Electrochemical synthesis of 1D ZnO

Electrochemical synthesis of 1D ZnO nanoarchitectures and their role in efficient photoelectrochemical splitting of water Avinash Rokade, Sachin Rondiya, Vidhika Sharma, Mohit Prasad, Habib Pathan & Sandesh Jadkar Journal of Solid State Electrochemistry Current Research and Development in Science and Technology ISSN 1432-8488 J Solid State Electrochem DOI 10.1007/s10008-016-3427-9

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Author's personal copy J Solid State Electrochem DOI 10.1007/s10008-016-3427-9

ORIGINAL PAPER

Electrochemical synthesis of 1D ZnO nanoarchitectures and their role in efficient photoelectrochemical splitting of water Avinash Rokade 1 & Sachin Rondiya 1 & Vidhika Sharma 2 & Mohit Prasad 2 & Habib Pathan 2 & Sandesh Jadkar 2

Received: 22 August 2016 / Revised: 22 September 2016 / Accepted: 4 October 2016 # Springer-Verlag Berlin Heidelberg 2016

Abstract Solution-based controlled morphological 1D ZnO nanorods (NRs) and nanotubes (NTs) were synthesized by a very simple and versatile electrodeposition method. The X-ray diffraction, UV-Vis spectroscopy, and scanning electron microscopy were used to characterize phase, composition quality, and optical properties of synthesized ZnO NRs and NTs. Growth mechanism, morphological evolutions, structural intactness of ZnO NRs, NTs, and their subsequent use as photoanode for efficient photoelectrochemical splitting of water are discussed in detail. ZnO NTs exhibited markedly enhanced photocurrent density of 0.67 mA/cm2 at 0.5 V vs SCE over NRs and also benefited from more negative flat band potential for hydrogen evolution. Keywords 1D ZnO . PEC cell . Electrodeposition . Nanorods . Nanotubes

Introduction To solve the global energy crisis problem, there is acute focus of research community on the development and production of clean and sustainable energy sources. Among all sustainable energy sources, solar-produced hydrogen fuel is envisioned as * Vidhika Sharma [email protected] * Sandesh Jadkar [email protected] 1

School of Energy Studies, Savitribai Phule Pune University, Pune 411 007, India

2

Department of Physics, Savitribai Phule Pune University, Pune 411 007, India

the most promising candidate, since both water and solar energy are abundantly available [1, 2]. To produce hydrogen in a sustainable and cost-effective manner, a photoelectrochemical (PEC) device offers an ingenious approach; however, it suffers from low solar-to-hydrogen conversion (STH) efficiency [3, 4]. Numerous semiconductors such as TiO2, ZnO, Fe2O3, and CdS have been investigated and modified to increase the STH conversion efficiency [5–8]. However, current STH efficiency is still low, due to limited photoresponse and high charge–carrier recombination properties [9]. Therefore, search for an efficient semiconductor for PEC water splitting is still going on. Among all semiconductors, ZnO is the most coveted semiconductor as it is a direct and large band gap material with a variety of electronic and opto-electronic applications [10, 11]. It has applications in field effect transistors [12], UV-sensitive and solar blind photodetectors [13], heat-protecting windows [14], front contact of liquid crystal displays [15], gas sensors [16], and in energy conversion [17]. With respect to solar hydrogen production, ZnO possess comparable photocatalytic properties as those of TiO2 [18, 19]. Typical electron mobility of ZnO is ∼155 cm 2 V −1 s −1 , 10–100 times higher than TiO2, which increases electron transfer efficiency with reduced recombination losses [20, 21]. ZnO has other superior inherent properties, such as high exciton binding energy (60 meV), suitable band edges, non-toxicity, low cost, and high electronic conductivity [22]. A major snag in ZnO is its wide band gap of 3.37 eV which limits its light absorption properties. Although conversion efficiency for ZnO is much lower, still its ease of crystallization and anisotropic growth makes it a viable alternative over other photocatalysts. Compared to ZnO, α-Fe2O3 has a very short excited state life time, poor minority charge– carrier mobility, and slow surface reaction kinetics.

Author's personal copy J Solid State Electrochem

Moreover, α-Fe2O3 requires an external bias to generate H2 as its conduction band edge does not straddle well with the water redox potential [23]. So, ZnO is an attractive candidate compared to hematite for PEC splitting of water. However, practical application of ZnO in solar water splitting is limited by its high band gap energy and poor stability in aqueous media [22, 24]. To increase the photoconversion efficiency of ZnO-based PEC cell, several strategies and methods have been introduced [25–32]. Among several methods deployed for realizing efficient photoanodes of ZnO, electrochemical deposition has attracted researchers as it is a very simple and powerful technique for the preparation of thin films with specific characteristics [33]. Compared with other methods such as chemical vapor deposition, plasma chemical vapor deposition, and self-assembly methods, electrochemical deposition has been proven to be highly economical and highly adaptable [34]. So, we were motivated to use electro-deposition method for our experimental study. In contrast to mesoporous ZnO thin films, onedimensional (1D) ZnO offers remarkable high charge– carrier separation efficiency and increased light trapping properties [28–32]. A detailed comparison of polycrystalline mesoporous thin films and 1D ZnO semiconductors and their potential application in photocatalytic degradation of methyl orange was reported by Ahmed et al. [29]. The morphological evolution of 1D ZnO during the formation is strongly dependent on the concentration, pH, temperature, and time [35, 36]. Moreover, the size and shape of 1D evolved nanostructure significantly affect the PEC water splitting properties [28–32]. Hsu et al. [32] reported the synthesis of 1D nanostuctures of ZnO by a simple chemical route method and compared the PEC performance with the ZnO nanosheets. The photoconversion efficiency reported for NTs is ∼0.08 %. In the present paper, we are comparing two different morphological evolved 1D nanostructure nanorods (NRs) and nanotubes (NTs) of ZnO, synthesized by a simple electrodeposition method and seeking their application in PEC splitting of water. Here, ZnO NRs were grown without a seed layer of ZnO nanoparticle. Fabrication of ZnO NTs from ZnO NRs was done by a very cost efficient, low temperature etching method. Electrodeposition method allowed us to control the size/density of the ZnO NRs and NTs on the surface by simply controlling the experimental parameters such as applied voltage, deposition time, and temperature. The photoconversion efficiency for our synthesized NRs and NTs is 0.29 and 0.50 %, respectively. To ensure the photostability and photo-degradation, chronoamperometry (CA) tests were also carried out for NRs and NTs.

Experimental Fabrication of ZnO nanorods and nanotubes Electrochemical deposition was carried out in a conventional three-electrode cell. The FTO coated glass substrate, Pt mesh, and saturated calomel electrode (SCE) were used as working, counter, and reference electrode, respectively. Prior to deposition, FTO-coated glass substrates were cleaned by a standard method (ultrasonication for 5 min in acetone, ethanol, and water, respectively). ZnO NRs were synthesized in an aqueous solution containing 5 mM zinc nitrate [Zn(NO3)2·6H2O] and 5 mM hexamethylenetetramine (CH2)6N4 at 75 °C electrolyte bath temperature. During the electrodeposition, applied potential and the duration of the deposition were −0.75 V and 4 h, respectively. After the deposition, thin films were rinsed in deionized (DI) water and then dried in air at room temperature. To obtain ZnO NTs, ZnO NRs array was immersed in an alkaline solution of 0.25 M KOH at 90 °C for 1 h. Finally, the samples were washed with DI water and dried at room temperature. The as-deposited thin films were further heated in air at 600 °C to obtain ZnO NTs from NRs. Characterization of films KLA-Tencor profilometer was employed to determine the thickness of ZnO samples. The surface morphology of thin films was observed by using JEOL JSM 6360A scanning electron microscope (SEM). The structure and crystallinity of ZnO samples were investigated by X-ray diffractometer (Bruker D8 Advance, Germany) equipped with CuKα irradiation (λ = 1.54 Å). Measurements were taken in the range of a 20–60° diffraction angle. To determine band gap, absorption band edges, electronic absorption spectra of ZnO thin films were recorded by using JASCO V-670 UV-VIS spectrophotometer in the range from 300 to 800 nm. Photoelectrochemical (PEC) study The photocurrent (J-V) measurements of ZnO NRs and NTs were carried out in a convenient three-electrode quartz cell to facilitate illumination on photoelectrode surface using AUTOLAB PGSTAT302N. ZnO working electrode was used in conjunction with platinum (Pt) counter electrode and SCE reference electrode. The PEC measurement of the photoelectrode was carried out in aqueous solution of 11 pH NaOH. The applied potential was varied from −1.0 to +1.0 V/ SCE, using 150 W Xenon lamp (PEC-L01). Illumination intensity of 100 mW/cm2 (AM 1.5) was used to record the J-V characteristics of ZnO NRs and NTs under darkness and illumination. The Mott-Schottky (MS) plot of ZnO samples (1/C2

Author's personal copy J Solid State Electrochem a1) Low Magnification

b1) Low Magnification

c1) Low Magnification

a2) High Magnification

b2) High Magnification

c2) High Magnification

Fig. 1 SEM micrographs of electrochemical deposited ZnO a nanorods, b, c nanotubes

vs electrode potential) was recorded under darkness at a frequency of 500 Hz to obtain flat band potential (Vfb) and donor density (Nd). Films were prepared in triplicate in same conditions, and five repetitive measurements were recorded. All chemicals used were of analytical grade, and double distilled water (DDW) was used throughout the experimental study.

Results and discussion The electrochemical deposition of ZnO NRs was carried out in an aqueous solution of Zn (NO3)2·6H2O and (CH2)6N4 (HMT) with Fig. 2 X-ray diffraction pattern of ZnO nanorods and nanotubes

the applied potential of −0.75 V for 4 h at 75 °C. The highly dense ZnO NRs (thickness, 2.2 μm) were successfully grown on FTO substratewithoutthepresenceofseedlayerasshowninFig.1(a1). Figure 1(a2) is its corresponding magnified view. To obtain uniform and stoichiometric ZnO films, electrodeposition conditions were optimized. The deposition was carried out by varying concentration and applied potentials from −0.1 to −1.3 V vs SCE. As the concentration increases, the average diameter, density, and inclination of the ZnO NRs increased. At large negative voltages, higher than −1.0 V, metallic Zn was deposited. Good quality films of ZnO were obtained at applied potential of −0.75 V [37]. The hexagonalstructure is formedevenly onthe entire surface with the average diameter of ∼400 nm. The reaction mechanism of 1D


ZnO is basically based on the formation of hydroxyl ion (OH−) by electrochemical reduction on to the surface of working electrode. The reaction mechanism can be written as follows: ðCH 2 Þ6 N 4 þ 6 H 2 O→6 HCHO þ 4 N H 3 þ

N H 3 þ H 2 O→N H 4 þ OH

−

Structural and optical parameter of ZnO NRs and NTs

Samples

Average diameter (nm)

Density/cm2

Band gap energy (eV)

ð1Þ

ZnO NRs

400

1.7 × 108

ð2Þ

ZnO NTs

250

2.4 × 108

3.22 3.20

In this reaction, HMT provided hydroxyl ion (OH−) to the reaction solution and precipitate with Zn+2 to form Zn(OH)2, becoming the precursor to ZnO at 75 °C via reactions: Zn2þ þ 2 OH − →ZnðOH Þ2

ð3Þ

ZnðOH Þ2 →ZnO þ H 2 O

ð4Þ

The hexagonal Wurtzite ZnO has polar (001) and non-polar crystal planes parallel to c-axis. Non-polar crystal planes have higher chemical stability and low surface energy compared to

Fig. 3 a UV-Vis absorption spectra and b band gap calculation (Tauc plot) diagram for ZnO nanorods and nanotubes

Table 1

polar crystal planes. To obtain ZnO NTs, (001) plane was removed by suspending ZnO NRs in the selective solution, as dissolution rate of polar crystal planes is faster than lateral non-polar planes [38]. However, dissolution rate is highly sensitive to the time and chemical bath temperature, as time increases not only the center part but also the walls have been dissolved as shown in Fig. 1(b1) and Fig. 1(c1). Figure 1(b2) and Fig. 1(c2) are their magnified view, respectively. The average diameter and wall thickness of NTs are ∼243 and ∼70 nm, respectively.


Figure 2 shows the X-ray diffraction pattern for ZnO NRs and NTs. The high intensity diffraction peak at 2θ∼31.8, 34.4, 36.2, and 47.5° confirmed the formation of highly crystalline ZnO with complete c-axis texturing [JCPDS card # 36–1451]. These results are consistent with SEM analysis. The existence of any other additional or characteristic peaks from Zn complexes is not seen, which confirms the purity of ZnO. Lattice constants of synthesized ZnO are a = 3.24 Å and c = 5.21 Å, calculated from the peak positions using the formula of hexagonal system. The obtained values are close to the earlier reported values [39]. Figure 3(a) depicts the UV-Visible optical absorption spectra of ZnO NRs and NTs in the range of 300 to 750 nm. Absorption spectra were analyzed by plotting (αhυ)2 vs hυ, based on relation: n ð5Þ αhν ¼ C hν−E g 2

where hυ is the photon energy, C a constant for direct transition, α is the optical absorption coefficient, and n depends on the nature of transition. The intercept on photon energy (hυ) axis corresponds to the optical band gap (Eg) [40]. The ZnO NRs and NTs exhibited efficient absorption in UV region. It was also observed that absorption in ZnO NTs has shifted slightly toward higher wavelength region as shown in Fig. 3(b). The band gap energy calculated from Tauc plot is tabulated in Table 1. Figure 4 describes the Mott-Schottky (MS) curves of ZnO NRs and NTs in order to understand the intrinsic properties of photoelectrode such as flat band potential (Vfb), charge–carrier density (Nd), and depletion layer width (nm). These were analyzed by measuring capacitance (C) of semiconductor at semiconductor/

Fig. 4 Mott-Schottky plots of ZnO nanorods and nanotubes

Table 2 Flat band potential (Vfb), charge–carrier density (Nd), depletion layer width (nm) values of ZnO NRs and NTs Samples

Flat band potential (Vfb) (V vs SCE )

Charge–carrier density (Nd) cm−3

Depletion layer width (nm)

ZnO NRs

−0.63

2.14 × 1019

ZnO NTs

−0.77

4.32 × 1019

9.18 6.73

electrolyte junction at different potential (V) using the following equations: 1 2 ðk B T Þ ð6Þ ¼ V−V fb − q C 2 q ε0 εs N D 2 q ε0 εs N D 1 =2 2εs ε0 V−V fb w¼ qN D

S¼

ð7Þ ð8Þ

where ε0 is the permittivity of free space, εs is the dielectric constant of semiconductor electrode, q is the electronic charge, T is the temperature (in Kelvin), k B is the Boltzman’s constant, w is the depletion layer width, and S is the slope of MS curve [41]. The extrapolation of the linear portion of 1/C2 at x-axis is considered as the values of Vfb and is given in Table 2. The ZnO NRs and NTs exhibited positive slopes, indicating that films are of ntype. The flat band potential values were shifted from −0.63 V/SCE for ZnO NRs to −0.77 V/SCE for ZnO NTs. However, in order to get a clear idea of band bending and charge transfer properties of ZnO NRs and NTs, a schematic band diagram is presented in Fig. 5. More negative values of Vfb indicate reduced recombination rate and better separation and transportation of charge–carrier

Author's personal copy J Solid State Electrochem Table 3 Fitting parameter of Nyquist plots calculated from equivalent circuit model

Fig. 5 Schematic band diagram of ZnO nanorods and nanotubes

at semiconductor/electrolyte junction. The donor density (Nd) has been calculated from the slope of MS plots. It is observed that Nd increased from 2.14 × 1019 cm−3 for ZnO NRs to 4.32 × 1019 cm−3 for ZnO NTs, due to improved conductivity. Therefore, the depletion layer width in ZnO NTs is less than ZnO NRs as shown in Table 2. A thin depletion layer enables the diffusion of photogenerated

Fig. 6 PEIS spectra of ZnO nanorods and nanotubes. Inset contains the equivalent circuit diagram

Samples

Rs (Ω)

Rp (Ω)

Cp (F)

ZnO NRs ZnO NTs

68 43

1232 1052

25 × 10−6 29 × 10−6

charge–carriers, which is a favorable characteristic for PEC activity. This phenomenon can also be attributed to the presence of more defect states (i.e., oxygen vacancies) in ZnO NTs, which increased the number of local donor energy levels and enhance the conductivity (i.e., charge– carrier concentration) in comparison with ZnO NRs. Due to the large surface area, one can expect that a large concentration of defect (oxygen vacancies or interstitial Zn) is contained in the wall of tube oxide [38]. Photoelectrochemical impedance spectroscopy (PEIS) was also carried out to analyze charge transfer process at semiconductor/electrolyte junction under illumination. The PEIS measurements were recorded in 11 pH NaOH electrolyte at 0.5 V, and the frequency was kept in the range of 0.1 to 100 KHz. The PEIS is extremely sensitive to light illumination; the radius of arc in PEIS spectra describes the junction resistance between semiconductor surface and electrolyte [42]. PEIS data obtained from Nyquist plots is simulated and fitted using equivalent electrical circuit. The electrical components used in the equivalent circuit model consist of solution resistance (Rs), charge transfer resistance (Rp), and the capacitance (Cp), which are shown in inset of Fig. 6. The values of

Author's personal copy J Solid State Electrochem Fig. 7 Linear scan voltammogram (LSV) of ZnO nanorods and nanotubes in dark and illumination

the components are estimated and summarized in Table 3. The smaller arc indicates higher charge–carrier efficiency (under light illumination). The diameter of the arc in NTs is relatively smaller than NRs as shown in Fig. 6. This suggests that ZnO NTs had reduced recombination rate and an accelerated charge transfer mechanism which is responsible for the enhanced photocurrent activity. Figure 7 shows photocurrent-voltage (J-V) behavior of ZnO NRs and NTs measured using linear sweep voltammetry (LSV). To measuretherate ofhydrogen productionbywatersplitting,J-V behavior is regarded as an indirect way, if the faradic efficiency of water oxidation is assumed unity. The experiments were carried out in a three-electrode cell under dark and illumination, employing a visible light source of 100 mW/cm2 (AM 1.5) in 11 pH NaOH. Both ZnO electrodes generated anodic

Fig. 8 Photoconversion efficiency plot of ZnO nanorods and nanotubes as a function of applied potential (vs SCE)

photocurrent, due to n-type characteristic, which has also been confirmed by MS and PEIS analysis. A very small current of ∼0.1 μA/cm2 was observed during measurement of photocurrent in dark because of non-faradic reaction, while under illumination, photocurrent increases and reaches 0.39 mA/cm2 at a bias of 0.5 V/SCE (in ZnO NRs). Moreover, no saturation was observed at positive bias, indicating better charge separation upon illumination. In comparison to ZnO NRs, the ZnO NTs showed significantly enhanced photocurrent of 0.67 mA/cm2 at 0.5 V/SCE. These results can be attributed to the high surface area of ZnO NTs present in vicinity of electrolyte which in turn helps in harvesting large number of photons. This leads to vectorial increase in the number of charge–carrier and photocurrent density. The results are supported by Mott-Schottky and PEIS analysis. In NTs, large surface area and smaller wall thickness enable the

Author's personal copy J Solid State Electrochem Table 4 Short circuit current (Jsc), open circuit voltage (Voc), and ABPE (%) values of ZnO NRs and NTs Samples

Short circuit current (Jsc) (mA/cm2)

Open circuit voltage (Voc) (V)

ABPE (%)

ZnO NRs

0.10

-0.33

0.29

ZnO NTs

0.27

-0.42

0.50

transportation of diffused photogenerated holes in electrolyte. Specific surface area of Z-NTs is larger than Z-NRs, which makes them attractive for PEC applications. Large surface area in Z-NTs is responsible for enhanced light absorption and charge–carrier concentration which results in higher photocurrent density. The densely packed Z-NT walls can absorb enough photons to improve thelight harvest efficiency.Furthermore,thephoto-evolved electrons can transport directly through the oriented tube walls to the conducting substrates. This greatly reduces the recombination losses of the photogenerated charge–carriers due to fewer grain boundaries in charge transportation process. The applied bias photon to current efficiency (ABPE) was calculated for ZnO NRs and NTs with visible light source of irradiance of 100 mW/ cm2 in the same electrolyte (11 pH NaOH) using the following equation: ABPE ¼

J Ph ð1:23−jV Bias jÞ PLight

ð9Þ

where JPh is the photocurrent density in mA/cm2, VBias is the applied external potential, and PLight is the illumination intensity of light source [43]. A maximum photon conversion efficiency of 0.50 % is obtained at 0.5 V/SCE for ZnO NTs as shown in Fig. 8. Compared to previous published report, we have obtained higher Fig. 9 Variation in photocurrent vs time in chronoamperometry stability test of ZnO photoanodes

conversion efficiency in the present experimentation [32]. The values of open circuit voltage, short-circuit current, and applied bias photon to current conversion efficiency (ABPE) were measured and are shown in Table 4. To study the stability of ZnO photoelectrode for long-term operation, 1-h chronoamperometry (CA) test was carried out in 11 pH NaOH at 0.5 V/SCE under chopped illumination of 100 mW/cm2. The minimal loss of photocurrent was observed in NTs or NRs, which indicates that photoelectrode is quite stable even after 1800-s operation (see Fig. 9).

Conclusion In summary, we have successfully fabricated 1D nanostructures (NRs and NTs) of ZnO on a FTO substrate through a facile electrodeposition method. The influence of two different morphologies of ZnO on PEC measurements for hydrogen production was studied and discussed in detail. ZnO NTs exhibit remarkably enhanced PEC response with enhanced photocurrent density, excellent stability, and much superior characteristics then NRs. These characteristics can be attributed to increased surface area. Our work provides a very simple, economical, and scalable method for designing ZnO NRs and NTs with high PEC response. There are still lots of challenges that need to be probed for both ZnO NRs and NTs. Since the surface morphology, surface area to volume ratio and optoelectronic properties can affect the performance of realized device. Therefore, continuous research efforts and new developments would open new prospects for different types of applications based on ZnO NRs and NTs such as chemical and biological sensors, novel optoelectronic, and photonic devices.

Author's personal copy J Solid State Electrochem Acknowledgments One of the authors, Dr. Sandesh Jadkar, is thankful to University Grants Commission, New Delhi, for special financial support under UPE program. Mr. Avinash Rokade is thankful to Ministry of New and Renewable Energy (MNRE), Government of India for National Renewable Energy (NRE) fellowship. Mr. Sachin Rondiya gratefully acknowledges the financial support from Dr. Babasaheb Ambedkar Research and Training Institute (BARTI), Pune, for the award of Junior Research Fellowship. Dr. Vidhika Sharma and Dr. Mohit Prasad are thankful to University Grants Commission, Government of India, New Delhi, for Dr. D. S. Kothari Postdoc Fellowship and financial assistance.

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