Graphene Quantum Dots-Driven Multiform Morphologies of β-NaYF4:Gd3+/Tb3+ Phosphors: The Underlying Mechanism and Their Optical Properties.

Dimension and shape tunable architectures of inorganic crystals are of extreme interest because of morphology-dependent modulation of the properties of the materials. Herein, for the first time, we present a novel impurity-driven strategy where we studied the influence of in situ incorporation of graphene quantum dots (GQDs) on the growth of β-NaYF4:Gd3+/Tb3+ phosphor crystals via a hydrothermal route. The GQDs function as a nucleation site and by changing the concentration of GQDs, the morphology of β-NaYF4:Gd3+/Tb3+ phosphors was changed from rod to flowerlike structure to disklike structure, without phase transformation. The influence of size and functionalization of GQDs on the size and shape of phosphor crystals were also systematically studied and discussed. Plausible mechanisms of formation of multiform morphologies are proposed based on the heterogeneous nucleation and growth. Most interestingly, the experimental results indicate that the photoluminescence properties of β-NaYF4:Gd3+/Tb3+ phosphor crystals are strongly dependent on the crystallite size and morphology. This study would be suggestive for the precisely controlled growth of inorganic crystals; consequently, it will open new avenues and thus may possess potential applications in the field of materials and biological sciences.

Introduction

Rare earth ion-doped phosphors possess potential applications such as solid-state lasers;[1] high-resolution display devices, security and brand protection, photodynamic therapy;[2] fluorescent labels for the detection of molecules;[3] in vivo imaging;[4,5] medical diagnostics, fingerprint detection, and sensors;[6,7] light-emitting devices;[8] solar cells;[9,10] and so forth which are better substitutes of conventional fluorophores and organic dyes.[11] Although the molar absorption coefficient of organic dyes and quantum dots is comparatively higher than that of the rare earth phosphors, high background noise, high blinking probability, poor photostability, and potential long-term toxicity hamper the use of these materials.[12,13] In contrast, the fascinating luminescence features of lanthanide ions arising from intra 4f transitions such as narrow bandwidth, long-lived emission, large Stokes and antiStokes shift, less blinking, photostability, and low autofluorescence offer excellent prospects for designing new luminescent materials with enhanced properties.[14,15] Among the phosphors, β-NaYF4 has been demonstrated as the most efficient host for upconversion (UC) and downconversion (DC) emissions because of its low phonon energy (∼350 cm–1), high chemical stability, good optical transparency, high radiative emission rates, and low nonradiative decay rates.[16]

In recent years, the design and the synthesis of inorganic nano-/microcrystals with well-defined morphologies and accurately tunable sizes remain the research focus as the precise control over shape and size allows manipulation of the physical, chemical, and biological properties of the nanocrystals as desired. Therefore, it is essential to develop efficient methods to fabricate multiform inorganic crystals to enhance their performance in existing applications. In the last decades, various efforts have been dedicated to synthesize the rare earth ion-doped phosphors in uniform but different sizes and shapes.[17] In the kinetic control of the growth process of crystals, various external factors, such as concentration and types of precursors, reaction time/temperature/pressure, types of solvents, pH of precursor solution, and organic additives, drastically influence the shape of the crystals.[18] All of these factors have been studied quite intensively. A large number of organic additives and shape-directing agents such as oleic acid,[19] polyethylene glycol,[5,20] trisodium citrate,[21] cetyltrimethylammonium bromide,[22] dodecyltrimethylammonium bromide,[23] ethylenediaminetetraacetic acid,[24] poly(4-styrenesulfonate),[25] and so forth are used to control the particle size and tune the crystallinity and morphology of the anisotropic crystals. These agents selectively bind to the different facets of the crystals and change their surface energy and chemical potential, which affects the orientation growth rate of different crystal facets, resulting in the formation of different architectures under various environments.[17] Adjusting the molar ratio of a complexing agent/rare earth ion can also tune the crystal size and shape.[26] Commercially available ligands such as polyvinylpyrrolidine and polyethylenimine (PEI) are used to control the particle growth of the nanocrystals and endow them with surface functionality for further modification.[27] Various impurity dopant ions such as K+, Li+, Zn2+, Sc3+, and Gd3+ have been used intensively to study the change in phase and morphology. In a study by Dou and Zhang in 2011, Li+-ion doping in NaYF4:Yb3+/Er3+ nanocrystals showed morphology change from nanorods to nanospheres along with the change in phase from hexagonal to cubic, when the doping percentage increased till 60%. While when K+ ions were doped in the NaYF4:Yb3+/Er3+ nanocrystals, morphology changed from elliptical shape to hexagonal prism and finally to nanorods as the K+ content was increased in the system.[28] Similar to other studies, a mechanism was proposed for the influence of the molar ratio of K+ to Y3+ on the anisotropic growth and morphology evolution of β-NaYF4 nanocrsytals.[29] In the same way, Li+ ions were incorporated in the KSc2F7:Yb3+/Er3+ nanocrystals, which substituted K+ or Sc3+ ions at lower doping concentration, whereas some of the Li+ ions went into the interstitial site of the crystal lattice at higher doping concentration. An increase in size of the nanocrystals with 60 mol % of Li+-ion doping was observed.[30] In 2013, collective effect of different alkali metals onto the formation of multiform morphologies of rare earth fluorides was also studied by Xue and co-workers.[31] While Chen et al. studied the influence of Gd3+ ions on the phase transformation of NaYF4 crystals from cubic to hexagonal phase and also observed decrease in luminescence intensity because of the decrease in the size of the nanorods with higher Gd3+ doping.[32] Hitherto, numerous methods have been reported to fabricate the anisotropic nanoparticles such as rods, plates, prisms, tubes, spheres, disks, and spindles, and so forth. These methods include solvothermal, coprecipitation, thermal decomposition, combustion, and sol–gel process, among which hydro-/solvothermal method is considered a relatively environment-friendly method which provides high crystallinity and monodispersity to the material with diverse controllable morphologies and architectures.[11] Apart from the homogeneous nucleation, the crystal growth strategies based on the heterogeneous nucleation are quite popular. The choice of substrates provides an ideal nucleation site by lowering the activation energy barrier for the phase to form. In fact, the heterogeneous nucleation makes it possible for some metastable crystalline states to form well below the supersaturation limit of the monomer concentration.

In this article, we have investigated the role of graphitic carbon materials with varying surface functionality, size, and shape as a nucleation site for the growth of NaYF4 nanocrystals. The rationale behind using these materials was due to their versatile application as the co-components in optically active hybrid materials as well as their role in defining the crystalline and optical properties of the crystals. Previously, our group reported the in situ insertion of graphene quantum dots (GQDs) during the growth of TiO2 particles. Because of the presence of GQDs, irregular structures of the TiO2–GQD hybrid were formed instead of regular rodlike structure TiO2 particles.[33] The carbon-based nanomaterials such as graphene, carbon nanotubes (CNTs), GQDs, and so forth have attracted significant attention owing to their advantageous properties such as high surface area, low density, minimal cost, environment friendliness, and high thermal stability. Carbon-based materials possess excellent mechanical strength, electrical and thermal conductivity, and optical properties.[34] The GQDs have grabbed more attention because of their chemical inertness, solubility, tunable photoluminescence (PL), long-term photobleaching resistance, biocompatibility, and low cytotoxicity.[35] The GQDs have carbon with sp2 hybridization having one or few layers of graphene sheets with lateral dimensions smaller than 100 nm[36] and oxygen-containing functional groups on their surfaces and at their edges. The GQDs are called zero-dimensional graphene materials. The quantum confinement of electrons has been demonstrated in GQDs and led to the practical applications in bioimaging, lasing, photovoltaics, and light-emitting diodes.[37]

Composites of carbon-based materials have become important for various applications because of their unique physical and chemical properties. Various composites with GQDs have been synthesized such as GQD with polyaniline composite films, which have applications in photonic devices.[38] The GQDs have also been used as a co-sensitizer in the hybrid dye-sensitized solar-cell architectures.[39,40] In other studies, the amine-functionalized GQDs (NH2-GQDs) were incorporated into the flexible and transparent clay host and cellulose nanofibers (CNFs). The resultant GQD@CNF–clay films exhibited PL; therefore, they were used as a material for blue light-emitting diodes to achieve white light emission.[41]

The development of multifunctional hybrid materials of lanthanide ion-doped phosphors is gearing tremendous interest for a broad range of potential applications in biological and material sciences because of their unique tunable electronic and magnetic properties.[42] It is worth mentioning that a very few reports of the composites of carbon-based materials and lanthanide ion-doped phosphors are known till date for their sensing or photocatalytic applications. New-generation nanocomposites of NaYF4:Yb3+/Er3+ and graphene oxide (GO) were fabricated with superior optical limiting performance,[43] followed by their mechanistic studies in the same year.[44] Composites of reduced GO and NaYF4:Yb3+/Er3+ were proposed to achieve good electrical conductivity, which consequently increased the efficiency of the solar cell by 10%.[45] Fabrication of optical pH sensors based on the flexible and biocompatible freestanding optical hybrid film, which is composed of GO and NaYF4:Yb3+/Er3+ nanoparticles (UCNPs), is reported. In this work, high surface area, mechanical stability, and luminescence quenching capability of GO are utilized as a sensing platform for pH sensing.[46] In another work, TiO2–NaYF4:Yb3+/Er3+–graphene composite photoanode was prepared, which improved the solar cell efficiency by increasing the interfacial electron transport of fluorine-doped tin oxide/TiO2.[47] For biomedical applications, multifunctional multiwalled CNT (MWCNT)–NaGdF4:Yb3+/Er3+/Eu3+ hybrid nanocomposite was developed for simultaneous magnetic and optical imaging by NaGdF4:Yb3+/Er3+/Eu3+ NP and photothermal conversion property from MWCNT.[48] Recently, a sensor based on GQDs and ssDNA-UCNP@SiO2 is developed for the detection of microRNA sequence.[49] Although the composites of Ln3+-doped phosphors and carbon-based nanomaterials have been explored, the in situ incorporation of carbon-based nanomaterials into the Ln3+-doped phosphors is not studied yet.

Herein, we report a novel strategy to incorporate carbon-based nanomaterials; for example, we used GQDs as a foreign impurity into the β-NaYF4:Gd3+/Tb3+ phosphor and studied their effect on the crystal phase, morphology, and optical properties of β-NaYF4:Gd3+/Tb3+. Interestingly, we observed that the in situ incorporation of GQDs into the β-NaYF4:Gd3+/Tb3+ phosphors has significantly influenced the morphology and consequently the optical properties of β-NaYF4:Gd3+/Tb3+ crystals. We have studied the effect of the concentration of GQDs, the type of surface chemical functionalization, and CNTs on the morphology and PL properties of the β-NaYF4:Gd3+/Tb3+ phosphors. To the best of our knowledge, for the first time, we are reporting the morphology tuning of inorganic crystals via incorporation of GQDs. This work has led to new opportunities of extending the use of this system in the area of architectural manipulation.

Results and Discussion

Intentional incorporation of foreign elements into the hosts has a significant influence on the nucleation and growth of nanocrystals (heterogeneous nucleation) by lowering the thermodynamic activation energy barrier for the nucleation events to take place, which is influenced by the several microscopic properties such as the crystal structure of foreign elements and the precipitate, lattice mismatch between two species, surface roughness, surface wetting properties, and so forth from microscopic to the atomistic details. Thus, it provides a unique approach to modify the crystallographic phase, size, morphology, and consequently the optical properties of nanomaterials. Here, we have incorporated varying concentrations of carboxylic and amine co-functionalized GQDs (mentioned as GQDs) in β-NaYF4:Gd3+/Tb3+ phosphor crystals in the reaction vessel during the synthesis itself. To investigate the effect of GQDs and other carbon-based materials on the morphology of β-NaYF4:Gd3+/Tb3+ phosphor crystals, we first characterized the as-prepared GQDs and then studied the structural, morphological, and optical properties of GQD-incorporated β-NaYF4:Gd3+/Tb3+ phosphor crystals.

Characterization of GQDs

The size and morphology of the as-prepared GQDs were characterized by transmission electron microscopy (TEM). Figure a shows the TEM images of the GQDs having size in the range of 3–5 nm. The concentration of the GQDs was calculated from the thermogravimetric analysis (TGA) at a heating rate of 10 °C/min under nitrogen atmosphere (Figure b), which was found to be 10 mg/mL. The UV–visible absorbance (Figure a) showed the presence of broad absorbance in the UV range with a knee around ∼230 nm because of π–π* transitions of sp2 C=C bonds present in the GQDs, which is in good agreement with the previous studies.[50] The inset of Figure a shows the photograph of the as-prepared GQDs aqueous solution taken under 365 nm UV light exposure. Detailed PL studies were carried out at different excitation wavelengths ranging from 250 to 550 nm. It was found that the PL intensity first increased till 330 nm and then decreased remarkably. The fluorescence emission peak of GQDs red-shifted from 440 to 570 nm when the excitation wavelength increased from 250 to 550 nm. The highest intensity of the PL was observed for the GQDs at 445 nm when excited at 330 nm. Raman spectroscopy was performed at an excitation wavelength of 633 nm. Figure b shows the Raman spectrum where two well-resolved peaks corresponding to D and G bands at ∼1340 and ∼1600 cm–1, respectively, were observed.

Figure 1

(a) TEM image of as-prepared GQDs showing a particle size of 3–5 nm and (b) TGA curve showing the weight fraction of GQDs under inert atmosphere.

Figure 2

(a) UV–visible absorbance spectra showing a broad UV absorption with a small knee at 230 nm and PL spectra at different excitation wavelengths from 250 to 550 nm while the inset shows the photograph of the GQD aqueous suspension taken under 365 nm UV light exposure and (b) Raman spectra of the as-prepared GQDs using 633 nm laser as a source.

The G band corresponds to the E2g phonon at the Brillouin zone center, whereas the disordered induced D band corresponds to the transverse optical phonons around the K-point of the Brillouin zone. The D band requires a defect for the momentum conservation. The intensity ratio ID/IG of D to G band is greater than 1, suggesting that the as-prepared GQDs have defects because of the dominant contributions from the edge states at the periphery. The 2D band is a second-order Raman process, which originates from in-plane breathing-like modes of the carbon rings.[51] The broadening of the 2D band at ∼2800 cm–1 may be due to the relaxation of the double-resonance Raman selection rules that are associated with the random orientation of GQDs with respect to each other.[52] The broadness of this peak increases with increase in the defect states in the system.

Structural and Morphological Investigations of β-NaYF4:Gd3+/Tb3+-xGQDs

The composition, crystallinity, and phase purity of the β-NaYF4:Gd3+/Tb3+-xGQD phosphors were first examined by X-ray diffraction (XRD). Figure shows the XRD patterns of the as-synthesized PEI-capped NaYF4:Gd3+/Tb3+, incorporated with different concentrations of GQDs (x = 0, 1, 3, 5, and 7 mL). The relative intensity of the peaks is changed compared with that of standard data, suggesting the probable anisotropic growth behavior of the particles. The rationale behind using PEI in the synthesis of NaYF4 is not only to provide monodispersity to the particles but also to prevent particles from agglomeration. Here, we used GQDs, in which, as referred above, 1 mL of suspension is equal to 10 mg of GQDs. The sharp diffraction peaks in all the samples can be indexed to the pure hexagonal phase β-NaYF4 (space group: P63/m) with calculated lattice parameters a = 5.9 Å and c = 3.5 Å, which are in good agreement with the reported data (JCPDS 16-0334). The absence of any other peak in XRD patterns indicates the high purity of as-prepared samples, implying that no secondary phase is formed. The pure hexagonal phase of all the samples reveals that the incorporation of GQDs at all concentrations does not induce the phase transformation in the β-NaYF4 crystal structure. It is worth mentioning that on comparing the peak intensities of the prepared samples, we found that there is a difference in the relative intensities based on (100), (101), (110), (002), and (201) peaks, indicating the existence of different preferential orientation growths at different GQD concentrations. The size, shape, and structure of the as-prepared samples were characterized by TEM and field-emission scanning electron microscopy (FESEM) (Figures and 5). The TEM images, as shown in Figure , show that the as-prepared PEI-capped β-NaYF4:Gd3+/Tb3+ crystals possess a rod-shaped structure with an average length of ∼280 nm and a diameter of ∼96 nm. It can be seen that the β-NaYF4:Gd3+/Tb3+-xGQD (x = 0) rods are highly uniform and monodispersed in nature. Interestingly, upon in situ incorporation of the GQDs into the PEI-capped β-NaYF4:Gd3+/Tb3+ phosphors during the synthesis, the variation in morphology was observed as a function of different concentrations of GQDs. As it can be seen from Figure b,c, at the lower concentrations of GQDs, that is, at x = 1 and x = 3, hexagonal prismatic structures were formed with average lengths/diameters of ∼225 nm/∼130 nm and ∼280 nm/∼250 nm, respectively. At x = 5, flower-shaped crystals were observed with a length and a diameter of ∼325 and ∼300 nm, respectively (Figure d). Whereas the crystallites observed at x = 7 are composed of disklike structures having an average length and a diameter of ∼260 and ∼620 nm, as shown in Figure e.

Figure 3

Comparison of XRD patterns of β-NaYF4:Gd3+/Tb3+ and β-NaYF4:Gd3+/Tb3+-xGQD phosphor crystals where x = (a) 0, (b) 1, (c) 3, (d) 5, and (e) 7 mL. The standard data of β-NaYF4 (JCPDS 16-0334) are also compared. Here, 1 mL of GQDs equals 10 mg by weight as calculated by the TGA curve. The incorporation of GQDs does not induce any phase transformation in the β-NaYF4 crystal structure.

Figure 4

TEM images of GQDs-incorporated β-NaYF4:Gd3+/Tb3+ particles show the modification of nucleation and growth of these particles when the concentration of GQDs was varied from x = 0 to 7 mL (a–e). The figures (f–j) show their respective SAED patterns. All samples were hydrothermally treated at 180 °C for 24 h.

Figure 5

FESEM images show the change in the morphology of β-NaYF4:Gd3+/Tb3+ after the incorporation of GQDs where the concentration of GQDs was varied from x = 0 to 7 mL (a–e). With increase in the concentration of GQDs, the aspect ratio (length/width) of the β-NaYF4:Gd3+/Tb3+ phosphors decreased. NaYF4 rods were formed at 0 mL of GQDs with an aspect ratio of 2.9 and disklike structures with an aspect ratio of 0.4 at 7 mL of GQDs.

These results strongly suggest that the morphology of the β-NaYF4:Gd3+/Tb3+-xGQD crystals (x = 0, 1, 3, 5, 7 mL) exhibited striking dependence on the different concentration of the GQDs, where the shape changed from rods (x = 0) to disklike structures (x = 7). The corresponding selected area electron diffraction patterns (SAEDs) and high-resolution TEM images shown in Figure f–j and insets, respectively, demonstrate that the as-synthesized β-NaYF4:Gd3+/Tb3+-xGQD crystals are highly crystalline in nature. Meanwhile, the lattice fringes with an interplanar spacing of respective planes of β-NaYF4:Gd3+/Tb3+-xGQD crystals are ascribed. The detailed observations with their proper facets at various concentrations of GQDs in the as-prepared samples can be clearly seen by the FESEM images (Figure ). The FESEM images laid more evidence that the incorporation of the GQDs had huge effect on the morphology of the as-prepared crystals.

It should be noted that the pH of the solution has not changed because of the addition of GQDs in the precursor solution. Therefore, the effect of pH has not influenced any changes of the product. Hence, these results clearly indicate that the size and morphology control in these experiments is closely related to the incorporation of GQDs because the concentration of GQDs was the only parameter changed in the system. The plausible mechanism for the modulation in the morphology of β-NaYF4:Gd3+/Tb3+ phosphors because of the incorporation of GQDs is discussed in the next section.

Growth Mechanism of GQDs-Incorporated β-NaYF4:Gd3+/Tb3+ Multiform Morphologies

For the better understanding of the formation processes of multiform morphologies of β-NaYF4:Gd3+/Tb3+ phosphor crystals via incorporation of GQDs during the synthesis, reaction samples were carefully investigated by taking the different concentrations of as-prepared GQDs. Here, we believe that GQD particles of nearly 5 nm serve as the nucleation sites for the nucleation and further growth of the crystals. Because the precursors used in the reaction are ionic and GQDs contain functional groups at the edges and on the surface, lanthanide ions interact with functional group specially −COOH and −OH groups and form a metal–GQD complex. Then, nuclei are formed upon the addition of a fluoride source. Interaction of GQDs with the precursor ions plays an important role, where the charge or the functional groups such as carboxylic acid, amine, hydroxyl, etc. present in the GQDs have a significant role in binding to the lanthanide ions. As the seeds grow, the GQDs bind to the particular surface of the phosphor seeds, governing the shape and size of the particles. With an increase in the GQDs concentration, their probability to bind to the surface increases which increases the growth of the crystals along the (100) plane, thereby giving rise to the lowest aspect ratio of the rods. Consequently, with a larger concentration of GQDs, disklike structures were formed having the lowest aspect ratio among all. We have formulated the above results on the same hypothesis as given by Yang et al. for the diethylene glycol molecule for the growth of KGdF4 nanocrystals.[53] Here, GQDs may have different functions—first, it can form a metal–GQD complex by which the growth rate of the nanoparticles is slowed down, thereby decreasing the crystallite size. This is similar to the mechanism reported by Zeng’s group, where a metal–oleic acid complex was formed, when oleic acid was used as a ligand for the preparation of NaREF4 nanocrystals.[54] Second, this may also slow down the diffusing rate of the cations and anions which may reduce the collisions’ probability of the ions in the system. Third, it can also cap the external surfaces of the nanoparticles, giving the nanoparticles a directional growth. By virtue of this, size control and shape evolution of the nanoparticles can be easily realized by changing the GQDs’ amount which governs the growth of the crystal in a particular direction.

Further, growth directional analysis was done by measuring the XRD intensity. In general, facets perpendicular to the fast direction of growth have smaller surface area; therefore, slow growing face will dominate the morphology having large surface area and thus will be more exposed to the environment. Hence, the more exposed plane will show higher intensity in XRD, which has slower growth. In Figure , variation in the normalized XRD intensity of different planes is plotted with varied concentration of GQDs used in the system. It can be seen that the intensity of the plane (100) decreases with higher concentration of co-functionalized GQDs, suggesting that the plane is less exposed and growth takes place along this direction, giving rise to disklike structures at 7 mL of GQDs. Similarly, when compared with other side planes in hexagonal structures (110) and (101), normalized intensity decreases, indicating the same. While in the case of the (002) plane (Figure d), the intensity increases with an increase in the concentration of GQDs, thus implying that growth direction is perpendicular to this plane.

Figure 6

Variation of normalized XRD intensity of different planes (a) 100, (b) 110, (c) 101, and (d) 002 in β-NaYF4:Gd3+/Tb3+ with the change in concentration of GQDs. Respective direction of planes is shown in the anisotropic hexagonal crystal structure.

Thence, it can be concluded that GQDs have the function of inhibiting the longitudinal growth along the ⟨0001⟩ direction with a relative enhancement of the growth along the ⟨10–10⟩ direction in the form of hexagonal prism first and then disklike structures. Schematic showing the growth direction and proposed mechanism has been depicted in Schemes and 2, respectively.

Scheme 1

Schematic Illustration of the Directional Growth of Anisotropic Structure of β-NaYF4:Gd3+/Tb3+-xGQDs Shown with Different Planes with the Change in Concentration of GQDs Where x Is Varied as x = 1 mL, x = 3 mL, x = 5 mL, and x = 7 mL

Scheme 2

Schematic Illustration of Heterogeneous Nucleation and Growth Process and the Effect of GQD Incorporation on the Morphology of Final β-NaYF4:Gd3+/Tb3+ Phosphors

Here, the GQDs provide nucleation sites for the heterogeneous nucleation as well as act as a capping agent and manipulate the growth of β-NaYF4:Gd3+/Tb3+ crystals.

To validate the influence of the GQDs on the resultant morphology, we carried out the controlled experiments by changing the dimension of foreign impurity and the functionality of the as-prepared GQDs.

Effect of Size of the Foreign Particles Introduced in the Reaction (GO vs CNT)

To study the effect of size of foreign particles incorporated during the synthesis of β-NaYF4:Gd3+/Tb3+ crystals and to examine whether the change in the morphology of β-NaYF4:Gd3+/Tb3+ phosphors is due to the incorporation of GQDs, a control experiment was performed. In three separate reactions of β-NaYF4:Gd3+/Tb3+ phosphors, GO, MWCNTs and carboxylic- functionalized MWCNTs (COOH-MWCNT) were incorporated. The synthesis procedure is similar to the preparation of β-NaYF4:Gd3+/Tb3+-xGQDs, except that GO, MWCNTs and COOH-MWCNT were incorporated instead of GQDs. The as-obtained products were named as β-NaYF4:Gd3+/Tb3+-xGO and β-NaYF4:Gd3+/Tb3+-xMWCNT and β-NaYF4:Gd3+/Tb3+-xCOOH-MWCNT. The composition and phase purity of the products were examined by XRD as shown in Figure S1 given in the Supporting Information, where the diffraction peaks of the samples can be indexed to the pure hexagonal phase of NaYF4 (JCPDS 16-0334). As represented in Figure , it was observed that due to the incorporation of GO sheets and MWCNT, the morphology of the β-NaYF4:Gd3+/Tb3+ phosphor crystals remained unchanged. These results infer that the size of the foreign particles introduced in the reaction plays a role in the morphology tuning of β-NaYF4:Gd3+/Tb3+ phosphors where only small GQD particles can tune the morphology, whereas large carbon materials: sheet or tubelike structures such as GO and MWCNT cannot influence and alter the shape of β-NaYF4:Gd3+/Tb3+ phosphors.

Figure 7

SEM images show the effect of different concentrations of GO (a–c), MWCNT (d–f), and COOH-MWCNT (g–i) on the growth morphology of β-NaYF4:Gd3+/Tb3+ phosphors. We did not observe any change in the morphology of β-NaYF4:Gd3+/Tb3+ phosphors because of the incorporation of these materials as foreign impurities.

Effect of Functionalization of GQDs on the β-NaYF4:Gd3+/Tb3+ Phosphor Crystal Growth

Furthermore, to analyze the effect of the functionalization of GQDs on the morphology of β-NaYF4:Gd3+/Tb3+ phosphors, we synthesized carboxylic-functionalized (COOH-GQDs) and amine-functionalized (NH2-GQDs) along with reduced GQDs (rGQDs). COOH-GQDs, NH2-GQDs, and rGQDs were characterized by TEM to see the particle size and UV–vis to see their absorbance. Figure compares the TEM images of COOH-GQDs, NH2-GQDs, and rGQDs, showing a relatively narrow size distribution (∼3–5 nm). To explore the optical properties, UV–vis spectra were recorded, showing the main absorbance peak at around 230 nm which is attributed to the π–π* transitions of sp2 C=C in as-synthesized COOH-GQDs, NH2-GQDs, and rGQDs, whereas a peak at around ∼300 nm is attributed to the n−π* transitions of C=O in the case of rGQDs.[55]

Figure 8

TEM images of (a) COOH- GQDs, (b) NH2- GQDs, and (c) rGQDs showing the average particle size in the range of 3–5 nm. In the case of rGQDs, particles are aggregated because of the less surface charge after the reduction reaction. Their respective UV–visible absorbance spectra are also shown below, showing the main absorbance peak at around 230 nm in as-synthesized COOH-GQDs, NH2-GQDs, and rGQDs, whereas a peak was observed at around ∼300 nm in rGQD alone.

The measured zeta potentials of COOH-GQDs, NH2-GQDs, and rGQDs were −32.8, −15.8, and −20.5 mV, respectively. The positive shift of the zeta potential for the NH2-GQDs and rGQDs indirectly indicates the introduction of amine groups because the introduced amine groups can counteract part of the electronegative effect and reduction in hydroxyl groups, respectively.[56] The concentration of the functionalized GQDs was calculated from the TGA at a heating rate of 10 °C/min under nitrogen atmosphere (Figure S2), which was found to be 10 mg/mL for all the types of GQDs. Furthermore, we performed the synthesis of β-NaYF4:Gd3+/Tb3+ by incorporating COOH-GQDs and NH2-GQDs in the reaction vessel. The phase purity and crystallinity of the as-prepared samples were monitored by XRD and compared in Figure S3A, where the diffraction peaks matched very well with the standard JCPDS data of β-NaYF4. No traces of impurity peaks were observed, indicating that all the samples crystallized in a single phase of β-NaYF4. It can be seen in Figure that functionalization has a notable effect on the morphology and size of the resultant products. Microrods with cracked ends were formed using COOH-GQDs having an average length of ∼15 μm at x = 5 mL, whereas the average size of the microrods increased to ∼30 μm when the concentration of COOH-GQDs increased further to 7 mL. In a study in 2006, during the growth of NaYF4 crystals, oleic acid selectively bound to the surface which was parallel to the c-axis, rendering the growth along the (001) direction and hence resulting in the formation of nanorods.[57] Supporting the above mechanism, in another study, it was concluded that the anionic form of oleic acid (OA–) preferentially binds to the RE3+ ions exposed more on the six symmetric facets of the hexagonal fluoride crystal, resulting in the formation of rods.[19] Applying the same hypothesis, we can conclude that the functional groups such as carboxylic and hydroxyl may bind to the side planes of the hexagonal prismatic structures where lanthanide ions are more exposed, giving rise to the formation of microrods; therefore, the preferential growth direction is along ⟨0001⟩. Additionally, growth direction was confirmed by normalized XRD intensity. As shown in Figure a, the intensity of different planes in β-NaYF4 changes when we increase the concentration of COOH-GQDs. The increment in the intensity of the (100), (110), and (101) planes suggested that the growth direction was perpendicular to these planes, thus rendering the growth along the (001) plane. Further, we have studied the effect of NH2-GQDs on the growth process of β-NaYF4. The crystallinity of the β-NaYF4:Gd3+/Tb3+-xNH2-GQDs was examined by XRD. The diffraction peaks in the XRD pattern shown in Figure S3B can be indexed to pure β-NaYF4.

Figure 9

FESEM images of β-NaYF4:Gd3+/Tb3+-xCOOH-GQDs (a–c) and β-NaYF4:Gd3+/Tb3+-xNH2-GQD (d–f) phosphors where the concentration of respective GQDs was varied from x = 1 mL, x = 3 mL, and x = 7 mL. Microrods were formed when COOH-GQDs were incorporated, while there was no change observed in the case of the incorporation of NH2-GQDs during the synthesis of β-NaYF4:Gd3+/Tb3+.

Figure 10

Variation of normalized XRD intensity of different planes in β-NaYF4:Gd3+/Tb3+ with the change in the concentration of (a) COOH-GQDs and (b) NH2-GQDs.

The presence of NH2-GQDs has no obvious impact on the morphology of the crystals, which can be seen in the FESEM images (Figure d–f). Moreover, the XRD intensity of different planes as shown in Figure b has not changed with the concentration of NH2-GQDs, confirming the unaltered morphology of β-NaYF4.

To study the effect of non-functionalized GQDs, we have used rGQDs during the synthesis of NaYF4. Oxygen-rich functional groups such as carboxyl, hydroxyl, and epoxy groups are introduced to the edges and onto the basal plane during the top-down synthesis,[58,59] which can be reduced using some of the reducing agents such as NaBH4, hydrazine hydrate, and so forth. Here, we have reduced COOH-GQDs with NaBH4 which reduces hydroxyl and epoxy groups on the basal plane of the GQDs while carboxyl moieties remain unreduced at the edges. As observed in the XRD pattern shown in Figure S3C, some of the diffraction peaks belong to the cubic phase of NaYF4 which matched very well with the standard face-centered cubic structure data (JCPDS 77-2042), whereas other peaks can be indexed to the pure hexagonal phase of NaYF4 (JCPDS 16-0334). We can see in Figure that similar microrods were formed when we performed the reaction with rGQDs having few carboxylic acid groups at the edges. Here, the proposed mechanism for the formation of microrods may be the same as that in the case of COOH-GQDs. Apart from microrods, small spherical particles were seen, which have been assigned as the cubic phase as indexed in the XRD pattern. Careful investigation revealed that the microrods were composed of solid interiors with small quasi-spheres attached to them. Together with the XRD pattern, it can be judged from SEM images that the microrods were hexagonal phase and small quasi-spherical nanoparticles were cubic. Although the formation of cubic phase in the system is unclear at present, we can provide the following explanation for the existence of cubic phase. Because the rGQDs contain less number of functional groups, it would lead to the availability of more number of lanthanide ions, which would provide fast nucleation and growth of NaYF4 crystals. However, there were no enough monomers to supply the growth of small particles under the same circumstances. Thus, small nanoparticles with cubic phase as well as microrods with hexagonal phase coexisted in the final products with rGQDs. The schematic and table showing corresponding morphologies obtained at different concentrations of GQDs, different functionalized GQDs, and different foreign impurities are represented in Scheme , and details of aspect ratios are summarized in Table .

Figure 11

SEM images of β-NaYF4:Gd3+/Tb3+ phosphors with varied concentration of rGQDs. Microrods were formed along with some small quasi-spherical particles with the in situ incorporation of rGQDs during the synthesis.

Scheme 3

Schematic Illustration of the Effect of Other Carbon-Based Materials and Differently Functionalized GQDs on the Morphology of NaYF4:Gd3+/Tb3+ Phosphors

Only the incorporation of COOH-GQDs and rGQDs has made a significant change in the morphology of the β-NaYF4:Gd3+/Tb3+ phosphor crystals.

Table 1

Summary of the Effect of GQDs and Other Carbon-Based Materials on the Morphology and Aspect Ratio of the Final β-NaYF4:Gd3+/Tb3+ Phosphors Where x Is Varied as x = 1 mL, x = 3 mL, and x = 7 mL in the Respective System

S. no	sample	concentration (mL)	morphology	aspect ratio (L/W)
1	β-NaYF4:Gd3+/Tb3+-xGQD	0	rods	2.9
		1	hexagonal prism	1.7
		3	hexagonal prism	1.2
		5	flowerlike structure	1.1
		7	disklike structure	0.4
2	β-NaYF4:Gd3+/Tb3+-xGO	1	rods	3.1
		3	rods	2.6
		7	rods	2.1
3	β-NaYF4:Gd3+/Tb3+-xMWCNT	1	rods	2.9
		3	rods	2.7
		7	rods	2.8
4	β-NaYF4:Gd3+/Tb3+-xCOOH-MWCNT	1	rods	2.4
		3	rods	2.3
		7	rods	2.3
5	β-NaYF4:Gd3+/Tb3+-xCOOH-GQDs	1	microrods	4.1
		3	microrods	9.8
		7	microrods	23.3
6	β-NaYF4:Gd3+/Tb3+-xNH2-GQDs	1	rods	3.1
		3	rods	2.8
		7	rods	2.4
7	β-NaYF4:Gd3+/Tb3+-xrGQDs	3	microrods	15.1
		7	microrods	20.5

PL Studies

Owing to the non-existence of d-electrons in Y3+ ([Kr] 4d0) in undoped NaYF4, the probability of emission is negligible in the host matrix. Figure compares the PL emission spectra of β-NaYF4:Gd3+/Tb3+ and β-NaYF4:Gd3+/Tb3+-xGQD (x = 1, 3, 5, and 7 mL) phosphor crystals. The luminescence of lanthanide ions mainly originates from the electron transition within 4f electronic configuration [Xe 4f; N = 0–14], which consists of complex energy levels because of Coulombic repulsion and spin–orbit coupling. The shielding of the 4f electrons of Ln3+ by the filled 5s2 and 5p6 subshells results in weak influence of external environment, which is responsible for their sharp and narrow emission spectra. Thus, the emissive electronic transitions are a characteristic feature of lanthanide dopant ions, and so, the incorporation of GQDs does not render any peak shift in the transition emission of Tb3+ ions and bands differ only in their relative intensities. Here, Gd3+ ion is used as a sensitizer to enhance the luminescence of Tb3+ ions. As Gd3+ ions exhibit strong absorption band at 273 nm because of its 8S7/2 → 6I11/2 transition, Gd3+ and Ln3+ together containing nanoparticles exhibit a very intense Ln3+ excitation band at 273 nm because of the 8S7/2 → 6IJ transition in Gd3+ ions followed by a nonradiative (nr) energy transfer to Ln3+ ions.[60,61] Thus, Gd3+ ions act as a sensitizer to enhance the emission of Tb3+ ion via nr energy transfer. The obtained emission spectra monitored at λex = 273 nm yielded intense green emissions in the region of 480–680 nm, which are due to the 5D4 → 7F (J = 3, 4, 5, 6) transitions of Tb3+ ions. Four prominent emission peaks centered at ∼488, ∼544, ∼584, and ∼619 nm originate from the transitions of 5D4 → 7F6, 5D4 → 7F5, 5D4 → 7F4, and 5D4 → 7F3, respectively.[22,62,63] The mechanism for DC in Gd3+–Tb3+ pair under excitation of 273 nm has been demonstrated in Figure S4 (Supporting Information). Among these transitions, the green emission 5D4 → 7F5 at ∼544 nm is the most intense emission, which corresponds to a magnetic dipole transition. Figure shows the comparative emission spectra of β-NaYF4:Gd3+/Tb3+ phosphors incorporated with different concentrations of GQDs. It can be seen that with the increasing concentration of GQDs, the PL intensity of the β-NaYF4:Gd3+/Tb3+ phosphors showed a remarkable enhancement. We believe that the change or increase in luminescence intensity is due to change in morphology and size of the β-NaYF4:Gd3+/Tb3+-xGQD crystals. Different morphologies of the host matrix greatly influence the emission intensity and the shape of the spectra.[63,64] It is evident from the spectra that among all the samples, the disklike structure obtained from β-NaYF4:Gd3+/Tb3+-xGQDs (x = 7) displayed the strongest emission intensity under similar measurement conditions shown in Figure . The β-NaYF4:Gd3+/Tb3+-xGQD (x = 0) rods possess the lowest emission intensity. Whereas the relative luminescence intensity of other β-NaYF4:Gd3+/Tb3+-xGQD crystals, that is, hexagonal microprisms obtained at x = 1 and x = 3 and flower-shaped crystals obtained at x = 5, lies in between the rods and the disklike structure. The reason behind the difference in relative PL intensity might be due to the difference in the surface area of different facets, defects, and crystallinities. Herein, the highest luminescence intensity of disklike structures might be due to their low surface area to volume ratio and consequently possesses low surface defects when compared to that of other anisotropic structures.[65,66] While the rods and other as-formed morphologies are predicted to have more surface defects owing to their high surface area.[67] The defects may act as the nonradiative recombination centers, which are thus responsible for luminescence quenching. Moreover, high surface area also renders greater Gd3+ or Tb3+ ion fraction on the particle surface, which again causes the enhanced nonradiative quenching and results in diminished emission intensity.[66,68] In addition, to further understand the variation in PL performance, the PL spectra of β-NaYF4:Gd3+/Tb3+-xGO, β-NaYF4:Gd3+/Tb3+-xMWCNT, and β-NaYF4:Gd3+/Tb3+-xCOOH-MWCNT (x = 7) crystals were also studied. Figure shows the comparative PL spectra of β-NaYF4:Gd3+/Tb3+-xGQDs (x = 0 and 7), β-NaYF4:Gd3+/Tb3+-xGO, β-NaYF4:Gd3+/Tb3+-xMWCNT, and β-NaYF4:Gd3+/Tb3+-xCOOH-MWCNT (x = 7) crystals. As discussed in section , the incorporation of other carbon materials such as GO and MWCNTs had a very little influence on the morphology and size of the β-NaYF4:Gd3+/Tb3+ crystals. As a consequence with negligible change in the morphologies, it can be clearly seen that the relative luminescence intensities hardly changed with the incorporation of the above-mentioned foreign impurities. However, when compared with that of the disklike structures, the PL intensity showed a remarkable change. The luminescence intensity of a disklike structure was observed to be eight times stronger than that of the β-NaYF4:Gd3+/Tb3+ crystals incorporated with GO and MWCNT and β-NaYF4:Gd3+/Tb3+ rods. To get further insights into the effect of the incorporation of GQDs on the luminescence intensity of β-NaYF4:Gd3+/Tb3+ phosphor crystals, the PL spectra of β-NaYF4:Gd3+/Tb3+ crystals incorporated with different functionalized GQDs, β-NaYF4:Gd3+/Tb3+-xCOOH-GQDs, and β-NaYF4:Gd3+/Tb3+-xNH2-GQDs were demonstrated (Figure ). As it is already discussed above that functionalization of GQDs plays an important role in the morphology tuning of the crystals, it could be clearly reflected in the PL spectra. In the case of β-NaYF4:Gd3+/Tb3+-xCOOH-GQD (x = 1, 3, and 7) crystals, with increasing concentration of COOH-GQDs, the size of as-formed microrods increased and thus the PL intensity also increased. Because the surface area to volume ratio increases with decrease in the size of the material, a phosphor with the largest size would possess the lowest surface area and the highest luminescence intensity.[69] Therefore, the β-NaYF4:Gd3+/Tb3+-xCOOH-GQD (x = 7) microrod (∼30 μm) possesses the highest luminescence intensity, while β-NaYF4:Gd3+/Tb3+-xCOOH-GQD (x = 1) microrods (∼15 μm) and β-NaYF4:Gd3+/Tb3+ rods have a lower PL intensity.

Figure 12

Comparison of static PL emission spectra at the excitation wavelength λex = 273 nm of β-NaYF4:Gd3+/Tb3+-xGQD phosphors with different concentrations of as-prepared GQDs.

Figure 13

Static PL emission spectra at the excitation wavelength λex = 273 nm of β-NaYF4:Gd3+/Tb3+ phosphors with different concentrations (x = 7 mL) of GO, MWCNTs and COOH-MWCNT. A comparison has been made with the β-NaYF4:Gd3+/Tb3+ phosphors incorporated with GQDs.

Figure 14

Comparative static PL emission spectra of β-NaYF4:Gd3+/Tb3+ phosphors with different concentrations of COOH-GQDS, NH2-GQDs, rGQDs, and β-NaYF4:Gd3+/Tb3+-xGQD phosphors at λex = 273 nm where x is varied as x = 1 mL, x = 3 mL, and x = 7 mL.

Negligible change in the PL intensity was observed owing to negligible change in size and morphology of β-NaYF4:Gd3+/Tb3+-xNH2-GQD (x = 1, 3, and 7) rod-shaped crystals (Figure ). PL spectra of NaYF4:Gd3+/Tb3+ with rGQDs were also recorded and compared with those of disklike structures. The emission intensity of NaYF4:Gd3+/Tb3+-xrGQDs was obviously lower than that of disklike structures but higher than that of the β-NaYF4:Gd3+/Tb3+-xCOOH-GQD microrods. Because NaYF4:Gd3+/Tb3+-xrGQDs attained cubic phase along with the hexagonal phase, that is, mixed phase, their emission intensity is expected to be lower because the mixed phase system is less luminescent than the pure hexagonal phase.[25,70,71]

However, the higher intensity of NaYF4:Gd3+/Tb3+-xrGQD microrods as compared with that of β-NaYF4:Gd3+/Tb3+-xCOOH-GQD microrods was due to their larger size (∼35 μm). However, the overall spectra revealed that the PL intensity of disklike structures remained the strongest emission intensity among all the samples. The luminescent properties of inorganic materials are dependent on several factors such as crystal structure around emitting ions, morphology, size, crystallinity, impurity doping, surface defects, surface adsorbed species, solvent molecules, ligands, and so forth.[72] In this study, although incorporation of GQDs had significant influence on the growth of β-NaYF4:Gd3+/Tb3+ crystals, but on the basis of the above analysis, among all the factors we reasonably believe that the variation in PL properties of the samples predominantly arises from their size and morphologies.

Conclusions

In summary, a novel impurity-driven strategy for the morphology tuning of β-NaYF4:Gd3+/Tb3+ phosphor crystals is presented, wherein GQDs were incorporated in situ into the β-NaYF4:Gd3+/Tb3+ crystal system to manipulate the growth of phosphor crystals. Consequently, the morphology of the crystals was drastically changed upon the incorporation of GQDs at different concentrations. The results were also compared with differently functionalized GQDs at varied concentrations. The plausible growth mechanism of β-NaYF4:Gd3+/Tb3+-xGQD crystals is proposed. The effect of two-dimensional and one-dimensional other carbon-based structures such as GO and MWCNT was investigated and compared with the results obtained with the incorporation of GQDs. It was found that the PL properties of β-NaYF4:Gd3+/Tb3+ phosphor crystals are strongly dependent on the crystallite size and morphology. To the best of our knowledge, for the first time, we have implied an approach to incorporate GQDs as an impurity to influence the growth of β-NaYF4:Gd3+/Tb3+ phosphor crystals to attain their multiform morphologies. The results presented here underline the important role that controlled morphological synthesis can play in optimizing the key properties of advanced functional materials and could be extended to other lanthanide-doped nanocrystals. This unique approach paves way to new opportunities for designing and tuning the rare earth phosphor crystals and their unique luminescence properties may render potential applications in the field of color displays, light-emitting diodes, solid-state lasers, and luminescent biological labels.

Methods

Materials

All the chemicals were of analytical grade and were used without further purification. Yttrium nitrate hexahydrate (Y(NO3)3·6H2O, 99.89%), gadolinium nitrate hexahydrate (Gd(NO3)3·6H2O, 99.89%), and terbium nitrate hexahydrate (Tb(NO3)3·6H2O, 99.89%) were purchased from Sigma-Aldrich Inc. Sodium chloride (NaCl, 99.9%) and potassium permanganate (KMnO4, 99.0%) were received from Thomas Baker. Ammonium fluoride (NH4F, >95%) and sodium nitrate (NaNO3, >99%) were received from Merck. The graphite powder was obtained from Loba Chemie. PEI (with Mw = 25 000 and Mn = 10 000), MWCNT, and COOH-MWCNT were received from Sigma-Aldrich Inc. Deionized (DI) water was used throughout the experiments.

Synthesis

Synthesis of GO

GO was prepared by using the modified Hummers method from the graphite powder.[73] Briefly, graphite powder (2 g) and sodium nitrate (1 g) were added to a 250 mL round-bottom flask at 0 °C. Then, 50 mL of concentrated H2SO4 was added slowly with stirring below 5 °C. The solution was then stirred for 30 min. KMnO4 (0.3 g) was then added to the solution below 10 °C. The solution was again stirred for 30 min. Then KMnO4 (7 g) was added over 1 h below 20 °C. After this, the solution was warmed to 35 ± 3 °C and was stirred for 2 h. Water (90 mL) was then slowly dripped into the paste, causing an increase in temperature to 70 °C. This diluted suspension was stirred at this temperature for 15 min. The mixture was treated with 10 mL of H2O2 (30%) and 55 mL of water to quench the reaction. The suspension turned bright yellow, and subsequently, the solution was filtered, resulting in light brown precipitate. The precipitate was then washed with warm solution of 3% HCl (150 mL). It was dried at 40 °C for 24 h in vacuum. The GO stock solution was obtained with a concentration of 4 mg/mL.

Synthesis of GQDs

Synthesis of Carboxylic and Amine Co-Functionalized GQDs (GQDs)

We synthesized GQDs using the previously reported method elsewhere.[56] To the above prepared stock solution of GO (15 mL), 40 mL of H2O2 (30%) and 10 mL of ammonia (25–28%) were added. This mixture was reacted at 80 °C for 24 h with vigorous stirring. The solution was evaporated at 65 °C to remove unreacted H2O2, ammonia, and water. Finally, ethanol was used to precipitate and wash the final GQD product. The GQDs obtained by this method had both the functional groups such as carboxylic and amine and were highly stable and dispersible in water.

Synthesis of COOH-GQDs

To the prepared GO stock solution (15 mL), 40 mL of H2O2 (30%) was added. This mixture was reacted at 80 °C for 24 h with vigorous stirring. The solution was evaporated at 65 °C to remove unreacted H2O2 and water. Finally, ethanol was used to precipitate and wash the final COOH-GQDs.

Synthesis of NH2-GQDs

NH2-GQDs were prepared using the hydrothermal method reported elsewhere.[74] For NH2-GQDs, 15 mL of GO stock solution, 15 mL of DI water, and 10 mL of ammonia were added together. The mixture was stirred for 30 min, followed by ultrasonication for another 30 min. The resultant solution was transferred into a 50 mL Teflon-lined autoclave and heated at 150 °C for 6 h by hydrothermal treatment. Next, the solution was cooled down to the room temperature, and NH2-GQDs were filtered through a 0.22 μm microporous membrane. The filtered solution was heated at 100 °C for another 1 h to remove excess ammonia in the mixture. The obtained solution was stored for further characterization.

Synthesis of rGQDs

COOH-GQDs were reduced by using sodium borohydride, following a previously reported method.[55] Briefly, to the above-prepared (section 2.2.2.2) COOH-GQDs (15 mL), 1 g of NaBH4 was added. The mixture was allowed to stir at room temperature for 2 h. When the color of the mixture changed from faded to light yellow, HNO3 was added to terminate the reaction. Finally, pH was tuned to 8. The resultant solution was filtered through a 0.22 μm microporous membrane and stored for further characterization.

Synthesis of Amine-Functionalized β-NaYF4:Gd3+/Tb3+

In a typical procedure[60,75] for the synthesis of β-NaYF4:15%Gd3+/5%Tb3+, solutions of Y(NO3)3, Gd(NO3)3 and Tb(NO3)3 (0.2 M) were added in a 10 mL solution of NaCl (0.2 M). The solution was continuously stirred for 30–40 min. Then 20 mL of ethanol was added, followed by the addition of PEI (5 wt %). Then, 0.5 M of NH4F was added dropwise to the resultant solution. The whole mixture was stirred for 30 min. Finally, the mixture was poured in a Teflon container with 80 mL capacity, and the reaction was set for 24 h hydrothermally at 180 °C.

Synthesis of β-NaYF4:Gd3+/Tb3+ Incorporated with GQDs

A similar procedure was followed to synthesize GQDs incorporated β-NaYF4:Gd3+/Tb3+ phosphors except that different concentrations of the as-prepared xGQDs (x = 0, 1, 3, 5, and 7 mL) were incorporated in situ in the initial solution reaction system.

Characterization Techniques

The phase purity and crystallinity of the as-prepared samples were characterized by powder XRD using a PANalytical X’PERT PRO instrument and the iron-filtered Cu Kα radiation (λ = 1.54 Å) in the 2θ range of 10°–80° with a step size of 0.02°. To analyze the shape and size of the samples, FESEM (FESEM: Hitachi S-4200) was used. The specific structural details and morphology were obtained by using a FEI Tecnai T20 transmission electron microscope operated at 200 keV accelerating voltage with Schottky field emitter source with maximum beam current (>100 nA) and small energy spread (0.8 eV or less). The powder samples obtained were dispersed in ethanol and then drop-casted on carbon-coated copper TEM grids with 200 mesh and loaded to a single tilt sample holder. UV–vis spectroscopy measurements were performed on a Jasco UV–vis–NIR (model V570) dual beam spectrometer operated at a resolution of 2 nm. PL spectra were acquired using a Cary eclipse fluorescence spectrophotometer, equipped with a 400 W Xe lamp as an excitation source and a Hamamatsu R928 photomultiplier tube as a detector. Raman spectra were recorded on an HR-800 Raman spectrophotometer (Jobin Yvon-HORIBA, France) using monochromatic radiation emitted by a He–Ne laser (633 nm), operating at 20 mW and with the accuracy of ±1 cm–1 in the range between 450 and 850 nm, equipped with thermoelectrically cooled (with Peltier junctions), multichannel, spectroscopic grade CCD detector (1024 × 256 pixels of 26 μm) with dark current lower than 0.002 electrons pixel–1 s–1. An objective of 50 XLD magnification was used to both focus and collect the signal from the powder sample dispersed on a glass slide. TGA was done using the SDT model Q600 of TA Instruments Inc. USA at a heating rate of 10 °C/min under nitrogen flow at 100 mL/min. A PALS Zeta Potential Analyzer Ver 3.54 (Brookhaven Instrument Corps.) was used to determine the electrophoretic mobilities. Mobilities were converted to zeta potentials (ζ) using the Smoluchowski model. DI water was used as the dispersion medium.