Department of Engineering Sciences

Materials for energy efficiency and environmental applications

The research group ”Materials for energy efficiency and environmental applications” is carrying out a  comprehensive thematic effort aimed at functional materials for the built environment, in particular green nanotechnologies for sustainable buildings. The focus is on next generation functional oxides in chromogenic (electrochromic and thermochromic) windows, sensors, photocatalysis, lighting and other applications. We also study novel multilayer and nanostructured oxides for solar energy control and light harvesting, which combine chromogenic, photocatalytic and sensor properties.

We study these subjects by an integrated approach encompassing thin film deposition, experimental studies of electromagnetic and surface (electro-) chemical properties of materials together with computational modelling.

Ongoing research areas

Read our publications

Surface coatings for smart windows

Smart windows make it possible to regulate the throughput of visible light and solar energy and can provide energy efficiency for buildings and at the same time give indoor comfort. We work with two approaches: electrochromic (EC) materials and devices, which allow electrical transmittance control, and thermochromic (TC) materials which alter their transmittance when the temperature is changed.

Electrochromic devices can be constructed as shown below. A typical device uses two EC thin films connected by an electrolyte layer. When a voltage is applied between two transparent electrical conductors, ions and electrons are shuttled between the EC films and if, for example, these are of tungsten oxide and nickel oxide, the optical transmittance can be changed as illustrated. The whole construction can be based on flexible plastic foils (as shown in the left-hand part of the first figure) and used for laminating window glass. The third figure shows full-scale EC windows in dark and clear states; these windows were produced by ChromoGenics AB, an Uppsala-based company and offspring of our research.

Konstruktionsprincip för ett elektrokromt "smart fönster"
Construction of a typical EC smart window foil-type device and its implementation for glass lamination.
Modulation of genomskinligheten hos elektrokromt fönster
Transmittance modulation for an EC device of the kind shown above.
Demonstration av elektrokroma fönster
 Full scale EC window prototypes installed at the premises of Uppsala-based company ChromoGenics AB.

Thermochromic materials can consist of thin films or nanoparticles, in both cases based on vanadium dioxide. TC materials change their optical and electrical properties at a temperature in the vicinity of room temperature. The figure below shows schematic data on wavelength-dependent transmittance and reflectance. Very different properties are achieved at room temperature (when the material is semiconducting) and at elevated temperature (when the material is metallic). These types of materials can be used in energy-effective windows which combine good transparency with a large temperature-dependent modulation of solar energy, as illustrated in the figure.

Modulering av de optiska egenskaperna hos en termokrom film
Spectral transmittance and reflectance for VO2-based TC thin films and nanoparticles.
Olika meteoder att modifiera de optiska egenskaperna hos en termokrom film
Relationship between transmittance of visible light (Tlum) and temperature-dependent modulation of solar energy throughput (ΔTsol).

An overall aim is to develop improved EC and TC materials and to make improved EC devices (which means that electrolytes and transparent electrical conductors are of interest too). Many different aspects are of interest, such as thin film deposition and characterization with physical and (electro)chemical techniques, optical measurements, device integration and device testing. Both experimental and theoretical/computational work is performed. The research has evolved during many years and has high international profile.

References:
G.B. Smith and C.G. Granqvist, Green Nanotechnology: Solutions for Sustainability and Energy in the Built Environment, CRC Press, Boca Raton, USA.
G.A. Niklasson and C.G. Granqvist, J. Mater. Chem. 17 (2007) 127.
C.G. Granqvist, Thin Solid Films 564, 1 (2014).
S.-Y. Li, G.A. Niklasson, and C.G. Granqvist, J. Appl. Phys. 115 (2014) 053513.

Optical materials for energy applications

Optical properties of thin films, nanoparticles and composites are an integral part of many of our projects on energy efficiency and environmental applications. In particular, many of the materials used for applications in energy-efficient buildings and solar energy utilization employ optical functionality. Fundamental studies of optical properties are necessary in order to improve and understand the functional properties of these materials. We have a top quality optical measurement laboratory with spectrophotometers for the ultraviolet, visible and infrared wavelength ranges, a spectroscopic ellipsometer and a photoluminescence setup with cryogenic cooling capability. The instruments enable us to measure spectral and angular light scattering and include a range of integrating sphere detectors. We participate in international collaborations to develop superior techniques and methods for accurate optical characterization. Additionally, we have large experience in analysis of optical properties of thin films and nanoparticles, including development of our own software.

A number of projects of current interest within this broad field are briefly summarized below.

(1)  Solar collectors for hot water production or space heating need a surface coating with a high solar absorptance and, at the same time, a low emittance of thermal re-radiation. Such coatings are referred to as spectrally selective solar absorbers. The currently most popular type consists of a composite of metal nanoparticles in a dielectric material. Detailed understanding of their optical properties is of importance for (a) the optimization solar absorptance and thermal emittance by model computations, and (b) to establish which compositional and structural changes during high temperature ageing that lead to degradation of the optical properties, and hence limit the performance of the coatings. Another issue of current interest is to produce solar collectors with architecturally desirable colors. Some examples of optical reflectance spectra are shown in the figure below.

Spectral reflectivity for TiAlOxNy films

Spectral reflectivity for TiAlOxNy films
Spectral reflectance for TiAlOxNy films made by sputter deposition onto Al. Different colors (shown) were obtained by selecting film thicknesses leading to reflectance maxima in the luminous wavelength range.

(2) Light scattering should be avoided in for example window coatings but can also add functionality to a material or a thin film. In both cases, it is important to characterize and understand the scattering. We are primarily studying light scattering from small particles and inhomogeneous materials. Some examples of current interest are pigmented polymers for radiative cooling, paints for solar absorbers and “cool roofs”, light diffusors for energy efficient lighting, as well as materials that can switch from a transparent to a light scattering white state. Light scattering can also be an aesthetically pleasing phenomenon as illustrated below.

Scattering of laser light from a structured surface.
Scattering of laser light from a structured glass.

 (3) We also have some projects related to the emerging field of plasmonics. Very thin semitransparent gold films can be used as transparent contacts to displays and electrochromic devices. Metal nanoparticles give rise to a large enhancements of electric fields, an effect that may be advantageous in for example photo-catalysis. Thin gold films can show a range of morphologies, as seen below.

Morphology in thin gold films with different thickness
Morphology of thin gold films, deposited onto SnO2:In-coated glass, with a mass thickness of 5 nm prepared at different (a) sputter deposition temperatures, and (b) annealing temperatures.

References:
E. Wäckelgård, G.A. Niklasson and C.G. Granqvist: Selectively solar-absorbing coatings, in Solar Energy: The State of the Art. ISES Position Papers, edited by J. Gordon (James & James, London, 2001), Ch. 3, pp. 109-144.
P.C. Lansåker, P. Petersson, G.A. Niklasson and C.G. Granqvist, Solar Energy Mater. Solar Cells 117 (2013) 462.

Solid state sensors for the indoor environment and health applications

Solid state sensors are necessary as supplementary tools for monitoring indoor air quality, and are increasingly being considered in medical research and diagnostics. We perform research on electronic gas sensors and optical sensors consisting of nanostructured metal oxide films and functionalized diamond films.

Resistive gas sensors contain electrically weakly coupled nanocrystals between electrical contacts on a substrate that normally is heated. Gas molecules can interact with surfaces of semiconductors and change their electrical conductivity, as shown schematically below. Accurate control of the nanostructure is needed to go beyond present state-of-the-art and achieve high sensitivity at low temperature and at the same time have low long-term resistance drift. Our research shows that nanoporous metal oxide based films may be tailored so that they can detect oxidizing and reducing gases with high specificity down to ppb concentration levels. An example of gas sensor performance is given in the figure.

Operating principle for a resistive gas sensor
Principle of operation of a resistive gas sensor.
Sensitivity for a WO3 based gas sensor
Sensitivity of WO3-based resistive gas sensor exposed to H2S gas.

Optical mid-infrared sensors are ideal for identifying the molecular identity of unknown species and are based on the unique vibrational fingerprint which each molecule exhibits. One can employ evanescent field mid-infrared spectroscopy using diamond-based materials as internal reflection element in advanced waveguide spectroscopy. In an even more advanced approach, nanocrystalline diamond-coated waveguides can be functionalized for selective adsorption of target molecules to enhance the signal-to-noise ratio. Using broad-band quantum cascade laser sources coupled by optics to diamond waveguide chips, ultra-sensitive evanescence IR spectroscopy can be performed, as illustrated below.

Diamond based waveguide
Top: Mid-infrared laser throughput of a thin diamond waveguide as a function of scanning distance perpendicular to the propagation axis. Bottom: Optical image of a diamond waveguide chip consisting of six waveguides, each with the same thickness but with different widths.

In one project we study new materials for resistive gas sensors (e.g. WO3, NiO and other oxides) capable of detecting indoor pollutants close to room temperature. Our goal is to develop better understanding of the relation between nanostructure, interfacial states and electrical properties. We are developing electrical analysis techniques to discriminate between various gases, for example by using noise spectroscopy or impedance spectroscopy. These spectral techniques may give possibilities to detect multiple gases by the same sensor. We are also exploring combinatorial sensor functionality with for example photo-catalytic properties as well as materials with light-dependent response.

In another project we study optical mid-infrared sensors with an aim to develop miniaturised sensor chips, which can be functionalized to selectively bind to targeted molecules and be combined in parallel configurations for recognition of multi-component gas mixtures using multivariate analysis tools. In another project we consider protein structures related to neurodegenerative diseases. Our intention is to utilize the combined effects of intrinsic sensitivity and selectivity from an optical mid-infrared waveguide (much more sensitive than internal reflection elements employed in standard infrared spectroscopy techniques) together with the low background noise associated with evanescent waves.

References:
R. Ionescu et al., Sensors Actuators B: Chem. 104 (2005) 132.
C. Luyo et al., Sensors Actuators B: Chem. 138 (2009) 14.
L. Österlund et al., EP 2271914.

Photocatalytic and advanced wetting materials

Semiconductor photo-catalysis is a broad research field which lies at the heart of modern sustainable technologies such as air and water cleaning, solar hydrogen production, wet solar cells, self-cleaning and antibacterial surface coatings. We perform research on new materials and structures with improved solar light-to-chemical energy efficiencies and functional surface properties, e.g., controlled wettability.

Photonic band gap materials are the photonic counterpart to semiconductors, where photons instead of electrons disperse and form forbidden bands wherein no photon can propagate. In one project we explore a new class of photo-catalytic materials where the semiconductor photo-catalyst material is incorporated in a photonic band gap structure in the form of an inverse opal, as shown below. Here the aim is to match the energy of the photonic band gap with the electronic band gap of the photo-catalytic material.

Inverse opal structure of Al2O3
Inverse opal Al2O3 with an “air hole” diameter of 160 nm. The photo-catalytic material is either deposited as nanoparticles with high surface area on the inner walls of the Al2O3 scaffold, or as additional layers deposited on the walls to modify the band gap properties of the inverse opal structure.

Photo-catalytic degradation of pollutants is a promising sustainable technology for air and water cleaning. Employing either the sun’s light or artificial light available in the indoor setting, the removal of hazardous compounds can be realized and yield nothing but water, carbon dioxide and trace mineral acids as waste products. We perform research on new concepts for utilizing photo-catalytic materials (e.g., pure, mixed, and doped TiO2, WO3 and Fe2O3 based materials) in the built environment, either as thin films, photonic band gap structures, multi-layer films with synergetic properties for UV absorption (photo-catalysis) and near-infrared absorption (heat), and nanoporous coatings. The design of a novel window-based device is shown below. In several projects, we fabricate materials and characterize their physicochemical properties. We use various types of in situ vibrational spectroscopy methods to study reaction kinetics on the photo-catalytic materials, and in many projects we compare these results with DFT calculations as shown below. Studies are performed on nanoporous materials as well as single crystals; from ultra-high vacuum to high pressures; from liquid nitrogen to several hundred degrees; with and without photon irradiation.

Operating principle for a photocatalytic window for indoor air cleaning
Photo-catalytic removal of hazardous pollutants present in the indoor environment by means of a photo-catalytic window.

Photocatalytic decomposition of acetaldehyde

Modelled adsorption and decomposition kinetics
Measured and modeled reaction kinetics of the adsorption and decomposition of acetaldehyde on TiO2.

Advanced wetting surfaces are increasingly being used as hygienic, easy-clean, anti-bacterial and aesthetic surfaces. Two main approaches can be distinguished: One which aims at superhydrofobic properties so that water and dirt do not stick to the surface or come off during rain when small water droplets roll off the surfaces. In another approach, the surface is intentionally made superhydrophilic to promote water film formation, which makes the pollutant flow off the surface by means of film flow. We have developed methods to chemically change the acid-basic properties of photo-catalytic oxide materials, and we have shown that SO2-modified TiO2 has unique oleophobic (oil-repelling) properties which prevent e.g. fatty acids present in human fingerprints to stick to such surfaces, as illustrated in the figure below.

Comparison of the amount of stearic acid on a SO2 modified surface
Comparisons of the amount of adsorbed stearic acid (a fatty acid) on a SO2-modified TiO2 film (several consecutive adsorption cycles) and a pure TiO2 film. No stearic acid adheres to the SO2-modified surface.

References:
A. Mattsson et al., J. Chem. Phys. 140 (2014) 034705.
Z. Topalian et al., J. Catal. 307 (2013) 265-274.
L. Österlund, Solid State Phenom. 162 (2010) 203-219.
Z. Topalian et al., ACS Appl. Mater. Interfaces 4 (2012) 672.

Electrochemistry and electronic structure

The electronic structure of solids constitutes a fundamental basis for understanding their optical and electrical properties. This subject is highly relevant for a number of our applied projects, especially regarding electrochromic, thermochromic and photo-catalytic materials. We earlier developed a novel technique to measure the electronic density of states of amorphous and nanocrystalline oxide materials. For this purpose one can use a number of electrochemical measurement techniques, for example chronopotentiometry, impedance spectroscopy and photo-electrochemical spectroscopy, as well as photoelectron- and vibrational spectroscopy methods.

The electronic density of states (DOS) of transition metal oxides is in focus for our interest since it dictates the optical and transport properties in these oxides. Effects of disorder on the electronic structure, and whether states are localized and extended, are significant. Surface states and bulk-type localized band gap states in transition metal oxides are of potential importance for their gas sensing and photo-catalytic properties. Our current work is focused on oxide materials of interest for applications as transparent conductors, electrochromic, thermochromic and photo-catalytic thin films. The figure below shows the electrochemically determined DOS of an electrochromic WO3 coating and compares it with a theoretical computation.

Expermintal determination of DOS in an amorphous WO3 film
The electrochemical DOS of an amorphous WO3 film is compared to results from calculations by density functional theory.

In one project we explore the advantages and limitations of electrochemical techniques to study DOS in oxides. The electrochemical density of states is compared with calculations of the electronic structure. Supplementary information on the density of states can be obtained from X-ray absorption and emission measurements at synchrotrons. Our work involves collaboration with theory groups in Sweden and abroad.

Referens:
M. Strömme, R. Ahuja and G.A. Niklasson, Phys. Rev. Lett. 93 (2004) 206403

Solar water splitting

The research is based on the use of the solar energy to produce hydrogen by photocatalytic splitting water. To systematically analyze the problem of converting water and solar radiation into hydrogen, the process can be divided into a number of subsequent steps. The overall process can be divided into:  absorption, charge separation, charge transport and catalysis. [1] Accordingly the external quantum efficiency, EQE, can be described according to equation 1, where LHE is the light harvesting efficiency, λ the wavelength, Φsep the charge separation efficiency, Φtrans the charge transport efficiency and Φcat the quantum efficiency of catalytic charge transfer to the desired red-ox species in the solution.  

EQE(λ) = LHE(λ) · Φsep(λ) · Φtrans(λ) · Φcat(λ)             (1)

The different processes are illustrated in the figure below where the charge separation and transport is merged in step ii), and the quantum efficiency of the catalytic charge transfer is divided into iii), the hydrogen evolution reaction (HER) and iv), the oxygen evolution reaction (OER).

The system can be taken from molecules or nanoparticles in solution, to electrodes with either n-type or p-type semiconductors and all the way to more to more conventional solar cell technologies (photovoltaic cell). Here, either the photoactive material is used as a source for the charge separation and converts the solar energy to electrical energy where the catalysis can be made directly on the surface or the photoactive material or by adding co-catalysts. The co-catalysts can be added in direct contact to the photoabsorber or separated on a different conductive substrate transforming the system to a PV-electrolysis setup. The gradual transformation from a particles in solution or on a photoelectrode in a photoelectrochemical cell (PEC) to PV-electroctrolysis is shown in the figure below.

Research on solar fuel
(Left) Schematic diagram over the vital parts of our research on solar fuel from photocatalytic water splitting in photoelectrochemical devices to PV-electrolysis. Figure from our work in [3] and (right) a CIGS based device under illumination in our lab with simulated AM1.5 solar spectrum showing 10% solar-to-hydrogen efficiency for unassisted water splitting [4].

Typical PV-cells used in our projects are CIGS or hybrid perovskite solar cells that both offers tunable band gaps in contrast to Si-based solar cells. This is important since it allows us to engineer the photoabsorber system to a specific catalyst. A CIGS solar cell is consist of a thin layers of p-type copper indium gallium selenide on a conducting substrate, a CdS buffer layer and then a thin ZnO layer followed by a transparent conducting oxide (ZnO:Al). A hybrid perovskite solar cell is formed by a perovskite-structured material with both organic and inorganic materials such as methylammonium lead halides. The electrochemical cell consists of two electrodes, a cathode and an anode, dipped into an electrolyte which is water and dissolved ions in it. On the surface of the anodic catalyst OER occurs meanwhile the HER occurs on the cathodic surface. The electrodes can be electrocatalytic (e.g., Pt, NiO and Fe-NiO) or photocatalytic (e.g., TiO2, α-Fe2O3 and WO3) materials. Electrocatalytic and photocatalytic materials generate catalyst activity when exposed to an electric field and a light, respectively.

The solar-to hydrogen (STH) efficiency is defined as

where ΔG0 = 237 kJ mol-1 is the Gibbs free energy stored in hydrogen molecules. The STH efficiency can also be written as

where the overall STH efficiency is calculated by multiplying the thermodynamic potential of the redox reaction (Vredox), the electrolysis current (IWE), and the Faradaic efficiency for hydrogen evolution (ηF) during the reaction and then dividing by the power of the incoming light (Pin). We are working with both nanoparticles in solution and on electrodes as well as the full system approaching PV-electrolysis where we have reached STH efficiency over 10% for unassisted solar water splitting for both PEC/PV and PV-electrolysis approaches.[3-5] We are working with improving the catalytic properties of earth-abundant nano-catalysts as well as band-edge tuning if CIGS and perovskite materials.

Contact: Senior lecturer Tomas Edvinsson

References
[1] Jacobsson T. J, Platzer-Björkman C., Edoff M and Edvinsson T.
CIGS as an efficient photocathode for solar hydrogen generation, Int J of Hydrogen Energy, 38, 2013, 15027-15035
[2] Jacobsson, T. J.; Edvinsson, T.,
A Spectroelectrochemical Method for Locating Fluorescence Trap States in Nanoparticles and Quantum Dots, J. Phys. Chem. C 2013, 117, 5497−5504
[3] Jacobsson, T. J.; Fjällström, V.; Edoff, M.; Edvinsson, T.,
Sustainable Solar Hydrogen Production: From Photo- Electrochemical Cells to PV-Electrolysis and Back Again, Energy & Environmental Science, 2014, 2014, 7, 2056-2070
[4] Jacobsson, T.J., Fjällström, V., Sahlberg, M., Edoff, M.,  Edvinsson, T.
A monolithic device for solar water splitting based on series interconnected thin film absorbers reaching over 10% solar-to-hydrogen efficiency. Energy and Environmental Science, 6,  2013, 3676-3683
[5] Jacobsson, T. J.; Fjällström, V.; Edoff, M.; Edvinsson, T.,
CIGS based devices for solar hydrogen production spanning from PEC-cells to PV-electrolyzers: A comparison of efficiency, stability and device topology
Solar Energy Materials & Solar Cells, 2015, 134, 185–193

Hybrid perovskite solar cells

Perovskites are materials with a crystal structure analogous to CaTiO3, with the general composition formula ABX3, where A is a large monovalent cation,  B a smaller sized cation, and X is an anion. The organometallic halide perovskites (OMHPs) that are investigated for photovoltaics are commonly summarized with the notation hybrid perovskites from their inclusion of both organic and inorganic ions. In these systems, A is a small organic dipolar cation, such as methylammonium (MA, CH3NH3+), formamidinium (FA, HC(NH2)2+), B is Pb2+, and X is a halide, mostly iodide.

(Left) Schematic representation of the hybrid perovskite structure of CH3NH3PbI3 and (right) the device structure of a hybrid perovskite solar cell. Figures are adopted from our work in [1] and [2].

The system is also commonly compared with an analogue system with the relatively large inorganic cation as A cation, such as Cs+.  Lead halide perovskites are semiconductors with a bandgap that depends on the halide and the cation composition where exchange of the A cation in APbI3 results in a bandgap of 1.45, 1.60 and 1.73 eV for A= FA, MA and Cs, respectively. Halogen exchange allow a larger variation and tuning of the bandgap where MAPbX3 has a bandgap of 1.6, 2.2 and 3.0 eV for X= I, Br and Cl, respectively. Intermixing of halides is here a promising strategy performing a tuning in between the band gaps of the pure halogens.[3]

Colour shift of hybrid halide pervoskites
(Top) Photos of hybrid halide perovskites with different amount of I/Br fractions, (bottom left) optical band gap dependence on the Br fraction, and (bottom right) PL spectra of perovskite films with different Br fraction. [3]

We also investigate more intriguing effects such as the photoinduced Stark effect and the mechanism of ion displacements in these materials.[4]

Stark effect
(Left)  First (1H), second (2H), and third order harmonics (3H) of a photoinduced Stark effect observed in the perovskite materials [4] and (right) calculated density of states and energy band dispersion and illustration of the light induced distortion of the Pb-I octahedral and ion movement in the perovskite materials.

Contact: Senior lecturer Tomas Edvinsson

References
[1] Jacobsson, T. J.; Pazoki, M.; Hagfeldt, A., Edvinsson, T.
Goldschmidt’s Rules and Strontium Replacement in Lead Halogen Perovskite Solar Cells: Theory and Preliminary Experiments on CH3NH3SrI3, J. Phys. Chem. C  2015, 119, 25673-25683
[2] Park B., SM Jain S.M., Zhang X., Hagfeldt A., Boschloo B., Edvinsson T.
Resonance Raman and Excitation Energy Dependent Charge Transfer Mechanism in Halide-Substituted Hybrid Perovskite Solar Cells, ACS Nano, 2015, 9, 2088–2101
[3] Park, B. Philippe, B., Jain, S.M., Zhang,X., Edvinsson,T., Rensmo,H., Zietz, B. Boschloo, G. Chemical engineering of methylammonium lead iodide/bromide perovskites: tuning of opto-electronic properties and photovoltaic performance, J. of Materials Chem. A2015, 3, 21760-21771
[4] Pazoki, M., Jacobsson, T. J., Kullgren, J., Johansson, E. M. J., Hagfeldt, A., Boschloo, G.,  Edvinsson, T.  
Photoinduced Stark Effects and Mechanism of Ion Displacement in Perovskite Solar Cell Materials, ACS Nano,  2017,  DOI: 10.1021/acsnano.6b07916