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MICROCRYSTALLINE CELLULOSE-NANOPARTICLE HYBRID NANOCOMPOSITES PREPARED BY REDUCTION OF INCORPORATED COPPER AND NICKEL IONS
Kotelnikova Nina Efimovna1 Mikhailidi Aleksandra Mikhailovna2 Vainio Ulla3
1 Prof., dr. sci. (chem.), leading researcher in Institute of Macromolecular Compounds,
Russian Academy of Sciences, St. Petersburg 2 Cand. sci. (chem.), assoc. prof. in St. Peterburg State University of Technology and Design, St. Petersburg, 3 Dr., Helmholtz-Zentrum Geesthach, Zentrum für Material- und Küstenforschung, Hamburg, Germany
Polymer nanocomposites which consist of metal nanophase dispersed throughout a polymer scaffold are one of the major application areas for nanoscale technology which has been studied in the last decades. This is due to their novel functional material properties, which differ from both the isolated atoms and the bulk phase. Interest in the properties of above objects has required the control of the particles' polydispersity, their size and shape, and their organization on the surface and in the bulk structure of the polymeric template.
A large number of physical and chemical methods for the preparation of nanomaterials has been developed. In recent years chemical reduction of metal ions in a polymer matrix has been widely used. In our previous studies we used microcrystalline cellulose (MCC) as a template to yield cellulose - inorganic hybrid nanocomposites, and to improve and expand the application of inorganic materials. The preparation of silver, platinum, palladium, copper and nickel nanoparticles has been successfully performed in the presence of a support matrix of MCC which played a role as a nanoreactor [1-4]. The synthesis of cellulose-metal nanocomposites has been carried out by different routes via interaction of metal ions with several reducers and in various media with the insoluble cellulose template. The preparation of metal nanoparticles along with control of their size, shape and oxidation state has been performed in order to study how the formation of metal or metal oxide nanoparticles, the crystallization and their average size are affected by a given reduction agent and by reaction conditions.
In this paper the main results on the chemical aspects of a modification of cellulose by chemical reduction of copper and nickel ions taken are considered. The techniques WAXS, ASAXS, XANES, XPS, SEM and TEM were applied to illustrate specific features of the metal nanoparticle incorporation into the cellulose support and to study the structure of the cellulose-metal nanocomposites.
Experimental part
Microcrystalline cellulose was used as a porous template for copper and nickel particles. The properties of MCC have been described elsewhere [1]. The DPv of MCC was 170. The pore volume, the pore radius and the specific
3 2
area were 2.16 cm /g, 20 ^m, and 230 m /g, respectively.
The synthesis procedure included diffusion of Cu2+ and Ni2+ ions from solutions of their salts CuSO4 or Cu(CH3COO)2 and NiSO4-7H2O or Ni(NO3^6H2O into the cellulose matrix and their reduction with reducers sodium boron hydride NaBH4, hydrazine sulfate N2H4^SO4 or hydrazine dihydrochloride N2H4-2HQ and potassium hypophosphite KHPO2-H2O in various media. The media included H2O or ammonium hydrate NH3-H2O; sometimes glycerol was added [1, 2].
A synthetic route to prepare MCC-metal composites depended on a reducer and on a media. Cellulose itself in an NH3-H2O medium can reduce Cu2+ to Cu1+ due to the end aldehyde groups in cellulose chains:
R-C=O + [Cu(NH3)4](OH)2 ^ R-COOH + Cu2O + NH3 + H2O, where R is cellulose.
When NaBH4 and N2H4H2SO4 are used Cu2+ ions can be reduced either to Cu1+ or to metallic copper Cu0:
Cu2+ + BH4- + H2O ^ Cu2O and/or Cu0 + B2O3 +H2 Cu2+ + N2H4 + OH- ^ Cu2O and/or Cu0 + NH3 + N2 + H2O
When NaBH4, KH2PO2^O and N2H4-2HCl are used Ni2+ ions can be reduced to metallic nickel Ni0. In both cases, copper (II) oxide CuO and nickel oxide NiO can also be obtained, due to the high capacity of Cu1+ and Ni0 for oxidation:
Ni2+ + BH4- + H2O ^ Ni0 and/or NiO + B(OH)4 + H2O Ni2+ + H2PO2- + H2O ^ Ni0 and/or NiO + H2PO3- + H2T + 2H+ Ni2+ + H2PO2- + OH- ^ Ni0 and/or NiO + H2PO3- + H2T Ni2+ + N2H4 + OH- ^ Ni0 and/or NiO + N2! + H2O
As a result MCC-Cu and MCC-Ni composites with various metal contents were prepared (MCC-Cu and MCC-Ni samples below).
Elemental analysis of the resulting compounds was carried out with a Hewlett-Packard C,H,N-analyser.
Wide-angle X-ray scattering (WAXS) measurements were applied to determine the degree of order in the matrix, the crystal structure of the metal particles, and the size of crystallites in the matrix and in the particles. The mean size of the crystallites was determined using the Scherrer formula [5].
Anomalous small-angle (ASAXS) X-ray scattering and X-ray absorption spectroscopy near edge structure (XANES) measurements were applied to make structural studies at the 1-100 nm length scale and to characterize the shape and size of homogeneous metal particles and the particle size distribution. These measurements have been carried out at the experimental station JUSIFA B1 at the Hamburg Synchrotron Radiation Laboratory (HASYLAB) in Germany [4].
X-ray photoelectron spectroscopy (XPS) was performed with a PHI 5400 spectrometer to analyze the surface and to evaluate the degree of metal oxidation on the surface of cellulose fibres. The spectra were calibrated by the C 1s line of hydrocarbon components with Ebond = 285.0 eV [1, 2].
Scanning (SEM) and transmission (TEM) electron microscopy methods were used to study the surface morphology, the shape and the particle size distribution in the micrometer range on the surface (SEM) and at the nanoscale in the bulk (TEM) of the MCC fibrils. The SEM study was performed with a JEOL JCM-35 CF instrument [1, 2]. The TEM study was performed with a ZEISS EM 10C electron microscope [1].
Results and Discussion
Experimental conditions, namely the type and the concentration of reducer, the reaction medium as well as the temperature of ions diffusion into the matrix and that of their reduction strongly affect the metal content in the bulk MCC-metal composites. The maximum Cu content in the bulk MCC-Cu samples was 13.0 w.% (NH3H2O medium, reducer cellulose itself) and the maximum Ni content in the bulk MCC-Ni samples was 12.8 w.% (NH3 H2O medium, reducer KH2PO2H2O) (Tab. 1).
Table 1.
Metal content in the bulk of MCC-metal nanocomposites (elemental analysis) and on the surface (XPS), and crystallite sizes ___of metal nanoparticles_
Reducer or medium Metal content in the bulk (max), Metal content on the surface XANES and WAXS results
w.% (max), w.% crystallite metal form crystallite size, nm
Cu Ni Cu Ni Cu Ni Cu Ni
NH3H2O 13.0 - 23.1 - CuO - 5.6-19.8 -
NaBH4 4.2 11.1 17.9 23.4 Cu2O Ni0&NiO 7.4-55 5-35
N2H4H2SO4 8.0 - 26.1 - Cu0&Cu2O - 12.3-53.5 4.7 to 53 -
N2H42HCI - 10.0 - 31.5 - Ni0 - 10.8-13.5
KH2PO2-H2O — 12.8 - 5.8 - Ni0* - 5-40
*Ni0 in amorphous form
WAXS, ASAXS and XANES
According to the WAXS results, the chemical processing of nanoparticle formation in the MCC template did not affect the crystalline arrangement of the cellulose matrix. Thus, the size of cellulose I crystallites (the thickness of MCC crystallites at the (200) direction) was estimated to be 7.2 + 0.1 nm and was the same in MCC-metal samples as in a pristine MCC sample.
The formation of copper nanoparticles was extremely sensitive to reactive conditions. X-ray intensity curves of MCC-Cu samples prepared with various reducers contained reflections of crystalline Cu0, CuO or Cu2O (Fig. 1, a) [3]. Synthesis in NH3-H2O medium (with cellulose itself as a reducer) yielded only crystalline CuO. The average size of crystallites depended on the reducer and on the experimental procedure, and was determined from the (111) reflections of Cu, CuO, or Cu2O (Table 1). The CuO nanoparticle size ranged from 5.6 nm to 19.8 nm. Reduction with NaBH4 yielded mainly crystalline Cu2O nanoparticles, the size of which varied from 7.4 to 55 nm. Reduction with NH4-H2SO4 yielded both crystalline Cu2O and Cu0 nanoparticles. The size of Cu2O nanoparticles ranged from 4.7 to 53 nm and that of Cu0 nanoparticles ranged from 12.3 to 53.5 nm.
The formation of Ni crystalline phases and the size of Ni crystallites in MCC-Ni samples also strongly depended on the reducer and its concentration. In the case of intensive reflections of Ni0 and NiO (Fig. 1, b) appeared in X-ray
diffraction patterns of the samples prepared with the reducer NH4-2HQ. The ratio of those crystalline phases mostly depended on the reducer concentration. The higher the concentration of N2H4-2HCl, the higher the amount of Ni0 in the samples. The average size of Ni0 crystallites determined from the Ni (111) reflection was in the range 10.8-13.5 nm.
The diffraction patterns of the MCC-Ni samples synthesized with the reducers KHPO2-H2O and NaBH4 showed reflections from cellulose as well as a broad and weak diffraction maximum at q = 3.09 A-1 (Fig. 1, c). The maximum could be either the (111) reflection of face-centered cubic Ni0, or the (011) reflection of hexagonal closed-packed Ni0 or of amorphous Ni0. The positions of those reflections were the same and it was not possible to conclude which crystalline phase dominates in the nanoparticles [4].
The oxidation state of Ni was determined from XANES results. The X-ray absorption measurements showed that the Ni Kedge was at the same position for the samples MCC-Ni as for the Ni foil (Fig. 2). XANES spectra of the samples prepared with NaBH4 exhibited features indicating that nickel was in Ni0 form and in NiO form [6]. The spectrum of the sample prepared with KHPO2-H2O had smoother features indicating that Ni0 was found in weakly ordered nanoparticles. The size of the Ni0 nanoparticles determined by ASAXS ranged from 5 to 40 nm.
Fig. 1 (a-c). X-ray intensity curves of MCC-Cu (a) and MCC-Ni (b and c) samples: a) Initial MCC sample (1) and MCC-Cu samples [reducers MCC (2) and NH4-H2SO4 (3)]; b) MCC-Ni samples with Ni content 6.9 w.% (1), 7.4 w.% (2), 10.0 w.% (3) (reducer N^^HCl); c) MCC-Ni samples with Ni content 8.8 w.% (1) (reducer NaBH4) and 8.8 w.% (2), 10.2 w.% (3), 12.8 w.% (4) (reducer KH2PO2-H2O).
Thus, the results obtained with WAXS indicated that the crystalline arrangement of the MCC template did not change during the formation of nanoparticles, i.e. the nanoparticles were anchored on the surface or in the amorphous parts of the microfibrils. The same phenomenon
has been already observed in previous studies on cellulose-metal nanocomposites [1, 2]. This means that cellulose plays a role as a nanoreactor for the formation of nanoparticles of copper and nickel.
Fig. 2. The normalized XANES spectra of a Ni foil (1) and of the samples MCC-Ni prepared with NaBH4 (2) and with KH2PO2-H2O (3)
943 6 941 1
935 6 933 6 931 6 929 6 Binding energy, eV
856 852 848 Etaidog energy, ev
Fig. 3. XPS spectra of the Cu 2p3/2 (a) and Ni 2p3/2 (b) lines. a) MCC-Cu samples (a) [reducers NaBH4 (1) and
N2H4-H2SO4 (2)]; b) MCC-Ni samples [reducers NaBH (1) and KH2PO2-H2O (2)].
X-ray photoelectron spectroscopy
XPS results showed that the metal content on the fibre surface in the MCC-metal samples was much higher than that in the bulk (Table 1). The only exception was MCC-Ni samples prepared with KHPO2-H2O. In the XPS spectra of MCC-Cu and MCC-Ni samples prepared with NaBH4, Cu1+ (in Cu2O) and Ni2+ (in NiO) predominate, respectively. In the samples prepared with N2H4H2SO4 Cu0 and Ni0 are mainly distributed on the surface (Fig. 3). A good correlation of these results with the determination of the crystalline phase of metals made with WAXS and XANES can be seen from the data listed in Table 1. These data also show that metals on the surface are only slightly subjected to furher oxidation. Thus, the MCC matrix protected metal nanoparticles from oxidation not only in the bulk but also on the fibre surface.
Scanning and transmission electron microscopy
SEM micrographs of MCC-Cu and MCC-Ni samples show their ^m-scale structure. The globular spheres mainly aggregated into larger agglomerates on the fibre surface of the samples (Fig. 4, a and b). TEM micrographs visualized the particle shape and size distribution in the bulk (Fig. 4, c). The size of nanoparticles in the bulk was much smaller than that on the surface. Thus, the average size of Cu0 particles on the surface was 500-600 nm [Fig. 4(a) and (d)]; Ni0 particles
ranged 120-380 nm (Fig. 4, b and e). However, the average size of copper nanoparticles in the bulk was only 5-25 nm as followed from the histogram (Fig. 4, f) and that of nickel nanoparticles was only 5-40 nm [4]. In the bulk and on the surface smaller particles clustered together to form larger aggregates of particles. The pores in the fibrous cellulose assisted separate growth of particles inside the fibres so that the particles were not as aggregated as on the surface of the fibres as seen by SEM.
Conclusions
Synthetic procedures have been successfully developed to incorporate copper and nickel nanoparticles of different size and shape into the MCC template. As a result, microcrystalline cellulose-nanoparticle hybrid nanocomposites have been prepared. The content of copper and nickel in the nanocomposites strongly depended on the experimental procedure, particularly on the reaction medium and the reducer type.
MCC did not exhibit any changes in supramolecular structure, i.e. it played the role of nanoreactor. Crystalline CuO, Cu2O and Cu0 nanoparticles were prepared with reducers cellulose itself, NaBH4 and NH4-H2SO4, correspondingly. Crystalline Ni0 and NiO nanoparticles were synthesized with
NH4-2HQ and NaBH4, whereas Ni0 nanoparticles in amorphous form were prepared with KHPO2-H2O.
Agglomerates of the metal particles formed on the fibre surface. The nanoparticles were found to be in the range of 555 nm. The MCC matrix protected metal nanoparticles from oxidation not only in the bulk but also on the fibre surface. The nanoparticles were found to be relatively stable.
The variations in experimental conditions provided the opportunity to prepare metal nanoparticles with the wide range of oxidation degree, of sizes and shape and to control their surface properties, stability, and reactivity. This allowed adjusting the properties of the resulting nanocomposites.
200 400 600 BOO 1000 1 20 185 250 315 3«0 445 510 575 4 6 12 16 20 24
Diameter of particles, nm Diameter of particles, nm Diameter of particles, nm
Fig. 4 (a-f). SEM images of Cu and Ni particles on the fibre surface of MCC-Cu samples (reducer N2H4H2SO4) (a) and MCC-Ni (reducer KH2PO2H2O) (b) and their histograms (d and e correspondingly). tEm image (c) and histogram (f) of Cu
nanoparticles in the bulk of MCC-Cu sample.
References
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ВЫСОКОЭФФЕКТИВНАЯ ЖИДКОСТНАЯ ХРОМАТОГРАФИЯ ПРОИЗВОДНЫХ 4-АМИНОХИНОЛИНА С АМПЕРОМЕТРИЧЕСКИМ ДЕТЕКТИРОВАНИЕМ
Некрасова Надежда Андреевна
Студентка 5 курса Самарского государственного университета, г. Самара
Курбатова Светлана Викторовна
Доктор хим. наук, профессор, декан химического факультета Самарского государственного университета, г. Самара
Климина Анастасия Владимировна Магистрант Самарского государственного университета, г. Самара
В высокоэффективной жидкостной хроматографии (ВЭЖХ) наряду с широким применением оптических детекторов за последние годы наметился значительный прогресс в развитии электрохимических методов детектирования. В настоящее время амперометрическое детектирование применяется при анализе пищевых продуктов, в судебно-медицинских экспертизах, в фармацевтике и биохимических исследованиях, а также в анализе загрязнений окружающей среды. Благодаря высокой чув-
ствительности и селективности такие детекторы с успехом используются для анализа широкого круга неорганических и органических веществ, способных окисляться или восстанавливаться [1, с. 110]. Интересными объектами с точки зрения хроматографирования с использованием амперометрического детектирования являются производные хинолина и, в частности, аминохинолины, способные окисляться на таком детекторе за счет наличия аминогруппы.
