Graphdiyne-confined synthesis of highly stable Cu₅₅ nano-clusters with white fluorescence

Abstract

Copper nanoclusters are known for their unique electronic, optical, and catalytic properties, but their size is difficult to accurately regulate and the optical stability is poor. Herein, we develop a novel method based on the confinement effect of graphdiyne to successfully synthesize size-controllable, uniformly distributed, and optically stable copper nanoclusters with graphdiyne (GDY–Cu₅₅ NCs). The GDY–Cu₅₅ NCs exhibit an exceptionally bright white fluorescence emission (320–700 nm). The white fluorescence emission property of GDY–Cu₅₅ NCs can be attributed to the fluorescence resonance energy transfer (FRET), where energy is transferred from Cu₅₅ NCs to graphdiyne. This work not only opens a novel avenue for preparing copper nanoclusters with photostability but also provides a new strategy for designing white-light-emitting nanomaterials and fluorescent graphdiyne composites.

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Introduction

Due to their inherent instability, research studies on synthesizing copper nanoclusters (Cu NCs) with controllable size, high stability and regulable performance are relatively scarce.^1–4 Copper, as a transition metal, is abundant in the Earth's crust. Copper nanoclusters possess unique electronic,⁵ optical,⁶ and catalytic properties.⁷ Particularly, they exhibit tunable photoluminescence (PL).⁸ Therefore, Cu nanoclusters are usually regarded as promising materials for bioimaging, sensing, and optoelectronic device applications.^9–13 In general, physical and wet chemical methods are two main strategies for synthesizing Cu NCs. Compared with physical manufacturing approaches (mechanical milling, laser ablation, and arc discharge synthesis),¹⁴ wet chemical synthesis of Cu NCs has been extensively studied in recent years for its scalable and modulable manufacturing properties. Top-down and bottom-up approaches are two main branches of wet chemical synthesis. The former approach normally uses excessive etchants to etch or digest large nanoparticles or nanocrystals into nanoclusters under certain pH and temperature with appropriate initial concentration,^15,16 while the latter approach mainly involves two steps: the reduction of Cu(II) ions, and the following aggregation of the as-obtained Cu atoms.^5,17,18 Both approaches can obtain scalable synthesis of Cu NCs with regulable optical properties.^19,20 However, the key challenges still remain in this field. The first challenge is the preparation of copper nanoclusters with uniform size distribution, which directly influences the absorption and fluorescence properties¹⁶ and plays an important role in investigating their optical properties.⁸ Moreover, Cu NCs usually suffer from poor optical stability owing to oxidation by air, which leads to the quenching of fluorescence.²¹ The widely adopted inert atmosphere with optimized conditions in the synthesis process merely ensures the successful preparation of Cu NCs but hardly prevents the oxidation afterwards.²² More importantly, the copper nanoclusters fabricated to date usually realize only monochromatic light emission with single size.^5,15–18,22 Developing white-light-emitting Cu NCs with broader spectral distribution, good spectral adjustability²³ and high fluorescence efficiency²⁴ is of great significance. Current research studies on white-light-emitting Cu NC materials usually employ strategies such as combining Cu NCs of various sizes or adjusting the functional groups of protective ligands that introduce chromophores to achieve mixed emissions.^25–28 However, the introduction of ligands may cause biological toxicity and affect the long-term stability of nanoclusters in which ligands such as thiols, phosphines and amines are necessary for maintaining stability and preventing aggregation and oxidation of copper nanoclusters. Furthermore, the electronic structure and optical properties of nanoclusters may vary owing to the interaction between functionalized ligands and the copper nanoclusters, resulting in reduced fluorescence efficiency. Therefore, understanding the fluorescence mechanism of Cu NCs is rather challenging. Precise control of the size of copper nanoclusters has to be achieved in order to systematically investigate their relationship with fluorescence properties.⁸

Graphdiyne is a two-dimensional carbon allotrope composed of sp and sp² hybridized carbon atoms, which possesses unique physicochemical properties and broad application prospects. Since its first synthesis by our research group in 2010, the study and application of graphdiyne have rapidly developed, becoming one of the frontier research areas in carbon materials.^29–32 Graphdiyne features a uniform natural porous structure, excellent chemical stability, and layered two-dimensional structure, which played a significant role in anchoring transition and noble metal atoms.³³ Due to the poor fluorescence properties of graphdiyne materials, there are fewer studies on the fluorescence properties of graphdiyne-based materials.^34–36 And none of the previous studies have demonstrated white-light-emitting graphdiyne-based materials.^34–40 Herein, we leverage the multifunctional molecular attributes of graphdiyne to confine the size of stable copper nanoclusters with controllable size and uniform distribution for white fluorescence emission. Using copper quantum dots as catalysts, we synthesized a novel kind of copper nanoclusters with graphdiyne (GDY–Cu₅₅ NCs) via the Glaser coupling method. The diacetylene bonds of graphdiyne can anchor Cu atoms and stabilize Cu NCs. Moreover, the unique pores and layered structure of graphdiyne can confine the size of Cu NCs, prevent unexpected aggregation and achieve controllable size and uniform distribution.⁴¹ We discovered for the first time that copper nanoclusters confined within graphdiyne exhibit broad-spectrum emission capabilities, high-quality white fluorescence, and enhanced stability. When excited at a wavelength of 280 nm, GDY–Cu₅₅ NCs exhibited an extraordinary broad-spectrum emission in the range of 320 nm to 700 nm. Especially, their maximum emission wavelength ranges from 397 nm to 498 nm, showing high-quality white fluorescence. This study breaks new ground for the application of graphdiyne in the optical field and opens up a new synthesis method for fast and convenient preparation of carbon-based white-light-emitting copper nanocluster composites.

Experimental section

Materials used

Cupric chloride dihydrate (CuCl₂·2H₂O) from Sinopharm; sodium dodecylbenzenesulfonate (SDBS) from Energy Chemical; tetrabutylammonium fluoride hydrate (T-BAF) from Innochem; hydrazinium hydrate (80%), pyridine, dichloromethane, methanol, tetrahydrofuran, and ethyl acetate from Concord; sodium chloride, sodium sulfate anhydrous, and copper from Sinopharm.

Characterization

X-ray diffraction (XRD) tests were carried out on a Rigaku SmartLab polycrystalline X-ray diffractometer with Cu radiation (λ = 1.54 Å). Transmission electron microscopy (TEM, JEOL JEM-2100F) and scanning electron microscopy (SEM, Hitachi SU 8020) images and spherical aberration-corrected transmission electron microscopy (AC-TEM JEM-ARM200F) were applied to obtain morphology information. Raman spectra were recorded on an NT-MDT NTEGRA Raman-AFM spectra system with an Ar laser of 473 nm. X-ray photoelectron spectroscopy (XPS) measurements were performed on a VG ESCALAB250XI X-ray photoelectron spectrometer with Al Kα radiation as an excitation source. Attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectra were recorded on a BRUKER VERTEX 70v vacuum FTIR spectrometer with an ATR module. UV-vis spectra were recorded with a Lambda 1050+. Photoluminescence spectra, fluorescence lifetime and quantum yield were acquired with a steady-state transient fluorescence spectrometer FLS980.

Synthesis of Cu QDs

11.3 mg CuCl₂·2(H₂O) and 19.7 mg SDBS were first dissolved in 160 ml deionized water and stirred continuously in an Ar atmosphere with the temperature gradually raised to 50 °C. Then 1 ml of hydrazinium hydrate (80%) was added and the reaction lasted for three hours. After centrifugation and drying, Cu QDs were obtained.

Synthesis of GDY–Cu₅₅ NCs

Hexakis[(trimethylsilyl)ethynyl] benzene was prepared according to the previous report.²⁹ 304.4 mg of Hexakis [(trimethylsilyl)ethynyl]benzene powder was dissolved in 50 ml of tetrahydrofuran and stirred continuously under an Ar atmosphere in ice water bath. 2 ml of T-BAF was added and the reaction lasted for 15 minutes to obtain hexaethynylbenzene (HEB). The reaction solution was then extracted and the supernatant was concentrated and dispersed in pyridine. On the other side, Cu QDs powder was ultrasonically dispersed in the pyridine solvent. HEB-Py was then slowly added dropwise to Cu QDs-Py under 100 °C for 24 hours and finally concentrated to obtain GDY–Cu₅₅ NCs.

Synthesis of GDY–Cu QDs and GDY–Cu NPs

The synthesis GDY–Cu QDs followed the method of preparing GDY–Cu NCs by using copper sheets instead of Cu QDs as the catalyst. GDY–Cu NPs were obtained via solid-phase synthesis. 149.5 mg of Hexakis[(trimethylsilyl)ethynyl]benzene powder was dissolved in 70 ml of dichloromethane and 2.5 ml of methanol and then stirred continuously under an Ar atmosphere in ice water bath. 2 ml of T-BAF was added and reacted for 25 minutes. HEB powder was obtained after filtering. On the other side, Cu QDs powder was ultrasonically dispersed in tetrahydrofuran and mixed with HEB. The solution was vacuumed by using a mechanical pump to remove the solvent and heated in a sand bath at 120 °C for 2 hours to obtain GDY–Cu NPs.

Results and discussion

Synthesis and Characterization of GDY–Cu₅₅ NCs

Fig. 1 shows the proposal process of GDY–Cu₅₅ NCs by graphdiyne-confined synthesis. Initially, copper quantum dots were prepared and transferred into pyridine. The surface of these copper quantum dots partially dissolved in pyridine to form catalytically active pyridine–copper complexes (Py–Cu) for further coupling, accompanied by the splitting of residual copper quantum dots into smaller copper nanoclusters. Hexaethynylbenzene was then added for the coupling reaction to yield three-dimensional graphdiyne. During the coupling process, the copper nanoclusters dispersed in pyridine solution were firmly confined within the three-dimensional porous graphdiyne substrate and finally formed GDY–Cu₅₅ NCs. Copper quantum dots were prepared through a bottom-up approach. Transmission electron microscopy (TEM) characterization (Fig. S1, ESI†) showed uniformly dispersed copper quantum dots with a relatively uniform size of 5 nm. A lattice spacing of 0.21 nm was observed, corresponding to the (111) plane of copper. In addition, spectroscopic characterization including UV-vis absorption and fluorescence measurements was performed on SDBS-protected Cu QDs, as presented in Fig. S2 (ESI†). The experimental data demonstrated the absence of fluorescence characteristics in the SDBS-protected Cu QDs.

Fig. 1 The synthesis process of GDY–Cu₅₅ NCs.

Subsequently, the as-prepared copper quantum dots were adopted as the copper sources and catalysts to synthesize GDY–Cu₅₅ NCs. Copper quantum dots were immersed in pyridine to trigger a ligand exchange process occurring between sodium dodecylbenzenesulfonate–copper (SDBS–Cu) quantum dots and pyridine solvent, accompanied by the partial dissolution of copper on the surface that formed pyridine–copper (Py–Cu) complexes and the deformation of copper quantum dots. Following this, hexaethynylbenzene monomers were introduced. The Py–Cu complex acted as the catalyst to promote the coupling of hexaethynylbenzene, and continuous magnetic stirring resulted in three-dimensional structured graphdiyne. Meanwhile, the three-dimensional graphdiyne in pyridine solution encapsulated the fragmented copper nanoclusters as graphdiyne possessed the ability to anchor metal atoms via the interaction between diacetylene bonds and d orbitals of Cu atoms. Such a unique interaction helped stabilize the Cu₅₅ NCs. Moreover, the uniformly distributed triangular pores in graphdiyne could prevent the aggregation and confine the size of Cu NCs, further ensuring the controllability of the size and quantity of the GDY–Cu₅₅ NCs. As a comparison, GDY–Cu₅₅ NCs were treated by 3 M HCl to completely remove the copper absorbed in GDY, allowing for a comparative analysis of the structural and property differences between the GDY–Cu₅₅ NCs and pure graphdiyne (GDY).

In Raman spectroscopy (Fig. 2(a)), peaks at 1382.2 cm⁻¹ and 1569.5 cm⁻¹ corresponded to the D-band and G-band of graphdiyne, respectively. In Fourier transform infrared (FTIR) spectroscopy (Fig. 2(b)), the peak located at 2097 cm⁻¹ could be assigned to the characteristic stretching vibration of the Cu–alkynyl bond (CuߝC≡C) in graphdiyne, while the peak at 1621 cm⁻¹ corresponded to the skeletal vibration of the benzene ring. Furthermore, we utilized X-ray photoelectron spectroscopy (XPS) to analyze the elemental composition of the synthesized materials (Fig. S3, ESI†). The full XPS spectrum of GDY–Cu₅₅ NCs exhibited characteristic peaks of C 1s, O 1s and Cu 2p. Fig. 2(c) shows the detailed C 1s spectrum, which could be divided into the O 1s peak at 532.1 eV, indicating the adsorption of a small amount of oxygen onto the graphdiyne, with four sub-peaks corresponding to 284.5 eV for C–C (sp²), 285.2 eV for C–C (sp), 286.9 eV for C–O, and 288.5 eV for C=O, respectively. The O 1s peak at 532.1 eV indicates the adsorption of a small amount of oxygen onto the graphdiyne. These results confirmed the successful synthesis of graphdiyne. Additionally, Cu 2p peaks displayed a single valence state of Cu(0) (Fig. 2(d)), with 932.7 eV corresponding to Cu 2p_3/2 and 952.6 eV corresponding to Cu 2p_1/2. After treated by HCl, graphdiyne showed no Cu 2p signals. In the X-ray diffraction (XRD) pattern (Fig. 2(e)), characteristic diffraction peaks of GDY–Cu₅₅ NCs were observed at 2θ = 23.4 degree and 42.8 degree, corresponding to the diffraction peak of graphdiyne and Cu (111) plane with significant broadening. In contrast, pure GDY did not exhibit the Cu (111) diffraction peak, suggesting the successful synthesis of nano-sized copper clusters. The ultraviolet absorption spectra of the GDY–Cu₅₅ NCs and pure GDY were compared, as shown in Fig. 2(f). GDY–Cu₅₅ NCs exhibited two absorption peaks at 308 and 390 nm, whereas the pure GDY exhibited distinct absorption peaks at 355, 373, and 396 nm, which could be attributed to the intrinsic absorption characteristics of the graphdiyne. This significant difference indicated that the introduction of copper clusters affected the optical properties of graphdiyne, which will be discussed in the next section in more details.

Fig. 2 Structural characterization of GDY–Cu₅₅ NCs and GDY: (a) Raman spectrum of GDY–Cu₅₅ NCs (black line) and GDY (red line); (b) FTIR spectrum of GDY–Cu₅₅ NCs (black line) and GDY (red line); (c) XPS spectrum of GDY–Cu₅₅ NCs: narrow scan for element C; (d) XPS spectrum of Cu 2p electrons in GDY–Cu₅₅ NCs (black line) and GDY (red line); (e) XRD spectrum of GDY–Cu₅₅ NCs (black line) and GDY (red line); (f) the UV-vis absorption spectrum of GDY–Cu₅₅ NCs (black line) and GDY (red line).

Electron miscroscopy techniques were subsequently adopted for morphology characterization. Scanning electron microscopy (SEM) images (Fig. 3(a)) revealed that the GDY–Cu₅₅ NCs exhibited a three-dimensional morphology. Energy-dispersive X-ray spectroscopy (EDS) demonstrated the uniform distribution of C, O, and Cu elements (Fig. 3(b)). Transmission electron microscopy (TEM) was then employed for further characterization of the morphology and distribution of copper clusters on the graphdiyne substrate.

Fig. 3 (a) SEM images and (b) elemental mapping analysis of GDY–Cu₅₅ NCs.

High-resolution TEM (Fig. 4(a) and (b)) (HR-TEM) images of GDY–Cu₅₅ NCs exhibited fringes of 0.36 nm, corresponding to the interlayer spacing of graphdiyne, which further confirms the successful preparation of graphdiyne. Analysis using EDS (Fig. 4(c)) indicated that copper was uniformly dispersed on graphdiyne, which was consistent with the SEM-EDS results. However, HR-TEM images showed no distinct copper quantum dots, indicating copper quantum dots fragmented into smaller, copper clusters encapsulated within the three-dimensional graphdiyne during the reaction. We subsequently utilized spherical aberration-corrected transmission electron microscopy to precisely characterize the morphology of the copper clusters. As shown in Fig. 4(d)–(i), aberration-corrected electron microscopy images displayed copper clusters in a disordered manner, yet uniformly dispersed on the graphdiyne substrate, with an average size of 1 nm to 2 nm. Combined with SEM and TEM characterization experiments, these results confirm that copper quantum dots fragmented into smaller, copper clusters encapsulated within the three-dimensional graphdiyne after the reaction, indicating that graphdiyne achieved domain-limited and uniform distribution for copper nanoclusters.

Fig. 4 (a), (b) TEM and HRTEM images and (c) elemental mapping analysis of GDY–Cu₅₅ NCs; (d)–(i) AC-TEM images of GDY–Cu₅₅ NCs.

We also synthesize graphdiyne copper composites under different conditions. As a result, none of those strategies realized the synthesis of GDY–Cu₅₅ NCs. When copper foils were employed as catalysts and templates, dense copper quantum dots with larger size were observed on the GDY substrate to obtain copper quantum dots with graphdiyne (GDY–Cu QDs). TEM images (Fig. S4a, ESI†) showed that the size of copper quantum dots ranged from 5 to 10 nm on the graphdiyne substrates. When physical mixing of copper quantum dots and pure GDY was employed under ultrasonication, copper quantum dots with the size of 5 nm could be observed adhering to the graphdiyne substrates. (Fig. S4b and c, ESI†). Lastly, we utilized a solid-phase synthesis method by mixing and heating copper quantum dots with HEB powder to obtain copper nanoparticles with graphdiyne (GDY–Cu NPs). TEM images (Fig. S4d, ESI†) revealed a more heterogeneous size dispersion of copper whose size ranged from 1 nm to 100 nm on graphdiyne with significant aggregation that formed larger copper nanoparticles. The structural characterization of GDY–Cu QDs and GDY–Cu NPs is shown in Fig. S5 (ESI†). Raman spectroscopy (Fig. S5a, ESI†) exhibited that the D and G band positions at 1350 cm⁻¹ and 1580 cm⁻¹, respectively, demonstrate a preserved graphdiyne structure after copper incorporation. In FTIR spectroscopy (Fig. S5b, ESI†), the peak located at 2104 cm⁻¹ could be attributed to the stretching vibration of diacetylene bonds, while the peak at 1708 cm⁻¹ corresponded to the skeletal vibration of the benzene ring. The XPS survey spectrum of GDY–Cu QDs and GDY–Cu NPs (Fig. S5c, ESI†) exhibited characteristic peaks of C 1s and Cu 2p. The high-resolution Cu 2p spectrum (Fig. S5d, ESI†) confirms the metallic state of copper in both GDY–Cu QDs and GDY–Cu NPs, with characteristic peaks at 932.6 eV (Cu 2p_3/2) and 952.5 eV (Cu 2p_1/2). The absence of satellite peaks rules out Cu²⁺ oxidation. These additional characterization studies collectively provide strong evidence for the successful synthesis and structural integrity of the graphdiyne copper composites. We assumed that the copper quantum dots formed acetylenic copper and further aggregated during heating to form larger copper nanoparticles dispersed on the graphdiyne substrate. We therefore concluded that the in situ coupling of graphdiyne with copper quantum dots was essential for the formation of GDY–Cu₅₅ NCs.

After examining various methods and comparing the outcomes, we concluded that only the proposed synthesis method utilizing copper quantum dot catalysis and the confinement of graphdiyne could successfully prepare GDY–Cu₅₅ NCs. This method leverages the unique properties of graphdiyne, such as its extensive diacetylene bond network, uniform porous structure, and excellent chemical stability, which firmly confined and protected the copper clusters from aggregation and oxidation. Subsequent characterization of the optical properties of this material revealed superior optical characteristics compared to both pristine graphdiyne materials and copper nanocluster materials previously reported.

Optical properties of GDY–Cu₅₅ NCs

In the preceding section, the successful synthesis of GDY–Cu₅₅ NCs was achieved using a graphdiyne-confining method. The resulted GDY–Cu₅₅ NCs possessed a three-dimensional structure and exhibited high dispersibility, allowing for uniform dispersion in the majority of organic solvents to form colloidal suspensions such as tetrahydrofuran (THF), acetone, and ethanol. After ultrasonication in tetrahydrofuran for 6 hours and subsequent settling, a colloidal solution with a Tyndall effect (Fig. S6, ESI†) can be obtained with a maximum concentration of 0.25 mg ml⁻¹. We discovered that the GDY–Cu₅₅ NCs dispersed in organic solvents display fluorescence properties under irradiation with a 365 nm ultraviolet light source. Fig. 5 illustrates the fluorescence emission properties of graphdiyne–copper composite series dispersed in THF. As shown in Fig. 5(a), GDY–Cu₅₅ NCs exhibited obvious bright, cold white fluorescence emission. As comparison, pure graphdiyne exhibited blue-purple fluorescence (Fig. 5(b)), while GDY–Cu QDs exhibited dull green fluorescence (Fig. 5(c)) and GDY–Cu NPs exhibited very weak fluorescence which was considered as solvent fluorescence (Fig. 5(d)).

Fig. 5 Images of (a) GDY–Cu₅₅ NCs; (b) GDY; (c) GDY–Cu QDs: (d) GDY–Cu NPs in THF under a 365 nm ultraviolet lamp.

We then tested the optical properties of the materials. First, we dispersed the GDY–Cu₅₅ NCs and pure GDY in tetrahydrofuran to form colloidal solution (0.1 mg ml⁻¹) using the aforementioned method. Then, UV-vis absorption and photoluminescence characterization were performed. The UV-vis spectra of GDY–Cu₅₅ NCs and GDY were very different. The UV-vis absorption spectra (Fig. 6(a)) indicated that the pure graphdiyne exhibited distinct absorption peaks at 355, 373, and 396 nm, which were thought to be the result of the intrinsic absorption bands of graphdiyne. However, the GDY–Cu₅₅ NCs exhibited two characteristic absorption peaks at 308 nm and 390 nm. The first peak could be assigned to the GDY-to-Cu charge transfer transition. The second peak was the result of the UV-vis absorption of Cu₅₅ NCs. A previous study⁷ has demonstrated the relationship between cluster size and UV-vis absorption spectrum. By comparing the results of spectroscopic and microscopic characterization, we thus concluded that the composition of nanoclusters was Cu₅₅.^7,42

Fig. 6 (a) UV-vis absorption spectrum of GDY–Cu₅₅ NCs (black line) compared with that of GDY (red line); (b) photoluminescence (PL) spectrum of GDY–Cu₅₅ NCs (black line) compared with that of GDY (red line) at an excitation wavelength 280 nm; (c) UV-vis absorption spectrum of GDY–Cu₅₅ NCs (black line) compared with those of GDY–Cu QD (green line) and GDY–Cu NP (blue line); (d) PL spectrum of GDY–Cu₅₅ NCs (black line) compared with those of GDY–Cu QD (green line) and GDY–Cu NP (blue line) at an excitation wavelength of 280 nm.

Subsequently, fluorescence spectroscopy was employed to assess the photoluminescence (PL) properties of the GDY–Cu₅₅ NCs and GDY with fixed excitation wavelength at 280 nm. The PL spectrum (Fig. 6(b)) indicated that the GDY–Cu₅₅ NCs possessed an ultra-broad emission peak shift from 320 nm to 700 nm, with the maximum emission wavelength range of 397 nm to 498 nm. The entire emission spectrum covers the wavelengths of blue, green, and part of the red light, leading to an exceptional white fluorescence emission. In contrast, the pure graphdiyne exhibited three main emission peaks at 408, 432, and 455 nm, corresponding to the distinct UV-visible absorption results. These peaks represented the intrinsic fluorescence emission property of pure graphdiyne. Furthermore, GDY–Cu₅₅ NCs exhibited significantly a stronger intensity of fluorescence and a broader emission range than pure graphdiyne (Fig. 6(b)), demonstrating its high-quality white fluorescence property.

We further characterized the UV-visible absorption and fluorescence emission of the graphdiyne–copper composites with various sizes and morphologies synthesized under different conditions as comparison. The materials were categorized into three size scales: copper nanoclusters with graphdiyne (GDY–Cu₅₅ NCs) which were synthesized using the proposed domain-limited method, copper quantum dots with graphdiyne (GDY–Cu QDs) which were synthesized by copper foil as catalysts and templates, and copper nanoparticles with graphdiyne (GDY–Cu NPs) which were synthesized by the solid-phase synthesis method. These materials were dispersed in tetrahydrofuran using the same method as described earlier. Owing to the weak absorbance and fluorescence emission of GDY–Cu composites synthesized by physical mixing, their corresponding properties were not further examined.

UV-vis absorption spectra of GDY–Cu composite series are displayed in Fig. 6(c). The UV-vis absorption spectra showed that GDY–Cu QDs exhibited an absorption peak at 279 nm, which was thought to be the result of GDY-to-Cu QDs charge transfer transition. GDY–Cu NPs exhibited no characteristic absorption peaks. Both materials did not exhibit characteristic absorption of copper because of the heterogeneous size distribution of copper in these two materials. In addition, the absorption peaks of heterogeneous size distribution of copper would overlap with those of graphdiyne which had a strong absorption capacity. In contrast, the copper nanoclusters in GDY–Cu₅₅ NCs had a more uniform size distribution and could still exhibit characteristic absorption peaks under the absorption coverage of graphdiyne.

We then conducted fluorescence emission tests of various GDY–Cu composites. The PL spectra of GDY–Cu QDs and GDY–Cu NPs were very different from GDY–Cu₅₅ NCs, and both GDY–Cu QDs and GDY–Cu NPs failed to exhibit white light emission property. GDY–Cu QDs exhibited a maximum fluorescence emission peak at 509 nm with an emission intensity of 362 nm to 687 nm. The emission intensity of GDY–Cu QDs was lower than that of GDY–Cu₅₅ NCs and the emission intensity range was also narrower. Because large and inhomogeneous sizes of copper (5–10 nm) in GDY–Cu QDs did not exhibit fluorescence properties, these quenched the fluorescence of graphdiyne. Due to the relatively large size of Cu QDs or Cu NPs, their electronic structure closely resembles that of bulk materials, characterized by a smaller bandgap. This facilitates the dissipation of energy through non-radiative pathways, such as thermal relaxation, preventing efficient energy transfer to graphdiyne. Consequently, this results in the quenching of fluorescence in the graphdiyne copper composites. Therefore, GDY–Cu NPs also hardly exhibited fluorescence emission (Fig. 6(d)). Considering the existence of larger copper particles in GDY–Cu NPs, the fluorescence of GDY was quenched by those copper particles in GDY–Cu NPs. The decreased intensity of fluorescence emission of GDY–Cu QDs and GDY–Cu NPs suggested that the white fluorescence embodied by GDY–Cu₅₅ NCs originated from the contribution of both small-sized copper nanoclusters (Cu₅₅ NCs) with uniform distribution and graphdiyne substrate. As a result, Cu₅₅ NCs promoted the fluorescence of graphdiyne, strengthened the emission intensity of graphdiyne with a broader range, and thus exhibited the white fluorescence property on a macroscopic scale. The distinct differences of fluorescence emission property among GDY–Cu₅₅ NCs, GDY–Cu QDs and GDY–Cu NPs indicated that GDY–Cu₅₅ NCs undergo different fluorescence mechanisms.

Next, we would further discuss the fluorescence mechanism of GDY–Cu₅₅ NCs. Based on the characterization results of GDY–Cu₅₅ NCs, we assumed that the white-light-emitting process was controlled by a fluorescence resonance energy transfer (FRET) mechanism. In general, the transfer of fluorescence energy from the donor to the acceptor occurs when there is some overlap between the emission spectrum of the donor and the absorption spectrum of the acceptor within suitable space distance (usually less than 100 Å).⁴³ We observed that the emission spectrum of the GDY–Cu₅₅ NCs overlapped partially with the absorption spectrum of pure graphdiyne in the range of 364 nm to 411 nm (Fig. S7, ESI†). Furthermore, electron microscopy results have demonstrated that copper clusters are firmly confined within the graphdiyne substrates with the distance close enough for the energy transfer process. Therefore, we considered that when excited at the wavelength of 280 nm, fluorescence energy emitted by Cu₅₅ NCs as the donor was absorbed by graphdiyne as an acceptor (364–411 nm), resulting in the FRET-induced emission of white fluorescence.

To verify the stability of GDY–Cu₅₅ NCs, the GDY–Cu₅₅ NC colloidal suspension was stored for long-term stability characterization. PL spectra of GDY–Cu₅₅ NCs tested at 21 and 54 days after being dispersed in THF are shown in Fig. 7(a). PL spectra showed no significant difference between freshly prepared GDY–Cu₅₅ NCs and those after 21 or 54 days. The results indicated that the graphdiyne-confined method effectively protected copper nanoclusters from oxidation and therefore achieved high photostability and long-term fluorescence emission ability. Fluorescence lifetime and quantum yield of the GDY–Cu₅₅ NCs were further tested. As shown in Fig. 7(b), GDY–Cu₅₅ NCs exhibited a fluorescence lifetime of 6.967 ns, while the quantum yield was 6.74% (Fig. S8, ESI†). The relatively low fluorescence lifetime and quantum yield indicated that GDY–Cu₅₅ NCs possessed rapid non-radiative relaxation characteristics, which reduced not only the lifetime of the excited state but also decreased the chemical reactivity of GDY–Cu₅₅ NCs in the excited state and contributed to the improved photostability of this material.

Fig. 7 (a) Photoluminescence (PL) spectrum of the GDY–Cu₅₅ NCs after 0, 21, and 54 days in THF; (b) PL decay profiles of the GDY–Cu₅₅ NCs in THF.

Conclusions

In summary, we designed a novel strategy that leveraged the multifunctional molecular attributes of graphdiyne to achieve confined size-controllable, uniformly distributed, and optically stable white-light-emitting copper nanoclusters with graphdiyne (GDY–Cu₅₅ NCs). The poor intrinsic photoluminescence properties of graphdiyne are improved by Cu₅₅ NCs. For the first time, we have discovered that the GDY–Cu₅₅ NCs exhibited an exceptionally broad emission band (320–700 nm) and bright white fluorescence. Optical studies indicated that the PL property of white GDY–Cu₅₅ NCs could be assigned to the fluorescence resonance energy transfer (FRET) mechanism. Such a mechanism was proved by the results of spectroscopic and microscopic characterization studies: the ultraviolet absorption spectrum of GDY and the photoluminescence spectrum of GDY–Cu₅₅ NCs showed partial overlap, while the distance between Cu₅₅ NCs and graphdiyne substrate was sufficiently small because copper clusters are domain-limited to grow in graphydiyne. This research opens new horizons for the application of graphdiyne in the field of optics and develops a novel approach for the preparation of white-light-emitting copper nanocluster composites.

Author contributions

Hongye Liu: formal analysis, investigation, writing – original draft, methodology, software, validation, visualization; Zecheng Xiong: software, validation; Hao Sun: software, validation, visualization; Wei Su: validation; Yang Huang: validation; Weiyue Jin: validation; Huibiao Liu: conceptualization, funding acquisition, methodology, project administration, resources, supervision, writing – review & editing.

Data availability

All data that support this study and its findings are available within the article and its ESI.† The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China [grant no. 22071251]; the Key Research Program of the Chinese Academy of Sciences [grant no. XD B0520201] and the National Key Research and Development Project of China [2022YFA1204500 and 2022YFA1204501].

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5qm00056d

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