Polyaniline Microrods

Article pubs.acs.org/Langmuir

p−n Heterojunction on Ordered ZnO Nanowires/Polyaniline Microrods Double Array Qunwei Tang,†,‡ Lin Lin,§ Xuan Zhao,† Kevin Huang,*,† and Jihuai Wu*,‡ †

Department of Mechanical Engineering, University of South Carolina, Columbia, South Carolina 29201, United States Institute of Materials Physical Chemistry, Huaqiao University, Quanzhou 362021, China § Department of Chemistry, Tongji University, Shanghai 200092, China ‡

ABSTRACT: Recently, there has been growing interest in the design of novel nano- and/or microscaled heterojunctions consisting of two distinctive ordered semiconductor arrays for highly efficient p−n diodes used in optical, optielectronic, and microelectronic devices. Here, we report the attainment of an ordered double array comprising of p-type polyaniline microrods and n-type ZnO nanowires by a controlled electrochemical deposition method. Extensive chemical and physical characterizations have been performed on the fabricated p−n heterojunction. The double-array p−n heterojunction exhibits good rectifying characteristics, the rectification ratio of which exhibits a minimum at an illumination density of 93 mW cm−2, making it suitable for high-sensitivity photodetectors. This research is expected to open up a new avenue for the development of highly efficient and sensitive p−n heterojunction diodes and possibly serve as the building blocks for future nanoelectronics.

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INTRODUCTION The microscopic-scale p−n heterojunctions are scientifically intriguing and technologically important to the development of high-efficiency and high-sensitivity diodes for applications in microelectronic and energy conversion devices.1,2 A great deal of research efforts have been devoted in recent years to this area, the majority of which are focused on n-type ZnO/p-type conducting polymers hybrids.3−6 One of the reported approaches to fabricating such a p−n structure is by filling the p-type conducting polymers into the gaps of oriented n-type ZnO nanostructures, forming a kind of hybrid films.3,7 The challenge is, however, that a large portion of the photogenerated electrons in the n-type semiconductors can be easily annihilated by the simultaneously photogenerated holes in the p-type semiconductors due to the short and random diffusion of electronic defects in the film, thus significantly decreasing the device efficiency.7,8 One solution to this impasse is to increase the diffusion length by creating independent pathways for electronic defects. To do so, two sets of oriented nano/ nanostructures or nano/micromatrices, with one layer being p-type and another being n-type conductor, should be aligned and well-bonded into one-dimensional array. In such an idealized p−n hereojunction structure, the probability of electrons being annihilated by holes or vice versa can be zero or at least substantially minimized. To date, very little work in this area has been reported because of the difficulties in growing microscale, not to mention nanoscale, ordered conducting polymer arrays on oriented ZnO nanostructures. © 2012 American Chemical Society

In this study, we report the attainment of a distinctive nano/ microscale bilayer structure consisting of ordered n-type ZnO nanowires and ordered p-type polyaniline (PANi) microrods by an electrochemical deposition method. This nano/microscale heterojunction diode structure is the first step to achieve the ultimate ideal nano/nanostructure. The multilayer structure of diode is schematically shown in Figure 1, where a glass is served as the substrate onto which an ITO bottom electrode, ordered ZnO nanowires, ordered PANi microrods and Au top electrode

Figure 1. Schematic of p−n heterojunctions formed on ordered ZnO nanowires/PANi microrods double array. Received: November 16, 2011 Revised: February 3, 2012 Published: February 4, 2012 3972

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coater (MSP-1S). The I−V characteristics were measured under a concomitant linear sweeping of voltage from −0.8 to +0.8 V at a rate of 50 mV s−1 and simulated solar light irradiation at an intensity ranging from 0 to 150 mW cm−2 generated from a 100 W Xe lamp (XQ-500W, Shanghai Photoelectricity Device Co., Shanghai, China). Other Characterizations. The morphologies of the fabricated double arrays were primarily observed and recorded by an S-4700 Hitachi cold field emission scanning electron microscopy (FESEM) combined with energy dispersive X-ray (EDX) analysis. The selected area diffraction (SAD) pattern was captured by a transmission electron microscopy (TEM, JEOL JEM-2010, Japan) operated at 200 kV. Fourier transform infrared (FTIR) spectra were obtained from a Nicolet Nexus 470 FTIR spectrophotometer. KBr was added into sample pellets, the signal of which was used as the reference. The UV− vis spectra of the samples dispersed in DI water were recorded with a UV-3100 UV−vis spectrophotometer (Shimadzu Corporation, Japan). The X-ray diffraction (XRD) was carried out using a Bruker D8 Advance X-ray diffractometer (Cu Kα radiation 0.1540 nm at 40 kV and 30 mA) from 2θ = 2°−80° at a speed of 5° min−1.

are sequentially deposited. During operation, incoming photons are intercepted and split by high-surface-area ZnO nanowires surface, with electrons being injected into the n-type ZnO nanowires array and holes being left in p-type PANi microrods array at the opposite side of the device.

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EXPERIMENTAL SECTION

Growth of Ordered ZnO Nanowires. ZnO nanowires array was synthesized on an ITO glass substrate (25 mm × 25 mm, sheet resistance 8 Ω cm−2, Beijing Building Material Factory, China) by chemical solution deposition. The ITO was first cleaned thoroughly by acetone and ethanol, each for 15 min, followed by drying in N2 gas and treatment in UV and O3 for 15 min to remove any residual organic components. A ZnO seed layer was then deposited by spin-coating an 100 μL Zn(Ac)2 ethanolic solution (10 mg 25 mL−1) for 30 s. The ZnO/ITO glass was then dried at 135 °C for 2 min. After repeating spin-coating twice, the coating/substrate was finally baked out at 310 °C for 20 min. The ZnO-seeded substrates was then immersed into a aqueous solution containing 4.386 mL of hexamethylenetetramine(methenamine) (HMTA, 25 mM), 20.734 mL of deionized water, 0.78 mL of saturated ammonia aqueous solution (18.4 M), 75 μL of polyethylenimine solution, and 4.5 mL of Zn(NO3)2 (0.05 M) aqueous solution heated at 88 °C. After 30 min, the substrate grown with arrays of ZnO/ITO nanowires were removed from the solution and thoroughly rinsed with DI water and dried with N2 gas. Growth of PANi Microrods Array on Ordered ZnO Nanowires. An electrochemical deposition method was applied to grow PANi microrods on the surface of ZnO nanowires array. To do so, a layer of aniline monomer of 0.3 mL was first spin-coated on the surface of ZnO nanowires array. With this as the working electrode, Ag/AgCl as the reference electrode, Pt foil as the counter electrode, and a mixture of 30 mL of 0.1 M HCl solution and 0.3 M aniline monomer as the electrolyte, the cyclic voltammetry method was applied from −0.1 to 1.0 V at a scanning rate of 100 mV s−1 for a total of 100 cycles. Electrical Characterization. To characterize the I−V behavior of the fabricated p−n heterojunction structure, a gold electrode was used as the top electrode made by sputtering Au for 1 min in a sputtering

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RESULTS AND DISCUSSION

Morphology. Figures 2a,b show the general morphology of the ZnO nanowires array grown on an ITO glass. The average diameter is around 60 nm and the length is 1 μm, yielding an aspect ratio (length/diameter) of ∼17. The electron diffraction shown in Figure 2c suggests a hexagonal wurtzite structure in the grown one-dimensional ZnO nanowires. According to ref 9, the hexagonal unit cell has six nonpolar prismatic faces capped by polar oxygen and zinc basal planes. Since the polar faces are electrostatically unstable Tasker type III surfaces, it makes the {0001} plane the highest energy of the low-index surfaces, i.e., c-axis as the crystal’s fastest growing direction. The chemical compositions of the grown ZnO are confirmed by EDX analysis in Figure 2d and XRD in Figure 2e; the latter indicates the positions of the diffraction peaks matching perfectly with the standard PDF (JCPDS 800075) of ZnO. No impurity phase is

Figure 2. (a) Top view and (b) cross-sectional SEM photographs, (c) SAD pattern, (d) EDX spectrum, and (e) XRD pattern of ordered ZnO nanowires array. 3973

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Figure 3. (a, b) Cross-sectional views of the ordered ZnO nanowires/PANi microrods double array; (c) the top-view of holes left from the scraped PANi microrods.

Figure 4. (a) Cross-sectional and (b) top-view SEM photographs of PANi film covered ZnO nanowire array prepared by a traditional method.

discernible within the detection limit of our X-ray diffractometer. The cross-sectional views of PANi grown on ZnO nanowires array as shown in Figures 3a,b indicate “rod” shape in micrometer scale, the dimension of which varies from 50 × 25 to 40 × 20 μm2 at a length of ∼50 μm. The formation of these unique “microrods” could be related to two major factors: application of a layer of aniline solution and use of high concentration aniline electrolytic solution. The latter assertion of high concentration of aniline solution playing a key role is further supported by the work of Liu and co-workers where ordered PANi nanowires were obtained with a programmable constant-current method in the presence of high concentration of aniline solution.10,11 A layer of PANi film was initially observed on the array of ZnO nanowires during the first few electrodeposition cycles, from which discrete PANi microrods were seen to grow during the following deposition cycles. The strong bonding between PANi microrods and PANi film can be inferred from Figure 3c taken at the surface of ZnO nanowires array after the PANi microrods were removed. The holes left on the top of ZnO nanowires array are the signs of that the PANi microrods were previously in good contact with the PANi film serving as the interlayer between PANi microrods

and ZnO nanowires. Therefore, it is reasonable to believe that thus fabricated p−n junction should possess good rectifying characteristic.12−14 More recently, Li and co-workers reported pioneering research on electrochemical-coupling layer-by-layer assembly of p−n heterojunction,14 demonstrating an excellent interface for studying nanoscale heterojunctions to achieve highperformance diodes. With traditional approach as described by other researchers,15,16 the array of ZnO nanowires is usually covered by a layer of compact and dense PANi as shown in Figure 4,3 not by discrete microrods found in this study. Spectroscopy Analysis. Spectroscopy techniques have been employed to examine the formation of functional groups and reaction mechanisms. The FTIR spectrum of the ordered ZnO nanowires/PANi microrods double array is shown in Figure 5a, and the attributes of the characteristic bands are summarized in Table 1. The observation of the bands at 800− 900 cm−1 indicates the occurrence of polymerization via a headto-tail mechanism. The oxide or metal surface are expected to interact with the conjugated structure of PANi, especially through the Q-ring (semiquinone radical cation), as has been reported in the case of nanocrystalline TiO217 and Au.18 Once the PANi has been grown on the top surface of ZnO nanowires array, the band belonging to CNH+ vibration move from 3974

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Figure 5. (a) FTIR and (b) UV−vis spectra of traditional PANi powders (red curves) and ordered ZnO nanowires/PANi microrods double arrays (blue curves). The inset shows the magnified IR spectrum in the range of 1500−800 cm−1.

respectively.19 The peaks at 472 nm and infrared region are due to the π → localized polaron band, indicating that the PANi is in its conducting form (i.e., the emeraldine salt form of PANi) similar to the PANi nanoparticles synthesized by a traditional chemical method. However, the red shift of the polaron transition band from around 940 nm in PANi nanoparticles to >1000 nm is due to the longer conjugation length in the ordered PANi microrods. From the red shift in the UV−vis absorption spectrum, H+ attached to the N sites in the backbone can be expected to increase the torsional angle appended by the trans− cis-azobenzene photoisomerization between adjacent rings. This will cause the extension of orbital overlap between the phenyl π electrons and the nitrogen lone pairs, decreasing the extent of π conjugation and resulting in an increase of energy level πq. The XRD patterns of the overall ordered ZnO nanowires/ PANi microrods double array are shown in Figure 6a. At the first glance, only peaks belonging to ZnO is detectable at 2θ = 31.7°, 34.4°, 36.1°, 47.5°, 56.6°, 62.9°, and 67.9°, corresponding to the diffraction planes of (100), (002), (101), (102), (110), (103), and (112), respectively.28 However, a close examination in Figure 6b reveals four more peaks at low angle region 2θ = 7° (001), 11.3° (011), 22.1° (100), and 25.8° (110). The peaks at 2θ = 11.3° and 25.8° are usually related to the periodicity parallel and perpendicular to the polymer chain, respectively.19,29 The peaks at 22.1° could be due to a periodicity caused by π−π stacking of rigid phenazine-like structures. The

Table 1. Attributes of the Characteristic Bands of the Ordered ZnO Nanowires/PANi Microrods Double Array bands position (cm−1) 1571 1497 1294 1238 1080 1035 690 800−900

attributes CC stretching deformation mode of quinoid (Q) rings CC stretching deformation mode of benzenoid (B) rings C−N stretching of the secondary aromatic amine C−N−C stretching vibration in the polaron structure19−25 vibration mode of CNH+ (semiquinone radical cation, IP•+) structures26,27 C−H out-of-plane bending of 1,2,4-rings C−H out-of-plane bending of the 1,2-rings para-substitution of the aromatic ring

1118 to 1080 cm−1, which signals the interaction between ZnO and PANi through semiquinone radical cation. During the reaction, the electron clouds in the conjugated CN bond transfer to the ordered ZnO nanowires as a result of the positively charged PANi and negatively charged ZnO nanowires. This charge transfer effect also implies that the PANi microrods and ZnO nanowires can act as donor and acceptor of electrons, respectively. The UV−vis absorption spectrum of the double array measured in DI water is shown in Figure 5b. The bands at around 252 and 364 nm are attributed to the π → π* transition and the polaron band → π* transition of PANi microrods,

Figure 6. XRD patterns of the ordered ZnO nanowires/PANi microrods double array (black curve) and PANi films-covered ZnO nanowires array shown in Figure 4 (red curve): (a) whole and (b) magnified views. 3975

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Figure 7. (a) I−V characteristics of the p−n heterojunction based on ordered ZnO nanowires/PANi microrods double array under various illumination intensities at room temperature. (b) The rectification ratio as a function of illumination intensity.

forward one, yielding the desirable characteristic of a photodiode. In other words, the illumination intensity should be controlled in the range of 0−93 mW cm−2 when being used as a photodiode. Overall, the ordered ZnO nanowires/PANi microrods double array heterojunction acts as a diode and exhibits a good rectifying behavior under visible illumination at room temperature. These characteristics are highly desirable for efficient and sensitive light photodetectors and optoelectronic devices. It is known that the rectifying characteristic of a p−n heterojunction are highly dependent on both the preparation methods and morphologies.14,35 An increase in number of the photogenerated electrons and holes yields a better rectifying characteristic of a p−n heterojunction36 as a result of efficient separation of photogenerated electron−hole pairs into free charge carriers at the interface between an electron-donor (p-type) and electronacceptor (n-type) material. The lifetime of photogenerated electrons is strongly dependent on diffusion length.37 For instance, the photogenerated electrons in oxide semiconductors, such as ZnO, have a trap-limited diffusion process,38,39 in which photogenerated electrons interact with traps as they undertake a random walk through the semiconductor film. To promote the separation of photogenerated electron−hole pairs in ZnO/PANi p−n heterojunction, increasing electron and hole diffusion lengths in n-type and p-type materials, respectively, by replacing the nanoparticle films of ZnO and PANi with arrays of aligned ZnO nanowires and PANi microrods, is one effective solution.12,13,37 The electron transport in the aligned arrays is expected to be several orders of magnitude faster than in random nanoparticle films.37

diffraction peaks at around 2θ = 11.3°, 22.1°, and 25.8° are generally observed in PANi products, but the peak at around 2θ = 7°, which is close to the PANi repetition unit, is observed only for highly ordered PANi structures.30 All these results confirm that the resultant PANi microrods array is in highly crystalline and emeraldine salt state. However, the typical diffraction peaks (red curve) belonging to hexagonal ZnO nanowires are not detectable in the traditional PANi-film covered ZnO nanowires array of Figure 4 due to the complete coverage of ZnO nanowires by the amorphous PANi film. Rectifying Behaviors. The I−V curves of the p−n heterojunction on the double array measured under various illumination intensities at room temperature are shown in Figure 7a, where the characteristic rectifying behaviors of a diode is clearly observed. Using the standard diode equation I = Is[exp(eV/nkT) − 1], where e is the electronic charge, V is the applied voltage, k is the Boltzmann constant, n is the ideality factor, and Is is the saturation current; the value of n is calculated as 5.6 for the fabricated ordered ZnO nanowires/PANi microrods double array diode. This value is compared with the reported ideality factor of 4.7 for ZnO/PANi hybrid film31 and 6.5 for ordered ZnO nanorods/PANi film.32 Theoretically speaking, any n value higher than 2 means that the diode is nonideal. The reason for the nonideality is probably caused by the presence of the surface defects in ZnO nanowires31 where these surface defects can act as gas adsorption sites to trap oxygen molecules.33 The chemisorbed oxygen can capture free electrons in the darkness from the conduction band of ZnO nanowires and form oxygen ions (O2−), creating a depletion layer at the interface of ZnO and PANi and resulting in a low current. Under visible light illumination, partial photogenerated holes migrate to the surface of ZnO nanowires and are neutralized by the negatively charged surface O2− ions,34 leaving the excited electrons unpaired. Therefore, there is an increased current flow through the junction and the external circuit for both the forward and reverse biases under visible light illumination. Another interesting phenomenon is that the rectification ratio (defined as the forward current divided by the backward current at a bias) as shown in Figure 7b shows a parabolic shape with a measured minimum at 93 mW cm−2. A parabolic equation of R = 1.501 × 10−4P2 + 0.0225P + 3.144 can be fitted to describe the relationship between the rectification ratio (R) and illumination intensity (P). From 0 to 93 mW cm−2, the degree of increase in backward current is greater than that of the

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CONCLUSIONS In summary, the results of this study have demonstrated a new type of p−n heterojunction based on ordered ZnO nanowires/ PANi microrods double array. The SEM observation reveals that the ordered n-type ZnO nanowires have an average diameter of 60 nm and a length of 1 μm. The layer of ordered p-type PANi microrods can be grown on the surface of ordered ZnO nanowires layer by a controlled electrochemical deposition route. Applying a layer of aniline monomer and highly concentrated aniline monomer play a key role in growing microrod-like PANi. The fabricated ordered ZnO nanowires/PANi microrods double array shows a good photodiode characteristic under visible light illumination. Because of the partial neutralization of photogenerated holes by O2− ions, the photocurrent flowing through 3976

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the junction and external circuit increases with both the forward and backward biases under a visible light illumination. The rectification ratio exhibits a minimum at ∼93 mW cm−2 as a function of illumination intensity. Further in-depth fundamental studies will offer new insights into understanding of mechanisms in the p−n heterojunction diodes consisting of ordered, continuous, and crystalline inorganic nanostructures and ordered conjugated polymers microstructures for modern highly efficient and sensitivity electronic and photovoltaic devices.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (K.H.); [email protected] (J.W.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of the National High Technology Research and Development Program of China (No. 2009AA03Z217), the National Natural Science Foundation of China (No. 90922028), and the University of South Carolina.

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