cu/ZnO and Cu/C3N4, the Cu2+ shakeup satellite peaks (938–946 eV) are absent in the other Cu/X catalysts, indicating the lack of Cu2+ species in these samples. Since the Cu 2p3/2 XPS cannot distinguish between Cu0 and Cu+, Auger Cu LMM spectra were utilized to quantify the fractions of Cu0 (BE ~ 566.7 eV) and Cu+ (BE ~ 569.3 eV) [21,39]. As shown in Figure 3B and Table 1, the fraction of Cu+ decreases from 88% for Cu/TiN to 38% for Cu/ZnO, while the fraction of Cu0 increases from 12% for Cu/TiN to 38% for Cu/TiO2. These results confirm that the supported metallic CuNPs undergo rapid oxidation to form a surface Cu2O layer upon brief exposure to air, in agreement with previous reports [8,40]. In the O 1s spectra (Figure 3C), peaks at BE ~ 530.0, 531.7, and 533.0 eV are assigned to lattice oxygen (OL), surface oxygen vacancy or adsorbed oxygen (OA), and adsorbed water (OW), respectively [41,42]. The fractions of OL and OA are listed in Table 1. Cu/TiO2 and Cu/ZnO exhibit a majority of OL species due to their oxide semiconductor nature, whereas Cu/BN and Cu/SiC show no OL species [35,43]. For Cu/C3N4, the 14% fraction of OL is due to the presence of CuOx, which is consistent with the HRTEM (Figure 2f) and Cu LMM (Figure 3B) results. The similar OL fraction (49%) observed for Cu/TiN and the TiN support suggests that the OL species originate from minor TiO2 impurities. Correspondingly, the OA fraction increases in the order: Cu/TiO2 (12%) < Cu/TiN (37%) < Cu/ZnO (40%) < Cu/BN (74%) < Cu/C3N4 (86%) < Cu/SiC (100%). Given the absence of OL and Cu2+ species in the Cu/BN and Cu/SiC catalysts, the OA species are most likely due to O2 adsorbed on support defects or on the surface of CuNPs. Notably, the presence of an inherent surface Cu2O layer and the lack of OL species in several Cu/X catalysts suggest the formation of Cu+-O2- species on the surface of plasmonic CuNPs, which can be restored under photocatalytic reaction conditions [8]. Therefore, it is plausible that O2 can be activated on Cu0 surfaces to produce Cu+-O2− species via LSPR-assisted electron transfer from Cu to O2 [44,45].
The light absorption properties of the Cu/X catalysts were investigated using diffuse reflectance UV-vis spectroscopy (Figure 4A). Comparative absorption spectra of the Cu/X catalysts and their corresponding supports are presented in Figure S3. After the deposition of CuNPs, all Cu/X catalysts exhibit significantly enhanced absorption in the visible light region. Notably, for Cu/TiO2, Cu/ZnO, and Cu/BN, the absorption range extends from the ultraviolet region (<400 nm) for the bare supports to the near-infrared region (>800 nm) upon CuNP deposition. To further elucidate the contribution of CuNPs, the UV-vis spectra of the Cu/X catalysts were analyzed using the spectra of the semiconductor supports as background, isolating the absorption features attributable to CuNPs (inset in Figure S3). For Cu/BN, Cu/TiN, and Cu/SiC catalysts, the CuNPs display strong visible light absorption with a characteristic LSPR peak in the range of 560–630 nm [46]. In contrast, the LSPR absorption bands for Cu/TiO2, Cu/ZnO, and Cu/C3N4 are red-shifted. This shift and broadening of the LSPR band have been reported to correlate with an increased proportion of Cu2O and CuO [28,31], which is consistent with the HRTEM and XPS results.
The optical bandgap energies (Eg) of the semiconductor supports and Cu/X catalysts were determined from Tauc plots (Figure 4B and Figure S4) derived using the Kubelka–Munk function [47]. In addition, valence band (VB) XPS measurements were employed to estimate the VB positions of the semiconductor supports (Figure 4C). Among the various supports, Eg values range from 0.9 eV for TiN to 5.2 eV for BN, while the estimated VB potentials increase from 1.61 V for C3N4 to 2.74 V for ZnO. Based on these values, the conduction band (CB) potentials were calculated using the formula ECB = EVB − Eg [48], resulting in CB potentials ranging from −2.46 V for BN to 1.18 V for TiN. The corresponding band structure diagram is illustrated in Figure 4D, and these results are consistent with previous reports for the bare semiconductor supports [32,43,47,48,49,50,51].
Under visible light irradiation, supports with narrower bandgaps (TiN, SiC, C3N4) can be photoexcited, whereas those with lager bandgaps (TiO2, ZnO, BN) cannot. As shown in Figure 4B and Table 1, the optical bandgaps of all Cu/X catalysts are narrower than those of the pristine supports. The pronounced enhancement in light absorption within the 500–800 nm range for Cu/X catalysts is attributed to the synergetic effects of Cu/Cu2O/X heterojunction formation and the LSPR of metallic CuNPs [28,29]. Given the small sizes of the CuNPs (2–6 nm), their LSPR is expected to decay primarily via Landau damping, whereby collective plasmon oscillations are non-radiatively converted into energetic electron–hole pairs that can be injected into the support or directly drive surface reactions [7,8,45]. The narrowing of the bandgap is expected to prolong the lifetime of photogenerated charge carriers and facilitate charge transfer between the CuNPs and the semiconductor supports [31], thereby improving the efficiency of visible-light-driven plasmonic photocatalytic reactions. It should be noted that, due to the complexity of the Cu/X nanostructures, a comprehensive understanding of their band structures would require more detailed photoelectrochemical characterization, which is beyond the scope of this study.

2.2. Photocatalytic Performance of the Cu/X Catalysts

The photocatalytic performance of Cu/X catalysts in the OHA reaction was systematically investigated (Figure 5). Phenylacetylene was chosen as the model substrate for the OHA reaction, which was carried out under white LED irradiation (λ > 400 nm, 0.2 W/cm2) and an O2 atmosphere at room temperature (RT ~ 30 °C) for 3h. Initially, the effect of solvent on the Cu/TiO2-photocatalyzed OHA reaction was examined to identify the optimal reaction medium (Figure 5B). Overall, alcohol-based protonic solvents outperformed other polar and nonpolar solvents. Methanol, in particular, afforded the highest yield of 1,3-diyne (93%). To confirm the solvent effect, Cu/ZnO and Cu/BN catalysts were tested in both methanol and ethanol, with methanol consistently providing superior results. Consequently, methanol was selected as the solvent for subsequent heterogeneous photocatalytic OHA reactions.
To clarify whether the photocatalytic activity enhancement originated primarily from the LSPR effect or from photothermal heating of CuNPs, we monitored the reaction temperature under visible-light irradiation and found it to rise to approximately 50 °C. Control experiments conducted without light at ~30 °C yielded less than 3% of the 1,3-diyne product for all Cu/X catalysts, while dark reactions at 50 °C produced slightly higher yields but still below 7% (Figure 5C). These clearly show that localized heating alone is insufficient to explain the high photocatalytic activity observed under illumination. In stark contrast, under identical visible-light irradiation, the yields followed the order: Cu/C3N4 (42%) < Cu/SiC (50%) < Cu/TiN (53%) < Cu/BN (65%) < Cu/ZnO (78%) < Cu/TiO2 (93%), far exceeding those under purely thermal conditions. Given that Cu+ is recognized as the thermocatalytic active species in OHA [21], the low dark activity is mainly due to the absence of base additives and the relatively low reaction temperature, which are insufficient to activate the reactants and to promote the formation of Cu+–phenylacetylide (Cu-PhAC) intermediates [52,53]. Therefore, the significantly enhanced yields under illumination suggest that LSPR excitation in CuNPs predominantly contributes to the reaction through Landau damping–induced hot-carrier generation, which efficiently activates O2 and drives the oxidative homocoupling process, rather than from phonon-mediated photothermal heating.
Although Cu/TiN (Eg = 0.79 eV), Cu/SiC (Eg = 1.96 eV), and Cu/C3N4 (Eg = 2.53 eV) possess bandgaps suitable for visible-light photoexcitation of the supports, their photocatalytic activities were relatively poor. In contrast, the supports of Cu/TiO2 (Eg = 2.90 eV), Cu/ZnO (Eg = 3.09 eV), and Cu/BN (Eg = 4.42 eV) cannot be directly photoexcited by visible light, yet these catalysts exhibited higher photocatalytic activity. Control experiments with bare semiconductor supports (without Cu loading) confirmed negligible activity (<1% yield) under visible-light irradiation, indicating that the supports alone do not drive the reaction. The low activity of Cu/C3N4 and Cu/SiC may be attributed to weak metal–support interactions, resulting in larger CuNP sizes (>4 nm) and excessive Cu2O/CuO species (see Figure 2f and Figure 3B). These factors can weaken the LSPR effect and hinder efficient electron transfer from the C3N4/SiC support to the CuNPs [7]. In contrast, the smaller CuNP sizes (<3 nm) in Cu/TiO2, Cu/ZnO, and Cu/BN can induce a stronger LSPR effect and facilitate charge separation and transfer within the Cu0/Cu2O heterojunctions [31]. Overall, the photocatalytic activity of Cu/X catalysts mainly depends on the size-dependent LSPR effect and the stable Cu0/Cu2O heterojunction of CuNPs. In this system, the wide-bandgap supports (TiO2, ZnO, BN) play a primarily structural and electronic-modulation role, providing strong Cu–support interactions that stabilize small-sized CuNPs and enhance hot-carrier utilization, rather than contributing directly via visible-light photoexcitation.
Since the LSPR effect is triggered when metallic CuNPs absorb photons from visible light [7], the photoinduced hot electron-driven OHA reaction is expected to be influenced by light intensity. To investigate this, the dependence of the photocatalytic activity of Cu/TiO2 on light intensity was examined. As shown in Figure 5D, the yield of 1,3-diyne increased linearly with the intensity of the white LED, which serves as experimental evidence for a photoexcited electron-driven reaction mechanism [21]. Generally, higher irradiance leads to higher yields, likely due to enhanced photoexcitation of the CuNPs [30].
A key advantage of heterogeneous photocatalytic OHA reactions is the catalyst recyclability. To assess the reusability of the Cu/X catalysts, they were separated by centrifugation, thoroughly washed with ethyl acetate and ethanol, dried under vacuum at 60 °C, and then directly reused in subsequent OHA reactions (Figure 5E). Remarkably, all Cu/X catalysts could be recycled and reused at least four times without additional reduction treatment. Although some degree of deactivation was observed, these catalysts still outperformed the previously reported Cu/Fe3O4/CuFe2O4 composite catalyst [21], which exhibited a dramatic decline in 1,3-diyne yield from 95% in the first cycle to 26% in the second cycle. Cu/TiO2 and Cu/BN maintained stable yields during the first two cycles, with slight deactivation occurring from the third cycle onward. In contrast, Cu/TiN, Cu/SiC and Cu/C3N4 showed gradual deactivation starting from the first cycle. Notably, Cu/ZnO demonstrated the best stability, with the yield of 1,3-diyne remaining constant at 62% over the last three cycles. The varying activity and stability of the Cu/X catalysts are likely related to the intrinsic metal-support interactions, which influence both the LSPR effect and the formation of heterojunctions.

2.3. Evidence for Metal-Support Interactions

The recycled Cu/X catalysts were characterized by HRTEM, AAS, and XPS to elucidate the differences in their activity and stability, and to provide evidence for metal-support interactions. HRTEM images of the recycled Cu/X catalysts after the first use are shown in Figure 6, and the average CuNP sizes are summarized in Table 2. Notably, both the (111) plane of Cu2O (lattice fringe of 0.25 nm) and the (111) plane of CuO (lattice fringe of 0.23 nm) were observed in close proximity to the (111) and (200) planes of metallic Cu0, indicating oxidation of the CuNPs during the photocatalytic OHA reaction. After recycling, the average CuNP sizes of Cu/TiO2, Cu/ZnO, and Cu/BN increased slightly, while those of Cu/TiN, Cu/SiC, and Cu/C3N4 decreased significantly. With the exception of Cu/C3N4 (4.3 nm), the other recycled Cu/X catalysts exhibited similar CuNP sizes (2.5~3.0 nm). This convergence in CuNP size suggests that the nanostructures are reconstructed during the photocatalytic OHA reaction. These findings are consistent with previous reports: (i) surface Cu+ species can undergo phenylacetylene-induced leaching from Cu2O nanoparticles, forming homogeneous copper catalytic species [53]; and (ii) soluble Cu+ species can be reduced back to metallic CuNPs by photogenerated electrons [8].
Copper contents of the recycled catalysts after the first and fourth cycles are listed in Table 2. Copper leaching was observed in each reaction cycle, with the most pronounced loss occurring during the first use. After four cycles, the percentage of Cu leaching increased in the following order: Cu/ZnO (23%) < Cu/TiO2 (35%) < Cu/BN (39%) < Cu/TiN (44%) < Cu/SiC (60%) < Cu/C3N4 (70%). This trend indicates that the Cu-support interactions are stronger in the first three catalysts compared to the latter three, aligning well with their activity and stability profiles. To elucidate the relationship between Cu leaching and the formation of the soluble Cu-PhAC complex, the Cu/TiO2 catalyst was separated after 1–3 h of reaction and analyzed by UV-vis and FT-IR spectroscopy (Figure S5). Both the UV-vis absorption at ~460 nm [54] and the FT-IR peaks at 1930 cm−1 (C≡C-Cu) and 1595, 1481, 1438 cm−1 (C=C of phenyl) [55] first increased and then decreased with reaction time. This consistent trend indicates initial accumulation of Cu-PhAC complexes on the catalyst surface, followed by their coupling and transformation into diyne products in the later reaction stages.
To examine whether homogeneous Cu-PhAC species in solution contribute significantly to the overall activity, a hot-filtration test was conducted using the Cu/TiO2 catalyst (Figure S6). In the second reuse cycle, the catalyst was removed by filtration after 1 h of visible-light irradiation, at which point the yield of 1,3-diyne was 49%. The filtrate was further irradiated for 2 h in the absence of the catalyst, and the yield only increased marginally to 55%. This slight yield increase indicates that leached Cu-PhAC species in the filtrate are unable to sustain the coupling reaction efficiently, confirming that the photocatalytic oxidative homocoupling primarily occurs on the catalyst surface via a heterogeneous pathway.
Based on the 1,3-diyne yield and copper content after the fourth cycle, the turnover frequencies (TOF) of the recycled Cu/X catalysts were calculated to be in the range of 2.3–4.0 h−1. Notably, the most stable Cu/ZnO catalyst achieved the highest photocatalytic activity even after four cycles (TOF ~ 4.0 h−1), which is 2.4-fold that of the freshly reduced CuFe2O4 catalyst [21]. These results demonstrate that the ZnO support exerts a unique stabilizing effect on the visible-light-activated Cu0-Cu2O-CuO composite (see Figure 6b), effectively retarding Cu leaching and facilitating the reduction in Cu+/Cu2+ back to Cu0. This stabilization helps maintain a balanced LSPR effect and ensures sustained photocatalytic activity.
XPS spectra of the recycled Cu/X catalysts (Figure 7) reveal that Cu2+ species are present in all recycled samples, consistent with the HRTEM observations. The XPS peak area ratios of Cu0/Cu, Cu+/Cu, OL/O, and OA/O are summarized in Table 2. The fraction of Cu0 in the recycled Cu/X catalysts decreases in the order: Cu/TiO2 (23%) > Cu/ZnO (21%) > Cu/C3N4 (20%) > Cu/BN (18%) > Cu/SiC (10%) > Cu/TiN (9%). The highest Cu0 fraction in recycled Cu/TiO2 correlates well with its superior photocatalytic activity in the second reaction cycle. Although the recycled Cu/C3N4 catalyst remains a relatively high Cu0 fraction, its activity drops sharply in the second cycle, likely due to significant Cu leaching. For both fresh and recycled Cu/SiC and Cu/TiN catalysts, the low Cu0 fractions (~10%) and high Cu leaching rates (>40%) explain their poor photocatalytic activity and stability. Notably, the Cu0 fraction in recycled Cu/ZnO increases from 14% (fresh) to 21% (recycled), whereas other recycled Cu/X catalysts show either a decrease or no change in Cu0 fraction. This unusual increase in Cu0 content further highlights the stabilizing effect of the ZnO support, which is also supported by the nearly unchanged OA and OL fractions. It is noteworthy that the successful identification of Cu0, Cu+, and Cu2+ species in this study was achieved through the combined use of HRTEM, Cu 2p XPS, and Auger Cu LMM XPS techniques. In contrast, most previous reports lacked Auger Cu LMM XPS analysis [23,24,25,26,27,28,29,30,31,32,33,34], which limited the accurate differentiation of copper oxidation states.
To elucidate the role of metal–support interactions in photocatalytic stability, XPS analysis was conducted to examine the electronic interactions between CuNPs and various supports before and after the OHA reaction. The Cu 2p spectra revealed that oxidation during the reaction caused discernible binding-energy shifts for all samples (Figure S7). Notably, the overlapping Cu0/Cu+ 2p3/2 feature at ~932.0 eV shifted toward lower binding energies for Cu/ZnO and Cu/SiC, whereas the corresponding feature for the other catalysts shifted toward higher binding energies. In addition, the core-level binding energies of both metal and non-metal elements in the TiO2, ZnO, and BN supports changed markedly after the reaction, while those in TiN, SiC, and C3N4 supports remained essentially unchanged. These observations indicate that TiO2, ZnO, and BN exhibit stronger electronic metal–support interactions with CuNPs, which are likely responsible for the superior stability of these catalysts during photocatalysis.
Building on the above findings, two crucial observations can be made: (i) SiC, TiN, and C3N4 semiconductors, with narrower bandgaps (<2.7 eV), can be photoexcited by visible light to generate conduction band electrons; (ii) however, their corresponding Cu/X catalysts exhibit lower Cu0 fractions and higher Cu leaching rates. This suggests that the conduction band electrons generated in these supports may not efficiently transfer to the CuNPs, thus failing to enhance the visible-light-driven photocatalytic OHA reactions. Excess photoexcited electrons may preferentially decompose the surface-bound Cu-PhAC complex, leading to a further decrease in catalytic efficiency. In contrast, TiO2, ZnO, and BN, with wider bandgaps (>3.0 eV), cannot be excited by visible light, yet their corresponding Cu/X catalysts display lower Cu leaching, higher Cu0 fraction, and superior photocatalytic activity. These results indicate that the photocatalytic activity in the OHA reaction is primarily governed by the synergistic effect of Cu0-induced LSPR and efficient electron transfer within the Cu0/Cu2O/X heterojunctions. Among them, ZnO demonstrates the strongest interaction with the Cu0-Cu2O-ZnO composite, making Cu/ZnO a particularly promising catalyst. While more complex nanostructured catalysts such as Cu@C-TiO2 [28], Cu-MOF/TiO2 [29], Cu@Cu2O/ZnO [31], and CuxO@TiO2 [56] have shown higher stability, the structure-performance relationships revealed by these simple Cu/X catalysts highlight the need for deeper mechanistic understanding to guide the rational design of more efficient and stable CuNP-based systems.
Based on the above discussion, the plausible mechanism for the visible-light photocatalytic OHA over various semiconductor-supported CuNPs mainly involves a LSPR-induced hot-electron process. Under visible-light irradiation, the LSPR effect of CuNPs generates energetic electrons that activate molecular oxygen to form superoxide anions (O2−, as indicated in Figure S8) [57,58], which act as a Lewis base to abstract the terminal alkyne hydrogen from phenylacetylene. The resulting deprotonated alkyne species subsequently binds to surface Cu+ centers, yielding Cu–PhAC intermediates. Coupling between two adjacent Cu–PhAC intermediates on the catalyst surface produces the 1,3-diyne product. The remaining isolated Cu+ species can be reduced back to Cu0 either by photogenerated electrons from the semiconductor (e.g., Cu2O) or by electrons supplied from the methanol sacrificial agent, thereby completing the catalytic cycle.
Metallic Cu within the Cu/X composites can induce pronounced LSPR effects, whereas Cu2O and CuO phases act as semiconductors capable of multifaceted electron-transfer interactions with the supports, resulting in a complex carrier-migration network. Previous studies have shown that metal-oxide-metal hybrid catalysts based on crystalline Cu2O (e.g., Cu2O-Pd) exhibit enhanced C-C coupling under visible light owing to more efficient charge separation [59]. This raises an important mechanistic question: how can the plasmonic enhancements from metallic Cu0, the semiconductor contributions from Cu2O, and potential photothermal effects during illumination be disentangled? In our present CuNP systems, Cu2O exists only as a few-atomic-layer-thick surface oxide, rather than the large crystalline nanoparticles typically used as photocatalysts [60,61], and its role in oxidative homocoupling may differ substantially. Ex situ XPS analyses before and after reaction suggest interphase charge transfer; however, the lack of in situ spectroscopic and photoelectrochemical data limits a definitive mechanistic deconvolution. Moreover, methanol produced a more pronounced promoting effect than ethanol, yet the underlying causes—whether acting as a sacrificial electron donor, stabilizing the Cu-PhAC intermediate, or engaging in other specific interactions—remain speculative. Future work will focus on the Cu/ZnO catalyst, integrating advanced photoelectrochemical studies, in situ spectroscopic characterization, and kinetic analyses to separately quantify LSPR-induced hot-carrier contributions, Cu2O-mediated semiconductor effects, and the promoting role of methanol. Such efforts will directly link mechanistic insight to structure-performance relationships, enabling the design of next-generation CuNP-based photocatalysts with improved stability and activity.

3. Methods

3.1. Catalyst Preparation

All chemicals and semiconductor nanomaterials (X = TiO2, ZnO, BN, TiN, SiC, and C3N4) used in this study were of analytical grade and purchased from Adamas Reagent Co., Ltd. The Cu/X catalysts, with a theoretical loading of 5 wt%, were prepared via a deposition-reduction method. For the preparation of Cu/TiO2, Cu/BN, Cu/TiN, and Cu/SiC catalysts, 0.5 g of the respective X powder was dispersed in an aqueous solution of Cu(NO3)2·3H2O (0.1g mL−1) at RT. After impregnation for 1 h, the mixture was dried under vacuum at 50 °C for 12 h. The resulting solid was then reduced under a flow of 10 vol% H2 balanced with N2 at 250 °C for 2 h to obtain the final catalyst. For Cu/ZnO and Cu/C3N4 catalysts, a modified procedure was employed. Specifically, 0.5 g of ZnO or C3N4 powder was dispersed into 50 mL of an aqueous Cu(NO3)2·3H2O solution (7 mM) under vigorous stirring for 30 min. Subsequently, NH3·H2O aqueous solution (5 wt%) was added dropwise until the pH reached 9.0. The suspension was stirred for an additional 12 h at RT, after which the solid was separated by centrifugation and washed several times with deionized water. The precursor was then redispersed in 60 mL of deionized water, and 30 mL of NaBH4 aqueous solution (70 mM) was added dropwise to completely reduce Cu2+ to metallic CuNPs. The final product was washed repeatedly with deionized water and ethanol and dried under vacuum at 50 °C overnight.

3.2. Catalyst Characterization

The crystalline phases of the catalyst were identified by XRD (Empyrean apparatus, 40 kV, 30 mA, PANalytical B.V., Almelo, The Netherlands) using Cu Kα radiation [62,63,64]. The structure and morphology of the catalyst were examined by SEM (FEI Nova NanoSEM 450, Eindhoven, the Netherlands) and HRTEM (FEI Tecnai G2 F30, Eindhoven, the Netherlands). Diffuse reflectance UV-vis spectra of the catalysts were collected with a Varian Cary 5000 spectrophotometer (Santa Clara, CA, USA) using BaSO4 as a blank reference. XPS was performed on an AXISULTRA DLD-600 W spectrometer (Shimazu-Kratos, Tokyo, Japan) with Al Kα irradiation, and the binding energies were calibrated by using the C 1s peak of contaminant carbon at 284.5 eV as an internal standard. Nitrogen adsorption–desorption isotherms were analyzed by a Micromeritics ASAP 2020V3.04 H nitrogen adsorption apparatus (Norcross, GA, USA). Copper content of the fresh and recycled Cu/X catalysts was detected by AAS (Perkin Elmer AA-300, Waltham, MA, USA) after treating the samples in nitric acid. FT-IR spectra were recorded on a Bruker VERTEX 70 FT-IR spectrometer (Dresden, Germany), using the KBr pellet method at a resolution of 4 cm−1. Electronic spin resonance (ESR) measurements were performed on a Bruker E500 spectrometer (Dresden, Germany) using 5, 5-dimethyl-1-pyrroline N-oxide (DMPO) as the spin-trapping agent to detect O2−• radical generated in the photocatalytic reaction system under visible-light irradiation (see Supporting Information for details).

3.3. Photocatalytic Oxidative Coupling of Phenylacetylene

All photocatalytic OHA experiments were conducted using a white light-emitting diode (PLS-LED 100, λ > 400 nm, Beijing Perfectlight Technology Co. Ltd., Beijing, China) as the light source, with the light intensity set at 0.2 W/cm2 unless otherwise specified. Typically, phenylacetylene (0.2 mmol), n-dodecane (0.1 mmol, used as an internal standard), and Cu/X catalyst (10 mol% Cu) were mixed with methanol (2 mL) and transferred into a 15 mL Pyrex glass tube (φ 15 mm) equipped with an oxygen balloon. The reaction mixture was irradiated with visible light under stirring for 3 h at ambient temperature. To terminate the reaction, ethyl acetate (5 mL) was added to the mixture. A portion of the diluted reaction mixture (2 mL) was then filtered through a Titan filter (pore size 0.22 μm) to remove catalyst particles. The products were qualitatively analyzed using an Agilent 7890A/5975C GC-MS (Santa Clara, CA, USA), and quantitatively determined by a Fuli 9070 GC-FID (Taizhou, China) employing the internal standard method. Notably, 1,3-diyne was obtained as the sole product with 100% selectivity, and the carbon balance was maintained at 100 ± 3% in all experiments.

4. Conclusions

In summary, a series of Cu/X catalysts was successfully constructed using representative semiconductor nanomaterials (X = TiO2, ZnO, BN, TiN, SiC, and C3N4) as supports for CuNPs and applied in visible-light-driven photocatalytic oxidative homocoupling of phenylacetylene. The study revealed that the photocatalytic activity and stability of these catalysts are closely related to the nature of the metal-support interactions. Supports with stronger interactions, such as ZnO and TiO2, effectively stabilized the Cu0 species, suppressed copper leaching, and maintained high photocatalytic performance. In particular, the Cu/ZnO catalysts demonstrated remarkable photocatalytic activity and durability, attributed to their unique ability to balance the LSPR effect and facilitate electron transfer within the Cu0-Cu2O-ZnO composite. This structural synergy minimized copper leaching and enabled excellent recyclability, preserving high activity across four successive cycles without requiring reduction treatment before reuse. Notably, after the fourth cycle, the Cu/ZnO catalyst still achieved a TOF of ~4.0 h−1, which is 2.4-fold higher than that of the freshly reduced CuFe2O4 catalyst reported in the literature, underscoring its superior long-term performance. These findings highlight the importance of rational support selection and structure-performance relationships in designing efficient and durable CuNP-based photocatalysts. The insights gained from this work provide valuable guidance for the future development of advanced heterogeneous photocatalysts for visible-light-driven organic transformations.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15111045/s1. Figure S1: XRD patterns of various semiconductor supports; Figure S2: SEM images of (a) Cu/TiO2, (b) Cu/ZnO, (c) Cu/BN, (d) Cu/TiN, (e) Cu/SiC, and (f) Cu/C3N4; Figure S3: Diffuse reflectance UV-vis spectra of various X, Cu/X, and isolated CuNPs (insert). The isolated spectra of CuNPs were obtained by measuring Cu/X using X as background; the band at ~570 nm is attributed to the LSPR peak of CuNPs; Figure S4. Optical bandgap energy derived from Tauc plots for various semiconductor supports. Figure S5. UV-vis (A) and FT-IR (B) spectra of in situ-generated Cu+-phenylacetylide on the surface of Cu/TiO2 catalyst; Figure S6. Hot filtration test by using fresh and recycled Cu/TiO2 catalysts with removal of the catalyst in the second cycle; Figure S7. XPS spectra of the fresh and recycled Cu/X catalysts: (A) Cu/TiO2, (B) Cu/ZnO, (C) Cu/BN, (D) Cu/TiN, (E) Cu/SiC, and (F) Cu/C3N4; Figure S8. DMPO spin-trapping ESR spectra of Cu/ZnO recorded at ambient temperature in methanol for DMPO-O2−• detection.

Author Contributions

Investigation, validation, writing—original draft, N.D.; investigation, data curation, editing, Y.W.; data curation, editing, Y.S.; conceptualization, resources, writing—review and editing, supervision, funding acquisition, P.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (21972050).

Data Availability Statement

The data supporting the findings of this study are available in the paper and its Supplementary Materials.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Wang, H.; Tian, Y.M.; König, B. Energy- and Atom-Efficient Chemical Synthesis with Endergonic Photocatalysis. Nat. Rev. Chem. 2022, 6, 745–755. [Google Scholar] [CrossRef]
  2. Wang, S.; Wang, J.; Wang, Y.; Sui, X.; Wu, S.; Dai, W.; Zhang, Z.; Ding, Z.; Long, J. Insight into the Selectivity-Determining Step of Various Photocatalytic CO2 Reduction Products by Inorganic Semiconductors. ACS Catal. 2024, 14, 10760–10788. [Google Scholar] [CrossRef]
  3. Ban, L.; Zhang, Y.; Sun, D.; Zhou, Y.; Li, Y.; Xu, C.; Yang, S.; Zhang, H. Photocatalytic Hydrogen Evolution Driven by Advanced Metal Sulfides from Sustainable Multilevel Biomass and Waste Plastics. Adv. Funct. Mater. 2025, 35, 2506114. [Google Scholar] [CrossRef]
  4. Linic, S.; Christopher, P.; Ingram, D.B. Plasmonic-Metal Nanostructures for Efficient Conversion of Solar to Chemical Energy. Nat. Mater. 2011, 10, 911–921. [Google Scholar] [CrossRef] [PubMed]
  5. Zhang, Y.; He, S.; Guo, W.; Hu, Y.; Huang, J.; Mulcahy, J.R.; Wei, W.D. Surface-Plasmon-Driven Hot Electron Photochemistry. Chem. Rev. 2018, 118, 2927–2954. [Google Scholar] [CrossRef] [PubMed]
  6. Mao, C.; Li, H.; Gu, H.; Wang, J.; Zou, Y.; Qi, G.; Xu, J.; Deng, F.; Shen, W.; Li, J.; et al. Beyond the Thermal Equilibrium Limit of Ammonia Synthesis with Dual Temperature Zone Catalyst Powered by Solar Light. Chem 2019, 5, 2702–2717. [Google Scholar] [CrossRef]
  7. Xin, Y.; Yu, K.; Zhang, L.; Yang, Y.; Yuan, H.; Li, H.; Wang, L.; Zeng, J. Copper-Based Plasmonic Catalysis: Recent Advances and Future Perspectives. Adv. Mater. 2021, 33, 2008145. [Google Scholar] [CrossRef]
  8. Sayed, M.; Yu, J.; Liu, G.; Jaroniec, M. Non-Noble Plasmonic Metal-Based Photocatalysts. Chem. Rev. 2022, 122, 10484–10537. [Google Scholar] [CrossRef]
  9. Siemsen, P.; Livingston, R.C.; Diederich, F. Acetylenic Coupling: A Powerful Tool in Molecular Construction. Angew. Chem. Int. Ed. 2000, 39, 2632–2657. [Google Scholar] [CrossRef]
  10. Shi, W.; Lei, A. 1,3-Diyne Chemistry: Synthesis and Derivations. Tetrahedron Lett. 2014, 55, 2763–2772. [Google Scholar] [CrossRef]
  11. Stefani, H.A.; Guarezemini, A.S.; Cella, R. Homocoupling Reactions of Alkynes, Alkenes and Alkyl compounds. Tetrahedron 2010, 66, 7871–7918. [Google Scholar] [CrossRef]
  12. Alonso, F.; Yus, M. Heterogeneous Catalytic Homocoupling of Terminal Alkynes. ACS Catal. 2012, 2, 1441–1451. [Google Scholar] [CrossRef]
  13. Lei, J.; Su, L.; Zeng, K.; Chen, T.; Qiu, R.; Zhou, Y.; Au, C.-T.; Yin, S.-F. Recent Advances of Catalytic Processes on the Transformation of Alkynes into Functional Compounds. Chem. Eng. Sci. 2017, 171, 404–425. [Google Scholar] [CrossRef]
  14. Chaabane, L.; Beyou, E.; Luneau, D.; Baouab, M.H.V. Functionalization of Graphene Oxide Sheets with Magnetite Nanoparticles for the Adsorption of Copper Ions and Investigation of its Potential Catalytic Activity toward the Homocoupling of Alkynes under Green Conditions. J. Catal. 2020, 388, 91–103. [Google Scholar] [CrossRef]
  15. Wan, W.; Lin, C.; Qiao, Y.; Chen, L.; Chen, J. Cu Catalyst Supported on Nitrogen and Phosphorus Co-Doped Carbon Nanosheets for Homocoupling of Terminal Alkynes Using CO2 as a Soft Oxidant. ACS Appl. Nano Mater. 2021, 4, 4839–4852. [Google Scholar] [CrossRef]
  16. Tang, J.; Jiao, B.; Chen, W.; Ruan, F.; Li, F.; Cui, P.; Wan, C.; Ha, M.N.; Nguyen, V.N.; Ke, Q. Revealing Efficient Catalytic Performance of N-CuOx for Aerobic Oxidative Coupling of Aliphatic alkynes: A Langmuir-Hinshelwood Reaction Mechanism. Nano Res. 2022, 15, 6076–6083. [Google Scholar] [CrossRef]
  17. Shen, Q.; Jin, C.; Xing, Y.; Jia, Z.; Zhang, Y.; Feng, G.; Wen, X. Oxidation Coupling of Terminal Alkynes over CuPd Bimetallic Alloy Enhanced by Optimized Charge Transfer and Alloy Structure. Chem. Eng. J. 2023, 470, 144193. [Google Scholar] [CrossRef]
  18. Muzalevskiy, V.M.; Shastin, A.V.; Tirkasheva, S.I.; Ziyadullaev, O.E.; Parmanov, A.B.; Nenajdenko, V.G. CCl4-TMEDA-CuCl—A Novel Convenient Catalytic System for Dimerization of Terminal Acetylenes in Mild Conditions. Catalysts 2023, 13, 1330. [Google Scholar] [CrossRef]
  19. Ding, R.; Xia, H.; Wu, Y.; Li, Y.; Yang, W.; Xu, H. Ultrafine Copper Oxide Nanoparticles Immobilized on Poly(4-vinylpyridine)-grafted UiO-66-NH2 as Heterogeneous Catalyst for Oxidative Homocoupling of Terminal Alkynes. Colloids Surf. A 2024, 703, 135328. [Google Scholar] [CrossRef]
  20. Sagadevan, A.; Charpe, V.P.; Hwang, K.C. Copper(I) Chloride Catalysed Room Temperature Csp-Csp Homocoupling of Terminal Alkynes Mediated by Visible Light. Catal. Sci. Technol. 2016, 6, 7688–7692. [Google Scholar] [CrossRef]
  21. Zhu, Y.; Deng, N.; Feng, M.; Liu, P. On the Comparable Activity in Plasmonic Photocatalytic and Thermocatalytic Oxidative Homocoupling of Alkynes over Prereduced Copper Ferrite. Chin. J. Catal. 2019, 40, 1505–1515. [Google Scholar] [CrossRef]
  22. Cai, J.; Li, Y.; Zhang, M.; Li, Z. Cooperation in Cu-MOF-74-Derived Cu-Cu2O-C Nanocomposites To Enable Efficient Visible-Light-Initiated Phenylacetylene Coupling. Inorg. Chem. 2019, 58, 7997–8002. [Google Scholar] [CrossRef] [PubMed]
  23. Yuan, Y.; Wang, Y.-Q.; Zhuang, G.; Li, Q.; Yang, F.-L.; Wang, X.; Han, X. Supporting a Cu@In2O3 Core-Shell Structure on N-doped Graphitic Carbon Cuboctahedral Cages for Efficient Photocatalytic Homo-Coupling of Terminal Alkynes. J. Mater. Chem. A 2021, 9, 24909–24914. [Google Scholar] [CrossRef]
  24. Liu, H.; Zhao, J.; Zhang, M. Dual Photoactive Species in CuPd for Light-switched Homo-coupling of Alkynes in Anaerobic and Base Free Condition. ChemCatChem 2024, 16, e202400715. [Google Scholar] [CrossRef]
  25. Yan, X.; Xu, T.; Zhan, W.; Yang, Y.; Yu, Y.; Yi, J.; He, X.; Yang, L.; Zhao, J.; Sun, L.; et al. Engineering Cu2O/Cu/N-C Interface to Induce Directional Migration of Charge for Driving Photocatalytic Homo-Coupling of Terminal alkynes. Nano Res. 2024, 17, 6895–6902. [Google Scholar] [CrossRef]
  26. Zhao, J.; Li, Y.; Zhu, Y.; Wang, Y.; Wang, C. Enhanced CO2 Photoreduction Activity of Black TiO2-Coated Cu Nanoparticles under Visible Light Irradiation: Role of Metallic Cu. Appl. Catal. A Gen. 2016, 510, 34–41. [Google Scholar] [CrossRef]
  27. DeSario, P.A.; Gordon, W.O.; Balboa, A.; Pennington, A.M.; Pitman, C.L.; McEntee, M.; Pietron, J.J. Photoenhanced Degradation of Sarin at Cu/TiO2 Composite Aerogels: Roles of Bandgap Excitation and Surface Plasmon Excitation. ACS Appl. Mater. Interfaces 2021, 13, 12550–12561. [Google Scholar] [CrossRef]
  28. Liu, P.; Dörfler, A.; Tabrizi, A.A.; Skokan, L.; Rawach, D.; Wang, P.; Peng, Z.; Zhang, J.; Ruediger, A.P.; Claverie, J.P. In Operando Photoswitching of Cu Oxidation States in Cu-Based Plasmonic Heterogeneous Photocatalysis for Efficient H2 Evolution. ACS Appl. Mater. Interfaces 2023, 15, 27832–27844. [Google Scholar] [CrossRef]
  29. Khan, A.; Pivert, M.L.; Ranjbari, A.; Dragoe, D.; Bahena-Uribe, D.; Colbeau-Justin, C.; Herrero, C.; Rutkowska-Zbik, D.; Deschamps, J.; Remita, H. Cu-Based MOF/TiO2 Composite Nanomaterials for Photocatalytic Hydrogen Generation and the Role of Copper. Adv. Funct. Mater. 2025, 2501736. [Google Scholar] [CrossRef]
  30. Huang, Y.; Liu, Z.; Gao, G.; Xiao, G.; Du, A.; Bottle, S.; Sarina, S.; Zhu, H. Stable Copper Nanoparticle Photocatalysts for Selective Epoxidation of Alkenes with Visible Light. ACS Catal. 2017, 7, 4975–4985. [Google Scholar] [CrossRef]
  31. Lou, Y.; Zhang, Y.; Cheng, L.; Chen, J.; Zhao, Y. A Stable Plasmonic Cu@Cu2O/ZnO Heterojunction for Enhanced Photocatalytic Hydrogen Generation. ChemSusChem 2018, 11, 1505–1511. [Google Scholar] [CrossRef] [PubMed]
  32. Yu, K.; Jiang, P.; Yuan, H.; He, R.; Zhu, W.; Wang, L. Cu-Based Nanocrystals on ZnO for Uranium Photoreduction: Plasmon-Assisted Activity and Entropy-Driven Stability. Appl. Catal. B Environ. 2021, 288, 119978. [Google Scholar] [CrossRef]
  33. Zhang, S.-W.; Li, S.; Zhang, B.-P.; Yu, D.; Zhang, Z.; Li, J.-F. Copper-Nanoparticle-Dispersed Amorphous BaTiO3 Thin Films as Hole-Trapping Centers: Enhanced Photocatalytic Activity and Stability. RSC Adv. 2019, 9, 5045–5052. [Google Scholar] [CrossRef] [PubMed]
  34. Li, Z.; Zhu, G.; Zhang, W.; Zhu, L.; Cao, B.; Gao, J.; Shi, X.; Huang, Y.; Liu, P.; Hojamberdiev, M. Dual-Functional Copper (Cu0/Cu2+)-Modified SrTiO3-δ Nanosheets with Enhanced Photothermal Catalytic Performance for CO2 Reduction and H2 Evolution. Chem. Eng. J. 2023, 452, 139378. [Google Scholar] [CrossRef]
  35. Li, M.-Y.; Lu, W.-D.; He, L.; Schüth, F.; Lu, A.-H. Tailoring the Surface Structure of Silicon Carbide Support for Copper Catalyzed Ethanol Dehydrogenation. ChemCatChem 2019, 11, 481–487. [Google Scholar] [CrossRef]
  36. Zhou, G.; Yang, F.; Zhu, X.; Feng, W.; Chen, D.; Fang, J. Copper-Copper Oxide Heterostructural Nanocrystals Anchored on g-C3N4 Nanosheets for Efficient Visible-Light-Driven Photo-Fenton-like Catalysis. Molecules 2025, 30, 144. [Google Scholar] [CrossRef]
  37. Liu, P.; Hensen, E.J.M. Highly Efficient and Robust Au/MgCuCr2O4 Catalyst for Gas-Phase Oxidation of Ethanol to Acetaldehyde. J. Am. Chem. Soc. 2013, 135, 14032–14035. [Google Scholar] [CrossRef]
  38. Yue, L.; Wang, J.; Liu, P. Promoting Effect of Copper Doping on LaMO3 (M = Mn, Fe, Co, Ni) Perovskite-Supported Gold Catalysts for Selective Gas-Phase Ethanol Oxidation. Catalysts 2025, 15, 176. [Google Scholar] [CrossRef]
  39. Wang, J.; Chen, L.; Yue, L.; Filot, I.A.W.; Hensen, E.J.M.; Liu, P. Unveiling the Au-Mn-Cu Synergy in Au/LaMnCuO3 Catalysts for Selective Ethanol Oxidation. Chin. J. Catal. 2025, 75, 34–48. [Google Scholar] [CrossRef]
  40. Dabera, G.D.M.R.; Walker, M.; Sanchez, A.M.; Pereira, H.J.; Beanland, R.; Hatton, R.A. Retarding Oxidation of Copper Nanoparticles Without Electrical Isolation and the Size Dependence of Work Function. Nat. Commun. 2017, 8, 1894. [Google Scholar] [CrossRef]
  41. Liu, P.; Li, T.; Chen, H.; Hensen, E.J.M. Optimization of Au0-Cu+ Synergy in Au/MgCuCr2O4 Catalysts for Aerobic Oxidation of Ethanol to Acetaldehyde. J. Catal. 2017, 347, 45–56. [Google Scholar] [CrossRef]
  42. Hu, W.; Li, D.; Yang, Y.; Li, T.; Chen, H.; Liu, P. Copper Ferrite Supported Gold Nanoparticles as Efficient and Recyclable Catalyst for Liquid-Phase Ethanol Oxidation. J. Catal. 2018, 357, 108–117. [Google Scholar] [CrossRef]
  43. Liu, Q.; Chen, C.; Du, M.; Wu, Y.; Ren, C.; Ding, K.; Song, M.; Huang, C. Porous Hexagonal Boron Nitride Sheets: Effect of Hydroxyl and Secondary Amino Groups on Photocatalytic Hydrogen Evolution. ACS Appl. Nano Mater. 2018, 1, 4566–4575. [Google Scholar] [CrossRef]
  44. Huang, Y.-F.; Zhang, M.; Zhao, L.-B.; Feng, J.-M.; Wu, D.-Y.; Ren, B.; Tian, Z. Activation of Oxygen on Gold and Silver Nanoparticles Assisted by Surface Plasmon Resonances. Angew. Chem. Int. Ed. 2014, 53, 2353–2357. [Google Scholar] [CrossRef] [PubMed]
  45. Linic, S.; Aslam, U.; Boerigter, C.; Morabito, M. Photochemical Transformations on Plasmonic Metal Nanoparticles. Nat. Mater. 2015, 14, 567–576. [Google Scholar] [CrossRef]
  46. DeSario, P.A.; Pietron, J.J.; Brintlinger, T.H.; McEntee, M.; Parker, J.F.; Baturina, O.; Stroud, R.M.; Rolison, D.R. Oxidation-Stable Plasmonic Copper Nanoparticles in Photocatalytic TiO2 Nanoarchitectures. Nanoscale 2017, 9, 11720–11729. [Google Scholar] [CrossRef]
  47. Zhou, T.; Wang, J.; Chen, S.; Bai, J.; Li, J.; Zhang, Y.; Li, L.; Xia, L.; Rahim, M.; Xu, Q.; et al. Bird-Nest Structured ZnO/TiO2 as a Direct Z-Scheme Photoanode with Enhanced Light Harvesting and Carriers Kinetics for Highly Efficient and Stable Photoelectrochemical Water Splitting. Appl. Catal. B Environ. 2020, 267, 118599. [Google Scholar] [CrossRef]
  48. Yang, Y.; Zhang, C.; Huang, D.; Zeng, G.; Huang, J.; Lai, C.; Zhou, C.; Wang, W.; Guo, H.; Xue, W.; et al. Boron Bitride Quantum Dots Decorated Ultrathin Porous g-C3N4: Intensified Exciton Dissociation and Charge Transfer for Promoting Visible-Light-Driven Molecular Oxygen Activation. Appl. Catal. B Environ. 2019, 245, 87–99. [Google Scholar] [CrossRef]
  49. Yu, W.; Difa Xu, D.; Peng, T. Enhanced Photocatalytic Activity of g-C3N4 for Selective CO2 Reduction to CH3OH via Facile Coupling of ZnO: A Direct Z-Scheme Mechanism. J. Mater. Chem. A 2015, 3, 19936–19947. [Google Scholar] [CrossRef]
  50. Hao, C.-H.; Guo, X.-N.; Pan, Y.-T.; Chen, S.; Jiao, Z.-F.; Yang, H.; Guo, X.-Y. Visible-Light-Driven Selective Photocatalytic Hydrogenation of Cinnamaldehyde over Au/SiC Catalysts. J. Am. Chem. Soc. 2016, 138, 9361–9364. [Google Scholar] [CrossRef]
  51. Liu, Y.; Peng, M.; Gao, K.; Fu, R.; Zhang, S.; Xiao, Y.; Guo, J.; Wang, Z.; Wang, H.; Zhao, Y.; et al. Boosting Photocatalytic Degradation of Levofloxacin over Plasmonic TiO2-x/TiN Heterostructure. Appl. Surf. Sci. 2024, 655, 159516. [Google Scholar] [CrossRef]
  52. Madasu, M.; Huang, M.H. Cu2O Polyhedra for Aryl Alkyne Homocoupling Reactions. Catal. Sci. Technol. 2020, 10, 6948–6952. [Google Scholar] [CrossRef]
  53. Pary, F.F.; Tirumala, R.T.A.; Andiappan, M.; Nelson, T.L. Copper(I) Oxide Nanoparticle-Mediated C–C Couplings for Synthesis of Polyphenylenediethynylenes: Evidence for a Homogeneous Catalytic Pathway. Catal. Sci. Technol. 2021, 11, 2414–2421. [Google Scholar] [CrossRef]
  54. Shanmugam, M.; Sagadevan, A.; Charpe, V.P.; Pampana, V.K.K.; Hwang, K.C. Cu2O Nanocrystals-Catalyzed Photoredox Sonogashira Coupling of Terminal Alkynes and Arylhalides Enhanced by CO2. ChemSusChem 2020, 13, 287–292. [Google Scholar] [CrossRef]
  55. Lin, Z.; Xiao, Z.; Liu, Y.; Wang, Y.; Chen, S.; Zhang, J.; Chen, Y.; Zhang, X.; Zhang, G.; Li, D.; et al. Insights into Copper(I) Phenylacetylide with in-situ Transformation of Oxygen and Enhanced Visible-Light Response for Water Decontamination: Cu–O Bond Promotes Exciton Dissociation and Charge Transfer. J. Colloid Interf. Sci. 2024, 671, 1–14. [Google Scholar] [CrossRef]
  56. Wang, K.; Bielan, Z.; Endo-Kimura, M.; Janczarek, M.; Zhang, D.; Kowalski, D.; Zielinska-Jurek, A.; Markowska-Szczupak, A.; Bunsho Ohtani, B.; Kowalska, E. On the Mechanism of Photocatalytic Reactions on CuxO@TiO2 Core-Shell Photocatalysts. J. Mater. Chem. A 2021, 9, 10135–10145. [Google Scholar] [CrossRef]
  57. Quan, X.; Guo, F.; Li, R.; Zhang, H.; Shen, Z.; Zhang, J. Palladium−Copper Alloy Nanoparticles Supported on Carbon Nitride Nanosheets as a Photocatalyst for Degradation of Ciprofloxacin. ACS Appl. Nano Mater. 2024, 7, 23228–23239. [Google Scholar] [CrossRef]
  58. Hu, T.; Yue, Z.; Wang, Y.; Yu, Y.; Chang, Y.; Pei, L.; Chen, W.; Han, P.; Martens, W.; Waclawik, E.R.; et al. Cu@CuOx/WO3 with Photo-Regulated Singlet Oxygen and Oxygen Adatoms Generation for Selective Photocatalytic Aromatic Amines to Imines. J. Colloid Interf. Sci. 2024, 663, 632–643. [Google Scholar] [CrossRef]
  59. Tirumala, R.T.A.; Gyawali, S.; Le, T.; Kumar, S.; Tan, S.; Wang, B.; Bristow, A.D.; Andiappan, M. Visible-Light-Driven Photocatalytic Carbon-Carbon Coupling Reaction under Atmospheric Temperature and Pressure Conditions Using Hybrid Cu2O-Pd Nanostructures. ACS Appl. Mater. Interfaces 2025, 17, 26651–26660. [Google Scholar] [CrossRef]
  60. Tirumala, R.T.A.; Ramakrishnan, S.B.; Mohammadparast, F.; Khatri, N.; Arumugam, S.M.; Tan, S.; Kalkan, A.K.; Andiappan, M. Structure-Property-Performance Relationships of Dielectric Cu2O Nanoparticles for Mie Resonance-Enhanced Dye Sensitization. ACS Appl. Nano Mater. 2022, 5, 6699–6707. [Google Scholar] [CrossRef]
  61. Tirumala, R.T.A.; Khatri, N.; Ramakrishnan, S.B.; Mohammadparast, F.; Khan, M.T.; Tan, S.; Wagle, P.; Puri, S.; McIlroy, D.N.; Kalkan, A.K.; et al. Tuning Catalytic Activity and Selectivity in Photocatalysis on MieResonant Cuprous Oxide Particles: Distinguishing Electromagnetic Field Enhancement Effect from the Heating Effect. ACS Sustainable Chem. Eng. 2023, 11, 15931–15940. [Google Scholar] [CrossRef]
  62. Simukaitis, M.; Stewart, S.; Sun, Y. Visible-Light-Driven Desulfurization Reaction Using Partially Oxidized TiN Nanoparticles as Photocatalysts. J. Phys. Chem. C 2025, 129, 7780–7786. [Google Scholar] [CrossRef]
  63. Jiang, L.; Yuan, X.; Zeng, G.; Wu, Z.; Liang, J.; Chen, X.; Leng, L.; Wang, H.; Wang, H. Metal-Free Efficient Photocatalyst for Stable Visible-Light Photocatalytic Degradation of Refractory Pollutant. Appl. Catal. B Environ. 2018, 221, 715–725. [Google Scholar] [CrossRef]
  64. Wang, B.; Wang, Y.; Li, J.; Guo, X.; Bai, G.; Tong, X.; Jin, G.; Guo, X. Photocatalytic Sonogashira Reaction over Silicon Carbide Supported Pd–Cu Alloy Nanoparticles under Visible Light Irradiation. Catal. Sci. Technol. 2018, 8, 3357–3362. [Google Scholar] [CrossRef]
Catalysts 15 01045 g001
Figure 1. XRD patterns of (a) Cu/TiO2, (b) Cu/ZnO, (c) Cu/BN, (d) Cu/TiN, (e) Cu/SiC, (f) Cu/C3N4. (A) wide 2θ range, (B) enlarged view of selected narrow 2θ region.
Figure 1. XRD patterns of (a) Cu/TiO2, (b) Cu/ZnO, (c) Cu/BN, (d) Cu/TiN, (e) Cu/SiC, (f) Cu/C3N4. (A) wide 2θ range, (B) enlarged view of selected narrow 2θ region.
Catalysts 15 01045 g001
Catalysts 15 01045 g002
Figure 2. HRTEM images of (a) Cu/TiO2, (b) Cu/ZnO, (c) Cu/BN, (d) Cu/TiN, (e) Cu/SiC, (f) Cu/C3N4.
Figure 2. HRTEM images of (a) Cu/TiO2, (b) Cu/ZnO, (c) Cu/BN, (d) Cu/TiN, (e) Cu/SiC, (f) Cu/C3N4.
Catalysts 15 01045 g002
Catalysts 15 01045 g003
Figure 3. Cu 2p3/2 (A), Auger Cu LMM (B), and O 1s (C) XPS spectra of various Cu/X samples: (a) Cu/TiO2, (b) Cu/ZnO, (c) Cu/BN, (d) Cu/TiN, (e) Cu/SiC, and (f) Cu/C3N4.
Figure 3. Cu 2p3/2 (A), Auger Cu LMM (B), and O 1s (C) XPS spectra of various Cu/X samples: (a) Cu/TiO2, (b) Cu/ZnO, (c) Cu/BN, (d) Cu/TiN, (e) Cu/SiC, and (f) Cu/C3N4.
Catalysts 15 01045 g003
Catalysts 15 01045 g004
Figure 4. UV-vis spectra (A) and optical bandgap energy derived from Tauc plots (B) for various Cu/X samples. Valence band XPS spectra of various X samples (C). Plausible band structure diagrams for various X samples (D).
Figure 4. UV-vis spectra (A) and optical bandgap energy derived from Tauc plots (B) for various Cu/X samples. Valence band XPS spectra of various X samples (C). Plausible band structure diagrams for various X samples (D).
Catalysts 15 01045 g004
Catalysts 15 01045 g005
Figure 5. (A) Photocatalytic equation and reaction conditions. (B) Solvent effect on the yield of photocatalytic oxidative homocoupling of phenylacetylene using Cu/TiO2, Cu/ZnO, and Cu/BN as catalysts. (C) Comparison of the catalytic activity of Cu/X catalysts in dark and light. (D) Influence of light intensity on the Cu/TiO2-photocatalyzed OHA reaction. (E) Recyclability of Cu/X catalysts.
Figure 5. (A) Photocatalytic equation and reaction conditions. (B) Solvent effect on the yield of photocatalytic oxidative homocoupling of phenylacetylene using Cu/TiO2, Cu/ZnO, and Cu/BN as catalysts. (C) Comparison of the catalytic activity of Cu/X catalysts in dark and light. (D) Influence of light intensity on the Cu/TiO2-photocatalyzed OHA reaction. (E) Recyclability of Cu/X catalysts.
Catalysts 15 01045 g005
Catalysts 15 01045 g006
Figure 6. HRTEM images of the recycled Cu/X catalysts: (a) Cu/TiO2, (b) Cu/ZnO, (c) Cu/BN, (d) Cu/TiN, (e) Cu/SiC, and (f) Cu/C3N4.
Figure 6. HRTEM images of the recycled Cu/X catalysts: (a) Cu/TiO2, (b) Cu/ZnO, (c) Cu/BN, (d) Cu/TiN, (e) Cu/SiC, and (f) Cu/C3N4.
Catalysts 15 01045 g006
Catalysts 15 01045 g007
Figure 7. Cu 2p3/2 (A), Auger Cu LMM (B), and O 1s (C) XPS spectra of the recycled Cu/X catalyst after the 1st cycle: (a) Cu/TiO2, (b) Cu/ZnO, (c) Cu/BN, (d) Cu/TiN, (e) Cu/SiC, and (f) Cu/C3N4.
Figure 7. Cu 2p3/2 (A), Auger Cu LMM (B), and O 1s (C) XPS spectra of the recycled Cu/X catalyst after the 1st cycle: (a) Cu/TiO2, (b) Cu/ZnO, (c) Cu/BN, (d) Cu/TiN, (e) Cu/SiC, and (f) Cu/C3N4.
Catalysts 15 01045 g007
Table 1. Textural, physicochemical, and optical properties of various Cu/X catalysts.
Table 1. Textural, physicochemical, and optical properties of various Cu/X catalysts.
CatalystdCu a
(nm)		[Cu] b
(wt%)		SBET
(m2/g)		XPS Peak Area Ratio (%)Eg c
(eV)
Cu0/CuCu+/CuOL/OOA/O
Cu/TiO22.34.946.8386288122.90 (3.18)
Cu/ZnO2.24.316.8143860403.09 (3.16)
Cu/BN2.55.16.817830744.42 (5.20)
Cu/TiN3.35.02.3128849370.79 (0.90)
Cu/SiC4.75.243.0158501001.96 (2.10)
Cu/C3N45.53.720.4244514862.53 (2.70)
a Based on the HRTEM result. b Based on the AAS result. c Calculated band gap Eg from UV-vis spectra, data in parentheses is the value for bare semiconductor.
Table 2. Structural, chemical, and photocatalytic properties of recycled Cu/X catalysts.
Table 2. Structural, chemical, and photocatalytic properties of recycled Cu/X catalysts.
CatalystdCu a
(nm)		Cu Content (wt%) bXPS Peak Area Ratio (%) cYield d
(%)		TOF e
(h−1)
Fresh1st4thCu0/CuCu+/CuOL/OOA/O
Cu/TiO22.84.94.33.223686328523.4
Cu/ZnO2.74.33.73.321185743624.0
Cu/BN3.05.14.43.11875091392.7
Cu/TiN2.65.03.82.89825040302.3
Cu/SiC2.85.23.52.11052397252.5
Cu/C3N44.33.72.71.12021793152.9
a Based on the HRTEM images after the 1st cycle. b Based on the AAS result. c Calculated from the XPS spectra after the 1st cycle. d Yield of 1,3-diyne in the 4th cycle. Photocatalytic conditions: phenylacetylene (0.2 mmol), n-dodecane (0.1 mmol, internal standard), catalyst (20 mg), methanol (2mL), white LED (λ > 400 nm, 0.2 W/cm2), stirred 3 h under O2 balloon at room temperature. e Turnover frequency (TOF) = (moles of 1,3-diyne produced)/(moles of Cu)/(reaction time, h), calculated from the 4th cycle.
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Deng, N.; Wu, Y.; Sun, Y.; Liu, P. Promotional Effect of Semiconductor-Supported Plasmonic Copper Nanoparticles in Visible-Light-Driven Photocatalytic Oxidative Homocoupling of Alkynes. Catalysts 2025, 15, 1045. https://doi.org/10.3390/catal15111045

AMA Style

Deng N, Wu Y, Sun Y, Liu P. Promotional Effect of Semiconductor-Supported Plasmonic Copper Nanoparticles in Visible-Light-Driven Photocatalytic Oxidative Homocoupling of Alkynes. Catalysts. 2025; 15(11):1045. https://doi.org/10.3390/catal15111045

Chicago/Turabian Style

Deng, Nan, Yaqi Wu, Yi Sun, and Peng Liu. 2025. "Promotional Effect of Semiconductor-Supported Plasmonic Copper Nanoparticles in Visible-Light-Driven Photocatalytic Oxidative Homocoupling of Alkynes" Catalysts 15, no. 11: 1045. https://doi.org/10.3390/catal15111045

APA Style

Deng, N., Wu, Y., Sun, Y., & Liu, P. (2025). Promotional Effect of Semiconductor-Supported Plasmonic Copper Nanoparticles in Visible-Light-Driven Photocatalytic Oxidative Homocoupling of Alkynes. Catalysts, 15(11), 1045. https://doi.org/10.3390/catal15111045

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop