Titanium dioxide nanoparticles as a highly active photocatalytic material

Transcription

1 Titanium dioxide nanoparticles as a highly active photocatalytic material 1

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3 Ultrafine (nanoparticle) TiO 2 production at Cinkarna Celje, Inc... 4 Photocatalytic degradation of organic pollutants and of NOx gases basics... 4 TiO 2 nanoparticle types produced at Cinkarna Celje, Inc Photocatalytic performance of CC TiO 2 nanoparticles... 7 Isopropanol test method...7 CLP semivolatile calibration mix decomposition...8 TiO 2 photocatalysis applications and competitive advantages

4 Ultrafine (nanoparticle) TiO2 production at Cinkarna Celje, Inc. Titanium dioxide is one of the most widely used inorganic materials in the world. The most common form is pigmentary titanium dioxide but in recent years there has been a growing demand for ultrafine titanium dioxide. Ultrafine titanium dioxide is known for its many versatile applications emanating from its very small particle size and semiconductor properties. These applications include UV absorbing transparent coatings, flip flop automobile coatings, plastic additives, cosmetic UV blockers and electronic components characterised by rutile ultrafine particles. Anatase based applications are even more versatile and include photocatalysis (self cleaning effect, decomposition of harmful nitrous oxides from automobile exhausts, water and air purification), photoelectrochromic windows, DSSC ( dye sensitised solar cells ) and many more. The elementary principle of our ultrafine TiO 2 is the sulphate synthesis process, which is upgraded for the synthesis of a final ultrafine product. At Cinkarna Celje we decided to strategically orient towards the production of ultrafine TiO 2 explicitly in water suspension form. For that reason we have already developed the synthesis methods for anatase and rutile ultrafine particles that we obtain in suspension form without any intermediate powder phase. This decision is based on mastering the fine particles which stay in the suspension and to ensure healthy working conditions for our employees and the users of product. With the production of ultrafine TiO 2 products exclusively in suspension form, we prevent environmental impacts (emission of nanoparticles). With our synthesis methods we can control reaction mechanisms and that gives us control over the most important parameters of ultrafine particles, namely particle size, surface treatment and the crystal structure. Those parameters enable us to adapt ultrafine TiO 2 particles characteristics to best fulfil the demands of the above mentioned applications. Photocatalytic degradation of organic pollutants and of NOx gases basics Titanium dioxide particles in nano form catalyse the oxidation of adsorbed molecules in the presence of incident light of adequate photon energy. The light sufficient to induce the photocatalytic effect is in the UV part of the sunlight spectra. The adsorption of the UV light induces charge separation upon which electrons and positive holes form. Both species may act to produce highly active radicals, namely the hydroxyl radical and the superoxide radical. The airborne pollutants molecules may be adsorbed onto the TiO 2 surface and react with these radicals and chemically decompose. Ideally, the photocatalytic reaction leads to the formation of carbon dioxide (CO 2 ) and water (H 2 O). 4

5 Figure 1: A general scheme of TiO 2 mediated photocatalytic performance. Both, the hydroxide radical, OH, and superoxide radical, O 2, perform in photocatalytic reactions. TiO2 nanoparticle types produced at Cinkarna Celje, Inc. Cinkarna Celje s highly versatile technology for nanoparticle production enables the preparation of different types of TiO 2 nanoparticles, which are characterised by their crystal structure, size and crystallinity. We produce the following TiO 2 nanoparticle types: 1. CCA 100 BS polycrystalline anatase nanoparticles in the form of a neutral water suspension. The nanoparticles are about 50 nm in diameter and are constituted of smaller (5nm in size) crystallites. The CCA 100 AS is an acidic counterpart of CCA 100 BS. Figure 2: (a) SEM image of CCA 100 BS anatase nanoparticles and (b) TEM image of an individual anatase nanoparticle exhibiting polycrystallinity. The Individual crystallite is about 5 nm in size. 5

6 2. CCA 200 BS monocrystalline anatase nanoparticles in the form of a neutral water suspension. The nanoparticles are about 40 nm in diameter. Figure 3: (a) SEM image of CCA 200 BS monocrystalline anatase nanoparticles and (b) TEM image of an individual anatase monocrystalline nanoparticle. 3. CCR 100 AS polycrystalline rutile nanoparticles in water suspension form. The nanoparticles are anisotropic and are about 80 nm in length and about 30 nm in width. The CCR type nanoparticles, although being very photocatalytically active, are usually intended for UV absorption based applications rather than photocatalytic based applications. Therefore they are produced with an additional inorganic surface coating that hinders their photocatalytic activity but does not alter the UV absorption characteristics. Figure 4: (a) SEM image of CCR type rutile nanoparticles and (b) TEM image of an individual rutile nanoparticle exhibiting polycrystallinity. The Individual crystallite is anisotropic and is about nm in length and 5 nm in width. 4. CCR 200 BS monocyrstalline rutile nanoparticles in water suspension form. The CCR 200 BS nanoparticles are anisotropic and are about nm in length and about nm in width. 6

7 Figure 5: (a) SEM image of CCR 200 BS monocrystalline rutile nanoparticles and (b) TEM image of an individual rutile monocrystalline nanoparticle. The Individual crystal is anisotropic and is about nm in length and nm in width. All of the stated TiO 2 nanoparticle types may be altered in their individual size and in the case of monocrystalline materials also to some extent in their basic electronic properties ( bandgap value) by doping them. This enables the production of a tailor made product that best suits the specific final application. Photocatalytic performance of CC TiO2 nanoparticles The photocatalytic performance of TiO 2 nanoparticles is dependent on various parameters, which determine the two basic processes of photocatalysis, namely pollutant molecule adsorption and charge separation upon UV photon absorption. The two processes determine whether the pollutant molecule will be thoroughly and quantitatively decomposed. Since the adsorption properties and charge separation are both dependent on the basic TiO 2 particle properties and also on the chemical properties of the pollutant, there is no universally recognised TiO 2 photocatalyst. Rather, there are many types of TiO 2 nanoparticles, each with different properties and consequently variable performance depending on the specific type of pollutant. That is why Cinkarna Celje established its own TiO 2 nanoparticle production technology in such a manner as to successfully prepare different materials for a wide spectrum of possible pollutants. Based on the final photocatalysis application we can therefore advise our customers on the appropriate solution for the specific problem they have encountered. Isopropanol test method One of the widely used experimental methods to determine the photocatalytic performance of raw materials is the determination of the rate of isopropanol decomposition into the acetone intermediate. The advantage of this testing method is also the possibility to change the illuminating source which can either be a UV or visible light source. This enables the determination of the photocatalytic activity of a specific TiO 2 material type under the two possible illumination sources and therefore the determination of visible light activity, which is one of the most important niches in photocatalysis research performed globally. 7

8 Sample VIS [ppm/h] UV [ppm/h] CCA 200 BS monocrystalline anatase 0,6 99 CCA 100 BS CCR 200 BS monocrystalline rutile (raw) CCR 200 BS monocrystalline rutile (doped) CCR 100 AS Table 1: Photocatalytic performance of various TiO 2 nanoparticle types (all transformed in powder form by drying) produced at Cinkarna Celje, Inc. The photocatalytic performance is separated into UV and visible light performance. The most photocatalytically active material by far is the doped CCR 200 type which is also very active under visible light conditions. One of the most common commercially available TiO 2 materials, P25, has a visible light and UV light photocatalytic performance of 17 and 320, respectively. CLP semivolatile calibration mix decomposition As was already mentioned, various methods used to determine the photocatalytic activity of nano TiO 2 differ in the final result because of the differences in the testing materials that influence both the pollutant adsorption and also charge separation. That is why there is currently no universal testing method to determine the photocatalytic activity of TiO 2 nanoparticles. It is therefore appropriate to test TiO 2 nanoparticles for a wide range of possible pollutants (organic molecules) to determine its overall activity. One way to do this is to test the decomposition of a wide range of organic molecules present in a CLP semivolatile calibration mix (Sigma Aldrich) which includes 64 different organic components ranging from chlorinated alkanes to various aromatic compounds. The degradation of the organic molecules present in CLP was analysed by GC/MSD analysis of various samples taken after a specific period of time upon UV illumination in the presence of 0.25 g/l TiO 2. 8

9 A b u n d a n c e T I C : P O N O V N O S C A N M I X. D T i m e --> Figure 6: A chromatograph of the CLP semivolatile calibration mix (Sigma Aldrich, 1 ml, 1000 μg/ml) before the degradation experiments. A b u n d a n c e T i m e - - > A b u n d a n c e I : T C D I S T D _ T i m e - - > A b u n d a n c e I : T C D ( * ) I S T D _ T i m e - - > A b u n d a n c e I : T C D ( * ) I S T D _ T i m e - - > A b u n d a n c e I : T C D ( * ) I S T D _ T i m e - - > I : T C D ( * ) I S T D _ SLIKA 4: Posnetek vzorcev testa št. 3 dne (CCA100AS; 0,25 g/l) Figure 7: Different chromatographs of the CLP semivolatile calibration mix (Sigma Aldrich, 1 ml, 1000 μg/ml) upon carrying out TiO 2 (CCA 100 AS, 0.25 g/l) decomposition experiments. 9

10 As shown in Figure 7, the polycrystalline TiO 2 nanoparticles (CCA 100 AS/BS type) exhibit the ability to effectively decompose a wide range of possible organic molecules (pollutants). The results for other TiO 2 types are similar. TiO2 photocatalysis applications and competitive advantages The ability to decompose a wide range of possible organic pollutants and NOx gases coupled with a highly versatile production process that enables the control over TiO 2 nanoparticle crystal structure, particle size and crystallinity provides the variable solutions in the form of tailor made materials for a given problem or application our customers may encounter. So far our TiO 2 photocatalysts have been successfully tested for the following applications: for various construction materials (i.e. concrete, concrete tiles, ceramic tiles, roof tiles,.) in the form of an additive or in the form of a thin layer. The final construction materials exhibit a high photocatalytic effect, which enables organic pollutant removal and NOx gas removal (air remediation). for various solutions for the remediation of polluted air produced by thermal power stations and heavy traffic for remediation of industrial waste water polluted by specific organic components for hydrophilic and photocatalytically active thin layers on various substrates (i.e. glass) easy to clean surfaces (a) (b) (c) Figure 8: Different possible applications of TiO 2 photocatalysts for air remediation by adding TiO 2 nanoparticles in roof or concrete tiles (a), (b) and for dirt removal by a thin hydrophilic and photocatalytically active layer on glass (c). 10

11 Besides successfully testing our versatile TiO 2 photocatalysts for various applications, our TiO 2 photocatalysts also have the following advantages: control over the basic material properties (particle size, crystal structure, crystallinity, bandgap value). water suspension form of finely dispersed TiO 2 nanoparticles, which eliminates the possibility of any dust formation and emissions. This removes the need for expensive and technologically complex solutions for dust handling, emission control and deagglomeration processes, which are necessary when using TiO 2 in powder form. The use of water TiO 2 suspension thus lowers the initial cost and technology needs for material handling and is also environmentally safe. 11

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