Sugar‐Based Electroless Copper Deposition on Pectin‐Coated Alumina Microparticles

A novel approach for fabricating copper‐coated alumina microparticles utilizing a pectin‐based coating subsequently followed by electroless deposition is reported. The biopolymer pectin is modified with phosphate groups covalently binding to the particle surface, while the polymer itself promotes the formation of metal centers on the surface due to the high affinity to metal cations. The sugar‐based (galactose, xylose, and glucose) electroless plating process in combination with an organic‐based treatment ensures a homogeneous metal coating method. As a benefit, precious metal activation of alumina particles and hazardous reducing agents typically employed are not required, creating a low‐cost and sustainable process. Metal plated micro particles show a uniform and homogeneous coating rendering them as ideal additives for application in metal matrix composites. These ceramic metal composites can be used in a wide range of applications where high‐strength metal components are needed.


Introduction
In the last decades metal matrix composites (MMCs) gained much interest, hence metal coated ceramic particles were in research focus creating higher quality products. [1,2]MMCs are composite materials containing metals with highly dispersed ceramic particles aiming for a strengthening of the material.Furthermore, enhanced high-temperature properties, wear resistance and increased chemical resistance can be achieved. [3,4][3][4][5] The fabrication of MMCs is usually carried out in a liquid or solid phase approach. [3,6]However, both methods face the challenge of particle aggregation due to the low wettability of the ceramic particles by metals because of DOI: 10.1002/admi.202300496 the high difference in the surface energy.This may lead to an inhomogeneous distribution of the ceramic particles which can cause porosity and overall poor mechanical properties. [7,8]Another issue is a possible chemical reaction on the interface between metal and ceramic particles. [9][15][16][17] Nowadays, electroless plating is a widespread method, because of its high versatility and fast and simple process allowing also for mass production.Electroless plating is an autocatalytic process, once it has been initiated, however, in order to get it started, surface activation is required.
Currently, precious metal activation of the ceramic particles is still widely used, [18][19][20] even though this method results in a high cost due to the usage of mainly palladium, silver, and gold.23][24] The most significant steps of our plating process are depicted in Figure 1.To improve the activation step we propose an organicbased version using the biopolymer pectin in comparison to the conventional Pd-based treatment.We chose this biopolymer due to its water solubility and its capability to complex doubly positive charged metal ions. [25]To attach it to the ceramic particle surface (e.g., alumina particles) the pectin is modified using sodium trimetaphosphate (STMP), [26] which adds phosphate groups to the polymer.

Pectin Coated Microparticles
The key feature of pectin is the ability of the carboxyl groups to form strong complexes with the metal ions added to the dispersion, creating metal centers on the surface. [25]After the addition of the reducing agent, the first metal nuclei are formed on the surface, leading to a further deposition process in an autocatalytic manner. [18]In this work, we use pectin and STMP treatment as a sustainable and cost-effective activation process compared to conventional methods.The pectin is modified with STMP as shown in Figure 2. [27,28] This enables covalent bonding to the surface.To prove the presence of pectin phosphate on the surface a fourier transformation infrared spectrometer (FTIR) was used.In Figure 3a) the spectra of the pristine Al 2 O 3 microparticles, pectin phosphate, and the activated particles can be observed.
[36][37] After combining STMP with pectin a clear shift of these peaks is visible in the spectrum of the pectin phosphate to 1410, 1100, and 1015 cm −1 .These peak shifts are indicative of the changed nature of the bond changing from P-O-P to P-O-C, which correlates to the shift to higher wavenumbers.
After the activation process, it is clearly visible in Figure 3b) that the spectrum of the pectin-activated Al 2 O 3 microparticles is a combination of both spectra from Al 2 O 3 and pectin respectively.Most significantly, the broad multiple bands between 900−600 cm −1 from the Al 2 O 3 microparticles can be found unchanged in the spectrum of the product.The spectrum of the pectin phosphate shows the most significant peaks resulting from the phosphate groups at 1410, 1100, and 1015 cm −1 .At least the two peaks at lower wavenumbers can be identified in the spectrum of the product.Due to the weak signals from pectin phosphate itself and the low concentration of pectin phosphate on the surface, it is difficult to resolve the resulting peaks.Nevertheless, a clear difference compared to the pristine Al 2 O 3 particles is visible which indicates the successful activation.

Copper-Coated Microparticles
Furthermore, we show an equally sustainable plating method, in comparison to conventional methods using hydrazine or formaldehyde.Our method is based on monosaccharides as reducing agents together with urea and sodium hydroxide.We assume a Maillard-type reaction between the sugars as reducing agents and urea as an amino compound. [29,30]33] In the first experiments, we tested three different monosaccharides as reducing agents, namely galactose, xylose, and glucose.This is proof that we are able to produce elemental copper with all of the before-named sugars.Further, we see that xylose gives the best result with a nearly pure copper (JCPDS 4-836) phase and no visible copper(I)-oxide in the X-ray diffraction (XRD) pattern from Figure 4.
In contrast to this, we observe that glucose gives a comparably worse result with a significant amount of side products.Even  though galactose is also suitable for copper(II) reduction, we performed all further experiments with xylose due to its best performance of all of the used sugars.
The assumed Maillard-type reaction usually involves a carbohydrate with an aldehyde functionality and an amino compound.Here, the aldose xylose acts as the carbohydrate that reacts in a condensation reaction with the amino moiety of urea.The meanwhile formed and unstable aldosylamine undergoes an Amadori rearrangement and forms an amino-1-desoxiketose that is shown as the second molecule in Figure 5.In the next step, this molecule undergoes an oxidation promoted by two single electron transfer steps from either Cu(II) or Cu(I) species resulting eventually in elemental copper and the corresponding imine. [32,33]he scanning electron microscopy (SEM) images in Figure 6 of alumina microparticles before A) and after B) the copper plating process show a visible copper coating on nearly all of the particles (also verified by energy dispersive X-ray spectroscopy (EDS) analysis).Independent from the pristine particle size the coating was successful.Histograms of the pristine alumina particles and the copper-coated particles are provided in Figure S1 (Supporting Information).Moreover, we observed a copper-to-alumina ratio of 9:1, which correlates with the theoretical values in relation to the educts.Figure 6b shows alumina particles coated by using a traditional hydrazine plating bath (described elsewhere [38] ).With the traditional method, we observe copper nanoparticles which are distributed between the alumina particles.These however seem to be not coated at all with copper, and if so, they appear to stick together in undefined agglomerates.Compared to this approach, our novel plating method is advantageous.
With SEM the thickness of the copper layer was determined to range from 290−360 nm for various sizes of alumina particles which is shown in the images from Figure S2 (Supporting Information).
The XRD patterns of the uncoated particles and the processed alumina microparticles are compared in Figure 7.The diffraction pattern of the Al 2 O 3 microparticles indicates the presence of the -alumina phase, while the copper-coated particles show almost only reflections at 43.2°, 50.4°, and 74.1°, corresponding to elemental copper.This shows the plating process was successful without any observable by-products like copper oxides.The pattern observed from the -alumina phase is barely visible anymore, which also indicates a full coating. [39,40]

Conclusion
In this paper, we reported a new sustainable method for the electroless plating of alumina microparticles combined with an organic activation of aluminum oxide particles.Therefore, a novel activation with a biopolymer pectin was described.Key feature is the capability of pectin to complex divalent metal ions which creates metal centers on the surface of the particles.This supports a uniform coating during the plating process.Experimental results show an activation with the pectin phosphate on the   aluminum oxide particles' surface.The plating method itself was reworked, creating a process using non-toxic and natural products like xylose and urea to perform the reduction.The microparticles, investigated by SEM, EDS, and XRD, show a homogenous coating of all particles.This procedure ensures a scalable way for the synthesis of metal-coated particles without the conventional expensive palladium activation and the highly toxic hydrazine reducing agent.However, the reaction still requires strong alkaline conditions which has further potential for improvements.Nevertheless, it must not be overlooked that sodium hydroxide has in general a way lower hazard potential This method might also be applied to particles of other ceramics.The metal-coated particles created in this procedure might be applicable for the fabrication of metal matrix composites, especially to create high-strength copper components for additive manufacturing and foundry.
Coating of Microparticles with Pectin: The Al 2 O 3 microparticles were prepared in NaOH solution to clean the surface and increase the number of hydroxy groups on the surface.This first step was carried out by suspending 10 g of Al 2 O 3 microparticles in 100 mL 1 m NaOH in deionized water with vigorous stirring overnight.Then the particles were washed with distilled water by centrifugation until the pH value was <9.
The pectin activation solution was prepared by dissolving 1.25 g of pectin in 25 mL of deionized water at 60 °C.Afterward, 25 mL 1 m NaOH solution was added dropwise into the pectin solution.Lastly, 24 mL 0.3 m STMP solution was added via a syringe for 24 h and stirred for at least 5 more hours.The solution was then transferred to a 7000 MWCO dialysis tube and placed in deionized water for 2 days.After the washing step the pectin solution was diluted by half with deionized water and eventually combined with the cleaned Al 2 O 3 microparticles and let rest for 24 h under stirring.After a further washing step with deionized water, the particles were ready for copper plating.The same procedure was performed for SiO 2 microparticles (Figure S3, Supporting Information), except the preconditioning in sodium hydroxide solution was left out because of the solubility of silica in bases.
Further, control experiments were performed where alumina particles were activated by just using pectin without STMP functionalization.Every step of the synthesis was the same except for the addition of STMP.The results are presented in Figure S4 (Supporting Information).
Electroless Copper Plating of Pectin Activated Particles: The electroless copper plating of the microparticles was performed in an aqueous solution.Therefore 20 mL of deionized water was added in a flask, heated to 90 °C, and kept for the rest of the reaction.Then 24 mL 2.8 m xylose solution was added under continuous bubbling of the solution with nitrogen gas.After 15 min, 24 mL 1.3 m copper(II)-chloride solution, 8 mL saturated urea solution, and 0.1 g microparticles (Al 2 O 3 /SiO 2 ) were added.To start the reaction 24 mL 10 m NaOH solution were added quickly into the solution.A visible color change from light blue to orange and eventually dark brown shows a successful reaction.The mixture was stirred for another 30 mins whereafter the product was removed and thoroughly washed with distilled water and ethanol via centrifugation.
Characterization: SEM images were taken with a Hitachi SU8020 using 2 kV acceleration voltage.EDS was carried out with a Silicon Drift Detector X-Max N from Oxford.FTIR spectra were obtained with the Bruker Vertex 70 (32 scans, 2 cm −1 resolution).X-ray diffractograms were recorded using Panalytical Empyrean with CuK 1 radiation.The alumina content was determined by weighing back the alumina microparticles after dissolving the copper from coated particles with nitric acid with subsequent washing with deionized water, ethanol, and a drying step.

Figure 1 .
Figure 1.Schematic illustration of the fabrication process where the particles are activated with pectin phosphate and then plated with copper via electroless plating.The complexation process due to the high affinity of the carboxy groups promotes the metal deposition on the pectin-coated particles.

Figure 2 .
Figure 2. Proposed reaction between pectin and STMP resulting in a condensation and a terminal phosphorylation of the biopolymer.

Figure 3 .
Figure 3. Transmission FTIR spectra of a) pectin, STMP, and die product pectin phosphate after the combination of both and b) pectin-coated Al 2 O 3 microparticles and pristine Al 2 O 3 and the prepared pectin phosphate as reference.

Figure 4 .
Figure 4. XRD pattern of copper reduction using galactose, xylose, and glucose as reducing agents.Black lines indicate elemental copper reference, whereas all other reflections represent a copper(I)-oxide phase.

Figure 5 .
Figure 5. Proposed mechanism of the Amadori-rearrangement with following copper-induced oxidation of the xylose derivative.

Figure 6 .
Figure 6.SEM images of A) pristine aluminum oxide microparticles, B) copper-coated particles from a conventional hydrazine plating bath, C) copperplated particles with xylose as reducing agent, and D) EDS analysis of (C) where turquoise indicates copper and red aluminum.