C-QDs@UiO-66-(COOH)2 Composite Film via Electrophoretic

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Cite This: Inorg. Chem. 2018, 57, 2447−2454

C‑QDs@UiO-66-(COOH)2 Composite Film via Electrophoretic Deposition for Temperature Sensing Ji-fei Feng,†,‡,§,∥ Shui-ying Gao,*,† Jianlin Shi,‡,§,∥ Tian-fu Liu,† and Rong Cao*,†,§,∥ †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China ‡ School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China § State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China ∥ University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: Temperature plays a crucial role in both scientific research and industry. However, traditional temperature sensors, such as liquid-filled thermometers, thermocouples, and transistors, require contact to obtain heat equilibrium between the probe and the samples during the measurement. In addition, traditional temperature sensors have limitations when being used to detect the temperature change of fast-moving samples at smaller scales. Herein, the carbon quantum dots (C-QDs) functionalized metal−organic framework (MOF) composite film, a novel contactless solid optical thermometer, has been prepared via electrophoretic deposition (EPD). Instead of terephthalic acid (H2BDC), 1′,2′,4′,5′-benzenetetracarboxylic (H4BTEC) acid was employed to construct a UiO-66 framework to present two uncoordinated carboxylic groups decorated on the pore surface. The uncoordinated carboxylic groups can generate negative charges, which facilitates the deposition of film on the positive electrode during the EPD process. Moreover, UiO-66-(COOH)2 MOFs can absorb C-QDs from the solution and prevent CQDs from aggregating, and the well-dispersed C-QDs impart fluorescence characteristics to composites. As-synthesized composite film was successfully used to detect temperature change in the range of 97−297 K with a relative sensitivity up to 1.3% K−1 at 297 K. for the preparation of MOF films, such as in situ growth,20−22 spin-coating,23 electrodeposition,24−26 and electrophoretic deposition.27−30 Among these methods, the electrophoretic deposition (EPD) method can be used for the preparation of MOF films without requiring the corresponding substrate. Moreover, the film can be rapidly and easily fabricated using this method. Carbon quantum dots (C-QDs) are small carbon particles with a size less than 10 nm. Compared with traditional semiconductors or quantum dots (i.e., CdSe, ZnSe, PbSe), CQDs have many advantages such as high aqueous solubility, chemical inertness, low toxicity, and good biocompatibility.31−41 Therefore, C-QDs are promising in the application of optronics, photocatalysis, and sensing. In recent years, fluorescent C-QDs@MOFs composites have attracted the attention of scientists. Lan et al. reported C-QDs@UMCM-1 composites by stepwise synthetic approach for hydrogen

1. INTRODUCTION Temperature is one of the most important factors in both scientific research and industry.1 Traditional thermometers usually function via physical contact and are not applied in strong magnetic or electronic situations. Due to these limitations, some lanthanide metal−organic frameworks (MOFs) or lanthanide functionalized MOFs have been developed for temperature sensing.2−10 MOFs are porous materials constructed from metal ions or clusters bridged by organic ligands to form one-, two-, or three-dimensional infinite networks.11−18 Considering the high cost of lanthanide metal, development of non-lanthanide MOFs or MOF composites for temperature sensing is necessary in future research. In addition, the majority researches on MOF-based thermometers are based on MOFs powder or crystals. However, due to the crystalline intrinsic property, MOF materials are not malleable as soft materials, which is limited to be further processed and applied. MOFs in the form of filmshaped architectures are more suitable in practical applications.19 Recently, there are some interesting methods reported © 2018 American Chemical Society

Received: October 9, 2017 Published: February 13, 2018 2447

DOI: 10.1021/acs.inorgchem.7b02595 Inorg. Chem. 2018, 57, 2447−2454

Article

Inorganic Chemistry Scheme 1. Fabrication of C-QDs@UiO-66-(COOH)2 Composite

storage and fluorescent sensing.42 Yan and co-workers reported Eu3+/C-QDs@MOF-253 composites for Hg2+ sensing.43,44 In addition, C-QDs@ZIF-8 composites have been used for drug delivery, fluorescence imaging, and chemical sensing.45,46 Therefore, based on these excellent works, C-QDs are a potential alternative material for functionalizing the MOFs. In addition, C-QDs have been proved to be a potential temperature sensing material in this paper. However, solid CQDs without aggregation are difficult to be obtained without the substrates. To overcome this drawback, we employed a UiO-66 based MOF, UiO-66-(COOH)2 with uncoordinated carboxylic group, to load the C-QDs. The results proved that the C-QDs can be functionalized on MOFs without aggregation to yield C-QDs@UiO-66-(COOH)2 composites, resulting in better temperature sensing performance. Moreover, the C-QDs@UiO-66-(COOH)2 composites were prepared into film on a zinc plate via the electrophoretic deposition approach in 5 min. Compared with the prototype of UiO-66, MOFs employed in this work have two uncoordination carboxylic acids remaining on each linker, which can generate negative charge on the surface of the MOFs and composites. Therefore, the composite can be deposited on the positive electrode during the electrophoretic deposition process. The as-prepared film can been used to accurately detect temperature change in the temperature range from 97 to 297 K with a relative sensitivity up to 1.3% K−1. In addition, asprepared film exhibited better temperature sensing performance than those of solo C-QDs (0.29% K−1) and C-QDs@UiO-66(COOH)2 composites (1.05% K−1). Moreover, the film thermometer can be reused 3 times without loss of fluorescence intensity.

spectroscopy was performed with KBr pellets on a VERTEX70 infrared spectrometer. Brunauer−Emmett−Teller (BET) specific surface areas were measured by the N2 adsorption method on an ASAP 2020 instrument. Thermogravimetric analysis (TGA) spectra were recorded on an SDT Q600 instrument in N2 flow between 30 and 1000 °C and with heating at a rate of 5 °C min−1. Photoluminescence spectra were measured on an FLS980 in the temperature range between 77 and 297 K. Microwave syntheses were carried out in a microwave oven (Initiator 8 EXP, 2450 MHz frequency, Biotage Corp). Inductively coupled plasma (ICP) data were obtained on an Inductively Coupled Plasma OES spectrometer (Ultima2 HORIBA Jobin Yvon). 2.3. Details and Description of the Experiments. 2.3.1. Preparation of UiO-66-(COOH)2. The UiO-66-(COOH)2 solid was synthesized according to the previously described procedure.47 In a round-bottom flask equipped with a reflux condenser and magnetic stirrer, 4.3 g (0.017 mol) of H4btec and 2.3 g (0. 01 mol) of zirconium tetrachloride (ZrCl4) were dispersed in 50 mL of deionized water at room temperature under stirring and then heated at 100 °C for 24 h. This process resulted in a white gel, which was filtered off and washed with distilled water several times. To remove the remaining free acid encapsulated within the pores as much as possible, the activation was further carried out by dispersing the sample in the distilled water (approximately 10 mL per 1 g of product) and heated at 100 °C for 16 h. The solid was then recovered via centrifugation, washed with water and acetone for several times, and dried at 70 °C under vacuum for overnight, yielding approximately 4 g of white powder. 2.3.2. Preparation of Carbon Quantum Dots (C-QDs). The zinc plate was cut into rectangle pieces (approximately 10 × 40 mm2) and cleaned with deionized water, ethanol, and acetone in an ultrasonic bath for 15 min. In total, 2.5 g of glucose was dispersed in 12.5 mL of deionized water and dissolved in an ultrasonic bath for 30 min and then transferred into a 50 mL microwave reaction tube. Then, the zinc plate was taken into the reaction tube. The tube was put into a microwave oven for 2 h at 120 °C. 2.3.3. Preparation of C-QDs@UiO-66-(COOH)2. A total of 25 mL of C-QDs solution was added into a 50 mL Florence flask equipped with a magnetic stirrer, and 200 mg of UiO-66-(COOH)2 was added into the C-QDs solution and then stirred for 14 h at the rate of a 2000 r/min at room temperature. The pale yellow powders were obtained via centrifugation and washed with deionized water and acetone for several times and dried overnight at 70 °C under vacuum. 2.3.4. Preparation of C-QD@UiO-66-(COOH)2 Composite Film Using Electrophoretic Deposition. The film was prepared according to the previously described procedure.42 The zinc plates were cut into rectangle pieces (approximately 10 × 20 mm2) and cleaned with deionized water, ethanol, and acetone in an ultrasonic bath for 15 min, respectively. In addition, 10 mg of C-QDs@UiO-66-(COOH)2 was dispersed in 15 mL of CH2Cl2 solution in an ultrasonic bath for 2 min. Two identical zinc substrates were dipped in the deposition solution (approximately 1 cm separation distance), and a constant DC voltage of 90 V was applied using an Agilent E3 612A DC power supply for 5 min. The film can be prepared on the positive electrode.

2. EXPERIMENTAL SECTION 2.1. Chemicals. All chemicals were commercially obtained and used without further purification. Pyromellitic acid (H4BTEC) was purchased from Tokyo Chemical Industry Co. LTD (Japan). Zirconium(IV) chloride (ZrCl4) was purchased from Strem Chemicals Industry. Europium nitrate hexahydrate (Eu(NO3)3·6H2O) was purchased from Energy Chemical. D(+)-Glucose (AR) was purchased from General-Reagent. Ethanol, acetone, dichloromethane (CH2Cl2), and zinc acetate dehydrate (Zn(CH3COO)2·2H2O) were obtained from Sinopharm Chemical Reagent Co. LTD (China). The zinc plate was purchased from Tianjin Fu Chen Chemical Reagent (China). 2.2. Measurements. Powder X-ray diffraction patterns (PXRD) were recorded on a Miniflex 600 with a Cu Kα radiation (λ = 1.5406 Å), and the data were obtained within the 2θ range of 5−50° with a rate of 0.1° min−1. Transmission electron microscopy (TEM) was performed on a JEOL-2010 FEI Tecnai G20 field-emission microscope operated at 200 kV. Scanning electron microscopy (SEM) patterns were obtained using Phenom G2. Fourier transform infrared 2448

DOI: 10.1021/acs.inorgchem.7b02595 Inorg. Chem. 2018, 57, 2447−2454

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Inorganic Chemistry

Figure 1. TEM images (scale bar 50 nm) of C-QDs (A) The inset: the HRTEM image of C-QDs, and the size distribution of the C-QDs (B). 2.3.5. Preparation of Eu@C-QDs@UiO-66-(COOH)2. In a 50 mL flat flask equipped with a stirrer, 230 mg of Eu(NO3)3·6H2O was dissolved in 25 mL of an ethanol solution in an ultrasonic bath. Then, 100 mg of C-QDs@UiO-66-(COOH)2 was added under stirring for 15 h at a rate of 2000 r/min in room temperature. Then the solid was obtained via centrifugation and cleaned with ethanol and acetone for several times and dried overnight at 70 °C under vacuum. The content of Eu and Zr measured by ICP was 8.93% and 20.15%, respectively.

with C-QDs with the maximum emission peaks at 480 nm, the C-QDs@UiO-66-(COOH)2 composite has maximum emission peaks at 475 nm with a blue shift of 5 nm, which indicates that the C-QD interacts with UiO-66-(COOH)2 by hydrogen bond or coordination interaction rather than by physical interaction (Figure 2A). However, the pristine UiO-66-(COOH)2 does not have an obvious emission peak at 480 or 475 nm and has a weak emission peak at approximately 420 nm. Therefore, green fluorescence of the C-QDs@UiO-66-(COOH)2 composite originates from the C-QDs rather than from UiO-66(COOH)2. The fluorescence characterization proves that the composite was synthesized. As shown in Figure 2B, FT-IR spectra showed that the hydroxyl group of C-QDs has a broad absorption peak at approximately 3400 cm−1, while the hydroxyl group in CQDs@UiO-66-(COOH)2 has a broad absorption peak at approximately 3386 cm−1, shifted 14 cm−1, which indicates that there are intermolecular hydrogen bonds between the C-QDs and MOFs. Moreover, the absorption peak at 1715 cm−1 for the free −COOH group in C-QDs@UiO-66-(COOH)2 composite is much weaker than that in UiO-66-(COOH)2, which indicated that the free carboxylic group in C-QDs@UiO66-(COOH)2 interacts with the hydroxyl group of C-QDs.50−53 As a result, the C-QDs can be functionalized in MOFs without aggregation. In addition, the results were further confirmed by the TEM pictures. As shown in Figure 2C,D, the C-QDs in the composite have a uniform particle size of approximately 3 nm and were uniformly distributed in the composite without aggregation. In addition, the interplanar crystal spacing of CQDs in the composite is approximately 2 Å, which is consistent with the C-QDs. UiO-66-(COOH)2 MOFs are a mesoporous material, which may allow C-QDs to be absorbed in MOFs, which was confirmed by the N2 sorption isothermal of UiO-66(COOH)2 with a type-IV BET isotherm, as shown in Figure 2E. Compared with pristine UiO-66-(COOH)2, the BET surface area of the C-QDs@UiO-66-(COOH)2 composite decreased from 590 to 180 m2·g−1, which indicates that no porous C-QDs had been successfully doped into the composite. In addition, the pristine UiO-66-(COOH)2 MOF had a wide pore size distribution from 1 to 120 nm. However, the pore size distribution of C-QDs@UiO-66-(COOH)2 was 1−40 nm, which indicated that the C-QDs had been functionalized into the MOFs (Figure S3). In addition, the crystal structure of the MOFs is not changed during the process, which was demonstrated by the powder X-ray diffraction patterns shown in Figure 2F.

3. RESULTS AND DISCUSSION 3.1. Characterizations of the C-QDs@UiO-66-(COOH)2 Composite. The C-QDs@UiO-66-(COOH)2 composite was prepared, as illustrated in Scheme 1, wherein green fluorescence C-QDs were fabricated by one-step reaction in a water solution via microwave synthesis. In addition, UiO-66-(COOH)2 was synthesized according to the previously described procedure. Due to the uncoordination carboxylic function groups in the pore surface, C-QDs can be functionalized on the UiO-66(COOH)2 MOFs. 3.1.1. Characterizations of Carbon Quantum Dots (CQDs). There are some interesting methods to prepare C-QDs, such as hydrothermal graphene oxide reduction, electrochemical and metal-catalyzed approaches, chemical synthesis, chemical exfoliation, and bottom-up synthesis.48,49 Among these methods, the bottom-up synthesis was explored to prepare C-QDs with uniform morphology and size distribution using low-cost raw materials, such as glucose, fructose via hydrothermal method by microwave assisted heating. Therefore, the C-QDs were synthesized here via a microwave assisted hydrothermal method in one step. As shown in Figure S1, the yield of C-QDs is very small without a zinc plate, which indicates that the zinc plate plays a crucial role during the preparation process. Moreover, the content of zinc ions in the as-synthesized C-QDs solution is approximately 33 ppm via the inductively coupled plasma analysis. As-synthesized C-QDs are mainly spherical, which was confirmed using the TEM images, as shown in Figure 1A. The size of C-QDs is approximately 3 nm. (Figure 1B). Moreover, the HRTEM image (the inset in Figure 1A) proves the high crystallinity of C-QDs and the interplanar crystal spacing is approximately 2 Å, which demonstrates the successful fabrication of C-QDs (Figure S2). 3.1.2. Characterizations of the C-QDs@UiO-66-(COOH)2 Composite. As shown in Figure 2A (inset), the UiO-66(COOH)2 is a white powder under nature light, whereas the CQDs@UiO-66-(COOH)2 composite is pale yellow and emits green fluorescence excited under 365 nm ultraviolet (UV) light, indicating that C-QDs were doped into composites. Compared 2449

DOI: 10.1021/acs.inorgchem.7b02595 Inorg. Chem. 2018, 57, 2447−2454

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Figure 2. (A) PL emission spectra of C-QDs, C-QDs@UiO-66-(COOH)2, and UiO-66-(COOH)2 dispersed in water solution excited at 365 nm. The inset shows the images of C-QDs@UiO-66-(COOH)2 (left) under natural light and C-QDs@UiO-66-(COOH)2 (right) excited at 365 nm of UV light. (B) FT-IR spectra of C-QDs, C-QDs@UiO-66-(COOH)2 composites, and UiO-66-(COOH)2. (C) The TEM image of C-QDs@UiO-66(COOH)2 composites. (D) The HRTEM image of C-QDs@UiO-66-(COOH)2 composites. (E) N2 sorption isotherms of UiO-66-(COOH)2 (red) and C-QDs@ UiO-66-(COOH)2 (pink). (F) Powder X-ray diffraction (PXRD) patterns of simulated UiO-66 (black), as-synthesized UiO-66(COOH)2 (blue) and C-QDs@UiO-66-(COOH)2 composites (red).

3.2. Characterization of the C-QDs@UiO-66-(COOH)2 Composite Film. The C-QDs@UiO-66-(COOH)2 composite film was fabricated via electrophoretic deposition, as shown in Scheme 2, wherein the composites were dispersed in a CH2Cl2 solution, and two identical zinc plates were used as positive and negative electrodes. Then, the composites can be deposited on the positive electrode under a DC 90 V field for 5 min. As shown in the scanning electron microscope image of CQDs@UiO-66-(COOH)2 (Figure 3A), the film is dense and continuous and the film thickness is approximately 100 μm (Figure 3B). In addition, the C-QDs@UiO-66-(COOH)2 film is isostructural with the composite powder, which indicates that C-QDs@UiO-66-(COOH)2 film was successfully fabricated without breaking the structure of MOFs during the EPD

Scheme 2. Preparation of C-QDs@UiO-66-(COOH)2 Film on the Zinc Plate Using Electrophoretic Deposition

process (Figure S4). Moreover, the optical properties are similar to those of the composites. 2450

DOI: 10.1021/acs.inorgchem.7b02595 Inorg. Chem. 2018, 57, 2447−2454

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intensity (Y) and temperature (T) in the range of 97−297 K, which can be fitted as a function of Y = 22.99 − 0.065T

(1)

with a correlation coefficient of 0.991. The thermometer property can be assessed by the relative sensitivity (S), which is defined as a function of S= Figure 3. SEM image (scale bar: 200 μm) of C-QDs@UiO-66(COOH)2 film (A) and cross-sectional image (scale bar: 200 μm) of C-QDs@UiO-66-(COOH)2 film on the zinc plate (B).

dY Y dT

(2)

Compared with the other reported materials, the relative sensitivity of the film thermometer in our work is higher, as shown in Figure S6. The relative sensitivity of our film thermometer is up to 1.3% K−1 at 297 K, which is higher than the sensitivity of a mixed-lanthanide MOF (0.31% K−1), a typical temperature sensor.2 The thermometer performance is also evaluated for the precision and accuracy which is related to the correlation coefficient of the fitting curve. On the basis of eq 1, the accuracy of the film thermometer is 0.0095, and the temperature precision is ±4.0 K. As expected, the as-prepared films exhibited a better temperature sensing performance than those of solo C-QDs (0.29% K−1) and C-QDs@UiO-66(COOH)2 composites (1.05% K−1) (Figures S7 and S8). To demonstrate the stability of the film thermometer, the photoluminescence spectra were conducted taking the film to three temperature cycles from 97 to 297 K. As shown in Figure 5, the emission curves were overlapped at the same temperature in cycles and the emission intensity of the film did not change, which indicates that the optical property of the as-prepared film was stable. The crystal phase of our film was not changed from the PXRD spectra conducted after three cycles of the photoluminescence test, as shown in Figure S4. As previously stated, the temperature-dependent photoluminescence microscopy of Eu@C-QDs@UiO-66-(COOH)2 was conducted, indicating that the material was also a good thermometer in the temperature range of 77−257 K, which has a higher relative sensitivity of 1.7% K−1 at 257 K based on CQDs in the composites and 1.3% K−1 at 257 K based on europium ions (615 nm, 5D0−7F2) (Figures 6 and S9 and S10). In addition, the as-prepared material could exhibit white fluorescence in the temperature range from 77 to 363 K upon excitation of 365 nm (Figure 6).

On the basis of the IR analysis in section 3.1 , the composite has the free carboxylic groups. Moreover, the function group can generate the negative charge on the surface of composites. On the basis of the EPD theory, the particles with charges can be deposited on the opposite electrode. Hence, the composite can be prepared on the positive electrode. Moreover, this is consistent with the experimental results. To further prove the existence of free carboxylic function in the composites, the Eu@C-QDs@UiO-66-(COOH)2 composites were synthesized based on coordination interaction between the carboxylic function group and europium. 3.3. Photoluminescence Measurement and Temperature Sensing. To assess the potential application of CQDs@UiO-66-(COOH)2 film as a thermometer, temperaturedependent photoluminescence properties were investigated. To prove the thermal stability of as-synthesized materials, the TG test was conducted. As shown in Figure S6, the C-QDs@UiO66-(COOH)2 was stable under 400 °C, which indicated the MOFs were not destroyed in the tested temperature range of 97−297 K (Figure S5). As shown in Figure 4A, emission intensity of the C-QDs@UiO-66-(COOH)2 film decreased in quantity with the increase of temperature from 97 to 297 K due to the thermal activation of nonradiative decay pathways. In addition, the emission intensity at 297 K has decreased to 31% of the emission intensity at 97 K. The thermometer parameter Y is defined as the emission intensity of the C-QDs@UiO-66-(COOH)2 film. From Figure 4B, there is a good linear relationship between the emission

Figure 4. (A) PL emission spectra of C-QDs@UiO-6-(COOH)2 film on the zinc plate recorded in the temperature range of 97−297 K. (B) Emission intensity of the C-QDs@UiO-66-(COOH)2 film as a function of temperature (black squares, left axis) with the fitting curve (red line, R2 = 0.991) and the relative sensitivity curve (blue line, right axis). 2451

DOI: 10.1021/acs.inorgchem.7b02595 Inorg. Chem. 2018, 57, 2447−2454

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Inorganic Chemistry

Figure 5. Emission spectra (A) and the reversible emission intensity (B) of C-QDs@UiO-66-(COOH)2 film after a number of temperature cycles excited at 365 nm.

Figure 6. Emission spectra of Eu@C-QDs@UiO-66-(COOH)2 and the corresponding CIE picture (the inset) recorded in the temperature range of 77−363 K upon excitation of 365 nm.

4. CONCLUSION For the first time, we reported a simple and rapid preparation of the C-QDs@UiO-66-(COOH)2 film using the electrophoretic deposition. The composite was deposited on the positive electrode due to the negative charge of the composites originating from the free carboxylic functional group of MOFs. The resultant as-synthesized film was successfully used as a thermometer in the temperature range of 97−297 K with a relative sensitivity up to 1.3% K−1 at 297 K. Herein, we present a new method to prepare the C-QDs@MOF composite film, which facilitates the practical application in some fields, such as temperature sensing, small molecule sensing, and lightemitting devices.

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the C-QDs@UiO-66-(COOH)2 and the MOF materials (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.Y.G.). *E-mail: [email protected] (R.C.). ORCID

Shui-ying Gao: 0000-0002-1711-1319 Jianlin Shi: 0000-0001-8790-195X Tian-fu Liu: 0000-0001-9096-6981 Rong Cao: 0000-0003-2384-791X

ASSOCIATED CONTENT

Notes

The authors declare no competing financial interest.

* Supporting Information S

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02595. The picture of C-QDs solution; the PXRD patterns of CQDs; C-QDs@UiO-66-(COOH)2 film before test and after test; the temperature-dependent on solo C-QDs, CQDs@UiO-66-(COOH)2 composite powders, and Eu@ C-QDs@UiO-66-(COOH)2; the relative sensitivity of

ACKNOWLEDGMENTS

This research was supported by the 973 Program (2014CB845605), NSFC (21331006, 2152010200, 51572260, and 21521061), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000), and the Key Research Program of Frontier Sciences of the Chinese Academy of Sciences (QYZD-J-SSW-SLH045). 2452

DOI: 10.1021/acs.inorgchem.7b02595 Inorg. Chem. 2018, 57, 2447−2454

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DEDICATION Dedicated to Prof. Xin-Tao Wu on the occasion of his 80th birthday.

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