Improved photocatalytic activity of multicompound inverse opal structures
Pham Dinh Khai
MASTER’S THESIS Material Chemistry
International Master’s Program for Research Chemists
638/2019
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Universal Studies on Chemistry
Improved photocatalytic activity of multicompound inverse opal structures University of Eastern Finland, Department of Chemistry
Supervisors: Associate professor Jarkko Saarinen Joensuu 30.11.2019
Abstract
Recently, inverse opal materials have been widely studied due to their special structural characteristics. Inverse opal structure has high peridiocally macropores with large surface area and well interconected pores that make them ideal catalyst materials with a higher electron transmission rate than opal structure. It can also improve light absorption and photochemical properties of the prepared material. With that being said, TiO2 inverse opals (IOs) is the most well-known in catalytic process as well as organic waste treatment field. Many studies on materials with better photocatalytic performance than TiO2 have been conducted, but most of them are only active under UV light.
Therefore, it is necessary to study the combination of materials to synthesize photocatalyst with higher efficiencies and the ability to absorb visible light.
In this study, IO structures of different compounds (SiO2, ZrO2, TiO2) and multicompound (SiO2- ZrO2, TiO2-ZrO2, SiO2- TiO2) have been successfully fabricated. SEM and EDS were employed to study the morphology and composition of the samples. To improve the photocatalytic activity of the IO structures, gold nanoparticles (AuNPs) were employed to introduce to the structure. AuNPs with a diameter of 12 nm were synthesized using hydrothermal reduction method and then characterized by STEM, EDS and UV-vis spectra. AuNPs was then successfully loaded into the IO structures without distorting the structure of IO.
From the photocatalyst and activity assessment in the UV light region, it was clear that ZrO2 IO, Au- ZrO2, SiO2-ZrO2 IO, Au-SiO2-ZrO2 IO showed a very low activity. For single compound IO, TiO2
IO was the best photocatalyst, whereas for the multicompound IO, both SiO2-TiO2 and TiO2-ZrO2
were the best catalyst in the photodegradation of organic compound. AuNPs enhanced the photocatalytic activity of samples by increasing the separation time of photogenerated holes and electrons and improving the light coupling of the material. In the visible light, Au-SiO2-TiO2 IO samples were inactive, while both Au-TiO2 and Au-TiO2-ZrO2 IO revealed photocatalytic activity.
This opens a way for further studies in the utilize the doping nanoparticles method to improve the photocatalytic activity of IO in visible light applications.
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Contents
Contents ... 2
Abbreviations ... 4
1. Introduction ... 5
2. Inverse opal structures ... 6
2.1. Manufacturing ... 6
2.1.1. Electrochemical deposition ... 6
2.1.2. Chemical vapor deposition (CVD) ... 7
2.1.3. Atomic layer deposition (ALD) ... 8
2.1.4. Sol-gel method ... 9
2.2. Applications of inverse opals ... 10
2.2.1. Photocatalysis ... 10
2.2.2. Sensor applications ... 10
2.2.3. Thermophotovoltaics and dye-sensitized solar cells ... 11
2.2.4. Battery electrodes ... 11
2.2.5. Optoelectronics and telecommunications ... 12
2.2.6. Current collector and separator for energy storage... 13
2.3. Multicompound structures... 13
3. Photocatalysis ... 15
3.1. Background ... 15
3.1.1. Photocatalysis mechanism... 15
3.1.2. Principles of photocatalytic reactions ... 16
3.1.3. Photodegradation of organics ... 16
3.2. Applications ... 17
3.2.1. Wastewater treatment ... 17
3.2.2. Photocatalysis application in concrete for self-cleaning purposes ... 18
3.2.3. Photocatalysis for air purification ... 18
3.2.4. Wettability patterning usingphotocatalysts ... 19
3.3. Different structures for photocatalytic activity ... 20
3.3.1. Doping semiconductor photocatalyst structure ... 20
3.3.2. Mixed metal oxide nanocomposites ... 21
3.3.3. Nanoporous nanocomposite materials ... 21
3.3.4. Polymeric nanocomposites ... 22
3.3.5. Carbon-based nanocomposites ... 22
3.4. Increasing photocatalytic activity... 22
3.4.1. Optimizing particle size ... 23
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3.4.2. Optimizing crystal structure ... 23
3.4.3. Surface modification ... 23
3.5. Photocatalytic activity characterization ... 24
4. Materials and methods ... 25
4.1. Materials ... 25
4.2. Synthesis of inverse opal structures ... 25
4.2.1. Synthesis of polystyrene (PS) spheres ... 25
4.2.2. Synthesis of polystyrene colloidal crystal thin film ... 26
4.2.3. Synthesis of TiO2 inverse opal ... 26
4.2.4. Synthesis of ZrO2 inverse opal ... 26
4.2.5. Synthesis of SiO2 inverse opal ... 26
4.2.6. Synthesis of multicompound TiO2-ZrO2 inverse opal ... 27
4.2.7. Synthesis of SiO2 -TiO2 inverse opal ... 27
4.3. Deposition of AuNPs into IO structures ... 27
4.3.1. Synthesis of AuNPs ... 27
4.3.2. Synthesis of IO structures doped AuNPs ... 27
4.4. Photocatalytic characterization ... 27
4.5. Photocatalytic activity assessment ... 28
5. Results and discussion ... 30
5.1. Single compound IO structures ... 30
5.1.1. PS spheres ... 30
5.1.2. IO structures ... 31
5.2. Multicompound IO structures ... 33
5.2.1. SiO2-ZrO2 IO ... 33
5.2.2. SiO2-TiO2 IO ... 33
5.2.3. TiO2-ZrO2 IO... 34
5.3. AuNPs doping IO structures ... 35
5.3.1. AuNPs ... 35
5.3.2. Au-ZrO2 IO ... 35
5.3.3. Au-TiO2 IO ... 36
5.3.4. Au-SiO2-ZrO2 IO... 37
5.3.5. Au-SiO2-TiO2 IO ... 37
5.3.6. Au-TiO2-ZiO2 IO... 38
5.4. Photocatalytic activity evaluation... 38
6. Conclusions ... 44
7. References ... 46
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Abbreviations
IO: Inverse opal PhC: photonic crystal
ALD: atomic layer deposition ED: electrochemical deposition CVD: chemical vapor deposition PS: polystyrene
AuNPs: gold nanoparticles EtOH: ethanol
TTIP: titanium (IV) isopropoxides TEOS: tetraethyl orthosilicate ZRP: zirconium (IV) propoxide
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1. Introduction
The development of the world requires new technologies for energy storage and conversion that should be inexpensive, high stability, and high efficiency using environmentally and friendly sources.
Energy storage systems are the core element of the energy conversion systems. This leads to the need of finding the materials that are able to store and convert energy from one form to another such as materials used for electrodes of a battery and photocatalysis for the producing of hydrogen fuel and for solar cells. Batteries need structural and compositional materials, which have high capacities with long lifespan with better performance of recharge, high density of energy, high stability and suitable shape for large technological systems. Photocatalytic reactions are essential for elimination of polluted environment and for the production of energy by water splitting. There is a need photocatalyst materials that have a large surface area, fast electron transfer, and a wide range of absorption spectra [1].
Photocatalysis, one type of heterogeneous catalyst has gained much interest in fields such as physics, chemistry, and surface science since it is a promising technique to solve an increasing environmental problem. Transition metal complexes were often used as a catalyst in a heterogeneous photocatalytic process. Until now, it is reported that over 190 semiconductors could be used as photocatalysts [2].
Semiconductor nanoporous structure, particularly inverse opal structures were found to be suitable candidates for photocatalysis process due to their special structural characteristics. They possess high interconnected porosity, uniform size, and periodically ordered pore as well as easy fabrication from opal template by reproducing its face-centered cubic (fcc). Therefore, they have been widely employed as photocatalysts, catalyst support materials and photoelectrochemical cells with notable applications in the solar fuel generation, decomposition of pollutants, and photocatalytic fabricated reactions [3].
The keyword photocatalysis first appeared in the 1910s, which means a reaction under the light promoted by the existence of a catalyst. In the early 1900s, the first experiment to study how light can effect on chemical reactions was conducted by Giacomo Ciamician [4]. In this experiment, blue light and red light were applied with excluding the role of heat and chemical reagents. The results illustrated that only the blue light excited the reaction. In the mid 1920s, ZnO was used in the formation reaction of Ag from Ag+ under light irradiation [3]. In the 1930s, most studies in photocatalysis was concentrated on TiO2. Then in 1938, TiO2 act as photosensitizer was first to decolorize dyes in the presence of oxygen [5]. However, until the 1970s most of attention focused on semiconductor photocatalysis with the first photoelectrochemical cell for splitting water with electrodes made from TiO2 coated Pt reported by Fujishima group [6]. Since then, many studies on materials with better photocatalytic performance than TiO2 have been conducted, but most of them are only active under UV light [7]. Therefore, it is necessary to study the combination of materials to synthesize photocatalyst with higher efficiencies and the ability of absorb visible light.
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2. Inverse opal structures
2.1. Manufacturing
Inverse opal structures are normally fabricated based on colloidal crystal templating through three steps as shown in Fig 1 [8]. First, preparation of the colloidal crystal template was done using monodisperse spheres of silica or polymer such as polystyrene and polymethylmethacrylate. In this stage, the synthesized monodisperse spheres are uniformly organized into an opal photonic crystal on the substrate surface through various possible methods: dip coating, evaporation, centrifugal sedimentation and self-assembly. The self-assembly method is widely used because of its flexible and simplicity. Then the precursor solution of dielectric materials is filled into the template by several methods listed below; in the final step, wet chemical etching or calcination were used to remove the template. By using this method, various inverse opal structure materials have been successfully synthesized such as TiO2 [9], ZrO2 [10], ZnO [11].
Figure 1. Synthesis inverse opal structure schematic [8].
2.1.1. Electrochemical deposition
Electrochemical deposition (ED) is a process in which a layer of the desired solid metal is deposited by electrochemical reaction from ions in the solution onto a surface. In ED, a set up with three electrodes: working, counter and reference is required (Fig. 2). Normally, the solution of desired metal is an electrolyte, the substrate is working electrode, and can use platinum counter electrode, Ag/AgCl reference electrode. The reaction can only occur when there are sufficient currents passing through the solution and based on the substrate voltage positive or negative with respect to counter electrode, the process can be carried out in anodic or cathodic mode. ED is a simple method with high efficiency. It is possible to carry out ED process at room temperature with low-cost equipment. ED
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allows precursor to fill the template from the bottom to the top, that can completely fill all the gaps in the template.
Figure 2. Schematic of ED process with experimental set up (a) and TiO2 deposition mechanism [12].
The three-electrode system (working electrode made from glass coated by indium tin oxide with arranged PS and reference electrode made from saturated calomel electrode) was used to electrochemically deposited zinc hydroxide nitrate hydrate (ZHNH) nanosheets [13]. By using the potential static method, zinc precursor was electrochemically deposited in zinc nitrate solution at ambient temperature for a duration of 8h. The ordered pore nanosheets were then obtained by calcination sample at 600°C for 2h to eliminate template. The same solution and method were used to co-deposited 3D inverse opals and 2D nanosheets with ordered pores. The solution was electrochemically deposited at different voltages with different reaction time for a duration of total 6 hours. The template was removed by the same calcination condition to obtain the pore periodicity on ZnO nanosheets.
2.1.2. Chemical vapor deposition (CVD)
CVD technique has been used for a long time in manufacturing from glasses for decorated container and become a fundamental technique in electronic industry. In CVD, the substrate is placed in a chamber, normally in vacuum condition and high temperature, then the vaporized precursors are routed into the chamber, where they react with the substrate surface and from a deposition layer (Fig.
3). By changing the precursor material and tailoring the reaction, one can get the desired layer deposited on wanted substrate. CVD method provides a high uniformity inverse opal structure with controllable filling rate, but the preparation temperature is high, and the equipment for the reaction is complex.
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Figure 3. Schematic of CVD process (http://www.seas.ucla.edu/prosurf/MOCVD.htm) Titanium dioxide thin films were obtained by CVD method using isopropyl titanate precursors carrying by oxygen flow to a reactor contained substrate at 60-210oC and 10 mbar. The crystallinity of TiO2 in this experiment was found to highly dependent on the reaction temperature [14]. Another example of TiO2 IO fabricated at ambient condition using CVD method has been reported by Moon group: the PS substrate first exposed with water for half hour then to TiCl4 precursors for a duration of 10 minutes, and the TiO2 layer thickness was easily controlled by varying the amount of TiCl4
exposure [15].
2.1.3. Atomic layer deposition (ALD)
ALD is a useful method to deposit a thin film for different applications. It is not only a key technique used in semiconductor processing, but also an advanced tool for nanotechnology research. ALD provides large surface uniformity and precisely controls the thickness of the thin film down to atomic level with high repeatability. The working principle of ALD method is that the first vapor or gaseous precursor reacts with the substrate surface, which leads to the formation of a layer on the surface then the second precursor is introduced to the formed layer and conduct a chemical reaction to form a stable film of desired precursors as shown in Fig 4. These steps can be repeated until reaching the required thickness. The main problem of ALD is that it is a slow process of around several nm thickness layer per min.
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Figure 4. ALD process flow. (a) Functionalization or functionalized surface substrate. (b) Reaction of precursor A with surface. (c) Inert carrier gas blows away excess precursor and reaction by-
products. (d) Reaction of precursor B with surface. (e) Inert carrier gas blows away excess precursor and reaction by-products. (f) Repeated step 2-5 until achieve the desired thickness [16].
Inverse opal ZnO using ALD method were successfully fabricated from diethyl zinc (DEZ) and water.
PS substrate was filled with the solution of diethyl zinc and water at 85oC and 10 Torr under nitrogen flow. The ALD growth was carried out by frequently pulses of DEZ then H2O, with N2 purging in between. At the end, clear photonic band gaps and strong photoluminescence were observed from the resulting ZnO structure [11]. Multilayer inverse opal TiO2/ZnS:Mn/TiO2 was possible to fabricate using ALD method to alternately deposit different material [17].
2.1.4. Sol-gel method
Sol-gel technique is widely applied in many current studies due to its simplicify and effectiveness. It is well-known in the practical work of metal oxides synthesis. In this method, the precursor is prepared in mixed solutions of desired metal, volatile solvent and catalyst (mostly water, acid) to boost the reaction and increase the adhesion of metal to the substrate surface. Then the substrate with template is filled with liquid precursor in ambient conditions. After that hydrolysis and condensation reactions occur on the substrate to form a stable and transparent sol. The sample is left at room temperature for a while for aging and drying purposes. Then, it is put into the oven and calcinated at high temperature to remove the template. The significant advantages of sol-gel method are low fabricate temperature, high purity and uniformity products, easily controlled added volume of precursors and reaction conditions as well as simple preparation process.
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The efficient of the filling process in sol-gel method is quite low because of the capillary force causing the sol particles to deposit on the outermost layer of the substrate. Moreover, there are a lot of the cracks on the surface of the material because of the shrinkage of precursors. Therefore, to improve the quality of the structure, it is important to repeat the infiltration process and use adequate precursor concentration [18].
ZrO2 IO thin films were synthesized by Zr precursor infiltration in to PMMA template [10]. Zr precursor was prepared by adding Zr (IV) propoxide in 1-propanol to methanol (1:1 in ratio) while vigorous stirring, and then the addition of water and concentrated HCl. The stirring continued until a clear and homogeneous sol appear, then diluted times with methanol for the purpose of getting better infiltration efficiency. In the infiltration step, several drops of the diluted Zirconium sol were introduced into the colloidal crystal film on coated glass slides at slight incline (about 5o) then was left at room temperature for a day. The samples were then calcined at 400oC to obtain ZrO2 IO with uniform and smooth surface.
2.2. Applications of inverse opals
2.2.1. Photocatalysis
Inverse opal structure is well known for its large surface area, which mean that more active catalytic sites and faster electron transfer can be found in IO structure. Moreover, the absorption of light of the material may be improved with inverse opal structure due to it has complete photonic band gap.
Therefore, the photochemical properties of the material can be enhanced. In a study of Chen and co- workers [19], they synthesized a number of TiO2 inverse opal samples of various band positions.
Their catalytic activity was evaluated by the photocatalytic decomposition of dyes. In this study, they found that the activity of photocatalytic decomposition could be increased by tailoring the band gap position close to the intrinsic absorption position of TiO2. In another experiment, the activity of TiO2 IO photocatalyst was tested by the oxidation reaction of phenol. A comparison between the activity of TiO2 IO and (i) commercial TiO2 powder P25; (ii) TiO2 macropore with disorganized pores; (iii) TiO2 IO without 3D arranged pores by crushing and ultrasonic also was conducted. It was found that the TiO2 IO had highest catalyst activity due to the fact that 3D ordered periodic structure exhibits excellent photocatalytic activity [20].
2.2.2. Sensor applications
Due to the light reflection characteristic at specific wavelengths, inverse opals are beneficial for sensing applications. By replacing one of the compounds in the inverse opal materials with a different refractive index material, the refractive index contrast will change, which produces a change in the photonic band gap of the IO material. For instance, amine-functionalized hydrogel IOs formed by filling dimethyl aminopropyl methacrylamide (DMAPMA) precursor into silica opal was employed
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in CO2 gas sensing [21]. When the as-prepared material immersed in pure water with CO2 gas was injected into the solution, it exhibited a significant color shift in diffraction peak.
Additionally, IO sensors are also reported to be promising in terms of biological applications [22].
They fabricated biocompatible silk inverse opals from silk fibroin and a thin film of Ag. Then the optical properties of this silk IOs was characterized using chicken breast tissue with the length from 2 to 5 mm. They stated that under a high scattering environment, it is possible to detect the photonic band gap.
2.2.3. Thermophotovoltaics and dye-sensitized solar cells
In thermophotovoltaic (TPV) systems, the radiation is emitted from a small band thermal emitter in a small energy’s range against the photovoltaic (PV) cell then changes to electricity. Thus, in order to improve efficiencies and reduce losses, the energies emitted range need to be equal to the energy of band gap of the PV cell. It has been reported that because of its high melting temperature permitting thermal emission in a larger range, tungsten IOs were found to be a potential material for selective emitters. However, tungsten IOs has a drawback of low thermal stability [23]. In order to improve this, oxide passivation layers should be introduced to act like a surface prevention, and by that structural unity is maintained. This can be done by using atomic layer deposition to deposit a layer of alumina or hafnia to tungsten IOs’ surface [24]. Their fabricated material has high thermal stability of up to 12 h at 1000oC when coated with alumina and at 1400oC when coated with hafnia.
Since the porosity promotes the infiltration of electrolyte and enhancements in the photo-collection efficiency through improved electron transfer, IO structures are promising candidate for applying in dye-sensitized solar cells (DSSCs) for solar energy harvesting. Normally, 3D IOs can be used to replace the semiconductor oxide of photoanode in DSSCs. This replacement has exhibited many potential in enhancing electron transport characteristics, because of shorter the distances of electron diffusion and efficient electrolyte filling process [25]. However, this also lead to lower levels absorption of dye due to lower specific area with respect to the traditional nanoparticle [26]. This limitation can be solved by the combination of the 3D IO layout and nanoparticles approach as reported by Park and co-workers [27]. In this study, the introduction of TiO2 1D nanorods into the TiO2 inverse opal’s surface by the hydrothermal method has increased the effective surface area up to 4 times higher than that of bare IO. This together with the enhancement of visible light scattering responsive and the enhancement in the absorption of dye resulted in the enhancement in many properties of DSSC compared with pure IO structure, containing the photocurrent and the short circuit current.
2.2.4. Battery electrodes
IO PCs have been extensively used in the battery field due to their open, continuously interconnected structure has been exhibited to enhance ionic conduction and efficient electronic during addition and
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elimination of Li, consequently enhancing rate and capabilities of power [28]. The study on IOs electrode was on ambigel form of sol-gel V2O5 by Sakamoto and Dunn [29]. They found that the synthesized IO structure with pores of diameter of 800 nm and the mesoporosity about 10- 30 nm has better capacities at higher discharge rates, which in turn enhanced mass transfer and higher electrolyte infiltration. The IOs structure would lead to the decrease of polarization and tortuosity induced by the transfer of mass through the material with respect to other original designations of electrode.
Recently, with the new discovery in the black TiO2, Kang group has studied hierarchically ordered porous black TiO2 (HOP-bTiO2) IO material in production of Li-O2 battery [30]. In this study, TiO2 materials were fabricated from the filling of self-assembled PS@Ni template (diameter around 1µm) with Ti precursor and H2 heat treatment for a duration of 10 h. At a current of 500 mA/g, electrode fabricated from HOP-bTiO2 IO material and LiI showed higher stable of about 340 cycles. The improvement in the electrical conductivity could improve transportation of mass from catalyst to the surfaces of Li2O2 and result in more surface of Li2O2 exposed to the electrolyte. The discovery of TiO2 IO helped to prevent the death of premature cell because of the complicated Li-O2 battery contained carbon side reactions, which extend its practical usage in the battery field.
2.2.5. Optoelectronics and telecommunications
IO architectures have its unique optical characteristics, which is ideally suitable for several photonics applications. The presence of the resultant specific wavelengths reflective, the localization possibilities and properties for restrained or concentrated discharge, combined with the capability to adjust these properties by varying the substances and construction makes IO materials become a promising candidate for many applications in optical devices [31].
Feng group [32] fabricated amplified and lasing emission of a resonant cavity by depositing a PMMA film doping tert-butyl rhodamine B in sandwiched layer of two polymer inverse opals, which operated as feedback mirrors. These polymer IOs were prepared by the infiltration of a photopolymerizable resin into a PS sphere template and then UV irradiation exposure to polymerize. Single mode lasing emission was produced when the gain medium was switched between the overlap of the photo- luminescence band of the dye molecules in the middle layer with the photonic stop band of the polymeric IOs.
Nelson group [33] studied the epitaxial growth of semiconductor materials in group III–V in three- dimensional IO nanostructures using the epitaxy of chosen area via an artificial opal template, particularly, the optical applications of GaAs IOs structure. The incorporation of a passivation layer of InGaAs between GaAs IO doping C and N decorating layers during the growth process via the PhC template was employed to fabricate a three-dimensional photonic crystal LED.
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2.2.6. Current collector and separator for energy storage
Recently, the application of IOs and porous materials for store energy has been developed. Since the porosity often promotes function of a separator, materials with 3D structure and IOs have been fabricated as cathode or anode materials, battery separators, and current collectors. The structured metallic IO was employed for 3D porous separator and current collector applications and was adopted for electrolyte storage between cathode and anode in lithium-ion battery. In theory, by introducing a three-dimensional conductive matrix into thin active consistently covered materials, and if the thickness approaching the limit of the solid-state diffusion, decreasing the resistance of polarization, and raising charge rate without negative effect the electrode’s conductivity, the charge rate can be improved to be very high. Zhang group [34] prepared an active material from Ni IO support using electrodeposition method to deposit Ni precursor into a polystyrene template with a diameter of 1.8 mm or 466 nm, at 2 mA/cm2 current. Before infiltration, Ni IO was electropolished in order to raise the total porosity to about 94%. The Ni framework was then electrodeposited with active cathode material MnO2 and lithiated by a molten solution of LiOH and LiNO3. It is found that, this MnO2
cathode with a thickness of around 30 nm was exhibited to remain high capacity at high temperature of about 76% when discharged at 185oC, and 38% when discharged at 1114oC. In this study, they found that the smaller sized of PS spheres lead to a higher density of energy at a specific active material’s thickness, because of the expansion of surface area of the system with smaller pores.
Another compound, Ni–Si composite was also studied to use as electrodes prepared by silicon electrodepositing to Ni IO scaffold [35]. In this study, they compared the battery performance of a Si on Ni nanocable array with the Si on Ni IO structure of the same mass of active material. The former material showed higher rate capabilities and better volumetric capacities the later. However, in a structure similar with Si anodes, both of them suffered a declination in capacity, which contribute to a SEI layer formed on the surface of silicon. By varying thickness of silicon film, they observed that at a thickness of about 50 nm, the capacity retention decrease. The insertion of Li can increase the silicon layer’s thickness, which leads to a decrease in the IO’s porosity. This in turn lower the infiltration of electrolyte and contact thereby decreasing the capacity values. Thus, the performance of IO material can be optimized by varying the thickness of silicon film and the size of IO pore.
2.3. Multicompound structures
Multicompound IO structures have been widely studied since it can counterbalance the shortcomings of the individual components. For example, TiO2 and Fe2O3 are the most suitable materials to be used as anode in photoelectrochemical devices. However, their limitations of poor electrical conductivity for Fe2O3 and poor light absorption for TiO2 have limited their applications in industry. The combination of TiO2 and Fe2O3 can lead to the characteristics of visible light-induced activity, long carrier lifetime and high charge mobility as reported by An group [36]. In this study, the modulated light absorption Fe2TiO5 inverse opals were fabricated using polystyrene (PS) photonic crystals template. The characterization of this synthesized material showed that the absorption edges overlap, and the maximum stop-bands overlap were possible for multiple scattering of visible light. They also found that,
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Fe2TiO5 inverse opals showed a higher photocurrent density than Fe2O3, TiO2 and disordered Fe2TiO5 film, due to the improvement of superior charge separation and light absorption. Moreover, photoanodes performed better with the presence of FeOOH as a co-catalyst.
Another example is the 2D TiO2–WO3 composite inverse opal by surfactant-free co-assembly method with PS template [37]. The composite TiO2–WO3 IO films were used to construct the photochromic a thickness of 510 nm. The electrons migrate from the dye into TiO2 conduction band and then spread into WO3 conduction band under irradiation of solar light. When electrons spread into WO3, Li+ from the ion conducting layer also propagated into WO3 because of charge accumulation, the color of WO3
changed from transparent to dark. Ma group [38] fabricated WO3/BiVO4 inverse opal photoanodes by swelling− falling mediated PS template fabricated method, and examined these photoanodes’
efficiencies in photoelectrochemical cells with solar light simulation. The study indicated that the synthesized photoanodes had a high periodically ordered macroporous nanostructure. Compared to the photocurrent produced by pure WO3 IO photoanodes its photocurrent is 40 times higher at a bias of 1.23 V vs RHE. The combination of BiVO4 and WO3 improves BiVO4 weak charge mobility property, which resulted in production of density of photocurrent about 3.3 mA/cm2 at a bias of 1.23 V vs RHE.
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3. Photocatalysis
3.1. Background
3.1.1. Photocatalysis mechanism
In principle, the rate of photocatalytic reaction depends on the catalyst and photon energy or wavelength. The higher photon energy flux will give higher electron-hole pair (EHP) generation rate, which in turn boosts the reaction rate. When semiconducting materials are employed as catalysts, the sensitized photoreactions will take place with the photoexcitation first happens in the catalyst substrate and then transfer energy, by mean of electron transfer, in a ground state molecule.
There are three main steps in the mechanism of the photocatalytic reaction as can be seen in Figure 5. Firstly, when a light with an energy of equivalent or higher than catalyst’s band gap energy is captivated, it will excite a valence band’s electron into the conduction band and left a hole. This pair of photoexcited charges is named a pair of electron-hole. Secondly, the holes from valence band conduct the oxidation reaction of the donor molecules and water molecules resulted in hydroxyl radical formations which then interact with pollutants compounds. Finally, the electrons in the conduction band interact with oxygen resulted in superoxide ion formation, which then conduct the reduction and oxidation reactions to break down the pollutants or any other compounds attached to the catalyst surface [39].
Figure 5. Semiconductor photocatalytic mechanism schematic [39].
16 3.1.2. Principles of photocatalytic reactions
In general, a photocatalyst absorbs of a photon with suitable energy producing radicals with high reactive properties, which can conduct organic compounds oxidization. A typical photocatalytic reaction progress is proposed as Equations 1-6 below [39]. In this case, TiO2 the most widely studied semiconductor catalyst is adopted as the substrate, with absorbed molecules are H2O and O2.
TiO2 + hv → TiO2 (e-cb + h+vb) (1)
TiO2 (e-cb + h+vb) → TiO2 + heat (2)
TiO2 + H2Oads → TiO2 + OH- + H+ (3)
e-cb + O2ads → O2- (4)
h+vb + OH- → OH• (5)
OH• + organic compound → CO2 + H2O + residuals (6)
From the Equations 3-6, it is notable that the reaction between adsorbed molecules and photocatalysts is important. The surface of photoactive semiconductor can appeal donor and acceptor electrons via both electrostatic and chemical forces including van der Waals, resulted the interactions of hydrogen bonding, dipole-dipole, the complexity of ion exchange, and the hydrophobicity of sorbate.
3.1.3. Photodegradation of organics
One of the useful targets of heterogeneous photocatalysis development is to break down organic compounds that are toxic for the environment and living species. It has been shown that photocatalytic oxidation processes (PCO) are beneficial in alternating many current purification processes namely UV-C irradiation, chlorination, and ozonation, which are often threatening and generate unfriendly by-products. Normally, although photocatalysts are not able to break down high amount of spoilage, they are able to destroy them as they accrue. For instance, cigarette smoke residue stains can be decomposed by room light if the wall is coated with catalyst. Additionally, in the presence of photocatalysts, the UV light created from fluorescent light would be able to decompose the odors that are harmful to humans and in the concentration of parts per million (ppm). Thus, the quantities of decomposed materials will increase if higher intensities of light are available [40].
Up to now, many organic compounds such as phenols, alkanes, aromatics, alcohols, halophenols, haloalkanes, cresols, polymers, cancer cells, surfactants, pesticides, herbicides, dyes, fungi, bacteria, viruses, molds, and highly resistant spores, which have been successfully degraded into CO2 and harmless minerals by photocatalytic process have been reported [3]. The degradation mechanism of
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organic substances is similar in all cases, just varying in the characteristics of photocatalyst such as the redox potentials of the photocatalyst, the number and strength of acid sites on surface [41]. For example, the photocatalytic decomposition of 2-propanol was used as a substrate model for studying kinetics because it can be photodegraded to form acetone, which can be identified by high sensitivity gas chromatography. Moreover, the chain reaction did not involve in the photodegradation of 2- propanol mechanism, but just a photon took part in the production of an acetone molecule. Therefore, the quantum yield is equivalent to the rate of the figure of acetone molecule productions and the figure of photon absorptions [41].
Besides owning the strong oxidation characteristics, PCO is a friendly method for purification of environment and disinfection purposes through their formation in either thin-films or powders. The fabrication procedure and properties are different between the two forms, depending on the organic degradants and the level of decomposition. For example, nanoparticle photocatalysts are often easily fabricated with high level of productivity and is able to use in aqueous solution mixtures as well as interlinked into fibers to intimately interacting with organics. Whereas, film formations own a benefit in water purification and bacteria destruction since it does not need a catalyst filtering process.
3.2. Applications
Due to its simple principle, photocatalysis can be applied for many purposes such as decomposition of many pollutants in wastewater, purification of air, antibacterial activity and in conjunction with filtration, can also get rid of metals (such as mercury) [42]. Due to this widely applicability, nanoparticle photocatalysts are adopted for air purification, self-cleaning surfaces in building materials and water treatment.
3.2.1. Wastewater treatment
In waste water treatment field, TiO2 is the most popular photocatalyst with respect to other semiconductor, while zinc oxide (ZnO) could be a substitute candidate for some applications [43]. A comprehensive review of nano‐TiO2 photocatalytic for environmental applications was published in 2008 [42]. In this review, they emphasized that nano‐TiO2 can be used to successfully remove contaminants. They also did a comparison on the degradation mechanism and the special properties between nano‐TiO2 and micro‐TiO2. They concluded that TiO2 should be considered as an attractive technology in terms of the efficiency of photoreaction, the ease of usage, and the promisingly and economically decomposition of contaminants. However, they suggested that in order to prevent any free nanoparticles in water phase, nano‐TiO2 particles should be accommodated into thin‐films or born on substrate.
Maldonado and coworkers [44] found that to improve the efficiency of a photocatalyst in water treatment field, many solar reactors for photocatalysis process in the water treatment field have been built based on non-concentrating collectors. Moreover, the application of the solar photocatalysis to
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passive microorganisms existing in water has forced the studies on making experimental systems to utilize this disinfection method.
3.2.2. Photocatalysis application in concrete for self-cleaning purposes
The aesthetic properties of cementitious materials, especially the color, often change because of the regular exposure to various atmospheric contaminants, microorganisms and different weather conditions. Additionally, their nature also produces rather high roughness and porosity, which facilitates the colored organic pollutants’ deposition and partially promotes the growth of biological organism. Some popular methods such as additives, regular paintings, sealers, chemical cleaners, and maintenance works have been employed in buildings in order to prevent and control the changes of cementitious materials’ colors.
The use of TiO2 photocatalysis on cement-related materials has facilitated the decomposition rate of not only organic compounds but also some inorganic compounds, which considered having negative effects on the properties of cementitious materials as well as the environment. Therefore, TiO2 could be adopted to improve the life span of cement-related materials, while decreasing the concentration of few contaminants in the atmosphere, particularly in tunnels, canyon streets, gas stations and parking lots as well as some industrial zones. TiO2coated ceramic tiles were successfully employed for the production of antiseptic effect in rooms of hospitals, classrooms, bathrooms and kitchens [45].
The adjunction of some metals such as Ag and Cu to TiO2 coated materials has exhibited the increase in the antibacterial properties [3]. In 1995, the hydrophilic behavior of composite TiO2-SiO2 film was accidentally discovered when the composite was exposed under UV irradiation [46]. In general, the hydrophilic behavior was stopped when TiO2 was used alone without any UV light. However, the combination of TiO2 and siloxane bonding compounds increased the surface’s hydrophilic effect for several days even in the dark [3]. Recently, the introduction of roughness to TiO2 particles by a photoelectrochemical etching method causing an adjustable hydrophilic and a hydrophobic behavior in the same surface is new detection that has been reported [47]. This discovery makes the application of TiO2 for self-cleaning purposes wider.
3.2.3. Photocatalysis for air purification
Air purification by PCO of many and inorganic gaseous (SOx, NOx, CO, H2S) and volatile organic compounds (VOCs) at moderately concentrations has been reported [48]. First, TiO2-based photocatalysts can effectively degrade various organic compounds, but some modification of pristine TiO2 such as doping, coupling, and shape controlling was required for photocatalysis enhancement.
Secondly, in terms of inorganic degradation, modified TiO2 photocatalysts with controlled size were successfully removed the pollutants under the wider range of light absorption than UV light. Finally, besides TiO2 some non-titania based photocatalysts has also gained much interest. ZnO is known as the second most common semiconductor photocatalyst. It shows a better photocatalytic activity than TiO2, but in terms of photocatalytic stability, it is good as TiO2 when the reactions are conducted in
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aqueous phase. Some modification of ZnO was also conducted to enhance its activity [49]. In term of formaldehyde decomposition, modified ZnO exhibited higher photodegradation efficiency than commercialized P25. This is because the higher surface energy of the polar planes of this adjusted ZnO has larger energy of the surface. Similarly, a composite of ZnO and graphene fabricated using a green one-pot hydrothermal technique was used for the acetaldehyde gas’s photodegradation. In this reaction, the CO2 production rate improved due to the increase of the electron transfer rate.
Several parameters should be taken into account for the development of novel photocatalysts for treatment of polluted air [48]. First, appropriate valence band position for the oxidation process at the photogenerated hole and conduction band for production the reduction reaction via electron reduction should be proper. Secondly, the selectivity of the photocatalysts need to be considered to eliminate the unwanted product such as carbonaceous material (soot, coke) which can deposit on the photocatalyst’s surface. Moreover, the stability of the catalyst should be prolonged by advanced surface modification. Finally, the range of absorption light as well as the immobilization of photocatalysts have attracted many scientists because of the efficient solar energy implication.
3.2.4. Wettability patterning usingphotocatalysts
Wettability patterns that are based on the powerful oxidization and superhydrophilicity of photocatalyst, have been employed in various application such as in offset printing and printed-circuit boards [50] as well as promising technique for fluid microchips applications [51]. With these applications, the desired hydrophobic–(super)hydrophilic wettability patterns are normally synthesized by using light illumination and a photosensitive polymer, which called photolithographic method [50]. In this process, a photosensitive polymer receives the geometric pattern from a photomask under the light. Then the chemical of the photosensitive polymer illuminated area is changed and treated by alkaline substances followed by imprinting the desired exposure pattern onto the substrate under the photosensitive polymer.
The most widely utilization of wettability patterns is offset printing, a technique that is widely applied in newspapers printing. In offset printing, a PS plate is required, which composed of a hydrophobic–
hydrophilic master plate on an aluminum substrate, which has been anodized. The wettability pattern was fabricated using photolithography technique as reported by Nishimoto group [50]. As noted, in the event of a positive photoresist, the exposed surfaces of the anodized aluminum are hydrophilic while the photosensitive polymer surfaces are hydrophobic.
In addition to the advantages of reducing the cost and saving resources as well as reduced environmental impacts, there are still some negatives of offset printing. Firstly, the photomask creating from plastic substrate needs to be arranged before printing and afterwards disposed of coming the printing process. In the development step, the wastes of alkaline chemical are also produced. Moreover, the life cycle of the plate is short because its soft surface might be brushed all along the printing. Finally, the plate needs to be discarded before the next coming the printing process.
In order to solve these issues, Suda and coworkers have studied the use of TiO2-coated aluminum
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substrate [52]. By using this substrate, the printing paper was successively achieved since the wettability patterns contained a hydrophobic area based on organics could be excluded by photodegradation with the presence of TiO2 photocatalyst. They also found that the removal of organic compounds by photocatalysis resulted in the easily cleaning of the wettability patterns under UV irradiation and the surfaces are able to be reused with new superhydrophilic–hydrophobic patterns, which will reduce the amount of printing plates’ disposal.
3.3. Different structures for photocatalytic activity
Semiconductor metal oxides, which has the band gap in the UV spectrum can be employed to produce a UV-generated system. Their electronic process takes place after absorbing UV light, which could be adopted for various photoreactions. This helps nanostructured semiconductor extensively used as a photocatalyst. However, because of their large band gap they are only activated by UV light, which is responsible for a small part of solar spectrum (around 4%) [53]. In order to create visible light- generated photocatalyst different structures are studied as discussed below.
3.3.1. Doping semiconductor photocatalyst structure
The purposed of addition a dopant is to expand the absorption spectra of semiconductor photocatalysis to the region of visible light [54]. It can be doped with single metal (cation-doped) atom, nonmetal (anion-doped) atom or with two kinds of atom called codoping.
Adding metal ion (cation-doped) can reduce the recombination rate of EHPs since metal ion can act as electron or hole trap [55]. The mechanism of lowering band gap in this case are reported in different ways. Zhang group [56] reported that the source of the band gap lowering came from the mixing of the (d) orbital’s conduction band in Ti and in the doped metal. Whereas Thimsen group [57] reported that the doped metal adds energy levels into TiO2 band gap then decrease the required energy of electrons excitation from the valence band to the conduction band. It has been reported that cation- doped also varies the activation energy of phase conversion [58]. However, the performance of cation-doped semiconductor photocatalyst is dependent on various aspects such as concentration of doped cation, distribution of dopants, d-electronic configurations and the location of dopant’s energy levels in the lattice [59].
Adding nonmetal atom, normally S, N, C, B, and F into the semiconductors, has been studied and become an efficient way to expand their absorption spectra to the visible light region. In metal oxide semiconductors’ lattice, it is reported that oxygen is replaced by the anion atom [60]. Sulfur was doped into TiO2 [61] using one-step low-temperature hydrothermal technique. In XPS spectra, sulfur atoms replaced oxygen atoms in TiO2 lattice shown by a peak represent for the bonding of Ti-S.
Another example is carbon-doped TiO2 [62] by solvothermal method. In this study, C-doped TiO2
shown better performance in the photodegradation of methylene blue than the un-doped TiO2. This
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result was explained by the high reactivity of EHPs photogeneration with the direct transition of charge-transfer involving both carbon and TiO2 phases.
Semiconductor doped with two kinds of atom has shown better photocatalytic performance than single-element doping [63]. However, the mechanism of cooping semiconductor system is complex and depends on the using elements. Valentin group [64] prepared, theoretically and experimentally studied N-F-codoped TiO2. They found that smaller oxygen defects were presented in N-F-codoped TiO2 samples, which increase photostability and photocatalytic activity of the sample.
3.3.2. Mixed metal oxide nanocomposites
The combination of two semiconductors or semiconductor with molecule which has a narrower band gap has improved the photocatalytic activity of the material, because of the increase in charge separation efficiency and extending the photoexcitation energy [65]. Biswas group [66] fabricated a thin film of CdS–TiO2 composite on indium tin oxide (ITO) substrate and a glass slide using chemical bath deposition method. The degradation of methanol was employed to examine the photocatalytic activity of as-prepared material. They found that the photocatalytic performance of fabricated material was improved because of the crystallinity enhancement of CdS and TiO2 layers and the raise the surface roughness of CdS after hardening at high vacuum. Moreover, Resta group [67] found that in visible light, CdS–TiO2 thin film has a greater absorption efficiency region compared to TiO2. Thin film was fabricated by a novel in situ method by using [Cd(SBz)2]MI (MI stand for 1- methylimidazole) as unimolecular precursor for CdS, then CdS was absorbed onto TiO2 pores. In this process, CdS was adopted as a sensitizer to form TiO2 film, which results in photocatalytic performance improvement.
Saravanan group fabricated ZnO-CdO, ZnO-Ag and ZnO-CdO-Ag composites [68]. They found that the coupling of ZnO with Ag improves photodecomposition performance of textile dyes because of higher surface area, while the ZnO-CdO combination gives higher charge carriers because of the delay of back interaction between the photoinduced charge carriers, which shift the optical band gap of ZnO-CdO combination towards and become more reactive to the UV–visible light [69]. The ZnO- CdO-Ag combination gives a ternary composite, which improves the photocatalytic activity of the combination. This can be explained by the increase of surface area. In addition, the presence of Ag in the composite forms an electron trap that boosts the separation of electron–hole pairs [70].
3.3.3. Nanoporous nanocomposite materials
Nanoporous solids have been applied in heterogeneous catalysis and photocatalysis due to their large surface area, three-dimensionally interconnected porous networks, high volume of pore and adjustable pore size, as well as nano-sized crystalline walls. Many studies have been conducted to study the mechanism of improved performance of photocatalysis by nanoporous nanocomposite materials. Porous AgCl/Ag nanocomposites was synthesized using a two-step approach with
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nanoporous silver prepared by dealloying alloy foil and then washed in mixed H2O2/HCl solution [71]. Its photocatalytic activity was determined by the degradation of methylic orange under the irradiation of visible light. It was found that the nanoporous compound performs very well as efficient and stable visible light photocatalysts. In another study, Deng group [72] fabricated silver nanoparticle (Ag-NPs) deposited on nanoporous ZnO microrods (n-ZnO MRs) by a two-step technique. ZnO micrometer rods were synthesized by solvothermal-assisted thermal treatment approach with the dimension in between from 90 nm to150 nm and the length from 0.5 to 3 m and composed of ZnO nanoparticles with diameter of 20 nm. The silver nanoparticles with diameter in 20-50 nm range were delivered onto ZnO microrods by photoreduction approach. The Ag-NPs/n- ZnO MRs showed better photocatalytic performance than original n-ZnO MRs in case of using UV and solar light irradiations. Its photostability also improved due to exceptional structure, the deposited Ag-NPs and the superior crystallinity of the ZnO rods. In this case, Ag-NPs acting as an electron wells promoted the charge separation.
3.3.4. Polymeric nanocomposites
Polymers can be used to broaden the absorption spectra of the semiconductor, which leads to the enhancement in the performance of photocatalysis under UV or visible light irradiation. For enhanced photocatalytic activity of semiconductors, the conduction and valence bands of polymer need to be either higher or lower than the conduction and valence bands of semiconductors. Otherwise, the spatial charge separation could not be obtained which responds for the performance enhancement [73]. Beside improving photocatalytic activity, the nanocomposite of polymer and metal also found to have and efficient in fuel production [74].
3.3.5. Carbon-based nanocomposites
Carbon quantum dots (CQDs) can improve the photocatalytic performance of titanium dioxide. In CQDs-TiO2 composite, carbon acts as photosensitizing agent and it also builds mid-gap energy levels in TiO2 [75]. Moreover, Liu group [76] combined CQDs with Au, Ag and Cu as photocatalyst for the green oxidation of cyclohexane. In this study, they found that those combinations shift the light absorption into the purple, red, and green light, respectively, and the highest performance was observed with Cu-CQDs. Besides CQDs, the composite of carbon nanotube or graphene with other materials was also studied. The introduction of graphene into semiconductor results in the conductivity improvement of the semiconductor and also the photocatalytic activity [77]. The combination of graphene and Ag as a photocatalyst has shown a promising results of organic molecules degradation and an increasing in charge mobility [78].
3.4. Increasing photocatalytic activity
There are several parameters that can be used to determine the efficiency of a photocatalyst such as the rate of EHPs generation, and energy levels of the incident light (photon) which used to excite the
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catalyst. The rate of generated EHPs can be modified by tailoring the surface area, band gap and size of photocatalytic particles [79].
3.4.1. Optimizing particle size
Several studies has been conducted to study the impact of TiO2 particles on it photocatalytic performance [80]. It has been reported that in TiO2 with large particles the amount of charge carrier’s recombination is major, while in particles with quantum sizes the surface recombination is dominant.
The recombination can be reduced by decreasing diameters of large particles, since the required distance for the charge carriers’ migration to the surface decreases. The decrease in the size of particle also results in a higher surface area. When the particles size is reduced approaching quantum sizes, the photogenerated electrons and holes agitate quite near to particle surface. Zhang group [81] found that electron-hole recombination becomes major in particles with quantum sizes as it is characteristically faster than the interfacial charge carrier process. It has been concluded that the TiO2
nanoparticles optimal size is the size with highest surface area, whereas the particle is still large enough to avoid quantum effects.
3.4.2. Optimizing crystal structure
The crystalline phase of pure and modified TiO2 alsohas proved has effect on its photocatalytic performance. Normally TiO2 crystals can be designed in either following polymorphs called anatase, rutile and brookite. Brookite having a band gap of 3.4eV is found to be less photoactive than anatase and rutile [82]. Moreover, it is impossible to find brookite in large amount; therefore, pure anatase, rutile and their transition phases are often examined. Since a phase equilibrium of the anatase-rutile transformation does not exist, no exact temperature could be given to the transition of phases, which mainly relies on synthesis approach, particle size, impurity, reaction environment and the amount of vacant oxygen in the TiO2 lattice [83].
3.4.3. Surface modification
Surface modification can improve the activity of semiconductor photocatalyst since it can lower the band gap energy of materials and expand their absorption spectra. Many studies on TiO2 IO modifications by doping nanoparticles (NPs) of metals in to the TiO2 structure has been conducted.
For instance, TiO2 IO doped Pt and Au NPs exhibited higher hydrogen production activity in comparison to original TiO2 crystals. This is due to the fact that the consolidation of IO structure, surface properties and doped nanoparticles greatly improved the behavior of materials in term of photoelectrochemical hydrogen production [84]. In the photocatalytic conversion of CO2, TiO2 IO, P25 and Pt NPs deposited TiO2 IO were used as catalyst. The reaction used commercial P25 catalyst produced lowest amount of CH4 production, while the reaction used TiO2 IO loaded Pt catalyst produced the highest CH4 volume, which was approximately 3.2 and 2.4 times higher than original
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TiO2 IO and P25, respectively. It is then believed that the better performance can be attributed to enhance visible light absorbance and reduced EHP recombination [85].
3.5. Photocatalytic activity characterization
There are eight established ISO standards used for evaluation of photocatalytic performance.
However, in this work, only two of them are mentioned, which are air and water purification standard.
In water purification standard, dyes are usually adopted as polluted compound model, because their concentration can be easily controlled by spectrometers. Moreover, around 1-20% of the total produced dyes in the world is disposed from many industries such as dying, printing, dyestuff producing, and textile. Methylene blue (MB) is a cationic dye, used largely in dying process of silk, wool and cotton. When MB exist in wastewater, it may increase the harm for human health such as vomiting, burns effect of eye, nausea, and diarrhea [86]. The photoabsorption by MB should be eliminated for the determination of photocatalytic performance of photocatalysts, because MB absorb light in the visible range. In addition, the use of MB as a model for performance assessment also depends on the purity of MB, the pH of the MB solution and mechanism of the process [87].
In air purification standard, normally acetaldehyde, nitric oxide and toluene are used for photocatalytic activity assessment because they are popular pollutants in the atmosphere [87]. They give high repeatability results even conducted by different labs simply by controlling precondition protocol as well as same experimental steps. Recently, acetylene adopted as volatile organic compound (VOC) model, is employed for the assessment of photocatalytic activity. The reason of using it are its simplest alkyne structure and it can be completely decomposed in static conditions by TiO2 under light irradiation to form CO2 and H2O. Moreover, the use of acetylene makes the comparison of the behavior and the efficiency of different forms of TiO2 in many carriers or powder easier [88].
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4. Materials and methods
4.1. Materials
Titanium (IV) isopropoxides (TTIP, 97%, Sigma- Aldrich), zirconium(IV) propoxide (70 wt % in 1- propanol, Sigma-Aldrich), and tetraethyl orthosilicate (TEOS, Aldrich, 98%) were used as precursors for the fabrication of TiO2, ZrO2, SiO2, TiO2-ZrO2 and SiO2-TiO2 IO structures.
Hydrochloric acid (37%, VWR chemicals) was used as catalyst and ethanol (99.5%, Etax aa) was used as solvent in the preparation of precursors. For the preparation of polystyrene, styrene (99%, Acros Organics), Ammonium persulfate (APS, 98%, Sigma-Aldrich) and sodium dodecyl sulfate (SDS as a capping agent, 98.5%, Sigma-Aldrich) were used. Gold (III) chloride trihydrate (HAuCl4·3H2O, ≥ 99.9%, Alfa Aesar), and sodium citrate (TSC, ≥ 99%, Sigma-Aldrich) were used to synthesize gold nanoparticles (AuNPs). Glass plates (Thermo Scientific) were used as a substrate for the fabrication of IO structures.
4.2. Synthesis of inverse opal structures
Inverse opal structure in this study was synthesized by template filling method. The precursors were fabricated by sol-gel approach. Briefly, the monodisperse spheres of polystyrene were uniformly organized into opal photonic crystal on the surface of microscope slide glass cleaned by distilled water through self-assembly method. Then the precursor solution of dielectric materials is filled into the template by dipping methods; in the final step, calcination was used to remove the template.
4.2.1. Synthesis of polystyrene (PS) spheres
Negatively charged PS spheres with a diameter of 250 nm (PS250) and 400 nm (PS400) were prepared. First, seed solution of 200 nm PS spheres was prepared by a free-radical emulsion polymerization, 200 mg of SDS and 160 ml of deionized water were put into rounded bottom flask and heated to 70oC while stirred at 500 rpm and under nitrogen atmosphere. Subsequently, 20 g of styrene was poured into the this solution at 70oC and stirred for half hour, then 20 ml of APS solution (200mg of APS dissolved in 20 ml of deionized water) was slowly dropped into the reactor. The reaction then was left at that condition for 20 h. 121.5 ml or 30.4 ml of 200 nm seed solution were diluted with 70 ml or 150 ml in a flask to make PS250 and PS400 solutions. Then 25 mg of SDS was added and the mixture was heated to 70oC under nitrogen flow. After stabilizing at 70oC, 20 ml solution of 28 mg APS and dionzied water was added. Finally, styrene (11.8 g or 21.6 g) was carefully added to the reactor solution and the reaction was maintained for 20 h.
