Optical properties of biomimetically produced spectrally selective coatings

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Solar Energy Vol. 69(Suppl.), Nos. 1–6, pp. 27–33, 2000  2001 Elsevier Science Ltd S 0 0 3 8 – 0 9 2 X ( 0 1 ) 0 0 0 0 2 – 0 All rights reserved. Printed in Great Britain 0038-092X / 00 / $ - see front matter

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OPTICAL PROPERTIES OF BIOMIMETICALLY PRODUCED SPECTRALLY SELECTIVE COATINGS R. JOERGER†, T. KLAUS-JOERGER, E. OLSSON and C. G. GRANQVIST ˚ ¨ Laboratory, P.O. Box 534, S-75121 Uppsala, Sweden Uppsala University, The Angstrom Received 20 June 2000; revised version accepted 11 December 2000 Communicated by VOLKER WITTWER

Abstract—Efficient photothermal conversion of solar energy requires spectrally selective surfaces. The purpose of this work is the development of new economically and ecologically benign coating materials by biological means and the evaluation of their optical properties. Our approach to produce wavelength selective composite materials structures relies on the use of metal accumulating microorganisms to produce metallic and metal ion containing nano-particles. Crystalline silver particles with distinct shapes are formed in the periplasmic space of the silver accumulating bacterial strain Pseudomonas stutzeri AG259. A non-metallic carbon host matrix for the silver particles is provided by the organic biomass of the bacteria. The innovation and relevance of our work lies in the biotechnological approach to materials science and in the relatively low investment and operating costs to produce the coating material. Through different heat treatment procedures adjustable optical properties are obtained. Optical spectroscopic measurements were carried out in the UV/ VIS / NIR and IR in order to characterise the material. The performance of the material and strategies to optimise the spectral selectivity of the coatings are discussed in terms of effective medium theories.  2001 Elsevier Science Ltd. All rights reserved.

relies on the use of self-assembling and biological or biomimetic (Sarikaya, 1999) principles. In previous work (Joerger et al., 2000a, 1999) we have developed an organic–metal composite material with strongly wavelength selective properties by a biomimetic technique. This method uses metal accumulating microorganisms to produce metallic and metal ion containing nanoparticles. Crystalline silver particles with distinct shapes are formed in the periplasmic space of the silver accumulating bacterial strain P. stutzeri AG259 (Klaus et al., 1999). A non-metallic carbon host matrix for the silver particles is provided by the organic biomass of the bacteria. After different heat treatment procedures (250– 5008C) the nano-particles are embedded in the organic carbon matrix and adjustable optical properties are obtained. Optical spectroscopic measurements were carried out in the UV/ VIS / NIR and IR in order to characterise the material. Strategies to optimise the optical performance of the coatings are discussed in terms of effective medium theories.

1. INTRODUCTION

Efficient photothermal conversion of solar energy requires spectrally selective surfaces with a high absorption of solar radiation and low emittance of thermal radiation (Joerger et al., 1999; Niklasson and Granqvist, 1991). Coatings and surfaces with such properties were subject to intense research and development in many laboratories around the world. An annotated bibliography, covering the period 1955–1981, was published in 1983 (Niklasson and Granqvist, 1983) Such thin film coatings are traditionally produced by physical or chemical methods such as PVD, CVD (physical or chemical vapour deposition) or electrochemically. The purpose of this work is the development of new economically and ecologically benign coating materials by complementing biological means and the investigation of their optical properties. New and interesting optical and electrical properties emerge when metallic and non-metallic (e.g. ceramic or organic) materials are assembled into so-called cermet (ceramic–metal composite) films. Our approach to produce composite structures

2. EXPERIMENTAL †

2.1. Sample preparation Author to whom correspondence should be addressed. Tel.: 146-18-471-7234; fax: 146-18-50-0131; e-mail: [email protected]

The bacterial strain P. stutzeri AG259 was grown for 48 h at 308C in the dark on Lennox L 27

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(LB) broth agar plates (11 H 2 O bidistillatum, 10 g Bacto Tryptone, 5 g yeast extract, 10 g NaC1, 10 ml 20% glucose (autoclaved separately), 15 g agar) that contained additional 5 mM AgNO 3 . For the silver free films, the AgNO 3 was omitted. Cells were grown as a dense layer on the agar surface and not as single colonies. Then the cells were removed carefully from the agar and suspended in distilled water. The cell walls were opened in a French press at 4 3 10 6 Pa while simultaneously cooled on ice. Cell fragments were separated on a sucrose gradient (10–50% sucrose) and centrifuged at 15 000 g for 30 min. The sediment was resuspended in A. dist. and 200 ml wereevenlydistributedonaluminumsubstratesoron slide glass to form a thin film covering a 2 3 3 cm 2 surface. The films were dried in air for 24 h at room temperature. A following heat treatment of the films was performed at temperatures from 2508C to 5008C.

2.2. Optical measurements The total near normal reflectance and transmittance was measured in the ultraviolet–visible–near infrared (UV–VIS–NIR) region (0.3–2.5 mm) with a Beckman UV 5240 spectrophotometer. The instrument was equipped with a 198851 bariumsulphate (BaSO 4 ) painted integrating sphere and a BaSO 4 plate was used as a reference. In the infrared wavelength range between 2.0 mm and 20 mm, the samples were measured with a Fourier transform infrared Bomem-Michelson 110 spectrometer. It was equipped with a Labsphere 4 / 5 in. Infragold integrating sphere. A cleaned aluminum plate was used as a reference.

3. RESULTS AND DISCUSSION

3.1. Heat treatment of the carbon matrix Optical properties of thin films with a thickness of a few micrometers were studied after different steps of heat treatment. The physical and chemical properties of the films depend in a predictable way on the special conditions of the heat treatment, i.e. temperature and duration of the heat treatment. The embedded silver particles are stable and no oxidation of the silver occurs up to a temperature of 4508C as could be seen from X-ray diffraction data (Joerger et al., 2000b). However, the carbonaceous matrix becomes more porous with increasing temperature and the film thickness decreases. This effect ensues most likely from oxidation of C and H atoms and the evolu-

tion of CO 2 and H 2 O. After 1 h at 4508C the matrix material was almost destroyed and large agglomerated silver particles remained between temperature stable residues of the matrix material. After 1 h at 5008C also the crystalline silver particles decay and another, yet undetermined, crystalline structure arises.

3.2. Analysis of the optical properties Fig. 1 displays near normal reflectance spectra of the matrix material alone (a) and (b) as well as spectra of silver containing films (c) and (d) on aluminum substrates after different steps of heat treatment. The optical properties are characterised by their very distinct wavelength selective absorption, which can be tailored to a large extent. The matrix material obtains its spectral selectivity only after the heat treatment. The optical band gap, responsible for the absorption in the short wavelength range, ensues from the loss of H and O atoms and a consequent formation of sp 2 coordinated C=C bonds during the heat treatment, as confirmed by photoelectron spectroscopy. The matrix material can be regarded as hydrogenated amorphous carbon (a-C:H) doped with further inorganic cell constituents, mainly phosphorous, sulphur, calcium, potassium and chlorine. Compared to conventional a-C:H (Joerger et al., 1998) produced from a CVD process (Gampp et al., 1994), the present material has a much lower band gap, i.e., the strong change in optical behaviour occurs at a longer wavelength. Furthermore, the transition from the highly absorbing to weakly absorbing spectral region is much narrower. These properties make the present material favourable for thermal solar energy conversion. However, a better performance could be reached if the absorption edge could be moved further towards the IR. It was found that the position of this transition region can be shifted to different wavelengths by specific heat treatment. The width of the transition region is governed by the metal volume fraction in the film. The spectral characteristics of the films can be discussed in terms of structural changes in the material composition that occur due to the different ways of heat treatment. Physical and chemical processes in the films can be connected with different heating times and temperatures as follows: The relatively high reflectance of the not heated, Ag free sample is due to the high reflectance of the aluminum substrate and the high transparency of the film itself. In the Ag containing film, the Ag particles cause strong absorp-

Optical properties of biomimetically produced spectrally selective coatings

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Fig. 1. Spectral total near normal reflectance for biomimetically produced films on aluminum substrates. The spectra refer to carbonaceous material without (part (a) and (b)) and with (part (c) and (d)) silver particles. The films were heat treated in air at different temperatures and for different durations as indicated.

tion predominantly at short wavelengths. However, also the heat treatment of the carbonaceous matrix produces strong absorption in the short wavelengths range. An increase of the extinction coefficient of the film in the spectral range l , 800 nm is due to chemical reactions in the carbon matrix. Reactions that most likely take place during the heat treatment are evaporation of water and other volatile compounds, dehydration and oxidation of carbon atoms, leading to the formation of CO 2 and a higher number of C–C (sp 3 co-ordinated) and C 5 C (sp 2 co-ordinated) bondings. Investigations in terms of Raman-, photoelectron-, and optical spectroscopy of structural changes in sputtered a-C:H films due to heat treatments are reported in the literature. In particular it is reported that neighbouring C–C single bonds break up and, under release of hydrogen- or carbon–hydrogen molecules, form C 5 C doublebonds (Craig and Harding, 1982). This leads to an increase of sp 2 co-ordinated carbon at the expense of sp 3 co-ordinated carbon (Dischler et al., 1983; Koidl et al., 1989; Oelhafen et al., 1984; Oelhafen and Ugolini, 1987). The p -electrons in the C=C double bonds are much weaker bound than the s -electrons. An increase of sp 2 co-ordination leads to a smaller optical band gap (Tamor et al., 1989; Yoshikawa et al., 1992) and thus in general to a higher extinction coefficient k c of the carbon matrix for short wavelengths. The increase of sp 2 co-ordinated carbon might even be catalysed in

the presence of metal clusters in the carbon matrix (Grischke, 1989). These transformations of the carbon matrix most likely account for the drastic changes in the optical spectra in the UV/ VIS and the shift of the absorption edge towards a longer wavelength, as it was observed in the present samples. However, upon further heat treatment, the absorption edge shifts back to a shorter wavelength. This effect is caused by a decreasing film thickness. Since the decrease in film thickness is due to the evaporation of CO 2 and H 2 O, the relative metal volume fraction increases. This causes a broadening of the absorption edge, as will be shown in the following section on effective medium calculations. It is interesting to note that the three discussed effects, the modification of the carbon matrix, the decrease in film thickness and the increase in metal volume fraction have different effects in different wavelength regions, which allows tailoring the optical properties so that one can obtain wavelength selective materials suitable for solar collector surfaces. The optical behaviour at selected wavelength is analysed in Fig. 2. Solid lines represent Ag containing films, dashed lines correspond to Ag free films. At short wavelengths (a) the heated films are completely opaque. Thus film thickness and increase of extinction coefficient k C of the carbon have no effect on reflectance. The extinction coefficient k C of the carbon is about an order of magnitude

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Fig. 2. Spectral total near normal reflectance for silver containing and silver free films on aluminum substrates plotted at selected wavelengths against heat treatment periods. Annealing was carried out as follows: (0) before heat treatment, (1) 1 h at 3008C, (2) 2 h at 3008C, (3) 2 h at 3008C130 min at 4008C, (4) 2 h at 3008C160 min at 4008C, (5) 2 h at 3008C190 min at 4008C, (6) 2 h at 3008C1120 min at 4008C, (7) 2 h at 3008C1150 min at 4008C.

smaller than its refractive index and thus negligible for the reflectance. In the not heated films, the Ag particles contribute to a major absorption. At longer wavelength, but still below the absorption edge (b), the situation is qualitatively similar to the one in (a), however the large extinction coefficient kAg of the Ag particles leads to an increasing reflectance with increasing metal volume fraction. At the absorption edge (c) the films are relatively transparent. An increase of k C increases the screening of the high reflectance from the Al substrate while a decreasing film thickness has the opposite effect. The former effect is dominant at the beginning of the heat treatment while the latter one is dominant at the end. The effect of decreasing film thickness is almost balanced out by an increasing metal concentration. Just above the absorption edge (d), increasing metal concentration has a stronger effect. Due to the relatively high level of the reflectance, originating from the high reflectance of the aluminum substrate and the high transmittance of the matrix material, the reflected light passes the layer twice,

therefore more light can be absorbed in the metal particles. Thus the reflectance of the Ag containing sample decreases strongly at the end of the heat treatment. Far above the absorption edge (e) the heat treatment has negligible effect on the extinction coefficient k C of the matrix material. Decreasing thickness and increasing metal concentration have opposite effects. At an even longer wavelength (f) the inclusion of small amounts of small metal particles has a negligible effect on the effective optical constants of the material. The increase of metal fraction is thus negligible compared to the decrease in film thickness. Therefore, the only remaining significant effect is the decreasing thickness.

3.3. Embedded silver particles The optical behaviour of our samples can be reconciled with the existence of a cermet type structure, as can be shown by calculations according to effective medium theories. Specifically the Bruggeman model (Bruggeman, 1935; Niklasson, 1991), which was earlier successfully applied to

Optical properties of biomimetically produced spectrally selective coatings

selectively solar absorbing nickel–alumina coatings (Andersson et al., 1980), may be used to demonstrate the effect of varying metal content in the films and different sizes and shapes of the embedded silver particles. Hexagonal, triangular and elongated rod-like shaped silver particles have been observed in the bacterial cells. However, there is no simple model describing the optical response of these geometries. Oblate and prolate spheroidal shapes may then be a good approximation, which can be treated readily. The model is expressed in analogy to the formulas given in Granqvist and Hunderi (1977): 1 1 2 f 1 ] fa 3 ´¯ 5 ´B ]]]] 2 1 2 f 2 ] fa 3

(1)

where f is the volume fraction of the metal, ´B is the dielectric function of the carbonaceous matrix material and ´¯ is the effective dielectric function of the composite material. a is proportional to the polarizability of the silver particles and for ellipsoidal shape it is given by

O

´A 2 ´¯ 1 3 ] ]]]] a5 3 i51 ´¯ 1 Li (´A 2 ´¯ )

31

by known notions (Granqvist and Hunderi, 1977), by adjusting the free electron contribution while leaving the interband part unchanged. We took ´A from the literature (Lynch and Hunter, 1985) and used our own optical data for carbonaceous films to derive ´B . Calculations for a 2 mm thick film on an aluminum substrate are shown in Fig. 3. Optical spectra were calculated by averaging interference patterns to account for inhomogeneous layer thickness. Part (a) refers to different volume fractions of silver. An increase in silver content shifts the absorption edge towards the IR and increases the reflectance below the absorption edge slightly, thus resulting in a broadening of the transition region. This effect was also observed in the experimental curves in Fig. 1b. Fig. 3b displays the effect of a reduced mean free path of the conduction electrons, which also tends to smear and displace the transition between high and low absorption to some extent. However, the reflectance just below the absorption edge is

(2)

where ´A is the dielectric function of the metal and the Li ’s are the depolarisation factors of the silver crystals, which depend on the axial ratio of the ellipsoidal particles and are given for prolate spheroids by L1 L2

5 5

S

D

1 2 e 2PS 1 1 e PS ]] ln ]] 2 2e PS 3 1 2 e PS 2e PS 1 2 L1 L3 5 ]] 2

(3)

and for oblate spheroids by L3 L1

5 5

1 1 e 2OS ]]] (e OS 2 arctan e OS ) 3 e OS 1 2 L3 L2 5 ]] 2

(4)

The eccentricities of the spheroids are given by 2 1/2

e PS 5 [1 2 (c /a) ]

e OS 5 [(a /c)2 2 1] 1 / 2

(5)

where a and c represents the major and minor axis of the spheroids. The size of the silver particles does not enter explicitly in Eq. (1), but can be incorporated in the theory by a size dependent dielectric function

Fig. 3. Spectral normal reflectance calculated from the Bruggeman effective medium theory applied to a 2 mm thick carbonaceous film containing silver particles. The films were taken to be backed by optically thick aluminum. Part (a) refers to films having the shown volume fraction of silver, and part (b) and (c) were obtained for material with 9 vol.% silver. Part (b) shows the effect of a limited mean free path of the conduction electrons and part (c) displays variations due to non-spherical particle shapes. The inset in (c) shows an elongated rod shaped and a triangular crystal observed in the bacterial cell.

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not increased, as it was for high metal content. The size effect becomes relevant for path limitations which are small compared to the particle sizes actually observed. Therefore it is likely that the silver particles, though crystalline, contain impurities serving as scattering centres for the free electrons. Fig. 3c demonstrates the effect of the particle shape. Deviation from spherical particle shapes results also in displacing the transition region towards the IR. Non-spherical particle shapes are thus favourable for thermal solar energy applications.

4. CONCLUSION

Our new biologically prepared coatings exhibit spectral selectivity, although they are not yet able to compete with those prepared by the more traditional techniques. The innovation and relevance of our work lies in the biotechnological approach to materials science and in the relatively low investment and operating costs to produce the coating material. Fig. 4 compares the performance of our biomimetically produced coating with the performance of selective paints (Gunde et al., 1994) ¨ ¨ and Huland a sputtered coating (Wackelgard tmark, 1998). An optimisation of the solar absorption performance of the biomimetic coating will be possible by shifting the absorption edge to-

Fig. 4. Spectral reflectance for metal–dielectric composite coatings backed by aluminum for three commercially available solar collector surfaces (curves 2, 3 and 4) and for the biomimetic material fabricated with P. stutzeri AG259 cells as a carbonaceous host matrix material (curve 1).

wards a longer wavelength around 2 to 3 mm. To accomplish this, different techniques can be used: e.g. by enhancing the metal concentration in the organic material and by controlling the metal particle shapes and sizes. The metal content is — to some extent — adjustable biologically via the metal accumulation in the microorganisms, which can be controlled by the cultivation condition for the bacteria. The possibility to control also particle size and shape distribution by cultivation parameters as pH, temperature etc. will be investigated in future research. Furthermore, we will evaluate the applicability of a multi-layer structure with organic material cultivated under specific conditions leading to a graded index type coating.

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