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Jun 29, 2017 - ABSTRACT: Carbon quantum dots (CQDs) have attracted consid- erable interest because of their advantages of low cost, nontoxicity, easy...
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Zinc-Reduced CQDs with Highly Improved Stability, Enhanced Fluorescence, and Refined Solid-State Applications Rong Miao,†,‡ Shaofei Zhang,†,‡ Jianfei Liu,†,‡ and Yu Fang*,†,§ †

Key Laboratory of Applied Surface and Colloid Chemistry of Ministry of Education, ‡School of Materials Science and Engineering, and §School of Chemistry and Chemical Engineering, Shaanxi Normal University Xi’an 710062, People’s Republic of China S Supporting Information *

ABSTRACT: Carbon quantum dots (CQDs) have attracted considerable interest because of their advantages of low cost, nontoxicity, easy preparation, and diverse optical properties. Photophysical properties and stability are two key issues that affect the further use of CQDs, especially for solid-state applications. Here, we report a facile and effective method to enhance the fluorescence and moisture-resistance of CQDs. CQDs were synthesized by the thermolysis of citric acid on a multigram scale and were readily used to react with zinc powder in an aqueous solution to produce zinc-reduced CQDs (Zn-CQDs). The process was operated under mild conditions and completed in 10 min. The fluorescence intensity of the CQDs increased more than three times after reacting with zinc. In contrast to the pristine CQD solids, which easily absorbed moisture and turned into viscous liquids, the as-prepared Zn-CQD blocks possessed an ordered porous structure and were immune to moisture in the air. They remained stable under ambient conditions for months. Comprehensive studies were carried out to explore the reaction mechanism and material properties. It was revealed that zinc likely served as a reductant in the process. On the basis of the satisfactory properties of the Zn-CQD blocks, their solid-state applications were studied. A Zn-CQD-based fluorescent film for temperature monitoring was fabricated with favorable linear response and reversibility. Moreover, the asobtained Zn-CQD blocks showed remarkable adsorption of particulate matter (PM). Therefore, it is expected that they may find practical use in the purification of PM-polluted air.

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treatments.18−20 However, these methods suffer from high reaction temperatures, time-consuming procedures, the use of toxic chemicals or complicated purification processes. Therefore, the development of a facile, fast, green, and effective method for CQD reduction is of great importance. Oxidized CQDs contain an abundance of carboxyl groups at their surface.5 The carboxyl groups facilitate the surface functionalization of CQDs. In detail, the carboxyl groups are usually transformed into reactive acyl chloride species and then reacted with molecules containing amino/hydroxy groups to get CQDs modified with desired molecules.21 However, the abundant carboxyl groups make CQDs highly hydroscopic, and in fact, many CQDs rapidly absorb moisture from ambient air. Thus, it is hard to keep CQDs totally dry in many cases, which is why most of the CQDs-based sensing platforms were designed and used in solution, while few CQDs-based sensing films have been reported.5 In addition, this disadvantage will cause difficulty for quantifying CQDs and brings inconvenience for solid-state applications.

INTRODUCTION As one of the most attractive carbon materials, carbon quantum dots (CQDs) have received great attention since they were discovered in 2004.1−3 Features of CQDs include their low cost, easy preparation, nontoxicity, and UV/vis-light absorption and luminescence.4,5 These features have facilitated their use in a variety of applications, from bioimaging and sensing in the solution phase to optoelectronics and energy-related devices in the solid state.6−10 Among these, applications of CQDs in solution are more widely studied than CQDs in the solid state, which is due to their generally high hydrophilicity and rapid moisture absorption.9 Thus, CQDs with enhanced luminescence and improved moisture-resistance would promote related applications of CQDs in the solid state. Although an accepted explanation for the fluorescence mechanism of CQDs has not been discovered, much attention has been paid to improve the fluorescence quantum yield (QY) of CQDs, and considerable progress has been achieved.5,6,10−13 Different molecules, such as triethylamine and sulfur-containing compounds, have been introduced into the CQD synthesis to obtain CQDs with enhanced fluorescence.14−17 Meanwhile, reduction of CQDs is an effective way to increase the fluorescence of the intrinsic CQDs. Methods used for CQD reduction mainly include hydrothermal, hydrazine, or NaBH4 © 2017 American Chemical Society

Received: April 17, 2017 Revised: June 29, 2017 Published: June 29, 2017 5957

DOI: 10.1021/acs.chemmater.7b01580 Chem. Mater. 2017, 29, 5957−5964

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Chemistry of Materials

Figure 1. TEM image (a) and size distribution (b) of the as-prepared CQDs. The inset picture in (b) shows the HRTEM image of the CQDs. quartz cell (with a path length of 1 cm). SEM images were acquired on a Quanta 200 scanning electron microscope (Philips-FEI). TEM images were obtained using a JEOL JEM-2100 transmission electron microscope at an acceleration voltage of 200 kV. All Fourier transforming infrared spectrum (FTIR) measurements were performed on a Bruker VERTEX 70 V infrared spectrometer. XPS results were measured by X-ray photoelectron spectrometer (AXIS ULTRA). Contact angles were measured on a Dataphysics OCA20 contact angle system at ambient temperature. XRD measurements were performed using a Bruker D8 Advance with Cu−Kα radiation (λ = 1.5418 Å) at a scan rate of 0.02° s−1. Nitrogen adsorption measurements were conducted on an ASAP 2020 HD88. PM adsorption curves were recorded on a homemade adsorption device. Synthesis of CQDs. CQDs were fabricated using a cheap and nontoxic starting material, citric acid, according to a previously reported procedure with some modifications.24 In a typical experiment, 10 g of citric acid was placed into a 50 mL round-bottomed flask and heated to 210 °C in an oil bath in air. When the temperature raised to approximately 175 °C, citric acid began to melt, and a colorless liquid was formed. Then, bubbles came out of the system, and the color gradually changed to light yellow and then orange and finally brown over approximately 8 h. Upon cooling, an orange-brown liquid with a high viscosity was obtained, and 20 mL of ethyl acetate was added to the flask. Then, the mixture was evaporated to dryness, and 4.23 g of solid orange CQDs were obtained. In contrast to the reported procedure on the CQD synthesis, the CQDs obtained in this work were not neutralized because the carboxyl groups on the CQD surface were used in a further procedure. Synthesis of the Zn-CQDs. The Zn-CQDs were synthesized via simple method under mild conditions: the as-obtained CQDs were dissolved in 80 mL of water, and 2.0 g of zinc powder was added into the CQD solution under vigorous stirring. After 10 min, the mixture was filtered with a 0.22 μm membrane, and the filtrate was collected. The color of the CQD solution became much lighter after the reaction. Then, 100 mL of methanol was added to the filtrate, and a light-yellow precipitate was observed. The precipitate was collected by filtration and then washed with methanol three times. Afterward, the precipitate was freeze-dried, and 3.25 g of Zn-CQD blocks with a light-yellow color were obtained.

Active metals, especially zinc, are favorable reducing agents in organic synthesis and replacement reactions.22,23 As a reductant, zinc exhibits a favorable reducing ability under acidic conditions.23 Considering the abundance of carboxyl groups on the surface of CQDs and the reducing ability of zinc, it is hypothesized that zinc will react with the carboxyl groups (−COOH) on the CQD surface and that this reaction will provide two advantages: (1) the reaction will result in reduction of the CQDs and thus result in enhanced fluorescence; and (2) zinc will oxidize to Zn2+, and the −COOH groups on the CQD surface will change into −COO¯, which may improve their stability via resisting moisture. On the basis of these considerations, we used zinc to reduce CQDs in aqueous solution. It was interesting to find that the fluorescence of the CQDs increased more than three times after reacting with zinc. The reduction was operated at room temperature (25 °C) in ambient atmosphere and finished in 10 min. Meanwhile, highly fluorescent Zn-CQD blocks with ordered porous internal structures were obtained through simple precipitation. The ordered porous internal structure and good stability of the as-prepared highly fluorescent Zn-CQD blocks facilitated their solid-state applications. The as-prepared Zn-CQD blocks were utilized to fabricate fluorescent films, and the films showed a linear and reversible response to temperature change (10 to 100 °C). Moreover, the Zn-CQD blocks exhibited superior adsorption capability for particulate matter (PM) than activated carbon and were used in polluted air purification.

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EXPERIMENTAL SECTION

Materials. Citric acid and all other materials used were obtained from J&K Chemicals. All solvents (ethanol, methanol, etc.) were of analytical grade and used without further purification or treatment. Activated carbon (AC, charcoal activated, granular) was of analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd. Smoke was generated by burning a cigarette. Water used in this work was acquired from a Milli-Q reference system. Instrumentation. UV−vis absorption spectra were recorded on a U-3900 UV−vis spectrophotometer (Hitachi). All fluorescence studies in this work were carried out using a time-correlated single-photon counting fluorescence spectrometer (Edinburgh Instruments FLS 920). Steady-state fluorescence excitation and emission spectra were recorded using a Xenon light as the excitation source. Absolute fluorescence quantum yields were determined through an integrating sphere method. Fluorescence lifetime measurements were performed on the same system using an EPLED-340 ps pulsed-diode laser with vertical polarization as an excitation source (405 nm). All samples used for the absorption and fluorescence measurements were prepared in a

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RESULTS AND DISCUSSION Characterization. To determine the shape and size of the prepared CQDs in this work, transmission electron microscopy (TEM) was used. Figure 1 shows the TEM images and the size distribution of the as-prepared CQDs. It was found that the asfabricated CQDs were spherical nanoparticles with uniform diameters of 2−5 nm. As shown in the high-resolution transmission electron microscopy (HRTEM) image (inset of Figure 1b), the as-prepared CQDs contained both amorphous carbon and graphitic regions. In addition, the HRTEM image

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DOI: 10.1021/acs.chemmater.7b01580 Chem. Mater. 2017, 29, 5957−5964

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Figure 2. Morphology of the as-prepared Zn-CQD blocks. Photographs of the Zn-CQDs under daylight (a) and UV light (b); SEM images of ZnCQDs with different magnifications (c, d); EDX elemental mapping (e−g); and EDX analysis (i) of the Zn-CQD blocks shown in h. The scale bar in h is 200 nm.

Figure 3. UV−vis and fluorescence properties of the as-prepared CQDs and Zn-CQDs. (a) UV−vis absorption spectra of CQDs (3.3 mg/mL; black line) and Zn-CQDs (3.3 mg/mL; red line). (b) Fluorescence excitation spectra (dashed lines) and emission spectra of CQDs (black line; λem = 500 nm, λex = 410 nm) and Zn-CQDs (red line; λem = 485 nm, λex = 400 nm). Spectra were recorded in 0.1 M citric acid- sodium citrate buffer solution (pH = 6.0). Photos of the solution of CQDs and Zn-CQDs under sunlight (c) and UV light (d).

prepared Zn-CQDs were highly emissive under UV light (Figure 2b), and they exhibited an ordered porous structure (Figure 2c,d,h). The SEM images show that the Zn-CQD blocks are characterized by uniform porous structures (Figure 2c,d). Meanwhile, the composition of the Zn-CQDs was confirmed by energy-dispersive X-ray spectroscopy (EDX) measurements (Figure 2e−h). As expected, C (74.03 wt %), O (18.70 wt %), and Zn (7.27 wt %) were the main constitutional elements in the Zn-CQD blocks. TEM and HRTEM images of the Zn-CQD blocks were also taken to further investigate the structure of the materials (Figure S2). The Zn-CQD blocks were flake-like in structure (Figure S2a) and composed of an abundance of CQD (Figure S2b). In addition, X-ray diffraction (XRD) patterns of the CQD solids and Zn-CQD blocks were

revealed that the lattice fringes of the graphitic regions correspond to the (100) intralayer spacing of 2.2 Å (Figure 1b, inset).24 TEM images of the CQDs after reduction were also taken (Figure S1 of the Supporting Information, SI), and no obvious changes in shape or size of the CQDs were observed before and after reduction. The TEM results prove that the CQDs were successfully synthesized, and the reduction process did not have much effect on the basic structure of the CQDs. In contrast to most of the reported CQDs, which are often prepared in solution or doped/mixed with other matrices due to their highly hydroscopic nature or aggregation-induced quenching (AIQ),25,26 the Zn-CQDs prepared in this work were loose blocks (Figure 2a). As is shown Figure 2, the as5959

DOI: 10.1021/acs.chemmater.7b01580 Chem. Mater. 2017, 29, 5957−5964

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Chemistry of Materials recorded. Both XRD patterns (Figure S3) of the CQD solids and Zn-CQD blocks showed d-spacings of 0.42 nm (2θ = 21.4°, Figure S3), indicating that the two species are similar and that they both have graphite-like structures.27 Meanwhile, it was noticed that a new peak corresponding to a d-spacing of 0.87 nm (2θ = 10.2°, red line in Figure S3) appeared in the pattern of the Zn-CQD blocks, which suggests that the Zn-CQD blocks have looser stacking than the CQD solids. To evaluate the porosity and surface area of the Zn-CQD blocks, a nitrogen physisorption test was conducted. The analysis of the obtained isotherm revealed a surface area of 26 m2/g. The adsorption and desorption characteristics mainly indicated an IUPAC type II curve and type H3 loop (Figure S4), which proves the presence of large mesopores and macropores inside the Zn-CQD blocks.28,29 The corresponding cumulative pore volumes were calculated from the desorption branch of the isotherm. No prevalent pore size was observed. In addition, the pore-size distribution was broad, and no plateau was reached. These results agree well with the SEM results, as depicted in Figure 2c,d, which also showed macropores with a broad pore-size distribution. Photophysical Properties. An aqueous solution of the asprepared CQDs had an orange-yellow appearance (Figure 3c, left) under sunlight, and weak green fluorescence was observed under UV light (Figure 3d, left). However, the color of the CQD solution changed to nearly colorless after reacting with zinc (Figure 3c, right), and the solution showed much stronger blue fluorescence under UV light (Figure 3c, right). Further inspection of the spectra is shown in Figure 3, where the UV− vis absorption spectrum of the CQDs shows an absorption band at ca. 340 nm, while the absorption band of the Zn-CQDs is at ca. 290 nm (Figure 3a). The maximum excitation wavelength of the CQDs is approximately 410 nm and the maximum emission wavelength of the CQDs is 500 nm (Figure 3b, black lines). However, the CQDs after reaction showed a much higher fluorescence intensity, and both the emission and the excitation spectra were blue-shifted by approximately 10− 15 nm (Figure 3b, red line). Meanwhile, the excitationdependent fluorescence emission spectra of the CQDs before and after reaction were recorded (Figure S5). The CQDs showed a gradually red-shifted fluorescence emission with a change of the excitation wavelength from 360 to 450 nm (Figure S5a). A similar observation was obtained for the CQDs after reduction with both the excitation and emission spectra blueshifted (Figure S5b), which is in accordance with the UV− vis and fluorescence results shown in Figure 3. To further testify the fluorescence behavior of the CQDs before and after reaction with zinc, the absolute fluorescence QYs and fluorescence lifetimes were measured (Figure S6). The results show that the fluorescence lifetime and QY for the pristine CQDs are ∼3.01 ns and ∼4.37%, respectively. As expected, the reacted CQDs showed a longer fluorescence lifetime (3.90 ns) and a much-enhanced fluorescence QY (17.95%), which is more than three times that of the pristine CQDs. All the above results show that fluorescence properties of the CQDs were greatly improved after the reaction with zinc. Hydrophilicity of CQDs and Zn-CQDs. As mentioned before, a disadvantage of CQDs is their moisture absorption behavior due to the abundant −COOH groups. It was found that the CQDs are orange solids when freshly prepared (Figure 4a), but they absorb moisture rapidly under ambient conditions and change into viscous dark-brown liquids (Figure 4b). There is a remarkable increase in the weight of the CQDs after 24 h of

Figure 4. Hydrophilicity of CQDs and Zn-CQDs. Photos of CQD solids freshly prepared (a, 0.3885 g) and after stored at ambient condition for 24 h (b, 0.4017 g). Photos in c and d show Zn-CQD blocks freshly prepared (c, 0.5358 g) and after stored at ambient condition for 24 h (d, 0.5361 g). The inset pictures show water contact angles of CQD solids (up) and Zn-CQD blocks (down), respectively.

storage (from 0.3885 to 0.4017 g, Figure 4a,b). However, there is little change (either in the morphology or weight) for the ZnCQDs after storage for 24 h (Figure 4c,d). Meanwhile, the CQDs and Zn-CQDs were pressed into tablets for contactangle measurements. The results showed that the CQDs (with a water contact angle of 9.8 ± 0.5°, top section in Figure 4) are much more hydrophilic than the Zn-CQDs (with a water contact angle of 29.6 ± 0.5°, bottom section in Figure 4), which can be used to explain the moisture absorption behaviors. The hydrophilicity studies demonstrate that the Zn-CQDs are more stable than the CQDs in the solid state, which would benefit further applications of the Zn-CQD blocks. Mechanistic Study. Zinc will oxidize into Zn2+ when it reacts with acids, and therefore, the effect of Zn2+ on the fluorescence of the CQDs was investigated. The fluorescence responses of the CQDs to Zn2+ and the Zn-CQDs to a wellknown metal-ion complexing agent (ethylenediamine tetraacetic acid disodium salt, EDTA) were studied (Figure S7a,b). The addition of Zn2+ did not increase the fluorescence emission of the CQDs (Figure S7a), and the fluorescence of the ZnCQDs was almost unchanged even in the presence of excess EDTA in the system (Figure S7b). In addition, the effect of ZnO on the fluorescence of the CQDs was investigated by measuring the emission spectra of the CQDs before and after ZnO treatment (Figure S7c). No obvious fluorescence changes after ZnO treatment were observed. All these results infer that the fluorescence enhancement of the Zn-CQDs was due to the presence of Zn2+. The fluorescence of CQDs has been reported to be sensitive to pH changes in most cases,4 and therefore, the fluorescence spectra of the CQDs under different pH values 5960

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Chemistry of Materials Scheme 1. Proposed Reaction Mechanism between CQDs and Zinc

(2−10) were recorded (Figure S7d). The fluorescence of the CQDs reached a peak value at a pH value of ∼6.0, with an increase of 0.5 times, which is much less than the increase after zinc treatment (Figures 3b and S6). The results from the pH effect studies also explain the reason why a citric acid-sodium citrate buffer system (pH = 6.0) was chosen for the fluorescence measurements in the previous tests (Figures 3, S5, and S6). On the basis of these results, it is reasonable to conclude that the enhanced fluorescence of the CQDs after reaction with zinc resulted from reduction of the CQDs by zinc. To confirm and study the surface structures of the CQDs and Zn-CQDs, FTIR spectroscopy was used, and the results are shown in Figures S8 and S9. As shown in Figure S8, both the CQDs and Zn-CQDs showed CH2 bending vibrations (∼1410 cm−1). In the FTIR spectrum of the CQDs, there are O−H stretching vibrations at ∼3068 and ∼2619 cm−1 and30 a CO vibrational absorption band at 1710 cm−1, indicating the surfaces of the CQDs are dominated by −COOH and −CO groups. However, in the FTIR spectrum of the Zn-CQDs, the O−H stretching vibrations shifted to ∼3425 cm−1, and the stretching vibration for the carboxylate groups was located at ∼1579 cm−1, which implies that −COO− and −OH groups are the predominate structures on the surface. From the difference between the surface structures of the two kinds of quantum dots, the following is inferred: (1) the −COOH groups on the CQD surfaces likely react with zinc to produce −COO− groups and (2) the −CO groups on the CQD surfaces are reduced to −C−OH groups. Moreover, the disappearance of the features of citric acid in the FTIR spectra of both the CQDs and Zn-CQDs suggests the complete thermolysis of citric acid (see Figures S8 and S9). To further elucidate the chemical composition and reaction mechanism between the CQDs and zinc, X-ray photoelectron spectroscopy (XPS) measurements were performed. Compared to the CQDs (black line in Figure S10), which are composed of carbon and oxygen, an abundance of zinc was found in the ZnCQDs (red line in Figure S10). From the high-resolution XPS spectrum of Zn 2p in the Zn-CQDs, the Zn 2p region exhibits two peaks at 1045.1 and 1021.9 eV,31 which reveals that Zn exists in the form of Zn (II) in the Zn-CQDs (inset in Figure S10). The high-resolution XPS spectra of O 1s in both the CQDs (Figure S11a) and Zn-CQDs (Figure S11b) exhibit mainly two contributions: the peak at 532.0 eV corresponds to O−C species, and the peak at 533.3 eV corresponds to OC species.32 It is obvious that the O−C/OC ratio is different in the two spectra: the CQDs possessed an O−C/OC ratio of

approximately 1:1 (Figure S11a), but the ratio increased to approximately 4:1 in the Zn-CQDs (Figure S11b). The XPS spectra of C 1s can be divided into three peaks (Figures S11c,d). The main peaks at 284.6, 286.2, and 288.3 eV are features of sp2-hybridized graphite-like carbon (C−C, sp2), C− O species and CO species, respectively.32 It is noted that the C−O/CO ratio in the Zn-CQDs (∼1:1) is higher than that in the CQDs (∼0.57:1). These XPS results suggest that zinc was oxidized into Zn (II) when reacted with the CQDs and that some −CO groups on the CQDs were reduced to −C− OH groups. A combination of the results from control experiments and the XPS and FTIR studies, a rational mechanism is proposed to understand the reaction between the CQDs and zinc (Scheme 1). Zinc reacted with the carboxyl groups on the CQD surface to produce reductive species, which further reduce the −CO groups into −C−OH groups. Meanwhile, Zn2+ and −COO− groups were produced in the system, which is similar to a zinc carboxylate species. When the organic liquid (for example, methanol) was added into the system after the reaction (aqueous solution), the solubility of the zinc-carboxylate-like structure decreased. Therefore, aggregation of the reduced CQDs was promoted due to complexation and/or electrostatic interactions between the produced Zn2+ and the −COO− groups (on the CQDs). Accordingly, the porous Zn-CQD blocks formed (Scheme 1). In this way, Zn-CQD blocks with ordered porous structures were produced. Solid-State Applications of the Zn-CQDs. On the basis of the favorable properties of the as-prepared Zn-CQD blocks (ordered porous structure and enhanced fluorescence and stability), the solid-state applications of the Zn-CQDs were explored. First, a fluorescent film for temperature monitoring was fabricated. For the convenience of a low-cost preparation procedure, filter paper was chosen as the substrate, and the fabrication details were as follows: a small piece of the Zn-CQD blocks (5 mg) were finely ground and dispersed in 2 mL of methanol; the mixture was sonicated (100 W) for 20 min and then filtered with a 0.22 μm membrane; then, the membrane loaded with Zn-CQDs was freeze-dried to obtain the Zn-CQDbased film. Photobleaching is one of the key issues in fluorescent sensing, and therefore, the photostability of the film was tested (Figure S12). The results show that the Zn-CQD film has satisfactory photochemical stability, showing less than a 2.4% 5961

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Figure 5. Fluorescence response of Zn-CQD film to temperature. (a): fluorescence spectra of the as-prepared Zn-CQD film under different temperature (10 °C, 20 °C, 30 °C, 40 °C, 50 °C, 60 °C, 70 °C, 80 °C, 90 °C, 100 °C; λex= 420 nm); the inset plot shows the linear relationship between fluorescent intensity of Zn-CQD film and temperature (λex= 420 nm, λem= 488 nm). (b): fluorescence emission intensity of Zn-CQD film when the temperature is alternated between 10 and 100 °C (λex= 420 nm, λem= 488 nm). (c): images of the Zn-CQD-based film under sunlight (left) and UV lamp (365 nm, the ones on the right) at different temperatures (20 °C, 40 °C, 60 °C, 80 °C, 100 °C). Note: fluorescence spectra and images were recorded immediately after the temperature reached the desired value.

Afterward, 0.12 mg of the Zn-CQD blocks were placed onto the sample holder made of a metal net in the PM adsorption system. The weight of the Zn-CQD blocks during the test was continuously monitored by an electronic balance connected to a data acquisition system. In this way, the dynamic adsorption process was monitored and recorded during the entire adsorption process. The adsorption capacity was calculated using the following equation:

decrease in its emission intensity after continuous illuminating at 400 nm for 2 h. To evaluate the ability of the Zn-CQD film to monitor temperature, the fluorescence emission spectra of the Zn-CQD film under different temperatures were collected (Figure 5a). The results showed that the fluorescence of the Zn-CQD film decreased upon increasing the temperature, and there was a good linear relationship between the fluorescence intensity of the film and the temperature (from 10 to 100 °C; inset plot of Figure 5a). Meanwhile, the temperature response of the ZnCQD film was almost fully reversible between 10 and 100 °C over the seven cycles performed (Figure 5b). Fluorescence images of the Zn-CQD film under different temperatures showed that the fluorescence emission of the Zn-CQD film decreased obviously with an increasing temperature (Figure 5c), indicating that the Zn-CQD film can be used for real-time monitoring of the surface temperature. Second, the porous Zn-CQD blocks were used for polluted air purification. With rapid industrialization, air pollution has become a serious problem worldwide, especially in developing countries.33,34 Combustion smoke (automobile tail gas and industrial smoke) is the main source of air pollution, and it causes PM pollution in air, which poses a great threat on human health.33−35 An effective method to purify polluted air is to use typical materials to remove the PM in polluted air.36,37 Thus, ideal adsorbing materials are highly demanded. Generally, PM is composed of small particles and liquid droplets.33,34,38 This indicates materials that are relatively hydrophilic would facilitate PM removal in polluted air. Considering the hydrophilicity and porous internal structure of the Zn-CQD blocks, the PM adsorption behavior of the ZnCQD blocks was studied on a homemade adsorption device (Figure S13).39 PM particles were generated from cigarette smoke by burning following the procedures of some previous reports.36,37 In a typical test, a cigarette connected to a minipump was lit and put into the PM adsorption system, as shown in Figure S13. When the combustion was complete, the system was kept closed for 10 min to cool to room temperature.

q = (m − m0 × 1000)/m0

where q is the adsorption capacity (mg/g), m is the weight of the Zn-CQD blocks after PM adsorption (mg), and m0 is the initial weight of the Zn-CQD blocks (g). Figure 6a shows the time-dependent PM adsorption curves of the Zn-CQD blocks. The PM adsorption capacity of the ZnCQD blocks (red line in Figure 6a) reached ∼370 mg/g, which is almost twice that of activated carbon (∼180 mg/g, black line in Figure 6a). Meanwhile, a control experiment was performed, and the result showed that the Zn-CQDs hardly adsorbed any mass in clean air (blue line in Figure 6a). These experiments indicate that the as-prepared Zn-CQD blocks are better than AC in PM adsorption. Thus, they were further used in polluted air purification. A test apparatus was set up, and it was found that the Zn-CQD blocks had a high efficiency for PM removal (Figure 6b, 6c and video S1). The hazy air (Figure 6b) turned clear (Figure 6c) quickly (45 s) after the power was turned on. In addition, XPS spectra of the Zn-CQD blocks before (black line in Figure S14) and after (red line in Figure S14) PM adsorption were recorded. Compared with the XPS spectrum of the original Zn-CQDs, the spectrum of the Zn-CQDs after utilization showed an obvious N 1s peak (approximately 400 eV), which confirms the adsorption of PM by the Zn-CQD blocks.36 All these results demonstrate that the Zn-CQD blocks are an efficient adsorbent for PM and that they are favorable candidates for PM-polluted air purification. 5962

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Furthermore, the highly fluorescent, stable, and low-cost characteristics as well as the favorable sensing and PM adsorption properties of the as-prepared Zn-CQD blocks lays a solid foundation for their further applications.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01580. Morphology of the Zn-CQDs, XRD characterization, photophysical behavior of the CQDs and Zn-CQDs, XPS studies, experimental setup for PM adsorption, and so forth (PDF) PM adsorption video (AVI)

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86 29 81530787. E-mail: [email protected] (Y.F.). ORCID

Yu Fang: 0000-0001-8490-8080 Author Contributions

All authors have given approval to the final version of the manuscript.

Figure 6. Applications of porous Zn-CQD blocks in polluted air purification. (a): Time-dependent adsorptions of PM by Zn-CQD blocks (red line) and the commercially AC (black line); The blue line shows the control experiment, where the Zn-CQD blocks was placed in clean air. Parts (b) and (c) are the photos of the aparatus for polluted air purification, which contains four parts: an erlenmeyer flask filled with smoke connected to a splash ball stuffed with Zn-CQD blocks in its inner part; a power source and a mini pump was utilized to ensure circulation of airflow in the closed system. Photos of the system before (b) and 45 s after (c) the adsorption process proceeded. “SNNU” is adapted with permission from Shaanxi Normal University. Copyright [1960/Shaanxi Normal University].

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge funding from the Natural Science Foundation of China (21527802, 21603139, 21673133), the 111 project (B14041), the China Postdoctoral Science Foundation (1202040102), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT-14R33).

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REFERENCES

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CONCLUSIONS The reaction between zinc and the carboxyl groups on the CQD surface was studied, and a novel, facile, and effective method for CQD reduction was developed. The reduction was operated at room temperature and was completed in 10 min. Fluorescence of the CQDs was enhanced more than three times after the reduction. Moreover, the Zn-CQD blocks with an ordered porous structure and enhanced fluorescence emission were obtained. Several techniques, including fluorescence, UV−vis absorption, TEM, XRD, SEM, FTIR, and XPS, were used to study and understand the reaction. The results revealed that some −CO groups on the CQD surface were likely reduced into −C−OH groups upon the reaction. On the basis of the favorable properties of the porous Zn-CQD blocks, they were successfully used in solid-state applications. First, a fluorescent film was fabricated with the Zn-CQDs as the active layer. The as-prepared film exhibited a favorable fluorescence response to temperature: there was a strong linear relationship between the fluorescence intensity of the film and the temperature (from 10 and 100 °C), and the fluorescence response was fully reversible over the tested cycles. Second, the porous Zn-CQD blocks exhibited excellent performance in PM adsorption, and they were successfully used in PM-polluted air purification. This study on CQDs and Zn-CQDs will be helpful to understand the origin of the fluorescence of CQDs. 5963

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Article

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