Solar Energy Materials & Solar Cells 105 (2012) 309–316

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Solar Energy Materials & Solar Cells

0927-02

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Thermochromic cobalt(II) chloro-complexes in different media:Possible application for auto-regulated solar protection

Slobodan Gadzuric n, Milan Vranes, Sanja Dozic

Department of Chemistry, Biochemistry and Environmental Protection, Faculty of Sciences, University of Novi Sad, Trg Dositeja Obradovica 3, 21000 Novi Sad, Serbia

a r t i c l e i n f o

Article history:

Received 10 January 2012

Received in revised form

13 June 2012

Accepted 20 June 2012Available online 18 July 2012

Keywords:

Thermochromism

Cobalt(II) complexes

Solar protection

48/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.solmat.2012.06.035

esponding author. Tel.: þ381 21 485 2744; f

ail address: slobodan.gadzuric@dh.uns.ac.rs (S

a b s t r a c t

The well known fact that cobalt(II) halide complexes in various media undergo changes of co-

ordination, absorbance and colour with the increase of temperature was combined with the low

melting point of new materials suitable for simultaneous control of temperature and light intensity.

Hence, the increase of absorbance with temperature acts as an auto-regulated shading protection from

overheating. In this work, the thermochromic behaviour of cobalt(II) chloro-complexes was investi-

gated by VIS–spectroscopy in four different ammonium nitrateþorganic component based solvents and

in ammonium nitrateþcalcium nitrate tetrahydrate melt. The selected organic components were

formamide (FA), N-methylformamide (NMF), N,N-dimethylformamide (DMF) and dimethylsulphoxide

(DMSO). In all binary mixtures the mole ratio of ammonium nitrate to the second component of the

system was 1:3 and they were studied as suitable solvents for the formation of cobalt(II) chloro-

complexes. Absorption spectra have been investigated in the visible spectral range 400–800 nm at two

different temperatures (298.15 and 323.15 K). The overall stability constants of all formed complexes

were calculated using a non-linear regression program at different temperatures: 308.15, 318.15,

328.15, 338.15 and 348.15 K. Based on these stability constants, the thermodynamic data, i.e. standard

enthalpy, entropy and Gibbs energy, were calculated for all complexation reactions. Influence of the

solvent composition on the complex stability and thermochromism was also discussed.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

As a consequence of using up classical energy sources andincreasing cost of fuel, there is a growing interest in the utilisationof so-called alternative energy sources, such as solar radiation, wind,cooling water, exhaust gases, off-peak electric energy, etc. So far, thesolar energy is mainly used in residential field (space heating andcooling, domestic water heating systems, etc.) and in agriculture(greenhouse heating, drying cereals and other agricultural products),but there are also several pilot solar electric power plants. Green-houses are especially convenient objects for use of solar energy [1].According to some estimates [2], up to 80% of thermal energyobtained from the fossil fuels used for heating buildings, may bereplaced by the solar energy. The possibility to combine thermallyresponsive chemical processes is studied in this work in order toattain simultaneous control of temperature and light intensity inagricultural greenhouses and in residential field. Since sunlight can bedirectly absorbed by many coloured substances to re-generate heatwith achievable temperature below 100 1C, it is increasingly impor-tant to develop new chemical systems that can store and utilise this

ll rights reserved.

ax: þ381 21 454 065.

. Gadzuric).

almost free and readily available solar heat. Research work in thisarea involves studies of new materials and their optical propertiesupon addition of some thermochromic complex compounds. Thethermochromic substance is usually a transition metal, such ascobalt(II) with mixed ligands, undergoing reversible changes ofoptical properties in response to a change of temperature, actingthus as a self-regulating shading [3]. Also, the change from pale pinkoctahedral to dark blue tetrahedral cobalt(II) geometry is a highlyendothermic process appropriate for energy storage in differentsolvents and media. Reversibility of the process may provide constanttemperature in the residential buildings or agricultural greenhousesused for cultivation of some delicate plants in fluctuating climateconditions.

The studies on the ionic association (complex formation) at anearly stage have mainly been restricted to aqueous solutions asreaction media [4,5]. However, it was found that aqueous mediaare not quite adequate, since the presence of predominantly weakdipole–dipole and ion–dipole interactions upon increasing thetemperature will spend some energy on the translational androtational motion of the molecules. The remaining energy input isinsufficient to change the geometry of the complexes and, there-fore, the colour. Also, the irreversible water evaporation uponheating is a predictable problem that can shorten the life cycle ofthe thermochromic devices based on aqueous solutions.

S. Gadzuric et al. / Solar Energy Materials & Solar Cells 105 (2012) 309–316310

Several studies were oriented toward inorganic molten salts asa potential medium for thermochromic cobalt(II) halide complexformation [6,7]. In these systems, dominated strong ion–ioninteractions are not thermally labile, but the disadvantage ofmolten salts was their high melting point, which was the mainreason for their low applicability.

In order to reduce the melting point of the system, but to keepthe properties of the molten ionic compounds, the attraction hasbeen paid to partially solvated melts. There are a large number oforganic compounds and eutectics with inorganic salts meltingwithin the temperature range appropriate for solar energy storage[8,9]. These mixtures usually have high latent heat of fusion, lowmelting point and have been considered as phase change

Table 1Variation of density at 298.15 and 323.15 K in the systems with different n(Cl�)/

n(Co2þ) mole ratios z.

z d at 298.15 K(g cm�3) d at 323.15 K(g cm�3)

NH4NO3 �3FA

2 1.26070 1.24074

5 1.26074 1.24080

20 1.26102 1.24112

50 1.26155 1.24196

80 1.26213 1.24273

NH4NO3 �3NMF

2 1.13276 1.11288

5 1.13310 1.11310

20 1.13449 1.11467

50 1.13460 1.11668

NH4NO3 �3DMF

2 1.07048 1.05001

5 1.07065 1.05023

NH4NO3 �3DMSO

2 1.19424 1.17345

5 1.19442 1.17365

20 1.19535 1.17464

NH4NO3 �3Ca(NO3)2 �4H2O

2 1.72451 1.70222

5 1.72314 1.70090

20 1.71893 1.69687

50 1.70976 1.68796

80 1.70342 1.68184

Fig. 1. Absorption spectra of cobalt(II) chloro-complexes in NH4NO3 �3FA at 298.15 a

materials (PCM) suitable for energy storage [10–12]. If a smallamount of thermochromic complex compound is added to theeutectic mixture, the composition and the geometry of thecomplex compound can be adjusted so that the absorptionspectra in visible spectral range exhibit low absorbance at roomtemperature and a pronounced increase of the absorbance withthe increase of temperature. Partially solvated and solvated meltswith melting point below 100 1C make the transition between themolten salts and the aqueous solutions. The nature of interactionsin these systems is very complex, but the wide range of theirapplications and practical importance caused them to become asignificant part of the research of many scientists.

Three classes of ionic systems with the melting point below100 1C can be distinguished.

(a)

nd 3

Fig.mole

The first class consists of inorganic salt and a small amount ofwater (so-called aqueous melts) or the eutectic of twodifferent inorganic salts. One of the most studied aqueousmelts suitable for cobalt(II) thermochromism was calciumnitrate tetrahydrate [13].

(b)

Eutectics of inorganic salt and organic component are thesecond class of low temperature melting ionic systems. Themost suitable organic components are those with high polar-ity and relative permittivity. At early stage, cobalt(II) chloro-complexes were studied in inorganic saltþacetamide mixturedue to some physical properties of acetamide resemblingwater (e.g. relative permittivity, density and dissociationconstant) [14,15]. Later on, acetamide was replaced by similar

23.15 K and several n(Cl�)/n(Co2þ) mole ratios z, m(Co2þ)¼0.01 mol kg�1.

z = 2 z = 5 z = 20 z = 50 z = 80

2. Colour of NH4NO3 �3FA mixtures at 298.15 K and different n(Cl�)/n(Co2þ)

ratios z, m(Co2þ)¼0.01 mol kg�1.

Fig.mole

Fig.

S. Gadzuric et al. / Solar Energy Materials & Solar Cells 105 (2012) 309–316 311

organic molecules such as dimethylsulphoxide [16,17] andformamide [18].

(c)

Recently, attention has been paid to the room temperatureionic liquids as a potential media for transition metal thermo-chromism. Ionic liquids are the excellent solvent candidatesdue to their unique physical and chemical properties (e.g.negligible vapour pressure, low toxicity, non-corrosivity, etc.)and thermochromic behaviour of chloro-nickel complexeswas already investigated in 1-hydroxylalkyl-3-methyl-imida-zolium based ionic liquids [19]. On the other hand, the highprice of pure ionic liquids significantly limits their widespreaduse. For this reason, inorganic saltþorganic compound binarymixtures are more practical as a solvent for thermochromiccomplexes.

z = 2 z = 5 z = 20 z = 50

Fig. 5. Colour of NH4NO3 �3NMF mixtures at 298.15 K and different n(Cl�)/

n(Co2þ) mole ratios z, m(Co2þ)¼0.01 mol kg�1.

In this work, we report thermochromic characterisation of fivereversible and stable thermochromic systems composed of ammo-nium nitrate and FA; NMF; DMF; DMSO and Ca(NO3)2 �4H2O. In allthese mixtures, the mole ratio x of ammonium nitrate/the secondcomponent (NH4NO3/FA; NMF; DMF; DMSO or Ca(NO3)2 �4H2O) wasadjusted to 3, because at x¼3 ammonium nitrate was soluble in allprepared mixtures. Influence of the chloride concentration, tempera-ture and solvent composition on the absorption spectra, thermo-chromic behaviour and cobalt(II) complex formation was studied.Stability constants of all formed complexes and the thermodynamicparameters were calculated in all investigated systems. Thermo-chromic effects in the systems containing organic component will

z = 2 z = 20z = 5 z = 50 z = 80

3. Colour of NH4NO3 �3FA mixtures at 323.15 K and different n(Cl�)/n(Co2þ)

ratios z, m(Co2þ)¼0.01 mol kg�1.

4. Absorption spectra of cobalt(II) chloro-complexes in NH4NO3 �3NMF at 298.15

be compared with those of the system consisting of two inorganicsalts, NH4NO3 �3(Ca(NO3)2 �4H2O), in terms of the nature of interac-tions in the mixtures (ion–ion, ion–dipole or dipole–dipole).

2. Experimental

All chemicals used were reagent grade Merck products andsome of them were used without further purification. Melts of thedesired composition were prepared from weighed amounts of dry

and 323.15 K and several n(Cl�)/n(Co2þ) mole ratios z, m(Co2þ)¼0.01 mol kg�1.

z = 5 z = 20z = 2 z = 50

Fig. 6. Colour of NH4NO3 �3NMF mixtures at 323.15 K and different n(Cl�)/

n(Co2þ) mole ratios z, m(Co2þ)¼0.01 mol kg�1.

z = 2 z = 5

Fig. 8. Colour of NH4NO3 �3DMF mixtures at 298.15 K and different n(Cl�)/

n(Co2þ) mole ratios z, m(Co2þ)¼0.01 mol kg�1.

z = 2 z = 5

Fig. 9. Colour of NH4NO3 �3DMF mixtures at 323.15 K and different n(Cl�)/

n(Co2þ) mole ratios z, m(Co2þ)¼0.01 mol kg�1.

S. Gadzuric et al. / Solar Energy Materials & Solar Cells 105 (2012) 309–316312

reagent grade ammonium nitrate and the second component.Prior to use, ammonium nitrate was dried for 8 h at 353.15 K.Ammonium chloride as a source of chloride ions is dried for 2 h at378.15 K. Calcium nitrate tetrahydrate was re-crystallised andanalysed for water content and then the water/calcium nitratemole ratio was adjusted to 4 by drying or addition of water.

The stock solution of cobalt(II) was prepared by dissolvinganhydrous cobalt(II) chloride in a known amount of the solventmelt. The anhydrous cobalt(II) chloride was obtained by dryingthe commercial cobalt(II) chloride hexahydrate for 2 h at393.15 K, until the colour was not changed from purple to blue.Exact concentration of cobalt(II) in the stock solution was deter-mined by the complexometric titration. Cobalt(II) nitratehexahydrate was not used as a source of cobalt(II) due to itsdecomposition during the drying process at elevated temperature.Since the complexation of cobalt(II) with chloride ions in thestudied systems is weak and requires high concentration ofchloride ions, the presence of chloride in small amounts doesnot affect the data evaluation for the complex formation.

The absorption spectra of the melts with variable chloride andcobalt(II) concentration were recorded in the wavelength range400–800 nm on a Secomam Anthelie Advanced 2 spectrophot-ometer with thermostated cell compartments at 298.15 and323.15 K. The temperature was kept constant at 70.1 K.

In order to calculate the molar absorption coefficients, stabilityconstants and the thermodynamic functions, all concentrationswere expressed in molarities (mol dm�3). The density of the meltsolutions, required for conversion of molalities into molarityscale, was determined to be 298.15 and 323.15 K at differentammonium chloride concentrations for all investigated systems.A vibrating tube densimeter, Rudolph Research Analytical DDM

2911, was used for density measurements. The accuracy orprecision of the densimeter was 70.00001 g cm�3. The instru-ment was automatically thermostated (Peltier-type) within70.02 K and was calibrated at the atmospheric pressure beforeeach series of measurments. The calibration was performed usingambient air and bi-distilled ultra-pure water. Our experimentalresults are presented in Table 1, and it can be seen that densitydecreases with increasing temperature, as expected. At the sametime, density values are higher if the chloride concentrationincreases.

Fig. 7. Absorption spectra of cobalt(II) chloro-complexes in NH4NO3 �3DMF at 298.15

3. Results and discussion

Absorption spectra of cobalt(II) were recorded in the presence ofchloride ions in five ammonium nitrate binary mixtures, namely:NH4NO3 �3FA; NH4NO3 �3NMF; NH4NO3 �3DMF; NH4NO3 �3DMSOand NH4NO3 �3(Ca(NO3)2 �4H2O) at two different temperatures,ambient (298.15 K) and 323.15 K. Spectral properties were examinedin this range, because it was also the range of the possible applicationof thermochromic effect for auto-regulated shading protection. Thelowest possible mole ratio z of chloride and cobalt(II) ions was 2, and

and 323.15 K and several n(Cl�)/n(Co2þ) mole ratios z, m(Co2þ)¼0.01 mol kg�1.

z = 2 z = 5 z = 20

Fig. 11. Colour of NH4NO3 �3DMSO mixtures at 298.15 and different n(Cl�)/

n(Co2þ) mole ratios z, m(Co2þ)¼0.01 mol kg�1.

z = 2 z = 5 z = 20

Fig. 12. Colour of NH4NO3 �3DMSO mixtures at 323.15 K and different n(Cl�)/

n(Co2þ) mole ratios z, m(Co2þ)¼0.01 mol kg�1.

S. Gadzuric et al. / Solar Energy Materials & Solar Cells 105 (2012) 309–316 313

the highest chloride concentration was determined by the solubilityof ammonium chloride in the investigated mixtures.

In all systems the overall molar absorption coefficientincreases with increasing temperature. Also, the increase ofchloride concentration caused pronounced increase of the overallmolar absorption coefficient, due to the changing of the cobalt(II)geometry from octahedral to tetrahedral. In the absence ofchloride ions (or at very low chloride concentration), the absorp-tion maximum occurs between 510 and 550 nm which corre-sponds to octahedral geometry and these complexes are colouredpale pink. The first value of 510 nm is a characteristic of hexa-co-ordinated aqua complex of cobalt(II) [20] [Ca(H2O)6]2þ , and thesecond value of 550 nm was obtained in KNO3–LiNO3 melt [6] foroctahedral [Co(NO3)4]2� . Our experimental values suggest thatstudied systems in this work are partially solvated melts andmake transition between diluted aqueous solutions and highlyconcentrated molten salts.

Addition of chloride ions caused pronounced shift towardlower energies and change of geometry from octahedral totetrahedral. The new maxima appeared in the range from 630to 720 nm, and they are typical for dark blue tetrahedral cobalt(II)complexes. It was found that the highest complex formed in allsystems was [CoCl4]2� .

Depending on the chloride concentration and the solventcomposition, the complexes with mixed ligands were also foundin the investigated mixtures. Severely distorted octahedral geo-metry was ascribed to these mixed complexes.

Absorption spectra in the system NH4NO3 �3FA: Absorptionspectra of cobalt(II) in the system ammonium nitrateþformamide at two different temperatures are presented in aFig. 1. For clarity and illustration, all presented mixtures withdifferent [Cl�]/[Co2þ] mole ratios z (2ozo80) were photo-graphed in thermostated cell at 298.15 and 323.15 K and areshown in Figs. 2 and 3. It is obvious that at room temperature allmixtures are coloured pale pink, with a slight change in colourupon heating at 323.15 K. More intense change was observed atz¼80, indicating that the thermochromic effect in NH4NO3 �3FAmixture occurs only at very high chloride concentrations.

Absorption spectra in the system NH4NO3 �3NMF: The max-imum chloride concentration in this mixture was achieved atz¼50. Absorption spectra at 298.15 and 323.15 K are presented inFig. 4. It is interesting that changes in the spectrum at z¼50 arefar more pronounced compared to those of the system containing

Fig. 10. Absorption spectra of cobalt(II) chloro-complexes in NH4NO3 �3DMSO at 298.1

formamide; thus the colour in this mixture is intensely blue(Figs. 5 and 6). The intensity of the colour is more obvious at323.15 K, which may refer to the fact that this system can beconsidered as a suitable thermochromic material. However, somedifficulties occur during the complexation of cobalt(II) and changeof the geometry from octahedral to tetrahedral and this fact willbe discussed later.

Absorption spectra in the system NH4NO3 �3DMF: Absorptionspectra in this system, together with the photos of the investi-gated mixtures, are presented in Figs. 7–9. Changes in the overallmolar absorption coefficients, colour and geometry are visible atextremely low chloride concentration (z¼5). This indicates thatlarge DMF molecules cannot occupy the co-ordination sphere ofcobalt(II), which caused easier formation of tetrahedral [CoCl4]2�.Therefore, the mixture where z¼5 was coloured dark blue.Further increasing of chloride concentration was possible, butthe absorption band and the absorption maxima will not be

5 and 323.15 K and several n(Cl�)/n(Co2þ) mole ratios z, m(Co2þ)¼0.01 mol kg�1.

z = 80 z = 5 z = 2 z = 20 z = 50

Fig. 14. Colour of NH4NO3 �3(Ca(NO3)2 �4 H2O) mixtures at 298.15 K and different

n(Cl�)/n(Co2þ) mole ratios z, m(Co2þ)¼0.01 mol kg�1.

z = 2 z = 5 z = 20 z = 50 z = 80

Fig. 15. Colour of NH4NO3 �3(Ca(NO3)2 �4H2O) mixtures at 323.15 K and different

n(Cl�)/n(Co2þ) mole ratios z, m(Co2þ)¼0.01 mol kg�1.

S. Gadzuric et al. / Solar Energy Materials & Solar Cells 105 (2012) 309–316314

changed, which is consistent with the properties of tetrahedralcobalt(II) complexes.

Absorption spectra in the system NH4NO3 �3DMSO: The systemwith the most prominent thermochromism is the one containingDMSO as the organic component. From Fig. 10 it can be seen thatthe shape of the absorption bands and the position of theabsorption maxima are similar to those in NH4NO3 �3DMF, butthe overall molar absorption coefficient has much higher value inthe system with DMSO. The blue colour was achieved at z¼20and the temperature influence on the mixtures with DMSO isshown in Figs. 11 and 12.

Absorption spectra in the system NH4NO3 �3(Ca(NO3)2 �4H2O):This system contains ammonium nitrate and an inorganic crystal-ohydrate. This mixture is liquid at room temperature, althoughthe melting points of the pure components are much higher.Therefore, these eutectics have been studied and described inseveral papers, due to their low melting points and possibility oftheir application [21–26]. Absorption spectra and the colour ofcobalt(II) complexes in this system are presented in Figs. 13–15.Unlike the previously described mixtures where organic mole-cules can compete with the chloride ions and occupy theco�ordination sphere of cobalt(II), an octahedral complex[Co(NO3)4]2� in the mixture with calcium nitrate tetrahydrate isformed only with the nitrate ions. During the formation ofoctahedral complexes, molecules of water can also participate,but due to a high concentration of nitrate ions, it was assumedthat only nitrate complex was formed.

Temperature influence on the colour intensity: The colour changein all investigated systems is evident when temperature increasesfrom the room temperature to 323.15 K. To obtain the thermo-chromic effect, it is necessary to adjust and find an optimalconcentration of all ligands: chloride ions, nitrate ions and organicsolvent. These concentrations must be neither very low nor veryhigh, because in both cases change of the colour during theheating and/or cooling of the mixture is negligible. Thus, theenergy spent to change the geometry of the complexes and at thesame time the colour, is insufficient for thermochromic behaviourof the system.

Influence of the relative permittivity of the organic solvent: Oneof the most important factor for thermochromism existence iscertainly correct choice of the solvent or medium. The choice isdetermined by the relative permittivity of the solvent and thus,

Fig. 13. Absorption spectra of cobalt(II) chloro-complexes in NH4NO3 �3(Ca(NO3)

m(Co2þ)¼0.01 mol kg�1.

by the interactions between the solvent molecules and compo-nents present in the system. The values of relative permittivitiesof FA, NMF, DMF and DMSO are 109.5, 182.4, 36.71 and 46.7,respectively at 298.15 K [27]. The systems containing FA and NMFwith large values of relative permittivity act as less suitablethermochromic materials, due to strong ion–dipole interactionsand strong cobalt(II) solvation with FA and NMF. Thus, thedesolvation process and formation of tetrahedral [CoCl4]2�

require a large amount of heat.On the other hand, DMF and DMSO molecules exhibit low

values of relative permittivity (weak ion–dipole interactions) andtheir co-ordination with cobalt(II) is weak, making easier forma-tion of tetrahedral [CoCl4]2� . In the case of DMF, the formation ofcobalt(II) chloro-complexes takes place at relatively low chlorideconcentrations, which limits the use of NH4NO3 �3DMF as asuitable thermochromic medium. In addition, both molecules

2 �4H2O) at 298.15 and 323.15 K and several n(Cl�)/n(Co2þ) mole ratios z,

Table 2

Stability constants for cobalt(II) complexes log(bmnp/(mol–1 dm3)4) in all investigated mixtures at different temperatures.

T/(K) 308.15 318.15 328.15 338.15 348.15

NH4NO3 � 3FA

[Co(NO3)2(FA)2] 2.1270.06 2.2770.05 2.4170.07 2.4670.04 2.6270.05

[CoCl2(FA)2] 3.3770.08 3.7970.04 3.8670.07 3.9470.04 4.0370.05

[CoCl4]2– 4.3770.14 4.9170.05 4.9970.08 5.5870.04 5.9470.02

NH4NO3 � 3NMF

[Co(NO3)4(NMF)2]2� 1.2370.06 1.7070.02 2.2570.02 2.3970.02 2.6170.02

[CoCl3(NO3)(NMF)2]2� 3.4170.06 4.0670.05 4.6070.03 4.8670.03 5.4470.02

[CoCl4]2� 6.4770.06 6.8970.02 7.6370.02 7.9370.02 8.3370.02

NH4NO3 � 3DMSO

[Co(NO3)2(DMSO)2] 3.1570.03 3.1870.06 3.4970.06 3.9370.02 –

[CoCl2(NO3)2]2� 6.0970.02 6.2670.06 6.6870.05 7.3370.01 –

[CoCl4]2� 8.1870.01 8.4670.06 8.8870.06 9.7270.01 –

NH4NO3 � 3(Ca(NO3)2 �4H2O)

[Co(NO3)4]2� 2.7770.03 2.9570.03 3.3270.03 – –

[CoCl2(NO3)2]2� 4.8670.02 5.0170.01 5.3070.01 – –

[CoCl4]2� 6.7770.02 7.0870.01 7.5370.01 – –

Table 3Standard enthalpy and entropy values for cobalt(II) chloride complex formation in

studied mixtures in the temperature range 308.15�348.15 K.

DHo (kJ mol�1) DSo (J mol�1 K�1)

NH4NO3 � 3FA

[Co(NO3)2(FA)2] 24.471.9 120.175.9

[CoCl2(FA)2] 30.277.4 165.0722.7

[CoCl4]2– 78.378.6 337.7726.3

NH4NO3 � 3NMF

[Co(NO3)4(NMF)2]2� 70.478.9 254.0727.0

[CoCl3(NO3)(NMF)2]2� 104.477.7 405.0723.0

[CoCl4]2– 97.777.0 441.0721.0

NH4NO3 � 3DMSO

[Co(NO3)2(DMSO)2] 53.4714.1 231.4743.6

[CoCl2(NO3)2]2� 83.3716.8 384.2752.0

[CoCl4]2� 101.3720.2 482.7762.6

NH4NO3 � 3Ca(NO3)2 �4H2O

[Co(NO3)4]2� 54.3711.3 223.2734.6

[Co(NO3)2Cl2]2� 44.978.8 233.9726.9

[CoCl4]2� 74.177.6 362.0723.0

S. Gadzuric et al. / Solar Energy Materials & Solar Cells 105 (2012) 309–316 315

cannot easily occupy the co-ordination sphere of cobalt(II) andform octahedral complexes, due to their size and steric interfer-ences. Therefore, in the systems with voluminous organic mole-cules with low relative permittivity, the octahedral complexesmay be easily converted into tetrahedrals at optimal chlorideconcentration.

3.1. Stability constants and thermodynamic parameters

The complex formation equilibrium in a mixed ligand system

MþmAþnB2MAmBn ð1Þ

can be characterised by an overall stability constant:

ßmn ¼½MAmBn�

½M�½A�m½B�nð2Þ

On the basis of the cobalt(II) co-ordination by halide ions inother solvents, we assumed that the complexes were mono-nuclear in cobalt(II) and that the maximum ligand co-ordinationfor cobalt(II) was 4.

For the computation of the stability constants ßmn and speciesspectra emn (l), the non-linear regression programs STAR [28] andHypSpec [29,30] were used. Part of these programs was used fordetermination of the number of absorbing species by factor analysis.

From a large number of trials to determine the most relevant complexspecies it was concluded that the complexes presented in Table 2were formed together with calculated stability constants. For thecalculation of the stability constants, series of 30 different solutionsfor each system (in total about 150 solutions) were prepared andabsorption spectra were recorded at several different temperatures.These temperatures are also indicated in Table 2. From Table 2, it isclear that the stability constants increase with increasing temperaturein all systems. It is also obvious that the most stable species werefound in the system NH4NO3 �3DMSO, which indicates that thissystem is the most suitable medium for the application of cobalt(II)thermochromic complexes.

Using the constants from Table 2, the standard thermody-namic functions for the complex formation in all investigatedmixtures can be calculated. The standard enthalpy and entropychanges DHo

mn and DSomn have been estimated from the tempera-

ture variation of

DGomn ¼2RT lnßmn ð3Þ

The thermodynamic parameters have been estimated by linearregression analysis as temperature independent constants in thestudied temperature range. Obtained values are presented inTable 3. It can be seen that the formation of all complexes is anendothermic process; thus only a small amount of the energy isavailable for changing the geometry from octahedral to tetrahe-dral. The formation of tetrahedral complexes is the mostendothermic and the most responsible for the thermochromicbehaviour of cobalt(II) complexes.

4. Conclusion

Thermochromic behaviour of cobalt(II) chloro-complexes infive binary mixtures was investigated. It was found that the sizeof the organic component and its relative permittivity have thebiggest influence on thermochromism of cobalt(II) complexes.Careful selection of the organic solvent and also the chlorideconcentration can be done to favour the change of the geometryfrom octahedral to tetrahedral. Changing of the geometry followsthe change of the melt colour from pale pink to dark blue. Hence,additional chemical energy can be stored in such systems.Because the effective working temperatures match very well withthat readily achievable under sunlight, these thermochromicsystems present an example of novel materials suitable forauto-regulated protection and energy storage.

S. Gadzuric et al. / Solar Energy Materials & Solar Cells 105 (2012) 309–316316

Acknowledgements

This work was financially supported by the Ministry of Scienceand Environmental Protection of Republic of Serbia under ProjectContract ON172012 and The Provincial Secretariat for Science andTechnological Development of APV, Contract 114-451-2373/2011-01.

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