Improvement in the Piezoelectric Performance of a ZnO

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Improvement in Piezoelectric Performance of ZnO Nanogenerator by a Combination of Chemical Doping and Interfacial Modification Caihong Liu, Aifang Yu, Mingzeng Peng, Ming Song, Wei Liu, Yang Zhang, and Junyi Zhai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00069 • Publication Date (Web): 16 Mar 2016 Downloaded from http://pubs.acs.org on March 22, 2016

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The Journal of Physical Chemistry

Improvement in Piezoelectric Performance of ZnO Nanogenerator by a Combination of Chemical Doping and Interfacial Modification Caihong Liu, Aifang Yu, Mingzeng Peng, Ming Song, Wei Liu, Yang Zhang* and Junyi Zhai*

Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences; National Center for Nanoscience and Technology (NCNST), Beijing, 100083, China

KEYWORDS: piezoelectric nanogenerator, chemical doping, interfacial modification, electron screening effect, piezocharge

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ABSTRACT: In this work, we investigate the improvement in piezoelectric performance of ZnO nanowires film by chemical doping and interfacial modification strategy. The Cl-doped ZnO nanowires films were synthesized by a modified hydrothermal approach and characterized by SEM, EDX, XRD, PL, XPS, TEM and I-V characterization. The effect of Cl-dopant with different doping concentration has been demonstrated by an improvement of the piezoelectric output performance due to the induced lattice strain along the ZnO c-axis, which significantly facilitates the piezocharges separation under the applied stress. The experimental results indicate the existence of lattice strain along doped ZnO nanowire polar axis resulted from the different ionic size between Cl and O. More importantly, by preparing Cl-doped ZnO nanowire film on the p-type CuO film, our experimental results reveal that the piezoelectric output voltage and current of Cl-doped ZnO nanowire film can be further enhanced due to CuO can reduce electron screening effect. We demonstrate that the utilization of chemical doping and interfacial modification can be considered as a compatible strategy for realizing high performance energy harvesting device.


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1. INTRODUCTION The pursuit of harvesting renewably generated electricity and then powering electronic devices may be a strategy to decline the over reliance of fossil fuel. Recently, the power generating devices based on piezoelectric nanogenerators (PENGs) and triboelectric nanogenerators (TENGs) have been extensively investigated in order to develop self-powered devices.1-7 With the recent developments of wearable technology, human-electronic interfacing and biomedical diagnostics, flexible and high performance PENG is urgent needed.8-11 Although Pb(ZrxTi1-x)O3 (PZT) is widely used in harvesting mechanical energy, which is hard, brittle and bio-incompatible and not fit the requirements of wearable/implantable applications.12-14 ZnO nanomaterial is one of idea candidates for the construction of flexible PENGs since firstly reported in 2006. 1D or 2D ZnO nanostructures have been synthesized by various techniques including thermal evaporation, chemical vapor deposition, pulsed laser deposition, radio-frequency magnetron sputter deposition and electrochemical deposition. Compared to vapor phase techniques, solution-based methods are more suitable for mass production at low cost and practical application. In order to improve the overall utilization efficiency of piezoelectric charge (piezocharges), it is indispensable to synthesize and investigate the p- and n-type doped ZnO and broaden the application of PENGs and piezotronic/piezo-phototronic device.15-18 Due to the poor stability of p-type doped ZnO materials, until recent report on the Sb-doped ZnO nanowire exhibited good stability and its application in 3 ACS Paragon Plus Environment


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mechanical energy harvesting and piezotronic transistor.6, 9, 19 The synthesis of n-type of ZnO materials could be facilely achieved by a modified hydrothermal method. For instance, the synthesis of Cl-doped ZnO nanowire array was reported by Wang et al, which demonstrated excellent photocurrent density and overall photoelectrochemical efficiency.20 However, the piezoelectric performance of ZnO-based nanogenerator and piezotronic device has been limited due to the intrinsic characteristic of n-type ZnO materials.21-23 One of the bottlenecks is electron screening effect which screens the piezoelectrically generated charges and considerably lowers the output voltage and current of ZnO-based PENG. Previous studies indicated the deposition of p-type polymer or semiconductors can reduce the excess electrons in ZnO.24-26 Recent work shed light on the enhancement of piezoelectric performance by the chemical doping of halogen elements which modulated the lattice strain along ZnO polar c-axis.27 However, it is difficult to evaluate the influence of dopants caused electron screening effect to the output performance of ZnO-based PENGs due to the simultaneous variation of the dopant size and doping concentration. In particular, it is of fundamental importance to investigate the optimal balance between piezocharges separation and charge screening effect, which both inevitably resulted from n-type dopants. The comprehensive study will further broaden the strategy of boosting the output performance of piezoelectric semiconductor for harvesting mechanical energy and piezotronic/piezo-phototronic devices. Herein, we report the synthesis, characterization of Cl-doped ZnO nanowire (NW) film by a modified hydrothermal method and highlighted the enhancement of 4 ACS Paragon Plus Environment

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the piezoelectric performance of Cl-doped ZnO NW film as flexible nanogenerator. We characterized the Cl-doped ZnO NW film by Scanning Electron Microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), X-ray Photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and Photoluminescence (PL) spectroscopy. The possible in-situ doping and growth mechanism is proposed as well. The output performance of as-fabricated Cl-doped ZnO NW film PENG was considerably superior to the undoped ZnO PENG in output voltage and current. The deposition of a CuO film under ZnO NW film suggests that the piezocharges separation triggered by dopant induced lattice strain without undesirable electron screening effect. This study provides a general strategy to modulate piezocharges separation in ZnO NW and optimize the output performance of flexible PENGs by an effective chemical-doping strategy.

2. EXPERTIMENTAL A polyethylene terephthalate (PET) substrate (250 µm thick) was cleaned with acetone, ethyl alcohol and deionized water, respectively. ZnO seed layer was deposited by a standard RF magnetron sputter deposition using a high purity ZnO target (99.99%) and Ar/O2 (flow rate was fixed at 8:1) as the sputtering gases. During the RF deposition process, the RF power was set at 100 W and deposition time was 30 minutes. The synthesis of Cl-doped ZnO NW film was carried out by a modified hydrothermal method using NaCl as chlorine medium. The nutrient solution consisted zinc nitrate and hexamethylenetetramine with equal concentrations (100 mM). After 5 ACS Paragon Plus Environment


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7-hour-reaction in an oven (at 95℃), the as-prepared ZnO NW films were carefully rinsed by deionized water to remove the chemical residuals on the top surface of the sample. Prior to the assembly of flexible PENGs, the undoped and Cl-doped ZnO NW films were placed in a 40 ℃ oven overnight. The fabrication of PENGs were fulfilled by sputtering 100 nm Cu (99.99% purity) on the top of ZnO NW film and on the bottom of PET. The photograph of ZnO-based PENG was presented in Figure S1 (Supporting Information). The effective dimension of the device used in this work is 1×1 cm2. In order to protect the top/bottom electrode and PENG during measurement, the PDMS layers were deposited on the top and bottom side of the PENG. The output voltage and current of the PENGs were characterized by LeCroy 610Zi oscilloscope, Stanford Research System SR 570 low noise current amplifier. The room temperature electrical property was measured by a semiconductor characterization system (Keithley 4200-SCS).

3. RESULTS AND DISCUSSION

The synthesis of undoped and Cl-doped ZnO NW films is illustrated in Figure 1(a). In order to synthesize Cl-doped ZnO NW film, nutrient solution with different NaCl concentrations (0, 4, 8 and 10 mM) was used in hydrothermal processing.


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Figure 1. (a) Schematic presentation of the modified hydrothermal synthesis of Cl-doped ZnO NW film, (b) SEM image of Cl-doped ZnO NW film, inset shows the cross-sectional view of ZnO NW film, (c) EDX spectrum of Cl-doped ZnO NW film.

Figure 1(b) and S2 show the surface morphology of Cl-doped and undoped ZnO NW film, respectively. The surface features of Cl-doped ZnO NW film are quite similar to the undoped ZnO NW film. The presentation of NaCl (10 mM) in nutrient solution does not influence the growth of ZnO NW film. The vertical aligned and close-packed nanowires can be observed from the cross section view, see inset of Figure 1(b). The cross-sectional view of ZnO NW film demonstrated that Cl-doped ZnO NW films synthesized in Cl medium with preferential perpendicular to the substrate plane. It suggests that the involvement of limited amount of Cl atoms in ZnO does not influence the growth ZnO NW film (will be discussed below). The presentation of Cl atoms in ZnO NW film was confirmed by EDX spectrum, as shown in Figure 1(c). The Zn (0.99, 8.6 and 9.5 KeV) and O (0.53 KeV) signals were



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observed in undoped ZnO NW film while and the additional Cl (2.63 KeV) signal was detected in Cl-doped ZnO NW film.

Figure 2. (a) XPS spectrum of Cl-doped ZnO NW film, (b) XRD spectrum of Cl-doped ZnO NW film, inset shows the enlarged view of ZnO (002) peak, (c) HRTEM image and (d) EDX spectrum of an individual Cl-doped ZnO nanowire which peeled from the textured film.

The incorporation of Cl atoms in ZnO NW film was examined by XPS measurement, see Figure 2(a). This data supported that Cl is doped in the ZnO NW film. The crystal structures of undoped and Cl-doped ZnO NW film were characterized by XRD measurement. As shown in Figure 2(b), the corresponding ZnO (002) peaks were clearly observed in all Cl-doped ZnO samples and found to gradually downshift from undoped ZnO sample (no NaCl in nutrient solution) to Cl-doped ZnO sample (containing 4, 8 and 10 mM NaCl in nutrient solution). The slight shift of ZnO (002) peak toward lower diffraction angles provided evidence that 8 ACS Paragon Plus Environment

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the incorporation of Cl atoms and the change of the lattice parameter which compared to the undoped ZnO.28 The lattice spacing of undoped ZnO and Cl-doped ZnO (with 10 mM NaCl in nutrient solution) calculated from the diffraction patterns are 5.206 Å and 5.221 Å, respectively. Additionally, one individual Cl-doped ZnO nanowire was peeled from the textured film and then characterized by high-resolution TEM. The lattice fringe of about 0.522 nm in Cl-doped ZnO nanowire is corresponding to the (0002) lattice spacing of wurtzite-structured ZnO, which proves the preferential growth direction of Cl-doped ZnO nanowires is [0001] direction. We proposed that the lattice expansion originated from Cl atoms in ZnO crystal due to the different ions radius between Cl- (1.81 Å) and O2- (1.40 Å) and lattice expansion induced by the incorporation of Cl atoms in ZnO NW film. According to the EDX characterization (see Figure 2(d)), Cl signal was observed and Cu signals were coming from sample holder. PL spectroscopy was employed in investigating the optical property of Cl-doped ZnO NW film. Figure S3 (Supporting Information) shows the PL spectra of undoped and Cl-doped ZnO NW film with different doping concentration, which excited by a He-Cd laser at 325 nm. The small amount of Cl species was doped into the ZnO NW film and then caused the ZnO lattice expansion and distortion.29,

30

Comparable

characteristic in UV emission was obtained in undoped and Cl-doped ZnO samples, while the considerable difference was shown in their visible part. Previous studies revealed that the PL study of ZnO oxygen vacancy emission wavelength at ~530 nm and the zinc vacancy emission wavelength emission at ~630 nm. The enhancement of 9 ACS Paragon Plus Environment


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visible part indirectly indicated the defect concentration of Cl-doped ZnO NW can be tuned by increasing the amount of Cl species in the nutrient solution during the hydrothermal synthesis process.

Figure 3. I-V curves of undoped and Cl-doped ZnO NW film with (a) Ohmic contact electrodes and (b) Schottky contact electrodes, the Cl-doped ZnO NW films were synthesized with different concentration of NaCl in nutrient solution. The electron transport property of the Cl-doped ZnO NW films was investigated. In order to form Ohmic contact with Cl-doped ZnO NW film, a pair of Al (100 nm) electrode was deposited. Previous study reported that heavily doped ZnO nanowire with Cl exhibited a metallic conductivity. However, Cl-doped ZnO NW film synthesized in our condition remained the n-type semiconductor characteristic, see Figure 3. The conductivity of Cl-doped ZnO can be tuned by changing the concentration of NaCl in nutrient solution. Similar results were observed in the case of Schottky contact between electrodes and ZnO film by replacing Al with Au as the electrodes in the device, see Figure 3(b). The current-voltage characteristic of Cl-doped ZnO film indicates that small amount of the incorporated Cl in the ZnO textured film as well.


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The ZnO NW array or film prefers to grow along the c-axis [0001] direction in the process of hydrothermal synthesis. For the preparation of Cl-doped ZnO NW film, Zn(OH)2+xx- (usually in the form of Zn(OH)42-) and Cl- formed Zn(OH)xCly(x+y-2)compound, and then diffused to the ZnO (002) surface. Then, Zn(OH)xCly(x+y-2)species might preferably absorb on the (002) surface due to its lower surface free energy than other facets (e.g. (100) and (101)) as well as with Zn(OH)2+xx-. Then, the dehydration of OH- resulted in the formation of Cl-doped ZnO NW. In fact, the amount of Zn(OH)xCly(x+y-2)- depended on the concentration of NaCl in the nutrient solution. The incorporation of Cl atom led to the rearrangement of ZnO crystal structure during the hydrothermal growth. For doping Cl in ZnO NW film, the influence of NaCl is much weaker than AlCl3 by solution-based approach. In our experimental

condition,

the

high

concentration

of

zinc

nitrate

and

hexamethylenetetramine (100 mM) provided enough amount of Zn(OH)2+xx- and Zn(OH)xCly(x+y-2)- species to form Cl-doped ZnO NW textured film structure. Employing NaCl as dopant precursor resulted in lower doping concentration (~0.2%) in ZnO NW film compared to the case of AlCl3 (~3.8%).20 Based on XRD and PL characterization, the incorporation of Cl species leads to slight lattice expansion and distortion in ZnO. When the NaCl concentration was higher than 12 mM, the morphology of ZnO nanostructure changed from 1D nanowire to 2D nanoplate which indicated that the nucleation process was considerably affected by the excess amount of Cl anions.31



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Fgiure 4. (a) Schematic diagram of the structure of Cl-doped ZnO NW film PENG, output voltage (b) and current (c) of undoped and Cl-doped ZnO NW film PENG, output voltage (d) and current (e) signals generated from two Cl-doped ZnO NW film PENG devices connected in serial and in parallel, respectively.

The output performance of piezoelectric nanogenerator strongly relies on the sandwiched materials between two electrodes. As the ZnO NW films are subjected to an applied stress along the c-axis, the positive and negative charge centers in ZnO become displaced with respect to each other. Therefore, the electric dipole in each ZnO nanowire is lined up in the same direction, thus resulting in a macroscopic dipole with positive and negative charges accumulated on Zn and O terminated facets, respectively. Previous works described that the electronic properties of semiconductor nanomaterials can be tuned via strain-engineering and strain-driven charge separation in semiconductor materials at the nanoscale, such as bent ZnO nanowires, buckled CdS nanowires and pyramidal clusters in carbon cages.32-38 Under the external force, the utilization of lattice strain for strengthening piezocharges separation can be 12 ACS Paragon Plus Environment

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considered as an alternative strategy in realizing high performance energy harvesting device. In fact, the experimentally observed piezoelectric potential of the n-type ZnO results from the competition of between piezocharges and free electrons in ZnO. Therefore, the dopant induced charges screening effect should be avoided for improving the efficiency of ZnO-based PENGs. The structure and performance of Cl-doped ZnO NW film PENG is schematically illustrated in Figure 4. The PET layer plays an important role in the design of PENG, which serving as flexible platform. Moreover, its insulating characteristic avoids the electron leakage through internal ZnO NW film. When applying a stress (1 MPa) to the surface of ZnO-based PENGs, the

positive

and

negative

piezocharges

are

generated

accordingly.

The

piezoelectrically generated charges flow through the external circuit from top electrode to bottom electrode. Once the external force is released, the piezoelectric potential dropped down dramatically due to the accumulated electrons on the bottom electrode flow back to the top electrode though the external circuit. The output performances of undoped and Cl-doped ZnO PENGs are shown in Figure 4(b) and (c). As the concentration of NaCl in nutrient solution increased to 10 mM, the optimized piezoelectric performance can be achieved in the aspects of Vpp (peak-to-peak voltage up to 4.9 V) and Ipp (peak-to-peak current up to 1030 nA/cm2), respectively. The measurement of the output of ZnO NW film PENG with Cl dopant was conducted under repeatedly applying a stress more than 30 cycles, see Figure S4 (Supporting Information). By constructively connecting the two devices either in series or in parallel, consistent enhancements in output voltage and current are 13 ACS Paragon Plus Environment


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observed, see Figure 4(d) and (e). The output of undoped and Cl-doped ZnO nanogenerators increases with increasing applied force, see Figures S5 and 6 in Supporting Information.

Figure 5. (a) The output performance of ZnO NW film PENGs dependence of Clcontent, (b) output voltage and (c) current of ZnO NW film PENGs with additional CuO film dependence of ZnO lattice strain. 14 ACS Paragon Plus Environment

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We found that the doped Cl atoms have a positive contribution to the enhancement of the output performance of ZnO-based PENGs. The output performances of fabricated Cl-doped ZnO PENGs (preparation with 10 mM NaCl in nutrient solution) are higher than the undoped ZnO PENG, see Figure 5(a). Under the applied stress, the incorporation of Cl atoms results in lattice strain along the ZnO polar c-axis which facilitates the piezocharges separation. The lattice strain in Cl-doped ZnO NW film was calculated according to previous study.39 It should be considered that the n-type characteristic of undoped and Cl-doped ZnO materials. It is known that the as-synthesized ZnO is naturally n-type semiconductor and the excess amount of free electrons can potentially screen the piezoelectrically generated charges and reduce the output performance of ZnO-based PENGs. In order to investigate the whether Cl atoms as dopant causes significant electron screening effect, we deposited a CuO layer (300 nm) under the undoped and Cl-doped ZnO NW films. With varying lattice strain in Cl-doped ZnO NW films, the piezoelectric output voltages and currents of ZnO-based PENGs were compared in Figure 5 (b) and (c). The comparable enhancement can be realized in all of Cl-doped ZnO NW films regardless of the different doping concentration. Because the low doping concentration in Cl-doped ZnO NW films, the p-type CuO can suppress the charges screening effect which originated from the n-type characteristic of ZnO. More importantly, the improvement in the device performance of ZnO-based PENGs can be attributed to a combination of chemical doping induced lattice strain and interfacial modification by 15 ACS Paragon Plus Environment


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p-type materials. After the employment of CuO layer, the enhanced output is ~2.2 V (peak-to-peak voltage) and ~1000 nA/cm2 (peak-to-peak current) for all of the fabricated Cl-doped ZnO NW film PENGs, see Figures 5b and c. It indicates that the contribution of the small amount of Cl dopant to the enhancement of PENGs mainly resulted from the lattice strain in Cl-doped ZnO NW film without causing undesirable electron screening effect. The band diagram, electron path and equivalent circuit of nanogenerators is represented in Figure S7 (Page 4 in Supporting Information). This fundamental understanding is useful in the design of ZnO-based PENG with higher energy harvesting efficiency. We summarize some of doped ZnO NW for

nanogenerator application, see Table S1 in Supporting Information.5, 17, 22, 40-43 However, the relationship between lattice strain and piezoelectric properties for different dopants is complicated, which is stated below according to previous works.9, 17, 22, 44 Firstly, the length of the ZnO NW will influence the output voltage, while the diameters of ZnO NW will influence the output current. Secondly, the dopants for ZnO can be classified into two group, n- and p- type. On the one hand, for n-type doping, the dopant can induce lattice strain along ZnO polar axis which may potentially enhance the piezoelectric performance. If the dopant with large ionic size and/or high doping concentration, the dopant may result in more lattice defects which increases concentration of free electrons in ZnO and decreases the piezoelectric output. On the other hand, for p-type doping, the dopant can reduce the screening effect inside ZnO NWs by free

electrons

to

piezoelectric

polarization.

Therefore,

the

enhanced 16

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piezoelectric performance is related to the type of dopant as well. Thirdly, different type of substrate and mechanical energy sources and illumination conditions may influence the piezoelectric output of ZnO-based nanogenerator. In this work, we highlight a combined approach for optimizing the performance of ZnO-based piezoelectric nanogenerator by chemical doping and interfacial modification strategy, which will be helpful for boosting the performance of flexible nanogenerator and piezotronic nanodevice, and offering valuable knowledge for other piezoelectric semiconducting nanomaterials.

4. CONCLUSIONS

In conclusion, we synthesized Cl-doped ZnO nanowire films and their piezoelectric outputs were enhanced by increasing doping concentration. The XRD and TEM characterization revealed that the involvement of Cl-dopant in ZnO can induce lattice strain along polar c-axis which facilitated the piezocharges separation under an external stress. Additionally, a p-type CuO layer was introduced to further improve the output performance of undoped and Cl-doped ZnO PENGs, which is attributed to its ability to reduce the electron screening effect. Our experimental results indicate that the chemical doping and interfacial modification have simultaneously positive impact on the piezoelectric performance of ZnO-based PENGs. In order to achieve maximum utilization of piezocharges under externally applied stress depends on both piezocharges separation strengthened by lattice strain and electron screening effect



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which was largely reduced by p-type material. Our results will be useful not only for ZnO but also for other piezoelectric semiconductors as well.

ASSOCIATED CONTENT

Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. PL spectra, photograph of flexible PENG, SEM images of undoped ZnO NW film. AUTHOR INFORMATION Corresponding Authors *Email: [email protected].

*Email: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by NSFC 51472056, the "thousands talents" program for pioneer researcher and his innovation team, China and the Recruitment Program of Global Youth Experts, China.

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Table of Content.


Improvement in the Piezoelectric Performance of a ZnO

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