Silver mirrors can be deposited on PET plastic via Tollens' reaction using appropriate surface sensitization.
the verdict
INSUFFICIENT LEANING
refutedsupported
the weight of evidence
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Available literature discusses silver mirror and Tollens reactions on generic polymer templates or other plastics, but lacks direct evidence establishing silver mirror deposition on PET plastic specifically using surface sensitization.
Is it possible to ‘explore’ metal’s intrinsic property—a cohesive interaction—which naturally transform M0 into an aggregate or a particle or film for driving oxidative C–C bond formation? With this intention, reduction of [Ag(NH3)2]+ to Ag0 with concurrent oxidation of different phenols/naphthols to biphenyls was undertaken. The work is originated during careful observation of an undergraduate experiment—Tollens’ test—where silver mirror film deposition takes place on the walls of borosilicate glass test tube. When the same reaction was carried out in polypropylene (plastic-Eppendorf) tube, we observed aggregation of Ag0 leading to floating Ag-particles but not silver film deposition. This prompted us to carry out challenging cross-coupling reaction by ONLY changing the surface of the reaction flask from glass to plastic to silicones. To our surprise, we observed good selective oxidative homo-coupling on Teflon surface while cross-coupling in Eppendorf vial. Thus, we propose that the formation of biphenyl is driven by the macroscopic growth of Ag0 into [Ag-particle] orchestrated by Ag…Ag cohesive interaction. To validate results, experiments were also performed on gram scale. More importantly, oxidation of β-naphthol carried out in quartz (chiral) tube which yielded slight enantioselective excess of BINOL. Details are discussed.
The work is originated during careful observation of an undergraduate experiment—Tollens’ test—where silver mirror film deposition takes place on the walls of borosilicate glass test tube. When the same reaction was carried out in polypropylene (plastic-Eppendorf) tube, we observed aggregation of Ag 0 leading to floating Ag-particles but not silver film deposition. This prompted us to carry out challenging cross-coupling reaction by ONLY changing the surface of the reaction flask from glass to plastic to silicones. To our surprise, we observed good selective oxidative homo-coupling on Teflon surface while cross-coupling in Eppendorf vial.
Introduction Role of [Ag(NH 3 ) 2 ] + for oxidative BINOL formation Bernard Tollens (1841–1918) realized the potential of silver complex, [Ag(NH 3 ) 2 ] + , to oxidize aldehyde group to carboxyl group, with concomitant reduction of Ag + to silver mirror/film (Ag 0 ), on the inner walls of the test tube 1 , 2 . The potential of this reaction has been exploited only in chemistry education—as a test to distinguish aldehydes from ketones—mainly due to its ease and wide applicability on variety of aldehydes. Interestingly, oxidation potential (+ 0.34 eV) for conversion of [Ag(NH 3 ) 2 ] + to Ag 0 matches well to that of direct oxidative C–C
In this observation we saw a potential in conducting a pair of redox reaction involving Ag + to Ag 0 and phenol to bisphenol as coupled reactions. This prompted us to carry out Tollens' test on β -naphthol, a common phenol employed for bisphenol formation, as the product bisnaphthol itself finds importance in asymmetric synthesis and material science 5 , 6 . The reaction gave a positive result, meaning formation of a silver mirror on the walls of test tube. The product, 1,1′-bi-2-naphthol (BINOL), in this reaction can be isolated by simple decantation! easy separation from the deposited silver film.
Recycling of generated silver film The deposited silver mirror film can be completely recovered (about 99% after 1st use) and reused by dissolving it with the addition of HNO 3 (10 N). Recycling of the ‘silver mirror’ was successfully carried out up to 5 cycles with more than 95% recyclability (presented in Fig. 1 ). Quantification of the amount of deposited silver using conductometric analysis was also carried out by titrating silver nitrate with standard KCl solution.
Oxidative coupling reaction between 2,6-dimethoxyphenol and 2,6-di- tert -butylphenol with [Ag(NH 3 ) 2 ] + resulted in silver mirror film formation on the walls of borosilicate glass tube along with the mixture of cross- and homo-coupled products. Interestingly, same reaction (without changing experimental conditions) when carried out in a plastic vial (eppendorf tube of polypropylene), furnished floating silver particles in the solution instead of silver mirror formation. This observation prompted us to pose a question, is the change in the growth of Silver mirror formation to silver particle has any influence on C–C oxidized product?
During these experiments, we realized that optimizing concentrations of the reactants to get transparent silver mirror film is the most critical part, that means generating possibly monolayer of silver film on glass surface is key. All these experiments were carried out by taking utmost care and were reproducible in normal reaction conditions; we did not make an effort to optimize reaction conditions for higher ee. Figure 5 Attempted observation of chiral silver film formation with concurrent oxidation of d-dextrose. Instead of ‘characterization’ of silver mirror film in a borosilicate glass vial, it is directly tested for observing enantioselective BINOL formation.
The key to the observation is carefully writing of the reaction, silver mirror test or Tollens’ test, in three explicit reversible equations and correlating metals intrinsic cohesive interaction with the molecular level interactions. Present work has a potential to open up role of dynamics of this in-situ formed metal’s (silver) aggregation, non-interacting catalytic force, in driving selective oxidative organic transformation. Experimental section General Silver nitrate was purchased commercially of 99.9999% purity. Potassium Chloride was purchased commercially of 99.999% of purity.
This reaction mixture was kept in the dark at 28–30 °C for 1 h. Solution in the vial was discarded. Vial was washed with water thrice. Ethanol washing was also given and kept 1–2 h for drying at room temperature. This silver coated surface was used instead of quartz for further C–C coupling reaction of BINOL using same procedure given for experiment 2.
Ultralight (<10 mg/cm3) cellular materials are desirable for thermal insulation; battery electrodes; catalyst supports; and acoustic, vibration, or shock energy damping. However, most of these ultralight materials, especially ultralight metal foams, are fabricated using either expensive materials or complicated procedures, which greatly limit their large-scale production and practical applications. Here we report a simple and versatile method to obtain ultralight monolithic metal foams. These materials are fabricated with a low-cost polymeric template and the method is based on the traditional silver mirror reaction and electroless plating. We have produced ultralight monolithic metal foams, such as silver, nickel, cobalt, and copper via this method. The resultant ultralight monolithic metal foams have remarkably low densities down to 7.4 mg/cm3 or 99.9% porosity. The metal foams have a long flat stress-train curve in compression tests and the densification strain εD of the Ni/Ag foam with a porosity of 99.8% can reach 82%. The plateau stress σpl was measured and found to be in agreement with the value predicted by the cellular solids theory.
However, most of these ultralight materials, especially ultralight metal foams, are fabricated using either expensive materials or complicated procedures, which greatly limit their large-scale production and practical applications. Here we report a simple and versatile method to obtain ultralight monolithic metal foams. These materials are fabricated with a low-cost polymeric template and the method is based on the traditional silver mirror reaction and electroless plating. We have produced ultralight monolithic metal foams, such as silver, nickel, cobalt, and copper via this method.
Herein, we report a novel and facile method to fabricate ultralight monolithic metal foams, such as silver, nickel, cobalt, and copper. These as-made monolithic metal foams have remarkably low densities down to 7.4 mg/cm 3 or 99.9% porosity. Results and Discussion Figure 1 illustrates the fabrication scheme of the ultralight Ag, Ni, Co, Cu foams. These materials are prepared by starting with a polymeric template, coating the template via the silver mirror reaction, then electroless plating (Ni, Co, Cu), and subsequently burning away the template. The template employed in this study is a polymer foam used as a household cleaning eraser which is cheap and easy to purchase in the market.
When the foam was heat-treated at 680 °C in the muffle furnace to burn away the polymer template, the particle size of the silver film which is 100–200 nm obtained by the
After the silver mirror reaction, the polymer template was coated by electroless nickel plating and subsequently the template was burned away in a muffle furnace, then, ultralight monolithic Ni/Ag foams were acquired ( Fig. 3a ). The porosity of this ultralight monolithic Ni/Ag foam can reach 99.9% (ρ = 7.4 mg/cm 3 , 13.4 mg in 1.79 cm 3 ). The foam has a three-dimensional network structure consisting of uniform slender filaments, analogous to the original polymer foam template ( Supplementary Fig. S1 ).
In the sandwich structure, the Ag coating was produced by the silver mirror reaction and the Ni films should remain as a supersaturated solid solution of phosphorous in a crystalline face-centered cubic nickel lattice after deposition 6 . Because the foam was heated at 700 °C in air in order to remove the polymer template and the Ni film is only about 100 nm thick, the nickel was all oxidized to generate NiO with Ni 2 P precipitates present and no peaks of any residual nickel were detected in the XRD pattern.
Similarly, the polymer foam acquired after the silver mirror reaction was coated by electroless cobalt or copper plating and subsequently burning away the polymer template, then, ultralight monolithic Co/Ag, Cu/Ag foams were synthesized ( Supplementary Fig. S7 ). EDS analysis of the Co/Ag foam showed that silver, cobalt and phosphorus are present. The film is a double layer structure (Co/Ag) and the thickness of the Co layer can also be easily controlled by changing the time of electroless plating. The copper particles of the Cu/Ag foam are coarse and no phosphorus exists in the metal film.
These materials are fabricated with a low-cost polymeric template and the method based on the traditional silver mirror reaction and electroless plating. We have produced ultralight monolithic metal foams, such as silver, nickel, cobalt, and copper via this method. The resultant ultralight monolithic metal foams have remarkably low densities down to 7.4 mg/cm 3 or 99.9% porosity. The densities of the metal foams can be easily controlled by changing the time of electroless plating. The metal foams have a long flat stress-train curve in the compression test and the densification strain ε D of the Ni/Ag foam with a porosity of 99.8% can reach 82%.
Finally, the sample was heated to 700 °C in an air atmosphere in a muffle furnace to burn away the polymer template, and the ultralight monolithic Ni/Ag foams were synthesized. Samples with a wall thickness (Ni) below ~100 nm could not remain the original size and shrank similar to the silver foam after burning away the polymer template. Synthesis of ultralight Co/Ag foams The polymer foam acquired after the silver mirror reaction was coated by electroless cobalt plating.
Synthesis of ultralight Cu/Ag foams The polymer foam acquired after the silver mirror reaction was coated by electroless copper plating. The samples were immersed in electroless copper plating solution with copper sulfate (6 g/L) as the copper source, sodium hypophosphite (28 g/L) as the reducing agent, and sodium citrate (15 g/L) and boracic acid (30 g/L) as complexing agents. The electroless copper plating bath was kept at pH 8.2 by addition of sodium hydroxide and plating was performed at 65 °C. Subsequently, the sample was rinsed in distilled water and completely dried in a drier at 120 °C for 30 minutes.
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