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In situ thermal synthesis of molybdenum oxide nanocrystals in thermoresponsive microgels
Colloids and Surfaces A 563 (2019) 130–140
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In situ thermal synthesis of molybdenum oxide nanocrystals in thermoresponsive microgels
T
Jing Penga, Dongyan Tanga, , Shuyue Jiaa, Yue Zhangb, Zhaojie Suna, Xu Yanga, Hongyun Zoua, Haitao Lva ⁎
a b
School of Chemistry and Chemical Engineering, Harbin Institute of Technology, No. 92, Xidazhi Street, Nangang District, Harbin City, Heilongjiang Province, China School of Foreign Languages, Harbin Institute of Technology, No. 92, Xidazhi Street, Nangang District, Harbin City, Heilongjiang Province, China
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Thermoresponsive Poly(N-vinyl caprolactam) Hybrid microgel In situ Molybdenum oxide
In situ formation of nanocrystals within thermoresponsive microgels has become a significant technology in the fields of catalysis and biomedicine to fabricate multifunctional hybrid nanostructures. Usually, the room temperature or cooler was set to control the formation of nanocrystals using microgels as template in such process. Here, the relatively higher temperature was used to synthesize molybdenum oxide nanocrystals where the poly (N-vinyl caprolactam) microgels were found to work as both the stabilizer and the template. Specifically, ethanol was added in the solution of the microgels to raise their volume phase transition temperature (VPTT). Later, a modified hydrothermal process was performed at 70 °C with precursor molybdic acid concentrated in the microgels matrix through the hydrogen bond between molybdic acid and N-vinyl caprolactam units. 2D nanoflakes, nanorods and nanoplatelets of molybdenum oxide were successfully synthesized. Specially, the microgels with the crosslinked degree of 2% exhibited well hybrid with controlled sizes and ideal confine of the molybdenum oxide nanoplatelets within microgels, along with strong photoluminescence intensity. These results emphasized the feasibility of poly(N-vinyl caprolactam) microgels as template and stabilizer at high temperature and provided a novel synthesis strategies for hybrid microgels applicable in wide areas of nanotechnology from catalysis, sensing to therapy.
⁎
Corresponding author. E-mail address: [email protected] (D. Tang).
https://doi.org/10.1016/j.colsurfa.2018.11.065 Received 26 September 2018; Received in revised form 26 November 2018; Accepted 27 November 2018 Available online 28 November 2018 0927-7757/ © 2018 Elsevier B.V. All rights reserved.
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1. Introduction
nanocrystals. On the other hand, poly(N-vinyl caprolactam) was recently mentioned that it exhibited a certain extent of coordination and weak reducibility in the formation of Au nanoparticles, both structurally and functionally similar to PVP [22,23]. It revealed that poly(N-vinyl caprolactam) microgels could be valuable as macromolecular stabilizer and soft colloidal templates for the nucleation and the growth of inorganic nanocrystals under the relative higher temperatures. In this paper, relatively higher temperature than the VPTT of PVCL microgels in water was considered to realize a hydrothermal procedure to compose in-situ hybrids. And the roles of poly(N-vinyl caprolactam) microgels as stabilizer and templates on the fabrication of hybrids were investigated. Specifically, molybdenum oxide was chosen as synthesized targets for its excellent optical properties as 2D transition metal oxide materials. The volume phase transition temperature of poly(Nvinyl caprolactam) microgels in the mixed solution of water and ethanol was discussed firstly to open the gate of utilizing PVCL microgels at relatively higher temperature. Then various synthesis parameters such as concentrations of precursor and crosslinked degrees of microgels were explored in the modified hydrothermal procedure of PVCL-MoO3 hybrid microgels. Microstructures of hybrid microgels and elements valence states as well as the interactions between nanocrystals and polymer were investigated. Furthermore, electronic structures of nanocrystals after the hybrid have also been discussed. Our results indicated that the high temperature can be utilized when using thermoresponsive microgels as template for the in situ generation of nanocrystals, which would diversify and functionalize hybrid microgels in a range of fields from biomedicine to catalysis.
Hybrid of inorganic nanocrystals with polymer have become one of the most powerful strategies to adopt some special application, such as catalysis [1], sensor [2] and biomedicine [3,4]. Hybrid nanostructures can avoid some inherent defects of inorganic nanocrystals, such as toxicity, hard target and unavoidable aggregation. Besides, it also integrates inorganic nanocrystals with the advantages in optical, electronic and magnetic fields and polymer with stimuli responsivity, designability and solubility [5–7] into one system with synergy effects. These properties make hybrid nanostructures become popular in various research fields, such as photothermal therapy, photoluminescence imaging and optical biosensors. To fabricate such hybrid nanostructures, various methods have been well-developed. One of common strategies is utilizing functional polymer to decorate the surface of inorganic nanocrystals. For example, exchange of oleylamine on the MoOx nanoparticles with linear polyethylene glycol(PEG) could lead to an hydrophilic surface and good biocompatibility [8]. Interaction of amphiphilic polymers with ligands through hydrophobic effect could also realize the hydrophilicity and functionalization of CdSe/ZnS quantum dots (QDs) [9]. Similar applications also include that of block copolymers [10]. Except that, solvents exchange between organic and water has also been utilized to tag nanoparticles into polymeric matrix [11]. Besides, small molecules with vinyl groups also was used to stabilize nanocrystals in the hydrothermal synthesis and further copolymerized with NIPAM and MBA through precipitation polymerization to form hybrid microgels [12]. These methods have their remarkable advantages since inorganic nanocrystals are prepared before hybrid, along with ideal morphologies and uniform sizes, and would exhibit excellent optical, electrical and magnetic properties. However, some important issues in these cases still need more careful consideration, such as nonideal loading, uncontrollable loading process, or weak bonding between inorganic nanocrystals and polymer. Another alternative is introducing polymer during the synthesis process of nanocrystals. Linear polyethylene glycol(PEG) and poly(vinylpyrrolidone) (PVP) are two most frequently used polymer in the solvothermal method [1]. They usually play two important roles in the preparation of nanoparticles: (1) ligands, bonding to the NP surface; (2) reducing agent, reducing the valence state of metal elements. Besides, they also exhibit good hydrophilicity and biocompatibility, which enormously stimulate their application in the biomedicine. However, their simple structures and uneasy designability for the special target and stimuli-responsivity restrict their application in the biomedicine area. In addition to linear polymer, microgels have also been reported as micro/nanoreactors for the in situ formation of nanoparticles. From the view of structure, microgels has irreplaceable advantages as template/ reactor for its highly swelling characteristic and crosslinked matrix [13]. Especially, thermoresponsive microgels, such as poly(N-vinyl caprolactam) and poly(N-isopropylacrylamide) (PNIPAM), could exhibit enhanced application value for their stimuli responsivity and low toxicity. For example, poly(N-vinyl caprolactam-co-acetoacetoxyethylmethacrylate) microgels had been used as micro/nanoreactors to prepare a series of inorganic nanocrystals, like ZoO [14], Fe3O4 [15,16], Ag [17], LnF3 [18], ZnS [19]. Winnik et al. utilized fully neutralized methacrylic acid (MAA) groups in the core of poly(NIPAM/MAA/ PEGMA) microgels to confine the growth of EuF3 nanoparticles [20]. Johannes et al. introduced additional surfactant to control the morphology of Au in the thermoresponsive core-shell microgels from nanoparticles to nanorods [21]. These researches focus on the influences of coordination groups, charged groups, and surfactants on the formation process of inorganic nanocrystals. To our best knowledge, most of them were performed at the room temperature or lower, not ever over their volume phase transition temperature (VPTT) in aqueous solutions. It is well known that the high temperature would benefit both thermodynamic and kinetic control in the nucleation and growth process of
2. Experimental 2.1. Materials N-vinyl caprolactam (VCL) was purchased from Sigma Co. Ltd and purified through recrystallization in n-hexane; potassium persulfate (KPS), N,N-methylene bisacrylamide (MBA), sodium dodecyl sulphate (SDS), molybdenum disulfide(MoS2), hydrogen peroxide (H2O2), and sodium bicarbonate(NaHCO3) were all analytically pure and were purchased from Aladdin Co., Ltd. 2.2. Preparation of poly(N-vinyl caprolactam) microgels PVCL microgels were prepared through precipitation polymerization according to the reference [14]. A typical process described as follows: appropriate amounts of VCL monomer, MBA as crosslinker, SDS as stabilizer, and NaHCO3 as pH controlling agent (as shown in Table 1) were dissolved in 60 mL ultrapure water. The synthesis was performed in a triple-wall glass reactor equipped with temperature control and magnetic stirrer. The solution was incubated and heated to 70 °C under nitrogen purging for 30 min to form stable emulsion. And after that, a solution of KPS initiator solved in 3 mL ethanol was injected into the previous solution to initiate the polymerization. The reaction was carried out for 6 h. The samples were purified using dialysis bag (8000 -12 000 KDA) for 4 days and dried as solid sample for further purpose. Table 1 Recipes for the precipitation polymerization of poly(N-vinyl caprolactam) microgels with different crosslinked degrees. Samples
NVCL/g
MBA/mg
KPS/mg
Diaa/nm
PDIa
PVCL1 PVCL2 PVCL5
0.93 0.92 0.89
10.5 21.0 52.5
30 30 30
680 620 940
0.357 0.167 0.074
a Diameters and PDI were obtained from DLS with concentrations of 0.1 mg/ mL at room temperature.
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Fig. 1. SEM images and hydrodynamic diameters of pure poly(N-vinyl caprolactam) microgels with the crosslinked degree of 1%(a, d, g), 2%(b, e, h), and 5%(c, f, i).
2.3. Synthesis of hybrid microgels
UV–vis absorption spectrum of hybrid microgels was recorded by the UV–vis spectrometer (760CRT, INEASA, CN). The observation of samples in the mixed solution of ultrapure water and ethanol were performed by fluorescence microscope (IBE 2000, COIC, CN). And the fluorescence spectrum data were obtained from fluorescence spectrophotometer (PE LS55, USA) at the concentration of 3.57 mg/L of molybdenum oxide.
Typically, 40 mg MoS2 power was dissolved in 14 mL ultrapure water. And under stirring, 6 mL H2O2 was dropwise added. The reaction was carried out until transparent yellow liquid appearing and kept stirring for another 24 h to consume the extra H2O2.Under stirring, 25 mg PVCL microgels was dissolved in a mixed solution of 7 mL ethanol and 3 mL ultrapure water using 20 mL reaction flask. Then certain amount of precursor of A was diluted to 4 mL and added into the previous solution slowly. The mixed solution was kept under stirring for another two hours to achieve equilibrium state. After that, the flask was sealed and heated at 70 °C in water bath. The reaction was carried out for 48 h. The samples were purified using dialysis bag (12 000 KDA) for 4 days and dried as solid samples for further use.
3. Results and discussion 3.1. Characterization of poly(N-vinyl caprolactam) microgels Poly(N-vinyl caprolactam) microgels with the different crosslinked degrees were synthesized through precipitation polymerization, using MBA as crosslinker and KPS as initiator. To avoid the hydrolysis of VCL, NaHCO3 was added to adjust the pH value and sustain the polymerization process [24]. The hydrodynamic diameters of the microgels of 620∼940 nm were obtained and characterized through dynamic light scattering (DLS) measurements, as shown in Fig. 1g–i. Relatively larger hydrodynamic diameters were observed because of their obvious swelling by bonding to water molecules. Smaller dried microgels with the diameters of 100–200 nm were observed by their scanning electron microscope images (Fig. 1a–f) after the collapse of the microgels. Thermoresponsive properties of poly(N-vinyl caprolactam) microgels in mixed solvents of ultrapure water and ethanol were investigated, as shown in Fig. 2a. To obtain a quantifiable data from the phase transition variations, the minimum in the first derivative curve of transmittance varying with temperature (seeing Fig. 2b) was obtained and used as cloud points values. The value of VPTT appeared a slight decrease with the amount of ethanol up to 30 vol% and then appeared a rapid and obvious increase with larger amount of ethanol in mixed solution. This decreased VPTT value could be ascribed to the partial collapse of PVCL microgels matrix as a result of hydrophobic hydration caused by the stronger interaction between water with ethanol other than with polymer. But more ethanol (above 30 vol%) with hydrophobic groups would strongly improve phase transition temperature of
2.4. Instruments and measurements The sizes and morphologies of both pure microgels and hybrid microgels were observed by scanning electron microscopy (SEM, Merlin Compact, Germany). Hydrodynamic diameters of microgels and hybrid microgels (without purification) were detected and analyzed with concentrations of 0.1 mg/mL at room temperature, by dynamic light scattering (DLS) measurements (Zetasizer nano ZS90, Malvern, UN). Cloud Points test was performed by recording the transmittances of microgels or hybrid microgels at 550 nm using UV–vis spectrometer (760CRT, INEASA, CN), equipped with an external heater. And cloud points values were identified by the minimum for the first derivative curve of transmittance varying with temperature. The characterization of compositions was confirmed by Fourier transform infrared spectroscopy on a PerkinElmer Spectrum 100 FT-IR spectrometer with KBr pellet. X-ray photoelectron spectroscopy (XPS, ThermoFisher ESCALAB 250Xi, USA) was utilized to identify elements and stoichiometry of hybrid microgels. Microstructures of hybrid microgels were observed by high resolution transmission electron microscopy (HRTEM, FEI Tecnai G2 F30, USA). States of hybrid microgels were analyzed by XRay diffraction (XRD, Panalytical Empyrean, Netherlands). Besides, 132
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Fig. 2. (a) Curves of percentage transmittance vs temperature for PVCL1 microgels in mixed solution of water and ethanol with ratio of 5 : 1–5. (b) Curves of volume percentage of ethanol vs cloud points of PVCL1 microgels in mixed solution of water and ethanol.
caprolactam) microgels with different crosslinked densities also were measured through the transmittance as a function of temperature (seeing Fig. S1). The lower initial transmittance with higher crosslinked degree was observed owing to their limited swelling state. With temperature increasing, they exhibited similar volume phase transition temperature (VPTT) of 32.6 °C because of intermolecular collapse between microgels. These results were significant and valuable because it indicated that higher temperature can be allowed when poly(N-vinyl caprolactam) was utilized as micro/nanoreactors in some chemical or physical processes that without the requirement of thermal responsivity. 3.2. Synthesis and characterization of hybrid microgels The loaded MoO3 hybrid microgels were prepared through a modified hydrothermal procedure, as outlined in Scheme 1. The procedure included three steps: oxidation, concentration and hydrothermal process. In the step of oxidation, hydrogen peroxide was used as oxidant to oxidize S2− and Mo4+ of MoS2 to the higher oxidation states of S6+ and Mo6+, which correspondingly generated sulfuric acid and yellow molybdic acid [2]. Molybdic acid(MoO2•(OH)2) was reported to have a monoclinic layered crystal structure (P21/n) and Stabilized by water molecules. In the second step, water molecules were replaced by Nvinyl caprolactam units within microgels through the hydrogen bonding between OH groups of molybdic acid and C]O groups of polymeric units. This rapid interaction could cause aggregation of microgels to form white emulsion when molybdic acid was added into microgels aqueous solution, as observed in Fig. S2a–b. This result also was confirmed by Fourier transform infrared spectroscopy, as shown in Fig. 3. Compared with pure PVCL microgels, the new peak at 1049.1 cm−1 was assigned to vibration mode of the MoeOH bending vibrations. The peaks at 973.9 and 894.8 cm-1 changed to that at 956.5 and 914.0 cm-1, respectively, because of the overlay effect of absorption band of vibration modes of Mo-O-Mo [28]. The C]O stretching vibration of N-vinyl caprolactam units deceased from 1643.1 cm-1 to 1637.3 cm-1, which can be the result of the hydrogen bonding effect between N-vinyl caprolactam units and molybdic acid. And the broad absorption peak at 3408 cm-1was ascribed to the stretching vibration of bonded OeH in molybdic acid. To avoid the aggregation, ethanol was introduced into microgels solution before adding molybdic acid to slow down the bonding rate between molybdic acid and poly(N-vinyl caprolactam). In the hydrothermal process, ethanol also played important roles. Firstly, it can rise significantly VPTT of poly(N-vinyl caprolactam)
Scheme 1. Concept for the in situ formation of the molybdenum oxide nanocrystals within the PVCL microgels through the modified hydrothermal treatment.
poly(N-vinyl caprolactam) microgels. Similar phase transition characteristic also were reported in the earlier study of linear homopolymer of poly(N-vinyl caprolactam) when solution contains methanol [25,26] or ethanol [27]. Thermoresponsive properties of poly(N-vinyl
Fig. 3. Fourier transform infrared spectroscopy of the pure PVCL microgels and PVCL/MoO2(OH)2 microgels. 133
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Fig. 4. Low and high magnification (a, b), and high-resolution (c) TEM images of the PVCL2-MoO3-0.5 hybrid microgels; (d) CONTIN plots of PVCL2-MoO3-0.5 in a mixture of water and ethanol with a volume ratio of 1:1.
microgels to remain their swelling states at high temperature. With the assistance of ethanol, PVCL microgels could significantly affect the concentration of molybdic acid, nucleation and growth of molybdenum oxide. Besides, it can also work as reductant to help to form crystals. These characteristics highly distinguish itself from most previous references in the past, where PVCL was utilized to provide both skeletons for the location of active sites like charged groups to attract metal cations and space for these ions undergoing precipitation reactions at room temperature or cooler [14–23]. With the addition of precursor prepared by 0.5 mg MoS2 into 2% crosslinked PVCL microgels, a hydrothermal process was performed at 70 °C. The dried sample was named as PVCL2-MoO3-0.5 hybrid microgels and exhibited shadow blue color. The morphology and sizes of sample were characterized by HRTEM and DLS, as shown in Fig. 4. From low and high magnification of TEM, a number of hybrid microgels at average diameters of 54 nm could be observed, indicating an effective hybrid of MoO3 nanocrystals and microgels. The aggregation should be ascribed to the dehydration during preparing the samples for TEM test. Compared to SEM image of PVCL2 microgels in Fig. 1, the average diameters of PVCL2-MoO3-0.5 hybrid microgels decreased a lot, which can be ascribed to the crosslinking effect of nanocrystals. This phenomenon has been reported in some references about the hybrid microgels [14–19]. The strong interaction of the C]O groups of VCL units in the polymeric matrix with OeH groups on the surface of MoO3 nanocrystals could force the MoO3 playing as crosslinker and shrink the microgels. On the other hand, the result from DLS (see Fig. 4d) showed little difference of hydrodynamic diameters before and after hybrid because of too low loading of MoO3 nanocrystals. And no
free nanocrystals can be found outside of the microgels. Furthermore, the insight observation of hybrid microgels was provided by HRTEM, as shown in Fig. 4c. Nanoplatelets at diameters from 2 nm to 10 nm were observed inside of microgels matrix, and the lattice spacing was 0.27 nm, corresponding to crystal plane (101) of α-MoO3. These results revealed that α-MoO3 nanoplatelets with limited sizes were well trapped inside of microgels and proved the feasibility of hydrothermal method for in situ formation of nanocrystals within microgels. Here, PVCL2 microgels not only concentrated precursor molybdic acid but also played as microgels template to provide confined space for the nucleation and growth of nanocrystals. To analyze the elemental valence states of hybrid microgels, X-ray photoelectron spectroscopy was utilized to detect the dialyzed sample PVCL1-MoO3-2.5, and results were shown in the Fig. 5a–d. According to the survey spectrum in Fig. 5a, C, N, O and Mo elements were recorded. And Mo elements processed atoms percentage of 0.76, slightly lower than that of the calculated Mo atoms percentage of 0.84 for the precursor, indicating well loading effects of molybdenum oxide within the microgels. This result also was confirmed by XRD result (Fig. S3), which showed the amorphous form of hybrid microgels. As shown in Table 2, in our work the calculated amounts of MoO3 for the microgels with crosslinked degrees of 1∼5% and diameters of 600∼1000 nm are in the range of 0.17∼0.84%. Low loading amount of molybdenum oxide in microgels revealed the possibility of the more loading of molybdenum oxide, owing to the permitting of the diffusion of precursor and the gowth of nanocrystals by the extra spaces of swelling microgels. After deconvolution and curve fitting, the high-resolution XPS spectrum of C1S in Fig. 5b shows three peaks at 284.8, 286, and 134
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Fig. 5. (a) XPS survey spectrum of the PVCL1-MoO3-2.5 hybrid microgels; and the high-resolution XPS spectra of C 1 s(b), O 1 s(c), Mo 3d(d). Table 2 Recipes for the in situ fabrication of the poly(N-vinyl caprolactam)-based hybrid microgels. Samplesa
V1preb./mL
H2O/mL
PVCL1/mg
PVCL2/mg
PVCL5/mg
PVCL1-MoO3-0.5 PVCL1-MoO3-1.5 PVCL1-MoO3-2.5 PVCL2-MoO3-0.5 PVCL5-MoO3-0.5
0.25 0.75 1.25 0.25 0.25
6.25 5.75 5.25 6.25 6.25
25 25 25 — —
— — — 25 —
— — — — 25
interactions between PVCL microgels and molybdenum oxide after the modified hydrothermal treatment. Furthermore, the poly(N-vinyl caprolactam) microgels with low crosslinked degree of 1% also was considered to be used as reactor to manipulate the morphology of nanocrystals. As shown in Fig. 6a–b, the aggregation of hybrid nanoparticles at sizes of about 1 μm was obtained without identifiable boundary by naked eyes. Markedly, plentiful nanoflakes ranging from ten to several hundred nanometers can be found, quite different from the nanoplatelets of PVCL2-MoO3-0.5 or the result without microgels(seeing Fig. S4), revealing that an continuous growth of the nanocrystals in the hydrothermal process happened. Furthermore, HRTEM image in Fig. 6c revealed these 2D nanoflakes exhibited a spacing of 0.39 nm, which can be corresponding to the (1 0 0) plane of layered α-MoO3. The formation of nanocrystal with large sizes could be ascribed to three factors: (1) large inner space offered by low crosslinked PVCL1 microgels; (2) stabilization provided by poly(N-vinyl caprolactam) polymeric chains to minimize the surface energy of nanocrystals; (3) relatively high synthesis temperature. Under these conditions, the concentrated molybdic acid within microgels could undergo rapid nucleation and then continuous growth, along with constant assumption of free precursor. On the other hand, the average hydrodynamic diameters in Fig. 6d is 237 nm, lower than pure microgels’, and much lower than the sizes of their TEM images in Fig. 6a, which indicated the aggregation happened during the preparation of the sample for TEM tests, not in the hydrothermal process. Besides, a small amount of free nanoflakes with sizes of 10 ∼ 100 nm have also been recorded by DLS detector, which indicated the microgels with crosslinked degree of 1% can hardly completely load nanocrystals in the
a The samples were synthesized with additional ethanol of 7 mL and performed at 70 °C for 24 h. b Precursor was prepared by the over oxidized MoS2 solution of 2 mg/mL.
287.5ev, corresponding to C elements of −CH2, CeN and C]O, respectively [29]. In the high resolution spectrum of O elements, peak at 530.6 ev was O1s spectrum of Mo-O-Mo [28]. Besides, peaks at 532.2 and 530.9 ev might respectively correspond to O of C]O with and without the interaction to Mo. Similar interactions also have been reported in the reference [30,31], where Tb(III) was bonding to O atoms of the carbonyl groups of poly(N-isopropylacrylamide) to form polymer-Tb(III) complex. The main peaks in Fig. 5d at 235.7 and 234.6 ev were the binding energies of 3d3/2 and 3d5/2 of molybdenum respectively. These two peaks have been commonly regard as features peaks of α-MoO3, in agreement with the analysis results from HRTEM (Fig. 4c). In addition, suspected small peaks at 232.6 and 231.5 ev might be the appearance of Mo(V) oxidation state, owing to the weak reducibility of N-vinyl caprolactam units. These results confirmed the composition of hybrid microgels, especially the coordination 135
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Fig. 6. Low(a) and high(b) magnification and high-resolution(c) TEM images of the PVCL1-MoO3-0.5 hybrid microgels; (d) CONTIN plots of the PVCL1-MoO3-0.5 hybrid microgels in a mixture of water and ethanol with a volume ratio of 1:1.
Fig. 7. (a) SEM image of the PVCL1-MoO3-2.5 hybrid microgels, the inset is TEM image; (b) CONTIN plots of PVCL1-MoO3-2.5 in a mixture of water and ethanol with a volume ratio of 1:1.
microgels matrix. These results reveal that the microgels with low crosslinked degree of 1% would partly lose their template effect, and work more like linear polymer to help the formation of nanocrystals with large sizes. Higher concentration of molybdic acid precursor than the previous experiment has also been used to synthesize molybdenum oxide nanocrystals in the modified hydrothermal procedure. From SEM image (Fig. 7a), aggregation of molybdenum oxide rods at lengths of hundreds
nanometers were observed, along with a little of nanospheres. The aggregation revealed that these nanorods might be mostly trapped in the microgels matrix, which also was indicated by DLS in Fig. 7b. Compared with PVCL1-MoO3-0.5 hybrid microgels, hybrid microgels with more molybdenum oxide shrunk more seriously. To further confirm the inner structure of hybrid microgels, a TEM image was recorded and the result was inset to SEM image. Large amount of nanorods can be found outside the microgels, as a result of easy movement of small 136
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Fig. 8. Curves of the normalized transmittances vs temperature for (a) the PVCL1 hybrid microgels synthesized with varying volume contents of precursor, the concentration of precursor is 2 mg/mL; (b) the PVCL hybrid microgels with varying crosslinked degrees of 1%, 2%, 5%; Plots of cloud points of the hybrid microgels vs volume contents of precursors (c) and crosslinked degrees (d) in aqueous solution.
nanorods from lowly crosslinked PVCL1 microgels. The formation of nanorods might be due to the scrolling of molybdenum oxide layers. With the addition of higher amount of MoO2(OH)2 precursor, nanocrystals kept growing until no more N-vinyl caprolactam units could be utilized to stabilize the surface of nanocrystals. The failure to stabilize the 2D nanoflakes finally led to the scrolling of molybdenum oxide layers along certain specific crystal directions to form nanorods. On the other hand, it also indicated that the microgels with low crosslinked degree of 1% appeared to have the limited capacity for the efficient loading of the nanocrystals, because of its larger porosity than that of the microgels with higher crosslinked degree of 2%.
sterically limit the collapse of hybrid microgels. The last factor that caused higher cloud points might be the restriction of inorganic nanocrystals on the movement of polymeric chains [32]. Furthermore, the effect of different crosslinked degrees of the microgels with the same amount of MoO3 contents also was investigated and calculated through the same method as mentioned above. The cloud points as a function of crosslinked degrees were shown in Fig. 8d. As the crosslinked degrees increased, the cloud points decreased slightly. The reason for the decrease of the cloud points might be that the high crosslinked degree of microgels would hinder the formation of MoO3 nanoparticles in PVCL microgels matrix; as a result, MoO3 nanoparticles dispersed in the aqueous solution. In the process of phase transition separation induced by heating, the free MoO3 nanoparticles worked as crosslinker and decreased the phase transition temperature of the hybrid microgels, as reported in the previously similar study [33]. In our colloidal stability test (Fig. S5), the absorbance at 325 nm for the aqueous solution of PVCL2-MoO3-0.5 hybrid microgels almost remained unchanged, without any observable precipitation even after the place for 15 days (as shown in the inset of Fig. S5). The results demonstrated that the hybrid microgels exhibited well colloidal stability in aqueous solutions.
3.3. Thermoresponsivity and colloidal stability of hybrid microgels In order to investigate the effect of the incorporation of MoO3 nanoparticles on the thermal responsivity of PVCL microgels, the transmittances of hybrid microgels in aqueous solution have been measured by UV–vis spectrophotometer as a function of the temperature, as shown in Fig. 8a–b. The cloud points’ values were obtained as the method previously. Fig. 8c showed the variations of the cloud points of hybrid microgels varied as a function of the precursor contents. An increase of cloud points can be observed with an increasing amount of MoO3 nanoparticles. The result might origin from multiple factors because of the loading of metal oxide nanoparticles. One factor could be the repulsive interactions between the MoO3 nanoparticles trapped in the microgels matrix. Moreover, the coordination bonding between metal oxide nanoparticles with carbonyl groups of VCL units could also weaken the thermal sensitivity of PVCL microgels. Besides, the existence of MoO3 nanoparticles within the microgels matrix would
3.4. Optical properties of hybrid microgels It is well-known that the inorganic nanocrystals with limited sizes of 1∼100 nm could exhibit strong luminescence from the deep UV to near-infrared wavelength owing to quantum confinement effect. Commonly, the wavelength and intensity of photoluminescence can be tuned by changing their sizes, surface states, solvents, pH and doping 137
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Fig. 9. (a) Absorption spectra of the hybrid microgels with the various loaded amount of molybdenum oxide; (b) The excitation-dependent photoluminescence emission spectra of the PVCL1-MoO3-0.5 hybrid microgels; Fluorescent images of the synthesized molybdenum oxide that without (c) and with (d) the use of microgels.
elements [34]. Specially, sizes and surface states have critical influence on their luminescent effects. Here, the optical properties of hybrid microgels were investigated by fluorescence spectra, and combining the data from HRTEM and XPS, we further discussed the relationship between the optical properties of hybrid microgels and their morphologies and surface states. The UV/vis absorption spectra of hybrid microgels loaded with varying amounts of molybdenum oxide were recorded and shown in Fig. 9a. From the absorbance spectra, α-MoO3 in the hybrid microgels were observed to have an absorption region of 200–450 nm, which can be attributed to the charge transfer of Mo-O band in MoO66− octahedron [35]. According to the previous reference [36], the optical gap energy of α-MoO3 was identified to be that of 2.85 eV. Moreover, as the concentrations of precursor in the synthesis of hybrid microgels increased, the absorption intensity also increased as expected. The photoluminescence of PVCL1-MoO3-0.5 hybrid microgels was measured under excitement waves of the wavelength from 275 nm to 425 nm, and the results were recorded in Fig. 9b. The photoluminescence spectra mainly exhibited two emission bands: near the band-edge of UV region (320–380 nm) and at visible emission band (410–470 nm). Specifically, the visible emission band can be separated into three peaks at 410, 450, 470 nm, respectively. The band at 410 nm can be attributed to Mo5+ dyz1-dxy1 transition and the band at 450 nm and 470 nm can ascribed to the deep level of Mo5+ dyz2-dxz2 transition. The strong deep level emission might originate from the crystal defects, such as structural or surface defects. On the other hand, the relatively wide UV emission band from 320 nm to 389 nm, as mentioned previously, might generate from the uneven size distribution of α-MoO3 nanoflakes [37]. The fluorescent photographs of MoO3 nanocrystals and PVCL1-MoO3-0.5 hybrid microgels (seeing Fig. 9c–d) also showed
their difference in background light. Hybrid microgels exhibited bright blue background light as a result of the restriction of MoO3 nanocrystals in microgels. The photoluminescence spectra of PVCL1-MoO3-2.5 hybrid microgels in the Fig. 10a and 10c indicated the similar emission band changes with remarkable decrease at the visible emission band. The decrease of photoluminescence intensities of the band at 450 nm might be ascribed to electronic coupling effect of PVCL1-MoO3-2.5 hybrid microgels as a result of the morphology change from nanoflakes of PVCL1-MoO3-0.5 to rods of PVCL1-MoO3-2.5 hybrid microgels. The result revealed that the formation of molybdenum oxide nanoflakes in the case of PVCL1MoO3-0.5 hybrid microgels involved the trap of poly(N-vinyl caprolactam) polymeric chain into interlayers of molybdenum oxide, which further increase the electronic decoupling of the layers. Contrast to that, the nanorods within the PVCL1-MoO3-2.5 hybrid microgels were dispersed in the aqueous solution, and showed weak interaction with poly (N-vinyl caprolactam) microgels. The influence of the crosslinked degrees of microgels template on the photoluminescence response of molybdenum oxide nanocrystal were also discussed, as shown in the Fig. 10b and d. The result indicated that the increasing crosslinked degree would decrease the photoluminescence intensity of visible emission band of α-MoO3. Combining with the results of TEM images of PVCL2-MoO3-0.5 (Fig. 4a–c), it can be inferred that the denser crosslinked microgels provided limited spaces for the growth of molybdenum oxide nanocrystal and further reduced the crystallinity of nanocrystals, which would response for the decay of photoluminescence intensity of α-MoO3. Similar results had also come out in the reference about molybdenum oxide nanoparticles [37] and ZoO thin film [38]. Besides, the limited light transmittance of microgels with a crosslinked degree of 2% and 5% might also be responsible for 138
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Fig. 10. The excitation-dependent photoluminescence emission of the PVCL1-MoO3-2.5(a) and the PVCL2-MoO3-0.5(b) hybrid microgels; And the photoluminescence emission of the hybrid microgels with varying loading of molybdenum oxide(c) and varying crosslinked degrees(d) at the excitement wavelength of 375 nm.
the decreased photoluminescence intensity, since it would decrease the transmittance of excitement light. To improve light sensitivity of nanocrystals, further strategies in the synthesis process, such as higher temperature, need to be considered to raise their crystallinity. Anyway, these results both showed the interaction between molybdenum oxide nanocrystals and PVCL microgels, not only owned the significant influence on morphologies of nanocrystals, but also could enhance their optical properties owing to electronic decoupling effect.
introduction of high reactive ligand groups, such as carboxyl or amine groups, still need to be considered if increased properties (e.g. plasmonic) for nanocrystals are expected. To conclude, a novel synthesis strategy for hybrid of nanocrystals within thermoresponsive microgels was introduced to overcome the limit of synthesis temperature and open up new possibilities for hybrid nanoparticles in nanotechnology.
4. Conclusions
The authors are grateful for the Excellent Academic Leaders Foundation of Harbin, China (No. 2014RFXXJ017), and the Open Project of State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (No. QA201610-02).
Acknowledgements
We reported the application of poly(N-vinyl caprolactam) microgels with different crosslinked degrees as micro/nanoreactors for in situ generation of molybdenum oxide nanocrystals through a modified hydrothermal process. Here, high synthesis temperature for in situ generation of nanocrystals within thermoresponsive microgels was firstly reported, realized by adjusting volume ratios of ethanol and water. In such hydrothermal process, poly(N-vinyl caprolactam) microgels worked as both template to offer confined spaces of crosslinked 3D matrix for size-controllable nanocrystals and stabilizer to control nucleation and growth of nanocrystals. Various structures of molybdenum oxide (e.g. 2D nanoflakes, nanorods and nanoplatelets) were synthesized under different synthesis parameters, along with changing luminescence intensity. Specially, the microgels with crosslinked degree of 2% showed an effective trap and a well size-controlled effect during the formation of the nanocrystals. These results highlight the feasibility of the synthesis temperature for in situ generation of nanocrystals, including the improvement of the precursor’s reactivity and the controllability to the formation process of nanocrystals. Despite these, the
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Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
In situ thermal synthesis of molybdenum oxide nanocrystals in thermoresponsive microgels
T
Jing Penga, Dongyan Tanga, , Shuyue Jiaa, Yue Zhangb, Zhaojie Suna, Xu Yanga, Hongyun Zoua, Haitao Lva ⁎
a b
School of Chemistry and Chemical Engineering, Harbin Institute of Technology, No. 92, Xidazhi Street, Nangang District, Harbin City, Heilongjiang Province, China School of Foreign Languages, Harbin Institute of Technology, No. 92, Xidazhi Street, Nangang District, Harbin City, Heilongjiang Province, China
GRAPHICAL ABSTRACT
ARTICLE INFO
ABSTRACT
Keywords: Thermoresponsive Poly(N-vinyl caprolactam) Hybrid microgel In situ Molybdenum oxide
In situ formation of nanocrystals within thermoresponsive microgels has become a significant technology in the fields of catalysis and biomedicine to fabricate multifunctional hybrid nanostructures. Usually, the room temperature or cooler was set to control the formation of nanocrystals using microgels as template in such process. Here, the relatively higher temperature was used to synthesize molybdenum oxide nanocrystals where the poly (N-vinyl caprolactam) microgels were found to work as both the stabilizer and the template. Specifically, ethanol was added in the solution of the microgels to raise their volume phase transition temperature (VPTT). Later, a modified hydrothermal process was performed at 70 °C with precursor molybdic acid concentrated in the microgels matrix through the hydrogen bond between molybdic acid and N-vinyl caprolactam units. 2D nanoflakes, nanorods and nanoplatelets of molybdenum oxide were successfully synthesized. Specially, the microgels with the crosslinked degree of 2% exhibited well hybrid with controlled sizes and ideal confine of the molybdenum oxide nanoplatelets within microgels, along with strong photoluminescence intensity. These results emphasized the feasibility of poly(N-vinyl caprolactam) microgels as template and stabilizer at high temperature and provided a novel synthesis strategies for hybrid microgels applicable in wide areas of nanotechnology from catalysis, sensing to therapy.
⁎
Corresponding author. E-mail address: [email protected] (D. Tang).
https://doi.org/10.1016/j.colsurfa.2018.11.065 Received 26 September 2018; Received in revised form 26 November 2018; Accepted 27 November 2018 Available online 28 November 2018 0927-7757/ © 2018 Elsevier B.V. All rights reserved.
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1. Introduction
nanocrystals. On the other hand, poly(N-vinyl caprolactam) was recently mentioned that it exhibited a certain extent of coordination and weak reducibility in the formation of Au nanoparticles, both structurally and functionally similar to PVP [22,23]. It revealed that poly(N-vinyl caprolactam) microgels could be valuable as macromolecular stabilizer and soft colloidal templates for the nucleation and the growth of inorganic nanocrystals under the relative higher temperatures. In this paper, relatively higher temperature than the VPTT of PVCL microgels in water was considered to realize a hydrothermal procedure to compose in-situ hybrids. And the roles of poly(N-vinyl caprolactam) microgels as stabilizer and templates on the fabrication of hybrids were investigated. Specifically, molybdenum oxide was chosen as synthesized targets for its excellent optical properties as 2D transition metal oxide materials. The volume phase transition temperature of poly(Nvinyl caprolactam) microgels in the mixed solution of water and ethanol was discussed firstly to open the gate of utilizing PVCL microgels at relatively higher temperature. Then various synthesis parameters such as concentrations of precursor and crosslinked degrees of microgels were explored in the modified hydrothermal procedure of PVCL-MoO3 hybrid microgels. Microstructures of hybrid microgels and elements valence states as well as the interactions between nanocrystals and polymer were investigated. Furthermore, electronic structures of nanocrystals after the hybrid have also been discussed. Our results indicated that the high temperature can be utilized when using thermoresponsive microgels as template for the in situ generation of nanocrystals, which would diversify and functionalize hybrid microgels in a range of fields from biomedicine to catalysis.
Hybrid of inorganic nanocrystals with polymer have become one of the most powerful strategies to adopt some special application, such as catalysis [1], sensor [2] and biomedicine [3,4]. Hybrid nanostructures can avoid some inherent defects of inorganic nanocrystals, such as toxicity, hard target and unavoidable aggregation. Besides, it also integrates inorganic nanocrystals with the advantages in optical, electronic and magnetic fields and polymer with stimuli responsivity, designability and solubility [5–7] into one system with synergy effects. These properties make hybrid nanostructures become popular in various research fields, such as photothermal therapy, photoluminescence imaging and optical biosensors. To fabricate such hybrid nanostructures, various methods have been well-developed. One of common strategies is utilizing functional polymer to decorate the surface of inorganic nanocrystals. For example, exchange of oleylamine on the MoOx nanoparticles with linear polyethylene glycol(PEG) could lead to an hydrophilic surface and good biocompatibility [8]. Interaction of amphiphilic polymers with ligands through hydrophobic effect could also realize the hydrophilicity and functionalization of CdSe/ZnS quantum dots (QDs) [9]. Similar applications also include that of block copolymers [10]. Except that, solvents exchange between organic and water has also been utilized to tag nanoparticles into polymeric matrix [11]. Besides, small molecules with vinyl groups also was used to stabilize nanocrystals in the hydrothermal synthesis and further copolymerized with NIPAM and MBA through precipitation polymerization to form hybrid microgels [12]. These methods have their remarkable advantages since inorganic nanocrystals are prepared before hybrid, along with ideal morphologies and uniform sizes, and would exhibit excellent optical, electrical and magnetic properties. However, some important issues in these cases still need more careful consideration, such as nonideal loading, uncontrollable loading process, or weak bonding between inorganic nanocrystals and polymer. Another alternative is introducing polymer during the synthesis process of nanocrystals. Linear polyethylene glycol(PEG) and poly(vinylpyrrolidone) (PVP) are two most frequently used polymer in the solvothermal method [1]. They usually play two important roles in the preparation of nanoparticles: (1) ligands, bonding to the NP surface; (2) reducing agent, reducing the valence state of metal elements. Besides, they also exhibit good hydrophilicity and biocompatibility, which enormously stimulate their application in the biomedicine. However, their simple structures and uneasy designability for the special target and stimuli-responsivity restrict their application in the biomedicine area. In addition to linear polymer, microgels have also been reported as micro/nanoreactors for the in situ formation of nanoparticles. From the view of structure, microgels has irreplaceable advantages as template/ reactor for its highly swelling characteristic and crosslinked matrix [13]. Especially, thermoresponsive microgels, such as poly(N-vinyl caprolactam) and poly(N-isopropylacrylamide) (PNIPAM), could exhibit enhanced application value for their stimuli responsivity and low toxicity. For example, poly(N-vinyl caprolactam-co-acetoacetoxyethylmethacrylate) microgels had been used as micro/nanoreactors to prepare a series of inorganic nanocrystals, like ZoO [14], Fe3O4 [15,16], Ag [17], LnF3 [18], ZnS [19]. Winnik et al. utilized fully neutralized methacrylic acid (MAA) groups in the core of poly(NIPAM/MAA/ PEGMA) microgels to confine the growth of EuF3 nanoparticles [20]. Johannes et al. introduced additional surfactant to control the morphology of Au in the thermoresponsive core-shell microgels from nanoparticles to nanorods [21]. These researches focus on the influences of coordination groups, charged groups, and surfactants on the formation process of inorganic nanocrystals. To our best knowledge, most of them were performed at the room temperature or lower, not ever over their volume phase transition temperature (VPTT) in aqueous solutions. It is well known that the high temperature would benefit both thermodynamic and kinetic control in the nucleation and growth process of
2. Experimental 2.1. Materials N-vinyl caprolactam (VCL) was purchased from Sigma Co. Ltd and purified through recrystallization in n-hexane; potassium persulfate (KPS), N,N-methylene bisacrylamide (MBA), sodium dodecyl sulphate (SDS), molybdenum disulfide(MoS2), hydrogen peroxide (H2O2), and sodium bicarbonate(NaHCO3) were all analytically pure and were purchased from Aladdin Co., Ltd. 2.2. Preparation of poly(N-vinyl caprolactam) microgels PVCL microgels were prepared through precipitation polymerization according to the reference [14]. A typical process described as follows: appropriate amounts of VCL monomer, MBA as crosslinker, SDS as stabilizer, and NaHCO3 as pH controlling agent (as shown in Table 1) were dissolved in 60 mL ultrapure water. The synthesis was performed in a triple-wall glass reactor equipped with temperature control and magnetic stirrer. The solution was incubated and heated to 70 °C under nitrogen purging for 30 min to form stable emulsion. And after that, a solution of KPS initiator solved in 3 mL ethanol was injected into the previous solution to initiate the polymerization. The reaction was carried out for 6 h. The samples were purified using dialysis bag (8000 -12 000 KDA) for 4 days and dried as solid sample for further purpose. Table 1 Recipes for the precipitation polymerization of poly(N-vinyl caprolactam) microgels with different crosslinked degrees. Samples
NVCL/g
MBA/mg
KPS/mg
Diaa/nm
PDIa
PVCL1 PVCL2 PVCL5
0.93 0.92 0.89
10.5 21.0 52.5
30 30 30
680 620 940
0.357 0.167 0.074
a Diameters and PDI were obtained from DLS with concentrations of 0.1 mg/ mL at room temperature.
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Fig. 1. SEM images and hydrodynamic diameters of pure poly(N-vinyl caprolactam) microgels with the crosslinked degree of 1%(a, d, g), 2%(b, e, h), and 5%(c, f, i).
2.3. Synthesis of hybrid microgels
UV–vis absorption spectrum of hybrid microgels was recorded by the UV–vis spectrometer (760CRT, INEASA, CN). The observation of samples in the mixed solution of ultrapure water and ethanol were performed by fluorescence microscope (IBE 2000, COIC, CN). And the fluorescence spectrum data were obtained from fluorescence spectrophotometer (PE LS55, USA) at the concentration of 3.57 mg/L of molybdenum oxide.
Typically, 40 mg MoS2 power was dissolved in 14 mL ultrapure water. And under stirring, 6 mL H2O2 was dropwise added. The reaction was carried out until transparent yellow liquid appearing and kept stirring for another 24 h to consume the extra H2O2.Under stirring, 25 mg PVCL microgels was dissolved in a mixed solution of 7 mL ethanol and 3 mL ultrapure water using 20 mL reaction flask. Then certain amount of precursor of A was diluted to 4 mL and added into the previous solution slowly. The mixed solution was kept under stirring for another two hours to achieve equilibrium state. After that, the flask was sealed and heated at 70 °C in water bath. The reaction was carried out for 48 h. The samples were purified using dialysis bag (12 000 KDA) for 4 days and dried as solid samples for further use.
3. Results and discussion 3.1. Characterization of poly(N-vinyl caprolactam) microgels Poly(N-vinyl caprolactam) microgels with the different crosslinked degrees were synthesized through precipitation polymerization, using MBA as crosslinker and KPS as initiator. To avoid the hydrolysis of VCL, NaHCO3 was added to adjust the pH value and sustain the polymerization process [24]. The hydrodynamic diameters of the microgels of 620∼940 nm were obtained and characterized through dynamic light scattering (DLS) measurements, as shown in Fig. 1g–i. Relatively larger hydrodynamic diameters were observed because of their obvious swelling by bonding to water molecules. Smaller dried microgels with the diameters of 100–200 nm were observed by their scanning electron microscope images (Fig. 1a–f) after the collapse of the microgels. Thermoresponsive properties of poly(N-vinyl caprolactam) microgels in mixed solvents of ultrapure water and ethanol were investigated, as shown in Fig. 2a. To obtain a quantifiable data from the phase transition variations, the minimum in the first derivative curve of transmittance varying with temperature (seeing Fig. 2b) was obtained and used as cloud points values. The value of VPTT appeared a slight decrease with the amount of ethanol up to 30 vol% and then appeared a rapid and obvious increase with larger amount of ethanol in mixed solution. This decreased VPTT value could be ascribed to the partial collapse of PVCL microgels matrix as a result of hydrophobic hydration caused by the stronger interaction between water with ethanol other than with polymer. But more ethanol (above 30 vol%) with hydrophobic groups would strongly improve phase transition temperature of
2.4. Instruments and measurements The sizes and morphologies of both pure microgels and hybrid microgels were observed by scanning electron microscopy (SEM, Merlin Compact, Germany). Hydrodynamic diameters of microgels and hybrid microgels (without purification) were detected and analyzed with concentrations of 0.1 mg/mL at room temperature, by dynamic light scattering (DLS) measurements (Zetasizer nano ZS90, Malvern, UN). Cloud Points test was performed by recording the transmittances of microgels or hybrid microgels at 550 nm using UV–vis spectrometer (760CRT, INEASA, CN), equipped with an external heater. And cloud points values were identified by the minimum for the first derivative curve of transmittance varying with temperature. The characterization of compositions was confirmed by Fourier transform infrared spectroscopy on a PerkinElmer Spectrum 100 FT-IR spectrometer with KBr pellet. X-ray photoelectron spectroscopy (XPS, ThermoFisher ESCALAB 250Xi, USA) was utilized to identify elements and stoichiometry of hybrid microgels. Microstructures of hybrid microgels were observed by high resolution transmission electron microscopy (HRTEM, FEI Tecnai G2 F30, USA). States of hybrid microgels were analyzed by XRay diffraction (XRD, Panalytical Empyrean, Netherlands). Besides, 132
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Fig. 2. (a) Curves of percentage transmittance vs temperature for PVCL1 microgels in mixed solution of water and ethanol with ratio of 5 : 1–5. (b) Curves of volume percentage of ethanol vs cloud points of PVCL1 microgels in mixed solution of water and ethanol.
caprolactam) microgels with different crosslinked densities also were measured through the transmittance as a function of temperature (seeing Fig. S1). The lower initial transmittance with higher crosslinked degree was observed owing to their limited swelling state. With temperature increasing, they exhibited similar volume phase transition temperature (VPTT) of 32.6 °C because of intermolecular collapse between microgels. These results were significant and valuable because it indicated that higher temperature can be allowed when poly(N-vinyl caprolactam) was utilized as micro/nanoreactors in some chemical or physical processes that without the requirement of thermal responsivity. 3.2. Synthesis and characterization of hybrid microgels The loaded MoO3 hybrid microgels were prepared through a modified hydrothermal procedure, as outlined in Scheme 1. The procedure included three steps: oxidation, concentration and hydrothermal process. In the step of oxidation, hydrogen peroxide was used as oxidant to oxidize S2− and Mo4+ of MoS2 to the higher oxidation states of S6+ and Mo6+, which correspondingly generated sulfuric acid and yellow molybdic acid [2]. Molybdic acid(MoO2•(OH)2) was reported to have a monoclinic layered crystal structure (P21/n) and Stabilized by water molecules. In the second step, water molecules were replaced by Nvinyl caprolactam units within microgels through the hydrogen bonding between OH groups of molybdic acid and C]O groups of polymeric units. This rapid interaction could cause aggregation of microgels to form white emulsion when molybdic acid was added into microgels aqueous solution, as observed in Fig. S2a–b. This result also was confirmed by Fourier transform infrared spectroscopy, as shown in Fig. 3. Compared with pure PVCL microgels, the new peak at 1049.1 cm−1 was assigned to vibration mode of the MoeOH bending vibrations. The peaks at 973.9 and 894.8 cm-1 changed to that at 956.5 and 914.0 cm-1, respectively, because of the overlay effect of absorption band of vibration modes of Mo-O-Mo [28]. The C]O stretching vibration of N-vinyl caprolactam units deceased from 1643.1 cm-1 to 1637.3 cm-1, which can be the result of the hydrogen bonding effect between N-vinyl caprolactam units and molybdic acid. And the broad absorption peak at 3408 cm-1was ascribed to the stretching vibration of bonded OeH in molybdic acid. To avoid the aggregation, ethanol was introduced into microgels solution before adding molybdic acid to slow down the bonding rate between molybdic acid and poly(N-vinyl caprolactam). In the hydrothermal process, ethanol also played important roles. Firstly, it can rise significantly VPTT of poly(N-vinyl caprolactam)
Scheme 1. Concept for the in situ formation of the molybdenum oxide nanocrystals within the PVCL microgels through the modified hydrothermal treatment.
poly(N-vinyl caprolactam) microgels. Similar phase transition characteristic also were reported in the earlier study of linear homopolymer of poly(N-vinyl caprolactam) when solution contains methanol [25,26] or ethanol [27]. Thermoresponsive properties of poly(N-vinyl
Fig. 3. Fourier transform infrared spectroscopy of the pure PVCL microgels and PVCL/MoO2(OH)2 microgels. 133
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Fig. 4. Low and high magnification (a, b), and high-resolution (c) TEM images of the PVCL2-MoO3-0.5 hybrid microgels; (d) CONTIN plots of PVCL2-MoO3-0.5 in a mixture of water and ethanol with a volume ratio of 1:1.
microgels to remain their swelling states at high temperature. With the assistance of ethanol, PVCL microgels could significantly affect the concentration of molybdic acid, nucleation and growth of molybdenum oxide. Besides, it can also work as reductant to help to form crystals. These characteristics highly distinguish itself from most previous references in the past, where PVCL was utilized to provide both skeletons for the location of active sites like charged groups to attract metal cations and space for these ions undergoing precipitation reactions at room temperature or cooler [14–23]. With the addition of precursor prepared by 0.5 mg MoS2 into 2% crosslinked PVCL microgels, a hydrothermal process was performed at 70 °C. The dried sample was named as PVCL2-MoO3-0.5 hybrid microgels and exhibited shadow blue color. The morphology and sizes of sample were characterized by HRTEM and DLS, as shown in Fig. 4. From low and high magnification of TEM, a number of hybrid microgels at average diameters of 54 nm could be observed, indicating an effective hybrid of MoO3 nanocrystals and microgels. The aggregation should be ascribed to the dehydration during preparing the samples for TEM test. Compared to SEM image of PVCL2 microgels in Fig. 1, the average diameters of PVCL2-MoO3-0.5 hybrid microgels decreased a lot, which can be ascribed to the crosslinking effect of nanocrystals. This phenomenon has been reported in some references about the hybrid microgels [14–19]. The strong interaction of the C]O groups of VCL units in the polymeric matrix with OeH groups on the surface of MoO3 nanocrystals could force the MoO3 playing as crosslinker and shrink the microgels. On the other hand, the result from DLS (see Fig. 4d) showed little difference of hydrodynamic diameters before and after hybrid because of too low loading of MoO3 nanocrystals. And no
free nanocrystals can be found outside of the microgels. Furthermore, the insight observation of hybrid microgels was provided by HRTEM, as shown in Fig. 4c. Nanoplatelets at diameters from 2 nm to 10 nm were observed inside of microgels matrix, and the lattice spacing was 0.27 nm, corresponding to crystal plane (101) of α-MoO3. These results revealed that α-MoO3 nanoplatelets with limited sizes were well trapped inside of microgels and proved the feasibility of hydrothermal method for in situ formation of nanocrystals within microgels. Here, PVCL2 microgels not only concentrated precursor molybdic acid but also played as microgels template to provide confined space for the nucleation and growth of nanocrystals. To analyze the elemental valence states of hybrid microgels, X-ray photoelectron spectroscopy was utilized to detect the dialyzed sample PVCL1-MoO3-2.5, and results were shown in the Fig. 5a–d. According to the survey spectrum in Fig. 5a, C, N, O and Mo elements were recorded. And Mo elements processed atoms percentage of 0.76, slightly lower than that of the calculated Mo atoms percentage of 0.84 for the precursor, indicating well loading effects of molybdenum oxide within the microgels. This result also was confirmed by XRD result (Fig. S3), which showed the amorphous form of hybrid microgels. As shown in Table 2, in our work the calculated amounts of MoO3 for the microgels with crosslinked degrees of 1∼5% and diameters of 600∼1000 nm are in the range of 0.17∼0.84%. Low loading amount of molybdenum oxide in microgels revealed the possibility of the more loading of molybdenum oxide, owing to the permitting of the diffusion of precursor and the gowth of nanocrystals by the extra spaces of swelling microgels. After deconvolution and curve fitting, the high-resolution XPS spectrum of C1S in Fig. 5b shows three peaks at 284.8, 286, and 134
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Fig. 5. (a) XPS survey spectrum of the PVCL1-MoO3-2.5 hybrid microgels; and the high-resolution XPS spectra of C 1 s(b), O 1 s(c), Mo 3d(d). Table 2 Recipes for the in situ fabrication of the poly(N-vinyl caprolactam)-based hybrid microgels. Samplesa
V1preb./mL
H2O/mL
PVCL1/mg
PVCL2/mg
PVCL5/mg
PVCL1-MoO3-0.5 PVCL1-MoO3-1.5 PVCL1-MoO3-2.5 PVCL2-MoO3-0.5 PVCL5-MoO3-0.5
0.25 0.75 1.25 0.25 0.25
6.25 5.75 5.25 6.25 6.25
25 25 25 — —
— — — 25 —
— — — — 25
interactions between PVCL microgels and molybdenum oxide after the modified hydrothermal treatment. Furthermore, the poly(N-vinyl caprolactam) microgels with low crosslinked degree of 1% also was considered to be used as reactor to manipulate the morphology of nanocrystals. As shown in Fig. 6a–b, the aggregation of hybrid nanoparticles at sizes of about 1 μm was obtained without identifiable boundary by naked eyes. Markedly, plentiful nanoflakes ranging from ten to several hundred nanometers can be found, quite different from the nanoplatelets of PVCL2-MoO3-0.5 or the result without microgels(seeing Fig. S4), revealing that an continuous growth of the nanocrystals in the hydrothermal process happened. Furthermore, HRTEM image in Fig. 6c revealed these 2D nanoflakes exhibited a spacing of 0.39 nm, which can be corresponding to the (1 0 0) plane of layered α-MoO3. The formation of nanocrystal with large sizes could be ascribed to three factors: (1) large inner space offered by low crosslinked PVCL1 microgels; (2) stabilization provided by poly(N-vinyl caprolactam) polymeric chains to minimize the surface energy of nanocrystals; (3) relatively high synthesis temperature. Under these conditions, the concentrated molybdic acid within microgels could undergo rapid nucleation and then continuous growth, along with constant assumption of free precursor. On the other hand, the average hydrodynamic diameters in Fig. 6d is 237 nm, lower than pure microgels’, and much lower than the sizes of their TEM images in Fig. 6a, which indicated the aggregation happened during the preparation of the sample for TEM tests, not in the hydrothermal process. Besides, a small amount of free nanoflakes with sizes of 10 ∼ 100 nm have also been recorded by DLS detector, which indicated the microgels with crosslinked degree of 1% can hardly completely load nanocrystals in the
a The samples were synthesized with additional ethanol of 7 mL and performed at 70 °C for 24 h. b Precursor was prepared by the over oxidized MoS2 solution of 2 mg/mL.
287.5ev, corresponding to C elements of −CH2, CeN and C]O, respectively [29]. In the high resolution spectrum of O elements, peak at 530.6 ev was O1s spectrum of Mo-O-Mo [28]. Besides, peaks at 532.2 and 530.9 ev might respectively correspond to O of C]O with and without the interaction to Mo. Similar interactions also have been reported in the reference [30,31], where Tb(III) was bonding to O atoms of the carbonyl groups of poly(N-isopropylacrylamide) to form polymer-Tb(III) complex. The main peaks in Fig. 5d at 235.7 and 234.6 ev were the binding energies of 3d3/2 and 3d5/2 of molybdenum respectively. These two peaks have been commonly regard as features peaks of α-MoO3, in agreement with the analysis results from HRTEM (Fig. 4c). In addition, suspected small peaks at 232.6 and 231.5 ev might be the appearance of Mo(V) oxidation state, owing to the weak reducibility of N-vinyl caprolactam units. These results confirmed the composition of hybrid microgels, especially the coordination 135
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Fig. 6. Low(a) and high(b) magnification and high-resolution(c) TEM images of the PVCL1-MoO3-0.5 hybrid microgels; (d) CONTIN plots of the PVCL1-MoO3-0.5 hybrid microgels in a mixture of water and ethanol with a volume ratio of 1:1.
Fig. 7. (a) SEM image of the PVCL1-MoO3-2.5 hybrid microgels, the inset is TEM image; (b) CONTIN plots of PVCL1-MoO3-2.5 in a mixture of water and ethanol with a volume ratio of 1:1.
microgels matrix. These results reveal that the microgels with low crosslinked degree of 1% would partly lose their template effect, and work more like linear polymer to help the formation of nanocrystals with large sizes. Higher concentration of molybdic acid precursor than the previous experiment has also been used to synthesize molybdenum oxide nanocrystals in the modified hydrothermal procedure. From SEM image (Fig. 7a), aggregation of molybdenum oxide rods at lengths of hundreds
nanometers were observed, along with a little of nanospheres. The aggregation revealed that these nanorods might be mostly trapped in the microgels matrix, which also was indicated by DLS in Fig. 7b. Compared with PVCL1-MoO3-0.5 hybrid microgels, hybrid microgels with more molybdenum oxide shrunk more seriously. To further confirm the inner structure of hybrid microgels, a TEM image was recorded and the result was inset to SEM image. Large amount of nanorods can be found outside the microgels, as a result of easy movement of small 136
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Fig. 8. Curves of the normalized transmittances vs temperature for (a) the PVCL1 hybrid microgels synthesized with varying volume contents of precursor, the concentration of precursor is 2 mg/mL; (b) the PVCL hybrid microgels with varying crosslinked degrees of 1%, 2%, 5%; Plots of cloud points of the hybrid microgels vs volume contents of precursors (c) and crosslinked degrees (d) in aqueous solution.
nanorods from lowly crosslinked PVCL1 microgels. The formation of nanorods might be due to the scrolling of molybdenum oxide layers. With the addition of higher amount of MoO2(OH)2 precursor, nanocrystals kept growing until no more N-vinyl caprolactam units could be utilized to stabilize the surface of nanocrystals. The failure to stabilize the 2D nanoflakes finally led to the scrolling of molybdenum oxide layers along certain specific crystal directions to form nanorods. On the other hand, it also indicated that the microgels with low crosslinked degree of 1% appeared to have the limited capacity for the efficient loading of the nanocrystals, because of its larger porosity than that of the microgels with higher crosslinked degree of 2%.
sterically limit the collapse of hybrid microgels. The last factor that caused higher cloud points might be the restriction of inorganic nanocrystals on the movement of polymeric chains [32]. Furthermore, the effect of different crosslinked degrees of the microgels with the same amount of MoO3 contents also was investigated and calculated through the same method as mentioned above. The cloud points as a function of crosslinked degrees were shown in Fig. 8d. As the crosslinked degrees increased, the cloud points decreased slightly. The reason for the decrease of the cloud points might be that the high crosslinked degree of microgels would hinder the formation of MoO3 nanoparticles in PVCL microgels matrix; as a result, MoO3 nanoparticles dispersed in the aqueous solution. In the process of phase transition separation induced by heating, the free MoO3 nanoparticles worked as crosslinker and decreased the phase transition temperature of the hybrid microgels, as reported in the previously similar study [33]. In our colloidal stability test (Fig. S5), the absorbance at 325 nm for the aqueous solution of PVCL2-MoO3-0.5 hybrid microgels almost remained unchanged, without any observable precipitation even after the place for 15 days (as shown in the inset of Fig. S5). The results demonstrated that the hybrid microgels exhibited well colloidal stability in aqueous solutions.
3.3. Thermoresponsivity and colloidal stability of hybrid microgels In order to investigate the effect of the incorporation of MoO3 nanoparticles on the thermal responsivity of PVCL microgels, the transmittances of hybrid microgels in aqueous solution have been measured by UV–vis spectrophotometer as a function of the temperature, as shown in Fig. 8a–b. The cloud points’ values were obtained as the method previously. Fig. 8c showed the variations of the cloud points of hybrid microgels varied as a function of the precursor contents. An increase of cloud points can be observed with an increasing amount of MoO3 nanoparticles. The result might origin from multiple factors because of the loading of metal oxide nanoparticles. One factor could be the repulsive interactions between the MoO3 nanoparticles trapped in the microgels matrix. Moreover, the coordination bonding between metal oxide nanoparticles with carbonyl groups of VCL units could also weaken the thermal sensitivity of PVCL microgels. Besides, the existence of MoO3 nanoparticles within the microgels matrix would
3.4. Optical properties of hybrid microgels It is well-known that the inorganic nanocrystals with limited sizes of 1∼100 nm could exhibit strong luminescence from the deep UV to near-infrared wavelength owing to quantum confinement effect. Commonly, the wavelength and intensity of photoluminescence can be tuned by changing their sizes, surface states, solvents, pH and doping 137
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Fig. 9. (a) Absorption spectra of the hybrid microgels with the various loaded amount of molybdenum oxide; (b) The excitation-dependent photoluminescence emission spectra of the PVCL1-MoO3-0.5 hybrid microgels; Fluorescent images of the synthesized molybdenum oxide that without (c) and with (d) the use of microgels.
elements [34]. Specially, sizes and surface states have critical influence on their luminescent effects. Here, the optical properties of hybrid microgels were investigated by fluorescence spectra, and combining the data from HRTEM and XPS, we further discussed the relationship between the optical properties of hybrid microgels and their morphologies and surface states. The UV/vis absorption spectra of hybrid microgels loaded with varying amounts of molybdenum oxide were recorded and shown in Fig. 9a. From the absorbance spectra, α-MoO3 in the hybrid microgels were observed to have an absorption region of 200–450 nm, which can be attributed to the charge transfer of Mo-O band in MoO66− octahedron [35]. According to the previous reference [36], the optical gap energy of α-MoO3 was identified to be that of 2.85 eV. Moreover, as the concentrations of precursor in the synthesis of hybrid microgels increased, the absorption intensity also increased as expected. The photoluminescence of PVCL1-MoO3-0.5 hybrid microgels was measured under excitement waves of the wavelength from 275 nm to 425 nm, and the results were recorded in Fig. 9b. The photoluminescence spectra mainly exhibited two emission bands: near the band-edge of UV region (320–380 nm) and at visible emission band (410–470 nm). Specifically, the visible emission band can be separated into three peaks at 410, 450, 470 nm, respectively. The band at 410 nm can be attributed to Mo5+ dyz1-dxy1 transition and the band at 450 nm and 470 nm can ascribed to the deep level of Mo5+ dyz2-dxz2 transition. The strong deep level emission might originate from the crystal defects, such as structural or surface defects. On the other hand, the relatively wide UV emission band from 320 nm to 389 nm, as mentioned previously, might generate from the uneven size distribution of α-MoO3 nanoflakes [37]. The fluorescent photographs of MoO3 nanocrystals and PVCL1-MoO3-0.5 hybrid microgels (seeing Fig. 9c–d) also showed
their difference in background light. Hybrid microgels exhibited bright blue background light as a result of the restriction of MoO3 nanocrystals in microgels. The photoluminescence spectra of PVCL1-MoO3-2.5 hybrid microgels in the Fig. 10a and 10c indicated the similar emission band changes with remarkable decrease at the visible emission band. The decrease of photoluminescence intensities of the band at 450 nm might be ascribed to electronic coupling effect of PVCL1-MoO3-2.5 hybrid microgels as a result of the morphology change from nanoflakes of PVCL1-MoO3-0.5 to rods of PVCL1-MoO3-2.5 hybrid microgels. The result revealed that the formation of molybdenum oxide nanoflakes in the case of PVCL1MoO3-0.5 hybrid microgels involved the trap of poly(N-vinyl caprolactam) polymeric chain into interlayers of molybdenum oxide, which further increase the electronic decoupling of the layers. Contrast to that, the nanorods within the PVCL1-MoO3-2.5 hybrid microgels were dispersed in the aqueous solution, and showed weak interaction with poly (N-vinyl caprolactam) microgels. The influence of the crosslinked degrees of microgels template on the photoluminescence response of molybdenum oxide nanocrystal were also discussed, as shown in the Fig. 10b and d. The result indicated that the increasing crosslinked degree would decrease the photoluminescence intensity of visible emission band of α-MoO3. Combining with the results of TEM images of PVCL2-MoO3-0.5 (Fig. 4a–c), it can be inferred that the denser crosslinked microgels provided limited spaces for the growth of molybdenum oxide nanocrystal and further reduced the crystallinity of nanocrystals, which would response for the decay of photoluminescence intensity of α-MoO3. Similar results had also come out in the reference about molybdenum oxide nanoparticles [37] and ZoO thin film [38]. Besides, the limited light transmittance of microgels with a crosslinked degree of 2% and 5% might also be responsible for 138
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Fig. 10. The excitation-dependent photoluminescence emission of the PVCL1-MoO3-2.5(a) and the PVCL2-MoO3-0.5(b) hybrid microgels; And the photoluminescence emission of the hybrid microgels with varying loading of molybdenum oxide(c) and varying crosslinked degrees(d) at the excitement wavelength of 375 nm.
the decreased photoluminescence intensity, since it would decrease the transmittance of excitement light. To improve light sensitivity of nanocrystals, further strategies in the synthesis process, such as higher temperature, need to be considered to raise their crystallinity. Anyway, these results both showed the interaction between molybdenum oxide nanocrystals and PVCL microgels, not only owned the significant influence on morphologies of nanocrystals, but also could enhance their optical properties owing to electronic decoupling effect.
introduction of high reactive ligand groups, such as carboxyl or amine groups, still need to be considered if increased properties (e.g. plasmonic) for nanocrystals are expected. To conclude, a novel synthesis strategy for hybrid of nanocrystals within thermoresponsive microgels was introduced to overcome the limit of synthesis temperature and open up new possibilities for hybrid nanoparticles in nanotechnology.
4. Conclusions
The authors are grateful for the Excellent Academic Leaders Foundation of Harbin, China (No. 2014RFXXJ017), and the Open Project of State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (No. QA201610-02).
Acknowledgements
We reported the application of poly(N-vinyl caprolactam) microgels with different crosslinked degrees as micro/nanoreactors for in situ generation of molybdenum oxide nanocrystals through a modified hydrothermal process. Here, high synthesis temperature for in situ generation of nanocrystals within thermoresponsive microgels was firstly reported, realized by adjusting volume ratios of ethanol and water. In such hydrothermal process, poly(N-vinyl caprolactam) microgels worked as both template to offer confined spaces of crosslinked 3D matrix for size-controllable nanocrystals and stabilizer to control nucleation and growth of nanocrystals. Various structures of molybdenum oxide (e.g. 2D nanoflakes, nanorods and nanoplatelets) were synthesized under different synthesis parameters, along with changing luminescence intensity. Specially, the microgels with crosslinked degree of 2% showed an effective trap and a well size-controlled effect during the formation of the nanocrystals. These results highlight the feasibility of the synthesis temperature for in situ generation of nanocrystals, including the improvement of the precursor’s reactivity and the controllability to the formation process of nanocrystals. Despite these, the
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