One-pot synthesis of silica-coated copper nanoparticles with high chemical and thermal stability

Accepted Manuscript One-pot Synthesis of Silica-coated Copper Nanoparticles with High Chemical and Thermal Stability Shohei Shiomi, Makoto Kawamori, Shunsuke Yagi, Eiichiro Matsubara PII: DOI: Reference:

S0021-9797(15)30128-4 http://dx.doi.org/10.1016/j.jcis.2015.08.033 YJCIS 20667

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

8 June 2015 25 July 2015 19 August 2015

Please cite this article as: S. Shiomi, M. Kawamori, S. Yagi, E. Matsubara, One-pot Synthesis of Silica-coated Copper Nanoparticles with High Chemical and Thermal Stability, Journal of Colloid and Interface Science (2015), doi: http://dx.doi.org/10.1016/j.jcis.2015.08.033

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One-pot Synthesis of Silica-coated Copper Nanoparticles with High Chemical and Thermal Stability

Shohei Shiomia,1,*, Makoto Kawamoria, Shunsuke Yagib, Eiichiro Matsubaraa a

Department of Materials Science and Engineering, Kyoto University, Kyoto 606-8501, Japan b

Nanoscience and Nanotechnology Research Center, Osaka Prefecture University, Osaka 599-8570, Japan

1

Present address: Kyoto Municipal Institute of Industrial Technology and Culture, Kyoto 600-8815, Japan *

Corresponding author. Tel: +81 75 753 5471, Fax: +81 75 753 5480. E-mail address: [email protected]

Abstract With the recent development of nanotechnology, enhancement of the stability of nanomaterials is becoming ever more important for their practical applications.

We studied the silica-coating of

Cu nanoparticles and the enhanced stability of silica-coated Cu nanoparticles to oxidation.

The

metallic nanoparticles are easily oxidized and agglomerated compared with the bulk metals because the nanoparticles possess large specific surfaces.

The Cu nanoparticle is one of the

most difficult nanoparticles to handle due to its absence of the oxidation stability.

In the

synthesis of silica-coated Cu nanoparticles via a sol-gel process using tetraethyl orthosilicate, the addition of NH3 as a catalyst of sol-gel reaction yielded homogeneous silica-coating.

However,

a large amount of Cu nanoparticles is instantly dissolved by forming complex ions in a NH 3 solution during and before the silica-coating process. of Cu nanoparticles.

This is the difficulty in the silica-coating

In the present work, the dissolution behavior of Cu nanoparticles was

electrochemically examined.

This electrochemistry-based optimization of reducing power of a

reaction bath enabled us to synthesize the silica-coated Cu nanoparticle via a consecutive

1

liquid-phase reaction which requires only basic equipment and involves no separate centrifuging or extraction step.

Cu nanoparticles coated by silica shells had the remarkable stability even in

the presence of a strong oxidizing agent.

Furthermore, we demonstrated that the highly stable

Cu nanoparticles can be applied to a red pigment using a unique red color of Cu nanoparticles because of its surface plasmon resonance.

Keywords Cu nanoparticles; Silica-coating; Electroless deposition; Sol-gel process.

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1. Introduction Noble metal nanoparticles show various characteristic properties, such as high catalytic activities, surface plasmon resonance, good electrical and thermal conductivities and low-temperature sintering [1−8].

These unique properties have led to the use of such materials

in wide applications, for example, optical sensors [1−3], biosensors [4], electronic materials [5,6], or colorants [7].

However, as nanoparticles possess large active surface areas, they can

agglomerate and oxidize more easily than their respective bulk materials [9,10]. The difficulty of handling nanoparticles in ambient atmosphere and at room temperature is a barrier to applications of metal nanoparticle systems.

To overcome this problem, nanoparticles are often

used in composite structures, for example as “supported” structures [11,12], “endohedral” structures [13,14], or “core-shell” structures [15−20].

Moreover, the synthesis technique of

“yolk-shell” structures [21,22] is also recently developed.

Among those structures, the

core-shell structure has been widely investigated because it provides high stability and the materials can be easily dispersed in solution.

Silica, as a coating layer, is remarkably

chemically and thermally stable, and silica-coatings maintain the unique color and optical properties of the metal core because of silica’s high optical transparency.

In the electronic

application, the silica-coating certainly has a weakness, because the silica shell shows strong electric insulation and prevent the agglomeration of metallic nanoparticles.

However, from

another viewpoint, the silica shell can give the insulation property to the metallic material with keeping the thermal conductivity of metal by improving the oxidation resistance.

This type of

material, for example, may have a potential for use as a filler of a molding compound for semiconductor packaging that needs both electrical insulation and high thermal conductivity. Moreover, silica can be fabricated by the sol-gel procedure which is a relatively simple chemical procedure [23].

Mine et al. successfully synthesized Au@SiO2 nanoparticles using

tetraethylorthosilicate (TEOS) and NH3 solutions, with the latter acting as a catalyst for the

3

sol-gel process [18].

Kobayashi et al. produced Ag@SiO2 nanoparticles with various SiO2 shell

thicknesses by a simple sol-gel process [19].

The fabrication of Ag nanowires with a

silica-coating by using a polyol method and the TEOS-based sol-gel process has also been reported [24]. In a similar fashion, uniform silica shells have been successfully fabricated on noble metal nanostructures in a solution using NH3 as a catalyst for the TEOS-based sol-gel process.

This silica-coating method is applicable for Ag which is not as stable as Au, because

the sol-gel process proceeds in a solution at room temperature.

However, the silica-coating of

base metals is much more difficult because of the instability of such metal particles. In the present work, we synthesized Cu nanoparticles in NH3 solution by an electroless deposition method [25,26], and formed a silica shell layer for Cu nanoparticles by a sol-gel method with TEOS.

Because Cu nanoparticles are much less stable than Au and Ag, it is highly

required to prevent the Cu nanoparticles from the oxidation or redissolution during the silica-coating process.

If we can understand the oxidation and dissolution mechanism of Cu

nanoparticles in the NH3 solution during the silica-coating process, the appropriate reaction bath condition can be determined to obtain the uniform core-shell structure.

Thus, we investigated

and clarified the oxidation and dissolution mechanism of the Cu nanoparticles from the electrochemical viewpoint, which enables us to optimize the reaction bath condition and synthesize the silica-coated Cu nanoparticles without the oxidation or dissolution of the Cu nanoparticles by the simple process involving no complicated equipment or steps such as centrifuging or extraction steps for the separation of Cu nanoparticles.

2. Experimental CuO powder was purchased from Kanto Chemical, Inc.

Aqueous ammonia (NH3, 28%),

hydrazine monohydrate (N2H4.H2O, 98%), gelatin fine powder, ethanol (C2H5OH, 99.5%),

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tetraethylorthosilicate (Si(OC2H5)4, (TEOS), 95%) and nitric acid (HNO3, 60%) were purchased from Nacalai Tesque, Inc. and used without further purification. Figure 1 shows the synthesis procedure used to obtain Cu@SiO2 nanoparticles.

CuO powder

(0.25 g) was dispersed in aqueous NH3 solution (28%, 19 ml) and gelatin (0.20 g) was added as a dispersing agent.

Hydrazine (2.0 ml) was mixed with NH3 solution (28%, 19 ml) and gelatin

(0.20 g) added. These two solutions were stirred separately at a rate of 500 rpm using a magnetic stirrer.

To minimizing the effect of dissolved oxygen, nitrogen gas was bubbled at a

rate of 50 cm3 min−1 for more than 30 min.

The temperature of the solutions was maintained at

323 K throughout the synthesis of the Cu nanoparticles. the reaction and had a total volume of about 40 ml.

The two solutions were mixed to start

In parallel to the above process, ethanol (32

ml) and TEOS (8 ml) were mixed and nitrogen gas was bubbled through the solution at a rate of 50 cm3 min−1 for more than 30 min.

This TEOS/ethanol solution was added to the suspension of

synthesized Cu nanoparticles at room temperature and stirred for more than 20 min. TEOS/ethanol solution was added about 30 min after the Cu nanoparticle deposition. gas was bubbled continuously throughout the entire process.

The

Nitrogen

The as-synthesized product was

also annealed in Ar at 973 K for 10 h. During the reaction, a copper-sputtered quartz crystal microbalance (QCM) electrode (SEIKO EG&G, QA-A9M-CU) was immersed in the solution and its frequency change was monitored [27−29].

The weight change of the electrode was calculated from the frequency of the quartz

substrate using the Sauerbrey equation;

m  

A ρqμ q 2 f0

2

f

5

where f0 is the frequency of the QCM substrate before the weight change, A is the active area of the QCM substrate (0.196 cm2), q is the density of quartz (2.648 g cm−3), and q is the shear modulus of quartz (

g cm−1 s−2).

A copper red glass (glaze) was prepared by mixing low-melting-temperature glass frit, Raku frit [30], (1.0 g) and Cu@SiO2 nanoparticles that had been annealed at 1173 K (0.04 g), painting the mixture on a white ceramic test piece, and heating the test piece at 1073 K in air.

The color

of the product was measured by a colorimeter (Nippon Denshoku, NF333 spectrophotometer).

3. Results and Discussion 3.1 Synthesis of Cu@SiO2 nanoparticles Figure 2a shows a scanning electron microscopy (SEM) image of the as-synthesized Cu nanoparticles having a diameter of about 30 nm which was determined by an image analysis for randomly selected 300 particles.

The silica-coating was applied to the Cu nanoparticles by

adding a TEOS/ethanol solution to the Cu nanoparticles dispersed in an aqueous NH3 solution immediately after the completion of Cu nanoparticle formation.

In this work, the addition of

TEOS/ethanol solution was suggested to be completed within 30 min after the Cu nanoparticle formation.

Here, NH3 acts as a catalyst for the production of a continuous and homogeneous

silica-coating in the sol-gel process.

Therefore, only adding a TEOS/ethanol solution to the Cu

nanoparticles suspension can provide the silica-coating on Cu nanoparticles.

Figure 2b shows

the SEM image of the silica-coated Cu nanoparticles with a diameter of about 120 nm.

As can

be seen in the SEM image, the core-shell structures are confirmed by the contrast difference between the cores and shells. nm.

This means that the thickness of the silica shell is about 40 to 50

The shell is actually composed of mesoporous silica, implying that the product still had a

relatively low stability. To form denser and less porous silica shells, the silica-coated Cu nanoparticles were heat-treated in an Ar atmosphere.

Figure 2c shows the silica-coated Cu

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nanoparticles after heating at 973 K for 10 h.

The contrast difference between the cores and

shells remained, and the diameter of the Cu@SiO2 particles decreased to approximately 100 nm, as shown in Figure 2c, indicating a contraction of the silica shells to about 30 to 40 nm because of the condensation of the silica shells during the thermal treatment.

Thermogravimetric

analysis of sol-gel-derived silica also showed the condensation of silica as shown in Figure 3. The weight of silica gradually decreased with heat treatment and finally became 78 % at 973 K. This weight loss corresponds to the decomposition of hydroxyl groups and condensation of silica with increasing temperature.

The curve was almost flat at 973 K, which indicates that the

condensation of silica almost finished around this temperature.

The silica shells seem to be a

little fused and agglomerated by the heat treatment, but most of the particles are separately dispersed, and the Cu cores are not agglomerated at all.

To check the stability of the

heat-treated Cu@SiO2 nanoparticles, both the heat- and non-treated Cu@SiO2 nanoparticles were immersed in aqueous HNO3 (3.0 mol dm−3) solution for 24 h. Transmission electron microscope (TEM) images and X-ray diffraction (XRD) patterns of the two sets of samples are shown in Figures 4a and 4b.

The heat-treated silica shells prevented the Cu nanoparticles from

undergoing oxidation and dissolution, as shown by the areas of dark contrast in Figure 4a that indicating metallic particles.

In contrast, the Cu cores of the untreated samples dissolved in the

aqueous HNO3 and hollow silica particles remained in the TEM images as shown in Figure 4a. The XRD pattern of heat-treated sample in Figure 4b confirms the fcc peaks of Cu, and also indicates that the Cu core was protected from oxidation by the heat-treated silica shells. Conversely, there are no Cu peaks in the XRD pattern of untreated sample in Figure 4b, indicating the absence of Cu cores.

This is consistent with the TEM image of untreated sample

in Figure 4a which shows that hollow silica shells without Cu cores.

In summary, we

demonstrated that the stability of Cu@SiO2 nanoparticles was greatly improved by heat treatment, and Cu core with a diameter of approximately 30 nm was protected by the coating with silica

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shells of approximately 30−40 nm thickness even in the aqueous HNO3 solution with pH smaller than 0.

3.2 Electrochemical consideration To obtain these uniformly coated Cu@SiO2 nanoparticles, the control of the reducing properties of the reaction bath to prevent oxidation of the Cu nanoparticles during the silica-coating is an important consideration.

Actually, the TEOS solution should be introduced

immediately after the completion of Cu nanoparticle formation for the silica-coating, because the Cu nanoparticles are easily oxidized and redissolved in NH3.

When the suspension of Cu

nanoparticles was left at room temperature, the Cu nanoparticles completely redissolved in NH3 solution within 24 h after deposition.

To examine this phenomenon in detail electrochemically,

the dissolution behavior of Cu and the reducing ability of the solution were monitored by simultaneous QCM and mixed potential measurements.

Figures 5a and 5b show the thickness

of the copper-sputtered QCM electrode in the solution directly after the synthesis and 24 h after deposition of Cu. The thickness was calculated from the weight change and the density of Cu (8.9 g cm−3).

Here, the minus sign indicates a decrease in thickness, and the rate of thickness

change corresponds to the dissolution rate of Cu nanoparticles.

Figures 5c and 5d show the

time-dependent mixed potentials in the solution as synthesized and after 24 h.

As shown in

Figure 5a, the weight change of Cu in the solution directly after synthesis remained unchanged for at least 30 min, indicating that the Cu nanoparticles were prevented from oxidizing and dissolving.

This is consistent with the results of lower mixed potential as shown in Figure 5c.

However, Figure 5b indicates that the Cu nanoparticles continuously dissolved in the solution which was left for 24 h.

The two pictures located above the blue line in Figure 5b are of the

QCM electrodes before and after the dissolution of Cu, which illustrate the dissolution of Cu on the QCM electrode.

Considering that the diameter of the synthesized Cu nanoparticles was

8

about 30 nm, the Cu nanoparticles were estimated to be completely dissolved within 1 min under these conditions.

The results of mixed potential measurements are also consistent with the

dissolution behavior depicted in Figure 5b.

As shown in Figure 5d, the mixed potential of this

solution got higher than that of the as-synthesized solution (Figure 5c), which indicates that Cu became unstable in this NH3 solution with time. To achieve a silica-coating on the Cu nanoparticles, the rate of silica formation is required to be faster than that of Cu dissolution. approach using QCM.

The rate of formation of silica was measured by a similar

Figure 6 shows the time-dependent deposition amount of products on a

QCM electrode during the fabrication of silica.

The deposition amount undergoes little or no

further change after 15 min, indicating completion of the reaction.

Because the Cu

nanoparticles remained stable for at least 30 min (Figure 5a), the silica shell formation could take place before dissolution of the Cu nanoparticles.

Based on these results of QCM and mixed

potential measurements, we quantitatively determined the hydrazine concentration (1.0 M) required to keep the Cu nanoparticles from undergoing dissolution during silica shell formation. Here, we try to discuss the reason for Cu dissolution and the accompanying increase in the mixed potential electrochemically.

Figure 7 schematically shows the current-potential curves of

the major reactions occurring during the deposition and dissolution of Cu, considering only hydrazine, Cu and dissolved oxygen as the chemical species present in the solution.

First, for

the synthesis of Cu nanoparticles, when excess hydrazine is added to the solution as a reducing agent, the mixed potential is mainly determined by the anodic reaction of hydrazine and the cathodic reaction of Cu(II) (Figure 7a).

The oxidation-reduction potential E of Cu(II)/Cu is

determined by the following Nernst equation;

ECu(II)/Cu  

1  0 1  G  RT ln 2 F  aCu(II) 

9

where F is the Faraday constant, G0 is the change in Gibbs free energy that accompanies the formation of 1 mole or 1 atm of a product from its component elements, respectively, R is the gas constant, T is the absolute temperature, and aCu(II) is the activity of Cu(II) ion.

Here, as the

reduction reaction of Cu proceeds, the mixed potential and oxidation-reduction potential of Cu(II)/Cu both become lower, because the activity of the Cu ionic species decreases.

When the

deposition of Cu ends, the mixed potential approaches the oxidation-reduction potential of hydrazine, and the oxidation-reduction potential of Cu(II)/Cu moves closer to the mixed potential (Figure 7b). This is inferred from the absence of a driving force (difference between the oxidation-reduction potential and mixed potential) for Cu deposition.

Over time, the hydrazine

gradually decomposes, and the reduction current of dissolved oxygen dominates.

Although

nitrogen gas bubbling was introduced, this could not completely eliminate the presence of dissolved oxygen.

At this stage, the mixed potential is determined mainly by the anodic

reaction of Cu(II)/Cu and the cathodic reaction of dissolved oxygen (Figure 7c), leading to Cu dissolution.

The oxidation-reduction potential of Cu(II)/Cu gets higher as the activity of the Cu

ionic species increases by following the Nernst equation.

In some previous reports, the

existence of the complex ion Cu(NH3)2+ was reported [31], but in this discussion, only Cu(II) is noted as a Cu ion for the simplicity.

In the case of considering Cu(I), the relation of

oxidation-reduction potentials and mixed potential is the same as Figure 7. The shifts in mixed potential and oxidation-reduction potential of Cu(II)/Cu which discussed in Figure 7 are depicted schematically in Figure 8a as a time-dependent description.

Figures 8b−f

show conceivable weight changes of the QCM substrate, corresponding to the driving force for Cu oxidation.

Although, it is impossible to monitor the exact value of the driving force for Cu

oxidation, QCM measurements enabled us to determine the dissolution rate of Cu which depends on this driving force.

The results shown in Figures 5a and 5b correspond to Figures 8c and

8d–f.

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3.3 Application of Cu@SiO2 nanoparticles to a red glaze Finally, we applied the stable Cu@SiO2 nanoparticles as a red glaze, taking advantage of the red color of the Cu nanoparticles.

A glaze is a glassy layer which covers pottery or ceramics to

give a colorful or a shiny appearance [32].

Generally, copper red glass, and particularly copper

red glazes for Japanese pottery, are fabricated by mixing the glass components and a Cu compound with a harmful additive agent such as Pb.

A complex heat-treatment process is

performed using a sequence of exposure to oxidizing and reducing atmospheres [33−38].

A red

color could be achieved using the stable Cu@SiO2 particles alone without any additives or complex heat treatments.

A mixture of a low-melting-temperature glass frit [30] and the

Cu@SiO2 nanoparticles was heated at 1073 K in air.

Figure 9 shows the appearance and color

values in L*a*b* color space [39,40] of the glaze containing Cu@SiO2 nanoparticles and a glaze containing non-coated Cu nanoparticles as a reference.

This indicates that the red color arose

from the Cu@SiO2 nanoparticles and the non-coated Cu nanoparticles were oxidized and ionized in the glass by the heat treatment.

We thereby demonstrated that the Cu@SiO2 nanoparticles

exhibited a high stability even at high temperatures up to 1073 K, in other words, the silica shell protected Cu core against the heat treatment in the air.

4. Conclusions In the present work, we demonstrated the synthesis of silica-coated Cu nanoparticles via a consecutive liquid-phase method.

The silica-coating of rather stable metal nanoparticles like Au

or Ag has been reported in previous works [18,19].

However, because Cu nanoparticles are

intrinsically unstable to the oxidation, the handling of Cu nanoparticles was very difficult. Especially, they were easily dissolved in the NH3 solution which was used as a catalyst of sol-gel process for silica-coating by forming the complex ion. Specifically, the re-dissolution of Cu

11

nanoparticles which were once deposited by the liquid phase reduction method occurred as the reducing ability of the reaction bath decreased.

Therefore, we electrochemically evaluated the

dissolution of Cu nanoparticles in the NH3 solution, and attempted the control of reaction bath condition for the prevention of Cu dissolution before and during the silica-coating process.

By

the mixed potential and QCM measurements, the relation between the dissolution behavior of Cu and the reducing ability of the reaction bath was quantitatively clarified, and when the hydrazine concentration was 1.0 M, the deposited Cu nanoparticles were sufficiently prevented from oxidation and dissolution during the silica-coating.

The chemical stability of annealed vs

as-synthesized Cu@SiO2 nanoparticles was evaluated by attempting dissolution in an aqueous HNO3 solution, and the annealed Cu@SiO2 nanoparticles showed the high stability to the oxidation.

We also produced a red glaze composed of Cu@SiO2 nanoparticles (copper red

glass), without the need for harmful additives such as Pb or the use of complicated processes involving the reducing atmospheres.

As we demonstrated in the synthesis of core-shell structure,

the electrochemical evaluation enables us to understand the oxidation and dissolution mechanism of metallic nanoparticles, and it supports the determination of appropriate reaction bath condition. For the study on the stability of not only Cu nanoparticles but also other unstable metallic nanoparticles and the synthesis of uniform core-shell structures, the electrochemistry-based approach we suggested in this work can be applied.

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Acknowledgements The authors thank Hajime Taguchi from Kyoto Municipal Institute of Industrial Technology and Culture for his help with glaze fabrication. We also express our gratitude to Associate Professor Tetsu Ichitsubo from Kyoto University for his discerning suggestions and discussions. This research was supported by a Grant-in-Aid for JSPS (Japan Society for the Promotion of Science) fellows, and Grant-in-Aid for Knowledge Cluster Initiative (Kyoto Nanotechnology Cluster), from the Japan Society for the Promotion of Science, Ministry of Education, Culture, Sports, Science and Technology of Japan.

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http://dx.doi.org/10.1111/j.1151-2916.1959.tb13566.x. [34] M. Wakamatsu, N. Takeuchi, H. Nagai, S. Ishida, J. Non-Cryst. Solids, 80 (1986) 412, http://dx.doi.org/10.1016/0022-3093(86)90425-4. [35] M. Wakamatsu, N. Takeuchi, H. Nagai, S. Ishida, J. Am. Ceram. Soc., 72 (1989) 16, http://dx.doi.org/10.1111/j.1151-2916.1989.tb05946.x. [36] N. Brun, L. Mazerolles, M. Pernot, J. Mater. Sci. Lett., 10 (1991) 1418, http://dx.doi.org/10.1007/BF00735696. [37] I. Nakai, C. Numako, H. Hosono, K. Yamasaki, J. Am. Ceram. Soc., 82 (1999) 689, http://dx.doi.org/10.1111/j.1151-2916.1999.tb01818.x. [38] M. O. Figueiredo, T. P. Silva, J. P. Veiga, Appl. Phys. A, 83 (2006) 209, http://dx.doi.org/10.1007/s00339-006-3509-0.

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[39] M. I. B. Bernardi, S. Cava, C.O. Paiva-Santos, E.R. Leite, C.A. Paskocimas, E. Longo, J. Eur. Ceram. Soc., 22 (2002) 2911, http://dx.doi.org/10.1016/S0955-2219(02)00057-2. [40] M. Martos, M. Martínez, E. Cordoncillo, P. Escribano, J. Eur. Ceram. Soc., 27 (2007) 4561, http://dx.doi.org/10.1016/j.jeurceramsoc.2007.03.030.

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Figure 1: Schematic diagram of the whole Cu@SiO2 nanoparticles synthesis procedure.

Figure 2: SEM images of (a) Cu nanoparticles, (b) Cu@SiO2 nanoparticles before heat treatment, and (c) Cu@SiO2 nanoparticles after treatment at 973 K.

Figure 3: Weight change of sol-gel-derived silica with the heat treatment up to 973 K measured by thermogravimetry (TG).

Figure 4: (a) TEM images and (b) XRD patterns of heat-treated Cu@SiO2 particles immersed in HNO3 and untreated Cu@SiO2 immersed in HNO3.

Figure 5: (a) Thickness of Cu-sputtered QCM electrodes immersed in a suspension of Cu nanoparticles, synthesized under reducing conditions employing 1.0 M hydrazine and (b) after the reducing power decreased (after 24 hours from the Cu nanoparticles deposition), (c) mixed potential in the Cu nanoparticles solution measured at the same time as (a), and (d) mixed potential in the Cu nanoparticles solution measured simultaneously with (b).

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Figure 6: Amount of silica deposited on a QCM substrate during the fabrication of silica shells.

Figure 7: Schematic illustration of current-potential curves and oxidation-reduction potentials of the main reactions at (a) the starting point of Cu deposition, (b) the end point of deposition of Cu and (c) the start point of dissolution of Cu with the decomposition of hydrazine.

The red point

indicates the mixed potential determined by total anodic and cathodic currents.

Figure 8: (a) Schematic diagram of the mixed potential and the oxidation-reduction potential through the deposition and dissolution of Cu nanoparticles as a function of time and the schematic diagrams of conceivable weight changes evaluated by quartz crystal microbalance (QCM) measurements (b) during the Cu nanoparticle deposition, (c) over the period where reducing ability is maintained, and (d)–(f) after dissolution is induced by the decomposition of the reducing agent.

Figure 9: Appearances and color values in L*a*b* color space of the glaze containing (a) the stable Cu@SiO2 nanoparticles and (b) non-coated Cu nanoparticles.

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Graphical abstract

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