Fabrication and Effective Application of Strategically Designed Nano-Structures

The numbers in brackets in black and pink represent the lattice planes of anatase TiO2 and gold NPs, respectively.---49 Figure 2.1.4 Optical properties of the P-25, TIO, st-TIO and Au/st-TIO structure prepared . from PS balls with different diameters. a) UV-vis absorption spectra of the samples. Schematic illustration of the fabrication process of the hybrid TiO2 (H-TiO2) structure. b) Top view SEM image of st-TIO for photon capture. 108 Figure 3.1.3 BET study of the surface area and pore size distribution of GO,. 3DGN.

-Ni, and 3DGN.---109 Figure 3.1.4 Characterization of materials used for solar desalination device. of the bare wood and GO, 3DGN-Ni, 3DGN on the wood.

Introduction (literature review)

Motivations

Fabrication of the periodic patterned structures

Self assembly
Lithography

There are several types of lithography techniques and their applications are even more varied, as shown in Table 1.1. The lithographic techniques are broadly divided into two types by the use of shadow masks; These 3D patterned nanostructures, made using different lithography techniques, are applied in four major areas such as electronics and microsystems, medical and biotechnology, optics and photonics, and environment and energy harvesting, as shown in Table 1.1.

Moreover, the rapid development of modern technology has created a number of applicable fields for 3D patterned nanostructures, and lithography technique should play an important role in innovation.

Figure 1.3 Self assembled block copolymers as various shaped micelles. 3

Advantages for efficient energy devices

Surface to volume ratio
Light trapping effect
Plasmonic effect

As a result of the multiple scattering effect, most energy of incident light can be absorbed by the structure. Reflection of incident light from the air to the particular structure can be calculated by equation (1) with the refractive indices of the two media forming the interface, the air and the structure here. This surface plasmon resonance phenomenon means that light energy is converted into surface plasmon and accumulated on the surface of the metal nanoparticles.

As the size of nanoparticles, the affected wavelength of the incident light can be changed as shown in Figure 1.11.

Figure 1.7 Surface area to volume ratio. The increment of surface area vs

Applications of solar energy conversion

Photoelectrochemical water splitting
Solar steam generation

Hydrogen fuel is one of the most effective resources for solving global problems related to energy. To improve the performance of the efficient conversion of sunlight energy, two of the main researched approaches recently, such as improving the light absorbance and charge transfer in photoelectrochemical water splitting system. Recently, nanostructured photoelectrode materials are very attractive and significant attention in the development of the photoelectrochemical water splitting system.

They should have competitive cost of abundant element for mass production in industrial fields.

Figure 1.12 The global energy potential comparing energy resources by their potential

Photoelectrochemical Water Splitting

Optimization for visible light photoelectrochemical water splitting: gold-coated

Introduction
Experimental procedures

Fabrication of TIO
Fabrication of st-TIO and Au-NP coated st-TIO
PEC measurements

Results and discussion
Conclusion
References

In addition, the apparent hydrogen generation capability of the Au/st-TIO structure is realized by the synergistic effect of photon trapping and the plasmon-enhanced transfer mechanism observed in the metal/semiconductor interface. The excellent photocatalytic activity of the properly designed Au/st-TIO structure provides great potential for light engineering in hydrogen production systems. The inset shows a close-up image of st-TIO with a larger air hole diameter of 290 nm and a beam diameter of ~40 nm coated with TiO2 textures of smaller diameter ~9 nm.

The close-up image in the inset confirms the single crystal phase of the as-prepared st-TIO with an interplanar spacing of 0.31 nm. b) TEM image of the Au/st-TIO structure. The optimal structure with the largest cross-sectional area achieved by integrating the small dimensions in the UV-absorption curve is st-TIO with a diameter of 350 nm, as shown in Table 2.1.1. The intensity of the photon trapping effect caused by the scattered light under the irradiation beam on the device was further investigated by comparing the scattered reflectance spectra of the control film P-25 and st-TIO with different dimensions.

To investigate the hydrogen generation activity of the samples, photoelectrochemical measurements were performed directly on P-25, TIO, st-TIO, and Au-NP coated st-TIO control samples. The photocurrent density of the 350 nm diameter st-TIO optimized as demonstrated in Figure 2.1.4 and Table 2.1.1 is approximately 2 times that of the P-25 control sample at an applied bias of + 0.5 V. The inset is magnified image of the IPCE data showing the rise of Au/st-TIO and st-TIO in the visible region.

st-TIO was further optimized as a visible-light hydrogen generation electrode by introducing Au NPs that facilitate surface plasmon-enhanced hydrogen generation. The photocurrent of Au/st-TIO was 2.58 times higher than that of the conventional P-25 photoelectrode, which is recognized for its outstanding efficiency.

Figure 2.1.1 Schematic illustration of the fabrication process for a Au/st-TIO structure

Towards visible light hydrogen generation: quantum dot-sensitization via

Introduction

Fabrication of p-P25 and H-TiO 2
Fabrication of CdSe/H-TiO 2
Characterizations
PEC measurements
Calculation of effective refractive index of antireflection layer

Conclusion
References

However, the small particles cannot efficiently collect light in the red and near-infrared regions of the light spectrum, and they also reflect a large amount of light due to the high refractive index of TiO2. Therefore, appropriate modification of the P25 photoelectrode is required and careful design of quantum dot-sensitized devices based on the light-harvesting mechanism is crucial to increase efficiency in photoconversion systems. Under simulated illumination with AM 1.5 G sunlight, the maximum photocurrent density of CdSe/H-TiO2 reaches ~ 16.2 mA/cm2, which is 35% higher than that of the optimized control sample (CdSe/P25), at an applied bias of 0.5 V vs. Ag/AgCl.

Based on the effective medium theory2,3, the effective refractive index n can be calculated as 1.629 with the following equation. The effect of introducing the light trapping layers into the P25 film was further exploited by passing light through the transmission spectra from top to bottom (Figure 2.2.3c). As expected from the low absorption coefficient of P25 in the visible region, a significant portion of the light passes through 6 mm thick P25 at the longer wavelength range, clearly demonstrating that no light harvesting has occurred (black curve).

However, when a 1 mm thick st-TIO layer was introduced into 5 mm thick P25 on the bottom, creating 6 mm thick hybrid samples (blue and red curves), the transmission of light is greatly reduced, indicating that most of the light is reabsorbed by the structure of the units. The absorption peaks ranging from 350 to 400 nm were assigned to the absorption of the conventional TiO2 nanoparticles. The image in the right panel is a close-up of the rectangular area in the left image.

Surprisingly, the photocurrent density of the optimized H-TiO2 reaches ~1.65 mA/cm2 at an applied bias voltage of 0.5 V,40 which is about 2.5 times that of the control sample P25. Importantly, under filtered exposure conditions (l > 420 nm), CdSe/H-TiO2 recorded a current density of ~14.2 mA/cm2, the highest value in the visible region, which can be attributed to the overlap of the excitation wavelength of CdSe with the wavelength where we notice a slight harvest.

Figure 2.2.1 Schematic illustrations and electron microscopy images of TiO 2 structures

Introduction

Preparation of patterned FTO (p-FTO)
Preparation of the ɑ-Fe 2 O 3 on p-FTO
PEC measurements

Conclusion
References

To determine the step distance and pinhole diameter of the patterns, the Lloyd's mirror angle and exposure time were varied. Fabrication of the worm-shaped ɑ-Fe2O3 layer followed the procedure previously reported by this group.1 30 ml of a 150 mM FeCl3 ∙ 6H2O aqueous solution was transferred to a Teflon-lined autoclave and the substrates were supported on the inside of the autoclave facing the wall from the p-FTO side. The exposed surface area of the working electrode was set to 0.25 cm2 by making a window with adhesive tape.

The morphology of p-FTO after RIE and the annealing steps to remove the SU-8 pattern residues are shown in Figure 2.3.2 and 2.3.2b. The detailed conditions of the RIE process are given in the experimental part of Supplementary Information. The worm-like ɑ-Fe2O3 is one of the well-known hematite structures that has shown excellent performance in PEC systems.8 The worm-like patterned hematite materials were grown closely on p-FTO, as shown in Figure 2.3.2c, proving strong potential for a large surface area and light scattering caused by the interactions between the periodically aligned pattern structure and the exposed light.

Light absorption is very important because it is closely related to the amount of light used to generate photocurrent in PEC cells. Ultimately, the significantly enhanced light scattering effect of p-FTO with a three-dimensional structure improves the light absorption of the catalyst, and thus increases the photocurrent under the visible UV region. The diameter of the semicircle in the middle frequency region is related to Rct, which corresponds to the interfacial charge transfer resistance between ɑ-Fe2O3.

The sudden appearance and decay of the photocurrent density with sharp rectangular shapes during the illumination on/off sequence implies the rapid conduction of photogenerated electrons from ɑ-Fe2O3 to p-FTO. This is important because the key problems of hematite photoanodes for photoelectrochemical water splitting have been effectively overcome by simple patterning of current collectors.

Figure 2.3.1 SEM images of SU-8 patterns on FTO; the top and cross-section view.

Solar Steam Generation

Mesoporous Three-Dimensional Graphene Networks for Highly Efficient Solar

Introduction

Materials
Preparation of GO Sheets
Preparation of 3DGN (3DGN-Ni)
Preparation of the Photoabsorbers on the Wood Piece
Characterization

Conclusion
References

The wooden piece's cylindrical container structures with a high aspect ratio enable it to float when partially immersed in a 3.5% NaCl aqueous solution (simulated seawater) during measurement of the solar desalination efficiency. The release of the generated vapor molecules is very important for the continuous and efficient generation of vapor molecules since the high heat capacity of water vapor molecules trapped in the structure of the photoabsorbing material inhibits the temperature increase of the photoabsorbing material. For a quantitative characterization of the photoabsorbing materials, the absorption spectra were measured in the broadband solar spectrum ranging from 300 to 2500 nm wavelength via UV-Vis-NIR spectrometry (Figure 3.1.4c).

While GO sheets exhibited an absorption value of about 90%, both 3DGN-Ni and 3DGN exhibit higher absorption exceeding 97% in the full spectrum range. A drop of water dropped on the surface of 3DGN flows into the 3DGN in a few seconds, confirming the hydrophilic surface property of the mesoporous structure. The surface temperature of bare wood and bulk water remained at 34.3 °C and 30.1 °C, respectively, since no photothermal conversion mechanism is operating.

By combining highly absorbent and mesoporous materials on the wooden column, a large temperature increase on the surface of the 3DGN under 1 sunlight resulted in the appearance of steam bubbles, indicating active water evaporation (Figure 3.1.8). 3DGN digital camera image and thermographic images of GO, 3DGN-Ni, and 3DGN deposited wooden posts on saltwater reveal the temperature differences between the top of the photoabsorber materials and the bulk of the saltwater after exposure to sunlight. The GO and 3DGN-Ni samples on the wooden columns present mass change rates of 1.42 and 1.49 kg/m2∙h, respectively, due to lower light absorption (Figure 3.1.4b and Figure 3.1.5) and lower content of pores with a smaller surface area, the photoabsorbers prevent rapid steam generation.

91.4% and 76.6% of the original value were retained after 20 cycles and 6 hours of continuous irradiation, proving a relatively good durability of our device (Figure 3.1.11). As shown in Figure 3.1.9d, the salinity of the vapor collected through the desalination process was well below the salinities of safe drinking water as defined by World Health Organization (WHO) and U.S. Environmental Protection Agency (EPA) standards7.