Presented here is a protocol of bathless electroplating, where a stagnant metal salt paste containing composite particles are reduced to form metal composites at high loading. This method addresses the challenges faced by other common forms of electroplating (jet, brush, bath) of embedding composites particles into the metal matrix.
Composite plating with particles embedded into the metal matrix can enhance the properties of the metal coating to make it more or less conductive, hard, durable, lubricated or fluorescent. However, it can be more challenging than metal plating, because the composite particles are either 1) not charged so they do not have a strong electrostatic attraction to the cathode, 2) are hygroscopic and are blocked by a hydration shell, or 3) too large to remain stagnate at the cathode while stirring. Here, we describe the details of a bathless plating method that involves anode and cathode nickel plates sandwiching an aqueous concentrated electrolyte paste containing large hygroscopic phosphorescent particles and a hydrophilic membrane. After applying a potential, the nickel metal is deposited around the stagnant phosphor particles, trapping them in the film. The composite coatings are characterized by optical microscopy for film roughness, thickness and composite surface loading. In addition, fluorescence spectroscopy can be used to quantify the illumination brightness of these films to assess the effects of various current densities, coating duration and phosphor loading.
Traditional electroplating is widely used to deposit thin films of a variety of metals, alloys, and metal-composites onto conductive surfaces to functionalize them for the intended application1,2,3,4,5,6,7,8,9,10,11,12. This method adds a metal finish to parts used in the manufacturing of aerospace, automotive, military, medical, and electronic equipment. The object to be plated, the cathode, is submerged in an aqueous bath containing metal salt precursors, which are reduced to metal at the surface of the object by the application of a chemical or electrical potential. Non-charged composite particles can be incorporated into the metal film by adding these to the bath during coating to enhance the film properties for increased hardness in the case of metal oxides and carbides, smoothness with polymers or lubrication with liquid oils12,13. However, because these particles lack an inherent attraction to the cathode, the ratio of composite that is incorporated into the metal remains low for bath plating13,14,15. This is especially problematic for large particles that do not adsorb to the cathode long enough to be embedded by the growing metal film. Additionally, hygroscopic particles solvate in aqueous solutions and their hydration shell acts as a physical barrier impeding contact with the cathode16.
Some promising methods have been shown to mitigate this effect by using dry non-polar solvents to remove the hydration barrier completely17, or by decorating the composite particles with charged surfactant molecules16 that disrupt the hydration shell to allow contact between the particle and the cathode. However, because these methods involve organic materials, carbon contamination is possible in the film and breakdown of these organic materials could occur at the electrodes. For example, the organic solvents used (DMSO2 and acetamide) are heated to 130 °C in an inert atmosphere for air-free coating; however, we found them to be unstable during coating in air. Due to resistive heating at the electrodes, redox reactions with organic materials may result in impurities or sites for heterogeneous nucleation and growth of metal nanoparticles18. As a result, there is a need for an organic-free aqueous electroplating method that addresses the long-standing challenge of particle-cathode adsorption. So far, metal-composite bath coating has been shown to embed particles up to a few micrometers in diameter19 and as high as 15 % loading16,17.
In response to this, we describe an inorganic bathless electrostamping method that forces composite particles to become embedded into the film at high surface coverages despite their large size and hygroscopic nature20. By removing the bath, the process does not involve containers of hazardous coating liquids and the object to be plated does not need to be submerged. Therefore, large, cumbersome or otherwise corrosion- or water-sensitive objects, can be plated or “stamped” in select areas with the composite material. In addition, the removal of excess water requires less clean-up of liquid hazardous waste.
Here, we demonstrate this method to produce bright fluorescent metal films by co-depositing non-toxic and air-stable europium and dysprosium doped, strontium aluminate (87 ± 30 µm) with nickel at high loadings (up to 80%). This comes in contrast to previous examples that were plated in a bath and therefore were limited to small (nanometers to a few micrometers) phosphors12. In addition, previously reported electrodeposited films fluoresce only under short-wave UV-light, with the exception of a recent report that grew 1 – 5 µm luminescent strontium aluminate crystals in an alumina film with plasma electrolyte oxidation21. Fluorescent metal films could have far-reaching applications in many industries involving dim-light environments including road sign illumination21, aircraft maintenance equipment location and identification20, automobile and toy decorations, invisible messages, product authentication22, safety lighting, mechanochromic stress identification10 and tribological wear visual inspection12,16. Despite these potential uses for glowing metal surfaces, this method could also be expanded to include additional large and/or hygroscopic composite particles to produce a new variety of metal-composite functional coatings that were previously not possible via electroplating.
1. Preparing coating salts
CAUTION: Nickel salts and boric acid are toxic and should be handled with proper personal protective equipment including nitrile gloves, goggles and a lab coat. Strong acids and bases should be handled in the fume hood, and all waste chemicals should be disposed of as hazardous waste.
2. Preparing the electrodes
3. Assembly and coating
4. Characterization with electrochemistry
5. Characterization with quantum yield fluorescence spectroscopy
6. Characterization with optical microscopy
After following this protocol, a thin coating of metal should become plated onto the cathode surface and contain the composite particles that were added to the coating paste. Fluorescent or colored particle incorporation can be observed by visual inspection as a result of a change in appearance compared to the uncoated surface (Figure 1A1-A3). To investigate the percent surface coverage of the composite particles and to observe the surface morphology of the coating, optical microscopy can be used (Figure 1). Samples can be observed top-down (Figure 1B1-B3 and 1C1-C3) or cut to reveal the cross-section (Figure 1D1-D3). This works for observing microscale composite particles and microscale features in the metal coating. However, nanoscale composite particles and nanoscale morphological features should be observed with a scanning electron microscope19,25,26,27. Imaging software can be used to calculate the sizes and surface coverage of the composite particles by including data from images of several areas over the coated surface.
The results of this analysis can be plotted to show the effects of applied current and/or coating duration on the amount of particle loading (Figure 2A). In general, particle surface coverage increases with time and with current density. For coatings under constant voltage, the surface coverage increases steadily up to 80% after 18 h at a rate of 0.066% per min. For constant current deposition, the particle surface coverage increases rapidly at low currents and then levels off at current densities above 0.025 A∙cm-2. The reason for this is likely due to the observation that coatings at 0.25 A∙cm-2 were rough (Figure 1A1) and a nonuniform coating corresponded to a runaway voltage due to increased resistance (Figure 2A). Therefore, an optimized current density was identified at 0.025 A∙cm-2 for nickel and strontium aluminate phosphor particles, but this parameter may need to be re-determined when changing the metal salt or the composite material.
The thickness of the films increases with time and current density as well, and interestingly the surface coverage is also correlated with thickness (Figure 2B). This is expected because as the film thickness increases, it has the capacity to contain more particles. Coating parameters like these can be monitored by electrochemistry under constant current with chronopotentiometry and under constant voltage with chronoamperometry (Figure 3A). Under constant voltage, the resistance increases steadily and, as a result of Ohm’s law, the current density decreases. Fluctuations in the current can be explained by periodic plating of layers rich in composite particles (high resistance) alternating with layers of metal atoms (low resistance). Under constant current, the resistance stays steady for about 30 minutes; however, at some point, the resistance increases rapidly and the coating becomes unstable. This can be observed in the optical microscope (Figure 1A1) where high currents can lead to inconsistent and rough coatings. Likewise, low currents also produce inconsistent coatings with low composite loading as a result of the low driving force to deposit (Figure 1A3). In contrast, optimized current densities are required for uniform coatings (Figure 1A2). Under optimized conditions, the current efficiencies remain high (85-95%) as calculated from Equations 1 and 2. However, at either high current densities (> 0.025 A∙cm-2) or long durations (> 1 h), we observed a decrease in coating efficiency as the particle loading increases. For example, the film thickness of this metal composite does not increase as would be expected for pure nickel despite higher applied currents (Figure 1D1-D3).
For fluorescent or phosphorescent metal-composite coatings, fluorescence spectroscopy can be used to quantify the brightness and the luminescence quantum yield (Figure 3B). An integrating sphere encloses the sample in a reflective dome and allows light to travel through two openings separated by 90°. The excitation light that travels to the blank sample (no coating) will have minimal absorbance and maximum reflection. Light reflects off of the sample, scatters around the integrating sphere and escapes through the opening behind the sample. The coating surface is angled at 45° from the excitation light and 315° from the detector. When the spectrum is repeated with a fluorescent sample, the excitation peak diminishes as a result of absorbance, and the emission peak appears. The shape of the emission peak(s) and the wavelength is recorded. In addition, the ratios of the peaks can be used to calculate the luminescence quantum yield following the equation given in Figure 3B. In general, the phosphorescence brightness of the film increases linearly with particle surface loading (Figure 1A1-A3), which is controlled by film thickness (Figure 2B). The quantum yield was found to correlate proportionally to the particle surface coverage by Gerwitz et al., and this reference includes more discussion on this topic20.
Reagent / Parameter | Watt’s Bath Plating23 | Electrostamping20 |
NiSO4.6 H2O (g/L) | 270 | 1325 |
NiCl2.6 H2O (g/L) | 60 | 238 |
H3BO3 (g/L) | 38 | 212 |
Composite (SrAl2O4:Eu2+, Dy3+) (g/L) | — | 238 |
H2O (g/L) | 812 | 21 |
Cathode Current Density (A cm-2) | 0.045 | 0.025 |
Composite Size Range (µm) | 0 – 11 | 0 – 100 |
Composite Loading (%) | 0 – 40 | 0 – 90 |
Table 1: Recipes for Traditional Bath Plating and Electrostamping. These are based on Watts’s nickel bath plating solution and corresponding electrostamping methods where the relative amounts of dry reagents are the same for each. However, the amount of added water is significantly different which increases the concentrations (g/L solution). Also included are typical current densities (A∙cm-2), possible composite particle sizes (µm) and loading amounts (%).
Figure 1: Representative images of composite coatings. This includes photos of 4 cm2 films in the dark 5 seconds after excitation with visible light > 400 nm was removed (A), top-view optical microscope images in the dark under an FTIC filter and continuous 470 ± 20 nm excitation (B), and under brightfield light conditions (C), including cross-section images (D), after coating for 30 minutes with applied current of 1.0 A (0.25 A∙cm−2) (1), 0.1 A (0.025 A∙cm−2) (2), and 0.01 A (2.5 × 10−3 A∙cm−2) (3). This figure has been modified from Gerwitz et al.20 Please click here to view a larger version of this figure.
Figure 2: Representative analysis of microscopy images. This includes the composite surface coverage (%) as a function of time for chronoamperometry (blue) and as a function of current density (A∙cm-2) for chronopotentiometry (red) during coating of a 4 cm2 nickel foil sample after 30 min (A). Additionally, the surface coverage can be plotted with the film thickness as measured from the cross-section (B). This figure has been modified from Gerwitz et al.20 Please click here to view a larger version of this figure.
Figure 3: Representative electrochemical analysis during the coating process and fluorescence quantum yield measurements. This includes chronoamperometry (blue) and chronopotentiometry (red) during coating of a 4 cm2 nickel foil sample over time (A). Solid state stage fluorescence spectra of a 4 cm2 nickel foil sample coated for 30 min at 0.025 A∙cm−2 under constant current, plotted with the spectra from the blank metal film (B). The ratios of the peak areas are used to calculate photoluminescence quantum yield using the equation provided where Aem, Aex and Ao are the peak areas at the emission wavelength of the sample, the excitation wavelength of the sample and the excitation wavelength of the blank, respectively. This figure has been modified from Gerwitz et al.20 Please click here to view a larger version of this figure.
Critical steps of electrostamping. Bathless electrostamping shares many of the same critical steps with traditional bath electroplating. These include proper cleaning of the electrodes, mixing metal ions into the electrolyte and applying and external or chemical (electroless plating) potential to cause reduction of metal onto the cathode. In addition, the oxidation of the anode and cathode should be avoided after acid activation by quickly rinsing with water and adding these electrodes to the setup.
Electrostamping compared to traditional bath plating. A few metal-composite fluorescent metal films have been reported from bath plating16,17,28. However, in order to overcome the low physisorption of the composite to the cathode, some inventive methods have been demonstrated. These include removing the water and using dry organic solvents17, decorating the composite particles with charged surfactants to disrupt the hydration shell16 or only attempting the process with non-hygroscopic particles22,28,29. However, these methods continue to be limited to low surface coverages, small particle sizes from nanometers to a few micrometers12, and the potential for organic material incorporation. In contrast, the electrostamping method introduced in this report traps the composite particles in a stagnant electrolyte paste during coating, which forces the composite particles to remain near the cathode long enough to become embedded into the nickel matrix as it grows around them. Table 1 compares the recipes of both bath plating and electrostamping and highlights the effect of removing the majority of the water from the solution. For a 4 cm2 coating, the volume of the paste is approximately 0.5 mL. Without a liquid bath, the coating paste can physically contain the composite particles to allow for larger composites to be coated and also at higher loading amounts. It should be noted that even though the concentrations are expressed in grams per liter of solution, electrostamping does not require a liquid bath.
Interestingly, the power consumption required for plating is also comparable to bath plating. For example, in a typical bath, 0.02 – 0.07 A∙cm-2 current densities are employed, which is a range that contains the 0.025 A∙cm-2 current found to be optimal for bathless electrostamping. The resistivity of the electrolyte also plays a role, where lower resistance values lead to lower voltage requirements as a result of Ohm’s Law. In bath plating, the electrolyte solution is often heated (80 – 90 °C) to decrease the resistance, which is also a source of power consumption. In contrast, for bathless coating, the concentration of metal salts in the electrolyte is significantly higher (Table 1), which leads to a low electrolyte resistance20 (14-35 Ω for the samples here), even at room temperature. This results in power requirements of 2.0 V, 0.025 A∙cm-2 and 50 mW∙cm-2 for electrostamping of the metal-composites (Figure 3A). Without the presence of composites, we would expect the power demand to be lower than traditional bath plating as a result of the lower resistivity of the concentrated electrolyte. However, since this method is used to deposit high levels (up to 80%) of uncharged particles in the metal matrix, their presence in the film inherently increases the resistivity of the cathode. This effect may explain the lower than expected thicknesses as the ceramic particles occupy more of the nickel surface, decreasing the available sites for nickel reduction and film growth.
Limitations of electrostamping. A limitation of this method includes the potential for non-uniform coating if the electrolyte paste does not reach the crevices of complex surfaces. Whereas bath electroplating can be used to deposit a uniform coating over the entire surface of an object, electrostamping provides a localized deposition. This may lead to an uneven thickness and roughness distribution. For some applications were dimensions are critical, this method may need additional adjustments (sanding, polishing etc.) after deposition to account for discrepancies in thickness. Additionally, as a result of bringing the plating system out of the bath, surface coverages of up to 80 ± 12% are possible20, even with large (87 ± 30 µm) hygroscopic composite particles and without organic reagents. However, at high loadings, a compromise must be made between durability and functionality. As the percentage of composite material increases, the film properties begin to deviate away from the original metal matrix toward the properties of the composite material, which is often a brittle powder. As a result, high coverages above 50% may not be practical for many applications that require durable metal films.
Electrostamping compared to localized jet and brush plating. Bath plating can be used to selectively etch/plate specific areas on conductive objects by protecting the rest of the object with masking tape as the cathode is submerged. However, like electrostamping, jet plating and brush plating can deposit metal onto localized areas without a bath. In jet plating, an electrical potential is established between the cathode (object to be coated) and a stream of jetting electrolyte as it passes from the anode nozzle30. A plating potential is established through the jet, and the metal salts can be reduced at the surface. Likewise, the surface can also be etched under reversed polarity. This method has applications in electronic fabrication where small deposits can be jet-printed onto a circuit board. Similarly, brush plating also involves a flow of electrolyte out of an anode covered in an absorbent cloth31. The anode brush holds the flowing electrolyte and when it contacts the cathode object, the metal salts are reduced at the surface to produce a highly controllable thickness. This method is used for repair of large metal parts and these parts are often spinning during plating to increase the uniformity of the coating.
Both jet and brush plating involve a flowing liquid electrolyte. For this reason, they are amenable to pure and alloy metal plating. However, these methods would not be ideal for composite-metal plating. The challenges facing embedding composite particles into the cathode film in a bath are also present with these methods, especially for large and hygroscopic particles. In order to promote the capture of these particles within the growing metal film, the solution flow should be stagnant. Electrostamping is a new technique that addresses this problem by coating metal and composites with no solution flow. For this reason, electrostamping stands out as a unique method for coating in general, which may become a more commonly accepted method for locally depositing pure metal as well as metal composites.
Troubleshooting electrostamping. Potential troubleshooting of absent or inconsistent coatings can be addressed by checking the following. 1) The metal electrodes may not be cleaned effectively with base so that a hydrophobic organic material barrier prevents plating. 2) The metal electrodes are not effectively activated with acid, or have been exposed to air for more than 5 min so that a less conductive metal oxide layer barrier forms and prevents plating. 3) The paste is not viscous enough and does not trap the composite particles or is too dry and causes a high resistance through the electrolyte.
Future applications of electrostamping. For many objects, electrostamping could be advantageous over traditional bath plating. For example, if the object to be coated is large, cumbersome, delicate or sensitive to corrosion, then a selected area can be “stamped” onto the side of the object. Examples may include the sides of appliances, road signs, metal fences, sides of buildings, covert messages onto walls, laptop computers, metal hand tools, or automobile and aircraft components. In addition to the potential versatility of this method to coat objects that were previously considered not possible, this method opens the door to future investigation of other metal-composite combinations, at higher surface loadings and with larger or hygroscopic particles. Here, we demonstrate this method for metal oxide composite/nickel metal matrix coatings; however, this method may motivate the investigation of multiple different metals and alloys, with a variety of composite particles that have not yet been realized.
The authors have nothing to disclose.
This work was supported by the Aircraft Equipment Reliability and Maintainability Improvement Program and the Patuxent Partnership. Townsend was supported by an ONR Faculty Research Fellowship. The authors also acknowledge the general support of the SMCM Chemistry and Biochemistry Department faculty and students, including support from the SMCM football team.
37% M Hydrochloric Acid (aq) | SigmaAldrich | 320331-500ML | corrosive – handle in fume hood |
70% Nitric Acid (aq) | SigmaAldrich | 438073-500ML | corrosive – handle in fume hood |
Barium magnesium aluminate, europium doped (s) | SigmaAldrich | 756512-25G | fine powder |
Boric Acid (s) | SigmaAldrich | B6768-500G | toxic |
Cotton Swab | Q-tips | Q-tips Cotton Swabs | |
ImageJ | National Institutes of Health | IJ 1.46r | free software |
Nickel (II) chloride hexahydrate (s) | SigmaAldrich | 223387-500G | toxic |
Nickel (II) sulfate hexahydrate (s) | SigmaAldrich | 227676-500G | toxic |
Nickel foil (s) | AliExpress | Ni99.999 | |
Nitrile gloves | Fisher Scientific | 19-149-863B | |
nylon membrane (s) | Tisch Scientific | RS10133 | |
Optical Microscope equipped with FTIC filter (470 ± 20 nm) | Nikon | Eclipse 80i | |
Plastic Wrap | Fisher Scientific | 22-305654 | |
Porcelain Mortar | Fisher Scientific | FB961A | |
Porcelain Pestle | Fisher Scientific | FB961K | |
Potassium Hydroxide (s) | SigmaAldrich | 221473-25G | corrosive |
Potentiostat with platinum wire | Gamry Instruments | 1000E | |
Scoopula | Fisher Scientific | 14-357Q | |
Spectrofluorometer | Photon Technology International | QM-40 | |
Strontium aluminate, europium and dysprosium doped (s) | GloNation | 756539-25G | powder |
Variable linear DC power supply | Tekpower | TP3005T | |
Yttrium oxide, europium doped (s) | SigmaAldrich | 756490-25G | fine powder |