The Development of New Catalytic Pigments Based on SiO2 Amorphous Photonic Crystals via Adding of Dual-Functional Black TiO2-x Nanoparticles.

Biomimetic synthesis of amorphous photonic crystals (APCs) is an effective approach to obtaining 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 a novel method to combine APCs with black TiO2-x to construct a noniridescent structural color pigments with high visibility and photocatalytic activity. Due to the absorption of incoherently 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 the slow light effect generated from the pseudoband gap of SiO2 APCs with TiO2-x absorbed full spectrum. The tone and color saturation of catalytic pigments is controlled by the diameter of SiO2 nanospheres and the ratio of TiO2-x nanoparticles, which provides a controllable application study in color-related fields as artwork, environmental coatings, and textiles.

Introduction

Color is an essential form of expression passed down through social civilization by integrating art and reality and expressing the complex emotions of human beings.[1,2] The human fascination with and pursuit of color never stop. From Dunhuang murals[3] to modern and fashionable decorative pigments,[4,5] humans continue to improve the color and visual effects of synthetic dyes. To find inspiration for synthetic pigments, people have paid attention to the brightly colored morpho butterfly,[6] peacock feathers,[7] and chameleons[8] that can switch colors instantly, which also inspired scientists to study the physics behind the appearance of colors. The sparkle of butterflies comes from the regular arrangement of scale cells. Bright color switching of the chameleon is caused by the combination of transparent guanine nanocrystals and melanosomes in the inner layer of the skin.[8−10] These submicron-scale materials are regularly arranged in a specific structure in space, and diffract, interfere, and scatter light at a particular wavelength, thus producing a structural color that will never fade.[10−13]

Structural color based on photonic crystals has bright color and an iridescent effect.[14,15] The angle-independent structural color prepared by the doping of melanin with amorphous photonic crystals (APCs) has been widely studied by scientists due to their soft colors and their ability to meet the visual needs of human beings.[16−18] The short-disordered APC structure produces incoherent scattering of light, which makes the color of the material white.[19] The current researchers introduced nanoscale carbon material (carbon, nanotube, acetylene black, and graphene) into the APC structure to absorb coherently scattered light or heat treatment in the presence of carbon in the reducing atmosphere to obtain noniridescent structural color when synthesizing photonic crystal structure units.[20−23] Pastel colors and timeless non-iridescent structural colors can replace traditional chemical colors in use. However, in the application process of home decoration coating, pigment, and other color decoration fields, it can still be used to reduce indoor environmental pollution.[24−28]

The well-known environmentally friendly material titanium dioxide (TiO2) has excellent photocatalytic properties. Still, the white TiO2 can only be excited to produce redox ability under ultraviolet light, which accounts for 5% of the light.[29−32] Therefore, the preparation of black TiO2- capable of full-spectrum absorption has become a research focus.[33,34] Hydrogen heat treatment of TiO2, Al/Zn reduction of P25, hydrogen plasma reduction of TiO2 nanotubes, and other preparation methods require high temperature and high pressure or are time-consuming, so the chemical reduction method is used to prepare TiO2-.[35−37] Black TiO2- particles effectively replace carbon black material and absorb coherently scattered light. Meanwhile, SiO2 as a structural unit of APCs can effectively increase the specific surface area of TiO2 to provide redox active sites. The black TiO2- has enhanced light absorption efficiency. The synergistic action of both can effectively promote the separation of electrons and holes and significantly enhance the photocatalytic effect on pollutants.[38−44]

Herein, black TiO2-, with excellent photocatalytic performance prepared after P25 was thermally reduced by NaBH4, was used as a black additive to reduce incoherent scattering in APCs. A certain amount of TiO2- was added to the substrate by blending. In ethanol-activated SiO2 nanospheres, TiO2- particles are distributed on the surface of the SiO2 nanospheres after stirring for a certain period time at room temperature. After drying in a constant temperature drying oven at 50 °C, SiO2/TiO2- APCs with photocatalytic properties and non-iridescent structural color can be obtained. The preparation process is indicated in the schematic diagram (Scheme ). Different colors can be adjusted by controlling the diameter of the SiO2 nanospheres, and the color saturation can be governed by the amount of TiO2- added. The concentration of TiO2- is related to photocatalytic performance. Photocatalytic pigments have both decorative and environmental purification functions, which can effectively degrade the refractory toxic organic compounds produced in the color coatings decoration process. Therefore, the introduction of black TiO2- not only provides a direction for the functionalization of structural colors but also improves photocatalytic activity.

Scheme 1

Schematic Illustration of Preparation Strategy for SiO2/TiO2- APCs Catalytic Pigments

Results and Discussion

Morphology and Structure of SiO2/TiO2- APCs

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.[10,45] To obtain vivid color from the colloid assembly, it is essential to control the size of colloidal particles. First, 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. The specific reagents used in the reaction are shown in Table . By controlling the concentration of ammonia and rate of TEOS hydrolysis, SiO2 nanospheres with uniform size and different particle sizes were obtained.

Table 1

Size of SiO2 Nanospheres

Sample	NH3·H2O	C2H5OH	TEOS	C2H5OH	Particle Size
1	4 mL	49 mL	3 mL	1 mL	221 nm
2	4 mL	45 mL	3 mL	5 mL	245 nm
3	4 mL	41 mL	3 mL	9 mL	287 nm

The SEM images of SiO2 nanospheres with different particle sizes are shown in Figure a–c. The particle sizes of the obtained SiO2 nanospheres are 221, 245, and 287 nm, respectively. SiO2 distribution is uniform, and the particle surface is smooth and flat. The structural unit of APCs was prepared. When the amounts of TEOS and ammonia are unchanged, the particle size of SiO2 depends on the ratio of ethanol. As the proportion of ethanol increases, the degree of alcoholysis for SiO2 in ethanol increases, and SiO2 crystal nuclei continue to grow, resulting in SiO2 nanospheres with increased particle size. Short-range disordered SiO2 APCs have a white appearance. They require black additives to absorb incoherently scattered light to eliminate the iridescent effect and achieve a single color used in pigments and inks.

Figure 1

SEM images of SiO2 nanospheres with different particle sizes (a–c). SEM (d–f) and TEM (g–i) images of TiO2- nanoparticles for different reductants (TiO2:NaBH4 = 4:0, 4:0.5, 4:0.75).

As a new black additive, TiO2- 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- were characterized by SEM, TEM, XRD, and Raman spectroscopy. The SEM and TEM of TiO2- obtained with different contents of reducing agent are shown in Figure d–i. The particle size of TiO2- without reducing agent is about 30 nm, and the dispersibility is better (Figure d–f). When 0.5 g of NaBH4 was added for reduction, a portion of TiO2 particles agglomerated, and the particle size increased. Some particles bonded together to form larger agglomerates. Figure g–i illustrates that before P25 is reduced, the TiO2 nanocrystals exhibit a high degree of crystallinity and good lattice characteristics. Figure h 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. Figure i indicates 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- lattice fringes became curved. With the thickening of the amorphous shell on the TiO2 surface, the ordered lattice structure is almost invisible in the TEM image, and the lattice almost disappears.

The XRD and Raman spectra of crystal structure are shown in Figure . The XRD spectrum of TiO2- sintered at 380 °C at different ratios of P25 and NaBH4 is illustrated in Figure b. At 380 °C, 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 the brookite phase appeared, indicating excess reducing agent at this time; TiO2 was completely reduced to form Ti2O3 with a lower valence state. The Raman scattering spectrum characterization further illustrates the crystal structure of TiO2- series samples, as testified in Figure c. Four peaks appeared in the Raman diffraction pattern of colored TiO2, 146, 394, 515, and 645 cm–1, indicating that TiO2- and P25 have typical anatase phases. With the decrease in the molar ratio of P25 and NaBH4, the degree of reduction of TiO2- increased. The 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 increased.

Figure 2

Macroscopic display of series samples (a). XRD patterns of TiO2- nanoparticles with different content of reductants (b). Raman diffraction spectrum of TiO2- (c). TiO2- samples and absorption spectra of reductants with different contents (d). Curve of (Ahv)1/2–hv (e). Photocatalytic properties of TiO2- obtained with different reducing agents (f).

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 the amorphous shell increased. The peak corresponding to Eg shifted to the right and the phase changed, which shows that 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, the order of crystal lattice decreases, and the crystallinity decreases. Meanwhile, the thickness of the amorphous shell increases, and the color of TiO2 deepens with the progress in reaction. Black TiO2- is more conducive to absorbing UV–vis light and acts as a black additive to absorb incoherent scattered light in APCs.

To rapidly and accurately analyze the light absorption mechanism of TiO2-, UV–vis diffuse reflectance analysis was performed for TiO2- samples prepared under different ratios. The results are demonstrated in Figure d. Untreated P25 showed strong absorption of UV light but weak absorption in the visible light range. TiO2- samples prepared by chemical reduction reaction not only showed 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- significantly changed. With the increase in NaBH4, the appearance of TiO2- gradually darkened (yellow-black blue-black), and the performance shows that TiO2- powder not only gradually increased the absorption of visible light but also gradually increased the absorption of light in the near-infrared region. Especially, when the ratio of TiO2- and NaBH4 was 4:0.5, the light absorption performance of TiO2- became significantly stronger in the full spectrum and showed the same absorption intensity for UV light, indicating that the reduction of TiO2 substantially improved the material’s absorption for different bands of light.

Calculation and analysis of bandgap showed that the reducing agent increased the reduction reaction of TiO2-, gradually darkening the appearance of catalyst. Meanwhile, the bandgap of catalysts decreased from 3.13 to 1.59 eV. The change in bandgap shown in Figure e indicates that the pigments have a corresponding response to the light of different bands. As the bandgap becomes smaller, the response of TiO2- to light gradually becomes stronger. It is imperative 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- changed from white to black. This is because 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 simultaneously, and the reaction degree increases, increasing the oxygen vacancy of TiO2-. As a result, the oxygen vacancy of TiO2- increases. This is manifested in continuous color deepening (yellow–black–blue–black) on the macro level, and in turn the amorphous shell structure becomes thicker. Microscopically, more oxygen vacancies become available. 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.[46] The photocatalytic data are shown in Figure f. TiO2- shows a relatively strong degradation efficiency of RhB. Moderate reduction of NaBH4 is beneficial to the improvement of photocatalytic efficiency.

Noniridescent Structural Color Analysis of SiO2/TiO2- APCs

SiO2/TiO2- APCs with photocatalytic activity and noniridescent structural colors were prepared from a mixture of TiO2- with excellent photocatalytic activity and activated 245 nm SiO2 nanospheres. Their microscopic morphology was characterized by SEM, TEM, and EDS analyses, as shown in Figure a–d. TiO2- nanoparticles are effectively loaded on the surface of SiO2 and absorb the incoherent scattering in SiO2 APCs, so that the non-iridescent structural colors generated by the pseudogaps of SiO2/TiO2- APCs appear bright. Owing to the presence of incoherent scattering in APCs, the saturation of structural color is low. In this study, 10% TiO2- nanoparticles were added to SiO2 colloidal particles 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 Figure e. The diameters of SiO2 colloidal nanospheres are 221, 245, and 287 nm. The colors of SiO2/TiO2- 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.

Figure 3

SEM of SiO2 and SiO2/TiO2- (a,b). TEM of SiO2/TiO2- (c). EDS element analysis (d). Photos of SiO2/TiO2- structural color with different particle sizes of SiO2 (e), illustration of SiO2 APCs and SiO2/TiO2- APCs in (e). SEM of SiO2/TiO2- (10%; (b): 30%; (c): 50%); the experimental sample is shown in the illustration (g). Reflection spectra of SiO2/TiO2- APCs (f and h).

Nanospheres act as a structural unit, and the particle size affects the wavelength position of reflection peak, conforming to the Bragg–Snell law.[25] 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. Figure f 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, 512, 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- added is too low, the color of powder will be uneven and white. Therefore, the effect of 10%, 30%, and 50% TiO2- 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-, particle aggregation occurred at 50% (Figure g). However, as shown in Figure h, after SiO2 with a particle size of 245 nm was introduced with different amounts of TiO2, the position of the reflection peak did not move. Thirty nanometer TiO2- doping in the gap of 287 nm SiO2 APCs does not affect the tone of the structure color, and it absorbs the incoherent scattering in APCs. 50% TiO2- doping level slightly reduces the reflectance of the structure color. Therefore, the addition amount of TiO2- can also be used to adjust the color saturation, increasing the application range of catalytic pigments.

Photocatalytic Activity and Enhanced Mechanism of SiO2/TiO2- APCs

The active sites of photocatalytic materials are essential for the production of redox reactions. The large specific surface area increases the number of active sites. Figure a shows the N2 adsorption–desorption isotherms of prepared TiO2- and SiO2/TiO2-. 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-, the specific surface area increases with the increase in loading, significantly higher than TiO2-. SiO2 provides dispersion sites for TiO2-, which can effectively increase the specific surface area of photocatalyst to increase the active site of photocatalytic reaction, and to facilitate the transmission of photogenerated carriers. The charge separation and transfer behavior of photocatalyst after light excitation can be analyzed by transient photocurrent and impedance measurements. Figure b shows the transient photocurrent response of the SiO2/TiO2- sample. After 30 s of visible-light excitation, all the samples produced a constant current. The current of SiO2/50 wt % TiO2- is 6.1 μA/cm2, indicating that it has good charge separation efficiency and migration rate, which can be further explored. The semicircle in a high-frequency EIS diagram is a characteristic of the charge transfer process. The radius of the EIS semicircle is equal to the semiconductor charge transfer resistance. Figure c shows that the surface charge of TiO2- after chemical reduction is lower, almost five times smaller than that of P25. Meanwhile, with the increase in TiO2- content in SiO2, the transfer resistance of SiO2/TiO2- gradually decreased, indicating that SiO2/TiO2- APCs can be used as photocatalysts. In summary, structural color and photocatalysis were achieved by integrating the functions of pigments. The UV–vis-NIR absorption spectrum is generally used to measure the light utilization rate of a substance. When the response to light is high, the utilization efficiency of the substance to light increases in the photocatalytic reaction. SiO2 has almost no absorption of light, while after adding TiO2-, SiO2/TiO2- has strong absorption characteristics in both UV and visible light (Figure d). With the increase of TiO2- addition, the absorption rate of SiO2/TiO2- 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-, and double reflection peaks appear in the reflection spectrum. The corresponding reflection wavelengths are 406 and 635 nm. The absorption intensity decreased at 406 and 635 nm, but peaks appeared at 375 and 561 nm, caused by the edge effect of the photonic bandgap. Therefore, the introduction of TiO2- can enhance the light absorption. At the same time, the existence of pseudogaps of APCs produces a local optical effect and slow photon effect and synergistic enhancement of photon utilization ratio. Figure e illustrates the photocatalytic degradation curves of SiO2/TiO2- prepared with 10%, 30%, and 50% introduction of RhB.

Figure 4

Nitrogen adsorption–desorption isotherm (a). Transient photocurrent responses (b). Nyquist plots of SiO2/TiO2- sample (c). UV–vis absorption spectra (d). Photodegradation of SiO2/TiO2- APCs (e,f).

The introduction of black TiO2- into SiO2 provided photocatalytic performance. SiO2 APCs have no photocatalytic activity due to full transmittance to the light structure color. As the percentage of TiO2- increased, the rate of photodegradation of pollutants under simulated sunlight gradually increased. The photocatalytic degradation rate of SiO2/50 wt % TiO2- is 0.03 min–1 (Figure f); compared with pure TiO2-, its photocatalytic degradation rate constant is 0.00775 min–1, smaller than that for SiO2/50 wt % TiO2-. As demonstrated in Scheme , the pseudogaps of APCs display the structural color, and the light localization effect and slow-light effect of pseudogaps promote photon absorption. Therefore, in this experiment, SiO2/TiO2- APCs have a synergistic catalytic effect due to the optical localization effect and slow-light effect produced by the pseudogaps, and TiO2- with a large surface area, effectively enhancing the photocatalytic performance by improving the photon absorption.

Scheme 2

Schematic Diagram of the Enhancement Mechanism for Structural Color and Photocatalytic Performance of SiO2/TiO2- APCs under Visible Light

Conclusion

By simulating the color rendering mechanism of the chameleon and promoting its functional application, we developed novel catalytic structural color pigments that not only provide vividly physical structural colors but also eliminate indoor pollutants. Black TiO2- 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- APCs. Compared with TiO2-, SiO2/TiO2- APCs exhibited excellent photocatalytic activity. TEM characterization showed that SiO2 nanospheres promoted the dispersion of TiO2- nanoparticles and provided active sites for catalytic reactions. DRS spectroscopy demonstrated that the slow-light effect produced by the pseudogaps of APCs promotes the photon utilization of catalytic pigment, improving the photoelectrochemical reaction and photocatalytic activity. Tone and color saturation of catalytic pigments can be controlled by adjusting the diameter of SiO2 nanospheres and the ratio of TiO2-. This provides a basis for applying catalytic pigments in artwork, home decoration, and wall printing to decorate public places. Compared with traditional chemical pigments, these catalytic structural colors with decorative and photocatalytic functions can effectively satisfy the needs of mental and physical health.

Experimental Methods

Fabrication of Dual-Functional SiO2/TiO2- APCs

The detailed information on the reagents and characterization used in this study is listed as follows. Monodispersed SiO2 was synthesized by the modified Stöber method.[47] Appropriate amounts of ethanol and tetraethoxysiliane (TEOS, 3 mL) were poured into the mixture of ethanol and ammonia (4 mL), and the solution was magnetically stirred at 40 °C to mix well. The homogeneous solution of ethanol and TEOS was added dropwise to the mixture of ethanol and ammonium hydroxide (NH3·H2O, 4 mL), 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 to obtain white SiO2 nanospheres. Second, the chemical reduction method was applied to prepare black TiO2-. P25 (4 g) and NaBH4 were mixed for 30 min at room temperature. The mixture was placed into a combustion boat, the reduction reaction was carried out in a vacuum tube furnace, and the temperature was increased to 380 °C at a rate of 7 C/min for 2 h. A series of samples are obtained by controlling different molar ratios of P25 and NaBH4 (4:0, 4:0.5, 4:0.75). The sample was washed with water to neutrality to remove unreacted NaBH4, and then dried in an oven at 45 °C to obtain powdered black TiO2-. Finally, specific amounts of black TiO2- nanoparticles were added to the SiO2 ethanol dispersion solution. A 50 mL dispersion was mixed by ultrasound for 4 h and stirred at room temperature for 7 h. Anhydrous ethanol and deionized water were centrifuged and dried in an oven at 50 °C to obtain SiO2/TiO2- APCs with excellent photocatalytic activity and non-iridescent structural color.

Characterization and Photocatalytic Property Measurement of SiO2/TiO2- APCs

The surface morphology of SiO2/TiO2- APCs was observed by using field emission scanning electron microscopy (FESEM) (Hitachi FE-S4800). The reflection spectra and absorption spectra of SiO2/TiO2- APCs pigments were measured by using a Cary 5000 UV–vis-NIR spectrometer (Agilen). The interplanar crystal spacing and crystalline structure of TiO2- nanoparticles were imaged by transmission electron microscopy (TEM, FEI Tecnai G2F20 S-TWIN) and X-ray diffraction (XRE; D/, ax-2200PC). The Raman scatting spectra of TiO2- were acquired with Renishaw-invia equipped with 532 nm red laser and CCD detector. The photocatalytic activity and photoelectrochemical analysis illustrate the ability to degrade pollutants. In the process of photocatalytic activity, 0.05 g SiO2/TiO2- APCs catalytic pigments were added into 50 mL RhB solution (5 × 10–6 M) each time, which is probed by visible light irradiation from a BL-GHX-V photocatalytic reactor connected with a 500 W xenon lamp. Photoelectrochemical measurement was performed on an electrochemical workstation (CHI760E) using a normative three-electrode system, equipped with the Ag/AgCl electrode and platinum plate as the reference electrode and counter electrode, and the prepared photocatalysts as the working electrode. Two milligrams of photocatalyst 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.