Cobalt nitrate hexahydrate

The oxides Mg_0.25Co_0.25Ni_0.25Zn_0.25O (RS-0), Mg_0.20Co_0.20Ni_0.20Cu_0.20Zn_0.20O (RS-20), Li_0.20Mg_0.16Co_0.16Ni_0.16Cu_0.16Zn_0.16O (Li-RS-16) and Mg_0.40Ni_0.40Zn_0.20O (MNZO) were synthesized with the sol-gel method. All chemicals (lithium nitrate, magnesium nitrate hexahydrate, cobalt nitrate hexahydrate, nickel nitrate hexahydrate, copper nitrate hemi(pentahydrate), zinc nitrate hexahydrate, citric acid, and urea) were purchased from Sigma-Aldrich. The feeding mole ratio of metal, citric acid, and urea were 1:2:2. The metal precursors, citric acid, and urea were dissolved in de-ionized water with diluted nitric acid under stirring. The solution was kept at ~95 °C until the gel formed and then dried at 170 °C in the oven overnight, followed by annealing at 1000 °C for 6 h at a 5 °C min⁻¹ ramping rate and cooling down naturally.

All reagents used in this work were analytical grade. Ultrapure water was used in the synthesis of cobalt ferrite (Milli Q, Merck Millipore, USA). The cobalt ferrite nanoparticles were obtained following the sol–gel method. Precursor solution containing 0.16 M of iron chloride tetrahydrate (FeCl₂·4H₂O-Sigma Aldrich-USA) and 0.08 M of cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O-Sigma Aldrich-USA) was stirred in a glove box chamber filled with N₂. After stirring, enough KNO₃ (Sigma Aldrich-USA) and CTAB (Sigma Aldrich-USA) were added to reach a final concentration of 0.8M and 0.16%w/v, respectively. After that, a coprecipitation was promoted adding drop by drop 2.0 M potassium hydroxide (KOH-Merck-USA) until pH = 11. Then, the solid and solution were placed in a closed jar and heated at 95 °C during 16 h in an aging process for selective oxidation. After that, the system was cooled down until room temperature is reached and the solid was then washed with distilled water using cycles of centrifugation (7500 rpm) and redispersion until pH = 7.

The reagents used in this work are the following: Titanium dioxide (Degussa P25, 75% anatase, 25% rutile); Nickel chloride hexahydrate (reagent grade, Sigma-Aldrich); Cobalt nitrate hexahydrate (reagent grade, Sigma-Aldrich); Hydrogen tetrachloroaurate trihydrate (reagent grade, Sigma-Aldrich); Chloroplatinic acid hydrate (reagent grade, Sigma-Aldrich); Silver nitrate (reagent grade, Sigma-Aldrich); Palladium nitrate dehydrate (reagent grade, Sigma-Aldrich); Methanol (anhydrous, ≥ 99.8% (HPLC), Sigma-Aldrich); Ammonia gas (anhydrous, BOC); Argon gas (99.99%, BOC); Helium gas (99.99%, BOC); Nitrogen gas (99.99%, BOC).

Iron nitrate nonahydrate (Fe(NO₃)₃·9H₂O), cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O), ammonium fluoride (NH₄F), urea (CO(NH₂)₂), anhydrous ethanol (CH₃CH₂OH) and poly(vinyl pyrrolidone) (PVP, M_w = 55,000) were purchased from Sigma Aldrich, Korea. Hydrochloric acid (HCl), potassium hydroxide (KOH) and sodium hydroxide (NaOH) were purchased from Samchun Chemicals, Korea. Ni foam with a 1.6 mm thickness was purchased from Alantum, Korea. Deionized (DI) water and double-distilled water (DDW) were used especially for the purposes of preparing solutions and washing.

Halloysite nanoclay, potassium tetrachloropalladate (K₄PdCl₄), copper nitrate trihydrate (Cu(NO₃)₂·3H₂O), cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O), nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O) and sodium borohydride (NaBH₄) were purchased from Sigma-Aldrich Co. Ltd. All reagents were used without further purification.

Cobalt nitrate hexahydrate (purity ≥ 99.99%, Co(NO₃)₂.6H₂O), urea (purity 99%), ethanol (laboratory reagent), 2-(2-Butoxyethoxy) ethyl acetate (purity ≥ 99.2%, binder), uric acid (purity ≥ 99%), potassium chloride (KCl), sodium chloride (NaCl), potassium hexacyanoferrate (purity ≥ 99%, K₃[Fe(CN)₆]), and phosphate buffer saline (pH = 7.4, PBS) solution were purchased from Sigma-Aldrich. All analytical grade chemicals were used for this work.