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Electrochromism of 3Ni(OH)2∙2H2O

 

Thin films of 3Ni(OH)2∙2H2O were prepared with chemical bath deposition. They exhibit electrochromism in alkaline solution followed by color change from colorless to dark brown.

Materials
Two 100 cm3 beakers, glass rod, two pieces FTO glass substrates (2,5 cm x 7,5 cm), hot plate stirrer, stirring bar, two wires with crocodile clips, power source, ohmmeter, aluminum foil.

Chemicals
NiSO4·7H2O, urea, 0.1 M  aqueous solution of NaOH

Experimental

Preparation of the thin film of 3Ni(OH)2∙2H2O:
The glass substrates for the deposition of the 3Ni(OH)2∙2H2O films are coated with fluorine-doped tin oxide commercially available. Nickel(II) hydroxide films are deposited from a chemical bath with optimized composition and process conditions.
For the preparation of about 60 mL of the bath solution 1 g of NiSO4·7H2O are weighed and placed in a 100 mL beaker. A mass of 4 g of urea and 60 mL water is added. With stirring the substances are dissolved. This is the way of preparing the bath solution  for film deposition. 

Fig. 1. Chemical bath

Fig. 2. Chemical bath after heating

The substrate is previously cleaned with liquid detergent and alcohol, and than is immersed in the beaker that plays a role of a chemical bath with deposition solution. The substrate is vertically supported against the walls. The obtained system is then placed on heating plate, heated and stirred slowly, up to 95 ºC with continuously stirring. In the begging the solution is bright green Fig.1 and later it gets darker Fig. 2. The achieved temperature is kept constant during the deposition. The deposition time varied depending on the desired film thickness. For the purpose of this experiment the deposition should be terminated after approximately 25 to 30 min (10-15 min after the reaching temperature of 95˚C).

Demonstration of the electrochromism:

  • Place 0.1 M NaOH solution in beaker or other glass vessel.
  • Immerse the clean substrate facing the wall of the beaker with non conductive side (check with ohmmeter) and put small piece of aluminum foil on the contact with the crocodile clip and connect the wire to the power source of direct current.
  • Immerse the substrate with the prepared thin film in the beaker facing the wall of the beaker with non conductive side (check with ohmmeter).
  • Put small piece of aluminum foil on the substrate contact with the crocodile clip (as shown on Fig. 3) and connect the wire to the power source of direct current.
  • Put the electric power on at 3 V.
  • When the substrate with the thin film is connected as anode it turns dark brown (Fig. 4).
  • Switch the polarity by changing the connection to the power source and observe the changes.

Fig. 3. Colorless state of the thin film

Fig. 4. Colored state of the thin film

Observation and Discussion

By changing the polarity from -3 V to +3 V color change can be observed. The films get black. The mechanisms proposed for electrochromism of nickel(II) hydroxide in alkaline solution can be summarized by the following reaction schemes.
Ni(OH)2(white-transparen) = NiOOH(dark brown)
Ni(OH)2(white-transparen) + OH- = NiOOH·H2O(dark brown) + e-

Safety Tips
Wear gloves when working with nickel salts. Avoid contact of the solution containing Ni(II) ions.