TiO2/Ti thin-film electrode manufacturing equipment and combined external circuit photoelectrical catalytic process for reducing silver ions

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Abstract

In this study, a nano-class TiO2/Ti thin-film electrode was made using the atmospheric pressure chemical vapor deposition (APCVD). The electrode was combined with an external circuit and anode bias to study the efficiencies of silver ions reduction and acetic acid decomposition. Results of SEM images and XRD patterns of the TiO2/Ti thin-film electrode surface show that the thin-film electrode made using the APCVD method can be as size as 30 nm with the TiO2 photoelectric catalyst in anatase crystal form. More obvious agglomeration of TiO2 particles was observed for shorter APCVD sprayed times. Results of the light response study show that the electrode has a rapid response time to 365 nm UV light to produce electricity with 7 μA/cm2 density. Additionally, results of the photocatalytic studies using the electrode combined with an external circuit in the photoelectrical catalytic studies to reduce silver ions reveal that with a reaction time of 180 min, the photocatalytic process will reduce 70% of silver ions in high-concentrated solution (1000 mg/l as Ag) and 93% silver ions in low-concentrated solution (108 mg/l as Ag). When the irradiation time is extended to 240 min, the silver reduction efficiency is as high as 99.8%. Higher solution pH is favorable to the photoelectrical reduction of silver while the anode bias does not benefit the silver reduction efficiency but favors the decomposition of acetic acid. However, the process decomposes less than 10% of acetic acid. The external circuit will transmit the photo-generated electrons to the cathode surface thus reducing the combination of electron and holes at the anode surface. As the reduction of precious metals is concerned, the external circuit is capable of avoiding the metal deposition at the catalyst surface to cause catalyst poison or light shielding that are know to reduce the photo utilization efficiency.

Introduction

Silver is a basic precious metal that has wide uses in electroplating, die casting alloy, coins, silverware, photographic film and chemical manufacturing. With advances in science and technology as well as improved living standards, consumption of precious metal increases tremendously. Data reported by Taiwan Directorate General of Customs reveal that the annual consumption of silver in Taiwan is 160 thousand tons Thus, recovery and reuse of silver from discarded silver-containing wastes is of great urgency. Silver ions combined with anions (S2O32−, SO32−, Br, Cl) in water to form complexes; long-term consumption of the water containing silver ions has been considered to cause serious health hazards. In drinking water, the silver concentration is limited to less than 0.05 mg/l while for industrial wastewater discharges, it is limited to 0.5 mg/l. Most wasted silver comes from the discarded photographic fixer, which also contains thiosulfate sodium sulfite, acetic acid, and exposed photographic film, from hospital and commercial darkrooms, printing circuit plate industry as well as the waste discharged by electroplating and electronic metal surface treating industries for processing lead frame. The photographic fixer waste discharged by hospitals, which contains more than 6000 mg/l silver, has been proclaimed by Taiwan Environmental Protection Administration a hazardous industrial waste and needs proper treatment. Although the photographic waste fixer discharge from commercial darkroom contains lower silver ions of 3000 mg/l or above, it must be subject to effective treatment for recovering the silver to avoid wasting precious resources and damaging environment.

The silver recovery methods commonly practiced domestically and internationally include: electro-dialysis reduction [1], [2], [3], [4], ion exchange [5], solvent extraction [6], [7], bio-adsorption [8], ion flotation [9] and homogenous photo catalytic reduction [10].

In recent years, the technology of producing nano photocatalyst has attended to maturity and widely applied for solving environmental pollution problems. In the application of heterocatalyst system, the oxidation/reduction semi-conductor photocatalyst TiO2 is attractive because of many advantages, e.g. chemically stable and non-toxic. Its band gap energy is 3.2 eV that covers the reduction potential of Ag+/Ag0 (+0.799 eV) and Ag(S2O3)23−/Ag0 (+0.01 eV) to effect photocatalytic reduction of silver ions.

Many researchers have applied suspended TiO2 particles to conduct the photocatalytic reduction of silver ions but encountered some problems. When the concentration of suspended TiO2 is too high, an ultraviolet shielding effect reduces the light efficiency to reduce the reduction capability of photocatalyst. Additionally, the reduced silver metal is mixed with the suspended TiO2 leading to subsequent difficulty in separated silver metal [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]. Titanium dioxide fixed on thin film has been applied to recover precious metal from aqueous solution and improve the quality of precious metal (gold, silver and platinum) covered TiO2 thin-film. The objective is to increase activities of the catalyst by adding silver in water instead of treating silver-containing wastewater. If not properly handled, this process will produce more silver-containing solution. Although the thin-film photocatalyst with TiO2 fixation will alleviate the problem of ultraviolet shielding effect in suspended TiO2 system, deposition of the recovered metal on the TiO2 thin-film surface will also cause ultraviolet shielding effect to poison the catalyst and difficult in recovering the metal [22], [23], [24], [25].

To overcome the problems encountered when using either the suspended TiO2 or the TiO2 membrane method, the atmospheric pressure chemical vapor deposition (APCVD) is applied to make a TiO2 thin-film electrode to be used in the photoelectrocatalytic (PEC) system. An external circuit is used to transmit the electrons generated photoelectrically at the anode to the graphite cathode surface for enhancing the reduction of silver ion into silver metal to achieve recovery, separation and purification of silver. Additionally, to reduce the accumulation of photo-generated holes at TiO2 thin-film surface and the recombination of photo-generate electron–hole pairs, a co-existed organic substance (e.g. acetic acid) is used as the hole scavenger thus increasing the efficiency of silver reduction. Finally, anode bias is collocated to assist in improving the photoelectrical efficiency to increase the silver reduction and organic matter decomposition rates.

Section snippets

Reagents

Tetraisopropyl orthotitanate (TTIP, Ti[OCH3(CH2)2]4), which is the precursor of photocatalyst, was prepared in the lab by mixing silver nitrate (AgNO3, Mallinckrodt, 99%) and Glacial acetic acid (CH3COOH, J.T. Baker, 99.9%) using deionized water (18.2 mΩ, Milli-Q).

Preparation of TiO2/Ti thin-film electrode

The TiO2/Ti thin-film electrode was prepared in the laboratory using the APCVD (atmospheric pressure chemical vapor deposition). After submerged in 9 M sulfuric acid for 1 h, the plate substrate was drip washed and then dried.

Results and discussion

Fig. 2 shows the surface structure SEM image of TiO2/Ti thin-film electrode prepared at 6-h APCVD spray time. The TiO2 membrane prepared using the APCVD method shows slight agglomeration in addition to reduced TiO2 particles size with increasing APCVD spray time. The electrode SEM images shows that after 2-h APCVD spray time, the particles display obvious agglomeration. Since the APCVD spray time is short, when the deposited TiO2 particles contact the titanium plate that has a larger thermal

Conclusions

The APCVD method can be applied to produce nano-level TiO2 photocatalyst; the APCVD apparatus is easily to set up and is less costly than Direct Current chemical vapor deposition (DCCVD), Micro-wave plasma chemical vapor deposition (MPCVD) or Direct Current magnetron sputtering equipment. Pure water provides oxygen atoms in the TiO2/Ti thin-film electrode process thus eliminating the need of pure oxygen and making the process easy to operate. The TiO2/Ti thin-film electrode combined with an

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