Haibo Guo

Naturally occurring iron oxide nanoparticles: morphology, surface chemistry and environmental stability

The widespread nanostructures of iron oxides and oxyhydroxides are important reagents in many biogeochemical processes in many parts of our planet and ecosystem. Their functions in various aspects are closely related to their shapes, sizes, and thermodynamic surroundings, and there is much that we can learn from these natural relationships. This review covers these subjects of several phases (ferrihydrite, goethite, hematite, magnetite, maghemite, lepidocrocite, akaganeite and schwertmannite) commonly found in water, soils and sediments. Due to surface passivation by ubiquitous water in aquatic and most terrestrial environments, the difference in formation energies of bulk phases can decrease substantially or change signs at the nanoscale because of the disproportionate surface effects. Phase transformations and the relative abundance are sensitive to changes in environmental conditions. Each of these phases (except maghemite) displays characteristic morphologies, while maghemite appears frequently to inherit the precursors morphology. We will see how an understanding of naturally occurring iron oxide nanostructures can provide useful insight for the production of synthetic iron oxide nanoparticles in technological settings.

Thermodynamic modelling of nanomorphologies of hematite and goethite

Iron oxide and oxyhydroxide nanoparticles are among the most important mobile and catalytic agents in a variety of biogeochemical environments, and are being increasingly synthesized for energy, electronic, catalyst, environmental and medical applications. The morphologies at nanoscale are relevant to the control of shapes and sizes, surface chemistry, and performance of these nanoparticles, as well as our understanding of naturally occurring processes. Therefore, we have begun to develop this understanding by studying the relationship between size, shape, and thermodynamic stability of unpassivated hematite (α-Fe2O3) and goethite (α-FeOOH) nanoparticles, using a robust thermodynamic morphology model with input parameters from reliable first-principles calculations and thermochemical data. The results revealed the thermodynamic stable shapes of hematite and goethite nanoparticles, and demonstrated that the phase transformation from goethite to hematite is highly dependent on the particle size and temperature. Goethite nanoparticles are thermodynamically stable with small sizes, compared to hematite, but the equilibrium transformation temperature increases rapidly with decreasing particle size. The morphology sensitive phase transformation predicted by our model is a step further towards a nanophase diagram of iron oxides and oxyhydroxides.

Modeling the iron oxides and oxyhydroxides for the prediction of environmentally sensitive phase transformations

structures may contain partially occupied sites or long-range ordering of vacancies, and some loose structures require proper description of weak interactions such as hydrogen bonding and dispersive forces. If structures and transformations are to be reliably predicted under different chemical conditions, each of these challenges must be overcome simultaneously while preserving a high level of numerical accuracy and physical sophistication. Here we present comparative studies of structure, magnetization, and elasticity properties of iron oxides and oxyhydroxides using density-functional-theory calculations with plane-wave (PW) and locally-confined-atomicorbital basis sets, which are implemented in VASP and SIESTA packages, respectively. We have selected hematite (α-Fe2O3), maghemite (γ-Fe2O3), goethite (α-FeOOH), lepidocrocite (γ-FeOOH), and magnetite (Fe3O4 )a s model systems from a total of 13 known iron oxides and oxyhydroxides, and we used the same convergence criteria and almost equivalent settings to make consistent comparisons. Our results show that both basis sets can reproduce the energetic stability and magnetic ordering, and are in agreement with experimental observations. There are advantages to choosing one basis set over the other, depending on the intended focus. In our case, we find the method using PW basis set the most appropriate, and we combine our results to construct the first phase diagram of iron oxides and oxyhydroxides in the space of competing chemical potentials, generated entirely from first principles.

Three-dimensional self-branching anatase TiO2 nanorods: morphology control, growth mechanism and dye-sensitized solar cell application

Complex three-dimensional (3D) hierarchical nanostructures based on well-defined low-dimensional nanobranches of different sizes and specific exposed facets are highly desirable to obtain tunable physicochemical properties. Herein, a facile, one-step hydrothermal method is employed to construct self-branching anatase TiO2 (SBAT) 3D hierarchical nanostructures. By simply controlling the reaction time and weight ratio of F127/TBAH, SBAT nanorods can be obtained with a large percentage of exposed {010} facets. Based on X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) analysis, a growth mechanism is proposed for the formation of such self-branching 3D nanostructures, which involves the formation of the L-shaped step edges on the [103] surfaces and the alignment of the crystal facets (103) of anatase nanocrystals with the (103) face on the tips of the main anatase TiO2 nanorods. The dye-sensitized solar cell assembled with the SBAT nanorods exhibits an outstanding power conversion efficiency of 7.17%, which is superior to that of the devices based on the 1D anatase TiO2 nanorods and P25 TiO2. This high performance can be attributed to the high dye-uptake density, large size and unique self-branching 3D hierarchical nanostructures built from 1D nanobranches growing epitaxially from the main rod.

Environmentally dependent stability of low-index hematite surfaces

Nanoparticulate hematite is a promising material for catalytic and photoelectrochemical applications, where the surfaces are engineered to improve efficiency in different chemical environments. In the presence of water, the surfaces are typically passivated by hydroxyl groups, which modify the surface stability and reactivity. We use density functional theory and first principles thermodynamics to investigate the low-index surfaces (001), (101), and (104) in hydrous environments. For each of the surfaces, we build various hydroxylation configurations and compare their thermodynamic stability under different environmental conditions (temperature, humidity, and supersaturation of oxygen). The results enable us to construct surface phase diagrams, which provide guidance to the selection of surface structures, and the control of environmental conditions for specific applications.

Can hematite nanoparticles be an environmental indicator

Environmental indicators play an important role in assessing the state of the environment and understanding the trend of environmental changes. Iron oxide nanostructures are widespread in multiple ecosystems, and are therefore suitable for providing environmental indicators with measurable physical or chemical properties that are sensitive to the environmental conditions. It has been found that the morphologies of natural hematite (α-Fe2O3) nanocrystals have large variations depending on thermodynamic and chemical conditions of their formation environments. To assess the applicability of nanomorphology as an environmental indicator, we construct a morphology map of hematite nanocrystals in hydrous environments, characterized with different temperatures, humidity and supersaturation of oxygen. Our model indicates that the fractional surface areas of morphologically significant facets change with humidity and temperature, and can be used to track the variation of these environmental conditions.

Surface phase diagram of hematite pseudocubes in hydrous environments

Hematite nanoparticles often display the pseudocubic morphology enclosed exclusively by the (012) surface. The surface chemistry on these facets is important to understand the formation and properties of these nanoparticles in varying chemical environments. The surface is typically terminated by hydroxyl groups in water or humid atmospheres, and the various terminations differ largely in composition, structure, thermodynamic stability, and chemical reactivity. When we compare a large variety of chemical configurations, we find that three specific terminations are thermodynamically stable under aerobic conditions. The termination by singly and triply coordinated hydroxyl groups (S-,T-OH) is stable at low temperatures and in hydrous environments, the stoichiometric clean termination is stable at high temperatures and in dry environments, and the termination by doubly coordinated hydroxyl groups (D-OH) is stable under intermediate conditions. The S-,T-OH termination can convert topologically to the clean surface by dissociative adsorption of water, whereas the conversion from these two terminations to the D-OH termination requires re-organization of several atomic layers at the surface. Therefore, the surface may reversibly change between S-,T-OH and the clean surface depending on the temperature and humidity. Based on these findings we have constructed surface phase diagrams to predict the termination types in different hydrous and humid environments.

Improved photovoltaic performance of dye-sensitized solar cells by carbon-ion implantation of tri-layer titania film electrodes

AbstractnTiO2 tri-layer structure films were modified by C-ions implantation for improving the photovoltaic performance of dye-sensitized solar cells (DSSCs), in which the structure of TiO2 changes from rutile to anatase and the sizes of TiO2 particles increase. The optimal concentration of ions implantation for C-implanted cells is 1xa0×xa01015xa0atom·cm−2, and the maximum conversion efficiency of 5.32xa0% is achieved (luminous intensity of 1 sun, light irradiance of AM1.5G), which is 25.2xa0% higher than that of unimplanted cell. The significant improvement in conversion efficiency by carbon-ion implantation is contributed to reducing charge recombination and enhancing the light-harvesting ability, as indicated from incident photon-to-collected electron conversion efficiency (IPCE) and ultraviolet-visible (UV-Vis) measurements. Furthermore, the charge carrier’s lifetime in the tri-layer titania films is prolonged after carbon-ion implantations.