First principles DFT study of hematite clusters on anatase TiO2 and the effect of oxygen vacancy

Abstract. The main focus of this thesis, photocatalysis, has attracted enormous attention to tackle the global energy and environmental issues. A design and synthesis of photocatalysts with high photocatalytic activities have increased the scientific interest worldwide in order to develop a green and environmentally friendly technology. Heterogeneous photocatalysis on titanium dioxide, TiO2, is a potential solution to the issues we are facing. However, the photocatalytic efficiency of TiO2 is greatly limited by the large band gap energy and the high recombination rate of the photogenerated electrons and holes. Therefore, attempts are made to, not only doping TiO2 with various elements, but also loading TiO2 surface with appropriate cocatalysts. A coupling of TiO2 with another semiconductor with a suitable band gap and a band edge position can enhance visible light adsorption by reducing the band gap and inhibit recombination phenomena of the charge carriers. In this thesis, hematite (\alpha-Fe2O3) clusters on anatase TiO2 (101) surface and further the effect of oxygen vacancy in the combined system were investigated. We used powerful density functional based calculations to model our systems. The simulations provided insight to phenomena occurring at the atomic level in the investigated systems and thus better understanding on photocatalytic processes on theoretical level. We found energetically favorable interaction between TiO2 and hematite clusters, thus enabling the modification of the surface properties. Due to the adsorption, impurity states arise, narrowing the band gap of TiO2. A notable charge transfer from the cluster to the surface was observed, which may play crucial role in the photocatalytic reactions. The band alignment of TiO2 and hematite showed a potential formation of a heterojunction that can promote the observed charge transfer and suppress the recombination rate of TiO2. A large Fe2O3 cluster size has been proposed to hinder the photocatalytic performance of TiO2. Our results did not completely confirm or refute this claim. They suggest that the larger cluster size can have an influence on the electronic properties and thus the photocatalytic performance. An introduction of oxygen vacancies, both at the TiO2 and at the cluster, further altered the properties of the combined system. Oxygen vacancies can further decrease the band gap of TiO2, thus expanding the visible light response of the system, and lead to formation of a different types of heterojunction between TiO2 and hematite. The effect of oxygen vacancy located at the cluster is not observed to be as significant in order to decrease the band gap. In the presence of oxygen vacancy at TiO2 we observed the direction of the charge transfer to reverse, occurring from the surface to the cluster and thus promoting the oxidization ability of TiO2. Overall, the results indicate that hematite clusters and further oxygen vacancy can modify the surface properties of TiO2 and the photocatalytic mechanism in the combined system. They also suggest that hematite as a cocatalyst can improve the photocatalytic performance of pristine TiO2 surface

Energetic particle fluxes in the exterior cusp and the high-latitude dayside magnetosphere: statistical results from the Cluster/RAPID instrument

In this paper we study the fluxes of energetic protons (30–4000 keV) and electrons (20–400 keV) in the exterior cusp and in the adjacent high-latitude dayside plasma sheet (HLPS) with the Cluster/RAPID instrument. Using two sample orbits we demonstrate that the Cluster observations at high latitudes can be dramatically different because the satellite orbit traverses different plasma regions for different external conditions. We make a statistical study of energetic particles in the exterior cusp and HLPS by analysing all outbound Cluster dayside passes in February and March, 2002 and 2003. The average particle fluxes in HLPS are roughly three (protons) or ten (electrons) times larger than in the exterior cusp. This is also true on those Cluster orbits where both regions are visited within a short time interval. Moreover, the total electron fluxes, as well as proton fluxes above some 100 keV, in these two regions correlate with each other. This is true even for fluxes in every energy channel when considered separately. The spectral indices of electron and proton fluxes are the same in the two regions. We also examine the possible dependence of particle fluxes at different energies on the external (solar wind and IMF) and internal (geomagnetic) conditions. The energetic proton fluxes (but not electron fluxes) in the cusp behave differently at low and high energies. At low energies (<70 keV), the fluxes increase strongly with the magnitude of IMF <i>B<sub>y</sub></i>. Instead, at higher energies the proton fluxes in the cusp depend on substorm/geomagnetic activity. In HLPS proton fluxes, irrespective of energy, depend strongly on the <i>K<sub>p</sub></i> and AE indices. The electron fluxes in HLPS depend both on the <<i>K<sub>p</sub></i> index and the solar wind speed. In the cusp the electron fluxes mainly depend on the solar wind speed, and are higher for northward than southward IMF. These results give strong evidence in favour of the idea that the high-latitude dayside plasma sheet is the main source of energetic particles in the exterior cusp. Energetic particles can reach HLPS from the near-Earth tail. The closed field lines of HLPS act as storage for these particles. Direct diffusion (for electrons and high-energy protons) and magnetic reconnection in the high-latitude magnetopause near HLPS (for low energy protons) control the number of particles released into the exterior cusp. Note that this explanation, in contrast to other suggested theories, works both for the energetic protons and electrons in the exterior cusp. <br><br><b>Keywords.</b> Magnetospheric physics (Magnetopause, cusp and boundary layers; Solar wind-magnetosphere interactions) – Space plasma physics (magnetic reconnection

Comparing the influence of sunspot activity and geomagnetic activity on winter surface climate

We compare here the effect of geomagnetic activity (using the aa index) and sunspot activity on surface climate using sea level pressure dataset from Hadley centre during northern winter. Previous studies using the multiple linear regression method have been limited to using sunspots as a solar activity predictor. Sunspots and total solar irradiance indicate a robust positive influence around the Aleutian Low. This is valid up to a lag of one year. However, geomagnetic activity yields a positive NAM pattern at high to polar latitudes and a positive signal around Azores High pressure region. Interestingly, while there is a positive signal around Azores High for a 2-year lag in sunspots, the strongest signal in this region is found for aa index at 1-year lag. There is also a weak but significant negative signature present around central Pacific for both sunspots and aa index. The combined influence of geomagnetic activity and Quasi Biannual Oscillation (QBO 30 hPa) produces a particularly strong response at mid to polar latitudes, much stronger than the combined influence of sunspots and QBO, which was mostly studied in previous studies so far. This signal is robust and insensitive to the selected time period during the last century. Our results provide a useful way for improving the prediction of winter weather at middle to high latitudes of the northern hemisphere

Tunnelointimikroskopia, STM

Tiivistelmä. Tunnelointimikroskopia (Scanning Tunneling Microscopy, STM) on kokeellinen pintatutkimusmenetelmä, jonka avulla pystytään kuvantamaan materiaalien pintojen atomitason rakennetta. STM-kuvan muodostamisessa hyödynnetään kärjen ja pinnan välillä havaittavaa kvanttimekaanista tunneloitumista. Kokeellisten kuvien teoreettinen ja laskennallinen mallintaminen on avain niiden ymmärtämiseen ja tulkitsemiseen. Tässä tutkielmassa tutustutaan tunnelointimikroskopiaan. Tarkoitus on perehtyä tutkimusmenetelmään kokonaisvaltaisesti. Selvitetään mikroskoopin toiminta ja rakenne sekä siihen liittyvä teoreettinen tausta. Ennen syventymistä tunnelointimikroskopian osuuteen käsitellään tunneloitumisilmiöön liittyvä teoria kvanttimekaniikan avulla

Transition to a weaker Sun: Changes in the solar atmosphere during the decay of the Modern Maximum

Context. The Sun experienced a period of unprecedented activity during the 20th century, now called the Modern Maximum (MM). The decay of the MM after its maximum in cycle 19 has changed the Sun, the heliosphere, and the planetary environments in many ways. However, studies disagree on whether this decay has proceeded synchronously in different solar parameters or not.Aims. One of the related key issues is if the relation between two long parameters of solar activity, the sunspot number and the solar 10.7 cm radio flux, has remained the same during this decay. A recent study argues that there is an inhomogeneity in the 10.7 cm radio flux in 1980, which leads to a step-like jump (“1980 jump”) in this relation. If true, this result would reduce the versatility of possible long-term studies of the Sun during the MM. Here we aim to show that the relation between sunspot number and 10.7 cm radio flux does indeed vary in time, not due to an inhomogeneous radio flux but due to physical changes in the solar atmosphere.Methods. We used radio flux measurements made in Japan at four different wavelengths, and studied their long-term relation with the sunspot number and the 10.7 cm radio flux during the decay of MM. We also used two other solar parameters, the MgII index and the number of solar active regions, in order to study the nature of the observed long-term changes in more detail.Results. We find that the 1980 jump is only the first of a series of 1–2-year “humps” that mainly occur during solar maxima. All five radio fluxes depict an increasing trend with respect to the sunspot number from the 1970s to 2010s. These results exclude the interpretation of the 1980 jump as an inhomogeneity in the 10.7 cm flux, and reestablish the 10.7 cm flux as a homogeneous measure of solar activity. The fluxes of the longer radio waves are found to increase with respect to the shorter waves, which suggests a long-term change in the solar radio spectrum. We also find that the MgII index of solar UV irradiance and the number of active regions also increased with respect to the sunspot number, further verifying the difference in the long-term evolution in chromospheric and photospheric parameters.Conclusions. Our results provide evidence for important structural changes in solar magnetic fields and the solar atmosphere during the decay of the MM, which have not been reliably documented so far. We also emphasize that the changing relation between the different (e.g., photospheric and chromospheric) solar parameters should be taken into account when using the sunspot number or any single parameter in long-term studies of solar activity

RED-PL, a Method for Deriving Product Requirements from a Product Line Requirements Model

International audienceSoftware product lines (SPL) modeling has proven to be an effective approach to reuse in software development. Several variability approaches were developed to plan requirements reuse, but only little of them actually address the issue of deriving product requirements. Indeed, while the modeling approaches sell on requirements reuse, the associated derivation techniques actually focus on deriving and reusing technical product data.This paper presents a method that intends to support requirements derivation.Its underlying principle is to take advantage of approaches made for reuse PL requirements and to complete them by a requirements development process by reuse for single products. The proposed approach matches users' product requirements with PL requirements models and derives a collection ofrequirements that is (i) consistent, and (ii) optimal with respect to users' priorities and company's constraints. The proposed methodological process was validated in an industrial setting by considering the requirement engineering phase of a product line of blood analyzers

Predicting delay factors when chipping wood at forest roadside landings

Chipping of bulky biomass assortments at roadside landings is a common and costly step in the biomass-to-energy supply chain. This operation normally involves one chipping unit and one or several transport trucks working together for simultaneous chipping and chip transport to a terminal or end user. Reducing the delay factors in these operations is a relevant ambition for lowering supply costs. A method to estimate organizational delay based on: (1) the capacity ratio between the transport and the chipper, (2) the use of buffer storage, and (3) the number of transport units involved is suggested here. Other delays will also be present, and some of these may relate to the working conditions at the landing. A method to set a landing functionality index based on characteristics of the forest landing is also suggested. A total of 14 roadside chipping operations were assessed and the operators were interviewed to address the impact of machinery configuration and landing characteristics on machine utilization. At most sites, the chipper was the more productive part, and the chipper utilization was to a large extent limited by organizational delay. Still the utilization of the transport units varied between 37 and 97%, of which some 36% of the variation was explained by the landing functionality index. Knowledge from the work presented here should be a good starting point for improving biomass supply planning and supply chain configuration.acceptedVersio

Using mutual information to investigate non-linear correlation between AE index, ULF Pc5 wave activity and electron precipitation

In this study, we use mutual information from information theory to investigate non-linear correlation between geomagnetic activity indicated by auroral electrojet (AE) index with both the global ultra low frequency (ULF) Pc5 wave power and medium energy (>= 30 keV) electron precipitation at the central outer radiation belt. To investigate the energy and magnetic local time (MLT) dependence of the non-linearity, we calculate the mutual information and Pearson correlation coefficient separately for three different energy ranges (30-100 keV, 100-300 keV and >= 300 keV) and four different MLT sectors (0-6, 6-12, 12-18, 18-24). We compare results from 2 years 2004 and 2007 representing geomagnetically more active and less active years, respectively. The correlation analysis between the AE index and electron precipitation shows a clear MLT and energy dependence in both active and quiet conditions. In the two lowest energy ranges of the medium energy electrons (30-100 keV and 100-300 keV) both non-linear correlation and Pearson correlation indicate strong dependence with the AE index in the dawn sector. The linear dependence indicated by the Pearson correlation coefficient decreases from dawn to dusk while the change in the non-linear correlation is smaller indicating an increase in the non-linearity from dawn to dusk. The non-linearity between the AE index and electron precipitation is larger at all MLT sectors except MLTs 6-12 during geomagnetically more active year when larger amount of the activity is driven by interplanetary coronal mass ejections (ICMEs) compared to lower activity year with high speed stream (HSS) and stream interaction region (SIR) driven activity. These results indicate that the processes leading to electron precipitation become more non-linear in the dusk and during geomagnetically more active times when the activity is driven by ICMEs. The non-linearity between the AE index and global ULF Pc5 activity is relatively low and seems not to be affected by the difference in the geomagnetic activity during the 2 years studied.Peer reviewe

Studies Needed to Address Public Health Challenges of the 2009 H1N1 Influenza Pandemic: Insights from Modeling

In light of the 2009 influenza pandemic and potential future pandemics, Maria Van Kerkhove and colleagues anticipate six public health challenges and the data needed to support sound public health decision making.The authors acknowledge support from the Bill & Melinda Gates Foundation (MDVK, CF, NMF); Royal Society (CF); Medical Research Council (MDVK, CF, PJW, NMF); EU FP7 programme (NMF); UK Health Protection Agency (PJW); US National Institutes of Health Models of Infectious Disease Agent Study program through cooperative agreement 1U54GM088588 (ML); NIH Director's Pioneer Award, DP1-OD000490-01 (DS); EU FP7 grant EMPERIE 223498 (DS); the Wellcome Trust (DS); 3R01TW008246-01S1 from Fogerty International Center and RAPIDD program from Fogerty International Center with the Science & Technology Directorate, Department of Homeland Security (SR); and the Institut de Veille Sanitaire Sanitaire funded by the French Ministry of Health (J-CD). The funders played no role in the decision to submit the article or in its preparation