Noble metal plasmonic resonance has been utilized in optical data storage widely for its excellent photo-transformation efficiency. TiO2 nanoporous films deposited with Ag nanoparticles present outstanding polarization-response and color-modulation ability. However, the low exposure-sensitivity at single wavelength inhibits their application in optical information processing, which is urgent to be improved by innovative methods. Here, we report that Ag nanoparticles were deposited efficiently via continuous laser irradiation in the TiO2 nanoporous film treated by tannic acid, presenting high-efficient monochromic absorption property. As a result, two sets of holograms were recorded sequentially at the same point of the film with orthogonal circular polarization configurations. The colored reconstruction of the mixed holograms was achieved by utilizing laser polarization state as chrominance segmentation channel. Our method provides a distinctive route for enhancing the photo-energy conversion efficiency of plasmonic nanoparticles, and paves a way to develop advanced display device.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Conversion of photo-energy has caused significant attention in the areas of photo-catalysis [1, 2], solar cells [3, 4], biotechnology [5, 6] and data storage [7, 8]. The enhancement of rate and efficiency in transformation between photon and electron is always the hot-issue which can be resolved by optimizing the absorption performance of recording media. Noble-metal nanoparticles (NPs) have unique electronic and optical properties which can be used as high-efficient photo-conversion media due to localized surface plasmon resonance (LSPR) [9, 10]. The photo-generated electrons can be transferred in a nanocomposite system consisted with the noble-metal NPs and semiconductor, which serves for high-performance nano-devices [11, 12].
Ag/TiO2 nanocomposite films have ability of presenting the same color as that of the irradiation light . The photochromic behavior of the film is related to the changes of the size and shape of Ag NPs based on LSPR [13–15]. Ag NPs are dissolved directionally to Ag+ ions during the resonant polarized-light irradiation. The absorption of the film around the excitation wavelength decreases after monochromic exposure, resulting in “spectral-hole burning”. The LSPR absorption band can be restored with UV excitation [16–18]. Thus the optical information can be stored and recycled in the photochromic film via the above photo-chemical reaction.
However, the rate and efficiency of the reaction rely on the uniformity and density of Ag NPs for LSPR, respectively. Under monochromic light irradiation, the Ag NPs with different morphologies have poor dissolution ability interfacing with TiO2. Although the methods of UV-reduction, mesoporous-template and sol-gel synthesis have been exploited [19–23], uniform sized noble metal NPs are still difficult to be obtained. In fact, efficient fabrication of the uniform Ag NPs inside nanoporous TiO2 and precise modulation of LSPR absorption are crucial for the plasmonic application in photo-energy conversion.
Very recently, we reported a simple method to achieve the uniform and small-sized Ag NPs in TiO2 nanoporous films by the thermal-reduction pretreated with tannic acid for blu-ray holographic storage . However, the reduction time is rather long. To resolve this problem, a propulsive work is reported here that visible laser was used to stimulate tannic acid for accelerating the reduction of Ag NPs. Lorentz model was adopted to characterize the LSPR absorption. Benefiting from the narrow-band optical absorption of the sample, multiplexed holograms were recorded at 473 nm in the film efficiently, and reconstructed independently by using polarization channels to load different color information based on polarization holography technique.
2. Fabrication and characterization of sample
2.1 Photochromic film preparation
TiO2 nanoporous film was fabricated on a glass substrate by dip-coating from the titania sol which was reported on our previous work . Tannic acid (TA) was used as an electron donor in reduction of Ag+ ions. The TA solution with the concentration of 0.002 mol/L and potassium carbonate solution with the concentration of 0.009 mol/L were mixed to the PH value of 8.5. Subsequently, the nanoporous film was immersed in the TA solution for 2 hours and turned to be yellow. The TA-adsorbed TiO2 was then immersed to the solution of 0.1 M silver nitrate (AgNO3) mixed with ethanol of equal volume for 10 minutes at 30°C, followed by the irradiation of the single-longitudinal mode and continuous laser (Changchun New Industries Optoelectronics Tech. Co. Ltd.), as shown in Fig. 1. The sample prepared by the co-action of TA and laser is abbreviated as STA + laser. For comparison, Ag NPs were also deposited in the nanoporous TiO2 film via the sole TA (STA) or laser (Slaser) reduction. The optical properties of the samples were characterized by UV-Vis spectrophotometer (UV-2600).
2.2 Photoelectrochemical experiment process
Photoelectrochemical measurements were conducted with PARSTAT 2273 potentiostat in a typical three-electrode configuration using the prepared samples on FTO as working electrode, an Ag/AgCl reference electrode and the home-made Pt black as the counter electrode. 0.5 M Na2SO4 aqueous was used as electrolyte (pH = 5.8). The illumination was provided by a solar simulator (Newport 370-RC), of which light intensity was adjusted to 100 mW/cm2, with a 450 nm filter.
2.3 Optical setup
The diffraction grating was recorded with two coherent s-polarized beams from a 473 nm laser with the power of 8 mW. A red laser generating 671 nm s-polarized light, was used as a probe source to monitor the holographic grating dynamics. The power of the 671 nm laser was set as 500 μW to minimize the destructive effect of readout radiation which in principle leads to photochemical reactions. The first-order diffracted signal was registered on a photo-diode interfaced with a computer. Diffraction efficiency of holographic gratings, taking Fresnel losses into account, can be calculated accordingly, which is defined as the ratio between intensities of the first-order diffracted beam and the incident beam after passing through the sample [26–28]. Besides, two sets of holograms were recorded at 473 nm sequentially by introducing beam-expander system under orthogonal circular-polarization mode. Quarter-wave plate was used to change the polarization states to right circular polarization (RCP) or left circular polarization (LCP) for the object and reference beams. The reconstructed colored holographic images were collected by a CMOS video camera. Red (671 nm) and green (532 nm) lasers are used as probe sources to read the mixed holographic images. The sketch map for optical setup is shown in the Fig. 2.
3. Spectral regulation
Figure 3(a) shows the differential absorption spectra of STA with reduction times of 10, 40 and 70 min. After extending a long reduction time by TA thermal treatment, the maximum value of differential absorbance can be increased from 0.34 to 1.02. Figure 3(b) shows the differential absorption spectra of Slaser with reduction times of 5, 10 and 15 min. The maximum value of the differential absorbance of 0.96 appears at the wavelength of 461 nm under the sole laser action for a quarter of an hour. Unfortunately, the Ag NPs deposited by the sole laser excitation present a rather wide LSPR band, indicating that the size distribution of the metal NPs is heterogeneous. Figure 3(c) shows the differential absorption spectra of STA + laser, for different laser irradiation times of 1, 3, 5, 7, 10 and 15 min. The maximum value of the LSPR absorbance at 425 nm can be increased significantly from 1.11 to 2.82. Commonly, the profile of absorption band is related to size homogeneity and population density of Ag NPs [29, 30]. The absorption spectra of the samples synthesized by the above three methods can be fitted by Lorentz model , which is expressed as followed:Figs. 3(a)-3(c) (solid lines). Here the maximum absorption intensity (Ac) and the peak width at half height (W) are related to the population density and size uniformity of Ag NPs, respectively. Thus the ratio R = Ac/W may characterize the plasmonic absorption band, which are gathered in Fig. 3(d). STA + laser has the outstanding LSPR property for monochromatic absorption, compared with the other two samples. The ratio R for STA is uphill to reach only the half value for STA + laser even after a long reduction time. The ratio R for Slaser is still the lowest one reaching a quarter of the value for STA + laser, even after extending the reduction time.
Figure 4(a) shows the top-view of SEM image for STA. A small amount of Ag NPs with spherical shape were observed on the surface of TiO2. The cross-sectional SEM image of STA is shown in the inset of Fig. 4(a). Cumulative volume fraction shows the Ag NPs (<20 nm) occupied a volume fraction of ~95%. The top-view of SEM image for Slaser is shown in Fig. 4(d). The Ag NPs with various morphologies appear on the surface of TiO2. A broad distribution of lateral diameter from 13 nm to 58 nm was obtained as illustrated in Fig. 4(e). The cross-sectional image of Slaser is shown in the inset of Fig. 4(d). The top-view of SEM image of STA + laser is shown in Fig. 4(g). The regular and spherical Ag NPs on the surface of TiO2 are well formed. The size distribution of the Ag NPs deposited on the TiO2 film varies from 8 nm to 24 nm as illustrated in Fig. 4(h). The cross-sectional EDXA of STA, Slaser and STA + laser are shown in Figs. 4(c), 4(f) and 4(i), respectively. Accordingly, the molar ratios of Ag are estimated via EDAX to be 7.32%, 16.39% and 17.15% relative to the sum of Ti and O for STA, Slaser and STA + laser, respectively. The mean size of the Ag NPs is ~14.8 nm for STA, ~15.6 nm for Slaser, ~24.8 nm for STA + laser. The concentration of Ag NPs is ~7.00 × 109/cm2 for STA, ~1.90 × 1010/cm2 for Slaser, ~2.50 × 1010/cm2 for STA + laser. Correlating to the spectral analysis mentioned above, we conclude that it is easier to achieve high-concentration and uniform Ag NPs for STA + laser than that for the other two kinds of samples.
The efficient Ag NPs growth for STA + laser may benefit from the sufficient electron supplement for Ag+ ions from the co-action of TA and laser. TA, as a plant polyphenol, plays a role of electron donor center and can be absorbed to TiO2 due to its phenolic hydroxyl groups [24, 32, 33]. The TA-reduced Ag NPs act as dispersed nucleation centers. Ag+ ions are easier to be reduced around these synthesis-sites. The further laser irradiation provides more electron-supplying routes, such as TiO2 →Ag+, TA→Ag+, TA→TiO2 →Ag+. Figure 5(a) describes the UV-Vis absorption spectra of the pure and the TA-adsorbed TiO2 films. It was found that the pure TiO2 film has the absorbance extending to near-UV region, which may be related to the destroyed periodic potential fields . While, the differential absorption spectrum of the TA molecules (inset of Fig. 5(a)) presents a wide absorption band from UV to visible region. That means both of TiO2 and TA can be excited by the near-UV laser, providing electrons for Ag NP growth. However, the excitation of TiO2 by the 405 nm laser results in the reduction of Ag+ ions at random spatial sites which weakens the uniformity of NP size and increases the absorbance at longer wavelength . Hence, the laser co-deposition with the sole service for TA-reduction is necessary to suppress the photo-excitation of the metal oxide semiconductor and also inhibit the free growth of plasmonic NPs. Accordingly other visible lasers of 457 nm, 532 nm and 671 nm were introduced into the photochemical reduction system. The differential absorption spectra of STA + laser by these lasers are shown in Fig. 5(b). Lorentz model was also applied and the ratio R versus laser wavelength is shown in the inset part of Fig. 5(b). After these visible lasers treatment, the metal LSPR absorption band was further shaped. The highest value of R = 0.017 appears for the green (532nm) laser co-deposition. In fact, these visible lasers not only construct electron-supplying path, but also plays a role of photo-dissolution for the size tailoring of Ag NPs. Resultantly, the LSPR absorbance longer than 600 nm was inhibited.
To reveal the mechanism of laser co-deposition of metal NPs in the semiconductor matrix, linear sweep voltammetry for TiO2 and TA was conducted with the light source of 450 nm, as shown in Fig. 6(a). Dark-current for the pure and the TA-adsorbed TiO2 electrodes could be neglected. The photo-current intensity of TA-absorbed TiO2 electrode is enhanced obviously compared to that of TiO2 electrode, confirming the transformation of photo-electron from TA to TiO2 under the visible irradiation. Accordingly, the schematic diagram of energy levels is shown in Fig. 6(b). Under the visible irradiation, the electrons of TiO2 are hardly excited from valence band to conduction band. Thus the released electrons for reduction of Ag+ ions can only come from TA molecules. In fact, the visible laser beam can pass through the TiO2 film and play a role in the widespread TA to accelerate the production of reducing electrons. Furthermore, the electron transformation from TA to TiO2 can also increase the reducing sites and provide a possible reduction channel for Ag+ ions. Thus the co-action of the visible laser and TA molecules helps to weaken the excessive growth of the NPs in the surface layer and maintain the loose nanohole structure which tends to be blocked by single laser action.
4. Holographic Dynamics
Holographic recording as an effective method can manifest the ability of photo-energy transformation of the optical absorption medium. Figure 7 shows the blue-light written holographic kinetics for STA + laser, STA and Slaser. The grating formation based on photochromism of Ag/TiO2 films has been discussed in our previous works [14, 17]. The interferogram consisting of alternate bright and dark stripes was formed at the intersection point by two coherent writing beams. The interference pattern may be replicated to the Ag/TiO2 film based on photo-dissolution of Ag NPs. In the area of laser irradiation, Ag NPs is dissolved, releasing free electron and Ag+ ions. The free electrons can be captured by oxygen molecules in the air resulting in appearance of oxygen ions. Further, Ag+ ions combined with oxygen ions to form stable Ag2O. The spatial alternated arrangement of Ag NPs and Ag2O can form amplitude- or phase-type hologram. The diffraction efficiency of STA + laser increases exponentially versus time, to a maximum value of 1.8% at 450 s and keeps constant. The diffraction efficiency of the STA with reduction time of 70 min crosses a maximum value at 48 s, followed by a slight decline and a secondary incensement after 330 s. While Slaser presents a similar holographic dynamic behavior to that for STA, but with the poorest information-recording ability. Our previous works have pointed out that multistage growth for the holographic grating in Ag/TiO2 films results from the photo-response of the metal NPs with different sizes . Only a system consisted with high-population and size-uniform NPs can achieve a fast and single exponential grating growth.
5. Colored Holographic Reconstruction
The high-efficiency information recording in STA + laser provides possibility for multiple hologram writing and reconstruction. Moreover, with the help of the high polarization sensitivity of photo-dissolution of Ag NPs, a pure polarization hologram can be achieved in the system consisting of noble metal NPs and metal oxides after orthogonal circular polarization holographic recording . For the pure vector hologram, the diffraction light intensity is close to the polarization state of the reading beam. Thus dividing color information via polarization channels can realize the colored holographic image reconstruction. Here, the blue laser beam (473 nm, 8mW) was still used as writing source, as shown in Fig. 8(a). The object beam of RCP with a “flower” shape was overlapped with the reference beam of LCP at the same point in the sample of STA + laser for 600 s, forming the first hologram. And then alternating the polarization states of the object and reference beams, and a “leaf” shaped information was loaded at the same point to the former hologram for 100 s. The equivalent diffraction intensities for the two holograms were obtained. Figure 8(b) shows the reconstruction of colored holograms with red and green laser beams (671 nm and 532 nm, respectively; both 1 mW). The incident angle of the reading beams was adjusted carefully. The polarization states of the red and green laser beams were set as LCP and RCP, respectively. A red flower with green leaf was clearly reconstructed with little crosstalk, which was captured by the COMS camera, as shown in Fig. 8(c).
Compared to UV-reduction, more nucleation sites of Ag NPs were achieved by the visible laser-assisted reduction. As a result, an excellent plasmonic absorption for monochromatic light was obtained. Further, the efficiency of holographic storage at single wavelength was enhanced, which provides the feasibility for multiplexed holographic storage. However, this method has limitations in spectra regulation at other wavelengths, which need to be improved by introducing multi-source excitation. The optical medium prepared by this method can also be applied to the laser battery in wireless-power-transmission system, low-level-light detector and smart nano-template.
TiO2 nanoporous films were loaded with high-density and uniform-size Ag NPs by the co-action of TA and laser efficiently, which presents outstanding photo-electron conversion ability. The visible continuous laser irradiation is able to accelerate the release of electrons from TA in reduction of Ag NPs. The LSPR absorption ability of the nanocomposite film was further enhanced by adjusting the wavelength of the exciting laser. Based on such properties, the efficiency of information storage in the laser co-synthesized film was improved significantly compared to that by the traditional method. Mixed polarization holograms were recorded at single-wavelength under orthogonal circular polarization configuration. Color holographic reconstruction was also achieved in diffraction of the green and red lights with orthogonal circular polarization states. This work provides a good research strategy for photo-energy conversion at single wavelength and advanced display.
The National Natural Science Foundation of China (10974027, 31271442, 51372036, 61007006), the 111 project (B13013), and the Fundamental Research Funds for the Central Universities (2412017FZ011).
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