effect of thickness on the photocatalytic properties of ... · solution, where the coatings were...

2019 16th International Conference on Electrical Engineering, Computing Science and Automatic Control (CCE). Mexico City, Mexico. September 11-13, 2019

978-1-7281-4840-3/19/$31.00 ©2019 IEEE

Effect of Thickness on the Photocatalytic Properties

of ZnO Coatings based on Nanoparticles

G. García-Zambrano, M. de la L Olvera, A. Maldonado Departamento de Ingeniería Eléctrica, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico

Nacional CINVESTAV-IPN, SEES, Apartado Postal 14740, México, D.F., 07000, México

Phone (55) 70828579 Fax (55) 57473978 Email: [email protected]

Abstract— In this work the results obtained from the study of Zinc Oxide nanoparticles (ZnO-NPs) coatings deposited on sodocalcic glass substrates for decoloration of Methylene Blue as a function of the thickness is reported. ZnO-NPs were synthesized by the Homogeneous Precipitation technique. The obtained powders were characterized by Transmission Electron Microscopy (TEM) and X-ray Diffraction (XRD), to study their morphological and structural properties, respectively. Subsequently, ZnO NPs coatings with different thicknesses were manufactured on sodocalcic glass plates by immersion. ZnO-NPs coatings were analyzed by profilometry, X-ray diffraction (XRD), secondary ion mass spectrometry (SIMS) and the photocatalytic activity performance was studied by bleaching of a standard dye of methylene blue (MB) with an initial concentration of 1x10-5 M. It was found that ZnO-NPs coatings shows reliability, low cost and reproducible characteristics for a potential application in degrading MB in water. Key words: ZnO nanoparticles; Coatings; Water treatment; Photocatalysis; Methylene Blue.

I. INTRODUCTION

Zinc oxide (ZnO) is a IIB -VIA type semiconductor material, of great interest for its study due to its broad bandgap (3.3 eV at room temperature) and a high bond excitation energy (60 meV) [1], among others. The difference of electronegativities between zinc and oxygen produces a high degree of ionization in its bond, becoming one of the most ionic compounds in this family [1]. From the structural point of view, it is formed by a zinc atom and an oxygen atom, which crystallizes in a cubic form of zinc blende, rock salt or hexagonal wurtzite, and has a coordination index of 4 [2]. ZnO shows also a white powder form. This characteristic is important in the luminescent, photoconductive, photochemical and photovoltaic properties of this compound, besides it has a high refractive index of 2 [3]. The various ZnO nanostructures have lead to potential applications reported in literature. In fact, its applications in photovoltaic energy, biomedical, sensors, detection of chemical and biological molecules, astronomy, optics, optoelectronics, electronics, nanosciences and nanotechnology make it an attractive material for the study and optimization of

synthesis methods to generate a possible industrial and commercial impact [4]. ZnO-NPs are now considered non-toxic, biosecure, ecofriendly and biocompatible, and have been applied in many biological applications of everyday life, such as drug vehicles, cosmetics and fillings in medical materials or devices [5]. It has also remained at the forefront due to its durability, high selectivity and biocompatibility [6]. For their unique properties, ZnO is an attractive material for a large number of disciplines and industry area. It is a material whose applications range from medicine, energy industry, rubber industry, pigments, paints, sensors, solar cells, optics, and optoelectronics, acoustic transducers [7], varistors [8], gas sensors [9], electrodes transparent [10], optical windows in solar cells [11], field emitting devices [12], transparent conductors [13-14], among others. Additionally, ZnO is considered a material of interest in nanotechnology, because in a nanostructured way it enhances the performance of traditional processes, creating new applications such as nano-photonics, nanoelectronics and nano-biotechnology [15]. Within the aforementioned applications we have the heterogeneous photocatalysis. It is defined as the photocatalysis that takes place at the interfacial boundary between two different phases (solid and liquid). Usually the catalyst is in the solid phase and the reagent / contaminant is in the liquid phase [16]. The role of the semiconductor oxides in photocatalysis process has been evidenced [17]. The electrons and holes photogenerated on the surface of the photocatalyst are capable of inducing reduction or oxidation reactions in pollutants dissolved in water. Simultaneously, an adsorption of the contaminants takes place and the photo-hole generates a highly oxidizing radical called hydroxyl (OH •) that will oxidize the adsorbed contaminant. The holes have an extremely positive oxidation potential and therefore, in theory, they must be able to oxidize almost all the chemical substances present even the electron of the oxygen of the water, which results in the formation of hydroxyl radicals [18]. Dyes are found among the most common water contaminants that can be degraded by the heterogeneous photocatalysis process. Textile industries produce lot of wastes that are thrown to rivers and sea, and have the property of modifying some important properties of water affecting the flora and fauna. Most dyes are organic molecules that are complex and resistant to many agents, such as the action of detergents. These dyes are widely used in many industries, for


example, in the textile industry, paper, leather tanning, plastics, food processing, cosmetics and rubber manufacturing, among others [19]. The photocatalytic decolorization of dyes is believed to take place according to the following mechanism [20]:

(1)

eCB- + O2→ O2

- (2)

hBV+ + OH− → OH• (3)

O2•− + H2O→ HO2•+ OH- (4) HO2•+ H2O → H2O2 + OH• (5) H2O2→ 2OH• (6) OH• +dye → CO2 +H2O (7)

Where h+

VB and e−CV are the holes in the valence band and the electron in the conduction band, respectively. Several reports claim that the rate of photocatalytic degradation of various dyes is fitted to a pseudofirst order kinetic model [21, 22, 23]:

(8)

(9) In the present research work, it is proposed a study the photocatalytic properties of ZnO-NPs coatings for bleaching of Methylene Blue, depending on the thickness. Beginning with the procedure of elaboration of ZnO-NPs, following with manufacture of ZnO-NPs coatings, and finally presenting the structural, morphological, optical, and the photocatalytic characterizations.

II. EXPERIMENTAL

ZnO nanopowders were manufactured by chemical precipitation starting from zinc acetate (Zn (CH3COO)2*2H2O) dissolved in deionized water (DI-H2O). As a precipitat agent, sodium hydroxide (NaOH) dissolved in methanol and deionized water was used at a ratio of 10:6 (CH4O : DI-H2O). The NaOH solution is stirred at 60 ° C for 30 min, and then, the zinc acetate solution is poured slowly onto the NaOH solution and stirred at 60 ° C for 90 min. Once the precipitate is obtained, it was separated in tubes of 50 ml in order to be centrifuged at 3500 RPM for 6 min in order to separate the solid from the remaining solvent. The centrifuged process was carried out twice more and then dried at 100 ° C, and finally thermally treated at 800 ° C for 1 hour (see fig.1). The structural and morphological properties of the ZnO NPs were studied by means of XRD and TEM. Subsequently, the ZnO NPs were dissolved in a typical sol gel solution, where the coatings were grown by means of electronically controlled immersion. Coatings of 2, 4, 6, 8 and 10 layers of this material were obtained. Each layer requires drying for 3 minutes at 250 ° C and for the last layer a heat treatment of 450 ° C at 1 Hour (see fig.2). These coatings were analyzed by XRD, SEM and SIMS. Finally, the photocatalytic properties of each one was studied, in order to know which was the coating with the optimal properties based on the characterizations obtained. Structural properties were analyzed by an X-Ray diffractometer, XRD (PANalytical model XPERT-PRO) using the Kα radiation of Cu (1.534 Å). Morphological properties by using a scanning electron microscopy, SEM (AURIGA and transmission electron microscopy, TEM (JEM- 1400) were studied. The ZnO-NPs Coatings composition were analyzed by a secondary ion mass spectroscopy

(S.I.M.S. model Cameca IMS-6F). The particle size was determined directly from the images by using the software of the equipment.

Fig. 1. Experimental procedure diagram to synthesize

ZnO-NPs powders.

Fig. 2. Experimental procedure diagram to manufacture ZnO-NPs coatings.

III. RESULTS AND DISCUSSION

X ray diffraction of ZnO- NPs In Fig. 3 the diffractograms of ZnO-NPs are presented. The diffraction peaks correspond to the ZnO wurtzite type structure, irrespective of the thickness. In the X-ray diffraction pattern, we can observe that the characteristic peaks appear correspond to planes (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), and (202) that fit well to the standard signals (Reference: 00-036-1451) of XRD. The diffraction planes correspond to a 2θ = 31.64, 34.32, 36.13, 47.38, 56.49, 62.8, 66.38, 67.89, 69.02, 72.48, and 76.86 °, respectively. The most intensive peak (101) is observed at 2θ= 36.2°. The crystallite sizes corresponding to powders, calculated by Scherrer´s formula (Eq. 10), ranged between 30 and 39 nm.

Dhkl (10)


Fig. 3. X-ray diffraction patterns for ZnO-NPs powders and the ZnO ASTM reference [24].

Morphology of ZnO- NPs

Fig. 4 shows the TEM images of the ZnO-NPs obtained by the homogeneous precipitation technique and calcined at 800 °C. From the images it is observed that the particles present agglomerates of different geometries, mostly with hexagonal and semi-circular wafers form of different sizes, estimated in a range between 20 and 100 nm. Once the ZnO-NPs have been analyzed, we are ready to manufacturing coatings on soda lime glass substrates by simple immersion and study the the corresponding structural and morphological properties.

Fig . 4. TEM micrographs of ZnO -NPs powders calcined at 800 °C for 2 h.

ZnO coatings thickness Fig. 5 presents the thickness values of the ZnO coatings, deposited from solutions with ZnO NPs, on glass substrates is shown. As can be seen, the thickness increases proportionally with the number of immersions; this result evidences a controlled covering with the immersions number. The slope of the graph depends on the NPs concentration dissolved in solution. Table 1 shows the thickness values coatings as a function of the number of layers, as well as a visual appreciation of the coatings color associated with the reflection of white light.

Fig. 5. Thickness values as a function of the number of ZnO-NPs layers deposited.

Table 1. Characteristic of ZnO coatings.

X ray diffraction of ZnO coatings

Fig. 6 shows the X-ray diffraction spectra of ZnO-NPs coatings. In the X-ray diffraction patterns, we can see the prevalence of the (002) planes and a minor contribution of (100) and (101) planes for samples of 4 and 6 layers. It is also observed that, once the ZnO NPs are fixed on sodocalcic glass substrates, the reflections corresponding to larger angles practically disappear. This is attributed to the fact that, during the coating process, the particles are reoriented to the minimum energy configuration, and then we have the predominance of the (002) planes. Because the processing of the coatings is done at a temperature much lower than 800 ˚C, it could be descarted some type of aglomeration or changes in the structure. The crystallite sizes corresponding to all ZnO coatings are calculated by Scherrer´s formula (Eq. 10), ranged between 30 and 32 nm.

Sample Number of

layers

Color of the sample against

backlight

Thickness (nm)

ZnO-2 2 Silver 90 ZnO-4 4 copper 130 ZnO-6 6 Violet 156 ZnO-8 8 Dark Blue 190

ZnO-10 10 Yellow 265


Fig. 6. XRD patterns of ZnO-NPs coatings with 2, 4, 6, 8, and 10 layers.

Composition of ZnO coating, SIMS

Fig. 7 shows the SIMS depth profile of elemental composition of a ZnO coating of approximately 156 nm in thickness. It can be observed that the SIMS spectroscopy detected the elements Hydrogen (H +), Oxygen (O2 +), Sodium (Na +) and Zinc (Zn +) of the deposited coating. We also find sulfur (S +) and carbon (C +), contaminants commonly presented in chemical deposition techniques carried out in open atmosphere. On the other hand, the high presence of silicon (Si+) is observed after 300 seconds of sputtering, this indicates that the 156 nm coating ends at this point and initiates the substrate, since the decay of the rest of elements, with the exception of Sodium is also observed. It should be mentioned that the profile of elements is qualitative, since quantification requires reference patterns that are not available. However, for our analysis we are only interested in knowing the elements present in the ZnO-NPs coating whose thickness is 156 nm.

Fig. 7. SIMS depth profile of a ZnO-NPs coating of 156 nm thickness.

Morphology of ZnO coating, SEM

Fig. 8 (a-c) shows the SEM images of the ZnO-NPs coatings of 2, 6, and 10 layers, with magnifications of 20 and 50Kx, A and B, respectively. It can be appreciated larger veins formed of smaller grains. Because this technique allows us to observe larger surfaces, we can observe the distribution of particles from different points of view, the formation of hexagonal faceted grains and mostly semi-circular of different sizes.

Grains with different sizes were observed, predominantly around 15 and 90 nm; they were directly identified from the images, also coinciding with the crystal sizes values estimated from the X-ray spectra. The larger grains are attributed to secondary formations of agglomerates, which takes place during the coating process. It was also observed that as more layers are deposited, the surface is more compact and uniform; the 10 layers coating presents the best uniformity.

(a)

(b)


(c)

Fig. 8. SEM images of a) 2, b) 6, and c) 10 ZnO layers. A) 20Kx, B) 50Kx.

Fig. 9a presents the results obtained from the decay in the concentration (C/C0) of AM as a function of time using the ZnO NPs coatings of 2, 6 and 8 layers, the optimum being obtained with a thickness of 6 layers ( 156 nm), showing a decrease in the 8 immersions coating (190 nm), and a slight increasing in the 256 nm, as shown in Table 3. Fig. 9b shows the reaction rate k obtained by the -Ln (C / C0) for ZnO-NPs coatings of 2, 6, and 8 immersions. Table 2 shows all the reaction rates obtained for ZnO-NPs coatings.

a)

b)

Fig. 9. MB degradation curves of the ZnO-NPs coatings. a) Irradiation time

vs C/C0. b) Irradiation time vs -Ln (C/C0).

Table 2. Reaction rate (k) of photolysis and ZnO-NPs coatings.

Fig. 10 shows the MB degradation in periods of 30 min per 3 h, showing a gradual decoloring until achieving an almost transparent solution after 180 min.

Fig. 10. Degradation of MB in periods of 30 min for 3 h in presence

of the catalyst. IV. CONCLUSIONS

ZnO-NPs were synthesized, obtaining a wurtzite hexagonal structure of high crystalline quality, in addition hexagonal and semicircular morphologies of various sizes between 20 nm and 90 nm were found. ZnO-NPs coatings were manufactured on glass substrates, using a dipping and sol-gel system to deposit the different numbers of layers of each film, obtaining good quality coatings with good homogeneity and excellent adhesion. The coatings show a preferential phase along the (002) plane, it is also shown that, with the exception of the coating of 8 immersions, all the coatings present high surface roughness. The elements detected in the coatings were hydrogen (H +), oxygen (O2 +), sodium (Na +), zinc (Zn +), sulfur (S +) and carbon (C +); the last two are contaminants typically present in chemical deposition techniques carried out in open atmospheres. In addition, Na, K and Si, coming from the sustrates, were present in the coatings. This indicates that the processing of coatings at temperatures of less than 400 °C is convenient, in order to avoid contamination.All the ZnO-NPs coatings showed good photocatalytic properties, since the MB degradation oscillated between 80 and 91 % in 3 h. The 6 immersions coatings showed the optimum value, with a reaction rate of 0.796. ACKNOWLEDGEMENTS

We thanks the technical support received from Emma Julia Luna Arredondo, M. A. Luna Arias, Adolfo Tavira, and Jorge Roque-De La Puente from LANE-Cinvestav.

Sample Reaction rate, k (h-1)Photolysis 0.131

2 layers (90 nm) 0.6574 layers (130 nm) 0.6736 layers (156 nm) 0.7968 layers (190 nm) 0.56210 layers (265 nm) 0.706


REFERENCES [1] Tiwana, P., Docampo, P., Johnston, M. B., Snaith, H. J., &

Herz, L. M; ” Electron mobility and injection dynamics inmesoporous ZnO, SnO2, and TiO2 films used in dye-sensitized solar cells”.ACS Nano,vol.5-6, 2011, 565158-5166.

[2] Drasar, C., Lostak, P., & Uher, C., “Doping and defect structure of tetradymite-type crystals”. Journal of Electronic Materials, vol. 39-9, 2010, 2162-2164.

[3] Adachi, S., “Physical Properties of III ‐ V Semiconductor Compounds: InP, InAs, GaAs, GaP, InGaAs, and InGaAsP”. John Wiley & Sons, Chichester, UK , 1992, XVIII, 318.

[4] Memic, Al-Hamedi, Khan, Azam, Habib, Salah, N., & Zahed. .,”High-energy ball milling technique for ZnO nanoparticles as antibacterial material”. International Journal of Nanomedicine, 2011, vol.6. pp. 863-869.

[5] Hilali, M. M., Rohatgi, A., & Asher, S. “Development of screen-printed silicon solar cells with high fill factors on 100 Ω/sq emitters”. IEEE Transactions on Electron Devices. 2004, vol. 51-6. pp. 948-955.

[6] Chopra, K. L., Major, S., & Pandya, D. K. “Transparent conductors-A status review”. Thin Solid Films. Thin Solid Films ,1983 , .Vol. 102-1, p.p 1-46.

[7] Lüth, H. Solid Surfaces, Interfaces and Thin Films. Springer, Fifth edition, 2010, pp 1-28.

[8] Aranovich, J. A., Golmayo, D., Fahrenbruch, A. L., & Bube, R. H. “Photovoltaic properties of ZnO/CdTe heterojunctions prepared by spray pyrolysis”. Journal of Applied Physics Applied Physics Letters Journal of Vacuum Science and Technology., 2010. Vol. 62-22.

[9] Stolt, L., Hedström, J., Kessler, J., Ruckh, M., Velthaus, K. O., & Schock, H. W. ZnO/CdS/CuInSe2 thin-film solar cells with improved performance. Applied Physics Letters. 1993, vo. 62-6.

[10] Webb, J. B., Williams, D. F., & Buchanan, M. “Transparent and highly conductive films of ZnO prepared by rf reactive magnetron sputtering”. Applied Physics Letters. 1981, vol. 39-8.

[11] Sato, H., Minami, T., Takata, S., & Yamada, T. “Transparent conducting p-type NiO thin films prepared by magnetron sputtering. Thin Solid Films”. Thin Solid Films, 1993, vol.236-1-2.

[12] Vaseem, M., Umar, A., & Hahn, Y.-B. (2009). “ZnO Nanoparticles: Growth, Properties, and Applications. In Metal Oxide Nanostructures and Their Applications”.Chapter 4, American scientific publishers, vol.55. pp.1-36

[13] Lupan, O., Shishiyanu, S., Ursaki, V., Khallaf, H., Chow, L., Shishiyanu, T.Railean, S. “Synthesis of nanostructured Al-doped zinc oxide films on Si for solar cells applications. Solar Energy Materials and Solar Cells”,2009 , vol. 93-8. Pages 1417-1422

[14] Cioffi, N., & Rai, M. Nano-antimicrobials: “Progress and prospects”. Springer-Verlag Berlin Heidelberg, 2012.

[15] Ramani, M., Ponnusamy, S., & Muthamizhchelvan, C. “Preliminary investigations on the antibacterial activity of zinc oxide nanostructures”. Journal of Nanoparticle Research. 2013, Springer Netherlands .

[16] Y. Anjaneyulu, N. Sreedhara Chary, and D. Samuel Suman Raj, “Decolourization of industrial effluents - Available methods and emerging technologies - A review,” Reviews in Environmental Science and Biotechnology. 2005.

[17] S. Kumar, K. Rao,” Zinc oxide based photocatalysis: tailoring

surface-bulk structure and related interfacial charge carrier dynamics for better environmental applications” Royal Science of Chemistry (Advances), 5, 3306 (2015).

[18] J. Xu, Y. Ao, D. Fu, C. Yuan, Applied Surface Science, vol. 254-10, 3033 (2008).

[19] A. Valentine Rupa, D. Manikandan, D. Divakar, T.Sivakumar, Journal of Hazardous Materials, vol. 147,906 (2007).

[20] Ong, C. B., Ng, L. Y., & Mohammad, A. W. “A review of ZnO nanoparticles as solar photocatalysts: Synthesis, mechanisms and applications”. Renewable and Sustainable Energy Reviews. 2018. Vol. 81, Part 1, 2018, Pages 536-551

[21] Konstantinou, I. K., & Albanis, T. A. "TiO2-assisted photocatalytic degradation of azo dyes in aqueous solution: Kinetic and mechanistic investigations: A review". Applied Catalysis B: Environmental. Volume 49, Issue 1, 20 April 2004, Pages 1-14

[22] Hachem, C., Bocquillon, F., Zahraa, O., & Bouchy, M. "Decolourization of textile industry wastewater by the photocatalytic degradation process" . Dyes and Pigments. Vol. 49- 2, 2001, Pages 117-125

[23] Lucarelli, L., Nadtochenko, V., & Kiwi, J. “Environmental photochemistry: quantitative adsorption and FTIR studies during the TiO2-photocatalyzed degradation of Orange II”. Langmuir. 2000 vol.16-3, 1102-1108.

[24] A. Valentine Rupa, D. Manikandan, D. Divakar, T. Sivakumar, Journal of Hazardous Materials, 147, 906 (2007). American Mineralogist Crystal Structure Database,Record R060027, [Online]. http://rruff.info/zincite/displa display=default/R060027

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