Characterization of copper–manganese-oxide thin films deposited by dip-coating
Introduction
Transition-metal oxides with spinel-like structure have been extensively studied for many decades due to their wide variety of physical and chemical properties. During the last years the spectral selectivity of some spinel coatings deposited on highly reflective metals has attracted considerable interest because this makes them promising materials for being used as absorbers in solar thermal collectors. A good selective absorber surface should display high absorptance (αs>0.95) for solar radiation and low emittance for the reradiated infrared part (εT<0.1). In this way, black paintings with CuMnFeOx [1], [2] and thin films of CuFeMnOx or CuCoMnOx [3], [4], [5] have displayed fairly good optical parameters (αs=0.9–0.92 and ε100=0.2–0.1) when deposited onto aluminum substrates and covered with a SiO2 antireflective coating. In a previous work we reported on the preparation of copper–manganese-oxide thin films by dip coating as selective absorbers for low-temperature applications [6]. A single layer of this material deposited on aluminum and subsequently covered by an antireflective coating of SiO2 leads to αs=0.94 solar absorptance and ε100=0.06 thermal emittance. These results show that copper–manganese-oxide could be a competitive material to be considered by the industry as a future selective absorber.
Being the first time that copper–manganese-oxide was prepared by dip coating, little was known about the relationship between deposition parameters and thin film properties. In our previous work only some preliminary studies were carried out about the influence of both metal precursor's ratio and withdrawal rate on the optical properties. However the effect of other deposition parameters like annealing temperature and time should also be taken into account. Therefore the aim of this work has been to study in more detail what happens during the film formation process, how annealing time and temperature influence the optical properties of the films and whether these properties are related to the oxidation states of Cu and Mn. For this purpose, NIR–vis–UV spectroscopy and X-ray photoelectron spectroscopy (XPS) analysis have been used as main characterization techniques. Electrical properties were also measured for the copper–manganese-oxide thin films. Although it is obvious that these properties are irrelevant for solar thermal applications we should not forget that copper–manganese-oxides are conductive materials for which the electrical properties are likely to be influenced by valence state and site distribution of Cu and Mn ions [7]. In this way electrical measurements could be very helpful for discussing the results of XPS analysis. Finally it is important to note here that mixed copper and manganese oxides can be used as catalysts (i.e. Hopcalite) for the removal or air pollutants like carbon monoxide and nitrous oxide [8]. The activity of these catalytic materials strongly depends on both manganese and copper oxidation states. Therefore any additional information about cation valence and site distribution would be very helpful if one some day copper–manganese-oxide thin films prepared by dip-coating were to be used as catalysts.
Section snippets
Experimental
Copper–manganese-oxide thin films were deposited on commercially available highly-polished Al foils (Anofol) from a solution containing copper (II) and manganese (II) nitrates dissolved in absolute ethanol at Cu/Mn=1 ratio. A complexing agent and a wetting additive were also added to stabilize the solution and improve the film adherence. This solution showed long-term stability, which made the absorber deposition both reproducible and reliable. (No more details about solution composition can be
Results and discussion
Copper–manganese-oxide thin films were prepared by using a dip-coating method developed in our group for metal oxides [9]. The influence of film composition on the optical properties was studied in a previous work in which they were deposited on aluminum substrates at various Cu/Mn ratios (0.5, 1, 2) [6]. In Fig. 1 the reflectance spectra of three samples annealed at 500 °C for 60 min, with similar thickness (∼90–100 nm), but different compositions (i.e. Cu/Mn ratio) are displayed, together with
Conclusions
In this work copper–manganese-oxide thin films have been deposited on aluminum substrates by dip-coating. The influence of annealing time and temperature on the optical properties and film composition has been studied. For annealing temperatures lower than 450 °C a mixture of copper (II) oxide and manganese (III and IV) oxides is obtained. However, when temperatures higher than 450 °C are used, a solid state red-ox reaction occurs. In this reaction copper (II) is partially reduced to copper (I)
Acknowledgments
The authors would like to acknowledge Dr. Rocio Barrio for the electrical characterization of copper–manganese-oxide thin films.
References (25)
- et al.
Sol. Energy Mater. Sol. Cells
(2001) - et al.
Sol. Energy Mater. Sol. Cells
(2003) - et al.
Renew. Energy
(2008) - et al.
Solid State Ion.
(1997) - et al.
J. Catal.
(1986) - et al.
Mater. Res. Bull.
(1985) - et al.
Solid State Ion.
(1985) - et al.
Mater. Res. Bull.
(1973) - et al.
J. Cryst. Growth
(2003) - et al.
Thin Solid Films
(1999)
Thermochimia Acta
Appl. Catal. A: Gen.
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