Introduction
Carbon dots (CDs) have recently received considerable attention in various fields including photocatalysis, bioimaging, sensing, and optoelectronics due to their stability, cost effectiveness, and low toxicity.[] Nevertheless, due to excessive Forster resonance energy transfer (FRET) or direct π–π interactions,[] most CDs suffer from severe luminescence quenching in the solid state, similar to certain luminescent organic molecules.[] To overcome this issue, some studies have focused on inhibiting the aggregation-caused quenching (ACQ) of luminescence. Further, many effective methods, such as the dispersion of CDs in a solid matrix,[] surface passivation, or postfunctionalization of CDs, have been developed for realizing luminescence from CDs in the solid state.[] Although these methods can significantly enhance the optical properties of CDs, the poor dispersion of the carbonized polymer dots (CPDs), and the effect of different matrices on the photoluminescence (PL) characteristics limit their application to some extent.
A CPD, which is a polymer/carbon hybrid material, can suppress the ACQ to a certain extent. Yang and coworkers[] prepared a CPD that provided a solid-state PL quantum yield (QY) of 8.5% using maleic acid and ethylenediamine as raw materials via a microwave-assisted method. A crosslink-enhanced emission (CEE) effect was proposed for the first time to clarify the reason for the enhanced PL intensity.[] Liu and coworkers[] prepared a poly(vinyl alcohol)-based nitrogen-doped CD that exhibited strong yellow-green luminescence in the solid state by one-pot hydrothermal treatment of poly(vinyl alcohol) and ethylenediamine. In our previous study, we demonstrated that CPDs could self-assemble into hyperbranched fractal nanocarbons (HFNs) with hyperbranched morphology due to their low surface ζ potentials. Both the CPDs and the HFNs exhibited bright luminescence in the solid state with absolute QYs of 8.0 and 11.7%, respectively. To date, the number of CPDs that show solid-state luminescence is limited.
Further, it is necessary to fabricate CDs that emit white light in the solid state for developing white solid-state light-emitting diodes (LEDs). Most CDs generally emit light of a single color (e.g., blue, green, yellow, or red).[] The emission wavelength of CDs can be regulated by tuning the particle size or surface groups.[] For achieving white light emission from CDs, strategies such as the mixing of red, green, and blue CDs,[] or blue and yellow CDs,[] have been reported. However, the main drawback of this method is the imbalance of the luminescence color due to the mismatched QYs and radiative reabsorption.[] Recently, some CDs that exhibit white light emission in solutions were developed.[] For example, Shen and coworkers[] prepared CDs that emit white light in both the solid and solution states with a QY of ≈2%. Due to the potential applicability of CDs showing white light emission in the solid state as phosphors in white LEDs, it is highly desirable to prepare white-light-emitting CDs with high QYs in the solid state.
Herein, we report a CPD that emits white light in the solid state with a QY of 7.5%. Melamine was selected as a nitrogen-rich dopant due to the presence of sp3- and sp2-hybridized carbon species and amino groups, which could lead to multiple cluster states. By varying the reaction time, uncarbonized polymer floccules (PFs), partially carbonized CPDs, or highly carbonized CPDs were obtained using trimethylolpropane tri(cyclic carbonate)ether (TPTE) as the precursor via a one-step solvothermal route. High-resolution (HR) transmission electron microscopy (TEM) revealed that the carbon core of the CPDs was surrounded by a large number of uncarbonized polymers. This structural feature restricted the ACQ effect. In addition, based on their solid-state white light emission properties, the as-prepared CPDs were used as a phosphor to successfully realize a white solid-state LED.
Results and Discussion
A new type of CPD was prepared using TPTE and melamine as precursors at a stoichiometric ratio via a solvothermal reaction at 230 °C for 12 h (Scheme ). Melamine not only provides a CN bond, but also reacts with the cyclic carbonate groups of TPTE to form hydroxyurethane structures. As a result, CPDs with abundant heteroatomic groups on the surface are formed. To investigate the formation process and further evolution of the CPDs, the carbonization of the precursors was also conducted for 6 and 18 h at 230 °C. TEM and HRTEM images (Figure ) revealed that the structure, morphology, and particle size of the resultant CPDs could be changed significantly by tuning the carbonization time. As shown in Figure , no lattice fringes were observed in the HRTEM of PFs obtained at 6 h, suggesting that carbonization did not occur after reaction for 6 h. The material obtained after an extended carbonization time of 12 h (denoted as CPD-12) showed scattered dandelion-like structures (Figure ) and some carbonized cores with lattice fringes (spacing: ≈0.21 nm) wrapped by amorphous polymer chains (Figure ). As the reaction time was increased further to 18 h, CPDs were obtained (denoted as CPD-18), and the HRTEM image showed clear lattice fringes with a spacing of ≈0.21 nm, corresponding to the [100] facet of graphitic carbon. The [002] interlayer distance of the graphitic structure was further confirmed by X-ray diffraction (XRD) analysis. The XRD patterns of both CPD-12 and CPD-18 showed a broad peak centered at 2θ of 23° (0.34 nm) (Figure ). In addition, CPD-18 showed a stronger diffraction peak than that of CPD-12, indicating a higher degree of carbonization. This result suggests that the shell layer carbonized further, resulting in the formation of regular spheres with an average diameter of 4.2 nm (Figure ); this indicates that the material carbonized gradually with an increase in the reaction time.
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The as-prepared PF, CPD-12, and CPD-18 were characterized by Fourier-transform infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS) to further explore the change in the structure. FT-IR spectra of PF, CPD-12, and CPD-18 exhibit peaks corresponding to the stretching vibrations of OH and NH (3661 and 3365 cm−1), symmetric (2971 and 2900 cm−1) and asymmetric stretching vibrations of CH (1453 cm−1), stretching vibrations of the CO bond of carbamate (1735 cm−1), stretching vibrations of CN (1393 cm−1), and asymmetric stretching vibrations of CO (1065 cm−1) (Figure ). The characteristic peak of the five-membered cyclic carbonate at 1790 cm−1 completely disappeared for PF, CPD-12, and CPD-18, and the characteristic peaks of carbamate appeared at 1735, 1393, and 1250 cm−1, indicating that the five-membered cyclic carbonate moieties reacted with the amino groups of melamine to form carbamate groups at 230 °C. Surprisingly, the peak of triazine was very weak at 1547 cm−1 and even disappeared in the spectrum of CPD-18. Further, elemental analysis was conducted to clarify the structural changes. It was found that the relative content of N was significantly reduced from the theoretical value of 15.00–6.87 wt% with an increase in the carbonization time from 6 to 12 and 18 h (Table S1, Supporting Information), indicating that the deamination reaction occurred during the carbonization process. Meanwhile, the intensity of the CN stretching vibration in the FT-IR spectrum increased with increasing reaction time, as shown in Figure ; this result confirms that the ring-opening of triazines occurred, resulting in the release of ammonia and formation of CN groups. Further, the surface composition of CPD-12 was investigated by XPS. In the survey scan (Figure ), C1s, N1s, and O1s signals are observed at 286.2, 399.4, and 532.1 eV, respectively. The high-resolution C1s XPS profile could be deconvoluted into six components corresponding to CC bond at 284.0 eV, CN bond at 285.2 eV, CO bond at 286.0 eV, CN bond at 286.8 eV, OCO bond at 287.8 eV, and HNCO bond at 289.2 eV (Figure ). The high-resolution N1s XPS signal could be fitted with three peaks at 399.2, 400.2, and 401.4 eV, corresponding to CN, OCN, and CNH groups, respectively (Figure ). The O1s spectrum shows two typical peaks at 532.0 and 533.1 eV for CO and CO bonds, respectively (Figure ).[] The XPS results are in good agreement with the FT-IR results.
Furthermore, to confirm that the surface of the carbon core was wrapped by a large number of polymer chains, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were conducted to study the thermal properties of PF, CPD-12, and CPD-18. As shown in Figure S1, Supporting Information, all three materials exhibited a three-step decomposition process with weight losses corresponding to the evaporation of water, decomposition of surface groups, and decomposition of the polymers and carbon cores.[] Due to the presence of a large number of oxygen atoms (Table S1, Supporting Information), the carbon core also decomposed with the decomposition of the poly(hydroxyurethane) moiety. The presence of polymer chains was further confirmed from the glass transition temperature (Tg) measured by DSC (Figure S2, Supporting Information). For PFs, two Tg values were observed at −30.9 and −14.1 °C, corresponding to the glass transition of linear and crosslinked poly(hydroxyurethane) units, respectively. For CPD-12, higher Tg values of −28.7 and −1.9 °C were observed because of further carbonization of crosslinked poly(hydroxyurethane), resulting in a decrease in the free volume of the peripheral poly(hydroxyurethane) units. As the carbonization degree increased, the rigidity of the structure was enhanced and a higher Tg of −15.1 °C was observed. Thus, the TGA and DSC results confirmed the presence of polymer chains on the surface of the carbon cores.
Previous studies have shown that most CDs possess large sp2 aromatic structures based on aromatic rings such as benzene and pyridine rings.[] Accordingly, regardless of the type of emission (molecular-state or core-state emission), most luminescence sites are derived from conjugated structures. Recently,[] many polymers that lack conventional conjugated luminophores have been shown to exhibit fluorescence. Examples include poly[(maleic anhydride)-alt-(vinyl acetate)],[] polyacrylonitrile,[] poly(ethylene glycol),[] and poly(hydroxyurethane).[] Clusterization-triggered emission is a widely recognized luminescence phenomenon in which clusters are formed via the aggregation of electron-rich groups such as N, S, O, CO, C N, and NHCOO.[] Such clusters are considered to be the emissive units; they generate delocalized electrons and further achieve emission.[] We have previously proposed a cluster-size distribution effect to explain the luminescence of crosslinked poly(hydroxyurethane), according to which CPDs form nonuniform clusters of different sizes and bandgaps.[] Based on this hypothesis, we propose that the luminescence of PF, CPD-12, and CPD-18 originates from clusters generated by the aggregation of CN and NHCOO on the surface of the carbon core and polymer chains. Due to different degrees of carbonization, the groups are aggregated in different ways and to different degrees, resulting in the generation of multiple clusters.
To further illustrate the type of clusters formed and the mechanism of clustering, PL spectra and time-resolved fluorescence decay curves were acquired. In a dilute ethanol suspension, PF, CPD-12, and CPD-18 exhibited blue fluorescence under 365 UV light, with excitation-dependent fluorescence (EDF) properties (Figure a-c and insets in Figure -c). As suggested previously,[] the EDF properties can be attributed to the generation of multiple clusters with different energy levels by the aggregation of heteroatomic groups such as CO, CO, CN, and NHCOO. This results in different emission colors and EDF characteristics. In ethanol, the PL peak maxima of all three materials are found to be located at 427 nm (Figure -c), indicating that their emission states in ethanol are derived from the same type of clusters. However, in the solid state, PF, CPD-12, and CPD-18 obviously showed different emission colors and PL spectra (Figure -f), despite their EDF properties. Uncarbonized PF showed a strong blue-green emission with a maximum emission peak at 439 nm and a shoulder peak at ≈459 nm when excited at 380 nm (Figure and inset). The former emission peak is attributed to the cluster formed by hydroxyurethane groups, according to our previous study,[] and the latter peak may be attributed to the cluster produced from hydroxyurethane and CN groups. For CPD-12 and CPD-18, the maximum PL peaks are located at 478 and 522 nm, respectively, indicating a clear red shift of the PL peak with increasing degree of carbonization (Figure ). In addition, the full widths at half maximum of CPD-12 and CPD-18 became broader, and white light emission was observed under 365 nm light irradiation using a UV lamp (inset in Figure ). The corresponding Commission International de L'Eclairage (CIE) coordinates calculated from the PL curves are located at (0.27, 0.30) and (0.30, 0.37), as shown in Figure ; both these belong to the white gamut. The PL spectra of solid PF, CPD-12, and CPD-18 (excitation wavelength: 360 nm) are compared in Figure . The emission intensity in the yellow region increased, whereas that in the blue region decreased with the increase in the carbonization time, indicating that the proportion of clusters with long-wavelength and short-wavelength emissions increased significantly with increasing carbonization time. Moreover, the two emission peaks located in the blue region did not disappear, indicating that the carbonization process induced the formation of additional clusters with short-wavelength emissions. As shown in the inset of Figure h, the blue-to-red spectral compositions of PF, CPD-12, and CPD-18 were calculated to be 67.6, 74.6, and 85.7%, respectively, which further demonstrates that a high carbonization degree could generate multiple cluster states, resulting in emissions over a wide range of visible light. Theoretically, the emission energy increases (i.e., blue shifts) with a decrease in the particle size, according to the quantum confinement effect.[] In contrast, the red-shifting and broadening of the emission peaks of CPD-12 and CPD-18 indicate that the luminescence originates from cluster states rather than core states. Here, the carbon core is considered to cause intense aggregation of the polymer chains and strong delocalization of electrons from heteroatoms or unsaturated double bonds (Scheme ). In detail, groups such as CN and hydroxyurethanes from polymer chains aggregated in a tight manner due to the restriction imposed by the carbon core. Meanwhile, the groups on the surface of the carbon core are involved in the formation of clusters, thus resulting in multiple clusters and the realization of long-wavelength emissions. In the UV–vis absorption spectra of the films of PF, CPD-12, and CPD-18, only broad absorption bands extending to the visible region rather than narrow absorption bands are observed, as shown in Figure S3, Supporting Information; this result indicates the existence of continuous multiple cluster states, not isolated emission states. However, quantitative analysis of these clusters is required to corroborate our hypothesis, which is however technically challenging.
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The absolute QYs of CPD-18 dispersed in ethanol and in the solid state were measured to be 8.3 and 1.4%, respectively; the low QY is related to the ACQ effect. In general, luminescence quenching in the solid state may originate from FRET or the self-absorption of the emitted photons, which is ascribed to particle aggregation in the solid state.[] Interestingly, the QYs of CPD-12 in the solution and solid states are as high as 25.1 and 7.5%, respectively, which might be attributed to two aspects. First, a relatively large number of poly(hydroxyurethane) chains that act as passivators are attached to the surface of the carbon core of CPD-12, and they resist the self-quenching of luminescence in the solid state. The thickness of the poly(hydroxyurethane) layer around the CPD-12 core was estimated to be ≈5–10 nm from the TEM image in Figure . Therefore, the distance between two cores would be 10–20 nm because of the steric hindrance of the poly(hydroxyurethane) chains. This value exceeds the acceptable separation for FRET, which is in the range of 1–10 nm.[] As a result, the self-quenching of luminescence in the solid state is restricted to a certain extent. Second, the carbon core plays a role in restricting the motion of molecular chains, thus enhancing the interactions between the heteroatomic groups of poly(hydroxyurethane) chains, as in the case of crosslinking or hydrogen bonding interactions.[] Therefore, we propose that multiple clusters of CPDs can be realized by carbonization. However, excessive carbonization leads to severe fluorescence quenching. Further, the time-resolved fluorescence decays of the materials in the solution and solid states were monitored under 405 nm excitation (Figure ). The average lifetime of PF in ethanol and in solid state was found to be 5.0 ns, indicating that the cluster-induced fluorescence had the same radiation transition rate in both the states. However, the average lifetimes of CPD-12 and CPD-18 in the solid state (CPD-12: 3.8 ns; CPD-18: 4.0 ns) were found to be shorter than those (CPD-12: 6.5 ns; CPD-18: 6.1 ns) in ethanol (Figure ), which is attributed to fluorescence quenching caused by an increased radiation transition rate. Further, the carbon cores in CPD-12 and CPD-18 probably induced the cluster to have ultrafast electron transfer rate in the solid state.
Based on the aforementioned analysis, we attribute the luminescence of CPDs to emissions induced by different types of clusters. Heteroatoms or groups with n or π electrons aggregate, leading to extended electron delocalization by through-space conjugation. This results in luminescence, as shown in Scheme . The formation of clusters is affected by the degree of carbonization and surface structures. For uncarbonized PF, clusters are generated only through interactions between the hydroxyurethane groups and CN groups of poly(hydroxyurethane) chains due to the lack of carbonization, resulting in low conjugation and a large bandgap.[] Thus, solid PF exhibits blue emission in the solid state. For CPD-12, carbonized regions emerge and induce the poly(hydroxyurethane) chains to be tightly packed and wrapped on the surface of the carbon core. In addition, heteroatomic groups distributed on the surface of the carbon core also participate in the formation of clusters. Therefore, through-space conjugation is enhanced and multiple clusters appear, and eventually, the different emission centers collectively lead to white light. In a previous study, Yang and coworkers proposed the CEE effect for the first time to explain the emission mechanisms of nonconjugated polymer dots or CPDs.[] Subsequently, the concept was extended to noncovalently bonded systems (such as those involving supramolecular interactions and ionic bonding) and confined domains.[] Similarly, the carbon core of CPD-12 acts as the crosslinking point for the poly(hydroxyurethane) chains and induces the aggregation of these chains. In the case of CPD-18, most poly(hydroxyurethane) chains are carbonized, and FRET begins to dominate, leading to a decrease in the luminescence intensity and QY.
White phosphor-based LEDs are widely used as energy-saving lighting devices because they exhibit superior characteristics over other lighting systems in terms of energy consumption, stability, and lifespan.[] CPD-12, which emits white light in the solid state, is an ideal candidate to be combined with the commercially available 365 nm UV InGaN chip for the fabrication of white LEDs (Figure ). We fabricated a white LED by combining CPD-12 with a UV chip. The LED based on CPD-12 emitted white light when the chip was powered with a voltage of 3.0 V (inset in Figure ), and the electroluminescence (EL) spectrum shows three distinct emission peaks located at 392, 424, and 560 nm (Figure ). The peak at 392 nm originates from light leaked from the UV chip, and the latter two peaks are derived from the emitted light of CPD-12, in agreement with the results of the PL spectrum. To further confirm the performance of the as-fabricated white LED, the CIE coordinates, correlated color temperature (CCT), and color rendering index (CRI) were determined (Figure ). The CIE coordinates are located at (0.332, 0.344), close to that of pure white light (0.333, 0.333) (Figure ), and the corresponding CCT value is 5519 K, corresponding to that of cold white light. Remarkably, the CRI is as high as 85, comparable with those of some CD-based white LEDs. Thus, the white LED based on CPD-12 sufficiently meets the requirements of the commercial standard.[] Moreover, the white LED exhibited excellent stability for 150 min (Figure ), which renders it promising for use in continuous tracking studies. Thus, it has important practical value.
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Conclusion
We developed a one-step route for the preparation of white-light-emitting CPDs with multiple cluster states using TPTE and melamine as precursors. A white LED with a high CRI of 85 was fabricated by combining CPD-12 as a single phosphor with a UV chip. Heteroatomic groups from poly(hydroxyurethane) units and carbon cores aggregated to form different types of clusters via spatial conjugation. Carbonization generates multiple clusters and induces strong electron delocalization to produce long-wavelength emissions. Our system provides a platform for combining nonconjugated luminescent polymers and CPDs. We expect that the multiple-cluster-induced emission theory can guide the development of multicolor CPDs in the future.
Experimental Section
Materials and Methods
TPTE was synthesized as described previously. Melamine, trimethylolpropane triglycidyl ether, and cetyltrimethylammonium bromide (CTAB) were purchased from Aladdin Chemical Co., Ltd. Ethanol was purchased from Sinopharm Chemical Reagent Co., Ltd. All the chemicals were of analytical grade and used without further purification.
UV–vis absorption spectra were recorded in a quartz cell on a Cary 100 Scan UV–vis spectrophotometer. FT-IR spectra were acquired using a Bruker TENSOR 27 spectrophotometer. XPS was carried out using an ESCALAB VG ESCALAB MARK II spectrometer using a monochromatic Mg Kα X-ray source for excitation (1253.6 eV). XPSPEARK version 4.1 was used to deconvolute the XPS spectra. The absolute PL QY was measured using a QM40-LS spectrophotometer equipped with a calibrated integrating sphere. TEM images were obtained on a JEM-2100 microscope at an accelerating voltage of 200 kV. Ultrathin carbon-coated grids (Beijing XXBR Technology Co., Ltd.) were used as the support for TEM samples. PL spectroscopy was conducted at 25 °C using a QM40-LS spectrophotometer with a quartz cell (10 × 10 × 45 mm3) sample holder for measurements on solutions and a quartz plate (25 × 10 × 1 mm3) holder for measurements on solids. All the fluorescence emission and excitation spectra were recorded using a 3/3 slit width. XRD patterns were obtained using Cu Kα radiation (XRD-7000). Elemental analysis was carried out on an ELEMENTAR vario EL cube to determine the composition. The fluorescence decay curves were recorded on an EDINBURGH FLS980 transient fluorescence and phosphorescence spectrometer. The multidimensional time-correlated single-photon counting method was used to estimate the fluorescence decay lifetime. Spectra Scan PR655 was used to analyze the spectra, CIE coordinates, CCT, and CRI of the white LEDs. DSC was carried out on a TAQ200 instrument (New Castle, DE) at the scanning rate of 10 °C min−1 under a N2 atmosphere, and the data from the second heating curve were collected. TGA was carried out on a Perkin-Elmer Pyris 1 instrument under a N2 atmosphere from room temperature to 800 °C at the heating rate of 10 °C min−1.
Preparation of PFs and CPDs
PFs, CPD-12, and CPD-18 were synthesized as follows: a mixture of melamine (87.0 mg, 0.69 mmol), TPTE (290.0 mg, 0.67 mmol), and ethanol (10 mL) was loaded into a sealed 25 mL Teflon-lined stainless autoclave and heated at 230 °C for 6, 12, or 18 h. After that, the mixture was cooled to room temperature, and the resultant suspension was filtered to remove the unreacted melamine, and ethanol was removed by vacuum distillation.
Fabrication of White LEDs
An InGaN-based 365 nm UV LED chip was selected as excitation source. CPD-12 was filled into a cup-shaped optical lens. Then, the optical lens was firmly fixed on top of the LED chip to obtain the white LEDs.
Acknowledgements
The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (52003254 and 51973190), the Natural Science Foundation of Shanxi Province (201901D211262, 201901D211282, and 201901D211225), the Science Foundation of North University of China (XJJ201925 and XJJ201819), the State Key Laboratory of Motor Vehicle Biofuel Technology (KFKT2020-002), the MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Zhejiang University (2020MSF01), and the Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi (2019L0597).
Conflict of Interest
The authors declare no conflict of interest.
Data Availability Statement
Research data are not shared.
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You have requested "on-the-fly" machine translation of selected content from our databases. This functionality is provided solely for your convenience and is in no way intended to replace human translation. Show full disclaimer
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Copyright John Wiley & Sons, Inc. 2021
Abstract
Despite the advancements in synthetic methods, it is still challenging to prepare white light‐emitting carbon dots for solid‐state white light‐emitting diodes (LEDs). Herein, the synthesis and characterization of a carbonized polymer dot (CPD) that emits white light in the solid state are presented. The reaction of an oxygen‐rich precursor, trimethylolpropane tri(cyclic carbonate) ether, with nitrogen‐rich melamine in an alcohol via a single‐step solvothermal method affords CPDs with hydroxyurethane and CN groups. The morphology of the CPDs can be regulated by changing the reaction time. Transmission electron microscopy reveals structural evolution from flocculent polymers to dandelion‐like and spherical particles with an increase in the carbonization time from 6 to 12 and 18 h, respectively. The dandelion‐like CPDs exhibit a relatively high quantum yield of 7.5% in the solid state, which is ascribed to the abundant surface poly(hydroxyurethane) chains that restrict the aggregation‐caused quenching of luminescence. It is proposed that multiple coexistent clusters generate different emission sites, thereby leading to white light emission. Loose clusters formed from hydroxyurethane and CN bonds, which have a low degree of conjugation, emit blue light, whereas compact clusters generated through interactions between the hydroxyurethane and CN bonds of poly(hydroxyurethane) and carbon cores emit yellow light.
You have requested "on-the-fly" machine translation of selected content from our databases. This functionality is provided solely for your convenience and is in no way intended to replace human translation. Show full disclaimer
Neither ProQuest nor its licensors make any representations or warranties with respect to the translations. The translations are automatically generated "AS IS" and "AS AVAILABLE" and are not retained in our systems. PROQUEST AND ITS LICENSORS SPECIFICALLY DISCLAIM ANY AND ALL EXPRESS OR IMPLIED WARRANTIES, INCLUDING WITHOUT LIMITATION, ANY WARRANTIES FOR AVAILABILITY, ACCURACY, TIMELINESS, COMPLETENESS, NON-INFRINGMENT, MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Your use of the translations is subject to all use restrictions contained in your Electronic Products License Agreement and by using the translation functionality you agree to forgo any and all claims against ProQuest or its licensors for your use of the translation functionality and any output derived there from. Hide full disclaimer
Details
1 State Key Laboratory of Motor Vehicle Biofuel Technology, Nanyang, P. R. China
2 MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou, P. R. China
3 School of Energy and Power Engineering, North University of China, Taiyuan, P. R. China
4 MOE Key Laboratory of Interface Science and Engineering in Advanced Materials, Research Centre on Advanced Materials Science and Technology, Taiyuan University of Technology, Taiyuan, P. R. China