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• In recent years, the escalating environmental pollution and energy crisis have spurred the vigorous development of photocatalytic technology, an environmentally friendly and cost-effective solution [1-4]. This technology stands out among alternatives due to its broad range of material sources, safe operation, and eco-friendly nature. Concurrently, systematic studies on the photocatalytic removal of nitrogen oxides (NO) and water splitting for hydrogen production have demonstrated that the photocatalyst is the crucial component of this technology [5,6]. Despite these advancements, the efficiency of most photocatalysts is hindered by the rapid recombination of electron-hole pairs and limited light absorption, presenting a significant challenge. The quest for a highly effective photocatalyst capable of facilitating photocatalytic water splitting for hydrogen generation and NO removal remains a pivotal objective for the realization of both "source control" and "end treatment" strategies [7,8].

  Metal-organic frameworks (MOFs), composed of metal ions or clusters and organic ligands through coordination self-assembly, have garnered significant attention in photocatalysis [9,10]. NH₂-MIL-125 (NM-125), a prominent MOFs, is extensively studied for its exceptional porous structure, facile functionalization and a suitable band gap [11]. However, its photocatalytic efficiency is constrained by challenges in electron-hole pair separation and limited visible light response. To overcome these limitations, various strategies especially defect engineering have been employed to enhance NM-125′s photocatalytic performance [12].

  Defect engineering has garnered considerable interest, which can effectively modify NM-125′s physical-chemical properties, including conductivity and exposed catalytic sites, positively impacting photocatalytic performance [13]. Partial substitution of 2-amino-terephthalic acid with monocarboxylic acid in NM-125 increases its cavity volume and conductivity, thereby facilitating a faster transfer rate of photogenerated carriers through linker defects and enhancing photocatalytic activity [14]. In previous work, we have demonstrated that NM-125 with link-agent defects synthesized by hydrothermal regulation, thus exhibited enhanced photocatalytic performance through increasing the conductivity and pore volume [15]. However, this substitution led to the reduction of light absorption range due to the absence of the photosensitive 2-amino-terephthalic acid ligand. To compensate for this loss in photosensitivity while maintaining enhanced conductivity, the additional cavity volume created by ligand defects offers an opportunity for introducing photosensitive guest substances (especially carbon quantum dots (CQDs)), providing a feasible avenue for improvement. CQDs are a class of carbon-based materials known for their favorable properties, including excellent light absorption and unique up-conversion photoluminescence [16]. Notably, CQDs can convert long-wavelength light, particularly near-infrared, into shorter wavelengths such as visible light through the up-conversion effect, enhancing light absorption [17,18]. Platinum-doped carbon quantum dots (Pt/CQDs) are synthesized by doping platinum ions into CQDs, leveraging the metal's unfilled orbitals and unique electron configurations to facilitate electron transfer [19]. By encapsulating Pt/CQDs within cavities of linker-defective NM-125, a dual-functional composite material could be fabricated, integrating photosensitivity and conductivity for enhanced photocatalytic performance [20].

  In this study, we introduced linker defects into NM-125 by incorporating glacial acetic acid as a regulator during synthesis, resulting in the defective NM-125. This defect engineering notably enhanced the conductivity and expanded its pore space. Subsequently, we encapsulated light-responsive Pt/CQDs within the enlarged pores of the linker-defective NM-125 by calcining reaction (Fig. 1a). To our knowledge, this represents the first instance of integrating a linker-defective NM-125 with superior conductivity and in-situ encapsulated light-responsive Pt/CQDs within the expanded cavities created by defect engineering, yielding an efficient dual-functional composite for photocatalytic hydrogen evolution and NO elimination.

  Figure 1

  

  Figure 1.  (a) Synthetic schematic diagram of samples. TEM image of (b) Pt/CQDs@NM-125-4 and (c) Pt/CQDs.

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  First, X-ray diffraction (XRD) analyses were conducted on the starting material NM-125 (Fig. S1 in Supporting information). Diffraction peaks at 2θ values of 6.8°, 9.8°, and 11.7° are associated with the (101), (002), and (211) crystal planes, respectively, confirming the successful synthesis of NM-125 [21]. The XRD patterns of NM-125, NM-125-X, and Pt/CQDs@NM-125-4 are nearly identical, suggesting that the introduction of defects via acetic acid and Pt/CQDs does not disrupt the underlying crystal structure of NM-125. Interestingly, NM-125-X exhibits a larger specific surface area and pore size compared to the original NM-125, as detailed in Table S1 (Supporting information), likely due to the increased pore space resulting from the escalated dosage of acetic acid [22]. Furthermore, Pt/CQDs@NM-125-4 has a specific surface area and pore volume of 712.94 m²/g and 0.31 cm³/g, respectively, which are considerably lower than those of NM-125-4, signifying the effective encapsulation of Pt/CQDs within the NM-125-4 framework. The platinum content in Pt/CQDs@NM-125-4, as determined by ICP-AES, was approximately 2.923%. Additionally, thermogravimetric analysis (TGA) was conducted to assess the weight loss of the synthesized samples (Fig. S2 in Supporting information), in which NM-125-X showed a significant weight loss, attributed to the monocarboxylic acid modulator. Prior studies have suggested that the structural defects in NM-125-X are due to missing linker units, potentially resulting from the partial substitution by the monocarboxylic acid modulator [23].

  Next, scanning electron microscopy (SEM) was utilized to examine the morphology of Pt/CQDs@NM-125-4. As depicted in Fig. S3 (Supporting information), the SEM image of Pt/CQDs@NM-125-4 reveals the presence of small holes on the surface, indicating a rougher texture compared to NM-125 and NM-125-4. Transmission electron microscopy (TEM) further confirmed the uniform distribution of Pt/CQDs within the Pt/CQDs@NM-125-4 (Fig. 1b). Pt/CQDs are ultra-small quantum dots with an average diameter of approximately 3 nm (Fig. 1c), aligning with the pore size determined by BET analysis. Additionally, elemental mapping analysis further demonstrated the even distribution of elements such as C, N, O, Ti, and Pt across the prepared samples (Fig. S4 in Supporting information).

  The elemental composition and surface chemical state of NM-125, NM-125-4 and Pt/CQDs@NM-125-4 were confirmed using X-ray photoelectron spectroscopy (XPS) analysis, as shown in Fig. 2a. The XPS spectra of all samples exhibited distinct peaks corresponding to C 1s, N 1s, O 1s, and Ti 2p. Notably, Pt/CQDs@NM-125-4 also displayed a significant peak for the Pt element. Fig. 2b presents the O 1s high-resolution spectrum, where the oxygen concentration attributed to carboxylates (Ti-O) in NM-125 and NM-125-4 were measured at 9.22% and 9.69%, respectively, suggesting the potential formation of ligand defects as an increase in oxygen content. Furthermore, the Pt 4f spectrum in Fig. 2c showed peaks at 76.68 and 73.37 eV, corresponding to Pt 4f_7/2 and Pt 4f_5/2 of Pt (Ⅱ), while peaks at 75.33 and 72.18 eV were assigned to Pt 4f_7/2 and Pt 4f_5/2 of Pt(0) [24].

  Figure 2

  

  Figure 2.  XPS spectra of NM-125, NM-125-4 and Pt/CQDs@NM-125-4 (a) survey, (b) O 1s and (c) Pt 4f. (d) Up-conversion photoluminescence (UCPL) spectra under different excitation wavelengths of the Pt/CQDs.

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  Fig. 2d, Pt/CQDs exhibited the most intense visible light emission at wavelengths of 450–490 nm and 510–550 nm when irradiated with near-infrared light at 700 and 800 nm, respectively. The observed up-conversion effect suggests that Pt/CQDs@NM-125-4 can absorb a broader spectrum of visible light, extending into the near-infrared region, thereby potentially enhancing photocatalytic activity.

  The optical and electrical properties of the synthesized samples were investigated using UV–vis, photoluminescence (PL), and photoelectrochemical analyses. As illustrated in Fig. S5a (Supporting information), NM-125-X exhibited a slightly diminished absorption peak intensity compared to NM-125, particularly between 300 nm and 400 nm, which may be attributed to the absence of the linker 2-amino-terephthalic acid. Furthermore, Pt/CQDs@NM-125-4 demonstrated enhanced light absorption in the visible spectrum due to the incorporation of Pt/CQDs through defect engineering. Among all samples, Pt/CQDs@NM-125-4 had the lowest PL intensity, as shown in Fig. S5b (Supporting information), suggesting that the defect engineering and Pt/CQDs introduction effectively enhanced the separation efficiency of electron-hole pairs. These results were consistent with the subsequent photoelectrochemical results of NM-125, NM-125-X and Pt/CQDs@NM-125-4 (Fig. S6 in Supporting information).

  Performance testing revealed that Pt/CQDs@NM-125-4 exhibited the highest NO elimination efficacy, reaching 52.12% (Fig. 3a), surpassing NM-125-4 at 44.96% and NM-125 at 30%. These results suggest that defect engineering and the incorporation of Pt/CQDs significantly enhance photocatalytic performance. Additionally, Pt/CQDs@NM-125-4 maintained its photocatalytic efficiency after five consecutive cycles (Fig. S7 in Supporting information), indicating its potential as an efficient and stable photocatalyst. Supplementary studies indicated a minimal NO₂ yield, effectively preventing secondary pollution (Fig. S9 in Supporting information). Free radical capture experiments (Fig. S10 in Supporting information) confirmed the pivotal role of h⁺ and ^•O₂^- in the photocatalytic NO oxidation by Pt/CQDs@NM-125-4, as evidenced by a significant decrease in NO removal efficiency upon the addition of potassium iodide (KI) or p-benzoquinone (PBQ). The hydrogen production rate for NM-125-X improved overall (Fig. 3b), with NM-125-4 achieving 20.46 mmol/g, demonstrating that the creation of suitable linker defects can boost photocatalytic hydrogen production. Notably, Pt/CQDs@NM-125-4 achieved the highest rate at 28.75 mmol/g, suggesting that further enhancements are possible by adding Pt/CQDs to a base of defect engineering. After NO removal reaction, the XRD and SEM images of Pt/CQDs@NM-125-4 post-reaction (Fig. S8 in Supporting information) confirmed that the structure remained largely intact. Moreover, when compared to other photocatalysts, Pt/CQDs@NM-125-4 demonstrated superior performance in photocatalytic hydrogen production and NO removal (Tables S4 and S5 in Supporting information).

  Figure 3

  

  Figure 3.  Photocatalytic activity of NM-125, NM-125-4 and Pt/CQDs@NM-125-4 (a) NO removal, (b) hydrogen evolution.

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  The primary intermediate radical species involved in photocatalytic NO removal were identified using electron spin resonance (ESR) spectroscopy [25]. Under dark conditions, no signal response was detected from the reaction mixture. However, upon light irradiation, characteristic peak intensity ratios of approximately 1:2:2:1 and 1:1:1:1 were observed, indicative of the formation of the ^•OH and ^•O₂^- active radical species, respectively (Fig. S11 in Supporting information). Notably, Pt/CQDs@NM-125-4 exhibited a markedly higher signal intensity than NM-125 and NM-125-4 under illuminated conditions. This suggests that the incorporation of Pt/CQDs following the generation of linker defects can significantly boost the generation of more active species.

  Fig. 4 presents real-time monitoring of NO adsorption variations for NM-125 and Pt/CQDs@NM-125-4 samples via in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). In the dark adsorption phase, peaks at 914, 1048, and 1174 cm^-1 (Fig. 4a) are associated with NO₂^-, nitrite, and chelated nitrite, respectively [26]. Additionally, peaks at 1320, 1415, and 2284 cm^-1 are assigned to cis-N₂O₂^2- and N₂O. The peak at 1643 cm^-1 is likely caused by the adsorption of NO on the catalyst surface, which is readily oxidized by O₂ to form NO₂, as indicated by the peak at 1360 cm^-1 [27,28]. Fig. 4c reveals that Pt/CQDs@NM-125-4 also exhibits these absorption bands, while the band at 1643 cm^-1 shows a notable enhancement response compared to the original NM-125. Furthermore, Pt/CQDs@NM-125-4 displays an additional nitrate absorption band at 830 cm^-1, suggesting a higher NO adsorption capacity than NM-125, which is beneficial for subsequent photocatalytic NO removal.

  Figure 4

  

  Figure 4.  In-situ DRIFTS spectra of NO adsorption and visible light reaction processes over NM-125 (a, b) and Pt/CQDs@NM-125-4 (c, d).

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  Upon visible light irradiation, the original NM-125 exhibited three new adsorption peaks at 870, 1193, and 1291 cm^-1, corresponding to NO₂^-, NO^-/NOH, and Bidentate nitrate (NO₃^-), respectively (Fig. 4b). Concurrently, Pt/CQDs@NM-125-4 displayed five new absorption peaks at 1241, 1291, 896, 964, and 1162 cm^-1 (Fig. 4d), which are associated with Bidentate nitrate (NO₃^-), and nitrates [29-31]. Examination of the two illuminated catalyzed reaction reveals that absorption peaks at 2284 cm^-1 disappeared, suggesting that accumulated N₂O was gradually consumed, while the electrons from absorbed NO were captured by photogenerated holes and transformed into NO⁺. Additionally, the absorption bands at 1360 cm^-1 for both NM-125 and Pt/CQDs@NM-125-4, corresponding to NO₂, were intensified. This enhancement is likely due to the participation of reactive oxygen species (ROS) in the photocatalytic oxidation process: 3NO + OH^- → NO₂ + NO^- + NOH. Moreover, Pt/CQDs@NM-125-4 demonstrated a significantly higher intensity of NO oxidation under visible light compared to the original NM-125. During the catalytic process, the photocatalytic NO oxidation led to an increased nitrate yield with a concomitant reduction in the formation of toxic intermediates, indicating selective oxidation of NO under visible light irradiation.

  Based on the calculation data and experimental results, we have proposed a plausible photocatalytic reaction mechanism for Pt/CQDs@NM-125-4 (Scheme S1 in Supporting information). Under visible light irradiation, the linker in Pt/CQDs@NM-125-4 is initially excited, generating photoelectrons that are then transferred to the Ti-oxo cluster through the conventional ligand-to-metal charge transfer (LMCT) pathway, as shown in Eq. S1 (Supporting information). Following this, the adsorbed O₂ on the sample surface, given its energy level of approximately −0.33 V [32], can readily acquire photogenerated electrons due to the more negative conduction band (E_CB) of Pt/CQDs@NM-125-4 at −1.95 V (Fig. S12 in Supporting information), forming reactive ^•O₂^-. The ^•O₂^- is crucial for the ultimate oxidation of NO to NO₃^-, as shown in Eqs. S2, and S5-S8 (Supporting information) [33]. Additionally, while the energy level of the hydroxyl radical (^•OH)/OH^- is around 2.27 V, the E_VB of Pt/CQDs@NM-125-4 is about 0.39 V, which is insufficient to generate ^•OH through direct oxidation of OH^-. Instead, ^•OH is produced via Eqs. S3 and S4 (Supporting information) [34,35]. Notably, NO can be oxidized by the generated ^•OH to form NO₃^-, as shown in Eqs. S9 and S10 (Supporting information). During photocatalytic hydrogen production, protons (H⁺) at the Pt cocatalyst center are reduced to form H₂, while TEOA is oxidized, as represented by Eqs. S11 and S12 (Supporting information). In the photocatalytic processes of NO elimination and H₂ production, the presence of ligand defects alters the energy band structure of the photocatalyst and accelerates the charge transfer rate. At the same time, when ligand defects are present in the NM-125, the excitation energy between the organic ligands and the metal clusters decreases, making it easier for electrons to be excited to the Ti-oxo clusters, which could be demonstrated by DFT calculations (Fig. S13 in Supporting information). In addition, the in-situ confined up-conversion Pt/CQDs enables the linker-defective NM-125 to utilize photogenerated charge carriers more efficiently, which enhances the photocatalytic performance by augmenting the response to visible light. After thorough research and analysis, we suggest possible reaction pathways in Table S6 (Supporting information).

  In summary, we synthesized linker-defective NM-125 by incorporating glacial acetic acid as a monocarboxylic acid regulator and successfully fabricated the dual-functional composite Pt/CQDs@NM-125-4 using an in-situ encapsulation strategy for photocatalytic hydrogen production and NO removal. Pt/CQDs@NM-125-4 exhibits superior NO elimination capabilities and a higher H₂ evolution rate under visible light compared to the original NM-125. Based on a comprehensive set of characterization and analysis tests, we propose a photocatalytic mechanism that highlights the synergistic effect between the improved conductivity of the linker-defective NM-125 and the photosensitivity of Pt/CQDs. This synergy significantly enhances photocatalytic performance, offering a promising approach to addressing environmental and energy challenges.

  Declaration of competing 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.

  CRediT authorship contribution statement

  Xingyan Liu: Writing – original draft, Investigation, Funding acquisition, Formal analysis, Conceptualization. Yue Li: Writing – original draft, Validation, Investigation, Formal analysis. Kaili Wu: Visualization, Resources, Methodology. Panpan Li: Writing – review & editing, Supervision, Project administration. Yonggang Xu: Methodology, Formal analysis. Xiaowei Li: Writing – review & editing, Supervision, Project administration, Funding acquisition. Junhao Zhou: Validation, Supervision. Youzhou He: Validation, Methodology. Min Fu: Visualization, Formal analysis. Guangming Jiang: Methodology, Investigation. Siping Wei: Writing – review & editing, Supervision, Project administration, Funding acquisition.

  Acknowledgments

  This work was supported by the National Natural Science Foundation of China (Nos. 22001026, 22171233, 22201193). Sichuan Science and Technology Program (No. 2023NSFSC0109), and the Fundamental Research Funds for the Central Universities and the Hundred Talent Program of Sichuan University (No. YJ2021158). We also would like to thank the Shiyanjia Lab (www.shiyanjia.com) for SEM test (No. 2410096780).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2025.110853.

  
  
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  Figure 1  (a) Synthetic schematic diagram of samples. TEM image of (b) Pt/CQDs@NM-125-4 and (c) Pt/CQDs.

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  Figure 2  XPS spectra of NM-125, NM-125-4 and Pt/CQDs@NM-125-4 (a) survey, (b) O 1s and (c) Pt 4f. (d) Up-conversion photoluminescence (UCPL) spectra under different excitation wavelengths of the Pt/CQDs.

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  Figure 3  Photocatalytic activity of NM-125, NM-125-4 and Pt/CQDs@NM-125-4 (a) NO removal, (b) hydrogen evolution.

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  Figure 4  In-situ DRIFTS spectra of NO adsorption and visible light reaction processes over NM-125 (a, b) and Pt/CQDs@NM-125-4 (c, d).

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