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Aqueous synthesis of (21̅0) oxygen terminated defect free hierarchical ZnO particles and their heat treatment for enhanced reactivity
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.ORCID iD: 0000-0001-5867-0531
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.ORCID iD: 0000-0003-2201-2806
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.ORCID iD: 0000-0002-0236-5420
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.ORCID iD: 0000-0003-2170-0076
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2016 (English)In: Langmuir, ISSN 0743-7463, E-ISSN 1520-5827, Vol. 32, no 42, 11002-11013 p.Article in journal (Refereed) Published
Abstract [en]

A controlled aqueous growth of 1 µm flower-shaped ZnO particles with a hierarchical subset of exposed nano-sheets represented by {21̅0} crystal faces, followed by annealing at temperatures up to 1000 °C, is presented. The flower-shaped particles showed superior photocatalytic performance compared to the crystal faces of 20 nm ZnO nanoparticles. The photocatalytic reaction rate of the flower-shaped particles before annealing was 2.4 times higher per m2 compared with that of the nanoparticles with double specific surface area. Crystal surface defects and nano-sized pores within the flower-shaped particles were revealed by porosity measurement and electron microscopy. A heat treatment at 400 °C was found to be optimal for removal of nanoporosity/surface defects and impurities while retaining the hierarchical superstructure. The heat treatment resulted in a photo-degradation efficiency that increased by an additional 43 %, although the specific surface area decreased from 16.7 to 13.0 m2g-1. The enhanced photocatalytic effect remained intact under both acidic and alkaline environments owing to the {21̅0} crystal surfaces, which were less prone to dissolution than the nanoparticles. The photocatalytic performance relied on primarily three factors: the removal of surface impurities, the oxygen termination of the {21̅0} crystal faces, and the promotion of charge carrier lifetime by removal of lattice defects acting as recombination centres. The synthesis presented is an entirely hydrocarbon- and surfactant free ('green') preparation scheme, and the formation of the flower-shaped particles was favored solely by optimization of the reaction temperature after the correct nitrate salts precursor concentrations had been established.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2016. Vol. 32, no 42, 11002-11013 p.
Keyword [en]
Carrier lifetime, Crystal defects, Crystal impurities, Heat treatment, Nanoparticles, Photodegradation, Porosity, Removal, Specific surface area, Zinc oxide, Alkaline environment, Defects and impurities, Photocatalytic effect, Photocatalytic performance, Photocatalytic reactions, Photodegradation efficiency, Porosity measurement, Recombination centers
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-193667DOI: 10.1021/acs.langmuir.6b03263ISI: 000386422300024PubMedID: 27689906Scopus ID: 2-s2.0-84994008685OAI: oai:DiVA.org:kth-193667DiVA: diva2:1033546
Funder
Swedish Foundation for Strategic Research , EM11-0022
Note

QC 20161007

Available from: 2016-10-07 Created: 2016-10-07 Last updated: 2017-11-30Bibliographically approved
In thesis
1. The synthesis, surface modification and use of metal-oxide nanoparticles in polyethylene for ultra-low transmission-loss HVDC cable insulation materials
Open this publication in new window or tab >>The synthesis, surface modification and use of metal-oxide nanoparticles in polyethylene for ultra-low transmission-loss HVDC cable insulation materials
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Polyethylene composites which contain low concentrations of metal-oxide nanoparticles e.g. ZnO and MgO are emerging materials for the use in insulations of extruded high-voltage direct-current (HVDC) cables. The challenge in the development of the composites with ultra-low electrical conductivity is to synthesize uniform and high-purity metal-oxide nanoparticles, which are functionalized with hydrophobic groups in order to make them compatible with polyethylene. The thesis reports different approaches to prepare this new generation of insulation materials.

Different reaction parameters/conditions – zinc salt precursor, precursor concentrations and reaction temperature – were varied in order to tailor the size and morphology of the ZnO nanoparticles. It was shown that different particle sizes and particle morphologies could be obtained by using different zinc salt precursors (acetate, nitrate, chloride or sulphate). It was shown that 60 °C was a suitable reaction temperature in order to yield particles with different morphologies ranging from nano-prisms to flower-shaped superstructures. For removal of reaction residuals from the particles surfaces, a novel cleaning method based on ultrasonication was developed, which was more efficient than traditional water-replacement cleaning. After cleaning, the presence of one atomic layer of zinc-hydroxy-salt complex (ZHS) on the nanoparticle surfaces was suggested by thermogravimetry and infrared spectroscopy. A method involving three steps – silane coating, heat treatment and silica layer etching – was used to remove the last trace of the ZHS species from the nanoparticle surface while preserving its clean and active hydroxylated surface. The surface chemistry of these nanoparticles was further tailored from hydroxyl groups to hydrophobic alkyl groups with different lengths by reactions involving methyltrimethoxysilane (C1), octyltriethoxysilane (C8) and octadecyltrimethoxysilane (C18).

MgO nanoparticles were prepared by aqueous precipitation of Mg(OH)2 followed by a partial transformation to MgO nanoparticles via heat treatment at 400 °C. The surface regions of the MgO nanoparticles convert into a hydroxide phase in humid media. A novel method to obtain large surface area MgO nanoparticles with a remarkable inertness to humidity was also presented. The method involved three steps:  (a) thermal decomposition of Mg(OH)2 at 400 °C; (b) silicone oxide coating of the nanoparticles to prevent inter-particle sintering and (c) a high temperature heat treatment at 1000 °C. These MgO nanoparticles showed essentially no sign of formed hydroxide phase even after extended exposure to humid air.

The functionalized metal-oxide nanoparticles showed only a minor adsorption of phenolic antioxidant, which is important in order to obtain nanocomposites with an adequate long-term stability. Tensile testing and scanning electron microscopy revealed that the surface-modified metal-oxide nanoparticles showed improved dispersion and interfacial adhesion in the polyethylene matrix with reference to that of unmodified metal-oxide nanoparticles. The highly “efficient” interfacial surface area induced by these modified nanoparticles created the traps for charge carriers at the polymer/particle interface thus reducing the DC conductivity by more than 1 order of magnitude than that of the pristine polyethylene.

Abstract [sv]

Polyetenkompositer med mycket låga halter av ZnO och MgO metalloxid nanopartiklar är en växande kategori material för användning som isolering av extruderade kablar avsedda för likriktad högspänning. En utmaning i utvecklingen av dessa material kan relateras till den praktiska kompositframställningen, vilken innefattar framställning av högrena metalloxid nanopartiklar som ytmodifieras med hydrofoba molekylstrukturer för att möjliggöra blandning med den hydrofoba polyetenplasten. Denna avhandling behandlar olika metoder för att framställa denna generation av isoleringsmaterial.

Vid syntesen av de rena nanopartiklarna krävdes optimering av ett antal olika reaktionsparametrar för att uppnå tillfredställande slutresultat i form av partikelstorlekar och partikelmorfologier. Dessa inkluderade val av zinksalt, zinksaltkoncentration vid utfällning, samt reaktionstemperatur vid framställningen. Experimenten avslöjade att olika partikelstorlekar och partikelmorfologier kunde framställas som endast korrelerat mot källan av zinkjonerna, och berodde av vilka motjoner som zinkatomerna haft i zinksaltet (acetat, nitrat, klorid eller sulfat). Optimering av reaktionstemperaturen visade att ca 60 °C utgjorde en lämplig start för utvärdering av synteserna, som resulterade i olika partikelmorfologier i form av pyramidformade nanopartiklar till blomformationer. Utöver de specifika reaktionsparametrarna utvecklades även en ny ultrasonikeringsmetod för att rena ytorna hos partiklarna från motjoner relaterade till de valda specifika salterna. Metodiken som visade sig avsevärt mer effektiv än sedvanlig rening att utfällda nanopartiklar via repetitivt vattenutbyte, och skapade förutsättningar etablering av kolloidal stabilitet och fragmentering av aggregat i vattensuspensionerna. Efter ultrasonikeringsreningen beräknades de kvarvarande zinkhydroxidsalterna (ZHS) utgöra endast ett atomlager ZHS utifrån termogravimetriska data kompletterade med infraröd spektroskopi. En metod att eliminera de kvarvarande ZHS-komplexen från ytan av partiklarna tillämpades/utvecklades, inkluderade ytbeläggning av partiklarna med silan, följt av värmebehandling samt etsning av den resulterande kiseloxidytan, för att uppnå en ren hydroxylyta på partiklarna. Ytkemin hos dessa partiklar modifierades från att bestå av hydroxylgrupper till att utgöras av hydrofoba alkylgrupper med olika längder relaterade metyltrimetoxysilan (C1), oktyltrietoxysilan (C8), eller oktadekyltrimetoxysilan (C18).

Även MgO nanopartiklar framställdes via vattenutfällning av Mg(OH)2 partiklar, vilka omvandlades till MgO nanopartiklar via en lågtemperatur värmebehandling vid 400°C. Ytan av dessa partiklar omvandlades dock till hydroxid i fuktig miljö. En ny metod att bibehålla den stora ytarean av MgO nanopartiklarna med anmärkningsvärd motståndskraft mot att omvandlas till hydroxid utvecklades således. Metoden består av (a) en låg temperatur omvandling av Mg(OH)2, (b) en kiseloxidytbehandling av nanopartiklarna för att undvika partikelsintring vid högre temperaturer och (c) en hög temperaturbehandling vid 1000 °C. De framställda partiklarna uppvisade ingen anmärkningsvärd känslighet mot luftfuktighet och bibehöll MgO sammansättningen efter exponering mot fukt.

De modifierade metalloxid nanopartiklarna visade mycket liten adsorption av fenoliska antioxidanter, vilket medförde en långtidsstabilitet hos polyeten nanokompositerna. De ytmodifierade metalloxidpartiklarna visade även förbättrade möjligheter för dispergering och yt-kompatibilitet med/i polyetenmatrisen i jämförelse med omodifierade metalloxidpartiklar, utifrån mätningar baserade på dragprovning och svepelektronmikroskopi. Slutligen, de utvecklade ytorna på de modifierade nanopartiklarna skapade ett polymer/nanopartikel gränssnitt som kunder fungera som laddningsansamlingsområden i nanokompositerna, vilket resulterade i en storleksordning minskad ledningsförmåga hos kompositerna jämfört med den rena polyetenen.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. 57 p.
Series
TRITA-CHE-Report, ISSN 1654-1081 ; 2016:30
Keyword
polyethylene, metal-oxide nanoparticles, heat treatment, surface coating, humidity resistance, interfacial adhesion, nanocomposite, HVDC insulation, polyeten, metalloxid nanopartiklar, värmebehandling, ytmodifiering av partiklarna, fukt inverkan, gränsskiktsvidhäftning, nanokomposit, HVDC isolering
National Category
Polymer Technologies
Research subject
Fibre and Polymer Science
Identifiers
urn:nbn:se:kth:diva-190808 (URN)978-91-7729-059-9 (ISBN)
Public defence
2016-09-23, Kollegiesalen, Brinellvägen 8, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Foundation for Strategic Research , EM11-0022
Note

QC 20160829

Available from: 2016-08-29 Created: 2016-08-16 Last updated: 2016-10-14Bibliographically approved

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