TW201306974A - Synthesis of nanoparticles comprising oxidation sensitive metals with tuned particle size and high oxidation stability - Google Patents

Abstract

Process for the synthesis of nanoparticles comprising oxidation sensitive metals, in particular copper comprising the following steps: Preparation and nucleation of citrate-capped Metal-hydroxide nanoparticles, reduction of the intermediate citrate-capped Metal-hydroxide nanoparticles to Metal0 by reduction via NaBH4 Cu0 nanoparticles with narrow size distribution are obtained by NaBH4-induced reduction of CuCl2.2H2O in diethylene glycol. The course of the reaction essentially involves an intermediate formation of Cu(OH)2 nanoparticles as well as the presence of citrate to control the nucleation of almost monodisperse and non-agglomerated Cu0 nanoparticles. The citrate-capped Cu0 nanoparticles of the invention are surprisingly stable against air oxidation. Via simple solvent evaporation, porous Cu0 thin-films are prepared on glass substrates that exhibit bulk-like sheet resistances of 0.23 - 0.42 Ω after vacuum sintering at 250 DEG C (bulk-Cu sheet under similar conditions with: 0.3 Ω ). With these features the as-prepared, citrate-capped Cu0 nanoparticles become highly relevant to electronic devices in particular thin-film electronics, thin-film sensors and high-power batteries.

Description

Synthesis of Nanoparticles Containing Oxidation-Sensitive Metals with Adjusted Particle Size and High Oxidation Stability

The present invention relates to the synthesis of nanoparticles comprising an oxidation-sensitive metal (especially Cu ⁰ ) having an adjusted particle size and high oxidation stability; and having a bulk-like conductivity The electrode comprising the prepared nanoparticle is included.

There is a high demand for silver and copper films in all types of printed electronics, such as radio frequency identification (RFID) tags, thin film transistors (TFTs), ultra high frequency (UHF) antennas, thin film keyboards, solar energy. Battery or battery tester. Therefore, silver is currently the most commonly used material. However, its dramatic increase in spot prices and concerns about its biological properties have strongly led to the use of copper as a substitute. Here, the problem that arises is the appropriate printing technology and precursor materials. The first is that it has not been solved by electroplating or chemical vapor deposition, and then by a waste plate etching to obtain an electrode of the structure (PC Andricacos, Interface , 1999, 32). Precursor inks containing organometallic copper compounds are limited in advanced, multi-step synthesis and high cost. In some cases, the toxicity of the precursors and organic solvents, the annealing process used to decompose the precursors and the organic solvents used to remove the volatiles, and the vacuum technique must be considered (Rickerby et al. Chem . Rev. , 2002, 102 , 1525). In general, the annealing process becomes a thorny problem mainly because the poisoning of the circuit and the partially reoxidized copper make the conductivity much lower than the whole. Furthermore, the annealing process eliminates the use of low cost substrates such as paper and plastic.

Different strategies have been used to create a thin film copper electrode prepared directly by common printing techniques such as dip coating or spin coating, ink jet, off-set or screen printing. It includes the use of nanoparticles containing nano-sized particles (BK Park et al. Thin Solid Films , 2007, 515 , 7706; S. Gamerith et al. Adv . Funct . Mater ., 2007, 17 , 3111; Y. Wu et al. J. Am . Chem . Soc ., 2006, 128 , 4202; HHLee et al. Nanotechnol ., 2005, 16 , 2436) molten metal suspension (WO 2007038987), and organometallic precursor molecular solution (PJ Smith et al. J) Mater . Sci ., 2006, 41 , 4153; Z. Liu et al. Thin Solid Films , 2005, 478 , 275; GGRozenberg et al. Appl . Phys . Lett ., 2002, 81 , 5249). Furthermore, self-recuding precursors and electrodeposition processes have been suggested as more sophisticated methods (JP 2008190020; T. Osaka et al. Electrochim . Acta , 2007, 53 , 271). Although the electrical properties produced are comparable to the overall copper specific resistance (1.7.10 ^-6 Ω cm) due to its advanced and expensive precursors and / or heat treatment at high temperatures (typically 200-250 ° C, for example The limitations imposed by the decomposition of precursors and the removal of volatiles remain unresolved. The nanoparticle path towards thin film copper electronics has so far been largely limited by the high reactivity of Cu ⁰ nanoparticles. Reoxidation will generally occur immediately after the reaction and during the purification step (M. Abdulla al-Mamun et al. Mater . Lett . 2009 , 63 , 2007) or within hours to days (P. Kanninen et al. J. Colloid Interface Sci . 2008 , 318 , 88). Recently, two literatures have reported that liquid phase synthesis of Cu ⁰ nanoparticles can be stable for several weeks without reoxidation, but it contains high molecular weight poly(N-vinylpyrrolidone) as a polymer stabilizer (PVP). (Y. Lee et al Nanotechnol. 2008 , 19 , 415604; V. Engels et al Dalton Trans. 2010 , 39 , 6496). A disadvantage of PVP is that its polymer will form a layer in the nanoparticle, which has at least a negative effect on the sintering of the resulting particles or even a reduction in sintering, and it is more difficult to remove the PVP coating after synthesis.

Therefore, there is a need for the synthesis of a simple nanoparticle having an oxidation-sensitive metal, particularly Cu ⁰ or In ⁰ (further called Me); it can control particle size, high oxidation stability, and no need Advanced precursors do not use fine inert conditions.

By preparing Me ⁰ containing nano particles in a simple synthesis, which is solved by reduction of metal salts in the presence of a reducing agent having a bis- or tri-functional organic carboxylic acid as a blocking agent in a high-boiling alcohol. This problem.

The method of the present invention is particularly suitable for a metal or alloy comprising an oxidation-reduction potential from -0.9 to +0.9 V, preferably -0.5 to +0.5 V, such as Cu ^II /Cu ⁰ (E ₀ = +0.34 V), In ^III /In ⁰ (E ₀ =-0.34 V), Zn ^II /Zn ⁰ (E ₀ =-0.76 V), Fe ^II /Fe ⁰ (E ₀ =-0.44 V), Ag ^I /Ag ⁰ (E ₀ = +0.80 V), Fe ^III /Fe ⁰ (E ₀ =-0.77 V), Sn ^II /Sn ⁰ (E ⁰ =-0.14 V), Bi ^III /Bi ⁰ (E ⁰ =+0.31 V), Pb ^II / Pb ⁰ (E ⁰ =−0.13).

The first object of the invention is therefore a method for the synthesis of nanoparticles comprising one or more oxidation-sensitive gold Genus, which is a reduction of a metal salt in the presence of a reducing agent having a bis- or tri-functional organic carboxylic acid as a blocking agent in a high-boiling alcohol.

Suitable metal salts may be Me-hydroxides or all salts which can be converted into metal hydroxides, in particular commercially available metal sulphates, metal nitrates, halometalates, such as metal chlorides. Like CuCl ₂ . 2H ₂ O, InCl ₃ . 4H ₂ O.

Suitable blocking agents may be bis- or tri-functional organic carboxylic acids such as citrate, oxalate, and the like.

Suitable reducing agents may, for example, be aluminum hydride salts such as lithium aluminum hydride (LiAlH ₄ ) or others, borohydride salts such as sodium borohydride or potassium borohydride, sulfite compounds, and joint saddles (wo [fu]-qi [ (Wolff-Kishner reduction method), diisobutylaluminum hydride (DIBAH), Lindlar catalyst, oxalic acid (C ₂ H ₂ O ₄ ), formic acid (HCOOH), ascorbic acid (C ₆ H ₈ O ₆ ), phosphite, hypophosphite and phosphorous acid.

The boiling point of a suitable high boiling point alcohol 100 ° C and generally selected from the group comprising: ethylene glycol, butanediol, diethylene glycol (DEG), diethylene glycol ether (such as diethylene glycol diethyl ether, di-glycol Ether), glycerol or polyethylene glycol (PEG), preferably DEG or a mixture thereof and mixed with ethanol or isopropanol.

The ratio of the Me-salt and the blocking agent is generally from 1:0.5 to 1:1.5, preferably from 1:0.6 to 1:0.8.

Suitable Me-salt concentrations in high boiling alcohols are generally from 0.001 to 0.1 M, preferably from 0.005 to 0.05 M. Generally having a viscosity of less than 50 mPas, preferably a uniform flowable reaction of less than 10 mPas Mixtures are preferred, especially in a microreactor system for continuous preparation.

The ratio of Me-salt: reducing agent is generally from 1:0.5 to 1:25, preferably from 1:2 to 1:10. The size of the nanoparticles can be controlled by the amount of reducing agent used.

For better control of the addition, a preferred method is to add the reducing agent in solution form to the high boiling alcohol or water used. Generally, the concentration is from 0.1 to 15.0 M, preferably from 0.1 to 0.5 M. It is advantageous to adjust the pH to help stabilize the reducing agent solution.

A homogeneous flowable reductant solution having a viscosity of less than 50 mPas, more preferably less than 10 mPas, is particularly preferred for continuous preparation in a microreactor system.

Therefore, non-bonded oxidized stable Me ⁰ nanoparticle having an average particle diameter of 1 to 160 nm, preferably 1 to 100 nm, and most preferably 10 to 50 nm can be obtained.

In particular, InCl ₃ initiated by indium borohydride in the presence of citrate as a blocking agent in DEG under one-pot synthesis. Reduction of 4H ₂ O gave In ⁰ .

Citrate capping has been known to control the nucleation and colloidal stability of nanoparticles. Citrate capping has been used for liquid phase synthesis of metals such as gold, cobalt, silver, palladium, platinum (PPEdwards et al. Angew . Chem . Int . Ed . 2007 , 46 , 5480; G. Mpourmpakiset al. Phys Rev. Lett . 2009 , 102 , 155505, JYPark ET AL: Nano Lett . 2008 , 8 , 2388, AM Schwartzberg et al. J. Phys . Chem . B 2006 , 110 , 19935). M. Samim et al. reported the synthesis of Cu ⁰ nanoparticles by controlling the reduction of the copper ion aqueous phase solution with sodium borohydride and capping the metal particles in situ with citrate ions. The prepared nanoparticles exhibit increased efficiencies as catalysts in Ullmann reactions, and the oxidative stability of the resulting nanoparticles is not mentioned ( Bull . Mater . Sci . 2007 , 30 , 535).

Qualitative experiments in Example 3 confirmed that citric acid capping was an important role in the formation of In ⁰ nanoparticle and oxidative stability. It has also been found that the In ⁰ nanoparticle particle size can be controlled by varying the amount of reducing agent used. Simply varying the reducing dose results in a nanoparticle particle size of 8 to 105 nm.

One-pot synthesis of CuCl _{2 in the} presence of citrate as a blocking agent in DEG. The NaHH _{4 of} 2H ₂ O is initially reduced to give oxidized stable Cu ⁰ nanoparticle. Thus the synthesis preferably comprises two steps: a) preparation and nucleation of a citric acid-terminated copper-containing nanoparticle (possibly Cu(OH) ₂ or Cu ₃ (C ₆ H ₅ O ₇ ) ₂ ) intermediate , b) reduction of the citrate-terminated copper-containing nanoparticle intermediate to Cu ⁰ by NaBH ₄ reduction. In general, the formation of a dark cyan/green suspension can be observed in step a). The suspensions, powders and films of the prepared Cu ⁰ have surprisingly high oxidative stability when stored in contact with air, even when heated to 120 ° C in air. The stability of the nanoparticles containing Cu ⁰ is the first application to see the electronic device and a thin film having high correlation.

The importance of citric acid capping for the formation of Cu ⁰ nanoparticles and oxidative stability is qualitatively apparent, mainly because the lack of citrate leads to a large number of copper particle agglomerates with particle sizes much larger than 100 nm. In the absence of citrate, the resulting copper particles are more unstable against air oxidation. X-ray powder diffraction (=XRD) analysis showed that copper oxide was produced when the powder was only in contact with air for several hours.

To verify the effect of intermediate nucleation on the particles, stability experiments were performed at different [OH-] concentrations. In general, if nucleation is avoided in the synthesis, the nanoparticles exhibit an increasing tendency to agglomerate and/or reoxidation. During the reaction, the presence of transitional copper-containing nanoparticles and citrate is included to control the nucleation of nearly monodisperse and non-agglomerated Cu ⁰ nanoparticles. The important surname for citrate is the OH-terminal surface of the copper-containing nanoparticle exchanged via its carboxyl group (Fig. 1). Immediately following the chemical transformation of the transitional copper-containing nanoparticles, the protective outer shell also ensures the proper stability of the Cu ⁰ nanoparticle. Therefore, non-caking Cu ⁰ nanoparticle having an average particle diameter of 1 to 160 nm, preferably 1 to 100 nm, and most preferably 10 to 50 nm can be obtained.

Therefore, it is preferred that the method of the present invention comprises two steps: preparation of the blocked metal-containing nanoparticles and nucleation, reduction of the reducing agent to allow the intermediate to be terminated with metal-containing nanoparticles. The restore becomes Me ⁰ .

A [OH-] concentration of 0.01 to 0.1 M, preferably 0.03 to 0.06 M, promotes Me(OH) _x nucleation.

Heating can also be used to promote nucleation, reduction, or both of the intermediate terminated metal-containing nanoparticles. In general, the process of the invention is from room temperature (typically 20 ° C) to 200 ° C, preferably 20 to 110 ° C The reaction is carried out at a reverse temperature.

The process of the present invention can be batched or continuous. In the case of continuous synthesis, the process of the invention is generally carried out under a microreactor system comprising elements selected from the group consisting of a pumping system, a micromixer, a retained microreactor and a heat exchanger, wherein The mixer and the retention microreactor may also include a micro heat exchanger and elements coupled to other elements such that the reaction mixture may flow continuously from one element to another.

In general, the nanoparticles prepared according to the process of the invention can be carefully washed by means of centrifugation/resuspension of the mixture in/from a suitable solvent, usually methanol, ethanol or isopropanol or mixture.

In the examples, Cu ⁰ nanoparticles are resuspended in isopropanol or DEG and typically form a dark black suspension. a properly diluted suspension (such as Cu ⁰ 1 wt-%) did not show any significant precipitation after a few weeks (Figure 3). If the particle concentration exceeds 1 wt-%, only a slight precipitation will occur in the suspension after a few days. The particle size, particle size distribution and degree of agglomeration of the prepared nanoparticles were detected by dynamic light scattering (Cu ⁰ in DEG, Figure 1). Figure 1 shows a narrow particle size distribution between 15 and 45 nm and an average particle size of 21 (4) nm. This value is also the same as the particle size of the original Cu(OH) ₂ nanoparticles. Surprisingly, the prepared and citric acid-terminated Cu ⁰ nanoparticles can also be converted to be highly stable against air oxidation. No oxide impurities were found in the Cu ⁰ powder sample even after 9 months of contact with air. Even heating to 120 ° C in the air is tolerable.

Therefore, another object of the present invention is to obtain nano particles comprising one or more oxidation-sensitive metals by the method of the present invention, and more particularly, the average particle size of the oxidized stable nanoparticles is from 1 to 160 nm. Most notably, the nanoparticles contain indium or copper.

A porous Cu ⁰ film having a specific electrical resistivity of 6.4.10 ^-4 Ω cm was prepared by vacuum sintering on a glass substrate at 250 ° C by simple solvent evaporation. Proof based on this concept can significantly improve thin layers (such as with regard to uniformity and density), and can be expected to increase conductivity when using this specialized printing technique. Further cost advantages using their formulations - such as inks and pastes - may occur in mask-less production of structured conductive electrodes under high quality printing techniques.

Another object of the present invention is an ink comprising the nanoparticles of the present invention.

The oxidatively stable citric acid terminated Cu ⁰ or In ⁰ nanoparticle of the present invention can be highly correlated with thin film electronic products, light emitting diodes, thin film solar cells, polymer solar cells, inductors or high power batteries.

Therefore, a still further object of the present invention is to provide a device comprising the nanoparticles of the present invention in a special electronic device, more preferably a printed electronic device.

Experimental part

The methods in the present invention are merely examples in the following examples, and are not to be construed as limiting.

Material properties and analysis tools

Scanning electron microscopy (SEM) was performed on a Zeiss Supra 40 VP using an acceleration voltage of 20 kV and a working distance of 4 mm to analyze the particle size distribution and shape of the nanoparticles. All samples were prepared by evaporating a single droplet of Cu ⁰ nanoparticle dispersion prepared in isopropanol under ambient temperature air.

Scanning transmission electron microscopy (STEM) was performed in the same electron microscope as described above for SEM analysis using an acceleration voltage of up to 30 kV and a working distance of 4 mm. STEM samples were prepared on a hollow carbon film copper mesh at room temperature. The average particle size of the nanoparticles was evaluated statistically with at least 200 particles. It should be noted that the different systemic particle size systems obtained by SEM and STEM imaging have different relationship with different electron detection modes and the reaction space of the first-order electrons and the sample. The SEM image was obtained by detecting the backscattered secondary electrons by the Everhart-Thornley detector, while the penetrating electrons were collected only in the STEM mode of operation.

Dynamic Light Scattering (DLS) was tested by Malvern Instruments' Nanosizer ZS (equipped with a 氦氖 laser (633 nm), non-invasive backscattered electrons at 173°, 256 test channels, polystyrene cell detection ). For the analysis, the prepared nanoparticles were dispersed in diethylene glycol by ultrasonic vibration for 15 minutes.

X-ray powder diffraction (XRD) analysis was performed using a Stoe Stadi-P diffractometer with a source of Ge-single-light Cu-K _α1 .

Fourier transform infrared spectroscopy (FT-IR) was recorded on a Bruker Vertex 70 FT-IR spectrometer using KBr sheets. For this purpose, 400 mg of dry KBr was carefully mashed and pressed into a thin ingot with a 1 mg sample. All spectra of the nanoparticles are corrected for scattering effects for comparison with the reference spectrum.

UV-Vis spectra were recorded on a Varian Cary Scan 100 using a suspension of Cu ⁰ nanoparticles in isopropanol. Since particles with a particle size below 50 nm are not expected to provide a significant contribution to particle scattering, the test is performed in a transmission geometry.

Thermal analysis and thermogravimetric analysis (DTA-TG) were tested using a Netzsch STA 409C instrument, using a corundum crucible to hold the sample and serving as a control. The sample was heated to 800 ° C in air or in nitrogen at a heating rate of 5 ° C min ^-1 .

The conductivity test was measured with a four-point probe using a Keithley system (485 Autoranging picoammeter, 199 System DMM/Scanner, 230 programmable voltage source). The shocks were placed in a column and held at a distance of 1.0 mm from each other. The sheet resistance (LBValdes, Proceedings IRE , 1952, 40 , 445) was calculated using the geometric correction factor π / ln 2 .

Example 1: Batch synthesis of Cu ⁰ Nanoparticle

The names of all chemicals used are as follows: copper (II) chloride dihydrate (99%, Riedel de Haën); sodium borohydride (95%, Riedel de Haën); sodium citrate dihydrate (99%, Acros); Ethylene glycol 99%, Merck); ethanol (98%, Seulberger); isopropanol (industrial grade, Seulberger); sodium hydroxide ( 99%, Riedel de Haën). The aqueous solutions were all prepared using deionized water.

Cu ⁰ Nanoparticle Synthesis: A typical synthesis is carried out during a reaction with a dynamic nitrogen purge to remove moisture and oxygen. 85.3 mg CuCl ₂ . 2H ₂ O and 100.0 mg of sodium citrate dihydrate were dissolved in 20 ml of diethylene glycol and heated to 100 ° C with vigorous stirring. During the heating process, the clear green solution turned into a green-dark cyan suspension, which represents the formation of nano-sized copper-containing nanoparticles (Fig. 1). Although the copper-containing nanoparticles become amorphous, their chemical composition is characterized by cyan and qualitatively strong and broad (OH)-vibration via the infrared spectrum (v(OH): 3600-3200 cm ^-1 ). And quantified. The nucleation of the copper-containing nanoparticle can be regulated by adding citrate as a surface blocking agent to the initial solution. This was also confirmed by FT-IR spectroscopy (Fig. 1). Furthermore, dynamic light scattering confirmed a narrow particle size distribution with an average particle size of 21 (5) nm.

At the same time, 10 ml of diethylene glycol was also heated to 100 °C. At this temperature, 75.7 mg of NaBH _{4 was} added and dissolved under vigorous stirring. When NaBH _{4 was} completely dissolved, this solution was added to the first prepared Cu(II) solution. With this action, the color of the suspension instantly turns dark brown due to the formation of copper nanoparticles. After naturally cooling to room temperature, the prepared nanoparticles were collected by centrifugation, and redispersed in a mixture of 1:1 ethanol and isopropyl alcohol and washed three times by centrifugation. Re-dispersion of the cleaned nanoparticles in isopropanol gave a colloidal stable suspension; the solid residue was dried under ambient air for 12 hours to obtain a powder sample. The Cu ⁰ nanoparticle yields a yield of 35 mg. Calculated by taking 10% citrate as a surface capping agent, this indicates that the yield is close to 100%.

In order to verify the effect of the precipitation of copper-containing nanoparticles on the stability of the particles, experiments at different pH values / [OH-] were carried out. In general, during the synthesis process, if precipitation of copper-containing nanoparticles is avoided, the nanoparticles will exhibit an increasing tendency to agglomerate and/or reoxidation. The important role of citrate as a blocking agent is mainly through the OH-terminated surface of the copper-containing nanoparticle passing through its carboxyl adsorption medium (Fig. 1). Just after the chemical conversion of the hydroxide, its protective outer shell also ensures the proper stability of the Cu ⁰ nanoparticle.

After synthesis and purification, the prepared Cu ⁰ nanoparticles are resuspended in isopropanol or DEG to form a dark black suspension. Suitable dilution suspension (eg Cu ⁰ 1 wt-%) did not show any significant sedimentation after a few weeks. If the particle concentration exceeds 1 wt-%, only a slight sedimentation will occur in the suspension after a few days. The particle size, particle size distribution and degree of agglomeration of Cu ⁰ in DEG can be detected by dynamic light scattering (Fig. 1). In this case, a narrow particle size distribution of between 15 and 45 nm and an average particle size of 21 (4) nm are obtained. This value is also better than the particle size of the original Cu(OH) ₂ nanoparticles. In addition to DLS, the particle size and particle size distribution of Cu ⁰ were confirmed to be between 20-30 nm by electron microscopy.

The X-ray powder diffraction pattern of the prepared Cu ⁰ showed all characteristic Bragg peaks of the copper element and confirmed no significant crystalline impurities such as Cu ₂ O, CuO, CuCO ₃ or Cu(OH) ₂ (cf. Figure 4). According to the full width at half maximum of the Bragg peak (111), its crystal size can be reduced to 23 nm via Scherrer formalism. This average particle size value is consistent with the results measured by DLS and SEM. Since the Cu ⁰ nanoparticle exhibits a characteristic surface plasma resonance in the visible spectrum, the presence of copper metal can also be confirmed by its optical properties. To this end, the UV-Vis spectrum of Cu ⁰ suspension in isopropanol was recorded (Fig. 2). Therefore, the maximum absorption value is measured at a wavelength of 614 nm, which is consistent with the literature data (ie, close to 600 nm) (PKKhanna et al. Mater . Lett . 2007 , 61 , 4711; Manjeet Singh, I. et al. Colloids and Surfaces A: Physicochem . Engineer . Aspects 2010 , 359 , 88.). Therefore, the optical properties indicate the surface of the nanoparticle composed of elemental copper.

The FT-IR spectrum of the prepared Cu ⁰ nanoparticle clearly shows the presence of citric acid as a surface-terminated. Although the intensity is weak, the characteristic vibration of 3650-3300 cm ^-1 (v(OH)), 1750-125 cm ^-1 (v(COO)) and the fingerprint area (1200-800 cm ^-1 ) are used as the control group. The map of sodium citrate is identical (Figure 1). Before drying and preparing the IR sample, the Cu ⁰ nanoparticle was carefully cleaned. It was also found that the citrate adsorbed on the surface of the nanoparticle during the reaction, and the copper surface was effectively stabilized and passivated. Sex.

The amount of citric acid adsorbed on the surface of the Cu ⁰ nanoparticles can be measured using thermogravimetric analysis (TG) (Fig. 3a). On sample preparation, the prepared Cu ⁰ nanoparticles were washed as previously described and carefully vacuum dried at room temperature to remove all excess solvent. First, a TG test was conducted under a nitrogen atmosphere to avoid oxidation of any of the nanoparticles. Here, a slight weight loss of 1.8% was observed at temperatures up to 220 °C. This is due to a small amount of solvent remaining on the surface of the particles and dehydration of citric acid (ie > 175 ° C). In addition, there was a 7.6% weight loss between 220 and 280 °C followed by a 1.7% weight loss to 410 °C. This finding forms methyl maleic anhydride or acetone according to thermal decomposition of surface-bonded citric acid, and the decomposition product simultaneously evaporates. It should be noted that the thermal cracking of citric acid shifts to higher temperatures because of the presence of less oxygen in nitrogen (M. Samim, NK Kaushik, A. Maitra, Bull . Mater . Sci . 2007 , 30 , 535). . When further heated to 800 ° C, the sample weight remained almost the same, which also means that the sample did not oxidize to Cu ₂ O or CuO under nitrogen. In summary, the TG of the citrate-terminated Cu ⁰ nanoparticle showed a total weight loss of 11.5%, which also confirmed that about 10 wt-% of the surface-terminated citric acid was present on the surface of the prepared Cu ⁰ nanoparticle. Both are quite a number.

The second TG test was conducted in air to evaluate the thermal oxidation of Cu ⁰ nanoparticles (Fig. 3b). Therefore, the Cu ⁰ sample was again washed as described above and carefully vacuum dried. Here, TG showed a first weight loss of 1.0% when the temperature was raised to 115 ° C, which also indicates a slight residual solvent loss. Unlike TG under nitrogen, a significant weight gain was observed over the next temperature range of 115-240 ° C (+13.0%) and 240-310 ° C (+5.5%). In the first step, the added weight was generated by oxidation of Cu ⁰ to Cu ₂ O (calculation: +12.6%), followed by oxidation to CuO (calculation: +12.6%). When the increase in weight is considered because of the citric acid-related weight loss -1.8% (20-220 ° C) and -7.5% (220-280 ° C), it is also an oxidation reaction that is expected to form. In addition, XRD analysis did confirm the presence of CuO in the TG thermal residue in air. Most notably, the citrate-terminated Cu ⁰ nanoparticles do not produce any oxides in air at temperatures up to 115 °C. In addition to the long-term stability at room temperature, this also emphasizes the outstanding stability of the prepared nanoparticles.

To quantify the stability of citric acid-terminated Cu ⁰ at room temperature and to determine its oxidative stability against air and moisture, an X-ray diffraction pattern was observed (Fig. 5). Therefore, the cleaned and dried Cu ⁰ sample is stored in an open plastic box in contact with air. During the total test period of 120 days, the Cu ⁰ powder sample was observed by XRD every 20 days to confirm whether there was copper oxide formation (ie, Cu(OH) ₂ , Cu ₂ O, CuO, CuCO ₃ ), and whether there was any more The possibility of oxidation. Accordingly, the powder diffraction pattern showed only a small amount of Cu ₂ O formation (Cu ₂ O(111) at 36.4 ° 2θ). For example, when preparing samples for X-ray diffraction analysis, the slight difference in signal-to-noise ratio is related to statistical differences. The most relevant source of error in this type of analysis is the formation of amorphous phase oxidation products. When any oxidation reaction occurs from outside the Cu ⁰ nanoparticle to the inside, the increased oxidized shell subsequently has the following two effects: a) the Bragg peak of the metal Cu ⁰ core changes wide during oxidation; b) with Cu ⁰ cumulative, related to the Bragg peak intensity of Cu ⁰ is reduced. Detailed observations of the strongest Bragg peaks of Cu ⁰ have shown that these two effects do not occur during the 9-month settling time (Figure 4). In summary, these findings have reliably demonstrated the stability of the prepared citric acid terminated Cu ⁰ nanoparticles in air at room temperature.

Cu ⁰ Layer formation of nanoparticles and their electrical conductivity

The conductive layer for preparing the nanoparticles described above can be prepared by applying a droplet onto a glass substrate. Thus, about 20 drops of a 1 wt% cleaned copper nanoparticle suspension in isopropanol were placed sequentially on the cleaned glass substrate. For control and rapid drying, the glass substrate was placed on a hot plate and heated to 50 ° C in air. The dried sample was then sintered in a vacuum at a temperature of up to 500 ° C for 30 minutes. Subsequent cooling to room temperature in a vacuum, the conductivity of the Cu ⁰ film can be measured by a four-point probe. To clarify the structure and composition of the Cu ⁰ film, a portion of the conductive layer on the glass substrate was scraped off using a spatula. The characteristics of the obtained sample cross section were observed using an electron microscope; the removed powder was analyzed using XRD and FT-IR.

All formed Cu ⁰ film sections (before and after sintering) were observed using an electron microscope. Therefore, a conductive layer having a thickness of 20 to 30 μm can be obtained in the deposited layer. Here, the first-order Cu ⁰ nanoparticles will naturally coagulate, but still maintain the original particle size of 20-30 nm clearly visible. Although the Cu ⁰ nanoparticles are in close contact with each other, the high resolution picture shows that the deposited film has obvious pores inside. The particle size and the thickness of the conductive layer are hardly affected when subjected to vacuum thermal sintering at temperatures up to 300 °C. On the contrary, the cross-section of the film sintered at 400 ° C and 500 ° C was significantly reduced to 16 μm and 9 μm, respectively. In addition, the formation of a sintering bottleneck between the individual particles becomes more pronounced. Both effects (reduced thickness of the conductive layer and the observed sintering bottleneck) indicate that sintering of the nanoparticle at temperatures above 300 °C will result in an infinite Cu ⁰ sponge. The crack area of the sintered layer even shows the growth of copper needles, which exhibit an initial particle size diameter and a length of several micrometers.

Sintering of the deposited Cu ⁰ film can be confirmed by the XRD pattern of the sintered layer (Fig. 5). At a temperature of up to 300 ° C, the full width at half maximum (FWHM) of the Bragg peak (111) remains consistent, while higher sintering temperatures are accompanied by a decrease in peak width and a point of growth for the grains. It should be noted that only a slight concentration of impurity phase such as Cu(OH) ₂ , Cu ₂ O, CuO or CuCO ₃ is observed. Depending on the sintering and grain growth, the color of the Cu ⁰ powder changes from dark black to the characteristic luster of the overall metal (Figure 5).

Finally, the conductivity of the deposited and sintered porous Cu ⁰ film can be measured via a four-point probe (Fig. 6).

Although the deposited Cu ⁰ film did not significantly differ from the conductivity of the Cu ⁰ conductive layer sintered at 100 ° C, it caused a sheet resistance of 1.0.10 ⁵ Ω when the temperature was raised to 200 ° C. When the deposited porous Cu ⁰ film is sintered at a vacuum of 300 ° C, the resistance drops sharply by 6-7 orders of magnitude and the sheet resistance becomes 0.5-0.7 Ω. Further increase in sintering temperature does not result in significant improvement (Figure 6). On the one hand, this finding can be attributed to citric acid capping, and on the other hand to the grain boundaries between individual Cu ⁰ nanoparticles. And the conformity between the runout of the electrical conductivity and the weight loss obtained from the thermogravimetric analysis (cf. Fig. 3) is in good agreement with the model and interpretation.

In order to directly compare the deposited porous Cu ⁰ film, a copper plate having a thickness of 1.0 mm hydrochloric acid was used for measurement under the same conditions as a control group. The sheet resistance of the unetched metal piece was measured to be 6570 Ω, and the resistance of the metal piece after etching was 0.27-0.35 Ω. This resistance is equivalent to 0.23-0.42 Ω of the porous Cu ⁰ film, and confirms the latter-like body conductivity. With a porous Cu ⁰ film having an average layer thickness of about 20 μm, the specific resistance was calculated to be 8.4.10 ^-4 -4.6.10 ^-4 Ω cm. Although this value is an excellent conductivity, it is clearly inconsistent with the literature of the copper block (1.7.10 ^-6 Ω cm). This is due to the difficulty in obtaining the "actual" thickness of the porous layer and the accuracy limit of the measurement method. Therefore, the SEM image shows a porosity of >50%. Conversely, it is estimated that an effective thickness of about 10 μm (non-porous) layer will produce a conductivity of 3.2.10 ^-4 Ω cm.

It should be noted that all experiments on the deposition of conductive layers have not optimized the parameters of actual industrial grade inks and their printing (such as viscosity, solid content, solvent form, substrate preconditioning), so that when applying professional printing technology It is expected to increase its conductivity.

In order to measure the oxidative stability of the sintered Cu ⁰ film, the layer was stored in air at room temperature for several months. The X-ray diffraction pattern of the 400 ° C. sintered layer was recorded after 7 months. Neither an increase in the intensity of the Bragg peak of Cu ₂ O nor a decrease in the Bragg peak of Cu ^{0 was} found.

Example 2: Continuous Synthesis of Cu ⁰ Nanoparticle

170.5 mg (1 mmol) CuCl ₂ was placed in a copper precursor container under dynamic nitrogen aeration. 2H ₂ O and 188.2 mg (0.64 mmol) of sodium citrate (dihydrate) were dissolved in 40 mL of diethylene glycol (DEG). The mixture was heated to an oil bath temperature of 100 ° C using an oil bath. A uniform green-blue suspension is obtained.

The reducing solution was prepared in a NaBH ₄ solution vessel, and 200 mg of NaOH was dissolved in 40 mL of diethylene glycol, so that a diethylene glycol sodium hydroxide solution was first obtained, followed by the addition of 151.3 mg (4 mmol) of sodium borohydride. . High pH helps stabilize the sodium borohydride solution. This solution should be prepared each time before use and must be stored at 4 °C.

Both the copper precursor and the NaBH ₄ reduction solution were pumped at room temperature into a capillary continuous microreaction system as shown in FIG. 7, which included a pump (Shimdadzu LC 8A Prep Pump) and a micromixer (used). A capillary microreactor with a magnet stirring volume of 50 mm ³ and a side inlet and a top outlet with PTFE coupled to a 2-10 m PTFE capillary with an inner diameter of 0.5-1 mm, and in the chamber The oil bath was heated at a flow rate of 2 mL/min.

A black suspension was observed at the exit of the microreaction system, which is the formation of Cu nanoparticles. Cu nanoparticles were collected by centrifugation and washing with a mixture of ethanol and isopropanol solution. The particle product is dried under vacuum at 50-100 °C.

The analysis results confirmed the same as the nanoparticles in Example 1.

Example 3: Batch Preparation In ⁰ Nanoparticle

The synthesis of In ⁰ nanoparticles was carried out under dynamic nitrogen aeration. All purification and analysis steps were performed under air. All chemicals used are as described above.

In ⁰ nanoparticles were prepared via a citric acid-auxiliary polyol reaction in which diethylene glycol (DEG, Merck, 99%) was used as the polyol and solvent. First, 2.5 mmol (735 mg) of InCl ₃ . 4H ₂ O (Aldrich, 99.9%) and 1.9 mmol (500 mg) of disodium citrate hydrate (Aldrich, 99%) and 100.0 mL of DEG were placed in a three-necked flask under dynamic nitrogen aeration and vigorous stirring. Heat to 100 ° C in an oil bath. Next, 25.0 mmol (945.0 mg, In ³⁺ : BH ₄ ^- = 1:10, sample A), 7.5 mmol (284 mg, In ³⁺ : BH ₄ ^- = 1:3, sample B) and 5.0 mmol (190) Mg, In ³⁺ : BH ₄ ^- =1: 2, NaBH ₄ (Riedel de Haën, 95%) of sample C) was dissolved in 2.0 mL of deionized water and quickly added to a clear, colorless DEG solution. In a few seconds, the colorless, transparent liquid turns yellow-brown and, at the end, becomes dark brown (sample A), brownish gray (sample B), and gray (sample C). The nanoparticles were collected by centrifugation of the resulting suspension, and finally redispersed and centrifuged in ethanol (samples B and C). The suspension of sample A was added to 50 mL of a saturated aqueous NaCl solution and stirred for 10 minutes. After centrifugation, the solid residue is redispersed in water and alcohol and centrifuged for cleaning. A colloidally stable suspension is obtained by resuspending the nanoparticles in ethanol. The nanoparticles were dried in air at room temperature to obtain a powder.

The particle size of the In ⁰ nanoparticle can be controlled by varying the amount of borohydride, ranging from 25.0 mmol (sample A) and 7.5 mmol (sample B) to 5.0 mmol (sample C).

The particle size and particle size distribution of the prepared In ⁰ nanoparticles were first measured by dynamic light scattering (DLS) (Fig. 8). The In ⁰ nanoparticles were then carefully washed by centrifugation/resuspension and finally resuspended in ethanol. DLS analysis demonstrated a narrow particle size distribution and a clear difference in mean flow dynamic diameter: 10 (2) nm (sample A), 58 (19) nm (sample B), and 91 (27) nm (sample C). According to the above information, the suspension A was optically transparent, whereas the samples B and C became more and more translucent (Fig. 8).

Scanning electron microscopy (SEM) confirmed its spherical shape and narrow particle size distribution. Here, the In ⁰ nanoparticle showed a diameter of 8 nm (sample A), 55 nm (sample B), and 105 nm (sample C). Based on this uniform shape, a partially densely coated monolayer can be obtained on the SEM test piece after evaporation through the solvent.

In ⁰ Excellent colloidal stability of nanoparticles prepared with citric acid terminated on the particle surface concerned. The FT-IR spectrum of the prepared In ⁰ clearly shows its presence as surface-terminated citric acid. Although the intensity is slightly weaker, at 3665-3300 cm ^-1 (v(OH)), 1650-1250 cm ^-1 (v(COO) characteristic vibration and fingerprint area (1250-800 cm ^-1 ) are compared with the control lemon The spectrum of the acid is consistent.

Considering the low-noble character of In ⁰ (E ₀ (In) = -0.34 V), rapid reoxidation of the nanoparticles is expected in the proton solvent and in suspension with air.

If the citric acid capping is gone and causes agglomeration and reoxidation, the previously prepared In ^{0 in} DEG does have limited stability: it can be observed that the reoxidation of In ⁰ becomes In in 12-24 hours. Complete bleaching and settling of OH) ₃ /In ₂ O ₃ .

In contrast, the citric acid-terminated In ⁰ nanoparticle prepared in the present invention surprisingly does not undergo reoxidation in either the suspension or the powder. This can be quantified by qualitative verification of its color and X-ray powder diffraction pattern (XRD) (Figure 9).

Therefore, the centrifuged and washed samples were dried in room temperature air. The XRD patterns of Samples B and C show all characteristic Bragg peaks of elemental indium and no (grain) impurities (such as In(OH) ₃ , In ₂ O ₃ ). Even when the In ⁰ powder was in contact with air for 8 weeks, no change or contamination occurred. Based on the integral width of the (101) reflection, the grain size can be estimated to be 80-90 nm via the Scherrer equation. This is in good agreement with the results obtained by DLS and SEM. The budget in sample C is meaningless because the Bragg peak expansion of particles over 100 nm in diameter is in the extended range of the instrument.

Figure 1. Photograph, particle size distribution, and FT-IR spectra during the reaction: (1) nucleation of Cu(OH) ₂ nanoparticles in the medium; (2) reduction to Cu ⁰ nanoparticles. The nanoparticles were resuspended in DEG, and their particle size distribution was analyzed by DLS; the FT-IR spectrum of citric acid-terminated Cu(OH) ₂ and Cu ⁰ nanoparticles (black line) was used as the control group. (grey line).

Figure 2. Optical properties of citric acid terminated Cu ⁰ nanoparticle: UV-Vis spectrum (left) and dilute suspension (1 wt-%) in isopropanol (right).

Figure 3. Thermal differential analysis (DTA) and thermogravimetric analysis (TG) of citric acid terminated Cu ⁰ nanoparticles were carried out under nitrogen (a), air (b) (total weight 16 mg).

Figure 4. XRD pattern of nano-sized Cu ⁰ powders stored at different times in contact with air: a) complete XRD pattern; b) detailed area around the main (111) Bragg peak and corresponding FWHM values (considered as copper blocks) Control group: ICCD-No. 3065-9026).

Figure 5. XRD patterns, photographs, and FT-IR spectra of sintered Cu ⁰ powder.

Figure 6. Sheet resistance of a vacuum-sintered porous Cu ⁰ film on a glass substrate at a high temperature of 500 ° C and compared to a copper piece.

Figure 7. Capillary-based microreactor system

Figure 8. Photograph of citric acid terminated In ⁰ nanoparticle and DLS analysis: suspension of samples A, B and C in ethanol.

Figure 9. Samples A, B, C, and Sample C were contacted with air in powder form for 8 weeks (D) of X-ray powder diffraction pattern of citric functionalized In ⁰ nanoparticle (in ⁰ blocks as a control) (E), ICDD-No. 3065-929).

Claims (12)

A method for synthesizing nanoparticles comprising one or more oxidation-sensitive metals, a reduction of a di- or tri-functional organocarboxylic acid as a blocking agent in a high-boiling alcohol via a metal salt and a reducing agent effect. The method of claim 1, wherein the metal salt and the reducing agent are used in a ratio of Me-salt: the reducing agent is 1:0.5 to 1:25. According to the method of claim 1 or 2, which comprises two steps: a) preparation and nucleation of the blocked Me-Nano particles, b) reduction of the intermediate via reduction of a reducing agent The capped Me nanoparticle is reduced to Me ⁰ . According to the method of claim 3, wherein the [OH-] concentration is 0.01 to 0.1 M to promote the nucleation of Me(OH) _x . Batch method or continuous type can be used according to the method of one of claims 1 to 4. The method according to any one of claims 1 to 5, wherein the oxidation-sensitive metal has a redox potential of from -0.9 to +0.9 V. The method of claim 6, wherein the oxidation-sensitive metal is indium, copper, zinc, iron, tin, antimony, lead or a mixture thereof. A nanoparticle comprising one or more oxidation-sensitive metals obtained by a method according to one of claims 1 to 7. The nanoparticle according to item 8 of the patent application has an average particle size of from 1 to 160 nm. The nanoparticle according to Item 8 or 9 of the patent application includes indium, copper, zinc, iron, tin, antimony, lead metal or a mixture thereof. An ink comprising nanoparticle according to one of items 8 to 10 of the patent application. A device comprising the nanoparticle according to one of items 8 to 10 of the patent application.