Aditya Narayan Panda for their regular evaluation of my research work and their valuable suggestions. I sincerely thank all the staff of the Central Instruments Facility for their assistance and management of several analytical instruments required during my research work.
Experimental Section
Schematic illustration of different functions of photoanodic architecture based on 2D porous ZnO disk structures, sensitized with 1D CdS nanowires and CuSbS2 nanobricks. A comparative photovoltaic study of as-synthesized 2-D porous ZnO structures with respect to 1-D ZnO structures (in the absence of xanthan gum) is performed by co-sensitization with in-situ grown 1-D CdS nanowires and ternary metal chalcogenide ie .
Photovoltaic properties of as-synthesized 3D ZnO HSs compared to its basic structural unit, i.e., ZnO NRs are investigated by sensitization with a bisthiazole-linked metal-free donor-acceptor dye; D1. Electrochemical impedance spectroscopic (EIS) analysis is performed to support a slower photogenerated electron-hole recombination rate and better charge transports in the 3D ZnO HSs based photovoltaic device.
Chapter 6: Multifunctional Hierarchical 3D ZnO Superstructures Directly Grown over FTO glass substrates: Enhanced Photovoltaic and Selective Sensing Applications
Traces (a, b) represent the field emission scanning electron microscope (FESEM) images of the in situ grown 3-D hierarchical ZnO superstructure of different regions at different magnifications. The inset of (B) shows the hierarchical 3-D ZnO superstructures grown in-situ, useful for effective light utilization by providing higher charge sites for sensitizer adsorption.
Introduction and Literature Survey
AN OVERVIEW ON PHOTOVOLTAIC TECHNOLOGY
Fabrication of high-efficiency, cheap and lightweight solar cells with large areas is the main concern in this category. Furthermore, these solar cells are able to meet Shockley and Queisser's theoretical upper limit for single-junction solar cells.7 Essentially, in 1991, O'Regan and M.
WORKING PRINCIPLE OF DSSC/QDSSC DEVICES
Electron injection process (process 2) must be faster than the recombination process (process 6) of the sensitizer for better kinetics of electron injection.1920 In general, it is observed that photoinduced electron injection of the dye molecules to metal oxide films is within reach. of femtosecond (10–15 s) while the recombination process of photoinduced electrons (relaxation process) is pragmatic in the range of 10–7 s.21 Thus, electron injection efficiency (ɸinj) is achieved very close to unity in the case of optimized DSSC devices. However, photo-excited electron injection process of a QDSSC device is found to be in the range of picoseconds (10–12 s), which is slower with respect to electron injection process of a DSSC device.22–23 In QD sensitizers, Time scales of direct band-to-band recombination and capture-mediated recombination processes of photoexcited electrons vary in the range from 10–11 to 10–6 s, depending mainly on specific semiconductor and nature of semiconductor surface.22 Commonly used semiconductor QDs ( IIVI) such as e.g. , CdS, CdSe, CdTe etc.
Different morphologies of ZnO are used as electron transport material in DSSC devices to improve the photovoltaic conversion efficiency, summarized in Table 1.3.2. Different photoanode architectures based on multiple ZnO morphologies are demonstrated for the improved photovoltaic performance of ZnO-based QDDSCs, shown in Table 1.3.3.
BASIC INTRODUCTION OF CHEMICAL SENSOR
- GAS SENSOR
- SEMICONDUTOR METAL OXIDES BASED CHEMIRESISTOR GAS SENSOR Chemiresistor sensor devices based on semiconductor metal oxides are extensively
Based on performance parameters, comparative studies of different types of gas sensors are summarized in table 1.4.1.136. The performance of a chemiresistor device can be evaluated based on the parameters which are responsiveness, selectivity, response/recovery time, reproducibility, repeatability and stability.
CONCLUSIONS AND OUTLOOK
MOTIVATION AND OBJECTIVES OF THE PRESENT WORK
Fabrication of chemire resistor devices using different morphologies of ZnO and evaluate the sensing performance for ammonia gas sensing. This chapter discusses the common synthetic methodologies used for the preparation of materials, basic instrumental techniques for material characterization, fabrication of semiconductor metal chalcogenide/dye-sensitized solar cells emphasizing the ZnO-based photoanode preparation.
Materials Synthesis Characterization Device Fabrication
INTRODUCTION
In the following chapters, various biomass-derived template agents are used to tune the morphology of the ZnO structure. Various steps involved in the synthesis and fabrication of quantum dot-sensitive solar cells (QDSSC) and dye-sensitive solar cells (DSSC) are described.
EXPERIMENTAL METODS
- MATERIALS AND REAGENTS USED
- CHARACTERIZATION OF AS-SYNTHESIZED MATERIALS, PHOTOVOLTAIC AND CHEMIRESISTOR DEVICES
- STEPS INVOLVED FOR THE FABRICATION OF A QDSSC AND DSSC (A) PHOTOANODE PREPARATION
- ESTIMATION OF SOLAR CELL PERFORMANCE
- ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY (EIS) ANALYSIS
- FABRICATION OF TWO-TERMINAL CHEMIRESISTOR DEVICE FOR VAPOR PHASE SENSING
- GAS SENSING ASSEMBLY
- GAS SENSING PERFORMANCE PARAMETERS
The efficiency of a photovoltaic device is sensitive to differences in both the power and the spectrum of the incident light. The response time of a sensor device is the time required to change 90% of the current analyte vapors.
The lower response and recovery times of the detection device are well suited for rapid detection of analyte vapors.
INTRODUCTION
Therefore, several groups have successfully synthesized various morphological structures of g-C3N4 such as nanosheets, nanoparticles, nanoribbons, nanobelts, nanorods, nanotubes, and hollow mesoporous spheres, solving the drawbacks associated with the usability of g-C3N4.15 , tuning the morphology of bulk g-C3N4 is a very challenging work due to its layered structure, produced during the polycondensation process. In this chapter, g-C3N4 NTs are synthesized through spinning of ultrasonically exfoliated g-C3N4 nanoflakes (NFs),29 generating a thermal gradient between the outer and inner surfaces of g-C3N4.
- SYNTHESIS OF gC 3 N 4 NANOFLAKES (NFs) AND NANOTUBES (NTs)
- SYNTHESIS OF ZINC OXIDE NANORODS (ZnO NRs)
- SYNTHESIS OF ZnO NANOPARTICLES (NPs)
- FABRICATION OF PHOTOANODES AND DEVICES
Photoanodes based on ZnO NR-g-C3N4 NF or ZnO NR-g-C3N4 NT composites were prepared by making a homogeneous paste of the composite powders. After this, the fabricated photoanode was shown as ZnO NR-g-C3N4 NF-CdS or ZnO NR-g-C3N4 NT-CdS.
RESULTS AND DISCUSSIONS
- POWDER X-RAY DIFFRACTION ANALYSIS
Similarly, photoanodes of ZnO NPs-based composites of g-C3N4 NFs and g-C3N4 NTs were fabricated using a similar procedure used for photoanodes of ZnO NRs-based composites. ZnO NPs-based photoanodes were fabricated by following the same procedure used for the fabrication of ZnO NRs-based photoanodes.
2 (degree)
- MATERIAL MORPHOLOGY
- FORMATION MECHANISM FOR TUBULAR STRUCTURES OF gC 3 N 4
- ENERGY DISPERSIVE X-RAY SPECTROSCOPIC ANALYSIS
- BET SURFACE AREA ANALYSIS
- UV–VISIBLE DIFFUSE REFLECTANCE SPECTRA
- STEADY STATE AND TIME RESOLVED PHOTOLUMINESCENCE ANALYSES Figure 3.3.8 (A) shows the steady state photoluminescence (PL) spectra for (a) g-C 3 N 4
Based on all the considerations related to texture modification in the morphology section, the probable mechanism for producing tubular structures of g-C3N4 NFs, i.e. g-C3N4 NTs, highlighted in scheme 3.3.1. Tubular nature of g-C3N4 NTs is likely reason for better light scattering which is not in the case of g-C3N4 NFs.
Wavelength (nm)
Time (ns)
FOURIER TRANSFORM INFRARED (FTIR) SPECTRA
In the FTIR spectrum of a pristine material such as trace ZnO NRs (a), a dominant peak centered at 531 cm1 corresponding to. The stretching vibration mode of ZnO.37 In trace (b), g-C3N4 NF revealed a characteristic peak at 810 cm1, which corresponds to the bending modes of the repeating units of s-triazine g-C3N4.38 Different strong peaks are observed in the region of 1200 1650 cm1, which relate to the typical stretching modes of CN heterocycles.
Wavenumber (cm -1 )
PHOTOVOLTAIC MEASUREMENTS OF SOLAR CELLS
Although the composite ratio (1:0.25, ZnO NR in g-C3N4) has a faster rate of charge injection (in TRPL analysis), but the efficiency value is lower due to the lower wealth of sites surfactants for loading CdS QDs. For devices having a weight ratio (1:0.75, ZnO NR to g-C3N4), the efficiency value further decreases relative to the composite-based devices (1:0.5), possibly due to an amount of addition of g-C3N4 in the composition, leads to non-uniform loading of CdS QD.
ELECTROCHEMCIAL IMPEDANCE SPECTROSCOPY ANALYSIS
Photoinduced electron lifetimes (e) can be estimated from characteristic maximum peak frequency (fmax) in the mid-frequency region. It is noteworthy that sensitization of CdS QDs in the photoanodes leads to the probability of deposition on both the scaffolds, i.e., g-C3N4 and ZnO NRs; thus, two routes are possible for charge transfer processes in the devices.
MORPHOLOGY DEPENDENT PHOTOVOLTAIC MEASUREMENTS
ZnO NRs provide a minimal population of grain boundary-induced trap sites and a longer diffusion length of photogenerated electrons, resulting in less recombination of photogenerated electrons yielding better photovoltaic performance compared to ZnO NP-based devices. It is observed that photoanodes composed of ZnO NRs showed higher IPCE compared to ZnO NPs for both g-C3N4 morphologies due to the stronger synergistic effect between ZnO NRs and g-C3N4.
STABILITY OF PHOTOVOLTAIC DEVICES
Based on the observations, we conclude that g-C3N4 NTs-based devices are more stable compared to g-C3N4 NFs-based devices.
CONCLUSIONS
Combined effect of p-type CuSbS2 / n- in situ-grown CdS coupled with hierarchical ZnO-type CdS coupled with hierarchical ZnO nano-disks for improved photovoltaic light harvesting efficiency. The comparative photovoltaic study of the synthesized 2D porous ZnO structures with respect to the 1D ZnO structures is carried out by co-sensitizing with in-situ grown 1D CdS nanowire array and ternary metal chalcogenide, viz.
INTRODUCTION
This shaping agent is mainly responsible for controlling the growth kinetics of high-energy facets of the wurtzite-ZnO crystal structure to obtain disc structures.
EXPERIMENTAL METHODS
- SYNTHESIS OF POROUS 2D ZnO DISK
- SYNTHESIS OF COPPER ANTIMONY SULFIDE NANOBRICKS
- FABRICATION OF PHOTOANODES AND DEVICES
After that, these photoanodic films were calcined at 450 °C for 30 min to remove organic impurities. Finally, the photoanodic films were calcined at 350 °C for 30 min in the presence of N2 atmosphere to obtain stable absorbent film in the crystalline phase.
RESULTS AND DISCUSSIONS
- POWDER X-RAY DIFFRACTION ANALYSIS
- MATERIAL MORPHOLOGY
FESEM image (a) of Figure 4.3.2 shows the microstructural features of as-synthesized porous 2D ZnO disk structures. Furthermore, structural features of as-synthesized 2D ZnO structures are confirmed by performing transmission electron microscopy (TEM) analysis.
2-D ZnO
Trace (b) represents the selected area electron diffraction (SAED) pattern, while trace (c) shows the high-resolution transmission electron microscopy (HRTEM) image of in-situ grown 1-D CdS structures. Trace (h) represents the SAED pattern and trace (i) shows the HRTEM image of co-sensitized composite material.
CuSbS 2 )
Trace (e) shows the SAED pattern while trace (f) illustrates the HRTEM image of the as-synthesized CuSbS2.
CdS)
ZnO) 0.35 nm
- TEM ELEMENTAL MAPPING AND ENERGY DISPERSIVE X-RAY SPECTROSCOPIC ANALYSES
- UV–VISIBLE DIFFUSE REFLECTANCE SPECTRA
- BET SURFACE AREA ANALYSIS
- RAMAN SPECTRA
- PHOTOVOLTAIC MEASUREMENTS OF SOLAR CELLS
- ELECTROCHEMCIAL IMPEDANCE SPECTROSCOPY ANALYSIS
- STABILITY OF PHOTOVOLTAIC DEVICES
- CONCLUSIONS
- REFERENCES
Based on these isotherms, the BET surface area was calculated for both ZnO structures, the porous 2D ZnO (synthesized in the presence of xanthan gum) and the 1D ZnO structure. Consequently, the better loading of the sensitizer particles advocates the improved light-harvesting ability of the photoanode film based on the porous 2D ZnO structures.
Enhanced Photovoltaic Performance Using Biomass Derived 3D ZnO Hierarchical Superstructures and a
INTRODUCTION
These superstructures are believed to be constructed in principle by the nonclassical self-oriented attachment of basic building blocks, where high-energy surfaces are eliminated by the epitaxial attachment of nanocrystals and are considered as a driving force for crystal growth. We have discussed the likely reaction mechanism for the formation of these superstructures under controlled reaction conditions.
EXPERIMENTAL METHODS
- SYNTHESIS OF 3D ZnO HIERARCHICAL SUPERSTRUCTURES
- SYNTHESIS OF DYE D1
- FABRICATION OF PHOTOANODES AND DEVICES
ZnO seed solution was prepared by dissolving 0.20 g of zinc acetate and 55 μL of ethanolamine in 3.0 mL of 2-methoxyethanol followed by overnight stirring. Filtered ZnO solution was spun onto the FTO and then heated at ∼200 °C for 15 min.
RESULTS AND DISCUSSIONS
- POWDER X-RAY DIFFRACTION ANALYSIS
- MATERIAL MORPHOLOGY
- GROWTH MECHANISM OF 3D ZnO SUPERSTRUCTURES
- UV–VISIBLE DIFFUSE REFLECTANCE SPECTRA
- STEADY STATE AND TIME RESOLVED PHOTOLUMINESCENCE ANALYSES Figure 5.3.6 (A) shows the steady state photoluminescence (PL) spectra for both the
- BET SURFACE AREA ANALYSIS, CHEMISORPTION AND NORMALISED DIFFUSED REFLECTANCE UV–VISIBLE ABSORPTION SPECTRA OF
Transmission electron microscopy (TEM) images (c–d) show the structural features of 1D ZnO NRs at different magnifications. We have noticed a reasonable blue shift (15 nm) in the absorption profile of 3D ZnO superstructures compared to 1D ZnO NRs.
ZnO NRs-D1
- ELECTROCHEMICAL AND COMPUTATIONAL STUDY OF DYE D1
- PHOTOVOLTAIC MEASUREMENTS OF SOLAR CELLS
- ELECTROCHEMCIAL IMPEDANCE SPECTROSCOPY ANALYSIS
- STABILITY OF PHOTOVOLTAIC DEVICES
- CONCLUSIONS
- REFERENCES
Efficient light confinement in the 3D ZnO superstructures-based photoanode is probably due to the hollow nature of ZnO superstructures shown in the inset of figure 5.3.10 (B). Diameter of the right semicircle (Rk), observed at medium frequency range which is mainly related to the charge transfer resistance (Rct).34 From figure 5.3.11 (A), as can be seen that diameter of semicircle (Rk) is significantly higher for 3D ZnO superstructures-based device (olive line) compared to ZnO NRs-based device (blue line).
Multifunctional Hierarchical 3D ZnO Superstructures Directly Grown over FTO Glass
INTRODUCTION
The fabrication of semiconductors with hierarchical morphologies based on one-dimensional (1D) nanostructures appears to be a capable way to significantly improve the charge transfers and surface area.1 Multifunctional three-dimensional (3D) hierarchical nanoarchitectures have the ability to vitalize the photo -electrical conversion efficiency due to their favorable properties such as high sensitizer loading, strong light scattering effect and efficient electron transport through 1D networks.24 In order to assemble the 1D nanostructures into a regularly ordered array, various growth patterns were utilized on based on their crystal growth behavior similar to template-derived homo- or hetero-epitaxial growth behavior.5 Crystal growth mechanism is essential to understand the epitaxial attachment of 1D nanocrystals to form the hierarchical structures via non-classical growth mechanism.6 10 Growth patterns of different forms are greatly influenced by different reaction conditions and internal structure of a given crystal. Gas sensors play an important role for the detection and monitoring of various toxic and harmful gases,15 which are generated as a result of rapid industrialization, burning of fuels and the use of agrochemicals.
EXPERIMENTAL METHODS
- INSITU GROWTH OF ZnO HETEROSTRUCTURES
- PHOTOVOLTAIC DEVICE FABRICATION
- TWO-TERMINAL CHEMIRESISTOR DEVICE FABRICATION
- VAPOR PHASE DETECTION
These FTO substrates grown on ZnO heterostructures were further used to fabricate the N719 dye-sensitized photoanode. In a second step, a similar synthetic procedure (as discussed for the in situ growth of ZnO heterostructures) was used to deposit the ZnO seed layer as well as to grow the in situ ZnO structures.
RESULTS AND DISCUSSIONS
- POWDER X-RAY DIFFRACTION ANALYSIS
- MATERIAL MORPHOLOGY
- DIFFUSE REFLECTANCE UVVISIBLE ABSORPTION SPECTRA ANALYSES Optical behaviour of both in-situ grown ZnO heterostructures are examined by recording
- STEADY STATE AND TIME RESOLVED PHOTOLUMINESCENCE ANALYSES Room temperature steady-state and dynamic photoluminescence (PL) analyses are
- PHOTOVOLTAIC MEASUREMENTS OF THE SOLAR CELLS
- ELECTROCHEMCIAL IMPEDANCE SPECTROSCOPY ANALYSIS
- OPEN CIRCUIT VOLTAGE DECAY (OCVD) AND TAFEL POLARIZATION CURVE
- PHOTO-STABILITY OF SOLAR DEVICE
- SENSING STUDIES OF CHEMIRESISITOR DEVICES
- STABILITY OF CHEMIRESISTOR DEVICE
The size distribution of these in situ grown hierarchical superstructures is observed in the micron range. Trace (f) shows the high-resolution transmission electron microscopy (HRTEM) image of the hierarchical 3D ZnO superstructure.
CONCLUSIONS
Based on various sensor experiments such as selectivity, sensitivity, LOD, recyclability, response/recovery time and stability, both sensor devices have been shown to be cost-effective, stable and capable for the detection of NH3 vapors under practical conditions.
Time (s)
THESIS SUMMARY
FUTURE SUGGESTIONS
In Chapter 6, we have reported the biomass-assisted in-situ growth of 3D ZnO multifunctional hierarchical superstructures grown directly on FTO substrates. We have utilized in-situ grown 3D ZnO hierarchical structures in photovoltaic as well as selective chemical vapor sensing.
