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Self-doped ZnO Microrods - High Temperature Stable Oxygen Deficient Platforms for Solar Photocatalysis Sanjay Gopal Ullattil, Pradeepan Periyat, Binu Naufal, and Manoj A. Lazar Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b01030 • Publication Date (Web): 10 May 2016 Downloaded from http://pubs.acs.org on May 20, 2016

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Industrial & Engineering Chemistry Research

Self-doped ZnO Microrods - High Temperature Stable Oxygen Deficient Platforms for Solar Photocatalysis Sanjay Gopal Ullattil, a Pradeepan Periyat, a,b* Binu Naufal,a Manoj Ainikalkannath Lazarc a

Department of Chemistry, University of Calicut, Kerala, India - 673635.

b

Department of Chemistry, Central University of Kerala, India-671314.

c

School of Chemistry, Monash University, Clayton Victoria 3800, Australia.

*

Corresponding author’s email address: [email protected]/[email protected]

Abstract Dopant free, solar active ZnO photocatalysts with oxygen vacancy richness were achieved by a solution processing strategy followed by calcination at various temperatures 300, 500, 700, 800 and 900 °C. All the ZnO nanocrystals possessed defective structures with copious surface oxygen vacancies directed towards notable visible light absorption around λ = 480 nm (band gap = 3.05 - 3.09 eV). The photocatalytic efficiencies of all ZnO samples were systematically examined under sunlight and UV illumination using methylene blue (MB) as a model system. ZnO calcined at 500 and 700 °C demonstrated microrod morphology with band gap energies of 3.08 and 3.09 eV respectively have shown the highest solar photocatalytic activity revealed the synergistic effect between oxygen vacancy and the rod morphology. ZnO calcined at 500 °C, having maximum surface oxygen vacancy sites

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degraded MB within 10 minutes whereas the commercial photocatalyst Degussa- P25 has taken 20 minutes under solar illumination. Key words ZnO, self-doping, oxygen vacancy, band gap, photocatalysis 1. Introduction Exploration through oxide semiconductor nanomaterials are revealed owing to its applications in gas sensors, Li ion batteries, antimicrobial systems, heterogeneous catalysis, electrochromic displays and photocatalysis.1-6 Among various semiconductor oxides, TiO2 and ZnO are the unequivocally recognized candidates for photocatalytic processes.6-8 Although TiO2 is universally considered as the most important photocatalyst, ZnO is also a suitable alternative due to their similar band gap energy (3.37 eV), a much higher electron mobility than exhibited by TiO2 and its lower cost.9 Moreover, in certain case, larger quantum efficiency and higher photocatalytic activity than TiO2 have been reported.10-13 Generally ZnO has wurtzite structure and its photocatalytic activity can be further enhanced by calcinations along with high temperature thermal stability. ZnO can be prepared by a variety of synthetic strategies include sol-gel,14 hydrothermal,15 solvothermal,16 chemical vapour deposition,17 physical vapour deposition18 and microwave19 methods. These ZnO nanoparticles that have been synthesized were provided the impetus for majority of scientific and technological applications such as in dye sensitized solar cells, photocatalysis and in varistors.20-22 Previous works on ZnO photocatalysis have been identified that the material morphology, surface structure, particle size and porosity are all important factors for optimizing the photocatalytic performance.23,24 Upto date many synthetic strategies have been discovered for tailoring specific particle morphologies of ZnO rods,25 wires,26 flowers,27 tetrapods28 and 2 ACS Paragon Plus Environment

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needles29 of nano/micro dimension. Amongst these, ZnO microrods have got great attention due to their high photocatalytic activity.30-33 The synthesis of ZnO microrods have been achieved in presence of templates such as cetyltrimethylammonium bromide (CTAB),34 polyvinylpyrrolidone,35

tetramine,36

hexamethylene

monoethanolamine37

and

diethanolamine.38 However, these approaches have drawbacks such as the templates are expensive and removal of template to obtain pure nanorod is difficult. These limitations demand new approaches involving template free synthesis of ZnO rods. Hence the synthesis of nanocrystalline ZnO with required morphology and functional properties without any templates is important in the field of material science. Even though photocatalysis depends on physicochemical properties such as size and shape, band gap energy is an improtant crucial player that controls the photoactivity of any materials to a large extent. ZnO has a wide band gap energy (3.37 eV) similar to TiO2 (3.2 eV), which means the light responding range was limited due to the wide band gap energy of ZnO that can only absorb UV light, (λ ZnO-300 > ZnO-700 > ZnO-800 > ZnO-500 > ZnO-900. The higher activity of Degussa-P25 under UV light is well studied and is mainly due to its wide absorption ability in the UV region (Supporting Information, Figure S9) and it contains a mixed phase (75% anatase and 25% rutile) with a high BET surface area around 45 m2/g. All these factors would have contributed towards the higher activity of Degussa-P25 under UV irradiation. 54,55,56 Figure 6 indicates the degradation of dye using Degussa-P25 and ZnO samples in presence of sunlight. In presence of sunlight, degradation follows the order ZnO-500 > ZnO-700 > 15 ACS Paragon Plus Environment


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Degussa-P25 > ZnO-300 > ZnO-800 > ZnO-900. The maximum photoactivity was observed in the case of ZnO-500 having the lowest band gap energy of 3.08 eV, which is twice active than that of standard Degussa-P25. The ZnO-500 took only 10 minutes for dye degradation whereas Degussa-P25 have taken 20 minutes. The higher solar photocatalytic activity of ZnO-500 and ZnO-700 than standard Degussa-P25 can be explained as follows. It is already reported that, with the increase in oxygen deficiency a drastic increase in photocatalytic performance can be introduced.57,58 The band gap energy of these samples are in the range of 3.05-3.09 eV which corresponds to visible light region in the electromagnetic spectrum. This narrower range of narrowed band gap energy along with increase in oxygen deficiency of as synthesized ZnO samples facilitated a peculiar way to show higher photocatalytic efficiency. The band gap narrowing in ZnO samples are undoubtedly due to the excess oxygen vacancies (which is evident from Raman spectra, Figure 3B) as a kind of self-doping mechanism present in the lattice.59 Also it is clear from Raman spectra (Figure 3B) that, since there is no dopant and the presence of Zn interstitials here, oxygen vacancy is the only key player that is responsible for band gap narrowing, hence higher visible light absorption and thus optimizing higher photocatalytic efficiency of ZnO samples. The maximum photocatalytic efficiency under sunlight was obtained for ZnO-500 (Supporting Information, Figure S10) which has relatively the maximum intense peak at 580 cm-1 (Figure 3B) further reveals the dependence of photocatalysis on oxygen vacancy concentration. Other samples such as ZnO-800 and ZnO900 shows lower photocatalytic activity. According to previous reports as the temperature increases ZnO loses its oxygen vacancy sites which led to noticeable shift of absorption peaks towards the lower absorption region, consequently an increment in band gap energy also occurs.57,58 However, here the higher wavelength absorption is retained along with 16 ACS Paragon Plus Environment

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slightly low oxygen vacancies at high temperatures as compared to the most active photocatalyst ZnO-500. Moreover on account of the densification of these samples occurs at high temperatures, the microrod morphology was disappeared. All these factors have contributed towards the lower photocatalytic activity of ZnO-800 and ZnO-900. Also, the ZnO precursor have lower sunlight photocatalytic activity due to its poor visible light absorption and weak crystallinity. A B C D E F

SUN LIGHT

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Figure 6: Photodegradation of methylene blue under natural sunlight and UV illumination using A) Degussa-P25 B) ZnO-300 C) ZnO-500 D) ZnO-700 E) ZnO-800 and F) ZnO-900. 4. Photocatalytic Reaction Mechanism Generally a photocatalytic process commence with the photoexcitation of electrons from valence band to conduction band access to the generation of holes at the valence band. As a result an electron-hole separation arises as follows. ZnO + hν

ecb¯ + hvb+

In defect free ZnO fast electron-hole recombination taking place. However, the recombination rate is reduced in defect rich ZnO. This is happening due to the presence of an 17 ACS Paragon Plus Environment


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electron acceptor, obviously here the surface oxygen vacancy sites (Vo) of as synthesized ZnO sample that can trap the photogenerated electrons. These oxygen vacancy sites of ZnO samples are highly beneficial for restraining electron-hole recombination leading to the formation of O2¯˙ radical by reduction as shown below. Vo¨ + ecb¯

Vo˙

Vo˙+ O2

O2¯˙

When the oxygen vacancy concentration is very high, the oxygen vacancy state will be above the valence band as a forbidden state as depicted in Figure 7. Zheng et al.60 have already reported that the separation efficiency of electron and hole is controlled by oxygen vacancies. Moreover some oxygen interstitials (Oi) are also generated (which is evident from PL spectra, Figure 4) in the as prepared ZnO samples during light absorption and temperature processing. These Oi combines with the hole generated at the valence band facilitate oxidation to OH˙ described by the following equations. Oi" + hvb+

Oi'

Oi' + OH¯

Oi" + OH˙

According to band bending approximation,60 oxygen vacancies are believed to be more efficient in suppressing electron-hole recombination and thus exhibiting high photocatalytic efficiency61 which is in correct agreement with the photocatalytic efficiency as shown in Figure 6, where ZnO-500 shows maximum photocatalytic efficiency due to maximum oxygen vacancy sites present as compared to ZnO-300, ZnO-700, ZnO-800 and ZnO-900. Figure 7 displays that the oxygen vacancy sites are occupied just above the valence band of ZnO nanocrystals as a kind of self doping mechanism (ZnO-500 with band gap 3.08 eV is shown). Consequently the valence band and the oxygen vacancy state overlap together leads to 18 ACS Paragon Plus Environment

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the narrowing of band gap which is far less than the defect free ZnO (3.37 eV). Thus by utilizing the natural sunlight, electrons are excited from the high energy valence band to the conduction band easily results in the formation of O2¯˙ and OH˙ by reduction and oxidation respectively photocatalyzes the mineralization of organic dye pollutants into CO2 and H2O.

Figure 7: Photocatalysis mechanism of Oxygen deficiency enriched ZnO nanocrystals (VB: Valence band, CB: Conduction band, SOV state: Surface oxygen vacancy state) 5. Conclusion ZnO nanocrystals having narrowed band gap energy with rich oxygen vacancy sites were achieved by a facile and dopant free, solution processing method that has been designed only by using zinc nitrate hexahydrate, ammonium hydroxide and nitric acid. As synthesized ZnO nanocrystals have narrowed band gap energies in the range 3.05 – 3.09 eV which is due to the presence of excess oxygen vacancy sites in the crystal lattice and these vacancy sites are retained even after annealing at high temperature, 900 °C. These ZnO nanocrystals were successfully applied as photocatalysts under normal sunlight due to their extended substantial absorption ranges upto 480 nm. Rod shaped ZnO-500 and ZnO-700 with band gap energies 19 ACS Paragon Plus Environment


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3.08 and 3.09 eV respectively have demonstrated greater photocatalytic efficiency than the commercially available photocatalyst Degussa-P25 which revealed the combined activity of morphology and oxygen deficiency character of ZnO. The Degussa-P25 has performed dye degradation by taking more than 20 minutes while ZnO-500 and ZnO-700 have completed the same within 10 minutes and 20 minutes respectively. These high temperature stable solar active ZnO nanocrystals will unambiguously serve as remarkable photocatalysts and in future they may pave the way as a photocatalytic self-cleaner in ceramic tiles since its fabrication needs high temperature processing (~900 °C) along with light harvesting capability. Supporting Information FTIR and XRD of the ZnO precursor, Raman spectra of all the ZnO samples including the ZnO precursor (range: 700 cm-1 to 1500 cm-1), PL spectrum of the ZnO precursor, Photographs of all ZnO samples including the ZnO precursor, UV-Visible absorption spectra of the ZnO precursor, Solar and UV Photoactivity of ZnO precursor (UV-Visible spectra, first order kinetics and degradation plots), UV-Visible spectrum of Degussa-P25, UV-Vis spectra of methylene blue degradation using all ZnO samples and Degussa-P25 under solar and UV illumination. Acknowledgement Sanjay Gopal Ullattil and Binu Naufal are grateful to UGC-BSR fellowship, F.7184/2007(BSR), FD Diary No. 717. Dr. Pradeepan Periyat is thankful to UGC, India (Grant no F.20-6(8)-2012(BSR), for providing financial support.


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