Two dimensions materials beyond graphene have gain great interest in recent years for their unique mechanical, electrical and chemical properties and have shown great potential in catalysis, phototherapy, bioimaging and sensors applications, etc. [1-8]. Two dimension quantum dots (2D-QDs), an emerging and exciting confined 2D nanosheet materials with outstanding photoluminescent and photothermal properties, are one of the rising stars in 2D materials family and have triggered widespread applications in energy, nanomedicine and physical fields [9-17].

Molybdenum oxide quantum dots (MoO₃ QDs) is one of the 2D-QDs reported in recent years and attract scientists for their excellent localized surface plasmon resonance (LSPR) properties with promising application in surface-enhanced Raman spectroscopy (SERS) field [18]. Besides, their widespread absorption wavelength could open their new era in photothermal and phototherapy fields. Although great progress has been made, there is no reported MoO₃ QDs with both effective photothermal and highly photoluminescent properties and thus impede their application in nanomedicine and bioimaging fields.

Here, for the first time, we reported a novel and straightforward strategy of preparing the modified MoO₃ QDs with both high photothermal and photoluminescent properties. The modified MoO₃ QDs shows excitation wavelength from 430-610 nm and emission wavelength at 550-750 nm with quantum yield (QY) up to 20%. To our knowledge, this is the first reported modified MoO₃ QDs with both fluorescence and photothermal properties. The structure of the modified MoO₃ QDs was further explored with X-ray photoelectron spectroscopy (XPS), Fourier Transform infrared spectroscopy (FTIR) and extended X-ray absorption fine structure (EXAFS) and it was founded that local defect and heteroatom doping effect are the major contributors for the outstanding photothermal and photoluminescent properties of modified MoO₃ QDs. This research provided a scope for the commercial application of the modified MoO₃ QDs, especially in nanomedicine and phototherapy field.

The fabrication process is illustrated in Fig. 1. Firstly, MoS₂ powder (50 mg) were sonicated in 30% ethanol solution (10 mL) for 2 h for good dispersion purpose, followed by adding 1.5 mL 30% H₂O₂ to oxidize MoS₂ to MoO₃ (Fig. 1a). With the strong oxidation reactivity of H₂O₂, MoS₂ particle was oxidized to form a loose MoO₃ sheet structure. Heated the autoclave to 40 ℃ and gradually injected CO₂ into the autoclave until pressure up to 20 MPa, then released CO₂ smoothly at a placid speed after 3 h. With the assistant of CO₂ molecules, proton could further combine with MoO₃ sheet to form HxMoO₃ quantum dots. Subsequently, the reacted solution was put into the centrifuge and centrifuged at 6000 rpm for 15 min in order to remove aggregates, and the supernatant was taken as the quantum dots solution prepared.

In the next step, Tobias acid (0.08 g) and o-phenylenediamine (0.8 g) were used as precursors, mixed together with ethanol (40 mL) and sealed in a Teflon-lined stainless-steel autoclave to prepare near infrared red CDs (Fig. 1b), which was kept in a furnace at 200 ℃ for 10 h. Finally, the components of the mixture were further separated by column chromatography. Weighing 22 g silica gel to activate 2 h under 80 ℃, dissolved in petroleum ether and packed. Column height is 5 cm. MoO₃ QDs and red CDs were under heat treatment together for 30 min at 60 ℃, with the purpose of grafting the MoO₃ QDs edging with red CDs. Then with the roman spectroscopy chromatography purification process, the oversize modified MoO₃ QDs were eliminated and thus the modified MoO₃ QDs with size at 2-10 nm can be obtained. (Fig. 1c).

The morphology of MoO₃ QDs, near infrared red CDs, and modified MoO₃ QDS were characterized by high resolution transmission electron microscopy (HRTEM) images. Fig. 2a reveals the MoO₃ QDs with uniform distribution at size between 2-8 nm. High resolution in Fig. 2b revealed low lattice diffraction fringe, which implied a low degree of the edges of MoO₃ QDs, the fringes with the interplanar spacing of ca. 0.35 nm was assigned to the (040) crystal plane of MoO₃ QDs (Fig. 2b) [19]. The HRTEM images of the red CDs were shown in Fig. 2c with uniform distribution sizes with the presence of (100) lattice planes of graphitic carbon with d-spacing ~0.21 nm (Fig. 2d). The modified MoO₃ QDs were shown in Fig. 2e. The sizes of the modified MoO₃ QDs became larger and the fringes lattice were at 0.351 nm (Fig. 2f) which revealed the modified MoO₃ QDs keep the same core of MoO₃ QDs, with enriched oxygen and nitrogen at the edges, which was a critical factor for high photoluminescent function.

To further confirm the MoO₃ core of the modified MoO₃ QDs, we carried out the SEM EDX mapping of the sample and clear Mo, C, O element were observed in Fig. 3a, From Fig. 3a, the chemical elemental distribution was observed via SEM-EDX mapping clearly, where the sample of MoO₃ QDs grafted red carbon dots were enrich in Mo, C and O. From the morphology of the modified MoO₃ QDs, carbon and oxygen atom distribution map were well matched with each other, implying the surface of MoO₃ QDs was homogeneously coated with red carbon dots. This was another evidence to prove that red carbon dots were successfully grafted onto MoO₃ QDs. Furthermore, this homogeneously coating was also expected crystallinity with enriched oxygen and hydrogen element at to bring some optical benefit. We further carried out EXAFS spectra in Fig. 3b. The Mo K-edge EXAFS spectra reveal that the near-edge absorption energy of Modified MoO₃ dots located between Mo foil and MoO₃, indicated the average electron density around Mo in Modified MoO₃ dots is higher than Mo foil but lower than MoO₃ [20]. It can be further confirmed that the distribution of the modified MoO₃ QDs were uniform and presents lamellar core structure [21]. Several electronic transitions at the absorption edge are strongly dependent on the geometry around the investigated element and can be symmetry forbidden. In Fig. 3b, the pre-edge peak appears as a clear shoulder for Mo oxides, suggesting that Mo atoms are octahedral geometry of MoO₃ in sample modified MoO₃ dots [22].

The photoluminescent of the red CDs and the modified MoO₃ QDs were seen in Figs. 3c and d. In Fig. 3c, it was clearly showed that red carbon dot exhibited three unique photoluminescent peaks at 600, 650 and 720 nm with intensity gradually decreasing. After grafting on MoO₃ QDs, the peak at 720 nm become relatively higher by comparing with peaks at 600 and 650 nm. It is an interesting phenomenon, which indirectly revealed the existence of photoelectron channel and interaction between red carbon dot and MoO₃ QDs. This evidence was consistent with UV absorption results of the MoO₃ QDs were seen in Fig. 3e. After modified with red carbon dots, the absorption peak of the modified MoO₃ QDs was found to be red shifted, which also provided a strong evidence to prove the strong interaction between red carbon dot and MoO₃ QDs. Due to this red shift, it effectively extended the absorption of near infrared range, providing a high photothermal efficiency. The lifetime of MoO₃ QDs and red CDs were shown in Fig. 3f, The lifetime of red carbon dots and the modified MoO₃ QDs diluted 100 times in 99.7% ethanol at room temperature are 15.28 ns, 12.62 ns, respectively. Compared with original MoO₃ QDs, the modified MoO₃ QDs exhibited a shorter lifetime. This can be explained by the existence of photoelectron channel and interaction between red carbon dot and MoO₃ QDs observed in photoluminescent results, which are expected to be benefit for electron-thermal transition for high photothermal efficiency.

When at 540 nm excited, the modified MoO₃ QDs exhibited two peaks at 600 nm and 650 nm respectively, with QY up to 28%. After modification of MoO₃, two peaks were still observed, which indicates that MoO₃ QDs grafted red carbon dots not only preserves the original peaks which means the new material retains the original strong red fluorescence performance, but there also appears a new peak at 700 nm. However, the peak at 700 nm is relatively weak. All of these proved that the modified MoO₃ QDs had excellent fluorescence performance, which broke through the previous lack of fluorescence of MoO₃ QDs.

In order to further prove the strong bonding of chemical elements between the modified MoO₃ QDs after grafting, we conducted XPS test (Figs. 4a and b). At first, from XPS survey spectra of the modified MoO₃ QDs in Fig. S1 (Supporting information) which shows the modified MoO₃ QDs includes mainly chemical elements O, N, C, Mo, S. The element Mo was from 2D material MoO₃ QDs, while the elements N, C, S were from red carbon dots. Fig. 4a is the Mo 3d spectra of the XPS, the peaks at 232.63 and 235.73 eV are ascribed to the 3d_3/2 and the 3d_5/2 orbital electrons of Mo⁶⁺, while the peaks at 231.53 and 234.58 eV are attributed to the 3d_3/2 and the 3d_5/2 orbital electrons of Mo⁵⁺. According to previous reports, the appearance of Mo⁵⁺ indicates the interaction between MoO₃ quantum dots and H⁺ ions. In our system, the red carbon dots were synthesized in ethanol solvent, when red carbon dots grafted onto the MoO₃ will combination with H⁺ in the original system. At the same time, the combination of non-metallic elements in the red carbon dots with molybdenum element will cause the molybdenum ions to lose electrons, so that part of the Mo⁵⁺ in the system will be converted into Mo⁶⁺, which facilitates the formation of lamellar 2D materials [18]. XPS analysis of the C 1s spectra shows the 284.63 eV, 285.28 eV and 286.03 eV binding energy signals referred to C—C, C—N and C—O in Fig. 4b. XPS analysis of the O 1s spectra in Fig. S2a (Supporting information) shows that peaks at 530.78, 531.33, 531.88 and 532.58 eV were attributed to Mo-O, C—O, O—H and C=O, respectively. Fig. S2b (Supporting information) shows that the peak at 399.13 eV was pyridinic N, the peak at 400.53 eV was pyrrolic N, and the peak at 401.38 eV contributed to the graphitic N. We also conducted XRD experiments of MoO₃ and modified MoO₃ to characterize the phase structure in Fig. S3 (Supporting information). From the XRD pattern, we can find the difference between MoO₃ and modified MoO₃ obviously. A clear crystallization formation was found in modified MoO₃, indicating that the modified MoO₃ QDs crystal is more stable [23].

To test the photothermal property of the modified MoO₃ QDs. We conducted photothermal curves of MoO₃ dots at 10 mg/mL, 5 mg/mL and 1 mg/mL respectively (Fig. 4c). The as prepared showed remarkable photothermal property. With an NIR laser at wavelength 808 nm for only 150 s, the 10 mg/mL solution temperature can increase up 72.2 ℃. And can further increase to 77.7 ℃ at 270 s expose time (Fig. 4d). Comparing with previous result, this is the most exciting material both photoluminescent and photothermal properties so far (Table S1 in Supporting information) [24-29]. The electron charge transfer from red CDs to MoO₃ dots was considered as a potential result for this fast speed photothermal property.

To further explain the mechanism, a schematic mechanism was proposed, as shown in Fig. 5. The light with wavelength > 430 nm could be converted into photoluminescent with wavelength between 550-750 nm light and emission by red CDs and thus contribute the light property of the modified MoO₃ QDs. And the reset wavelength can be absorbed by red CDs, leading to the charge transfer from red CDs to MoO₃ QDs. And the electron-hole pairs were photogenerated on the surface of MoO₃ QDs [30-32]. In addition, benefited from the heterojunction between MoO₃ QDs and red carbon dots, the modified MoO₃ QDs gained a remarkable photothermal property.

Here, for the first time, we fabricated MoO₃ dots with both photothermal and photoluminescent properties. The QY of the modified MoO₃ QDs is nearly 20% with photoluminescent wavelength between 550-750 nm. Significantly, the modified MoO₃ QDs exhibited remarkable photothermal properties in increasing temperature up to 77.7 ℃ at 270 s with concentration of 10 mg/mL, which made a photothermal record as reported so far. The fast speed electron transfer between red CDs to MoO₃ QDs were considered as one of the main reasons for the photothermal and photoluminescent properties of the modified MoO₃ QDs. With those outstanding properties, the modified MoO₃ QDs are believed to have a great potential in bioimaging and phototherapy field.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Nos. 51575528, 51875577), Beijing Nova Program Interdisciplinary Studies Cooperative Project (No. Z181100006218138), Science Foundation of China University of Petroleum-Beijing (Nos. 2462019QNXZ02, 2462018BJC004) and the Research Program of Yongchuan Science and Technology Commission (Ycstc, No. 2018nb1402).

Appendix A. Supplementary data

Supplementarymaterial related to this article canbefound, in the online version, at doi:https://doi.org/10.1016/j.cclet.2019.11.010.