Franklinite-Zincochromite-Gahnite Solid Solutions for High NIR Reflectance Red Ceramic Pigments and Photocatalytic Activity With Visible Light

2.1. Samples Preparation

Solid state or ceramic procedure and ammonia coprecipitation route were used for the synthesis of the ceramic pigments. In the ceramic method (CE), precursors, oxides or carbonates, supplied by ALDRICH, with a particle size between 0.3–5 µm were mechanically homogenized in an electric grinder (20,000 rpm) for 5 min. The mixture was fired at the corresponding temperatures and soaking times. In the coprecipitation method (CO), soluble precursors, nitrates of the corresponding cations, supplied by ALDRICH, for to prepare 10 g of the final product were solved in 200 mL of water, then ammonia 30% was dropped maintaining 70°C the temperature in continuous stirring until gelation of the mixture. The gel is aged during 24 h at room temperature and dried in oven at 110°C. Finally, the dried gels were fired at the corresponding temperatures and soaking times.

2.2. Samples Characterization

X-ray Diffraction (XRD) was carried out on a Siemens D5000 diffractometer using Cu K_α radiation (10–70° 2θ range, scan rate 0.02 °2θ, 4 s per step and 40 kV and 20 mA conditions). From XRD results the volume of the cubic cell is calculated using the equation (1):

$$\frac{1}{d^2}=\frac{h^2’+k^2+l^2}{a^2}$$

(1)

where d is interplanar distance of the (hkl) lattice plane, obtained from Braggs’ law, and a is the cubic cell edge.

L*a*b* colour parameters of powders and glazed samples were measured following the CIE-L*a*b* (“Commission Internationale de l’Éclairage”) colorimetric method using a X-Rite SP60 spectrometer, with standard lighting D65 and 10° observer. In this method, L* is a measure of lightness (100 = white, 0 = black) and a* and b* of colour parameters (-a* = green, +a* = red, -b* = blue, +b* = yellow) [20].

UV-Vis-NIR spectra of both fired powder and glazed samples were collected using a Jasco V670 spectrometer through diffuse reflectance technique, which gives data in absorbance using arbitrary units (A) or in reflectance units (R (%)). Using the Kubelka–Munk transformation model, the bandgap of the samples was estimated using the Tauc method [21]. The total solar reflectance R, the solar reflectance in the NIR range R_NIR or the solar reflectance in the Vis range are evaluated from UV-Vis-NIR spectra through the diffuse reflectance technique, as the integral of the measured spectral reflectance and the solar irradiance divided by the integral of the solar irradiance is in the range of 350 to 2500 nm for R, 700 to 2500 nm for R_NIR or 350–700 nm for R_Vis as in the equation (2):

$$\mathrm{R}=\frac{{{\displaystyle \int }}_{350}^{2500}\mathrm{r}\left(\mathsf{\lambda }\right)\mathrm{i}\left(\mathsf{\lambda }\right)\mathrm{d}\mathsf{\lambda }}{{{\displaystyle \int }}_{350}^{2500}\mathrm{i}\left(\mathsf{\lambda }\right)\mathrm{d}\mathsf{\lambda }}$$

(2)

where r(λ) is the spectral reflectance (Wm⁻²) measured by UV-Vis-NIR spectroscopy and i(λ) is the standard solar irradiation (Wm⁻² nm⁻¹) according to the American Society for Testing and Materials (ASTM) Standard G173-03.

The photoactivity of powders was evaluated by the degradation of the azo dye Orange II (OII) in an aqueous solution. The photocatalytic tests were carried out by adding a catalyst loading of 0.5 g/l to an OII solution of 0.6 10^-4 mol/L, which was prepared by dissolving Orange II (C₁₆H₁₁N₂NaO₄S) in a pH 7.42 phosphate buffer medium (NaH₂PO₄, H₂O 3.31 g and Na₂HPO₄.7H₂O 33.77 g solved in 1 litter of water). The irradiation source was a white LED lamp (Philips, 19 W, 1300 lumens) with an estimated irradiance of 70 mW/cm². Before measurement, the suspensions were stirred in the dark for 15 min to reach sorption/desorption equilibrium. The evolution of the reaction was followed by sampling every 15 minutes. Orange II concentrations in different samples were determined using UV-Vis spectroscopy at 485 nm. Commercial TiO₂ Degussa P25 (Eg=3.15 nm that falls out of visible range [22]) was used as a reference for comparison with home-prepared samples; likewise, the test with red reference supplied by FERRO SP ((S_0.5Se_0.5)Cd@ZrSiO₄) was also carried out.

Photocatalytic degradation followed the Langmuir-Hinshelwood model of heterogeneous catalysis, which considers the reaction between two species adsorbed on the catalyst (adsorbates), versus the so-called Eley-Rideal model, which postulates the reaction between an adsorbate and an incoming molecule [23].

Microstructure characterization of samples was carried out by Scanning Electron Microscopy (SEM) using a JEOL 7001F electron microscope and a Transmission Electron Microscopy (TEM) using a 2000 kV JEOL JEM 2100 (following conventional preparation and imaging techniques). The chemical composition and homogeneity of the samples was determined by semi-quantitative elemental analysis with an EDX analyser (supplied by Oxford University) attached to the SEM microscope.

2.3. Application of Pigments

The colouring capacity of the pigments was studied in three matrices: (a) 5wt.% glazed in double firing frit with some lead (6wt.% PbO) 1000°C, (b) 5wt.% glazed in single firing frit 1080°C, (c) 5wt.% glazed in porcelain single firing frit 1200°C.

4.1. Franklinite-Zincochromite Zn(Fe_2-xCr_x)O₄ solid solutions

Figure 1 shows the Franklinite-Zincochromite Zn(Fe_2-xCr_x)O₄ solid solutions synthesized at 1000ºC/3h and 5wt% glazed in a single fire frit (1080°C) and a porcelain single firing frit (1200°C), with indication of the corresponding CIEL*a*b* values. Red powders with a* higher than 20 were obtained, except for x=2, corresponding to pure zincochromite, which shows a grey shade. Glazed samples also show red shades (a* between 17-23) except for x=0 (pure franklinite, yellow coloured) and x=2 (pure zincochromite, grey coloured). In the porcelain frit at 1200°C, all samples show dark behaviour with low red parameter, indicating the unstabilization of the pigments at this temperature and aggressive glaze medium. The best red colour in the single firing frit at 1080°C was x=0.2 sample with L*a*b*=37.5/23.4/28.9.

Figure 2 shows the X-ray diffractograms (XRD) of Franklinite-Zincochromite Zn(Fe_2-xCr_x)O₄ solid solutions synthesized at 1000ºC/3h with the evolution of the volume of cubic cell of the corresponding spinel with x enclosed. A single phase associated to the corresponding cubic spinel is detected in all samples, except for x=1.8 that exhibits weak peaks associated to ZnO. It can be observed that the diffraction peaks shift to higher °2ϴ degrees associated to a decrease of the interplanar distances d in equation (1), in accordance with the replacing of Fe³⁺ (Shannon-Prewit radius in octahedral site=1.03Å) by the smaller Cr³⁺ (0.755 Å); then a decrease of the cubic cell edge of the spinel with x is detected. In effect, from equation (1), the cell volume evolution with x, enclosed in Figure 2, shows that the cell volume decreases progressively with x accomplishing rigorously the Vegard’s law, except for the x=1.8, in agreement with the presence of residual ZnO in its XRD diffractogram, that can be explained by a reduction of some Fe³⁺ to Fe²⁺ which is located in tetrahedral site of spinel (Shannon-Prewit radius in octahedral site=0.77Å) replacing Zn²⁺ (0.74 Å) that attenuates the decreases associated to the replacement of Fe³⁺ by Cr³⁺ [24].

Figure 3 shows the UV-Vis-NIR diffuse reflectance spectra of Franklinite-Zincochromite Zn(Fe_2-xCr_x)O₄ solid solutions. In the case of fired powders at 1000ºC/3h (Figure 3A), all spectra are similar, except for x=1.8 and 2 associated to ZnO segregation and in the x=1.8 and the absence of iron in x=2 (pure zincochromite). The reflectance decreases with x from 30% for x=2 to 80% for x=0 in the NIR range. For x=0 sample (franklinite) two intense absorbance bands (minimums of reflectance) centred at 1180 and 770 nm with a wide band of absorption between 400-500 nm appears, associated with the transitions ⁶A_1g(S) → ⁴A_1g,⁴E(G), which overlaps with the charge transference band at 400–500 nm of Fe(III) in octahedral coordination. The entrance of chromium shifts lo lower wavelength the band at 1180 nm and two bands at 720 and 680 nm appears. Finally for x=2 (zincochromite) the bands at 1180 and 790 nm disappears, and minimums of reflectance are detected at 680, 570, 440 and 250 nm respectively associated to octahedral Cr(III). The bands at around 770 and 1200 nm are typical characteristics of the zinc ferrite, which are attributed to ⁶A_1g(S) → ⁴T_2g(G) and 6A_1g(S) → ⁴T_1g(S) for d-d electron transition of Fe³⁺. These Fe³⁺ bands overlap with those of Cr³⁺ in octahedral coordination, namely: (a) three main parity-forbidden transitions (⁴A₂(⁴F) → ⁴T₂(⁴F) at 570 nm and ⁴A₂(⁴F) → ⁴T₁(⁴F) at 440 nm, which are partially overlapped, and ⁴A₂(⁴F) → ⁴T₂(⁴P) at 240 nm, which overlaps with Cr³⁺-O²⁻ charge transfer), (b) two weak Cr³⁺ spin-forbidden transitions (⁴A₂(⁴F) → ²T₁(²G) and ⁴A₂(⁴F) → ²E(²G) that overlap at 670 and 710 nm [6,17].

In the case of 5wt.% glazed samples in a single fire frit 1080ºC (Figure 3B), The performance is similar to that of powders, although the evolution of the intensities in the NIR range shows the highest reflectance for x = 0.5 and 1 samples, the NIR reflectance of pure franklinite (x=0) significatively decreases in the glaze.

In effect, the R_NIR reflectance measurements (enclosed in Figure 1) of Zn(Fe_2-xCr_x)O₄ solid solutions (1000ºC/3h) and 5wt.% glazed samples in a single fire frit 1080ºC show the decrease of R_NIR with x in powders, but show certain overlapping in values for x=1, 0.5 and 0.2 in glazed samples. Considering the criteria of high colouring capacity and high NIR reflectance at minimum Cr amount, the x=0.2 sample results the optimal composition for a cool pigment based on Franklinite-Zincochromite system.

4.2. Gahnite-Zincochromite Zn(Al_2-xCr_x)O₄ solid solutions

Figure 4 shows the characterization of Gahnite-Zincochromite Zn(Al_2-xCr_x)O₄ solid solutions synthesized by the ammonia coprecipitation method (CO) described above with L*a*b* and R_Vis/R_NIR/R values enclosed. Zn(NO₃)₂.6H₂O, Al(NO₃)₃.6H₂O and Cr(NO₃)₃.9H₂O were used as precursors of Zn, Al and Cr respectively. The raw dried gels show a homogeneous appearance with a purple-blue colour that intensifies with the chromium amount (L* decreases, a* greenish and b* blueish). Powders fired at 1000°C/1h show that the colour intensifies (L* decreases) and reddish (a* increases) with chromium amount.

The samples 5wt.% glazed in double firing frit with some lead (6wt.% PbO) of 1000ºC (Figure 4C) show that the colour intensifies (L* decreases) and reddish (a* increases) with chromium amount, producing interesting red shades in samples between x=0-2 and 0.5. Lower chromium amounts give light colours and x=0.75 sample produces a dark grey colour. The glazing in single firing frit 1080°C and porcelain single firing frit 1200°C give the best colour for x=0.5 sample.

Figure 5 shows the X-ray diffractograms (XRD) of Gahnite-Zincochromite Zn(Al_2-xCr_x)O₄ solid solutions with the evolution of the volume of the cubic cell of the corresponding spinel with x. Only peaks associated to the corresponding cubic spinel are detected. It can be observed that the diffraction peaks shift to lower °2ϴ degrees associated to an increase of the interplanar distances d in equation (1), in accordance with the replacing Al³⁺ (Shannon-Prewit radius in octahedral site=0.675Å) by the bigger Cr³⁺ (0.755 Å); therefore. an increase of the cubic cell edge of the spinel with x is espected. In effect, the cell volume evolution with x, enclosed in Figure 5, shows that the cell volume increases progressively with x accomplishing rigorously the Vegard’s law in the studied range x=0-0.75 [22].

Figure 6 shows the UV-Vis-NIR absorbance spectra of Gahnite-Zincochromite Zn(Al_2-xCr_x)O₄ solid solutions synthesized by ammonia coprecipitation fired at 1000ºC/1h. Clearly the absorbance increases with the amount of chromium x. Intense bands centred at 240, 430, 570 and 700 nm respectively are observed, which are associated to Cr³⁺ in octahedral coordination, namely: (a) three main parity-forbidden transitions (⁴A₂(⁴F) → ⁴T₂(⁴F) at 570 nm and ⁴A₂(⁴F) → ⁴T₁(⁴F) at 430 nm, which are partially overlapped, and ⁴A₂(⁴F) → ⁴T₂(⁴P) at 240 nm, which overlaps with Cr³⁺-O²⁻ charge transfer), (b) two weak Cr³⁺ spin-forbidden transitions (⁴A₂(⁴F) → ²T₁(²G) and ⁴A₂(⁴F) → ²E(²G) that overlap at around 700 nm [6,17].

The bands shift to higher wavelength with the amount of chromium x in the sample, indicating a weakness of the crystal field associated to entrance of the bigger Cr³⁺ (0.755 Å), replacing Al³⁺ (Shannon-Prewit radius in octahedral site=0.675Å) in a small and constrained lattice of ZnAl₂O₄. The progressive increase of absorbance and shifting to red wavelength intensifies the colour and the a* parameter increases, but at x=0.75 the absorption is excessive and the colour darken producing dark brown shades as is observed in Figure 4.

The NIR reflectance of samples (enclosed in Figure 4 for powder and 5wt.% glazed in double firing 1000°C) is relatively high, with values higher than 45%, and the sample x=0.5 shows the higher value.

Following the criteria of high colouring capacity and high NIR reflectance at minimum Cr amount, the x=0.5 sample results the optimal composition for a cool pigment based on Gahnite-Zincochromite system. In Figure 7 is showed the microstructure of this optimal sample. Figure 7A shows the morphology by TEM of nanoparticles around 25 nm forming the raw gel. Figure 7B shows the SEM image and EDX composition mapping analysis of the particles of the same sample fired at 1000°C/1h: rough aggregates of particles of 1-3 µm can be observed in which the distribution of the Zn, Al and Cr appears homogeneous.

4.3. Gahnite-Zincochromite-Franklinite Zn(Al_1.5-xCr_0,5Fe_x)O₄ solid solutions

In order to optimize the ternary system with Al, Cr and Fe, compositions based on the above optimal sample, Zn(Al_1.5-xCr_0,5Fe_x)O₄ x=0-0.3 were synthesised. Figure 8A shows the image (x40) of raw dried gels which show a homogeneous appearance with a purple-blue colour for x=0 and reddish progressively with the iron addition. Powders fired at 1000°C/1h show that the colour intensifies (L* decreases) and reddish (a* increases) with iron amount.

The 5wt.% glazed samples in a single firing frit which maturate at 1080°C (Figure 8C) show that the colour slightly intensifies (L* decreases one point) and reddish (a* increases) with iron amount, producing interesting red shades in samples from pink of x=0 to red x=0.2, the sample x=0.3 darkens and the a* value drops slightly. On the other hand, in the samples 5wt.% glazed in a porcelain firing frit which maturate at 1200°C (Figure 8D) the colour intensifies (L* decreases) and reddish (a* increases) with iron amount up to the sample x=0.3, which darkens and the red a* value falls. The glazing in single firing frit 1080°C and porcelain single firing frit 1200ºC give the best colour for x=0.2 sample (Zn(Al_1.3Cr_0,5Fe_0.2)O₄).

Figure 9 shows the XRD diffractograms of Zn(Al_1.5-xCr_0,5Fe_x)O₄ coprecipitated samples fired at 1000°C/1h: only peaks of the corresponding spinel are detected. Figure 10 shows the UV-Vis-NIR diffuse reflectance spectra of the same Zn(Al_1.5-xCr_0.5Fe_x)O₄ samples. The absorbance increases with x. Bands at 240, 450-500, 570, 670, 710, 780 (shoulder) and 1200 nm can be observed: as discussed above for Franklinite-Zincochromite samples, bands centred at 1200 and 780 nm with the wide band of absorption between 400-500 nm are associated with the transitions ⁶A_1g(S) → ⁴A_1g,⁴E(G), which overlaps with the charge transference band at 400–500 nm of Fe³⁺ in octahedral coordination. These Fe³⁺ bands overlap with those of Cr³⁺ in octahedral coordination: (a) three main parity-forbidden transitions (⁴A₂(⁴F) → ⁴T₂(⁴F) at 570 nm and ⁴A₂(⁴F) → ⁴T₁(⁴F) at 440 nm, which are partially overlapped, and ⁴A₂(⁴F) → ⁴T₂(⁴P) at 240 nm, which overlaps with Cr³⁺-O²⁻ charge transfer), (b) two weak Cr³⁺ spin-forbidden transitions (⁴A₂(⁴F) → ²T₁(²G) and ⁴A₂(⁴F) → ²E(²G) that overlap at 670 and 710 nm [6,17].

Associated to the UV-Vis-NIR reflectance spectra, the R_NIR of samples enclosed in Figure 8 shows a slightly increase with the iron amount, from 46 to 52% in powders and around 58% for all samples with iron addition.

Considering the criteria of high colouring capacity and high NIR reflectance at minimum Cr amount, the x=0.2 sample results the optimal composition for a cool pigment based on Franklinite-Gahnite-Zincochromite system.

Figure 11A shows the TEM images of the gel of this optimal sample. In this case the amorphous gel shows homogeneous cubic nanoparticles with edge between 50-90 nm. On the other hand, Fig 11B shows the particles of powder fired at 1000°C/1h: fine cubic particles with an edge around 1 µm can be observed.

4.4. Smmary of optimized samples compared with a reference

Figure 12 shows the summary of the three reddish optimized samples in the analysed systems compared with cadmium sulfoselenide encapsulated in zircon ((S_0.5Se_0.5)Cd@ZrSiO₄) considered the best red ceramic pigment [28], but considered toxic due to the presence of sulfoselenide The colour of powders (Fig 12A) changes from an intense red (L*=32 and a*=22) for Franklinite-Zincochromite, to a lighter pink shade for Gahnite-Zincochromite sample and Franklinite-Zincochromite-Gahnite system (L*=51 and 47, a*=12 and 18 respectively) which are higher intense (lower L*) but lower reddish (lower a*) than cadmium sulfoselenide in zircon reference (L*=53, a*=23). In glazed samples at relative low temperature (Fig 12B) the evolution is similar, and the spinel colours (with intense clear brown, pink and red shades respectively) show higher intensity and lower reddish shade than the red reference, however in porcelain frit at higher temperature (1200°C), the reference is colourless (Figure 12C).

Figure 13 compare the UV-Vis-NIR diffuse reflectance spectra of the optimized samples (A powders samples and B glazed samples at low temperature) that shows those already discussed above bands of absorption (minimum of reflectance) associated to Cr³⁺ in octahedral coordination, overlapped with the bands of Fe³⁺ in the same coordination when is present. NIR reflectance is high in all powders (between 45-51%) but slightly lower than reference (R_NIR=58). In the case of glazed samples, the R_NIR of samples and reference are similar (in the range 54-60%).

Figure 14 shows the Tauc plots for the three optimized powders that shows a direct semiconductor behaviour with bandgap very similar, around 2.0 eV (corresponding to visible light of 620 nm), the sulfoselenide zircon reference shows a bandgap of 1.77 eV. Figure 15 displays the photodegradation test of Orange II with the optimized powders and in Table 1 summarise the photodegradation parameters of the test. The results indicate a poor activity of all samples, only Cr-ZnFe₂O₄ shows a value lower than 300 min of the half degradation time t_1/2, and in addition, cyclability is not good and samples loss the activity (t_1/2>400 min) in the third cycle.

In accordance with Casbeer et al. [26], the moderate results obtained with the microparticles of the studied high NIR reflective reddish pigments and the relative success of Cr-franklinite, can be explained because, although ferrites result effective when have been applied as photocatalysts alone, is as nanoparticles, in composite with other photocatalysts as well as with other oxidants such as H₂O₂, its activity is more efficient. Thereby, Zhu et al. [27] reports that magnetic zinc ferrite fine nanoparticles, synthesized by a soft chemical solution process, showed high activity, good reusability and easy separation ability for visible-light-induced Orange II degradation, but in an integrated ZnFe₂O₄/PMS (peroxymonosulfate, 2KHSO₅·KHSO₄·K₂SO₄, OXONE) aqueous system.

3. Conclusions

Franklinite-zincochromite-gahnite solid solutions have been prepared by ceramic or coprecipitation method and its pigmenting capacity as cool ceramic pigments in different media (double and single firing frits and porcelain frit) was studied. Following the criteria of high red colouring capacity and high NIR reflectance at minimum Cr amount, Zn(Fe_1.8Cr_0.2), Zn(Al_1.5Cr_0.5) and Zn(Al_1.3Cr_0.5Fe_0.2)O₄ result the optimal compositions for an intense reddish cool pigment based on Franklinite-Zincochromite, Gahnite-Zincochromite and Franklinite-Zincochromite-Gahnite systems respectively, showing CIEL*a*b* around L*a*b*=38/23/29 (clear brown), 60/15/11 (pink) and 51/23/25 (red) respectively in 5wt% glazed samples in a single firing frit. The NIR reflectance of the optimized samples is high, reaching values between 45-51% in powder and 54-59% in glazed samples. All powders show direct semiconductor behaviour with bandgap around 2 eV which falls in the visible range (620 nm). Its associated photocatalytic activity over the azoic pigment Orange II with white visible light irradiation is moderate in all cases, and the result with Franklinite-Zincochromite Zn(Fe_1.8Cr_0.2) stands out, but in accordance with literature it should be necessary reduce the particle size, forms composites with other photocatalysts, as well as with other oxidants such as H₂O₂ to increase its photocatalytic activity, but then the pigmenting capacity would be limited.