Characterization of copper–manganese-oxide thin films deposited by dip-coating

doi:10.1016/j.solmat.2008.04.011

Solar Energy Materials and Solar Cells

Volume 92, Issue 10, October 2008, Pages 1211-1216

https://doi.org/10.1016/j.solmat.2008.04.011 Get rights and content

Abstract

Thin films of copper–manganese oxide were prepared by dip-coating method and annealed between 400 and 500 °C for periods of time from 15 to 90 min. The influence of both annealing time and temperature on film composition and optical properties was studied. X-ray photoelectron spectroscopy (XPS) measurements indicated that the initially formed mixture of copper and manganese oxides leads to Cu_1.5Mn_1.5O₄ through a solid state red-ox reaction. This reaction takes place at temperatures higher than 450 °C and the full conversion of single oxides into Cu_1.5Mn_1.5O₄ needs at least 60 min to proceed. In relation to the optical properties we found that low solar absorptance (α_s∼0.6) can be associated with the mixture of single oxides whereas the formation of Cu_1.5Mn_1.5O₄ dramatically increases solar absorptance values (α_s∼0.9). The presence of Mn³⁺ and Mn⁴⁺ in octahedral sites and Cu⁺ and Cu²⁺ in tetrahedral sites is also evidenced by XPS and can explain the high conductivity of films where Cu_1.5Mn_1.5O₄ is present.

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 (α_s>0.95) for solar radiation and low emittance for the reradiated infrared part (ε_T<0.1). In this way, black paintings with CuMnFeO_x [1], [2] and thin films of CuFeMnO_x or CuCoMnO_x [3], [4], [5] have displayed fairly good optical parameters (α_s=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 α_s=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.

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Characterization of copper–manganese-oxide thin films deposited by dip-coating

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgments

Sol. Energy Mater. Sol. Cells

Sol. Energy Mater. Sol. Cells

Renew. Energy

Solid State Ion.

J. Catal.

Mater. Res. Bull.

Solid State Ion.

Mater. Res. Bull.

J. Cryst. Growth

Thin Solid Films

Thermochimia Acta

Appl. Catal. A: Gen.