Room temperature photo-induced deposition of platinum mirrors and nano-layers from simple Pt(II) precursor complexes in water–methanol solutions

Abstract

Pure platinum mirror-like nano-layers (200–500 nm thick) can reproducibly be deposited onto conductive FTO glass (as well as other substrates) by a remarkably simple photo-induced method at room temperature (20 ± 3 °C) from a simple Pt(II) precursor in aqueous solution. Such Pt mirrors may be prepared by controlled illumination with intense polychromatic white light from a 5 W (4000 K) LED light source of a dilute solution containing 1 mM K₂[PtCl₄] precursor salt, dissolved in water/methanol solutions. By contrast, the use of [PtCl₆]²⁻ as a precursor does not lead to stable Pt mirror formation under similar conditions. Preliminary ¹⁹⁵Pt NMR results indicate that the chemical speciation of the precursor Pt(II) complexes plays a critical role in this photo-induced Pt nano-layer formation.

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The important applications of platinum-based nano-structured materials as electro-catalysts for use in proton exchange membrane fuel cells (PEMFCs) aimed at replacing conventional carbon-based energy sources, have stimulated considerable research activity in the last two decades.^1–3 Much of this work has focused on the preparation of Pt nanoparticles with defined size and shape by controlled reduction using a variety of methods ranging from hydrothermal, chemical and electrochemical reduction, sol–gel methodology, and so-called electroless techniques.^1–11 Generally the preparation of platinum nanoparticles employs commercially available Pt(IV) salts such as H₂[PtCl₆]·xH₂O and K₂[PtCl₆], or less frequently the Pt(II) salt K₂[PtCl₄] as precursor compounds.¹ Over the years, a plethora of electroless methods for Pt nanoparticle synthesis for use as catalysts has emerged, based on the chemical reduction from aqueous solution of mainly [PtCl₆]²⁻ anions and to a lesser extent [PtCl₄]²⁻ as precursors. A variety of reducing agents inter alia molecular hydrogen, citric acid, hydrazine, BH₄⁻, including alcohols such as ethanol, propanol, ethylene-glycol and polyols, often in the presence of surface-active reagents for size and shape control of Pt-nanoparticles have been used.^1–4,7 Numerous electroless ‘recipes’ using variable conditions of pH, pressure and generally higher temperatures ranging from >80 °C to exceeding 200 °C can be found in the literature.^1,3 Moreover conventional electrochemical plating methods for Pt, Pd and Rh to yield thin, porous and non-porous layers onto a variety of substrates has been reviewed a decade ago.⁵ Such methods require careful control of conditions in view of the relatively complex chemical speciation of Pt(II/IV), Pd(II) and Rh(III/I) as precursor compounds, even in ‘simple’ formulations of electroplating solutions. In the latter electrochemical methods concomitant hydrogen evolution resulting in consequential metal-embrittlement is however, still a problem.⁵ Apart from the well-known chemical vapour deposition (CVD) methodology requiring high temperatures, a survey of the open literature shows that electroless methods particularly at low temperatures for the preparation of uniform, stable Pt nano-layers, are relatively rare.³ The improved catalytic performance of nanostructured platinum films deposited by CVD methodology (using RF and DC sputtering methods under high vacuum at 200 °C) onto for example CaCu₃Ti₄O₁₂ oxides as more active electrodes, suggests however potential advantages of electroless platinum deposition as nano-layers.⁶ High temperature Pt-layer deposition by CVD methodology (≥200 °C) may, however be a technical disadvantage in the deposition of platinum nano-layers onto temperature sensitive substrates and devices. This is reflected by a recent comparative review of various methods for the preparation of electro-catalysts with low Pt-loading onto more delicate substrates such as carbon nano-tube materials for potential use in PEMFCs, suggesting a need for further work in the realm of electro-less Pt deposition, particularly at low temperatures.⁴

Pernstich et al. recently reported an electroless deposition of ultra-thin platinum mirrors (nano-films) onto a variety of substrates, ranging in thickness from 3–5 to 60–65 nm, using reductive thermolysis for 22–24 h of cis-dichlorobis(styrene)-platinum(II) in toluene at 80 °C.¹⁰ A comparable electroless thermolysis method using concentrated aqueous solutions of tetraamino-platinum(II)-dichloride at higher temperatures and high pressures (170–180 °C 8–10 atmospheres) in a sealed tube was described a decade ago.¹¹ This results in the formation of thicker (∼140 nm) pure platinum foils composed of 200–400 nm Pt-nanoparticle agglomerates.

Although reports of the influence of intense visible light on the hydrolytic decomposition of dilute solutions of K₂[PtCl₆] emerged more than a century ago,¹² detailed studies of photo-activated reduction of various platinum containing precursors are rare in the open literature.^13–16 Recently renewed interest in photo-chemically induced reductive formation of platinum nanoparticles in particular, is evident in the literature.^17–21

We here report the development of a remarkably simple photo-induced method for producing reflective Pt-mirrors (or nano-layers) directly onto fluorine-doped tin oxide (FTO) glass at room temperature (20 ± 3 °C) from a simple Pt(II) precursor in aqueous solution, using the simple apparatus in a darkened laboratory in the shown schematically in Fig. 1.

Fig. 1 A schematic representation of the apparatus used for the photo-induced Pt-mirror nano-layer deposition onto 2 mm strips of cleaned FTO glass. The light source was a standard commercial 5 W “cool white” (4000 K) Light Emitting Diode (LED).

Good Pt-mirrors can reproducibly be prepared by controlled illumination of a dilute solution containing 1 mM K₂[PtCl₄]²⁻ precursor salt dissolved in a 0.8 : 1 methanol : water (wt/wt) mixture with intense polychromatic white-light from a 5 W (4000 K) LED light source for typically 6–24 h (experimental details in ESI†). We find that the best-quality reflective platinum mirrors are deposited directly onto a strip of clean FTO glass (2.3 × 10 × 100 mm), by allowing polychromatic light to cascade (via internal reflection) vertically down the FTO glass immersed into the platinum containing precursor water–methanol solution without stirring. It is advantageous to prevent excessive stray light from reaching the solution by means of a screen, so preventing too rapid uncontrolled reduction leading to Pt nanoparticles. Under these conditions the Pt-mirrors are reproducibly and preferentially deposited onto the more conductive (7 Ω sq⁻¹) side of the FTO glass strip, which is useful for characterization of the Pt-mirror by electron microscopy (vide infra).§ Nevertheless under slightly modified conditions Pt-mirrors can also be deposited by controlled direct illumination onto other clean, grease-free substrates such as borosilicate glass, etched soda glass, polypropylene and even organic materials such as cellulose.

The reflective nature of the Pt-mirror deposited on a 15 × 15 mm square of FTO glass is shown in Fig. 2(a). Moreover, although the Pt-mirror adheres well to the FTO substrate, it can readily be removed intact from FTO glass by means of adhesive tape resulting in a flexible strip as shown in Fig. 2(b). Examples of Pt-mirrors deposited onto other substrates such as a borosilicate beaker and a polypropylene centrifuge tube, using slightly modified photochemical depositions are shown in Fig. 2(c and d) respectively.

Fig. 2 Pt-mirrors deposited by exposure with white polychromatic light (λ > 324 nm) for 17–24 h at room temperature. (a) A 1 × 1 cm square of FTO glass illustrating its reflectivity of a logo; (b) a 1 cm wide strip of a ca. 500 nm thick layer of Pt-mirror removed from FTO glass using adhesive tape; (c) Pt-mirror deposited onto a conventional borosilicate laboratory glass beaker; (d) Pt-layer deposited onto a polypropylene centrifuge tube.

The interesting microstructure of a Pt-mirror deposited on FTO glass is shown in Fig. 3(a–h). An optical microscope image of shinny pure platinum ‘flakes’ of submicron thicknesses removed from FTO glass, show these flakes to have a smooth, highly reflective side and slightly matt side as is visible in Fig. 3(a). The smooth reflective side of these flakes corresponds to the Pt-mirror directly in contact with the FTO glass template as initially exposed to polychromatic light (λ > 324 nm) passing down the FTO glass strip, immersed into the precursor solution as shown in Fig. 1. The matt side of these ultrathin platinum flakes corresponds to that side exposed to the precursor solution during the growth of the Pt-mirror. The remarkable micro-morphological differences between the ‘smooth’ and the ‘matt’ sides of the Pt nano-layers are confirmed by scanning electron microscopy (SEM) images at higher magnification Fig. 3(c–f). These SEM images clearly show that the micro-morphology of the smooth, reflective side of the Pt-mirror closely matches the micromorphology of virgin FTO glass (Fig. 3(c and d)). This suggests that during the initial phase of photo-reductive Pt-mirror deposition the Pt-mirror apparently mimics the surface structure of clean FTO surface (compare the SEM image of clean FTO glass, Fig. S1, ESI†). On the other hand, the matt side of the Pt-mirror shown in Fig. 3(e and f) is clearly composed of a densely packed aggregate of relatively uniform Pt-nanoparticles (average diameter ca. 211 ± 40 nm). This suggests that the initially photo-induced ultrathin Pt-mirror deposited onto the FTO template may act as an auto-catalyst to further reductive formation and deposition of Pt-nanoparticles onto an initial Pt layer. The overall thickness and quality of the Pt-mirrors obtained, is determined by several factors, including the time of light exposure, the effective wavelength range of the light (ranging from ca. 324–594 nm), the concentration of precursor complex, as well as the relative amount of methanol in water. Higher concentrations (>5 mM) of [PtCl₄]²⁻ and methanol : water (wt/wt) ratios > 1 result in poorer quality Pt-mirrors, indicated by competitive Pt-nanoparticle formation as well as too rapid mirror deposition resulting in crack formation causing peeling of the Pt-mirror from the FTO substrate, possibly due to internal stress fracturing.

Fig. 3 Optical and SEM images of the Pt-mirror on and removed from a strip of FTO glass template. (a) Optical microscopic image pure Pt-flakes removed from FTO glass shows shiny and matt sides; (b) a low magnification SEM image of the reflective side of the Pt-mirror removed from FTO glass; (c) a SEM image of the reflective side of Pt-flakes showing that the micromorphology of the Pt-flake mimics the FTO glass as template; (d) SEM image at higher magnification of a “crack” in a Pt-flake directly in contact with the FTO glass template; (e and f) SEM images of the matt side of the Pt-layer exposed to the precursor solution during its deposition under the influence of light. This is suggestive of an auto-catalytic growth of densely packed Pt nanoparticles onto the Pt-mirror initially formed on the FTO glass template; (g) SEM estimate of the average thickness of the Pt-mirror on FTO template; (h) SEM view of Pt-mirror removed from FTO glass template.

Under optimum conditions white LED light irradiation of dilute solutions of [PtCl₄]²⁻ in methanol water mixtures (CH₃OH : H₂O mole ratio of ca. 0.45 : 1, for between 17 and 24 h at room temperature) results reproducibly in good Pt-mirror deposition onto FTO glass with average thickness 569 ± 30 nm, as determined from a series of SEM images shown in Fig. 3(g and h). The effect of differing wavelength ranges of visible light on Pt-mirror formation was qualitatively examined by filtering the polychromatic LED light though a series of photographic red, yellow and blue optical filters. Thus ‘red’ light (λ > 594 nm) results in practically no mirror formation, whereas ‘yellow’ and ‘blue’ light (λ > 475 nm and λ > 329 nm respectively, Fig. S2, ESI†), result in Pt-mirror formation under identical conditions. Thinner Pt-mirrors may similarly be prepared using shorter light irradiation times, and potentially by better control of the intensity and wavelength band of the light used; detailed quantitative studies of these aspects are underway.

The Pt-mirrors deposited onto FTO glass were fully characterized by SEM in conjunction with electron beam-excited EDS (Fig. S3, ESI†), thermo-gravimetric analysis (TGA) (Fig. S4, ESI†), X-ray diffraction (XRD), and laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), all of which confirm that the reflective metal nano-layer essentially consists of pure platinum. The XRD pattern of a fragment of Pt-mirror is typical of a face-centered cubic (fcc) platinum metal lattice (Fig. S5a, ESI†), with Pt(111) and Pt(200) reflections similar to those observed for well-defined platinum nanocrystals previously reported.^22,23 Static contact-angle (SCA) measurements a 1 μL water droplet placed on the matt side of a dry Pt-mirror results in a contact angle of 117° for the droplet (Fig. S5b, ESI†). The static contact-angle of 117° indicates a significant degree of hydrophobicity associated with the micro-roughness of matt side of the Pt-mirror, consistent with the Wenzel's or a Cassie–Baxter's model of hydrophobicity, although a SCA of 117° falls short of ‘super-hydrophobicity’ generally defined by contact angles of >150°.²⁴

Definitive evidence of the purity and regularity of thickness of the photo-deposited Pt-mirrors onto FTO glass is obtained from LA-ICP-MS analysis using a Resonetics Resolution S155-LR excimer pulsed (20 ns) laser ablation system, equipped with a COMexPro pulsed laser source emitting at a wavelength of 193 nm, coupled to an Agilent 7500ce quadrupole ICP-Mass Spectrometer. A series of highly reproducible, normalized ¹⁹⁴Pt⁺, ¹⁹²Pt⁺ and ¹²⁰Sn⁺ ion-counts per second are obtained after 10 s ablation spots along a strip of Pt-mirror coated FTO glass, confirming the purity and regularity of thickness of the Pt-mirror (Fig. S6a, ESI†). The stable Pt isotopes ¹⁹⁴Pt (0.32967) and ¹⁹²Pt (0.00782) were chosen for ICP-MS detection to avoid possible isobaric interferences, while the experimentally determined ratio ¹⁹⁴Pt⁺/¹⁹²Pt⁺ of (41.00 ± 2.46) obtained closely matches the expected ¹⁹⁴Pt/¹⁹²Pt isotope ratio of 41.65–42.16.²⁵ This data confirms that the laser pulses rapidly and completely ablates the Pt-mirror and the underlying Sn from the tin-oxide coated FTO glass template. A typical time-resolved profile of ablated material, shows that the appearance of ¹⁹⁴Pt⁺ and ¹⁹²Pt⁺ ion-count signals precedes the appearance of an intense ¹²⁰Sn⁺ signal by only ca. 0.72 s (Fig. S6b, ESI†). Interestingly, the time dependent ¹⁹⁴Pt⁺ and ¹⁹²Pt⁺ ion-count profiles show two successive peak maxima during the 10 s ablation interval, followed by a gradual return of the ion-count signal to the pre-pulse values during the 45 s washout period. Two Pt maxima may suggest that the Pt-mirror actually consists of two Pt nano-layers, a thinner denser layer initially directly deposited onto the FTO glass, and a more porous layer consisting of (subsequently) deposited Pt-nanoparticles, as suggested by Fig. 3(e and f). This observation is similar to the observed two-layer micromorphology reported by Pernstich et al.¹⁰ in the thermolytic Pt-mirror formation, but in our case needs further verification.

The mechanism of this interesting photo-induced platinum mirror or nano-layer deposition is undoubtedly fairly complex and beyond the scope of this communication; nevertheless it is clear that the chemical speciation of the Pt(II) precursor is critical in the success of this methodology. In particular we find the use of Pt(IV) as the [PtCl₆]²⁻ does not lead to platinum mirror/nano-layer formation under comparable conditions. Preliminary ¹⁹⁵Pt NMR work suggests that the redox-active precursor species leading to Pt-mirror formation is not the relatively stable, kinetically inert [PtCl₄]²⁻ anion predominant in precursor solutions (or even its aquated [PtCl₃(H₂O)]⁻ counterpart), but most probably the [PtCl₃(CH₃OH)]⁻ anion which is formed only in very low concentrations under the influence of polychromatic light in these solutions. A 128.8 MHz ¹⁹⁵Pt NMR spectrum of 100 mM K₂[PtCl₄] in a water–methanol solution (10 : 3 v/v)¶ freshly prepared under subdued ambient light, recorded at 20 °C shows the presence of only three resonances at −1606, −1170 ppm and at −1192 ppm (relative intensities of 1 : 0.25 : 0.01, respectively) in Fig. 4 (25 h acquisition time in the dark). The ¹⁹⁵Pt NMR peaks at −1606 and −1170 ppm are due to the known [PtCl₄]²⁻ and aquated [PtCl₃(H₂O)]⁻ anions respectively (cf. −1539 & −1185 ppm in 1 M HClO₄ at 23 °C).²⁶ However the low intensity peak at −1192 ppm has not to our knowledge been previously reported in the literature; on the basis of chemical shift trend-analysis,²⁷ we assign this ¹⁹⁵Pt NMR peak to the [PtCl₃(CH₃OH)]⁻ precursor species, which is postulated to be the photo-chemically active species.

Fig. 4 A 128.8 MHz ¹⁹⁵Pt NMR spectrum of a freshly (made under subdued light) solution of 100 mM K₂[PtCl₄] in water/methanol (10 : 3 v/v to ensure adequate solubility of the K₂[PtCl₄]) at 20 °C after 25 h acquisition to ensure an adequate signal to noise ratio. The three resonances at δ(¹⁹⁵Pt) −1606, −1170 and −1192 ppm of intensity 1.0 : 0.25 : 0.01 are assigned to the known [PtCl₄]²⁻, [PtCl₃(H₂O)]⁻ and a previously undetected [PtCl₃(CH₃OH)]⁻ species respectively. The latter complex disappears from solution on allowing the solution to stand in the dark for further 24 h, with a concomitant formation of a traces of Pt⁰ deposited in the NMR tube.

Independent experiments show that addition of even a slight excess of Cl⁻ ions to our precursor solutions, effectively inhibits any photo-induced Pt-mirror formation under these conditions. The importance of chemical speciation in a study of photocatalytic reduction of Pt(II/IV) in TiO₂ suspensions, aimed at the recovery of traces of Pt from aqueous solutions, has also been reported recently.²¹ Clearly the [PtCl₄]²⁻ species cannot be the direct photo-active species resulting in the photo-induced Pt-mirror formation under our conditions; in the presence of a slight excess of Cl⁻ ions, mass action is likely to suppress that formation of significant concentrations of either [PtCl₃(H₂O)]⁻ and/or presumably the photo-chemically active [PtCl₃(CH₃OH)]⁻ anions. In our methanol–water solutions containing only [PtCl₄]²⁻, it is likely that a slight excess of Cl⁻ ions inhibit the formation of significant concentrations of [PtCl₃(H₂O)]⁻ and the potential redox-active [PtCl₃(HOR)]⁻ species by mass action thus preventing Pt⁰ formation. Significantly we find that the Pt(IV) species [PtCl₆]²⁻ as precursor under identical conditions does not lead to stable Pt-mirror formation. This hypothesis is corroborated by a ¹⁹⁵Pt NMR experiment of a 0.3 mM H₂PtCl₆·xH₂O methanol solution with in situ irradiation by blue-green laser light (λ = 405 nm). Over a period of ca. 24 h the [PtCl₆]²⁻ species undergoes a clean two-electron photo-induced reduction to the [PtCl₄]²⁻ anion, as reflected by the time dependent disappearance of the δ(¹⁹⁵Pt) resonance at ∼66 ± 1 ppm with concomitant growth of a peak at δ(¹⁹⁵Pt) = −1575 ± 1 ppm clearly due to the [PtCl₄]²⁻ anion at 25 °C (Fig. S7(a and b) ESI†). Moreover this solution is stable for more than 32–48 h in the dark, showing no evidence of further reduction to metallic platinum, or the development of other species in solution. These findings are consistent with observations by Bocarsly et al. some three decades ago, concerning the reduction of [PtCl₆]²⁻ by ethanol and/or 2-propanol under ex situ irradiation, using an argon-ion laser 488 nm source. This reportedly resulted in mainly [PtCl₄]²⁻ as photoproduct (and necessarily two Cl⁻ ions per reduction equivalent) followed by a long induction period, prior to further reduction of [PtCl₄]²⁻ resulting in Pt⁰ (nano)particles formation under their conditions.¹⁶ Significantly, Bocarsly et al. reported that the addition of Cl⁻ or [PtCl₆]²⁻ inhibits a ‘dark’ reduction of [PtCl₄]²⁻ by ethanol/2-propanol, postulating that subsequent further reduction of “[PtCl₄]²⁻” to Pt⁰ proceeded via a slow thermal redox reaction. Moreover, addition of Pt⁰ apparently had a catalytic effect on further [PtCl₄]²⁻ reduction to Pt⁰ particles by ethanol.

Regardless of the reduction mechanism postulated, it was suggested that a reversible substitution of a chlorido ligand by ethanol/2-propanol must occur prior to reduction of [PtCl₄]²⁻ under their conditions, implying that at least species such as [PtCl₃ (ethanol/2-propanol)]⁻ might be involved. Our preliminary ¹⁹⁵Pt NMR studies of the interesting photo-induced Pt-mirror formation from methanol–water solution containing the [PtCl₄]²⁻ complex as precursor in the absence of additional Cl⁻ ions, provides for the first time to our knowledge, direct evidence of the formation of the likely key intermediate [PtCl₃(HOCH₃)]⁻ species. We postulate that controlled illumination of dilute solutions of [PtCl₄]²⁻ in water–methanol solutions leads to a slow photo-induced formation of the [PtCl₃(HOCH₃)]⁻ species at low concentrations, followed by facile Pt-mirror formation onto suitable substrates under very mild conditions. Further work aimed at elucidating details concerning this fascinating photo-induced Pt-mirror formation form water–methanol mixtures, and its deposition onto a variety of other substrates is underway.

In conclusion, good Pt-mirrors can reproducibly be deposited onto FTO glass and other substrates, by the controlled illumination with polychromatic LED light of a dilute methanol–water solutions containing low concentrations of [PtCl₄]²⁻ anions, under exceptionally mild conditions at room temperature (20 ± 3 °C). To our knowledge, this simple method of controlled photo-induced mirror-like Pt-nanolayer deposition under very mild conditions at room temperature onto a variety of substrates has not been previously described in the literature. In our view, this methodology may pave the way for Pt-nanolayer deposition or “photo-printing” onto heat sensitive, delicate substrates and devices.

Acknowledgements

We gratefully acknowledge financial assistance form the NRF (Grant No. 96077) and a Stellenbosch University (Post-Doctoral Fellowship to Dr Liang Xian).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Experimental details for the Pt-mirror formation and their full characterization and supplementary figures. See DOI: 10.1039/c6ra03318k

‡ On leave from Chemical Engineering Institute, Northwest University for Nationalities, Lanzhou 730030, China.

§ It is not clear whether the tin-oxide layer plays a role in the preferential Pt deposition onto FTO glass, although this is doubtful since Pt-mirror formation takes place onto a variety of other substrates. We speculate that the preferential deposition onto the FTO glass may be ascribed to the micromorphology (Fig. 3c and d) of this layer as well as possibly greater hydrophilicity of the relatively conductive FTO layer.

¶ NMR receptivity constraints of ¹⁹⁵Pt NMR, even in a 14.1 Tesla field, prevent recording an NMR spectrum of a 1 mM [PtCl₄]²⁻ concentration within a reasonable time, while solubility limitations of K₂[PtCl₄] necessitated a water richer mixture of water–methanol than used in the photo-reductive Pt mirror deposition.

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