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As a result of hydrogen-band interactions, blue-light-emitting luminescent carbon dots (CDs) synthesized by one-pot hydrothermal treatment were successfully assembled into mesoporous aluminas (MAs). Blue-light-emitting CDs/MAs compounds with mesoporous structures, narrow pore size distribution, high thermal stability, and large surface areas were obtained. Furthermore, the obtained CDs/MAs compounds possessed high sensitivity and selectivity for oxygen, and excellent photochemical stability. The assembled compounds can potentially be applied to oxygen sensing.
© 2017 Optical Society of America
1. Introduction
Monitoring the gas concentration is critical in many industrial fields and in daily life [1,2]. Typical applications of gas sensors have safety issues with regard to the detection of toxic or explosive gases in industrial processes and the supervision of air quality [3, 4]. The general sensing principle of gas sensors is based on the interactions between gas molecules and the surfaces of solid materials [5]. This interaction affects both sensitivity and reaction/regeneration time. Most gas sensors require very low amounts (sub-milligrams) of sensor-active materials [6]. Meanwhile, solid materials with large surface-to-volume ratios are naturally particularly promising for use as a matrix [7,8]. Therefore, current research areas in the gas sensor field focus on luminescent molecules and new functional solid materials [9, 10]. Much of the work so far has demonstrated that mesoporous materials still own a well-defined surface property [11, 12].
According to the International Union of Pure and Applied Chemistry (IUPAC), mesoporous materials are classified as porous materials if their pore sizes between 2 nm and 50 nm [13]. Considerable research efforts have been devoted to mesostructured materials. The first successful synthesis of mesoporous molecular sieves launched a new era in the investigation of inorganic molecular sieves [14]. Subsequently, ordered mesoporous silica materials were first reported in 1992 [15]. Somorjai et al. reported on the synthesis of ordered mesoporous alumina (MAs) through a sol-gel route [16]. Yan et al. presented an easily accessible, reproducible, and high-throughput method to synthesize highly ordered MAs [17]. Compared with conventional mesoporous solid materials, alumina is a type of inorganic materials with one of the largest volume and a well-known catalyst support firstly. Pure alumina has low surface areas at temperature calcinations between 700 K and 1000 K [18], MAs present high thermal stability, large surface areas, narrow pore-size distribution, and tunable pore sizes over a wide range [17, 19]. As a result of their large surface areas and specific gas diffusion phenomena within the pores, it is feasible that the detection of gas molecules possesses high sensitivity under low-concentration conditions. Generally, materials with higher specific surface areas exhibit higher sensitivities, which indicate their promising application in gas sensors [20].
Luminescent carbon dots (CDs) have been regarded as fluorescent carbon materials, evenly. CDs have attracted widespread attention in many fields such as bioimaging, biosensing, and optoelectronic devices [22–24], owning to their chemical inertness, widely available precursors, eco-friendly preparation, low photobleaching, low toxicity, and excellent biocompatibility [21]. CDs usually have photoluminescent quantum yields (QYs) lower than 1.0%, and significant self-quenching in the aggregation state, which hamper their applications [25, 26]. Thus, it is essential to discover an ideal luminescent CD-based solid material, to prevent the phonon quenching effect in the solid state.
In this work, we first synthesized ordered MAs through a simple sol-gel route. The MAs had high thermal stability even at 700 °C, large surface areas, and narrow pore sizes. Then, luminescent CDs were produced through a one-pot hydrothermal treatment of polyvinyl alcohol (PVA) and ethylenediamine (EDA). Interestingly, on the basis of the hydrogen-band interaction, CDs can be successfully assembled into MAs by a simple stirring process. In oxygen sensing applications, sensing tests of the obtained CDs/MAs compounds were presented at different water vapor (H2O), carbon dioxide (CO2), oxygen (O2), and nitrogen (N2) volume fractions. As a result, the obtained CDs/MAs compounds can maintain outstanding luminescent stability, high selectivity, and sensitivity to oxygen, and exhibit a reversible oxygen sensing signal. This work opens new avenues for oxygen-sensing materials.
2. Experimental method
Materials and chemical reagents
Tetrahydrofuran (THF), ethylenediamine (EDA), diethylenetriamine (DETA), tetraethylenepentamine (TEPA), polyvinyl alcohol (PVA), acetone, dimethyl formamide (DMF) and methanol, ethanol (99.7%) were purchased from Guangdong Guanghua Technology Co. Ltd. All solvents are analytical grade. The degree of polymerization of polyvinyl alcohol (PVA) is 1750 ± 50 (molecular weight: 77000 ± 2200). Pluronic P123 (EO20PO70EO20, Maverage = 5800) was purchased from Aldrich Ltd. Acetic acid (CH3COOH), anhydrous citric acid (99.5%, AR), and hydrochloric acid (HCl) were purchased from Aladdin Shanghai, China. Nitrogen (99.999%) and water vapour (99.999%) were purchased from Guangzhou Shenyin Gas Co. Lit. Carbon dioxide (99.999%) and oxygen (99.999%) were purchased from Guangdong Huate Gas Co. Ltd. De-ionized water was used throughout this work.
Synthesis of MAs and CDs/MAs compound
- 1. Synthesis of MAs: Fig. 2 shows the preparation process for mesoporous aluminas (MAs). According to a reported method [17], 1.0 g of Pluronic P123 was dissolved in 15 mL of ethanol at room temperature, and transparent liquid was obtained. Then, anhydrous citric acid was added to the solution with vigorous stirring [26]. Next, 37% (wt.) hydrochloric acid and glacial acetic acid were regulated to a pH of 3-4. 10 mmol of aluminum isopropoxide was added immediately to the mixture with vigorous stirring at room temperature for 10 h. The prepared sample was transferred to a water bath at 40 °C by stirring. A transparent sol was obtained. The transparent sol was placed in an atmosphere of 60 °C to undergo solvent evaporation. Two days later, white solid-state powders were collected. The calcination process was carried out by slowly increasing the temperature from room temperature to 400 °C (a ramping rate of 1 °C min−1), and then heating at 400 °C for 4 h in air. After that, a high-temperature treatment at 700 °C was carried out in air for 2 h with a temperature ramp of 10 °C min−1.
- 2. Assembled CDs/MAs compound: The method to obtain carbon dots (CDs) was based on a recent study [27]. Figure 2 reveals the preparation process for the CDs/MAs compound. In a typical procedure [28], 0.1 g of MAs was mixed with different amounts of CDs solution, and the mass ratio of CDs/MAs was adjusted to 2.5%, 5%, 10%, 15%, 20%, 25%, 30% and 35%. After stirring for 5 h, the compounds were with centrifugation for 15 min (5000 rpm min−1). The obtained precipitation was filtered with a Buchner funnel and dried at 333 K in an oven.
Characterizations
The UV-Vis absorption spectra were recorded with an ultraviolet-visible spectrofluorometer (UV-2550, Shimadzu). The crystal structure of the samples was analyzed by small-angle powder X-ray diffraction (SAXRD, Rigaku) with Cu Kα radiation (λ = 1.5418 Å). The data were collected in a 2θ range from 0.6° to 4° at a scanning step of 0.02° and a scanning rate of 0.2° min−1. The particle morphology was analyzed using a scanning electron microscope (SEM, XL-30; Philips, North Billerica, MA) and a transmission electron microscope (TEM, TECNAI12, Holland). The infrared spectrum was recorded on a Fourier spectrophotometer equipped with an integrating sphere, using KBr as a reference. Nitrogen physisorption isotherms were measured using a Micromeritics ASAP 2020HD88 system. The pore size distributions were calculated from the discrete Fourier transform (DFT) pore size distribution. Photoluminescence (PL) spectra were obtained with a Hitachi F-7000 fluorescence spectrophotometer equipped with a monochromator (resolution: 0.2 nm) and a 150-W Xe lamp as the excitation source, with the emission at 353 nm as the excitation wavelength. X-ray photoelectron spectroscopy (XPS) was conducted on a Kratos AXIS ULTRA DLD.
3. Results and discussion
In line with the experimental materials and chemical reagent analysis, the FT-IR spectra of the CDs, MAs and the CDs/MAs compound verify the existence of abundant -OH and -NH2, as shown in Fig. 1(a), because of the strong and broad stretching vibration (νs) at 3600-3050 and 1220-1020 cm−1. The surface of the CDs and the CDs/MAs compound connect large amounts of polyvinyl alcohol (PVA) chains owing to the C-H (νs) at approximately 2950-2850 cm−1 and -OH [27]. The bands at approximately 620 cm−1 and 742 cm−1 indicate Al-O (νs) [29]. These results support the existence of Al-O-Al in the inorganic framework. Furthermore, the characteristic bands at approximately 1395 cm−1 and 1462 cm−1 indicate the existence of polythylene oxide (PEO) groups and PEO groups that belong to the hydrophilic surfactant [30]. In addition, the XPS data confirms that these are the carbon, oxygen, and aluminium elements in the MAs, as shown in Fig. 1(b). The XPS analysis further demonstrates the above conjecture.
By combining Fig. 1 and Fig. 2, we can speculate on the forming mechanism of the CDs/MAs compound. Firstly, PEO groups that belong to the hydrophilic surfactant, the hydrophilic surfactant acts as a structural guide and the surfactant agent forms a fibrous mesoporous structure. Hydrogen bonding occurs between the surface oxygen atoms of the PEO chain and the surface hydroxyls of reactants. The surfactant agent assembles into reactants forming the precursor of the MAs. Then, the surfactant agent is removed by calcinations at high temperature. The ordered MAs are obtained with p6mm hexagonal symmetry, which is consistent with the TEM observation in Fig. 4(a). On the basis of the FT-IR spectra, the forming mechanism of the CDs/MAs is attributed to hydrogen bonding interactions in Fig. 2(b).
Fig. 2 (a) Schematic illustration for preparation of CDs/MAs compound. (b) Schematic drawing of carbon dot and MAs with functional hydroxyl surface.
The relevant characteristics of the MAs have been demonstrated. Figure 3(a) shows the small-angle X-ray diffraction (XRD) patterns of the MAs and the CDs/MAs compounds. We observe the very obvious diffraction peak at approximately 0.5, illustrating a worm-hole structure after high-temperature calcinations [31]. As we can see in Fig. 4(a), the materials exhibit a mesoporous structure of p6mm hexagonal symmetry with high thermal stability. Evidence for the formation of the γ-Al2O3 phase is provided by wide-angle XRD patterns in Fig. 3(b), owing to the distinctive peaks of the γ-Al2O3 phase (37°, 46°and 67° are seen from JCPDS Card No. 11-0661) [32]. With the increase in mass fractions of CDs, all samples appear in the γ-Al2O3 phase. The combination of the small- and wide-angle XRD data convinces us that mesoporous γ-alumina is ordered with crystalline walls.
Fig. 3 (a) Small-angle and (b) wide-angle XRD patterns of as-synthesized MAs, and x CDs/MAs (a; b, x = 2.5%; c, x = 5.0%; d, x = 10%; e, x = 15%; f, x = 20%, respectively).
Fig. 4 (a) TEM image of as-synthesized MAs. (b) TEM image of as-synthesized CDs/MAs compound, and HRTEM image of a carbon dot (inset). (c) TEM image of CDs coating from aqueous solution. (d) SEM image of as-synthesized CDs/MAs compound.
Figure 4(a) shows a TEM image of the as-prepared MAs. The wormhole mesoporous structures are retained after high-temperature calcination. It is clear that the sample processes uniform and hexagonally ordered pores with a pore size in the range of 8 to 10 nm with worm-like pore morphology. A TEM image of the CDs/MAs compound indicates several CD-like dots with pore sizes in a range from 5 to 6 nm in Fig. 4(b). The sizes match the average diameter of the CDs [Fig. 4(c)]. CDs dispersed in water with uniform spherical particles, and the average size of the CDs, are approximately 5 nm, as shown in Fig. 4(c).
For further validation, an even closer observation [inset in Fig. 4(b)] reveals high crystallinity with a lattice fringe distance of 0.254 nm, corresponding to that of the (100) facet of graphitic carbon [33]. An SEM investigation [Fig. 4(d)] presents a structure of samples with many amorphous wrinkles, but the surface of wrinkles is not observed the existence of the CDs. This result further demonstrates that the CDs can be successfully assembled into the pores of the CDs/MAs compound.
As the mass ratio of the CDs/MAs increases, the pore-size distribution curves of the CDs/MAs can be seen from Fig. 5(a). Narrow pore-size distributions are maintained in a range of mesoporous pore sizes. The nitrogen adsorption-desorption isotherms [Fig. 5(b)] of the CDs/MAs compound represent yield type-IV curves with H1-shaped hysteresis loops, indicating their mesostructures and cylindrical pores [34].
Fig. 5 (a) Pore-size distribution curves. (b) Nitrogen adsorption-desorption isotherms of surfactant-extracted MAs and CDs/MAs (2.5%, 5%, 10%, 15%, and 20%), respectively.
Table 1 summarizes the Brunauer-Emmett-Teller (BET) surface area, pore size, and pore volume of the pure MAs and the CDs/MAs compound under different mass ratios. The obtained MAs compound possess a large BET surface area of 409 m2 g−1, a wide pore volume of 0.54 cm3 g−1, and a narrow pore size of 7.6 nm. As the mass ratios of the CDs/MAs increase, the BET surface area, pore size, and pore volume all decrease. Uniform CDs can still enter these pores, and the obtained CDs/MAs compound may be successfully synthesized. The pore size is smallest when the mass ratio of the assembled compounds is 15%, but it still displays a high BET surface area of 205 m2 g−1, wide pore volume of 0.21 cm3 g−1, and narrow pore size of 3.2 nm. By comparison, the CDs/MAs (15%) assembled material possesses the maximum doping of the CDs. The large surface areas and narrow pore-size distributions of the CDs/MAs compound underscore their potential applications in gas sensors [35].
Table 1. Textural data of MAs and the CDs/MAs compound with different mass ratios.
The QY of the CD solution is approximately 33% from calculations based on [36]. The relevant optical properties of the CDs/MAs compound were investigated by photoluminescence (PL) spectra detection. The CDs solution emits bright blue light under UV light, as can be seen from the inset of the Fig. 6(b). In addition, Fig. 6(a) demonstrates the characteristic emission (red line) and excitation (black line) spectra of the assembled CDs/MAs material with 353-nm excitation and 418-nm emission wavelengths respectively under pure nitrogen atmosphere. As shown in Fig. 6(b), the emission spectra of the CDs/MAs with different relative concentrations (mass ratio: 2.5%, 5.0%, 10%, 15%, 20%, 25%, 30%, and 35%) under 353-nm excitation are exhibited in pure nitrogen. As the mass ratios of the CDs/MAs compound increase, the luminescence intensities clearly increase as well, but the mass ratio of CDs/MAs is up to 15% or evenly exceeding the point, which inevitably causes a decrease in the luminescence intensity. Thus, the most optimal condition of CDs/MAs is 15%. This is consistent with the observation from Table 1.
Fig. 6 (a) Excitation (black line, em: 418 nm) and emission (red line, ex: 353 nm) spectra of the CDs/MAs sample under pure nitrogen atmosphere. (b) Emission spectra of the CDs/MAs composites with different mass ratios in pure nitrogen atmosphere (ex: 353 nm). Inset: Photographs of CDs solution in daylight (left) and UV light of 365 nm (right).
MAs can be applied in oxygen sensing, involving high surface areas and narrow pore sizes. In order to verify our findings, Fig. 7 shows that the relative fluorescent intensity is stable in pure oxygen gas or pure nitrogen, but drops rapidly when switching between pure nitrogen and pure oxygen. The response times of the CDs/MAs compounds are relatively short, which illustrates that the assembled materials have extremely high sensitivity to oxygen. Stable features can be attributed to the highly ordered channel structure of mesoporous carriers, which can also effectively promote gas diffusion [37].
Fig. 7 Relative intensity and response time varieties for CDs/MAs compound during switching between 100% nitrogen and 100% oxygen, (a, 2.5%; b, 5%; c, 10%; d, 15%; and e, 20%, respectively).
The high selectivity to oxygen of the assembled compounds is further demonstrated in Fig. 8(a), 8(b), and 8(c). The fluorescence intensity of the CDs/MAs (20%) changes slightly at water vapor (H2O) or carbon dioxide (CO2) with various volume fractions, as shown in Fig. 8(a) and 8(b). As shown in Fig. 8(c), the fluorescence intensity of the CDs/MAs compound clearly decreases when the volume ratio of oxygen (O2) and nitrogen (N2) is tuned from 0 to 100% in intervals of 10% of oxygen. There is no obvious change in the spectrum shape or peak position. Furthermore, with the oxygen volume increasing from 0 to 50%, the fluorescence intensity has clear tendency to decline. It illustrates that the CDs/MAs compound has effectively sensibility to oxygen. Nevertheless, the emission intensity does not vary rapidly under oxygen volumes of 50% or 100%. We suppose that the number of oxygen molecules entering the pore network is limited owing to physical hindrances and constraints in which loaded CDs block the network. In view of the latter case, the CDs/MAs compound has no response to nitrogen. Thus, the assembled materials have significant potential applications in oxygen sensing [38].
Fig. 8 (a), (b), and (c) Emission spectra of the CDs/MAs (20%) compound at O2, H2O, and CO2 atmosphere with various volume fractions (λex = 353 nm). (d) Typical luminescence intensity-based Stern-Volmer plots for the CDs/MAs compound with different mass ratios at various O2 volume fractions atmosphere. Solid lines present the best fit using the two-site model Eq. (2).
As demonstrated in Fig. 8(d), I0/I is the fluorescence intensity ratio of different CDs/MAs sample at different oxygen volumes atmosphere from 0 to 100% in intervals of 10% of oxygen and 100% of nitrogen. In addition, I0/I is introduced to distinguish between the effects of different oxygen molecules in homogeneous media and the effects of negligible matrices on the extent of emission intensity quenching of the assembled compounds. In an ideal solution, the fitting curve of I0/I or τ0/τ scattered points should be a straight line with a slope of KSV. This relationship is applied to the Stern-Volmer (SV) equation [39]:
where I and τ denote the luminescence intensity and the decay lifetime. 0 denotes no oxygen. KSV is an SV constant. k is the bimolecular rate constant, representing the effective collision between the luminophore and the quencher, and PO2 describes the partial pressure of oxygen at 1 atm of pressure. However, our experimental data tends to result in a down curved nonlinear curve. This occurs because the luminescent molecules are not uniformly distributed in the pore channels of the MAs. Specifically, luminescent molecules enter various sites with diverse KSV and τ0 values. This nonlinear relationship cannot strictly conform to the SV equation, but the SV plots of the assembled compounds may be well fitted by using a Demas two-site mode [40].where f01 and f02 reveal the degree of contribution of luminescent intensity of the luminescent molecules located at two different sites. KSVi indicates the quenching constants for the two sites.Based on the result analysis in Fig. 7, we demonstrated that the emission intensities of the assembled samples boast a reversely rapid response time and recovery time during the conversion of pure oxygen and pure nitrogen. Specifically, a response time of 90%, i.e., t↓ (90%, N2 → O2), refers to the demanded time in which the luminescent intensity of the CDs/MAs compound is reduced to 10% from 100% to 0 nitrogen process. A 90% recovery time, i.e., t↑ (90% O2 → N2), is defined as the time required for the luminescent intensity of the compounds to be restored to its original 90% from 0 to 100% nitrogen [41].
The values of the response time (I0/I100), t↓, t↑, parameters of the SV model and Demas model, and the oxygen-quenching fitting parameters of the MAs/CDs compounds are listed in Table 2.
Table 2. Values of response time (I0/I100), 95% response time (t↓), 95% recovery time, Stern-Volmer model and Demas model oxygen-quenching fitting parameters of MAs/CDs compounds.
In conclusion, the assembled materials have relatively high sensibility for oxygen, and the oxygen-quenching fitting parameters of the Demas model are more accurate than the Stern-Volmer model. Thus, the assembled materials can be potentially applied in oxygen sensing.
4. Conclusions
We have demonstrated the systematic fabrication of highly ordered MAs with high thermal stability, narrow pore-size distribution, and large surface areas. In addition, synthesized blue- emitting CDs can be successfully dispersed in the pores of the MAs by a simple and effective strategy. Assembled CDs/MAs compounds with different mass ratios with blue emission under UV light were obtained. MAs are with large surface-to-volume ratios, indicating their excellent advantage in oxygen sensing. In addition, this assembled material has extremely high sensitivity and selectivity to oxygen. Furthermore, the response time is relatively short at approximately 4-8 s. We propose that this research can open the new avenue in oxygen-sensing materials.
Funding
This work was supported by National Natural Science Foundation of China (NSFC) (51203053, 21571067); Guangzhou Science and Technology Program Project (GSTP) (201605112017209); Guangdong Natural Science Foundation (GNSF) (S2013030012842).
Acknowledgments
Authors thank Prof. P. Smet for his comment and check of the manuscript.
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