Supramolecular Templates. Nanoporous Metal Oxides. Structure Replication: Porous Silica as "Hard Template" conventional synthesis:

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1 Nanoporous Metal Oxides as Gas Sensors Institute of Inorganic and Analytical Chemistry Giessen Introduction conventional synthesis: self assembly of amphiphilic molecules supramolecular aggregates serve as templates uniform nanopores (2 10 nm) after template removal by thermal combustion suitable for SiO 2 / aluminosilicates AlPO 4 (some few metal oxides) Synthesis Gas Sensing (Background) Examples Humidity Sensor Summary Supramolecular Templates + Structure Replication: Porous Silica as "Hard Template" Introduction Synthesis Gas Sensing (Background) Examples Humidity Sensor Summary It's All Been Done Before... porous silica e.g. SBA 15 pore filling with precursor formation of solid in pores template removal* negative "replica" e.g. sucrose 900 C vacuum porous carbon RYOO et al Pt(II) salt 200 C H 2 porous platinum RYOO, TERASAKI et al STUCKY et al metal salt C porous metal oxide ZHAO et al air * removal of silica matrix by etching with HF or NaOH TIEMANN, "Repeated Templating" (Review), Chem. Mater some million years ago: "petrified" shell, bone, wood "hard structure matrices" + silicic acid structured SiO 2 (but different kinetics)

2 Introduction Synthesis Gas Sensing (Background) Examples Humidity Sensor Summary In Detail In 2 O 3 : TEM / SAED porous matrix (e.g. 'KIT 6' silica) impregnation with metal salt porous poröse matrix Matrix "wet impregnation" dilute metal salt solution "incipient wetness" repeat as required dry saturated metal salt solution oxide formation C metal oxidesilica composite (tempering at 800 C, inert gas) matrix removal porous replica In 2 O 3 : XRD / Physisorption In 2 O 3 : SEM In 2 O 3 : direct replication from SiO 2 possible (silica removal with NaOH) In 2 O 3 with pore system particles approx. spherical particle size: nm In 2 O 3 with hexagonal pore system large single crystalline domains: ca. 30 nm ( bixbyite structure) WAGNER/SAUERWALD/KOHL/WAITZ/WEIDMANN/TIEMANN, Thin Solid Films, in press

3 In 2 O 3 : Growth In 2 O 3 : Growth (KIT 6 silica matrix) 1 cycle 2 cycles formation of In 2 O 3 'islands' within porous silica matrix droplet formation of In(NO 3 ) 3 in the molten state? 3 cycles HAFFER/WAITZ/TIEMANN, submitted In 2 O 3 : Growth Background: Gas Sensing (SBA 15 silica matrix) ambient conditions (air): with analyte gas: from "SBA 15 80" from "SBA " n type semiconductor (e.g. SnO 2, In 2 O 3 ): oxygen vacancies (electron donors) chemisorption of oxygen: extrinsic surface acceptor states i.e. surface near electrons immobilized depletion layer surface chemical reaction with gas molecules (reducing or oxidizing) conduction electron density changes depletion layer thinner / thicker resistance changes (measurable)

4 Background: Gas Sensing "Classical" Way: Sol Gel Synthesis industrial i plants toxic gases explosives domestic living air quality ovens environment pollutants ozone automobiles lambda probe air condition spherical particles of few nanometers inter particle voids: mesopores pore size: ill defined, broadly distributed poor control affected by particle size ripening is a problem affected by particle connectivity sintering is a problem TIEMANN, "Porous Metal Oxides as Gas Sensors" (Review), Chem. Eur. J Goals for the Synthesis of Nanoporous Sensors Gas Diffusion through Nanopores uniform pore sizes arbitrary choice of pores size choice of pore geometry crystallinity thermal stability control of particle morphology (films, monoliths, ) more elaborate synthesis concepts needed micropores (< 2 nm): surface diffusion gas molecules "wander along the pore walls" mesopores (2 50 nm): mean free path pore width diffusion strongly pore size dependent Knudsen diffusion coefficient: macropores (> 50 nm): molecular diffusion mean free path < pore width diffusion as in "free space"

5 Sensor Preparation SnO 2 : CO Sensor substrates withptelectrodes dispersion of sensor material (powder) e.g. in water drop coating drying(ambient temperature) 60 hours tempering (e.g. 300 C) porous SnO 2 commercial SnO 2 based sensor porous SnO 2 : sensitivity to CO comparable to commercial sensor much lower crosssensitivity to water sensing layer substrate (Al 2 O 3 ) Pt electrodes rel. humidity: 10 % 50 % 90 % porous SnO 2 commercial sensor (SnO 2 ) heating 3 mm WAGNER/KOHL/FRÖBA/TIEMANN, Sensors 2006 Spherical SnO 2 : CH 4 Sensor Spherical SnO 2 : CH 4 Sensor porous SnO 2 spheres porous SnO 2 (non spherical) Spherical porous SnO 2 from Åbo*: well defined dfi dmorphology (here: 5µm) controllable gas diffusion high sensitivity to methane ( , % rel. hum.) (Spherical porous SnO 2 from Åbo) Ideas: defined percolation paths between electrodes electrode patterns by design manipulation of single SnO 2 spheres miniaturization suppression of 'macroporosity' (i.e. Inter particle voids) reproducibility * SMÅTT/SCHÜWER/JÄRN/LINDNER/LINDÉN, Microporous Mesoporous Mater. 2006

6 In 2 O 3 : O 3 Sensor operation temperature = 250 CC 2 ppm O 3 (blue) in O 2 /Ar (80:20) (source: dielectric barrier discharge) measurable: electronic resistance (reference resistor: 2.2 G ) sensitivity = (R gas / R air ) 1 mesoporous In 2 O 3 vs. bulk In 2 O 3 (red): more stable baseline signal faster response (t 90 = 40 s vs. 300 s) stronger sensitivity room temperature: strong sensitivity (porous >> bulk) but response/decay much too slow In 2 O 3 : O 3 Sensor In 2 O 3 : O 3 Sensor In 2 O 3 : O 3 Sensor room temperature: strong sensitivity (porous >> bulk) but response/decay much too slow blue LED (460 nm): faster response faster decay photoreduction UV off O 2 /N 2 switching off UV lamp under synthetic air (21 % O 2 ): recovery faster (ca. 15 hours) under pure N 2 atmosphere (absence of O 2 ): slower (ca. 30 hours) optical band gap (bulk In 2 O 3 ): 3.7 ev, LED energy (460 nm) only 2.7 ev not sufficient for generation of electron hole pair chemical effect more likely N 2 UV induced changes in oxygen balance in surface near regions?

7 In 2 O 3 : CH 4 Sensor In 2 O 3 : CH 4 Sensor BET surface area: 90 m 2 /g 75 m 2 /g 70 m 2 /g 52 m 2 /g 38 m 2 /g motivation: explosion prevention (e.g. g European norm EN 50194) 0 6 CH 4, 50 % rel. hum., 300 C various In 2 O 3 samples: different pore sizes different "pore wall thicknesses" sensitivity depends linearly on BET area how about pore size / pore wall thickness? sensitivity must be normalized to the BET surface area first clear correlation with pore size (diffusion?) unclear for pore wall thickness nanostructure / porosity does matter! WAITZ/WAGNER/SAUERWALD/KOHL/TIEMANN, Adv. Funct. Mater SiO 2 : Capacitive Humidity Sensor SiO 2 : Capacitive Humidity Sensor hygroscopic material as the dielectric in a capacitor capacitance changes with humidity electrode common concept (90 % of commercial devices) dielectric but problematic at high temperature (> 200 C): stability of dielectric (usually ypolymers, ceramics) high sensitivity needed: relative humidity very low (!) at high T electrode sensor preparation: pellet (23 15mm) 1.5 of mesoporous silica (e.g. SBA 15, 25 kbar) 50 nm porous gold layers (by vapor deposition) motivation: humidity sensor for industrial dryers (textiles), joint project with Setex corp. (Germany)

8 SiO 2 : Capacitive Humidity Sensor SiO 2 : Capacitive Humidity Sensor some example measurements at 90 C: (SBA 15, 9 nm pores) (pore size / pore shape have little impact) long term field test: prototype tested for 3 months stable performance real conditions: industrial dryer for textile processing (dyeing, impregnation, finishing) ca. 120 C aggressive chemicals sensor (gas, aerosol) desired: lifetime > 1 year (not tested yet) KROTZKY/ROGGENBUCK/TIEMANN/BECKER/SAUERWALD/KOHL, almost submitted Summary Thanks synthesis concept (structure replication): new ordered nanoporous materials porous silica or carbon serve as matrices large surface areas & defined pore systems properties of porous semiconducting metal oxides: catalysis(mgo, CeO 2 ) perspectives: optical properties p (ZnO) new systems (perovskites, spinels, ITO, ) gas sensing (In 2 O 3, Co 3 O 4, SnO 2 ) particle morphology photovoltaics magnetism new sensors, sensing mechanisms THOMAS WAITZ STEFANIE HAFFER BENJAMIN BECKER THORSTEN WAGNER (JAN ROGGENBUCK) MICHAEL FRÖBA DIETER KOHL, TILMANSAUERWALD (University of Hamburg) (Physics Dept. Univ. Giessen) PETER KLAR (Physics Dept. Univ. Giessen) JAN HENRIK SMÅTT, MIKA LINDÉN (Åbo Akademi, Finland) JAN HANSS (Augsburg University) SANGAM CHATTERJEE (Marburg University) TANYA TSONCHEVA, CHRISTO MINCHEV (Bulgarian Academy Sofia) FRANK BECKER (SETEX, Mengerskirchen)