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44403-G5
Synthesis of Inorganic Electrodes with Controlled Interfacial Structures by Crystal Engineering in Electrodeposition

Kyoung-Shin Choi, Purdue University

The overall goal of our research is to produce various inorganic polycrystalline electrodes with optimum interfacial structures that can enhance desired functional properties.  We achieve this goal by precisely tuning the individual shapes of particles composing polycrystalline electrodes and studying their morphological effects on physical and chemical properties. . Electrodeposition is used as the main synthetic method as it can produce a variety of materials as thin film-type electrodes.  The most significant achievements made during the second budget year are summarized below. 

(1) Synthesis of Cu₂O Electrodes Composed of Dendritic Crystals

We have demonstrated a new strategy of exploiting and manipulating dendritic growth of Cu₂O to produce photoelectrodes with high surface areas and good electrical continuity.  The dendritic branching growth of Cu₂O allowed for facile substrate coverage and high surface roughness without growing a thick film.  The resulting electrodes generated significantly improved photocurrent compared to the electrodes composed of micron-size faceted crystals that produced negligible short-circuit photocurrent. 

In order to further enhance photocurrent, various methods to increase dendritic crystal size were investigated.   Increasing crystal size reduces crystal-crystal boundary areas and ensures good electrical continuity in the larger domains of the electrode.  This can minimize recombination losses and improve charge transport properties.  Deposition potential, Cu²⁺ concentration, and acetate concentration were altered to regulate the deposition overpotential, h, which has a direct impact on nucleation density and, therefore, on crystal size.  Increasing crystal size consistently resulted in the improvement of photocurrent regardless of the method used to regulate crystal size.  The electrode showing the highest photocurrent was composed of dendritic crystals that laterally expanded ca. 12000 mm² while the thickness of the electrode was kept below 5 mm (Figure 1).  This electrode generated more than 20 times higher photocurrent (0.45 mA/cm²) than the electrode containing the smallest crystal size produced in our study with the average lateral size of 100 mm² (0.02 mA/cm²).  The significant increase in photocurrent achieved simply by controlling the crystal sizes in dendritic branching growth, without involving any compositional changes, implies an enormous potential for morphology tailoring in improving properties of polycrystalline electrodes.  

Figure 1. SEM image of dendritic Cu₂O electrode composed of dendritic crystals that laterally expanded ca. 12000 mm² (left).  Its low magnification (middle) and side view (right) SEM images are also shown.

(2) Synthesis of Highly Transparent Nanocrystalline a-Fe₂O₃ electrodes

Ferric oxide (a-Fe₂O₃, hematite) is an n-type semiconductor highly desirable for use in solar energy conversion due to its bandgap (E_g = ~2.2 eV) that allows for utilizing the significant portion of the solar energy spectrum.   We have developed a new anodic deposition route to prepare a-Fe₂O₃ electrodes using a slightly acidic plating solution (pH 4.1).  In this pH, Fe²⁺ ions are soluble without the need of adding complexing agents, which simplifies the compositions of the plating solution.  Deposition in this medium produced amorphous g-FeOOH films that can be converted to highly transparent nanocrystalline a-Fe₂O₃ films by annealing.

a-Fe₂O₃ films with varying thickness were produced by changing deposition times in order to investigate the effect of film thickness on photon absorption and photocurrent generation.  Films deposited for 1min, 2 min, 4 min, 8 min, and 16 min resulted in films with the average thickness of 70 nm, 180 nm, 320 nm, 430 nm, and 680 nm, respectively, judging from their cross sectional SEM images.  The photographs and UV-vis absorption spectra of the a-Fe₂O₃ films with varying thicknesses are shown in Figure 2a-b.  These films showed similar absorption features (i.e. onset of band gap transition) but absorption increased gradually as the film thickness increased.

            The short-circuit photocurrent, which was measured using a 60:40 solution of propylene carbonate: acetonitrile containing 0.5 M tetrabutylammonium iodide and 0.04 M iodine, gradually increased as the film thickness increased due to enhanced photon absorption and surface areas.  However, when the film thickness exceeds 460 nm, photocurrent decreased because severe aggregation of Fe₂O₃ particles at the Fe₂O₃/substrate interface increased recombination losses in the film.

Figure 2.  (a) Photographs, (b) UV-vis absorption spectra, and (c) short-circuit photocurrents of a-Fe₂O₃ films with varying thickness (1:70 nm, 2:180 nm, 3:320 nm, 4:430 nm, and 5:680 nm).

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