Ultrasound assisted large scale fabrication of superhydrophilic anodized SnOx films with highly uniformed nanoporous arrays

Journal Pre-proof Ultrasound assisted large scale fabrication of superhydrophilic anodized SnOx films with highly uniformed nanoporous arrays

Zhaoqing Gao, Jinwei Cao, Hussain Muhammad Muzammal, Chen Wang, Hao Sun, Dong Chong, Haitao Ma, Yunpeng Wang PII:

S0254-0584(19)31350-1

DOI:

https://doi.org/10.1016/j.matchemphys.2019.122540

Reference:

MAC 122540

To appear in:

Materials Chemistry and Physics

Received Date:

11 October 2019

Accepted Date:

10 December 2019

Please cite this article as: Zhaoqing Gao, Jinwei Cao, Hussain Muhammad Muzammal, Chen Wang, Hao Sun, Dong Chong, Haitao Ma, Yunpeng Wang, Ultrasound assisted large scale fabrication of superhydrophilic anodized SnOx films with highly uniformed nanoporous arrays,

Materials Chemistry and Physics (2019), https://doi.org/10.1016/j.matchemphys.2019.122540

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Journal Pre-proof Ultrasound assisted large scale fabrication of superhydrophilic anodized SnOx films with highly uniformed nanoporous arrays

Zhaoqing Gao, Jinwei Cao, Hussain Muhammad Muzammal, Chen Wang, Hao Sun, Dong Chong, Haitao Ma*, Yunpeng Wang* School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China Corresponding author 1 Email: [email protected] Corresponding author 2 Email: [email protected]

ABSTRACT: In this paper, the effect of ultrasonic field on the morphology, optical and wetting properties of anodized SnOx film was revealed. X-ray diffraction, field emission scanning electron microscopy, ultraviolet-visible-near-infrared spectroscopy, electrochemical impedance spectroscopy and contact angle analyzer measurements were employed to characterize the as-prepared obtained films. Based on the analysis of these obtained results, it can be confirmed that the application of ultrasonic field has a significant influence on the reactive activity of Sn crystal plane, interface charge transfer, total current (including ionic current and electronic current), microstructure uniformity, and optical absorption property of anodized SnOx films. This study opens a new window to prepare highly uniformed SnOx films with excellent semiconductor performance and superhydrophilic properties by introducing the ultrasonic field to anodic oxidation process. Keywords: tin oxide film; ultrasound-assisted agitation; superhydrophilicity; charge transfer; nanoporous array

1. Introduction From the past decades to the latest days, metal oxides as the promising semiconductor material have increasingly attracted much attention from many researchers in different fields [1-5]. Tin oxide has received much attention because of its potential properties such as faster electron transport rate, tolerance to UV 1

Journal Pre-proof illumination resulting from the larger band gap (3.6 eV) [6], and gas-sensing properties[7]. Owing to its wide band gap of SnO2 material, Bouznit et al. [8] and Lee et al. [9] conducted some interesting work and they proposed different effective solutions to improve the performance of SnO2-based films in the application of photocurrent generation. Nowadays, there are two main development directions to improve tin oxide film properties [8-13]: one is to modify tin oxides by doping the hetero-structure, and the other is to decrease the grain size of tin oxide with nanoporous microstructure. Nanoporous SnO2 materials can be used for various applications in photovoltics [14], gas-sensors [15-17], SnO2-supported palladium catalysts [18]. Tin oxides can be prepared by chemical vapor deposition [19], chemical spray [8], ultrasonic spraying pyrolysis [20], anodic oxidation [21], sol-gel [22], solvothermal methods [23], magnetron sputtering [13,24], pulsed laser deposition (PLD) [25], thermal evaporation [26], etc. Additionally, the required complex process and expensive equipment are also their drawbacks for some of those synthesis methods. Therefore, it is very necessary to develop simple, low-cost, and pollution-free methods to fabricate SnO2 nanostructured materials. Anodic oxidation is an efficient and simple method to prepare tin oxide film at a large scale, however, some problems in this field are still needed to solve. For example, it is inevitable to occur some microcracks in the semiconductor tin oxide film obtained by anodic oxidation method [21,26,27], and therefore some of its intrinsic functional characteristics can not be given full play. Although the number of micro-cracks can be reduced by optimizing the electrochemical process parameters or adding other additives, the uniformity of the micro-structure of the film is still relatively poor. It can be expected that ultrasonic field has a positive effect on promoting chemical reaction and improving the micro-structured homogeneity of the synthesized material, due to its intrinsic ultrasonic cavitation effect, mechanical effect and thermal effect. According to some previous scientific references, it can be learned that ultrasonic cavitation has been used in many fields, such as interface metallurgical brazing [28], electroforming [29], electroplating [30,31] and anodic oxidation reaction [14,32], etc. The wetting behavior of solid surfaces as one of the most important aspects of 2

Journal Pre-proof surface chemistry plays a very significant role in many practical applications. Functional surfaces with special wetting properties hold great promise for the various applications of different industrial products. As we know, the wetting behavior of solid surface is governed by their surface micro-structure and chemical composition. For a given material with a constant intrinsic surface energy, the surface wettability can be modulated by changing its surface geometrical structure. There are many methods to design different functional surfaces with special wettability by constructing various geometrical structure, such as template synthesis, phase separation, electrochemical deposition and crystallization control, etc [33-35]. Among these methods, one-step simple anodization method is simple and inexpensive technology. Moreover, it can be employed to construct highly uniformed nano-structured arrays for these applications where interface materials with superwettability are needed [36]. As reported in previous references, the stable superhydrophilic inorganic material without light irradiation mainly included the lower lotus surface [37], Cu(OH)2 [37,38], hierarchically meso-porous silica nano-particles [39], cysteic acid functionalized alumina [40]. Generally, the surface energy of the stable super-hydrophilic inorganic interfaces should have a very high surface free energy [33, 34]. Tin oxides films with nanoporous microstructure can be successfully controlled using anodic oxidation technology. Highly uniformed nanochannel arrays may have a very high solid surface free energy due to its relatively high specific area which may be used as a selective material with superhydrophilic properties. Previous studies indicated that there were two main problems to be solved for anodized tin oxide films which were detrimental to the electronic conductivity and reliable long durability [21,26,27]. One was that the nanostructure of anodized tin oxide films was not homogeneous and ordered. The other was that some micro-cracks in anodized tin oxide films were extremely difficult to eliminate. It should be noted that these two shortcomings were mainly attributed to the intense evolution and release of oxygen during anodization process [21,41]. Taken into consideration the unique effect of ultrasound field on the release velocity of oxygen, it is very interesting to explore the influence of ultrasound field on the 3

Journal Pre-proof morphology and properties as well as the chemical reaction during anodization process of tin substrates. It can be expected that anodic oxidation technology modified by ultrasound may be a potential and effective way to prepare anodized SnOx films with high quality in a large scale. In this paper, emphasis is placed on the influence of cavitation and mechanical effects on liquid phase mass transfer and charge transfer. Based on the obtained results, new insights will be proposed to deeply understand the underlying influence mechanism of ultrasonic field on the process of reaction and formation of highly uniformed nano-porous SnOx films. One-step ultrasonic-modified anodic oxidation will become a very significant method to design the high-performance vertically aligned SnOx nanochannel arrays for the promising photovoltaic applications. Moreover, this study will explore the feasibility to fabricate highly uniformed tin oxide films in a large scale by the modified anodic oxidation and investigate the wetting behavior of water and the optical properties of the as-prepared oxide films. To our knowledge, this study represents the first attempt using nanoporous SnOx as a interface material to evaluate its wettability, which may extend the promising application scope of semiconductor SnOx material in multiple interdisciplinary subjects.

2. Experimental 2.1 Preparation of Tin oxides film The nanoporous tin oxide layers were synthesized using a modified one-step electrochemical anodic oxidation method. The commercially available metallic tin foil was purchased from Guantai Metal Material Co., Ltd. (China) and employed as the starting material. Tin foils (99.99% purity, 0.2 mm thick, 2.0 cm× 1.5 cm in size) were anodized in a 0.5 M oxalic acid aqueous (H2C2O4) solution and 1.0 M sodium hydroxide (NaOH) solution, respectively. The back of tin foil sample was sealed with insulating paint and the active surface area was 3 cm2. Before anodizing, tin foils were cleaned by sequential ultrasonication in baths containing the following solutions: a 1:1 mixture of ethanol and acetone, distilled water, and then were dried with nitrogen. 4

Journal Pre-proof Tin foils were used as received without any further treatment except cleaning. The synthesis of SnOx film was conducted at 26± 1.5℃ using a two-electrode electrochemical cell (with a tin foil as anode and graphite sheet as cathode) with a applied constant DC potential of 5 V. The oxidation time was 600 s and the distance between the tin foil and graphite sheet was 3 cm. The anodic oxidation process was carried out with the ultrasonic agitation (40kHz, 360W, F-060S, FUYANG, China) and without the ultrasonic agitation, respectively. In order to simplify the analytical complexity of the effect of ultrasonic field agitation on the actual anodic oxidation reaction process, a constant temperature water bath cooling system was introduced to the anodic oxidation process with the ultrasonic-assisted agitation. The main intention of introducing a constant temperature water bath cooling system was to eliminate the thermal effect of ultrasonic field on the actual anodic oxidation reaction. 2.2 Instruments and Characterization The phase composition of the as-prepared samples was determined by using X-ray diffractometer (Rigaku D/MAX-Ultima, Japan) operated at a voltage of 40 kV and a current of 40 mA, utilizing Cu Kα radiation (λ = 1.54178 Å, 2θ = 30°–100°, scan step of 0.02°, scan rate of 2°/ min). In order to uncover the effect of ultrasonic field on the preferential reactive-active plane of Sn substrate during anodic oxidation reaction, XRD analysis technique was used to determine Sn surface plane orientation evolution before and after anodic oxidation. The texture coefficient (TC) was employed to evaluate the texture evolution of the matrix material beneath the oxide film using a semi-quantitative XRD analysis technology. The texture coefficient of the (hkl) reflection plane is defined by [42,43] :

TC hkl  

0 I hkl I hkl  1   I hkl    0  n   I hkl

  

(1)

where I(hkl) and I0(hkl) are the integrated intensities of (hkl) reflections measured for experimental specimen and a standard powder sample, respectively. ∑ means the summation, and n represents the number of diffraction peaks in XRD patterns. 5

Journal Pre-proof Compared with the method mentioned in the previous reference [44], the employed method (formula1) in this paper is reliable to evaluate the change trend of the texture of these single-phase materials and to obtain the preferred orientation information of the solid materials in half quantification. The surface morphology of as-prepared samples was characterized by high-resolution Zeiss-Supra 55 field emission scanning electron microscopy (FE-SEM). The ultraviolet-visible-near-infrared (UV-VIS-NIR) diffusion absorption performances were recorded by a spectrophotometer (UV-3600, Shimadzu) using BaSO4 as the reference. Electrochemical test was conducted on the VMP3 (EG&G) electrochemical workstation using a three-electrode system, i.e., the original substrate and as-prepared samples used as working electrodes, platinum net as counter electrode and saturated calomel electrode (SCE) as reference electrode, respectively. All the electrochemical measurements were carried out in in a 0.5 M oxalic acid aqueous solution and 1.0 M sodium hydroxide solution at 25 ± 1℃, respectively. All the electrochemical tests were conducted out with the ultrasonic agitation (40 kHz, 360 W) and without the ultrasonic agitation, respectively. The open circuit potential (OCP) was obtained for 10 min before A.C. impedance test. The A.C. impedance test was carried out with OCP of A.C. voltage from 0.01 Hz to 105 Hz frequency. Software ZView was employed to simulate equivalent circuits of electrochemical impedance spectroscopy EIS. The static contact angle of 4 μL deionized water on the clean surface of these as-prepared SnOx films were measured by contact angle analyzer (DSA 100, KRUSS, Germany). The condition for measuring the static contact angle was a humidity of 48.1 ± 0.8% and a temperature of 26.5 ± 0.2 ℃.

3. Results and discussion 3.1 Structural analysis

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Fig.1 XRD patterns of samples obtained by anodic oxidation at the potential of 5 V under ultrasonic-free and ultrasonic conditions: (a) in 0.5 M oxalic acid solution; (b) 1 M NaOH solution.

The X-ray diffraction patterns of the as-prepared tin oxides were shown in Fig.1. For the as-anodized tin samples, all the sharp peaks of X-ray diffraction patterns corresponded to tin substrates, which indicated that the application of ultrasound field could not change the amorphous properties of anodized tin oxide films. The macro-texture evolution of tin crystal plane after anodic oxidation reaction was semi-quantitatively calculated by XRD analysis method, and the corresponding results were listed in Table 1. Texture coefficient of the (hkl) reflection plane in the original tin substrate was employed as an evaluation standard, the variation of TC(hkl) before and after anodic oxidation reaction can indirectly reflect the crystalline plane reaction activity of tin matrix. It can be found from Table 1 that the TC(101), TC(220) of tin substrates after anodization in H2C2O4 solution and NaOH solution were lower than that of original tin substrates, indicating that the reactive activity of (101) and (220) crystal planes in the two different electrolyte solutions was higher. The decrease of TC(431) and TC(440) confirmed that the reactive activity of (431) and (440) crystal planes was improved by the application of ultrasound field in the oxalic acid solution and sodium hydroxide electrolyte solution. The increase of TC(211) and TC(312) induced by ultrasound-assisted agitation indicated that ultrasound stirring in oxalic acid solution could not improve the reactive activity of (211) and (312) crystal planes. However, the TC(112) and TC(103) of (112) and (103) crystal planes decreased after ultrasound treatment in NaOH 7

Journal Pre-proof solution, indicating that the reactive activity of (112) and (103) crystal planes was enhanced by the ultrasonic field. Table 1 The texture coefficient of the (hkl) reflection plane for different samples TC(hkl)

pristine

H2C2O4

H2C2O4

NaOH

NaOH

substrate

ultrasonic-free

ultrasonic

ultrasonic-free

ultrasonic

TC(200)

0.033

0.060

0.053

0.076

0.046

TC(101)

0.509

0.504

0.468

0.408

0.316

TC(220)

2.810

1.140

0.241

0.421

2.182

TC(211)

0.455

0.444

0.876

0.238

0.214

TC(301)

0.158

0.421

0.396

0.027

0.907

TC(112)

0.509

1.512

0.963

0.804

0.350

TC(321)

0.455

0.744

0.732

0.467

1.972

TC(420)

0.247

0.552

1.033

0.528

0.745

TC(312)

0.571

0.468

0.912

0.384

0.305

TC(431)

0.710

0.432

0.456

0.516

0.919

TC(103)

1.500

2.093

3.012

3.373

1.302

TC(440)

4.050

3.624

2.840

4.780

2.751

3.2 Morphology of SnOx films The top-view SEM images of anodized samples prepared in different electrolytes solutions were shown in Fig.2 (0.5M oxalic acid solution) and Fig.3 (1M sodium hydroxide solution), respectively. Compared with the results from ultrasonic-free conditions (Fig.2a and Fig.3a) and ultrasonic condition (Fig.2b and Fig.3b), it clearly indicated that tin oxide thin films with fewer microcracks could be obtained by ultrasound-assisted anodic oxidation. The decrease of micro-cracks in quantity was closely related to the reduce of stress stored in tin oxide films during the process of anodic oxidation. Moreover, as shown in Fig.2c and Fig.2d, the application of ultrasound in oxalic acid solution seems to reduce the probability of pores with larger size, and to a certain extent, the uniformity of pore size was improved. The surface morphology of ultrasonic-free tin oxide film obtained in sodium hydroxide solution is not smooth (Fig.3c). However, compared the results of Fig.3c and Fig.3d, it can be found that although the nanopore sizes of the as-prepared tin oxide film obtained from 8

Journal Pre-proof ultrasonic condition did not seem to obviously change, the uniformity of pore sizes and the surface smoothness of the SnOx film was obviously improved. The average size of nanopore on the tin oxide film can be estimated using a software of Image-Pro Plus. The estimated average pore size of anodized SnOx films prepared in samples 0.5 M oxalic acid solution was about 47±9 nm under the ultrasound condition and about 41± 3 nm under without ultrasound condition, respectively. The estimated average pore size of anodized prepared in samples 1 M sodium hydroxide solution was about 45± 5 nm under ultrasound-free agitation condition and about 38± 2 nm under ultrasound-assisted agitation condition, respectively. The homogeneity improvement of the tin oxide film (Fig.2d and Fig.3d) may be attributed to the acoustic cavitation effect and mechanical effect of ultrasound field. Using the conventional anodization process (ultrasonic-free), vigorous oxygen evolution during the corresponding anodic oxidation process easily led to the non-homogeneous distribution of chemical etching and field-assisted dissolution on the metal tin surface [14,21], and thus resulting in the formation of a non-uniform nanoporous microstructure. It should be noted that the application of ultrasound assisted the oxygen gas to escape rapidly and smoothly, causing the homogeneous distribution of chemical etching throughout the whole Sn foil surface [14,21]. Moreover, ultrasonic field may make the chemical etching of the formed tin oxide layers more effective [32]. Besides the ideal plane, there are many defects on the real metal surface such as helix dislocation, surface cavity, snarl stepl, adatom, single atom step and step hole. At the early stage of the reaction without ultrasound, the reactive-active sites of the anodic oxidation reaction are relatively less and the electrode reaction were mainly controlled by the surface defects with high surface energy (such as dislocations, holes and atomic steps) due to their low activation energy required for the electrochemical reaction. At this time, both active and inert regions exist on the electrode surface, which may be one of the main reasons for the non-uniformed morphology of the oxide film without ultrasonic field. It is noteworthy that, during the process of tin anodization under ultrasonic-assisted agitation condition, the random collapse of a large number of bubbles exists. Therefore, the ultrasonic cavitation effect results in 9

Journal Pre-proof the local regions with transient high temperature and high-speed micro-jet, which may increase the number reactive sites on the anode tin surface. Under the combined action of ultrasonic cavitation effect and mechanical effect, the surface activity of electrodes increased dramatically due to the transient response for the vibration and expanding of chemical bonds of the electrode surface and reaction particles in solution. Accordingly, the number of active sites and reactive micro-areas increased. Therefore, under the action of ultrasonic field, the tin substrate surface is composed of many electrochemical reaction micro-cells, which may be the main reason to explain the obvious uniformity improvement of tin oxide films.

Fig. 2 FE-SEM images of tin oxide nanochannel arrays prepared by the conventional and modified anodic oxidation process in 0.5 M oxalic acid solution : (a,c) without ultrasound, (b,d) with ultrasound, (c,d) is the local magnification of (a,b) respectively; (e) surface morphology of original tin foil substrate.

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Fig. 3. FE-SEM images of tin oxide nanochannel arrays prepared by the conventional and modified anodic oxidation process in 1.0 M sodium hydroxide solution : (a, c) without ultrasound; (b, d) with ultrasound; (c, d) is the local magnification of (a, b), respectively; (e) surface morphology of original tin foil substrate.

3.3 Optical properties

Fig. 4. UV-VIS-NIR optical absorption patterns of anodic tin oxide layers obtained at ultrasonic and ultrasonic-free conditions grown in different electrolyte solutions: (a) 0.5 M oxalic acid solution; (b) 1 M sodium hydroxide solution. 11

Journal Pre-proof The UV-VIS-NIR absorption spectra of anodic tin oxide films obtained by one-step anodic oxidation at both electrolytes are shown in Fig.4. As we know, the configurations of extra-nuclear electrons of O2-, Sn4+, Sn2+ are 1s22s22p6, [Kr]4d10 and [Kr]4d105s2, respectively. Therefore, the absorption in the UV region corresponds to the direct charge transfer transitions from O2- 2p to Sn4+ 4d and/or Sn2+ 5s charge. However, the absorption in the visible region is due to the Sn4+ 4d → 4d and / or Sn2+ 5s → 5s spin forbidden transition excitation (indirect transition) [32]. It should be noted that optical property of films is closely related to surface morphology and thickness of films. It can be found from Fig.4a that the as-prepared SnOx films in oxalic acid solution (in ultrasonic-free and ultrasonic conditions) possess wide-band absorption varying from the UV to the visible region, but it seems that ultrasonic agitation has a little influence on the optical absorption properties of anodic SnOx film prepared in oxalic acid solution. It can be found from Fig.4b that the absorption intensity of tin oxide film prepared in ultrasound field was significantly higher than that in ultrasonic-free condition due to the comprehensively combined effect of surface morphology and film thickness. In addition, as shown in Fig.4b, the existence of interference fringes of absorption spectra in the visible and near-infrared region confirmed that the anodic SnOx films were highly homogeneous [45]. It can be expected that the highly uniformed nanoporous array structure in this study can be effectively applied in photovoltaic devices, sensors, capacitors, and lithium ion batteries. 3.4 Effect of ultrasonic field on electrode reaction process The current density vs time curves recorded during anodization of Sn samples at 5 V in the oxalic acid solution and sodium hydroxide solution were shown in Fig.5a and Fig.5b, respectively. As was shown in Fig.5, when the potential (5 V) was applied, a significant decrease in current density for both agitation conditions (ultrasonic agitation and ultrasonic-free agitation) was observed immediately at the beginning stage of the anodic oxidation process owing to the formation of the compact passive layer on the surface of Sn foil [46, 47]. Current transient value during anodic oxidation of Sn substrate could be divided into three steps: step (i) 12

Journal Pre-proof formation of compact SnOx film; step (ii) nucleation of nanopores resulting from chemical etching and field-assisted dissolution of the as-formed SnOx layer; step (iii) formation of nanoporous channel structure. Hoar and Yahalom [48] found that the current of oxide growing decreased exponentially when the barrier oxide grew. A local current density minimum appeared in Fig.5a and Fig.5b after about 10-20 s of anodization followed by a slight current increase due to the pore formation within the oxide layer. It can be found from Fig.5 that the average steady-state current density under ultrasonic-free agitation condition was higher than that under ultrasonic agitation condition, which was suitable for these anodization processes in the both electrolyte solutions. The phenomenon that the application of ultrasonic field could reduce the average steady-state current density may be attributed to the influence of ultrasonic cavitation effect and mechanical effect on the liquid phase mass transfer process during the anodization reaction. There are three main means to influence the mass transfer process in liquid phase: electromigration, convection (including natural convection and forced convection), diffusion. In this study, the electromigration direction of OH- induced by electronic field force was the same as that of OH- induced by electrochemical reaction. Both the electromigration and diffusion played a positive effect to increase the current in the anodic oxidation process. Under the ultrasonic agitation condition, on the one hand, the forced convection caused by the application of ultrasonic field could reduce the concentration difference of each component in different regions and then the chemical potential gradient decreased, which might not promote the liquid-phase mass transfer of OH- in solution. On the other hand, the nucleation, coalescence growth and collapse of cavitation bubbles were random. The random breakage of bubbles led to the local high temperature and high speed of electrolyte micro-jets. Consequently, the existence of micro-jet with high moving speed may interfere with the liquid-phase mass transfer process of OH- and then, as a response to this disturbance, the current density decreased. In addition, it should be noted that ultrasonic field has a so-called depolarization effect [22], which might weaken the directional electromigration of OH-. Therefore, based on the analysis

13

Journal Pre-proof mentioned above, the cooperation of multiple factors resulted by the applied ultrasonic field caused the decrease of current density.

Fig.5. Current density-time curves of the anodization of tin oxides films in different electrolyte solutions : (a) 0.5 M oxalic acid solution; (b) 1.0 M sodium hydroxide solution

According to previous references, the total current density could be separated into ionic current density (jion) and electronic current density (je) [48-50]. The experimental data of current density-time during anodic oxidation can be well fitted to the theoretical functional expressions as follow [50]:

j  jion  je

 Ae

2 t   lc1  1 e T1  

BU 2 t     T2  l 1  e c 2      

    

2 t     T1   lc1  1 e    

 je 0 e

2 t     T2   lc 2  1 e    

     

(2)

Where A, B and θ are constants. lc1 and lc2 are the critical thickness as a part of the total barrier oxide thickness, respectively. T1 and T2 is the time when the barrier oxide in part l1 and l2 reaches the critical thickness, respectively. U is the constant voltage and t is the anodizing time. Fig.5 indicated that ultrasonic agitation had a little influence on the change of ionic current density. However, it had a very important effect on the electronic current density. Chong et al. [50] suggested that the a small fluctuation of the current density (even less than 4 mAcm-2) could lead to the obviously different ionic current density and electronic density. In this paper, however, the difference of the total anodizing current density caused by the application of ultrasonic field in different electrolyte solutions was higher than 20-40 mAcm-2, and therefore, it can be safely concluded that the application of ultrasound during anodic oxidation reaction would have a striking influence on the electronic density. As we 14

Journal Pre-proof know, the ionic current density nearly equals the oxide growing current density and the electronic current represents the pore forming current density [50]. Consequently, there is no doubt that the applied ultrasonic field has a critical influence on the corresponding anodic oxidation reaction as well as the microstructure of the as-prepared SnOx films because of its strong interference on the value of electronic density.

Fig.6 Nyquist curves of pristine substrate (a,c) and tin oxide films (b,d) at open circuit potential in different electrolyte solutions under ultrasonic and free-ultrasonic conditions : (a, b) 0.5 M oxalic acid solution; (c, d) 1.0 M sodium hydroxide solution.

The A.C. impedance plots at open circuit potential with and without the ultrasonic agitation was shown in Fig.6 and the equivalent circuit for the electrode–electrolyte interface was displayed in the inset. Rs was the solution resistance between the working electrode and the reference electrode, and CPE was the constant phase angle element. Rct was the faradaic charge transfer resistance. As we all know, semicircle in high frequency region is mainly controlled by electrochemical polarization; however, a linear variation in low frequency region are mainly controlled by diffusion. It can be found from Fig.6a and Fig.6c original tin substrates have a smaller radius in high frequency region due to the good electrical conductivity of pristine tin substrate, indicating that the pristine Sn substrate has a low 15

Journal Pre-proof internal resistance of electrode reaction, a smaller internal resistance of charge transfer and an internal Faraday resistance of charge transfer [51-53]. Compared with the Nyquist impedance curves of these pristine tin substrates (Fig.6a and Fig.6c), it clearly showed that anodized tin oxide films have a larger Bode radius (Fig.6b and Fig.6d) in high frequency due to their poor electrical conductivity of semiconductor tin oxide film. Based on the obtained electrochemical impedance spectroscopy (EIS) data, the fitted values of equivalent circuit parameters of pristine substrates and as-synthesized tin oxides films were presented in Table 2, Table 3, respectively. It can be found from the statistical results of Table 2 and Table 3, ultrasonic field has a little effect on the solution resistance (Rs). However, the application of ultrasound field generated an apparent increase in the Faradaic charge transfer resistance and the Warburg impedance. Meanwhile, it should be noted that the ultrasound-assisted agitation led to the decrease of CPE, which confirmed that ultrasound field would have a significant influence on distribution of physical properties of the system software and the system heterogeneity [54]. Table 2 Fitted values of equivalent circuit parameters for original substrates in H2C2O4 and NaOH solution under ultrasonic and free-ultrasonic conditions. Condition

Rs(Ω)

CPE(μF)

Rct(Ω)

Zw(Ωs1/2)

H2C2O4 ultrasonic-free H2C2O4 ultrasonic NaOH ultrasonic-free NaOH ultrasonic

30±0.3 31±0.6 10±0.4 12±0.7

80±0.3 42±0.5 67±0.6 28±0.4

53±6.4 106±9.7 39±5.2 60±8.2

304±8.5 515±7.7 110±6.8 206±13.6

Table 3 Fitted values of equivalent circuit parameters for as-prepared anodized samples in H2C2O4 and NaOH solution under ultrasonic and free-ultrasonic conditions. Condition

Rs(Ω)

CPE(μF)

Rct(Ω)

Zw(Ωs1/2)

H2C2O4 ultrasonic-free H2C2O4 ultrasonic

32±0.2 30±0.4

209±0.5 224±0.2

188±6.3 250±8.5

30±3.2 160±4.6

NaOH ultrasonic-free NaOH ultrasonic

12±0.5 13±0.8

241±0.1 78±0.3

134±4.9 172±7.1

102±5.3 206±13.6

The premise for electron transfer in the process of electrode reaction is that the electronic energy levels of the electrode and reaction particles should be excited to 16

Journal Pre-proof reach near the Fermi level [55]. The essence of electrode polarization is the shift of Fermi level, which can change the excitation state required for tunneling transition of free electrons. Therefore, the activation energy of electron transfer is changed by electrode polarization, which is the main reason to explain why the electrode potential can affect the variation of activation energy and reaction speed. It should be pointed out that the polarization of the electrode will cause the Fermi level change of free electrons, but the Fermi level of the reaction particles in the electrolyte solution is nearly unchanged [55]. The decrease of anode potential induced by the depolarization effect of ultrasonic field may cause the decrease of the electrode electrons potential energy which is lower than the Fermi level of the reaction particles in the electrolyte solution. For this reason, only when the electrode electron level reaches near the Fermi level of the reaction particle in the electrolyte, the electronic tunnel transition can be successfully realized. Therefore, at the interface between electrodes and electrolyte solution, the application of ultrasonic field may be not beneficial to the tunneling transition effect of free electrons, and then the Faraday charge transfer resistance increases. It should be pointed out that the mechanical effect and cavitation effect of ultrasound also play a significant role in determination for the electron transfer during the electrode reaction. In this study, the influence of ultrasonic field on the interaction between surface atoms (or even the near-surface reaction atoms) of anode electrode and particles (including ions and molecules) in electrolyte mainly can be divided into two aspects [55]: one is the change of solvation state of reaction particles; the other is the change of relative atom positions in reaction system, namely, the vibration and expansion of chemical bonds and so on. As we all know, the solvation degree fluctuation of reaction particles (rearrangement of solvent molecule in solvation layer) can change the electronic energy level of particles, thus providing a convenient opportunity for their free electrons to reach Fermi level. Moreover, the increase of electroactive sites for charge transfer may decrease the charge transfer resistance, attributing to the application of ultrasonic-assisted agitation. In short, the aforementioned effects of different factors induced by ultrasonic agitation for the variation of interface charge transfer resistance were as follows: the changes of 17

Journal Pre-proof solvation degree and atomic position in the system as well as the increase of electrode surface electroactive sites played a positive role in the elevation of Rct value, while the depolarization effect generated a negative influence on the increase of Rct. In this study, the synergistic effect of these four factors may be the main reason for the increase of the charge transfer resistance under ultrasonic field. 3.5 Wettability of SnOx films

Fig.7 Time-dependent change curves of water contact angle on anodized tin oxide films prepared at different conditions：(a) 0.5 M oxalic acid solution, ultrasonic (red) and ultrasonic-free (black) ; (b) 1.0 M sodium hydroxide, ultrasonic (red) and ultrasonic-free (black).

Time-dependent changes of water contact angle on tin oxide films were shown in panels a and b in Fig.7, respectively. Clearly, the water contact angles on tin oxide films prepared in oxalic acid solution decreased quickly when the wetting test time was increased from 0 to 1s (Fig.7a), indicating a relatively high spreading speed of water droplet. However, the water contact angles on the tin oxide film prepared in sodium hydroxide solution decreased slowly when the wetting test time was increased from 0 to 50 s (Fig.7b), indicating a relatively low spreading speed of water droplet. Interestingly, tin oxide films with superhydrophilic properties (water contact angle is less than 5°) can be prepared using ultrasonic-assisted agitation in oxalic acid solution (Fig. 7a). Fig.7a and Fig.7b illustrated that the application of ultrasonic field can significantly improve the hydrophilic properties of tin oxide films in the two employed electrolyte solutions (0.5 M oxalic acid solution and 1.0 M sodium hydroxide solution), which can be attributed to the enhancement of the apparent surface energy of the modified anodized tin oxide films. The wetting behavior of solid 18

Journal Pre-proof surface depends on its surface geometrical structure and its intrinsic surface energy which is closely related to the crystallization degree and surface composition of solid materials. Different water contact angles of anodized tin oxide films prepared at ultrasonic and ultrasonic-free conditions may be attributed to its surface morphology and intrinsic surface energy of anodized tin oxide films. In this paper, the XRD results (Fig.1) indicated that these as-prepared tin oxides films were amorphous. Therefore, the low crystallinity or amorphous properties of anodized tin oxide films made it very difficult to offer some direct evidence to support our speculation from the perspective of crystallography. Fig.2 and Fig.3 confirmed that the application of ultrasonic field can substantially improve the microstructure homogeneity of tin oxide film, although it decreased the structure complexity of morphology. It should be noted that anodized tin oxide films fabricated under ultrasonic and ultrasonic-free conditions may have some structure difference in the local short-range order. Under the similar anodization condition (the anodization process was also conducted in 1.0 M sodium hydroxide solution), Zaraska et al. [56] suggested that an absorption edge of anodized tin oxides film between 200 and 450 nm corresponded to the excitonic band gap of tin oxides mixture (SnO and SnO2). The results of Fig.4a and Fig.4b indicated that the absorption region edge of ultrasound-modified tin oxides film was significantly red-shifted compared with that of ultrasonic-free one, indicating that the application of ultrasonic field can change its surface composition and microstructure of anodized tin oxide films [56]. As we know, the contact angle of a solid material is closely related to its surface energy. For the anodized tin oxide film, the solid surface energy is mainly determined by the surface mciro-/nanostructure and its intrinsic surface composition. It should be noted that the water contact angle in Fig.7(a) was much lower than that in Fig.7(b), which could be attributed to the synergistic effect of different morphology and different surface compositions. For a given solid surface, the higher the surface free energy, the better the wettability. Hence, the evolution of water contact angle confirmed that the application of ultrasound field can improve the surface free energy of anodized tin oxide films. 4. Conclusions 19

Journal Pre-proof In summary, we have fabricated a superhydrophilic tin oxide film using a facile, efficient, and inexpensive ultrasound-assisted anodic oxidation technology. The microstructure homogeneity of tin oxide films can be substantially improved due to the acoustic cavitation effect and mechanical effect of ultrasonic field. Under the combined interaction of ultrasonic cavitation effect and mechanical effect, the surface activity of electrodes increased dramatically due to the transient response for the vibration and expanding of chemical bonds of the electrode surface and reaction particles in the electrolyte solution. Accordingly, the number of active sites and reactive micro-areas increased. Compared with the anodic process under ultrasonic-free condition, the cooperation of multiple factors induced by the applied ultrasonic field resulted in the decrease of electronic current density. The optical absorption intensity of tin oxide film prepared in sodium hydroxide electrolyte solution under ultrasound-assisted agitation was significantly higher than that under ultrasonic-free condition. Ultrasound-assisted agitation could decrease the interface charge transfer resistance (Rct) during the anodized process. The application of ultrasound field can improve the surface free energy of anodized tin oxide film. Especially, the superhydrophilic tin oxide film can be successfully prepared in oxalic acid solution using ultrasound modified anodic oxidation process. It can be expected that the highly uniformed nanoporous array structure in this study can be

applied in

photovoltic devices, sensors, capacitors, and lithium ion batteries as well as other interdisciplinary subjects. ACKNOWLEDGMENTS This project was supported by the Natural Science Foundation of Liaoning Province, China (Grant No. 20170540163) and “the Fundamental Research Funds for the Central Universities” China (Project No. DUT18GJ207). REFERENCES [1] D. Selloum, A. Henni, A. Karar, A. Tabchouche, O. Bacha, S. Tingry, F. Rosei, Effects of Fe concentration on properties of ZnO nanostructures and their application to photocurrent generation, Solid State Sci. 92 (2019) 76-80. [2] A. Henni, N. Harfouche, A. Karar, D. Zerrouki, F.X. Perrin, F. Rosei, Synthesis of graphene-ZnO nanocomposites by a one-step electrochemical deposition for efficient photocatalytic degradation of organic pollutant, Solid State Sci. 98 (2019) 106039. [3] A. Henni, A. Merrouche, L. Telli, S. Walter, A. Azizi, N. Fenineche, Effect of 20

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Declaration of interests ☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

There is no potential competing interests

Journal Pre-proof Highlights:

●Ultrasound field decreases interface charge transfer resistance ●Ultrasound modified anodization significantly improves SnOx film homogeneity ●Anodized SnOx films with superhydrophility can be fabricated in large-scale ●Ultrasound field increased the number of active sites and reactive micro-cells