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Optical properties of silver sulphide thin films formed on evaporated Ag by a simple sulphurization method
Thin Solid Films 518 (2010) 1835–1838
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Optical properties of silver sulphide thin films formed on evaporated Ag by a simple sulphurization method E. Barrera-Calva a,⁎, M. Ortega-López b, A. Avila-García b, Y. Matsumoto-Kwabara b a b
Departamento de Ingeniería de Procesos e hidráulica, Universidad Autónoma Metropolitana — Iztapalapa, Av. Purísima Esq. Michoacán, Col. Vicentina, México, D.F., 09340, Mexico Departamento de Ingeniería Eléctrica, Centro de Investigación y de Estudios Avanzados del IPN, México DF 07360, Mexico
a r t i c l e
i n f o
Available online 19 September 2009 Keywords: Silver sulphide thin films Optical properties Solar selective tandems
a b s t r a c t Silver sulphide (Ag2S) thin films were grown on the surface of silver films (Ag) deposited on glass substrate by using a simple chemical sulphurization method. According to X-ray diffraction analysis, the Ag2S thin films display low intensity peaks at 34.48°, 36.56°, and 44.28°, corresponding to diffraction from (100), (112) and (103) planes of the acanthite phase (monoclinic). A model of the type Ag2S/Ag/glass was deduced from spectroscopic ellipsometric measurements. Also, the optical constants (n, k) of the system were determined. Furthermore, the optical properties as solar selective absorber for collector applications were assessed. The optical reflectance of the Ag2S/Ag thin film systems exhibits the expected behavior for an ideal selective absorber, showing a low reflectance in the wavelength range below 2 µm and a high reflectance for wavelengths higher than that value. An absorptance about 70% and an emittance about 3% or less were calculated for several samples. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Solar selective thin films are currently investigated for applications in photothermal converters. Among diverse selective absorber surface coatings, semiconductor–metal tandems have attracted considerable interest because thin films of an ample variety of semiconductors can easily be deposited on metal sheets by chemical [1,2] or physical [1–3] methods. Efficient photothermal conversion requires selective absorbing surfaces that exhibit high absorptance (α) and low thermal emittance (ε), which are dimensionless parameters (0<α, ε < 1). In terms of the reflectance, an ideal selective surface displays zero reflectance for wavelengths lower than 2.5 µm and unitary reflectance for wavelengths above 2.5 µm. This calls for semiconductors with band gaps from 0.5 eV (2.5 μm) to 1.26 eV (1.0 μm), which absorb shortwavelength radiation and the underlying metal provides low emittance to give the desired spectral selectivity semiconductor–metal tandems. On other hand, the metal silver coatings have important applications in a number of solar devices for photothermal conversion. It has been used as the reflecting film of concentrator mirrors, anti-emitting barrier of the flat plate solar collectors, and a component of the selective absorbing system in vacuum–tube photothermal converters based on glass material [4]. Actually, methods such as reactive evaporation, chemical vapor deposition (CVD), and materials like TiNOx, NiO, etc. have proven their usefulness to manufacture commercial high efficiency photothermal converters [1–4], however there exist the interest to develop both, novel
⁎ Corresponding author. Tel.: +52 5804 4644/4645; fax: +52 5804 4900. E-mail address: [email protected] (E. Barrera-Calva). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.09.043
selective materials and inexpensive techniques, that lead to an economical coating process, while maintaining the high performance of solar converter devices. In earlier works [5,6], the silver sulphide formation was analyzed by means of chemical bath deposition and electrodeposition respectively and more recently the silver sulphide nanoparticles preparation, using relatively complex preparation technique [7,8]. In this work, we propose a simple chemical method to form silver sulphide at the surface of an evaporated silver layer on glass substrates. The resultant films were characterized regarding their microstructure and optical properties in the UV–Visible interval, by means of spectroscopic ellipsometry technique. In addition, the IR reflectance of Ag2S/Ag system was measured to determine the optical properties of Ag2S as selective coating. 2. Experimental The silver layers used in this work were deposited on glass slide substrates at 10− 7 Torr, using a high vacuum evaporator system. Prior to silver deposition process, the substrates were cleaned with detergent and soaked in absolute ethanol under ultrasonic agitation for 5 min. The silver sulphide (Ag2S) formation was performed in a 50 ml beaker containing 15 ml of sodium sulphide (Na2S) aqueous solution. A silver coated glass was fixed on the beaker top, and then the beaker was wrapped with a plastic film (Parafilm). The above process was done at room temperature using 0.25, 0.5 and 1.0 M sodium sulphide aqueous solutions. The samples reported in this work were prepared using 0.5 M solution for time periods from 1 to 8 days. The thickness of evaporated silver layers was determined using a Talistep Dek Tack II surface profiler. The thickness and the surface morphology of formed silver sulphide layers were evaluated by atomic force microscope, AFM (Qscope,
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Fig. 1. Atomic force micrograph of a step of the metallic silver samples on glass that allowed determining the film thickness of the samples.
Quesant Co), whereas their phase composition was determined from X-ray diffraction studies using a Siemens D-500 diffractometer. The optical constants (refractive index and extinction coefficient) of Ag2S were obtained from spectroscopic ellipsometry (SE) measurements in the UV–Visible region, using a phase modulated Jobin Yvon Uvisel DH10 ellipsometer. The ellipsometric data were recorded at an incidence angle of 70° and analyzed using the Delta-Psi software Version 2.0. The selective parameters of the Ag2S/Ag tandem were determined from the specular reflectance data in the region 400–25,000 nm, which was measured in a Nicolet Nexus spectrophotometer. The solar optical properties, solar absorptance αs, and thermal emittance ε, were estimated by means of the use of Duffie and Beckman [9] methodology for some representative samples. 3. Results and discussion According to profile measurements performed on several silver samples coated on glass substrates, all samples have an average film thickness of 205 nm. This thickness was comparable with that measured using an atomic force microscope (Fig. 1). The film thickness (205 nm) was determined by an analysis in the vertical coordinate using the AFM equipment software. When silver films are exposed to the sodium sulphide bath at different concentrations for several days, a sulphidric
Fig. 2. XRD spectra of a representative sample of silver sulphide/Ag/glass. The XRD spectra give a clear evidence of the monoclinic Ag2S phase. The signal of silver coatings can also be seen.
Fig. 3. AFM micrograph of the silver sulphide/Ag/glass sample with 1 day sulphurization treatment.
acid atmosphere generates and reacts with the silver film, slowly growing a silver sulphide thin film. The mirror like silver thin films is transformed to grey, blue and black silver sulphide coatings depending on the solution concentration and the time duration. We have observed that the diluted solutions (0.25 M) after several days give rise to only grey–blue thin films of silver sulphide having a low solar absorptance. On the contrary, when the samples are treated with highly concentrated solutions (1 M), the Ag layer tends to completely transform into silver sulphide, damaging the silver in contact with the glass substrate, and thus making the back surface of the silver films lose the mirror quality. For this reason, the samples treated with low and high concentrations are not reported in this work. The thin films prepared with a 0.5 M sodium sulphide solution provide the best black silver sulphide, so the samples discussed in this paper correspond to this experimental condition. The phase structure of the prepared representative samples was studied with an X-ray diffractometer. Fig. 2 shows the typical XRD for the silver sulphide/Ag/glass samples. The diffraction spectra confirm the monoclinic phase of Ag2S that is produced by the chemical reaction between metallic silver and the sulphidric acid. The diffraction spectra show strong signal of the metallic silver indicating that not all silver turns into silver sulphide. Figs. 3 and 4 show the AFM morphology of Ag2S/Ag samples prepared during 1 and 4 days of chemical treatment with a 0.5 M of Na2S solution. It is noticeable that the flat silver surface morphology becomes textured, with some three-dimensional grains appearing after 1-day treatment with sulphidric acid (Fig. 3). After 4 days of treatment, the texture of the samples becomes conical which makes them a potential absorbing material. In order to obtain the fundamental optical properties and to study the microstructure, some representative thin films were analyzed by spectroscopic ellipsometry in the wavelength range of 220 to 700 nm. The ellipsometric parameters Is and Ic determined from the measurement of the silver
Fig. 4. AFM micrograph of the silver sulphide/Ag/glass sample with 4 days of sulphurization treatment.
E. Barrera-Calva et al. / Thin Solid Films 518 (2010) 1835–1838
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Fig. 5. Is and Ic ellipsometric parameters for the Ag2S/Ag/glass sample, prepared by the 4 day sulphurization process. Dotted line represents experimental data and continuous line is the classical dispersion model.
sulphide/Ag thin film are shown in Fig. 5. A classical dispersion model is used to model the silver sulphide film, whereas for the silver films the data of a previously studied standard sample is assumed [10]. After fitting the above models to the experimental data, a good correlation is obtained (χ2 = 1.27), the fitting curves are also shown in Fig. 5. When inverting the fitted Is and Ic ellipsometric results, it is possible to find out the values of refractive index (n) and the extinction coefficient (k), as depicted in Fig. 6. The corresponding microstructure calculated from the ellipsometric simulations using the classical dispersion model is depicted in Fig. 7. The film thickness values obtained after this modeling are in reasonable agreement with those observed from AFM images. Accordingly the silver layer of fixed optical properties, together with a layer of pure Ag2S and another layer made up of Ag2S and a void fraction (surface roughness) was estimated as well. Fig. 6 shows that the values of refractive index and the extinction coefficient for most of the wavelength region are higher for samples prepared by one day sulphurization, which could be due to the influence of thicker silver layer underneath. Assuming a direct forbidden transition model in the estimation of the fundamental optical properties, the optical band gap energy for the monoclinic silver sulphide film is determined as about 1 eV. Fig. 8 and the inset show the graph of the spectral reflectance curve for silver sulphide thin films prepared from 0.5 M solution in the case of 1 and 4 days sulphurisation treatment. A sudden increase in the reflectance value for both samples in the 1 to 2 µm range is observed, which is in agreement with the estimated band gap value. In general, the reflectance value is low below 1.5 µm and attains value close to 1 for wavelengths above 1.5 µm. These measurements also permit to evaluate the solar absorptance (0.4–2 µm) and the thermal emittance of the silver sulphide thin films (2–25 µm).
Fig. 6. Optical properties (n and k) obtained after fitting for a representative Ag2S/Ag thin film on glass substrate. a) Sample obtained by 1 day sulphurization process, and b) sample prepared with 4 days sulphurization.
Fig. 7. Proposed tandem microstructure used as a model of the Ag2S/Ag/glass samples that yields the best fit to the experimental data; a) sample obtained by 1 day sulphurization process, and b) sample prepared with 4 days sulphurization.
From the spectral reflectance measurements (ρ), the solar absorptance of the samples (α) was estimated using Eq. (1) and the Duffie and Beckmann methodology [9]. Here the spectra are evaluated from 0.4 to 2.5 µm. In the evaluation of thermal emittance (ε), an equation similar to Eq. (1) was used, but the integration region was from 2.5 to 25 µm and instead of α, the calculated optical property was ε. α = ð1−ρÞ
ð1Þ
Table 1 shows the typical values of the optical properties, including the selectivity S (=α / ε) and the RMS roughness of silver sulphide/Ag/ glass samples prepared from 0.5 M Na2S bath. Samples with four day sulphurization treatment provide the highest solar selectivity value while the roughness parameter is around 55 nm. 4. Conclusions The silver sulphide thin films prepared on metallic silver/glass substrates are potential photothermal materials that can be used for
Fig. 8. Spectral reflectance (%) of the Ag2S/Ag/glass samples in the 0.4–25 μm wavelength range, where the solar absorptance and the thermal emittance are determined. a) Sample obtained by 1 day sulphurization process, and b) sample prepared with 4 days sulphurization.
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Table 1 Values of optical properties such as solar absorptance (α), thermal emittance (ε), selectivity (S), and RMS of silver sulphide/Ag/glass samples. Ag2S/Ag thin film
α
ε
S
RMS (nm)
1 day 4 days 7 days
0.7 0.71 0.72
~ 0.01 ~ 0.01 ~ 0.05
~ 70 ~ 71 ~ 15
40 55 65
Concentration of the Na2S bath was 0.5 M.
the development of coatings for flat plate and vacuum tube solar collectors. The samples exhibited very high solar selectivity which depends on the extent of sulphurization of the metallic silver layer. Preliminary studies indicate that the thermal stability of silver sulphide needs to be improved, because after exposing silver sulphide to temperatures of the order of 150 °C the thin films experiment a color change affecting surface morphology and optical properties. A possible way to improve the thermal stability of these films could be
the utilization of some protective coatings based on thin silicon or tin oxide [11]. Further studies to improve the quality of these materials are in progress in our laboratory. References [1] O.P. Agnihotri, B.K. Gupta, Solar Selective Surfaces, Wiley Interscience, New York, 1981. [2] C.G. Granqvist, Materials Science for Solar Energy Conversion System, Pergamon Press, Oxford, 1991. [3] C.E Kennedy, Review of Mid- to High-Temperature Solar Selective Absorber Materials, National Renewable Energy Laboratory, NRE/TP520-31267, USA, 2002. [4] F.A. Peuser, K.H. Remmers, M. Schnauss, Solar Thermal System, James and James, London, 2002. [5] D.L. Douglass, Sol. Energy Mater. (ISSN: 0165-1633) 10 (1984) 1. [6] A.J. Varkey, Sol. Energy Mater. 21 (1991) 291. [7] A. Núñez Rodríguez, M.T.S. Nair, P.K. Nair, Semicond. Sci. Technol. 20 (6) (2005) 576. [8] D. Karashanova, N. Starbov, Appl. Surf. Sci. 252 (8) (2006) 3011. [9] J.A. Duffie, W.A. Beckman, Solar Engineering of Thermal Processes, 2nd ed., Wiley Interscience, New York, 1991, p. 54. [10] E.D. Palik, Handbook Optical Constants, Academic Press, New York, 1985. [11] C.E. Barrera, L.M. Ortega, J. Mendez, R. Olayo, J. Morales, Res. Lett. Mater. Sci. (2008) (ID 190920).
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Optical properties of silver sulphide thin films formed on evaporated Ag by a simple sulphurization method E. Barrera-Calva a,⁎, M. Ortega-López b, A. Avila-García b, Y. Matsumoto-Kwabara b a b
Departamento de Ingeniería de Procesos e hidráulica, Universidad Autónoma Metropolitana — Iztapalapa, Av. Purísima Esq. Michoacán, Col. Vicentina, México, D.F., 09340, Mexico Departamento de Ingeniería Eléctrica, Centro de Investigación y de Estudios Avanzados del IPN, México DF 07360, Mexico
a r t i c l e
i n f o
Available online 19 September 2009 Keywords: Silver sulphide thin films Optical properties Solar selective tandems
a b s t r a c t Silver sulphide (Ag2S) thin films were grown on the surface of silver films (Ag) deposited on glass substrate by using a simple chemical sulphurization method. According to X-ray diffraction analysis, the Ag2S thin films display low intensity peaks at 34.48°, 36.56°, and 44.28°, corresponding to diffraction from (100), (112) and (103) planes of the acanthite phase (monoclinic). A model of the type Ag2S/Ag/glass was deduced from spectroscopic ellipsometric measurements. Also, the optical constants (n, k) of the system were determined. Furthermore, the optical properties as solar selective absorber for collector applications were assessed. The optical reflectance of the Ag2S/Ag thin film systems exhibits the expected behavior for an ideal selective absorber, showing a low reflectance in the wavelength range below 2 µm and a high reflectance for wavelengths higher than that value. An absorptance about 70% and an emittance about 3% or less were calculated for several samples. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Solar selective thin films are currently investigated for applications in photothermal converters. Among diverse selective absorber surface coatings, semiconductor–metal tandems have attracted considerable interest because thin films of an ample variety of semiconductors can easily be deposited on metal sheets by chemical [1,2] or physical [1–3] methods. Efficient photothermal conversion requires selective absorbing surfaces that exhibit high absorptance (α) and low thermal emittance (ε), which are dimensionless parameters (0<α, ε < 1). In terms of the reflectance, an ideal selective surface displays zero reflectance for wavelengths lower than 2.5 µm and unitary reflectance for wavelengths above 2.5 µm. This calls for semiconductors with band gaps from 0.5 eV (2.5 μm) to 1.26 eV (1.0 μm), which absorb shortwavelength radiation and the underlying metal provides low emittance to give the desired spectral selectivity semiconductor–metal tandems. On other hand, the metal silver coatings have important applications in a number of solar devices for photothermal conversion. It has been used as the reflecting film of concentrator mirrors, anti-emitting barrier of the flat plate solar collectors, and a component of the selective absorbing system in vacuum–tube photothermal converters based on glass material [4]. Actually, methods such as reactive evaporation, chemical vapor deposition (CVD), and materials like TiNOx, NiO, etc. have proven their usefulness to manufacture commercial high efficiency photothermal converters [1–4], however there exist the interest to develop both, novel
⁎ Corresponding author. Tel.: +52 5804 4644/4645; fax: +52 5804 4900. E-mail address: [email protected] (E. Barrera-Calva). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.09.043
selective materials and inexpensive techniques, that lead to an economical coating process, while maintaining the high performance of solar converter devices. In earlier works [5,6], the silver sulphide formation was analyzed by means of chemical bath deposition and electrodeposition respectively and more recently the silver sulphide nanoparticles preparation, using relatively complex preparation technique [7,8]. In this work, we propose a simple chemical method to form silver sulphide at the surface of an evaporated silver layer on glass substrates. The resultant films were characterized regarding their microstructure and optical properties in the UV–Visible interval, by means of spectroscopic ellipsometry technique. In addition, the IR reflectance of Ag2S/Ag system was measured to determine the optical properties of Ag2S as selective coating. 2. Experimental The silver layers used in this work were deposited on glass slide substrates at 10− 7 Torr, using a high vacuum evaporator system. Prior to silver deposition process, the substrates were cleaned with detergent and soaked in absolute ethanol under ultrasonic agitation for 5 min. The silver sulphide (Ag2S) formation was performed in a 50 ml beaker containing 15 ml of sodium sulphide (Na2S) aqueous solution. A silver coated glass was fixed on the beaker top, and then the beaker was wrapped with a plastic film (Parafilm). The above process was done at room temperature using 0.25, 0.5 and 1.0 M sodium sulphide aqueous solutions. The samples reported in this work were prepared using 0.5 M solution for time periods from 1 to 8 days. The thickness of evaporated silver layers was determined using a Talistep Dek Tack II surface profiler. The thickness and the surface morphology of formed silver sulphide layers were evaluated by atomic force microscope, AFM (Qscope,
1836
E. Barrera-Calva et al. / Thin Solid Films 518 (2010) 1835–1838
Fig. 1. Atomic force micrograph of a step of the metallic silver samples on glass that allowed determining the film thickness of the samples.
Quesant Co), whereas their phase composition was determined from X-ray diffraction studies using a Siemens D-500 diffractometer. The optical constants (refractive index and extinction coefficient) of Ag2S were obtained from spectroscopic ellipsometry (SE) measurements in the UV–Visible region, using a phase modulated Jobin Yvon Uvisel DH10 ellipsometer. The ellipsometric data were recorded at an incidence angle of 70° and analyzed using the Delta-Psi software Version 2.0. The selective parameters of the Ag2S/Ag tandem were determined from the specular reflectance data in the region 400–25,000 nm, which was measured in a Nicolet Nexus spectrophotometer. The solar optical properties, solar absorptance αs, and thermal emittance ε, were estimated by means of the use of Duffie and Beckman [9] methodology for some representative samples. 3. Results and discussion According to profile measurements performed on several silver samples coated on glass substrates, all samples have an average film thickness of 205 nm. This thickness was comparable with that measured using an atomic force microscope (Fig. 1). The film thickness (205 nm) was determined by an analysis in the vertical coordinate using the AFM equipment software. When silver films are exposed to the sodium sulphide bath at different concentrations for several days, a sulphidric
Fig. 2. XRD spectra of a representative sample of silver sulphide/Ag/glass. The XRD spectra give a clear evidence of the monoclinic Ag2S phase. The signal of silver coatings can also be seen.
Fig. 3. AFM micrograph of the silver sulphide/Ag/glass sample with 1 day sulphurization treatment.
acid atmosphere generates and reacts with the silver film, slowly growing a silver sulphide thin film. The mirror like silver thin films is transformed to grey, blue and black silver sulphide coatings depending on the solution concentration and the time duration. We have observed that the diluted solutions (0.25 M) after several days give rise to only grey–blue thin films of silver sulphide having a low solar absorptance. On the contrary, when the samples are treated with highly concentrated solutions (1 M), the Ag layer tends to completely transform into silver sulphide, damaging the silver in contact with the glass substrate, and thus making the back surface of the silver films lose the mirror quality. For this reason, the samples treated with low and high concentrations are not reported in this work. The thin films prepared with a 0.5 M sodium sulphide solution provide the best black silver sulphide, so the samples discussed in this paper correspond to this experimental condition. The phase structure of the prepared representative samples was studied with an X-ray diffractometer. Fig. 2 shows the typical XRD for the silver sulphide/Ag/glass samples. The diffraction spectra confirm the monoclinic phase of Ag2S that is produced by the chemical reaction between metallic silver and the sulphidric acid. The diffraction spectra show strong signal of the metallic silver indicating that not all silver turns into silver sulphide. Figs. 3 and 4 show the AFM morphology of Ag2S/Ag samples prepared during 1 and 4 days of chemical treatment with a 0.5 M of Na2S solution. It is noticeable that the flat silver surface morphology becomes textured, with some three-dimensional grains appearing after 1-day treatment with sulphidric acid (Fig. 3). After 4 days of treatment, the texture of the samples becomes conical which makes them a potential absorbing material. In order to obtain the fundamental optical properties and to study the microstructure, some representative thin films were analyzed by spectroscopic ellipsometry in the wavelength range of 220 to 700 nm. The ellipsometric parameters Is and Ic determined from the measurement of the silver
Fig. 4. AFM micrograph of the silver sulphide/Ag/glass sample with 4 days of sulphurization treatment.
E. Barrera-Calva et al. / Thin Solid Films 518 (2010) 1835–1838
1837
Fig. 5. Is and Ic ellipsometric parameters for the Ag2S/Ag/glass sample, prepared by the 4 day sulphurization process. Dotted line represents experimental data and continuous line is the classical dispersion model.
sulphide/Ag thin film are shown in Fig. 5. A classical dispersion model is used to model the silver sulphide film, whereas for the silver films the data of a previously studied standard sample is assumed [10]. After fitting the above models to the experimental data, a good correlation is obtained (χ2 = 1.27), the fitting curves are also shown in Fig. 5. When inverting the fitted Is and Ic ellipsometric results, it is possible to find out the values of refractive index (n) and the extinction coefficient (k), as depicted in Fig. 6. The corresponding microstructure calculated from the ellipsometric simulations using the classical dispersion model is depicted in Fig. 7. The film thickness values obtained after this modeling are in reasonable agreement with those observed from AFM images. Accordingly the silver layer of fixed optical properties, together with a layer of pure Ag2S and another layer made up of Ag2S and a void fraction (surface roughness) was estimated as well. Fig. 6 shows that the values of refractive index and the extinction coefficient for most of the wavelength region are higher for samples prepared by one day sulphurization, which could be due to the influence of thicker silver layer underneath. Assuming a direct forbidden transition model in the estimation of the fundamental optical properties, the optical band gap energy for the monoclinic silver sulphide film is determined as about 1 eV. Fig. 8 and the inset show the graph of the spectral reflectance curve for silver sulphide thin films prepared from 0.5 M solution in the case of 1 and 4 days sulphurisation treatment. A sudden increase in the reflectance value for both samples in the 1 to 2 µm range is observed, which is in agreement with the estimated band gap value. In general, the reflectance value is low below 1.5 µm and attains value close to 1 for wavelengths above 1.5 µm. These measurements also permit to evaluate the solar absorptance (0.4–2 µm) and the thermal emittance of the silver sulphide thin films (2–25 µm).
Fig. 6. Optical properties (n and k) obtained after fitting for a representative Ag2S/Ag thin film on glass substrate. a) Sample obtained by 1 day sulphurization process, and b) sample prepared with 4 days sulphurization.
Fig. 7. Proposed tandem microstructure used as a model of the Ag2S/Ag/glass samples that yields the best fit to the experimental data; a) sample obtained by 1 day sulphurization process, and b) sample prepared with 4 days sulphurization.
From the spectral reflectance measurements (ρ), the solar absorptance of the samples (α) was estimated using Eq. (1) and the Duffie and Beckmann methodology [9]. Here the spectra are evaluated from 0.4 to 2.5 µm. In the evaluation of thermal emittance (ε), an equation similar to Eq. (1) was used, but the integration region was from 2.5 to 25 µm and instead of α, the calculated optical property was ε. α = ð1−ρÞ
ð1Þ
Table 1 shows the typical values of the optical properties, including the selectivity S (=α / ε) and the RMS roughness of silver sulphide/Ag/ glass samples prepared from 0.5 M Na2S bath. Samples with four day sulphurization treatment provide the highest solar selectivity value while the roughness parameter is around 55 nm. 4. Conclusions The silver sulphide thin films prepared on metallic silver/glass substrates are potential photothermal materials that can be used for
Fig. 8. Spectral reflectance (%) of the Ag2S/Ag/glass samples in the 0.4–25 μm wavelength range, where the solar absorptance and the thermal emittance are determined. a) Sample obtained by 1 day sulphurization process, and b) sample prepared with 4 days sulphurization.
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Table 1 Values of optical properties such as solar absorptance (α), thermal emittance (ε), selectivity (S), and RMS of silver sulphide/Ag/glass samples. Ag2S/Ag thin film
α
ε
S
RMS (nm)
1 day 4 days 7 days
0.7 0.71 0.72
~ 0.01 ~ 0.01 ~ 0.05
~ 70 ~ 71 ~ 15
40 55 65
Concentration of the Na2S bath was 0.5 M.
the development of coatings for flat plate and vacuum tube solar collectors. The samples exhibited very high solar selectivity which depends on the extent of sulphurization of the metallic silver layer. Preliminary studies indicate that the thermal stability of silver sulphide needs to be improved, because after exposing silver sulphide to temperatures of the order of 150 °C the thin films experiment a color change affecting surface morphology and optical properties. A possible way to improve the thermal stability of these films could be
the utilization of some protective coatings based on thin silicon or tin oxide [11]. Further studies to improve the quality of these materials are in progress in our laboratory. References [1] O.P. Agnihotri, B.K. Gupta, Solar Selective Surfaces, Wiley Interscience, New York, 1981. [2] C.G. Granqvist, Materials Science for Solar Energy Conversion System, Pergamon Press, Oxford, 1991. [3] C.E Kennedy, Review of Mid- to High-Temperature Solar Selective Absorber Materials, National Renewable Energy Laboratory, NRE/TP520-31267, USA, 2002. [4] F.A. Peuser, K.H. Remmers, M. Schnauss, Solar Thermal System, James and James, London, 2002. [5] D.L. Douglass, Sol. Energy Mater. (ISSN: 0165-1633) 10 (1984) 1. [6] A.J. Varkey, Sol. Energy Mater. 21 (1991) 291. [7] A. Núñez Rodríguez, M.T.S. Nair, P.K. Nair, Semicond. Sci. Technol. 20 (6) (2005) 576. [8] D. Karashanova, N. Starbov, Appl. Surf. Sci. 252 (8) (2006) 3011. [9] J.A. Duffie, W.A. Beckman, Solar Engineering of Thermal Processes, 2nd ed., Wiley Interscience, New York, 1991, p. 54. [10] E.D. Palik, Handbook Optical Constants, Academic Press, New York, 1985. [11] C.E. Barrera, L.M. Ortega, J. Mendez, R. Olayo, J. Morales, Res. Lett. Mater. Sci. (2008) (ID 190920).