Charge compensation in trivalent cation doped bulk rutile TiO2

Doping of TiO2 is a very active field, with a particularly large effort expended using density functional theory (DFT) to model doped TiO2; this interest has arisen from the potential for doping to be used in tuning the band gap of TiO2 for photocatalytic applications. Doping is also of importance for modifying the reactivity of an oxide. Finally, dopants can also be unintentionally incorporated into an oxide during processing, giving unexpected electronic properties. To unravel properly how doping impacts on the properties of a metal oxide requires a modelling approach that can describe such systems consistently. Unfortunately, DFT, as used in the majority of studies, is not suitable for application here and in many cases cannot even yield a qualitatively consistent description. In this paper we investigate the doping of bulk rutile TiO2 with trivalent cations, Al, Ga and In, using DFT, DFT corrected for on-site Coulomb interactions (DFT + U, with U on oxygen 2p states) and hybrid DFT (the screened exchange HSE06 exchange correlation functional) in an effort to better understand the performance of DFT in describing such fundamental doping scenarios and to analyse the process of charge compensation with these dopants. With all dopants, DFT delocalizes the oxygen hole polaron that results from substitution of Ti with the lower valence cation. DFT also finds an undistorted geometry and does not produce the characteristic polaron state in the band gap. DFT + U and hybrid DFT both localize the polaron, and this is accompanied by a distortion to the structure around the oxygen hole site. DFT + U and HSE06 both give a polaron state in the band gap. The band gap underestimation present in DFT + U means that the offset of the gap state from both the valence and the conduction band cannot be properly described, while the hybrid DFT offsets should be correct. We have investigated dopant charge compensation by formation of oxygen vacancies. Due to the large number of calculations required, we use DFT + U for these studies. We find that the most stable oxygen vacancy site has either a very small positive formation energy or is negative, so under typical experimental conditions, anion vacancy formation will compensate for the dopant

Lead oxide-modified TiO2 photocatalyst: tuning light absorption and charge carrier separation by lead oxidation state

Modification of TiO2 with metal oxide nanoclusters such as FeOx, NiOx has been shown to be a promising approach to the design of new photocatalysts with visible light absorption and improved electron–hole separation. To study further the factors that determine the photocatalytic properties of structures of this type, we present in this paper a first principles density functional theory (DFT) investigation of TiO2 rutile(110) and anatase(001) modified with PbO and PbO2 nanoclusters, with Pb2+ and Pb4+ oxidation states. This allows us to unravel the effect of the Pb oxidation state on the photocatalytic properties of PbOx-modified TiO2. The nanoclusters adsorb strongly at all TiO2 surfaces, creating new Pb–O and Ti–O interfacial bonds. Modification with PbO and PbO2 nanoclusters introduces new states in the original band gap of rutile and anatase. However the oxidation state of Pb has a dramatic impact on the nature of the modifications of the band edges of TiO2 and on the electron–hole separation mechanism. PbO nanocluster modification leads to an upwards shift of the valence band which reduces the band gap and upon photoexcitation results in hole localisation on the PbO nanocluster and electron localisation on the surface. By contrast, for PbO2 nanocluster modification the hole will be localised on the TiO2 surface and the electron on the nanocluster, thus giving rise to two different band gap reduction and electron–hole separation mechanisms. We find no crystal structure sensitivity, with both rutile and anatase surfaces showing similar properties upon modification with PbOx. In summary the photocatalytic properties of heterostructures of TiO2 with oxide nanoclusters can be tuned by oxidation state of the modifying metal oxide, with the possibility of a reduced band gap causing visible light activation and a reduction in charge carrier recombination

Reactivity of sub 1 nm supported clusters: (TiO2)(n) clusters supported on rutile TiO2 (110)

Metal oxide clusters of sub-nm dimensions dispersed on a metal oxide support are an important class of catalytic materials for a number of key chemical reactions, showing enhanced reactivity over the corresponding bulk oxide. In this paper we present the results of a density functional theory study of small sub-nm TiO2 clusters, Ti2O4, Ti3O6 and Ti4O8 supported on the rutile (110) surface. We find that all three clusters adsorb strongly with adsorption energies ranging from -3 eV to -4.5 eV. The more stable adsorption structures show a larger number of new Ti-O bonds formed between the cluster and the surface. These new bonds increase the coordination of cluster Ti and O as well as surface oxygen, so that each has more neighbours. The electronic structure shows that the top of the valence band is made up of cluster derived states, while the conduction band is made up of Ti 3d states from the surface, resulting in a reduction of the effective band gap and spatial separation of electrons and holes after photon absorption, which shows their potential utility in photocatalysis. To examine reactivity, we study the formation of oxygen vacancies in the cluster-support system. The most stable oxygen vacancy sites on the cluster show formation energies that are significantly lower than in bulk TiO2, demonstrating the usefulness of this composite system for redox catalysis

SnO-nanocluster modified anatase TiO2 photocatalyst: exploiting the Sn(II) lone pair for a new photocatalyst material with visible light absorption and charge carrier separation

Modifying TiO2 to design new photocatalysts with visible light absorption and reduced charge carrier recombination for photocatalytic depollution or water splitting is a very active field. A promising approach is to deposit small nanoclusters of a metal oxide on a semiconducting oxide such as TiO2 or ZnGa2O4. In this paper we present a first principles density functional theory (DFT) investigation of a novel concept in photocatalyst materials design: Sn(II)O nanoclusters supported on TiO2 anatase (001) and demonstrate that the presence of the Sn(II)-O lone pair in the nanoclusters gives a new approach to engineering key properties for photocatalysis. The modification of anatase with Sn(II)O reduces the band gap over unmodified anatase, thus activating the material to visible light. This arises from the upwards shift of the valence band, due to the presence of the Sn 5s-O 2p lone pair in the nanocluster. Enhanced charge separation, which is key for photocatalytic efficiency, arises from the separation of electrons and holes onto the anatase surface and the Sn(II)O nanocluster. This work realises a new strategy of exploiting the lone pair in elements such as Sn to raise the VB edge of modified TiO2 and enhance charge separation in new photocatalyst materials

Molecular metal oxide cluster-surface modified titanium (IV) dioxide photocatalysts

The surface modification of TiO2 with molecular sized metal oxide clusters has recently been shown to be a promising approach for providing TiO2 with visible-light activity and/or improved UV activity. This short review summarizes the effects of the surface modification of TiO2 with the oxides of iron and tin selected from d- and p-blocks, respectively, on the photocatalytic activity. Fe(acac)(3) and [Sn(acac)(2)]Cl-2 chemisorption on the TiO2 surface occurs by ligand-exchange and ion-exchange, respectively. Taking advantage of the strong adsorption, we formed extremely small metal oxide clusters on TiO2 by the chemisorption-calcination cycle (CCC) technique with their loading amount strictly controlled. The iron oxide surface modification of P-25 (anatase/rutile = 4: 1, w/w, Degussa) gives rise to a high level of visible-light activity and a concomitant increase in the UV-light activity for the degradation of model organic pollutants. On the other hand, only the UV-light activity is increased by the tin oxide surface modification of ST-01 (anatase, Ishihara Sangyo). This striking difference can be rationalized on the basis of the material characterization and DFT calculations, which show that FeOx surface modification of rutile leads to visible-light activity, while SnO2-modified anatase enhances only the UV-light activity. We propose the mechanisms behind the FeOx and SnO2 surface modification, where the surface-to-bulk and bulk-to-surface interfacial electron transfer are taken into account in the former and the latter, respectively.Research Front (Open Access

Harnessing Evaluation and Learning for Equity and Impact Insights for Foundation Executives

Grantmaking foundations are increasingly using E&L functions in diverse ways. A 2019 survey from the Center for Evaluation Innovation found that 42 percent of foundations had a dedicated E&L unit or department that functioned separately from the program department, up from 34 percent in 2015.The survey also shows that E&L staff fill numerous and evolving roles including directing and managing evaluation work within the foundation; supporting broader team and organizational learning efforts, including equity work; supporting strategy development and review; and providing advice or coaching about evaluation to other staff. Beyond the foundation's walls, many E&L teams contribute to the fields of evaluation and philanthropy by sharing actionable knowledge or learning strategies with peer organizations. Field-building increases the influence and impact of a foundation by helping to advance philanthropic thought leadership and E&L practices more broadly.But leveraging the power of evaluation and learning is easier said than done. This guide is a resource for foundation executives interested in harnessing the power of evaluation and learning for impact. It was developed by Engage R+D with support from The James Irvine Foundation, Ewing Marion Kauffman Foundation, Center for Evaluation Innovation, and Kresge Foundation. It is based on our study of the E&L function across these three diverse foundations, all of which champion the Equitable Evaluation Initiative and are on their own equity journeys

A first principles investigation of Bi2O3-modified TiO2 for visible light activated photocatalysis: the role of TiO2 crystal form and the Bi3+ stereochemical lone pair

Modification of TiO2 with metal oxide nanoclusters is a novel strategy for the design of new photocatalysts with visible light activity. This paper presents a first principles density functional theory (DFT) analysis of the effect of modifying TiO2 rutile (110) and anatase (101) and (001) surfaces with Bi2O3 nanoclusters on the band gap and the nature of the photoexcited state. We show that band gap modifications over unmodified TiO2 depend on the crystal form: modifying rutile (110) results in new Bi2O3 derived states that shift the valence band upwards. On anatase surfaces, there is little effect due to modification with Bi2O3 nanoclusters, but an enhanced UV activity would be expected. Analysis of electron and hole localisation in a model photoexcited state shows enhanced charge separation in Bi2O3-modified rutile (110) but not in Bi2O3-modified anatase. The effect of the Bi3+ lone-pair on the properties of Bi2O3-modified TiO2 contrasts with SnO-modified TiO2, consistent with the weaker lone pair in Bi2O3 compared with SnO

Evaluation and Learning at Foundations: A Field Guide

This brief grew out of conversations with evaluation and learning leaders working in foundations across the United States about both the value of evaluation and learning in philanthropy, and the challenges of implementing this function well across diverse institutional contexts. Our intent is to provide practical guidance that new and existing leaders can use to navigate their roles in support of more effective and equitable philanthropy. It is based on indepth case studies of the Irvine, Kauffman, and Kresge Foundations along with our own experience partnering with foundations on evaluation, strategy, and learning efforts

Metal oxide nanocluster-modified TiO2 as solar activated photocatalyst materials

In this review we describe our work on new TiO2 based photocatalysts. The key concept in our work is to form new composite structures by the modification of rutile and anatase TiO2 with nanoclusters of metal oxides and our density functional theory (DFT) level simulations are validated by experimental work synthesizing and characterizing surface-modified TiO2. We use DFT to show that nanoclusters of different metal oxides, TiO2, SnO/SnO2, PbO/PbO2, NiO and CuO can be adsorbed at rutile and anatase surfaces and can induce red shifts in the absorption edge to enable visible light absorption which is the first key requirement for a practical photocatalyst. We furthermore determine the origin of the red shift and discuss the factors influencing this shift and the fate of excited electrons and holes. For p-block metal oxides we show how the oxidation state of Sn and Pb can be used to tune both the magnitude of the red shift and also its mechanism. Finally, aiming to make our models more realistic, we present some new results on the stability of water at rutile and anatase surfaces and the effect of water on oxygen vacancy formation and on nanocluster modification. These nanocluster-modified TiO2 structures form the basis of a new class of photocatalysts which will be useful in oxidation reactions and with the suitable choice of nanocluster modifier can be applied to CO2 reduction