New Hybrid Materials Synthesized with Different Dyes by Sol-Gel Method
One well-known method for hybrids synthesis with incorporated organic dyes is sol-gel method, which is based on the concept of molecular manipulation to design ceramics, glasses, and composites. The low-temperature process allows for the incorporation of guest organic molecules within the inorganic matrix, as well as for the synthesis of hybrid networks in which the organic and inorganic phases are interpenetrating. The aim of the work presented in this paper was the preparation of the gels with three different dyes, at different molar ratios by using the hydrolytic sol-gel process. The interaction of the dye and the oxide was examined by UV-vis spectroscopy and FT-IR. The thermal stability of the hybrid organic-inorganic xerogel formed here was studied by thermal analysis. The micrographs obtained by scanning electron microscopy (SEM) revealed the high density of the films. Such characteristics indicate the possible application of these films in solar cells.
The sol-gel technology, by which inorganic or composite organic-inorganic materials are made at relatively low temperature, consists of hydrolysis of the constituent molecular precursors and subsequent polycondensation to glass-like form. The technology allows the incorporation of organic and inorganic additives. Based on the sol-gel process, a large number of sophisticated materials have been prepared and studied during last decades. The first successful attempt to incorporate organic dyes and to stabilize them within a sol-gel matrix started in 1984, by Reisfeld et al. [1–6]. The use of organic dyes in a hybrid organic-inorganic matrix has better characteristics than an organic matrix because the dye colored hybrid films have enhanced mechanical properties like higher hardness and stronger abrasion resistance . With respect to inorganic sol-gel films, hybrid films also provide several advantages such as much better adhesion, transparency, flexibility, reduced surface roughness, and better refractive index matching [8–12].
Organic dye incorporated silica gels have many applications for nonlinear optical materials, laser, chemical sensors, color filters, luminescent solar concentrators, high density memories, and coating for glass bottles which can be easily recycled [13–17]. The sol-gel method is one of promising methods to prepare such materials. The concentration, coordination state, and dispersion of dye molecules in gels are important factors for optical applications. Organic dyes incorporated in inorganic silica gels by the sol-gel method are monomeric at low concentration but tend to form dimers in higher concentrated solutions .
The applications of the dyes include coloring textiles paper, leather, plastic, food, medicines, cosmetics, and solar and photovoltaic cells [19, 20]. Dye-sensitized solar cells (DSSCs) are one of their main applications nowadays, which have attracted a lot of interest, as a new option in photovoltaics area (especially as potential materials for photovoltaic cells) [21–25]. Dye-sensitized solar cells are more suitable to be used at large scale in comparison with the classical photovoltaics because they are much more cheaper and easier to be prepared [24, 26, 27].
The so-called organic-inorganic hybrid nanocomposites may present properties that do not depend solely on each component individually but also on interface phenomena between these phases, which can lead to synergistic effects. For example, the sol-gel method offers a molecular-level mixing of the calcium and phosphorus precursors, in comparison with conventional methods. In most cases, phosphorus compounds-oxide, triethylphosphate, and diethylphosphite are employing as phosphorus precursors in water or organic solvents phase. Polyphosphate association to aluminium, calcium, magnesium, iron, copper, or nickel ions in aqueous solution can lead to gelation under appropriate conditions.
Incorporating organic dyes with inorganic and hybrid products, obtained through the sol-gel process (Scheme 1), allows the achievement of colored layers for optics applications such as filters, transformers of solar energy, nonlinear optics, and laser environments.
The aim of the work described here was the preparation of the gels with three different incorporated dyes, by using sol-gel method. The gel is a solid jelly-like material that can have properties ranging from soft and weak to hard, with a lot of potential applications, from cosmetics (e.g., gels containing hyaluronic acid for skin [28–30], shower gels, and hair gels) to solar cells and even flame retardants .
2.1. Materials and Methods
Phenylphosphonic acid (PPA) was purchased from Fluka, and butyl-zirconate Zr[O(CH2)3CH3]4 from Sigma-Aldrich. Acids dyes chromatable added to the gels based on zirconium were synthesized in our laboratory, as follows: acid dye blue-violet chromatable (C1), acid dye green chromatable (C2), and acid dye Bordeaux chromatable (C3).
In the present work, eight syntheses (S1–S8) were performed by using the three different dyes previously mentioned (C1–C3, Figure 1), at different molar ratios, as shown in Table 1.
2.2. Sol-Gel Method
Phenylphosphonic acid (PPA) was dissolved in 10 ml ethanol for the synthesis of S1 and S2 (over which the dyes C1–C3 corresponding to the syntheses S3–S8 were added) depending on the molar ratios specified in Table 1. Then, the butyl-zirconate was added also in the quantity corresponding to the respective molar ratios, under magnetic stirring at room temperature, for 1.5–2 hours. After two hours, the reaction mass started gelling. The obtained gel was left to dry for 2-3 days at a temperature of 40°C in an oven to evaporate the ethanol. The resulting compounds were further analyzed with several techniques (FT-IR, TGA, SEM, TEM, and UV-vis).
(i)Jasco FT-IR 4200 Spectrometer was used for recording the IR spectra.(ii)Thermal analysis TGA-DTG-DTA (Thermogravimetry) was carried out by changing the temperature between 20°C and 900°C, by using an 851-LF 1100-Mettler Toledo apparatus in airflow. It has a sensitivity of 1 μg. The maximum amount of the sample which could be measured is 5 g.(iii)SEM and TEM characterizations were made using a Jeol JSM 6400 Scanning Microscope.(iv)UV-visible spectra were recorded on a JASCO UV-vis spectrometer, V-650 model (Japan). Absorption, excitations, and emission spectra were registered in environmental conditions (temperature: °C), on 1 cm path length cells.
3. Results and Discussion
In Figure 2, the presence of bands near 3420, 3750, and 1627 cm−1 was assigned to asymmetric and symmetric OH stretching modes and physically adsorbed water, respectively, and also the absorption bands from 1540 cm−1 and 3735 cm−1 come from the vibrations of intercalated water . The disappearance of the broad bands characteristic to the vibrations in phenylphosphonic acid at 2700 cm−1 and 2200 cm−1 indicates the condensation of the P-O-H groups .
The P-O-M bands are usually broad (indicative of the delocalization of electrons) and generally fall with 900–1200 cm−1 range . The formation of a P-O-M bond may be accompanied by the disappearance of the P=O and P-O-H bonds, depending on the mode of binding, which can be mono-, bi-, or tridentate. For example, a phosphonic acid bound in tridentate form should result in just a broad band within 900–1200 cm−1 and the total disappearance of the normal P=O and P-O=H bands. The presence of either the P=O band or the P-O-H bands indicates the presence of mono- or bidentate binding, or a mixture of the two [35–37].
The intense band at 1047 cm−1 is assigned to asymmetrical stretching vibration of P-O which confirms the formation of bonds P-O-Zr and the shoulder at 1150 cm−1 is ascribed to symmetrical stretching vibration of P-O  from free P-OH groups in hybrids samples. These free -P-OH groups suggested the possibility of further condensation of Ph-P-OH species within the hybrid framework. The bands at 1388 cm−1 and 1440 cm−1 could be assigned to the characteristic bond P=O and indicate the presence of P=O in hybrid structure .
The band at 1482 cm−1 was ascribed to P-C stretching vibration which is overlapped with C-H bending of CH2 groups  and one at 1627 cm−1 is due to the stretching vibration of phenylphosphonic acid. The P-C stretching modes at 1482 cm−1 and around 750 cm−1 were assigned to the presence of P-C aromatic, observed in all hybrids.
The bands at 563 cm−1 and 694 cm−1 match well with the calculated vibration frequency of the Zr-O species  and could overlap with out-of-plane P-OH bending.
UV-vis characterization (Figure 3) was performed to determine the absorbance of the obtained hybrid materials.
From UV-vis spectra, it can be observed that, for hybrid materials S1 and S2, the maximum wavelengths values are in ultraviolet domain. The low absorption takes place at wavelengths λ = 222 nm and λ = 265 nm for S1 and λ = 225 nm and λ = 265 nm for S2. This is explained by the white color of these gels. When overlapping them, it can be easily observed that they are almost identical. Only the absorbance value differs. This is due to the different molar ratio PPA/Zr(OBu)4. In S1 and S2 UV-vis spectra, no absorbance peaks in visible domain were noticed.
In the materials in which the dyes are incorporated (S3–S8), in UV-vis spectra, the maximum wavelength values are shifted to visible range, between 505 and 643 nm. Moreover, the curve shape is influenced by the auxochromes and chromophores substituents, grafted on basic structure of dyes. The dyes substituents showed significant influence on the obtained UV-vis spectra. The maximum absorbance was observed for hybrid S3 at PPA/Zr(OBu)4 molar ration 2/1, followed by the hybrid S4 at PPA/Zr(OBu)4 molar ration 1/1, with the same C1 dye. This indicates that C1 dye could possibly have a greater efficiency when applied to dye-sensitized solar cells (DSSC).
From the SEM images, it was concluded that the all obtained hybrids here are similar, and their morphology is amorphous and, when the dyes are present, are well dispersed. In Figure 4, SEM images of the synthesized materials S1, S4, S6, and S7 are shown.
Also, good porosity showed by hybrids S1–S8 pointed out to facile diffusion of redox mediators within the layer. This porosity is mandatory in the application in DSSC to obtain a good interaction with surface-bound dye (sensitizer). The TEM images (Figure 5) also indicate a porous structure and good dye dispersion in the hybrid framework of the obtained materials (dye molecules penetrate inside the pores).
The high percent of the macropores evidenced by TEM microscopy was confirmed by N2 adsorptions desorption.
The TG data for S1–S8 hybrids are presented in Figure 6. The TG data indicates for all hybrids a continuous weight loss till 900°C, associated with the loss of humidity, alcohol, and oxidation of organic part of components.
In the temperature domain 25–150°C, the weight loss is situated in the range of 4.3–6.3% and corresponds to the loss of humidity and alcohol present in sample as a result of sol-gel reaction. In the temperature range 150–350°C, the weight loss corresponds to the release of alcohol as a result of advanced condensation process and is situated in the range of 4.4–5.8%. Based on the weight loss in this stage and assuming that butyl alcohol is released, it was calculated that the second hydroxyl group from phosphonate unit could react in this stage in a proportion of maximum 35% for S1 and 34% for S2. In the case of S3–S8 sample and taking in account the dye weight loss in this step, the release of butanol as a result of second hydroxyl group reaction from phosphonate unit indicates a slightly low value than S1 and S2, about 27–32%. The weight loss increased to 23.3–29.1% and is massive in the temperature domain 350–550°C as a result of oxidation of organic part of components and finalization of second hydroxyl group condensation with the release butyl alcohol. In 350–550°C, the phenyl group is principally susceptible to decomposition. In this temperature domain, the literature presents some studies regarding decomposition of benzene and toluene and compound into small fragments followed by further oxidation to CO2 and water [42, 43]. After extraction of weight loss due to dye decomposition and the oxidation of phenyl groups to CO2 and water, the total amount of the second hydroxyl group capable of reacting reached 86% for S1, 77% for S2, and for lower 30–60% for S3–S8. In the case of S3–S8, the amount of second hydroxyl group capable of reacting is decreased by the presence of dye. The oxidation process in this domain takes place with the highest rate and the corresponding temperature is situated between 517 and 528°C (Td max).
The oxidation process continues till 900°C. In the temperature domain 25–900°C, the weight loss is situated in the range of 1.5–7.8% and corresponds to the further oxidation of organic part at CO2. In the case of S3–S8, the weight loss was increased by the presence of dye (Table 2).
We synthesized new hybrid materials by the incorporation witch three different dyes, at different molar ratios using the hydrolytic sol-gel process. The new obtained hybrid materials (gels) were investigated by using UV-vis spectra, FT-IR, TG, SEM, and TEM methods. Based on the obtained UV-vis spectra, we can conclude that the dyes had significant influence, indicating that C1 dye has a greater efficiency when applied to dye-sensitized solar cells (DSSC). By TEM and SEM microscopy, we can see that hybrids S1–S8 show good porosity, which indicates facile diffusion of redox mediators within the layer. This porosity is necessary and mandatory in the application in DSSC to obtain a good interaction with surface-bound dye (sensitizer).
TGA data indicates in case of samples S3 and S5 that adding dye increases the loss of mass, and also in case of samples S4 and S6 the addition of dye is lower and so the loss of mass is lower. All of these characteristics indicate the possible application of these films in solar cells. The transparent colored hybrid films have a very good optical quality with color intensity depending on the added amount of organic dye in the films. The dye incorporated gels leads to many applications as sensors, biosensors, solar concentrators for solar cells in the fields of photovoltaics, and so on.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this article.
The authors acknowledge the Research Programme 2, Project 2.2, Institute of Chemistry Timisoara of the Romanian Academy.
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