Biomimetic synthesis of amorphous photonic crystals (APCs) is a significant approach to obtain non-iridescent structural colors. However, the structural colors of artificially prepared APCs are dim or even white due to the influence of incoherent scattering. In this paper, we present an innovative method was proposed to combine APCs with black TiO2-x to construct non-iridescent structural color pigments with high visibility and photocatalytic activity. Due to absorption of incoherent scattered light by black TiO2-x, the color saturation of structural colors has been significantly increased. In addition, the utilization rate of photogenic carriers was effectively enhanced by slow light effect generated from pseudo-band gap of SiO2 APCs with TiO2-x absorbed full spectrum. The tone and color saturation of catalytic pigments are controlled by the diameter of SiO2 nanospheres and the ration of TiO2-x nanoparticles, which provides a controllable application study in color related fields such as artwork, environmental coatings and textiles.
Research Article
Development of New Catalytic Pigments: Dual Functional SiO2/TiO2-x Amorphous Photonic Crystals for Non-Iridescent Structural Color and Enhanced Photocatalytic Activity
https://doi.org/10.21203/rs.3.rs-853239/v1
This work is licensed under a CC BY 4.0 License
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Catalytic pigments
SiO2/TiO2-x amorphous photonic crystals
non-iridescent structural colors
black TiO2-x
photocatalysis
Color is an important form of expression in inheriting social civilization by integrating art and reality and expressing complex emotions of human beings [1]. Human fascination and pursuit of color has never stopped. From Dunhuang murals [2] to modern and fashionable decorative pigments [3, 4], humans have continued to improve the color and visual effects of synthetic pigments. To find inspiration for synthetic pigments, they paid attention to the brightly colored morpho butterfly [5], peacock feathers [6], and chameleons [7] that can switch colors instantly. This also inspired scientists to study the physics of the appearance of colors. The sparkle of butterflies originates from the regular arrangement of scale cells. Bright color switching of chameleon is caused by a combination of transparent guanine nanocrystals and melanosomes in the inner layer of skin [7, 8]. Submicron-scale materials are regularly arranged in a specific structure in space by the diffraction, interference, and scattering of light at a specific wavelength, thus producing a structural color that never fades [9–11].
Structural color based on photonic crystals has bright color and iridescent effect [12, 13]. The independent structural color prepared by the doping of melanin with amorphous photonic crystals (APCs) has been widely studied by scientists owing to their soft colors and ability to satisfy the visual needs of human beings [14–17]. A short-disordered APCs structure produces incoherent scattering of light, making the color of the material white [18]. Current studies have introduced nanoscale carbon black material (carbon, nanotube, acetylene black, and graphene) into the APCs structure as the backside to absorb coherent scattered light, or heat treatment in the presence of carbon in a reducing atmosphere to obtain non-iridescent structural color when synthesizing photonic crystal structure units [19–22]. Pastel colors and timeless non-iridescent structural colors can replace traditional chemical colors in use, but in the application of home decoration coating, pigment, and other color decoration fields, it is still unable to reduce indoor environmental pollution [23–25].
Well-known environment-friendly material titanium dioxide (TiO2) has excellent photocatalytic properties, but white TiO2 can only be excited to provide redox ability under UV light accounting for 5% of the light [26]. Therefore, preparation of black TiO2-x capable of full-spectrum absorption has become a research focus [27]. Hydrogen heat treatment of TiO2, Al/Zn reduction of P25, hydrogen plasma reduction of TiO2 nanotubes, and other preparation methods require high temperature and pressure and time-consuming, so the chemical reduction method is used to prepare TiO2−x [28–31]. Black TiO2−x particles effectively replace carbon black material and absorb coherent scattered light. Moreover, as a structural unit of APCs, SiO2 can effectively increase the specific surface area of TiO2, and black TiO2−x with enhanced light absorption efficiency and large specific surface area can effectively promote the separation of electrons and holes, and significantly enhance the photocatalytic effect on pollutants [32].
Herein, black TiO2−x with excellent photocatalytic performance was prepared by thermally reducing P25 with NaBH4 for a certain period of time, and it was used as a black additive to reduce the incoherent scattering in APCs. A certain amount of TiO2−x was added to the substrate by blending. In ethanol-activated SiO2 nanospheres, TiO2−x particles were distributed on the surface of SiO2 nanospheres after stirring for a certain period of time at room temperature. After drying in a constant-temperature drying oven at 50 ℃, SiO2/TiO2−x APCs with photocatalytic properties and noniridescent structural color were obtained. Different colors can be adjusted by controlling the diameter of SiO2 nanospheres, and the color saturation can be controlled by varying the amount of TiO2−x. The concentration of TiO2−x is related to the photocatalytic performance. Therefore, introduction of black TiO2−x not only provides a direction for the functionalization of structural colors, but also improves the photocatalytic activity.
2.1 Materials
Tetraethoxysilane (TEOS, 98%), ethanol (99%), and ammonium hydroxide (28–30%) were purchased from Sinopharm Chemical Reagent, China. P25 (98%) and sodium metaborate (NaBH4, 97%) were purchased from Tianjin Kermel Chemical Reagents. Deionized water (18.2 MΩ cm resistivity) was used in the experiments. All the other chemical reagents were of analytical grade and used as received without further purification.
2.2 Preparation of monodispersed SiO2 nanospheres
Monodispersed SiO2 nanospheres were synthesized using a modified Stöber method [33]. Appropriate amounts of ethanol (0–10 mL) and TEOS (3 mL) were poured into a mixture of ethanol (40–50 mL) and ammonia (4 mL), and the solution was magnetically stirred at 40 °C and mixed well. A homogeneous solution of ethanol and TEOS was added dropwise to the mixture of ethanol and ammonia, and the dropping rate was controlled at 0.2 mL/s. After the addition was completed, the system was continuously stirred at 40°C and 500 rpm for 4 h. Finally, the uniform solution obtained was centrifuged, washed, dispersed in ethanol, and dried in an oven at 50 °C to obtain white SiO2 nanospheres.
2.3 Synthesis of black TiO2−x nanoparticles
Chemical reduction method was used to prepare black TiO2 − x P25 (4 g) and NaBH4 were mixed at room temperature for 30 min. The mixture was added into a combustion boat; the reduction reaction was carried out in a vacuum tube furnace, and the temperature was increased to 380 ℃ at a rate of 7 ℃/min for 2 h. A series of samples was obtained by controlling different molar ratios of P25 and NaBH4 (4:0, 4:0.5, 4:0.75). The samples were washed with water to neutrality to remove unreacted NaBH4 and then dried in an oven at 45 ℃, producing powdered black TiO2−x.
2.4 Fabrication of colorful SiO2/TiO2−x APCs
Certain amounts of black TiO2−x nanoparticles were added to the SiO2 ethanol dispersion solution. Then, 50 mL of the dispersion solution was dispersed by ultrasonication for 4 h and stirred at room temperature for 7 h. The reaction process is shown schematically in Scheme 1. Anhydrous ethanol and deionized water were centrifuged, washed, and dried in an oven at 50 ℃ to obtain a SiO2/TiO2−x APCs with excellent photocatalytic activity and non-iridescent structural color.
2.5 Characterization and property measurement
The surface morphology of SiO2/TiO2−x APCs was observed using a field-emission scanning electron microscopy (FESEM, Hitachi FE-S4800). The reflection and absorption spectra of SiO2/TiO2−x APC pigments were measured using a Cary 5000 UV-Vis-NIR spectrometer (Agilent), covering a wavelength range from 250 nm to 800 nm. The interplanar crystal spacing and crystalline structure of TiO2−x nanoparticles were determined by transmission electron microscopy (TEM, FEI Tecnai G2F20S-TWIN) and X-ray diffraction (XRD; D/max-2200PC) analyses. The Raman scatting spectra of TiO2−x was acquired using a Renishaw-in Via instrument equipped with a 532 nm red laser and CCD detector. The optical photographs of synthesis samples were recorded using an optical microscope (Leica DM2500 M) connected with a CCD camera.
Photocatalytic activity and photoelectrochemical properties were determined to illustrate the ability of catalyst to degrade pollutants. During the measurement of photocatalytic activity, 0.05 g SiO2/TiO2−x APC catalytic pigments were added to a 50 mL RhB solution (5×10−6 M) each time, which was evaluated by visible-light irradiation from a BL-GHX-V photocatalytic reactor connected with a 500 W xenon lamp. To verify the adsorption and degradation balance, the solution was stirred magnetically under dark conditions for 60 minutes. After a period of illumination, 5 mL suspension was taken out every 30 minutes and the photocatalyst was removed by centrifugal separation. The degradation product was detected by UV-Vis spectrophotometer. Photoelectrochemical measurement was performed on electrochemical workstation (CHI760E) using a normative three-electrode, equipped with the Ag/AgCl electrode and platinum plate as the reference electrode and counter electrode, and the prepared photocatalysts as the working electrode. 2 mg photocatalysts was added to the mixture of 0.5 mL dimethylformamide and 1 mL ethanol to acquire the emulsion, which was coated in 1.0 cm3 FTO glass as the working electrode. Electrolyte is 0.5 M NaSO4 aqueous solution. The photocurrent curve was remarked by amperage under intermittent irradiation. Electrochemical impedance spectroscopy (EIS) was obtained in the frequency range of 0.01–100,000 Hz.
3.1 Morphology and structure of SiO2 nanospheres
According to the Bragg diffraction law, the particle size of SiO2 is related to the photonic bandgap of photonic crystals; photonic bandgaps produced iridescent structural colors [10, 34]. To obtain vivid color from colloid assembly, it is important to control the size of colloidal particles. Firstly, SiO2 nanospheres with different particle sizes were prepared using the improved Stöber method. The reaction mainly includes hydrolysis and polycondensation. The amount of TEOS (3 mL) and ammonia (4 mL) is fixed. By controlling the concentration of ammonia and rate of TEOS hydrolysis, SiO2 nanospheres with uniform size and different particle sizes were obtained. The specific reagents used in the reaction are shown in Table 1.
Table 1 Diameters of SiO2 nanospheres
The SEM images of SiO2 nanospheres with different particle sizes are shown in Fig. 1. The particle sizes of obtained SiO2 nanospheres are 221 nm, 245 nm, and 287 nm. SiO2 distribution is uniform, and the particle surface is smooth and flat. The structural unit of APCs was prepared. When the amount of TEOS and ammonia is unchanged, the particle size of SiO2 depends on the ratio of ethanol. As the ratio of ethanol increases, the degree of alcoholysis of SiO2 in ethanol increases, and SiO2 crystal nuclei continue to grow, resulting in SiO2 nanospheres with increased particle size.
3.2 Structure and properties of TiO2-x nanoparticles
3.2.1 Crystal structure and morphology of TiO2-x nanoparticles
Short-range disordered SiO2 APCs have a white appearance and require black additives to absorb incoherent scattered light to eliminate the iridescent effect and achieve a single color that can be used in pigments and inks. As a new black additive, TiO2-x is produced by the vacuum heat treatment of P25 (white TiO2) and NaBH4 in a tube furnace to hypoxia. The morphology and structure of TiO2-x was characterized by SEM, TEM, XRD analysis, and Raman spectroscopy. The SEM and TEM of TiO2-x obtained with different contents of reducing agent are shown in Fig. 2. The particle size of TiO2-x without reducing agent is about 30 nm, and the dispersibility is better (Fig. 2a-c). When 0.5 g of NaBH4 was added for reduction, a part of the particles agglomerated, and the particle size increased. Some particles bonded together to form larger agglomerates. Fig. 3A-C illustrates that before P25 is reduced, the TiO2 nanocrystals exhibit a high degree of crystallinity and good lattice characteristics. Fig. 3-5B testifies that the sample had a P25/NaBH4 mass ratio of 4:0.5 and reacted at 380 °C for 3 h. Samples showed a clear core-shell structure after NaBH4 reduction. A highly crystalline inner core with an amorphous outer shell was observed. Fig. 3-6C shows that when the mass ratio was increased to 4:0.75, the proportion of amorphous shell in the sample in the entire crystal grain increased, and the thickness of shell also increased. As the degree of reduction increased, the inside of TiO2-x lattice fringes became curved. With the thickening of amorphous shell on TiO2 surface, the ordered lattice structure is almost invisible in the TEM image, and the lattice almost disappears.
XRD and Raman spectra of crystal structure are shown in Fig. 3. The XRD spectrum of TiO2-x sintered at 380 °C for different ratios of P25 and NaBH4 is illustrated in Fig. 3a. the diffraction peaks of two samples with a ratio of 4:0 and 4:0.5 appear at 2θ = 25.4°, 37.8°, 47.5°, 53.9°, 54.9°, and 62.4°, corresponding to anatase (101), (004), (200), (105), (211), and (204) crystal faces of mineral TiO2 (JCPDS 21-1272), respectively. When the ratio of P25 to NaBH4 is 4:0.75, the diffraction peak of brookite phase appeared, indicating that excess reducing agent, TiO2 was completely reduced to form Ti2O3 with a lower valence state.
Raman scattering spectrum further illustrates the crystal structure of TiO2-x series samples, as testified in Fig. 3b. Four peaks appeared in the Raman diffraction pattern of TiO2-x, 146, 394, 515, and 645 cm-1, indicating that TiO2-x and P25 have typical anatase phases. With the decrease in the molar ratio of P25 and NaBH4, the degree of reduction of TiO2-x increased. Concentration of oxygen vacancies gradually increased, and the thickness of amorphous shell increased. This is consistent with the phenomenon observed by TEM, indicating that with the increase in the degree of reaction, the concentration of oxygen vacancies and the thickness of amorphous shell both increased. The peak corresponding to Eg shifted to the right, and the phase changed. This also shows that when the molar mass ratio of TiO2 to NaBH4 is 4:0.5, the anatase phase and other phase samples exist.
In a word, as the mass ratio decreases, the degree of TiO2 reduction increases and order of crystal lattice decreases, and the crystallinity decreases. Meanwhile, the thickness of amorphous shell increases and color of TiO2 deepens with the progress in reaction. Black TiO2-x is more conducive to absorbing UV-Vis light and acts as a black additive to absorb incoherent scattered light in APCs.
3.2.2 Photocatalytic activity and enhanced mechanism of TiO2-x nanoparticles
To rapidly and accurately analyze the light absorption mechanism of TiO2-x, UV-Vis diffuse reflectance analysis was performed for TiO2-x samples prepared under different ratios. The results are demonstrated in Fig. 4a. Untreated P25 showed strong absorption of UV light, but weak absorption in the visible-light range. TiO2-x sample prepared by chemical reduction reaction showed not only strong absorption of UV light, but also significantly increased the absorption of visible light, consistent with the results of other studies where it was found that an amorphous shell and Ti3+ enhance the absorbance through a synergistic effect. After improving the experimental method, the absorbance of TiO2-x significantly changed. With the increase in NaBH4, the appearance of TiO2-x gradually darkened (yellow-black blue-black), and the performance shows that TiO2-x powder not only gradually increased the absorption of visible light, but also gradually increased the absorption of light in the NIR region. Especially, When the ratio of TiO2-x and NaBH4 is 4:0.5, the light absorption performance of TiO2-x becomes significantly stronger in the full spectrum and shows the same absorption intensity for UV light, indicating that the reduction of TiO2 significantly improves the absorption for different bands of light.
Calculation and analysis of band gap showed that the reducing agent increased the reduction reaction of TiO2-x, gradually darkening the appearance of catalyst. Meanwhile, the bandgap of catalysts decreased from 3.13 eV to 0.61 eV. The change in band gap shown in Fig. 4b indicates that the material has a corresponding response to the light of different bands. As the band gap becomes smaller, the response of TiO2-x to light gradually becomes stronger. It is very important to acquire vividly visible structural colors.
In this experiment, by sequentially adjusting the ratio of P25 and NaBH4 to 4:0, 4:0.5, and 4:0.75 and reducing in a vacuum tube furnace at 380 °C for 3 h, the colors of prepared TiO2-x changed from white to black. With the increase in the amount of NaBH4, the concentration of reducing hydrogen increases by the thermal decomposition of NaBH4, thereby increasing the concentration of intermediate (reducing hydrogen) required for the reduction reaction. When the concentration of intermediates increases, the reduction of iron dioxide intensifies at the same time, and the reaction degree increases. Thus, increasing the oxygen vacancy of TiO2-x, the oxygen vacancy of TiO2-x increases. Which manifested in continuous color deepening (yellow-black-blue-black) on the macro level (inset of Fig. 4a), and the amorphous shell structure becomes thicker in turn. However, an appropriate oxygen vacancy can promote the separation of electrons and holes, reduce the combination rate of electrons and holes, and improve the utilization rate of light [35], The photocatalytic data are shown in Fig. 4c. TiO2-x shows a relatively strong degradation efficiency of RhB, it is indicated that the appropriate oxygen vacancy concentration can effectively promote photocatalytic reaction in this experiment.
3.3 Morphology and structure of SiO2/TiO2-x APCs
SiO2/TiO2-x APCs with photocatalytic activity and non-iridescent structural colors were prepared from a mixture of TiO2-x with excellent photocatalytic activity and activated 245 nm SiO2 nanospheres. Their microscopic morphology was characterized by SEM, TEM, and EDS analyses, as shown in Fig. 5. Since the activated SiO2 provides a site for TiO2-x to support and avoid agglomeration, it can be clearly seen that Ti, Si and O elements are evenly distributed in the microstructures.
3.4 Non-iridescent structural color analysis of SiO2/TiO2-x APCs
Owing to the presence of incoherent scattering in APCs, the saturation of structural color is low to white. In this study, 10% TiO2-x nanoparticles were added to SiO2 colloidal particles of different diameters and mixed with 10 mL ethanol to form a mixture, which was uniformly dispersed by ultrasonication and dried at 45 °C. Blue, green, and purple structural color materials were obtained, as shown in Fig. 6A-C. The particle diameters of SiO2 colloidal nanospheres are 221 nm, 245 nm, and 287 nm. The colors of SiO2/TiO2-x pigments prepared under different particle diameters of SiO2 nanospheres are quite different, indicating that SiO2 APCs have different colors. This is because the microstructure of APCs is long-range disordered and short-range ordered. Nanospheres act as a structural unit, and the particle size affects the wavelength position of reflection peak, conforming to the Bragg-Snell law: . In addition, when the size of SiO2 nanospheres gradually increases, the color of prepared structural color pigment starts to shift in the direction of long-wavelength light. Fig. 6d shows that with the continuous increase in particle size, the position of reflection peak of powder moves to the long wavelength direction, and its reflection spectrum shows obvious characteristic peaks at 439 nm, 512 nm, and 635 nm. The total amount of reflection on both sides is low, consistent with the color observed with the naked eye. If the amount of TiO2-x added is too low, the color of powder will be uneven and white. Therefore, the effect of 10%, 30%, and 50% TiO2-x addition on the color was evaluated. With the increase in the amount of addition, the color gradually changed to dark green. By observing the morphology with SEM, it was found that with the increase in the amount of TiO2-x, particle aggregation occurred at 50%. However, as shown in Fig. 6e, after SiO2 with a particle size of 245 nm was introduced with different amounts of TiO2, the position of reflection peak did not move. Therefore, the introduction of TiO2-x has slight effect on the tonal change of structural color, but absorbs the incoherent scattering in APCs. Thus, the structural color has no angle dependence, and the saturation of color is improved.
3.5 Photocatalytic activity and enhanced mechanism of SiO2/TiO2-xAPCs
The active sites of photocatalysis are important for the production of redox reaction. A large specific surface area increases the number of active sites. Fig. 7a shows the N2 adsorption-desorption isotherms of prepared TiO2-x and SiO2/TiO2-x. According to the IUPAC classification, all the samples have similar type IV isotherms, and the relative pressure range is 0.7-0.95, producing a H3 hysteresis loop, a characteristic of mesoscopic materials [26]. In the range of (10, 30, and 50 wt%) TiO2-x, the specific surface area increases with the increase in loading, significantly higher than TiO2-x. Therefore, SiO2 provides dispersion sites for TiO2-x, which can effectively increase the specific surface area of photocatalyst, increase the active sites, and promote the transport of photogenerated carriers. The charge separation and transfer behavior of photocatalyst after light excitation can be analyzed by transient photocurrent and impedance measurements. Fig. 7b shows the transient photocurrent response of SiO2/TiO2-x sample. After 30 s of visible-light excitation, all the samples produced a constant current. The current of SiO2/50 wt% TiO2-x is 3.1 µA/cm2, indicating that it has good charge separation efficiency and migration rate, which can be further explored. The semicircle in high-frequency EIS diagram is a characteristic of charge transfer process, and the radius of EIS semicircle is equal to the semiconductor charge transfer resistance. Fig. 7c shows that the surface charge of TiO2-x after chemical reduction is less, almost 5 times smaller than that of P25. Meanwhile, with the increase in TiO2-x content in SiO2, the transfer resistance of SiO2/TiO2-x gradually decreased, indicating that SiO2/TiO2-x APCs can be used as photocatalysts. In summary, structural color and photocatalysis were achieved by integrating the functions of catalyst.
UV-Vis-NIR absorption spectrum is generally used to measure the light utilization rate of a substance. When the response of a substance to light is high, the utilization efficiency of substance to light increases in the photocatalytic reaction. SiO2 has almost no absorption of light, while after adding TiO2-x, SiO2/TiO2-x has strong absorption characteristics in both UV and visible light (Fig. 8a). With the increase of TiO2-x addition, the absorption rate of SiO2/TiO2-x gradually increased. Because a pseudo-band gap exists in the amorphous photonic crystal, its structural color is purple-red relative to 287 nm SiO2/TiO2-x, and double reflection peaks appear in the reflection spectrum. The corresponding reflection wavelengths are 406 nm and 635 nm. The absorption intensity decreased at 406 nm and 635 nm, but peaks appeared at 375 nm and 561 nm due to the edge effect of photonic band gap. Therefore, the introduction of TiO2-x can enhance the light absorption, while the existence of pseudo-band gap of APCs produces a local optical effect and slow photon effect and synergistic enhancement of photon utilization ratio.
Fig. 8b shows the photocatalytic degradation curves of SiO2/TiO2-x prepared with 10%, 30%, and 50% introduction of RhB. The introduction of black TiO2-x into SiO2 provided photocatalytic performance. SiO2/TiO2-x shows amorphous photonic crystals structure color. As the percentage of TiO2-x increased, the rate of photodegradation of pollutants under simulated sunlight gradually increased. The photocatalytic degradation rate constant of SiO2/50 wt% TiO2-x is 0.01363 min-1, which can be attributed to photocatalysis (Fig. 8c). Excellent performance requires sufficient photocatalyst support. Moreover, compared with pure TiO2-x, its photocatalytic degradation rate constant is 0.00775 min-1, smaller than SiO2/50 wt% TiO2-x. Therefore, in this experiment, the prepared SiO2/TiO2-x APCs has a synergistic effect due to the optical localization effect and blue-edge slow photon effect produced by the pseudo-band gap and TiO2-x with a large surface area, thus effectively enhancing the photocatalytic performance by improving the photon absorption.
In summary, we developed a novel catalytic structural color pigment that not only provides vivid physical structural colors, but also eliminates indoor pollutants. Black TiO2-x with full-spectrum absorption prepared by reducing white TiO2 with NaBH4 was used as an additive to prepare noniridescent structural colors, achieving one-step synthesis of dual functional SiO2/TiO2-x APCs. Compared with pure TiO2-x, colored SiO2/TiO2-x APCs exhibited excellent photocatalytic activity. SEM and TEM characterization showed that SiO2 nanospheres promoted the dispersion of TiO2-x nanoparticles and provided active sites for catalytic reactions. DRS spectroscopy showed that the slow-light effect produced by the pseudo-band gap of APCs promotes the photon utilization of catalytic colorant, thereby improving the photoelectrochemical reaction and photocatalytic activity. The hue and color saturation of catalytic color can be controlled by controlling the diameter of SiO2 nanospheres and the ratio of TiO2-x. This provides a basis for the application of catalytic colorants in artworks, home decoration, and wall printing to decorate public places. Compared with traditional chemical colorants, these catalytic structural colors with decorative and photocatalytic functions can effectively satisfy the needs of mental and physical health.
Conflicts of interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
All authors have given approval to the final version of the manuscript. This work was supported by the National Key R&D Program of China (2019YFC1520202) and the Natural Science Foundation of China (No. 51702193, 51502165, 51972201).
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Scheme 1. Schematic of the preparation process of SiO2/TiO2-x APCs
