Nanoporous metal oxides templated by nanocrystalline cellulose

Porous materials play an important role in numerous environmental applications including energy storage, energy conversion and environmental remediation systems. Reducing structural features down to the nanoscale drastically alters materials properties and leads to the enhancement of materials performance. The successful fabrication of efficient functional materials requires a high degree of control over their morphology addressing the needs of target applications. The goal of this work was to develop a versatile general approach towards the synthesis of nanoporous metal oxides by using biogenic cellulose nanocrystals. Nanocrystalline cellulose (NCC) is an abundant biological nanomaterial that can be extracted from natural bulk celluloses. The present thesis demonstrates that the unique properties of NCC enable the efficient synthesis of porous titania and iron oxide (hematite) thin films by using sacrificial templating with cellulose nanocrystals. In particular, this study reveals the mechanism of metal oxide formation in the presence of cellulose, as well as the effect of NCC-templated porous scaffolds on titania performance in photocatalysis and dye sensitized solar cells. Chapter 1 provides general information about properties, application areas and common synthesis methods of nanoporous metal oxides, with an emphasis put on titanium oxide materials and biotemplating approaches. Chapter 2 discusses the basic principles of analytical methods employed to characterize porous nanomaterials. Chapters 3‒6 reveal the experimental procedures towards NCC-templated porous titania and hematite thin films, their characterization and their applications. First, the extraction of cellulose crystals from bulk celluloses is discussed. Different cellulose sources, as well as variable hydrolysis parameters have been employed to define the optimal procedure for the NCC preparation. Cotton fibers have provided the best results regarding the crystallinity, purity and shape of extracted cellulose crystals. Furthermore, repeated washings have been shown to narrow down the size distribution and to improve the crystallinity of cotton NCC. Chapter 4 focuses on the synthesis of porous titania thin films assisted by nanocrystalline cellulose. The tunable porosity of titania thin films is a key factor for successful applications in photovoltaics, sensing and photocatalysis. To synthesize NCC-templated titania, the cellulose nanocrystals are introduced to a titania precursor solution. The colloidal mixtures can be directly spin- or dip- coated on glass, silicon and transparent conducting oxide (TCO) substrates and then calcined to remove the template and to crystallize the titania porous network. The obtained structures are highly porous anatase morphologies having well-defined, narrow pore size distribution. We show that by varying the titania-to-template ratio it is possible to tune the surface area, pore size, pore anisotropy and dimensions of titania crystallites in the films. Post-treatment at high humidity and subsequent slow template removal promote pore widening; this treatment is also beneficial for the multilayer deposition of thick films. The NCC-templated mesoporous titania films show very high activity in the photocatalytic NO (nitrogen(II) oxide) conversion and in the degradation of 4-chlorophenol. Furthermore, the films are successfully applied as anodes in dye-sensitized solar cells. Chapter 5 presents a strategy toward enhancement of the photocatalytic activity of NCC-templated titania thin films by introducing solvothermally synthesized preformed anatase nanoparticles into a sol-gel based biotemplated titania scaffold. The synthesis is based on the self-assembly of two types of precursors, namely crystalline and sol-gel titania, directed by the biogenic NCC template. Due to the shape persistence of the template, the NCC-templated titania scaffolds can accommodate large amounts of preformed titania without a significant reduction of the film porosity. The resulting dual source titania thin films containing different amounts of preformed crystalline species were investigated with time resolved microwave conductivity (TRMC) measurements and tested in the photocatalytic conversion of 4-chlorophenol. The gradual addition of preformed nanoparticles leads to a consistent increase of the mean size of titania crystalline domains, whereas the porosity of the composite is well-preserved due to the rigid nature of the NCC template. The microwave conductivity studies establish increased photoconductivity of the films containing preformed anatase nanoparticles, in comparison to that of films made without the nanoparticles. The synergistic features of the dual source titania, namely the improved crystalline properties brought by the preformed nanocrystals in combination with the high surface area provided by the NCC-templated sol-gel titania, result in a very high photocatalytic activity of the films in the photocatalytic decomposition of 4-chlorophenol. In quantitative terms, the dual source titania films prepared with 75% nanoparticles exhibit a first order degradation rate constant of 0.53 h-1, strongly outperforming the activity of commercial P90 nanopowder showing a rate constant of 0.17 h-1 under the same conditions. We have also adapted the NCC templating protocol for the fabrication of porous iron oxide (hematite) thin films. Chapter 6 discusses the formation of porous iron oxide nanostructures via sol-gel transformations of molecular precursors in the confined space of self-organized cellulose nanocrystals used as a shape-persistent template. The obtained structures are highly porous hematite morphologies featuring pronounced anisotropic porosity. The character of the porous nanostructure depends on the iron salt used as precursor and on the heat treatment, respectively. Moreover, a post-synthetic hydrothermal treatment of the NCC/iron salt composites strongly affects the crystal growth, as well as the porous nanomorphology of the obtained hematite scaffolds. We demonstrate that the hydrothermal treatment alters the crystallization mechanism of the molecular iron precursors, which proceeds via the formation of anisotropic iron oxyhydroxide species. The present study reveals that the nanocellulose templating technique enables a straightforward fabrication of a variety of porous crystalline scaffolds with well-defined mesoporous structure. For the first time the NCC has been used for the fabrication of homogeneous porous metal oxide films on different substrates, in contrast to the previously reported powders or free-standing membranes. The versatility and flexibility of the NCC templating approach offers broad perspectives towards the generalization of this method for the fabrication of different types of nanoporous metal oxides

Influence of the preparation method on planar perovskite CH3NH3PbI3-xClx solar cell performance and hysteresis

The present research has been supported by the State Research Programme “LATENERGI” and National Research Programme “Multifunctional Materials and Composites, Photonics and Nanotechnology (IMIS2)”.Organometal halide perovskites are promising materials for lowcost, high-efficiency solar cells. The method of perovskite layer deposition and the interfacial layers play an important role in determining the efficiency of perovskite solar cells (PSCs). In the paper, we demonstrate inverted planar perovskite solar cells where perovskite layers are deposited by two-step modified interdiffusion and one-step methods. We also demonstrate how PSC parameters change by doping of charge transport layers (CTL). We used dimethylsupoxide (DMSO) as dopant for the hole transport layer (PEDOT:PSS) but for the electron transport layer [6,6]-phenyl C61 butyric acid methyl ester (PCBM)) we used N,N-dimethyl-N-octadecyl(3-aminopropyl)trimethoxysilyl chloride (DMOAP). The highest main PSC parameters (PCE, EQE, VOC) were obtained for cells prepared by the one-step method with fast crystallization and doped CTLs but higher fill factor (FF) and shunt resistance (Rsh) values were obtained for cells prepared by the two-step method with undoped CTLs.Institute of Solid State Physics, University of Latvia as the Center of Excellence has received funding from the European Union’s Horizon 2020 Framework Programme H2020-WIDESPREAD-01-2016-2017-TeamingPhase2 under grant agreement No. 739508, project CAMART

Research data supporting Co-assembly of Cellulose Nanocrystals and Silk Fibroin into Photonic Cholesteric Films

The dataset is compressed into “.zip” files grouped in folders corresponding to the figures in which they were reported. All spectra were exported in format that are accessible with free software, and sometimes in additional format that require a license (Matlab, Origin, MS Excel).EPSRC (1525292), MSCA Individual fellowship (BINGO 743543), the Office of Naval research, USA (N000141812258), National Science Foundation, USA (award CMMI-1752172)

Influence of the orientation of methylammonium lead iodide perovskite crystals on solar cell performance

Perovskite solar cells are emerging as serious candidates for thin film photovoltaics with power conversion efficiencies already exceeding 16%. Devices based on a planar heterojunction architecture, where the MAPbI(3) perovskite film is simply sandwiched between two charge selective extraction contacts, can be processed at low temperatures (<150 degrees C), making them particularly attractive for tandem and flexible applications. However, in this configuration, the perovskite crystals formed are more or less randomly oriented on the surface. Our results show that by increasing the conversion step temperature from room temperature to 60 degrees C, the perovskite crystal orientation on the substrate can be controlled. We find that films with a preferential orientation of the long axis of the tetragonal unit cell parallel to the substrate achieve the highest short circuit currents and correspondingly the highest photovoltaic performance

Cellulose Nanocrystal-Templated Tin Dioxide Thin Films for Gas Sensing.

Porous tin dioxide is an important low-cost semiconductor applied in electronics, gas sensors, and biosensors. Here, we present a versatile template-assisted synthesis of nanostructured tin dioxide thin films using cellulose nanocrystals (CNCs). We demonstrate that the structural features of CNC-templated tin dioxide films strongly depend on the precursor composition. The precursor properties were studied by using low-temperature nuclear magnetic resonance spectroscopy of tin tetrachloride in solution. We demonstrate that it is possible to optimize the precursor conditions to obtain homogeneous precursor mixtures and therefore highly porous thin films with pore dimensions in the range of 10-20 nm (ABET = 46-64 m2 g-1, measured on powder). Finally, by exploiting the high surface area of the material, we developed a resistive gas sensor based on CNC-templated tin dioxide. The sensor shows high sensitivity to carbon monoxide (CO) in ppm concentrations and low cross-sensitivity to humidity. Most importantly, the sensing kinetics are remarkably fast; both the response to the analyte gas and the signal decay after gas exposure occur within a few seconds, faster than in standard SnO2-based CO sensors. This is attributed to the high gas accessibility of the very thin porous film

Influence of the orientation of methylammonium lead iodide perovskite crystals on solar cell performance

Perovskite solar cells are emerging as serious candidates for thin film photovoltaics with power conversion efficiencies already exceeding 16%. Devices based on a planar heterojunction architecture, where the MAPbI3 perovskite film is simply sandwiched between two charge selective extraction contacts, can be processed at low temperatures (&lt;150 °C), making them particularly attractive for tandem and flexible applications. However, in this configuration, the perovskite crystals formed are more or less randomly oriented on the surface. Our results show that by increasing the conversion step temperature from room temperature to 60 °C, the perovskite crystal orientation on the substrate can be controlled. We find that films with a preferential orientation of the long axis of the tetragonal unit cell parallel to the substrate achieve the highest short circuit currents and correspondingly the highest photovoltaic performance

Co‐Assembly of Cellulose Nanocrystals and Silk Fibroin into Photonic Cholesteric Films

Abstract: Controlled self‐assembly of bio‐sourced nanocolloids is of high importance for the development of sustainable and low‐cost functional materials but controlling nanocomposite fabrication with both satisfactory optical properties and composition remains challenging. Silk fibroin (SF) and cellulose nanocrystals (CNCs) have independently demonstrated their ability to produce high‐quality photonic materials, in part due to their low absorbance and their transparency in the visible range. While SF is able to replicate inverse structures by high‐resolution nano‐templating, CNCs can spontaneously assemble into cholesteric liquid crystalline structures that are retained upon solvent evaporation, yielding photonic films. In this work, the conditions of successful co‐assembly of regenerated SF, extracted from silkworm silk, with CNCs extracted from cotton, are investigated. Their co‐assembly is investigated for various relative concentration ratios and pH, combining polarized optical microscopy and spectroscopy, SEM, and other characterization techniques (XRD, ATR‐FTIR, TGA). The appearance of photonic properties is observed when CNC and SF are assembled at pH ≥ 4.15, highlighting the importance of suppressing attractive electrostatic interactions between the two species for an organized structure to emerge. Beyond its fundamental motivations for colloidal co‐assembly with structural proteins, this work is relevant to design sustainable optical materials compatible with food packaging coatings and edible coloring pigments

Tailoring the Morphology of Mesoporous Titania Thin Films through Biotemplating with Nanocrystalline Cellulose

The tunable porosity of titania thin films is a key factor for successful applications in photovoltaics, sensing, and photocatalysis. Here, we report on nanocrystalline cellulose (NCC) as a novel shape-persistent templating agent enabling the straightforward synthesis of mesoporous titania thin films. The obtained structures are highly porous anatase morphologies having well-defined, narrow pore size distributions. By varying the titania-to-template ratio, it is possible to tune the surface area, pore size, pore anisotropy, and dimensions of titania crystallites in the films. Moreover, a post-treatment at high humidity and subsequent slow template removal can be used to achieve pore widening; this treatment is also beneficial for the multilayer deposition of thick films. The resulting homogeneous transparent films can be directly spin- or dip- coated on glass, silicon, and transparent conducting oxide (TCO) substrates. The mesoporous titania films show very high activity in the photocatalytic NO conversion and in the degradation of 4-chlorophenol. Furthermore, the films can be successfully applied as anodes in dye-sensitized solar cells

Nanocellulose-Assisted Formation of Porous Hematite Nanostructures

We report the formation of porous iron oxide (hematite) nanostructures via sol–gel transformations of molecular precursors in the confined space of self-organized nanocrystalline cellulose (NCC) used as a shape-persistent template. The obtained structures are highly porous α-Fe₂O₃ (hematite) morphologies with a well-defined anisotropic porosity. The character of the porous nanostructure depends on the iron salt used as the precursor and the heat treatment. Moreover, a postsynthetic hydrothermal treatment of the NCC/iron salt composites strongly affects the crystal growth as well as the porous nanomorphology of the obtained hematite scaffolds. We demonstrate that the hydrothermal treatment alters the crystallization mechanism of the molecular iron precursors, which proceeds via the formation of anisotropic iron oxyhydroxide species. The nanocellulose templating technique established here enables the straightforward fabrication of a variety of mesoporous crystalline iron oxide scaffolds with defined porous structure and is particularly attractive for the processing of porous hematite films on different substrates

Research data supporting "Cellulose Nanocrystal-Templated Tin Dioxide Thin Films for Gas Sensing"

the dataset provides the raw data for a variety of characterisation: Photographs, transmittance spectra, 119Sn NMR, SEM, TEM, FFT, sensor measurements, XRD, sorption. A detailed summary (.pdf) is provided to complement the data.The German Science Fundation (DFG) for funding in the context of the Nanosystems Initiative Munich (NIM) Excellence Cluster