One-step solvent-free synthesis of carbon dot-based layered composites exhibiting color-tunable photoluminescence

We here report a practical and green approach to the development of luminescent composites through in situ solvent-free formation of carbon dots on layered inorganic compounds. The composites exhibit higher solid-state photoluminescence than those prepared by mixing of synthesized carbon dots and layered clay minerals. Tuning of the emission color of the composites has also been achieved by the addition of small molecules into phloroglucinol as starting materials for carbonization. The carbon dots synthesized in clay compounds in the solvent-free conditions are well-dispersed to obtain homogeneous composites. Furthermore, we have demonstrated that highly luminescent carbon dots are formed by carbonization in the presence of layered inorganic compounds. The one-step solvent-free approach presented in this work may allow not only facile, economical, and sustainable production of nanostructured carbon dot-based composites but also improvement of their luminescence properties.

Herein, we report on one-step solvent-free synthesis of CDbased luminescent layered composites (Fig. 1). The in situ preparation of CDs on layered inorganic compounds was expected to allow the dispersion and controlled synthesis of CDs to achieve efficient luminescence of the composites in the solid states. Tuning of the luminescence color was also intended by addition of low-molecular-weight compounds into phloroglucinol as starting materials for carbonization. Furthermore, the structures of CDs formed in the layered compounds were examined to understand luminescence properties of the composites.

Material design
Phloroglucinol was selected as a carbon source for the formation of emissive CDs. 10 Carbonization of phloroglucinol on saponite under solvent-free conditions was expected to directly provide solid-state luminescent CD-based composites with thermal stability (P-CD1/saponite). Combination of carbon sources and doping of heteroatoms for the preparation of carbon-based nanomaterials have been shown to control their functions. [1][2][3][4][5][6][61][62][63][64] Thus, addition of resorcinol, boric acid, and phosphorus pentoxide to phloroglucinol followed by thermal treatment in the presence of saponite would lead to tuning of the emission color of the CD-based composites (PR-CD1/saponite, PB-CD1/saponite, and PP-CD1/saponite, respectively) ( Fig. 1). The composites obtained by the in situ formation of CDs were considered to exhibit efficient luminescence because of homogeneous dispersion of CDs in the composites. We also envisioned that the carbonization in layered inorganic compounds may control the formation of p-conjugated structures of CDs. 34 For comparison, the P-CD2/saponite, PR-CD2/ saponite, PB-CD2/saponite, and PP-CD2/saponite composites obtained by mixing of synthesized CDs and saponite were designed to study the luminescence properties.

Thermal properties of the carbon dots and their composites
The CD-based layered composites exhibited better thermal stability than CDs without clay minerals ( Fig. 2 and S1 †). Fig. 2 shows thermogravimetric (TG) curves of the P-CD1(10)/ saponite, P-CD1(21)/saponite, and P-CD1(40)/saponite composites containing 10, 21, 40 wt% of CDs, respectively. Aer dehydration of adsorbed water around 150 C, weight losses of the decomposition of CDs were observed from 300 C to 500 C for the P-CD1/saponite composites. Another weight loss around 700 C was characteristic of dehydroxylation of the clay minerals. 65 For P-CD1, which was extracted from the P-CD1(21)/saponite composite, the TG curve showed a gradual weight loss from 100 C (Fig. 2). The decomposition temperature of P-CD1 was much lower than those of CDs in the P-CD1/saponite composites. Hybridization of CDs with clay minerals led to the improvement of thermal stability of the composites.

Hybrid structures of the composites
The nanostructures of the CD-based composites were examined by X-ray diffraction (XRD) measurements (Fig. 3). The basal spacing of saponite treated at 200 C under N 2 ow for 3 h was about 10.5Å (Fig. 3, top), whereas the spacing of the P-CD1(21)/ saponite composite increased up to about 14.6Å by the formation of CDs (Fig. 3, bottom). In addition, the layered structure of the P-CD1(21)/saponite composite was slightly disordered as compared to that of saponite because the peak at 14.6Å was broader. These results suggest that the CDs were formed in the interlayer space of saponite. [33][34][35]37,39 As a comparison with the P-CD1(21)/saponite composite, the structure of the P-CD2(21)/saponite composite, which was prepared by mixing of P-CD2 and saponite, was also studied.   that the layered assembly of saponite was preserved aer the formation of CDs. The cross-sectional SEM images of a particle of the P-CD1(21)/saponite composite are shown in Fig. 4b. The cross section was analyzed by energy dispersive X-ray (EDX) spectrometer to characterize the distribution of CDs inside the particles of the composites ( Fig. 4c and d). The mapping images of C Ka and Si Ka X-ray suggest that CDs were dispersed inside the P-CD1(21)/saponite composite. For the P-CD2(21)/saponite composite, SEM and EDX analysis revealed the deposition of aggregates of CDs on saponite (see ESI, Fig. S5 †). These observations were consistent with the results of XRD measurements of the P-CD2(21)/saponite composites.
The effects of the ratios of CDs on specic surface area of the P-CD1/saponite and P-CD2/saponite composites were investigated. Fig. 5 shows Brunauer-Emmett-Teller (BET) specic surface area of these composites with different ratios of CDs. The specic surface area of the P-CD1/saponite composites remarkably decreased with an increasing weight percentage of CDs (Fig. 5, solid line). This observation suggests that the CDs were incorporated into the porous structure of saponite to decrease the surface area of the composites. It should be noted that specic surface area of the P-CD1(21)/saponite composite was over 100 times lower than that of the P-CD2(21)/saponite composite, even though the ratio of CDs in each composite was almost the same. These results indicate that CDs synthesized in situ were efficiently embedded in the pores including the interlayer space of saponite, which prevented aggregation of CDs in the composites.

Luminescence properties of the composites
The composites obtained by the in situ synthesis exhibited luminescence in the solid states under the illumination of UV light (Fig. 6a-d). For example, the P-CD1(10)/saponite composite showed blue-green emission (Fig. 6a). It is noteworthy that tuning of the luminescence color was achieved by the introduction of additives into the starting materials for the synthesis of CD-based composites. The PR-CD1/saponite composite prepared by using phloroglucinol and resorcinol exhibited blue luminescence (Fig. 6b), whereas the PB-CD1/ saponite composite prepared by using phloroglucinol and boric acid displayed yellow-green luminescence (Fig. 6c). The yellow-   orange emission was observed for the PP-CD1/saponite composite prepared by using phloroglucinol and phosphorus pentoxide (Fig. 6d).
The excitation and emission spectra of the PR-CD1/saponite, PB-CD1/saponite, and PP-CD1/saponite composites in the solid states were obtained to study the photophysical properties (Fig. 6e). The PR-CD1/saponite composite exhibited an emission band centered at 500 nm, which was blue-shied from that for the P-CD1(10)/saponite composite (525 nm, see ESI, Fig. S6 †). This difference was likely due to the formation of CDs containing smaller p-conjugated structures by the combination of resorcinol and phloroglucinol as carbon sources. 10 The PB-CD1/saponite composite exhibited a slightly red-shied emission at 535 nm as compared to the P-CD1(10)/saponite composite. Further bathochromic shi was observed in the emission for the PP-CD1/saponite composite probably due to the introduction of phosphorus into the CDs through carbonization. [66][67][68] On the other hand, the UV-vis spectra of PB-CD1/ saponite and PP-CD1/saponite composites showed small absorption peaks at 507 and 550 nm, respectively (see ESI, Fig. S7 †). The wavelengths of these peaks corresponded to those of the peaks in the excitation spectra (Fig. 6e).
The photoluminescence quantum yields (F PL ) of the composites were measured. The P-CD1(10)/saponite, P-CD1(21)/ saponite, PR-CD1/saponite, PB-CD1/saponite, and PP-CD1/ saponite composites exhibited the photoluminescence efficiency of 0.04, 0.05, 0.07, 0.29, and 0.15 in the solid states, respectively. The enhanced emission of the PB-CD1/saponite and PP-CD1/saponite composites was probably attributed to the xation of molecular motion through the interactions between the CDs and saponite. In contrast, the P-CD2(10)/saponite, P-CD2(21)/saponite, PR-CD2/saponite, PB-CD2/saponite, and PP-CD2/saponite composites, which were prepared by mixing of synthesized CDs and saponite, showed no signicant emission by visual inspection under UV light irradiation. The F PL values of these composites were less than 0.01. The higher F PL values of the composites prepared by the in situ synthesis of CDs on saponite were partly due to the suppression of aggregationcaused quenching through the dispersion of uorescent CDs in the composites.

Structure and luminescence properties of the carbon dots
The structures of the CDs were examined to provide further insight into the higher luminescence efficiency of the composites prepared by the one-step solvent-free method. For this purpose, the X-ray photoelectron spectroscopy (XPS) of P-CD1 and P-CD2 was carried out to study the chemical state of elements. Notably, the high-resolution XPS analysis of C 1s peaks showed that a peak ascribed to C-C bonds was more distinct for P-CD1 (Fig. 7). The percentage of C-C peaks in P-CD1 and P-CD2 was estimated to be 48% and 37%, respectively (see ESI, Fig. S8 †). The C/O ratio of P-CD1 and P-CD2 was almost the same (2.3 and 2.2, respectively), while that of phloroglucinol was 1.5. These results indicate that condensation of phloroglucinol for the formation of P-CD1 was affected by saponite to give more C-C bonds in the chemical structures. It was previously reported that carbonization of polyacrylonitrile in the two-dimensional space between montmorillonite lamellae produced graphite-like structures more efficiently. 34 The carbonization of phloroglucinol conned in the interlayer space of saponite may result in the formation of larger p-conjugated structures with more C-C bonds for P-CD1. 10 Emission properties of P-CD1 and P-CD2 in solution were explored. The methanol solution of P-CD1 displayed green emission (Fig. 8a, top), while that of P-CD2 exhibited blue emission (Fig. 8a, bottom). The wavelengths of the excitation and emission maxima of P-CD1 were red-shied by 79 and 87 nm relative to those of P-CD2, respectively (Fig. 8b). The absorption of visible light for P-CD1 was also stronger than that for P-CD2 (see ESI, Fig. S9 †). These results support the formation of extended p-conjugated structures for P-CD1 as proposed by XPS analysis. The F PL value of the methanol solution of P-CD1 was 0.20, which was much higher than that of P-CD2 (0.07). For P-CD1 and P-CD2 in the solid states, no emission was  observed under UV light irradiation. Overall, the efficient luminescence properties of the composites prepared by the onestep solvent-free approach can be attributed to the preferential formation of p-conjugated structures of CDs as well as dispersion of CDs in the composites.

Conclusions
In conclusion, we have demonstrated practical one-step solventfree synthesis of CD-based luminescent layered composites by thermal treatment of the mixtures of carbon sources and saponite. The emission color of the composites was tuned by addition of small molecules into phloroglucinol as starting materials for carbonization. SEM observation revealed that CDs synthesized in situ were well-dispersed in saponite, which led to the solid-state emission and high thermal stability of the composites. Moreover, the solvent-free in situ synthesis of CDs on saponite allowed the formation of CDs with enhanced emission efficiency. The facile and green preparation of nanostructured CD-based composites would have potential towards sustainable development of new optical functional materials.

Experimental section
Materials A synthetic saponite (Sumecton SA) was purchased from Kunimine Industries. Other reagents were purchased from Tokyo Kasei, Aldrich, FUJIFILM Wako Pure Chemical, and used as received without further purication.
PR-CD2/saponite composites. A ground mixture of 0.02 g phloroglucinol dihydrate and 0.1 g resorcinol was heated at 200 C under N 2 ow for 3 h to form PR-CD2. The PR-CD2/ saponite composite was obtained by grinding 0.05 g PR-CD2 with 0.45 g saponite.
PB-CD2/saponite composites. A ground mixture of 0.15 g phloroglucinol dihydrate and 0.057 g boric acid was heated at 200 C under N 2 ow for 3 h to form PB-CD2. The PB-CD2/ saponite composite was obtained by grinding 0.1 g PB-CD2 with 0.9 g saponite.
PP-CD2/saponite composites. A ground mixture of 0.15 g phloroglucinol dihydrate and 0.13 g phosphorus pentoxide was heated at 200 C under N 2 ow for 3 h to form PP-CD2. The PP-CD2/saponite composite was obtained by grinding 0.1 g PP-CD2 with 0.9 g saponite.

Extraction of carbon dots from the carbon dot/saponite composites
Carbon dot P-CD1 was obtained by extraction from the P-CD1(21)/saponite composite with methanol. Aer removing precipitates by centrifugation, the methanol solution was dried under vacuum to give P-CD1 as a brown solid.

Characterization
Thermogravimetric (TG) measurements were performed with Mettler TGA 2 up to 800 C at a heating rate of 10 C min À1 under air ow (40 mL min À1 ). Powder X-ray diffraction (XRD) measurements were carried out with PANalytical X'PERT-PRO MPD using Ni-ltered Cu Ka radiation. The morphologies of the composites were observed with a Hitachi SU8220 eldemission scanning electron microscope (SEM) operated at 1 kV. Energy dispersive X-ray (EDX) analysis was performed using a Bruker XFlash 5060FQ energy dispersive X-ray spectroscope. Brunauer-Emmett-Teller (BET) specic surface area was measured using Quantachrome Monosorb. The adsorbed gas was N 2 (20%) and He (80%) at 77 K. Prior to the BET surface area measurements, the samples were dried under the gas ow at 150 C for 10 min. UV-vis absorption measurements were conducted with Hitachi U-4100 and U-3900H spectrophotometers. Excitation and emission spectra and absolute photoluminescence quantum yields (F PL ) were obtained with JASCO FP-8500 spectrouorometer equipped with an integrating sphere unit ILF-835. X-ray photoelectron spectroscopy (XPS) was carried out using JEOL JPS-9010MC.

Conflicts of interest
There are no conicts to declare.