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High-Performance Photoelectronic Sensor Using Mesostructured ZnO Nanowires Liping Chen, Jiabao Cui, Xia Sheng, Tengfeng Xie, Tao Xu, and Xinjian Feng ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.7b00477 • Publication Date (Web): 19 Oct 2017 Downloaded from http://pubs.acs.org on October 20, 2017
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High-Performance Photoelectronic Sensor Using Mesostructured ZnO Nanowires Liping Chen,† Jiabao Cui,‡ Xia Sheng,† Tengfeng Xie,‡ Tao Xu§ and Xinjian Feng*,† †
College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China College of Chemistry, Jilin University, Changchun 130012, China § Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, Illinois 60115, United States ‡
Supporting Information
ABSTRACT: Semiconductor photoelectrodes that simultaneously possess rapid charge transport and high surface area are highly desirable for efficient charges generation and collection in photoelectrochemical devices. Herein, we report mesostructured ZnO nanowires (NWs) that not only demonstrate a surface area as high as 50.7 m2/g, comparable to that of conventional nanoparticles (NPs), but also exhibit a 100 times faster electron transport rate than that in NP films. Moreover, using the comparison between NWs and NPs as an exploratory platform, we show that the synergistic effect between rapid charge transport and high surface area leads to a high performance photoelectronic formaldehyde sensor that exhibits a detection limit of as low as 5 ppb and a response of 1223% (at 10 ppm), which are, respectively, over 100 times lower and 20 times higher than those of conventional NPsbased device. Our work establishes a foundational pathway towards better photoelectronic system by materials-design. KEYWORDS: ZnO, Nanowire, Surface area, Charge transport, Photoelectronic sensor Nanostructured electrodes that simultaneously possess rapid charge transport and high surface area are highly desirable for efficient charge carrier generation and collection in solar cells, water splitting, and photoelectronic sensing devices.1-5 Onedimensional (1D) nanostructures, such as single-crystal ZnO and TiO2 nanowire (NWs) that offer directed charge transport pathway have recently received substantial attentions.6-10 The two to three orders of magnitude faster electron transport rate of 1D NWs make it an advantageous electrode architecture over conventional mesoporous films that comprised of randomly packed nanoparticles (NP). 10-12 However, the relatively low surface area is a key issue impedes their wide applications. In order to increase the surface area of 1D NWs a number of approaches have been reported, such as increase the length of NWs, prepare NWs with multichannel, tubular and branched nanostructures.8, 12-18 Yet despite these efforts, their surface area is still cannot be comparable with that of NP films. Herein, we report the fabrication of mesostructured singlecrystal-like ZnO NWs that simultaneously possess rapid charge transport rate and high surface area. The NWs have a surface area of 50.7 m2/g, comparable to that of commonly used ZnO NPs, but a charge transport rate of over 100 times faster than that in the crystalline-randomly-oriented NP films. Moreover, we show that these unique properties lead to a high performance photoelectronic formaldehyde sensor with a detection limit of 5 parts-perbillion (ppb) and a response of 1223% (at 10 ppm), which is, respectively, over 100 times lower and 20 times higher than those of commercially available ZnO NPs-based device. Indeed, the detec-
tion of trace level chemical regents is ongoing importance because many volatile chemicals that are widely used in industries and urban construction can result in serious health problems when inhaled or exposed to even at ppb levels.5, 19-23 Among numerous detecting methods, semiconductor-based photoelectronic sensor has recently been considered as a promising alternative to conventional approaches for chemical gas detection owing to their high stability, high sensitivity, low detection limit, low operating temperature, and suitability for portable device fabrication.24-26 Figure 1a and 1b are typical field emission scanning electron microscopy (FE-SEM) cross-sectional view of ZnO NWs that fabricated from the fluoride doped tin oxide (FTO) coated transparent glass substrate. The NWs grow uniformly in a large scale with a length of about 15 µm. The length can be tuned to be longer than several tens of micrometers (see supporting information). Each of these NWs has a rough surface with a diameter in the range of 50-220 nm (Figure 1b). Figure 1c presents the X-ray diffraction (XRD) pattern. All peaks can be indexed to wurtzite structured ZnO powder (JCPDS card No. 65-3411) and previously reported [0001] oriented ZnO NW.27 The (10-10) diffraction peak is much stronger than the others and only a very week (0002) peak is observed.
Figure 1. (a) and (b) are FE-SEM cross-sectional views of ZnO NW arrays fabricated on FTO-coated transparent glass substrate at low and high magnifications, respectively. The NWs have a length of about 15 µm and a high density of mesopores. (c) XRD pattern of the as-prepared ZnO NW arrays.
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Figure 2. Structural characterization of the mesostructured ZnO NWs. (a) TEM image at low magnification. (b) Magnified TEM image of NWs, indicating that the NW has a mesoporous structure across the entire length and width. (c) and (d) are, respectively, TEM images recorded from c and d areas that marked by white boxes in (b), showing that pores with nanoscale size are homogeneously distributed across the NW. (e) and (f) are HR-TEM images of the single pore marked by white boxes in (c) and (d), respectively, ZnO [10-10] lattice with a d-spacing of 0.28 nm is clearly observed; inserts shown in (e) and (f) are their corresponding fast Fourier transform (FFT) patterns.
Figure 3. (a) Nitrogen adsorption-desorption isotherms and the corresponding pore size distribution within NWs. Inset is the pore-size distribution diagram, which shows that the NWs possess mesostructure with sizes ranging from 2 to 20 nm and a peak size of about 5 nm. (b) Dependence of electron diffusion coefficient on the incident photon flux based on ZnO NWs, NRs and NPs. Figure 2a is a typical transmission electron microcopy (TEM) image of the as-prepared ZnO NWs at low magnification, which shows that the NWs have a mesostructure and length up to tens of micrometers. The nanopores distribute homogeneously
across the entire length and width of NW as evidenced in Figure 2b. Further magnified TEM images (Figure 2c and 2d) taken from the center and edge areas of the NWs, marked in the corresponding white boxes in Figure 2b, show that the size of the pores lies in the range of 2 to 20 nm, but are predominatelyin the range of 4 to 8 nm. Furthermore, high-resolution transmission electron microscopy (HR-TEM) images of the single pores taken from Figure 2c and 2d show continuous, single-crystal ZnO lattice across the nanopores (Figure 2e and 2f). The almost identical crystalline orientation (FFT patterns shown in the insets of Figure 2e and 2f) of different areas indicates that the as-prepared mesostructured ZnO NWs are highly crystallized and have a unique crystallographic orientation of [10-10], which is consistent with XRD pattern shown in Figure 1c. ZnO NWs with such mesostructure were synthesized via thermal transformation from single-crystal Zn(OH)F NWs that prepared via a low temperature chemical synthesis approach, whose morphology and crystal structures are shown in Figure S-1. Such conversion is thermodynamically favorable since Zn(OH)F is unstable during high temperature treatment.28-29 When heated at above 400 ºC in an oxygen atmosphere, fluoride and hydrogen ions are removed from the lattice, which leads to diffusion of reaction atoms, contraction of nanocrystal and the formation of more compacted ZnO NWs with internal nanopores. The surface area of thus-prepared mesoporous ZnO NWs was further investigated. As shown in Figure 3a, the Brunauer-Emmett-Teller (BET) surface area of NWs after being scratched off the substrate is most measured to be 50.7 m2/g, which is comparable to that of commonly used NPs such as TiO2 P25 (~50 m2/g) and ZnO (~60 m2/g, 20-40 nm diameter). The pore size distribution was further characterized using the Barrett-Joyner-Halenda (BJH) method. As shown in the insert of Figure 3a, the pore size of mesostructure is centered at about 5 nm, which is close to that revealed from the TEM characterization shown in Figure 2c and 2d. High surface area electrode materials are normally associated with mesoporous films that comprised of randomly packed NPs, and thus have significantly low charge transport. To understand the electron transport property of the as-fabricated ZnO NWs, intensity-modulated photocurrent spectroscopy (IMPS) experiments were further conducted.30-31 Control experiments based on
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ZnO nanorods (NRs) (Figure S-2) that prepared using classical
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Figure 4. (a) Schematic diagram of the photoelectronic sensing device. ZnO NWs are immobilized on the ITO substrate with comb-like laser-etched insulating gaps. Under ultra-violate (UV) light illumination, the change in electrode photocurrent upon the introduction of HCHO gas into the test chamber will be recorded. (b) SEM image of ZnO NWs covered comb-like photoelectrodes. (c) Photocurrent of the device upon exposed to HCHO with concentrations ranging from 1 ppm to 200 ppm. Insert in (c) shows the operating procedures and the sensing behavior. (d) Plots of the electrode response versus HCHO concentration fitted using Langmuir isotherm adsorption for ZnO NWs (red circle), ZnO NRs (blue pentagon) and NPs (black square) based electrodes. (e-g) Detection limit test of ZnO NWs, NRs and NPsbased devices. approach and commercial available NPs were also conducted. Figure 3b shows the dependence of electron diffusion coefficient (D) on the incident photon flux. The D value of NPs film displays a typical power-law dependence on the light intensity.31 In contrast,the D value of NWs and NRs shows no dependence on the incident photon flux and is about one to two orders of magnitude higher than that of NP films over a broad incident photon flux range. These results confirm that the as-prepared mesostructured ZnO NWs possess not only high surface area but also rapid electron transport dynamics. Mesostructure ZnO NWs with such unique properties will be of broad academic and industrial interest. In this work, for demonstration study, we show their outstanding performance in photoelectronic sensing system. Figure 4a illustrates the configuration of the sensing system, in which the ZnO NWs are immobilized on an indium doped tin (ITO) oxide electrodes with comblike laser-etched insulating gaps. Detailed morphologies of the electrodes are shown in Figure 4b and Figure S-3. Controlled experiment using ZnO NPs with comparable high surface area but much lower charge transport dynamics, and ZnO NRs with similar fast electron transport but much lower surface area was also conducted. HCHO, one of the most commonly accessible hazardous volatile organic agents was selected as the target analyte.
Under ultra-violate (UV) light illumination (365 nm, 10 mW cm) and a circuit voltage of 10 V, upon exposed to HCHO gas in air ambient at room temperature, the change in electrode photocurrent response correlated directly to the surface photoelectrochemical reaction is recorded. As shown in Figure 4c, the photocurrents of ZnO NWs-based system keep increasing with the increase of HCHO concentrations up to a value of about 200 ppm. Figure 4d (red curve) plots the calculated relative photocurrent response versus (vs.) HCHO concentrations. The curve is linear at low concentration, but tended to saturate at high HCHO concentration. The data points can be well fitted with Langmuir isotherm adsorption32 using following equation: Response = 188.63/(1+147.79/C), where C represents the HCHO concentration, which can also be confirmed by the liner fitting of Response-1 vs. C-1, as shown in the inset of Figure 4d. Although similar trend between responsive photocurrent and HCHO concentration were also observed on NPs and NRs based probing systems as shown in Figure S-4 and Figure S-5, the relative response of mesoporous NWs is, respectively, 20 and 15 times higher than that of NPs and NRs (for example, at 10 ppm HCHO) as shown in Figure 4d. Bare ITO electrode is also tested in the same condition and no sensing response is observed (data not shown here). More importantly, as shown in Figure 4e, the 2
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NWs exhibits a decent response even at a concentration as low as 5 ppb, which meets the trace detection standard of 80 ppb re-
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quired
by
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World
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Table 1 Sensitivity Performance of the Photoelectronic Formaldehyde Sensors Sensor materials
Morphology
Response (10 ppm)
References
ZnO
Mesostructured Nanowire
1223%
This work
#
CdSO4–ZnO
Nanoparticle
100%
33
Ag-ZnO
Nanorod
35.6%
34
SnSO4-ZnO
Nanoparticle
200%#
35
Fe-ZnO
Nanoflower
50%
36
CdS-ZnO
Nanowire
#
125%
37
#
In2O3-ZnO
Nanoflower
50%
38
Ni-ZnO
Nanofiber
50%#
39
ZnO
3D hollow sphere
547%
40
Note: # Denotes a value not explicitly stated in the study, but approximated from a graphical plot. Organization.23 The detection limit of our NWs-based device is, respectively more than 100 and 20 times lower than that of NPs (500 ppb, Figure 4f) and NRs (100 ppb, Figure 4g) based-ones under the same testing condition. Table 1 compares the sensitivity of representative metal oxide semiconductors based HCHO sensors reported in literature. It can be clearly seen that our mesostructured ZnO NWs sensor has the best performance. In general, ZnO is an n-type semiconductor. When irradiated with UV light in air ambience, photoelectrons will be generated and seized by the surface absorbed molecular O2, leading to the formation of reactive oxygen species (ROS, e.g. O2•‒). Subsequently, the ROS can oxidize neighboring molecular analytes such as formaldehyde via reaction: O2•‒ + HCHO→CO2 + H2O+e– , leading to the release of electrons (informational charges) back to the electrode and the increase in photocurrent.41 To confirm this argument, controlled experiments in argon atmosphere were carried out. As shown in Figure S-6a, when exposed to UV light, the background photocurrent is about 8 times higher than that recorded in air (Figure S-6c), and the current does not increase as HCHO gas is introduced (Figure S-6b). The much higher background photocurrent recorded in argon atmosphere indicates that few photo-generated electrons are trapped in the absence of surface absorbed O2, and thus no subsequent HCHO oxidation reaction, informational carrier generation and photocurrent response. Sensing test without UV light illumination was also conducted for the NWs based device, which is shown in Figure S7. When UV light illumination was removed from the sensing system, the current response to HCHO is undetectable. This result proves that the UV light activation is indispensable for our room temperature sensor device. On the basis of these results, we can see that both surface area and electron transport dynamics of the semiconductor electrode play a pivotal role in the photoelectronic device performance. Explicitly, high surface area of mesostructured ZnO NWs offers a large number of adsorption sites for O2 and HCHO molecules, namely producing adequate informational electrons under light illumination; on the other hand, the high electron transport in the highly crystalline-oriented ZnO NWs favors for the efficient collection of these informational electrons, namely, collecting adequate informational electrons. That is, the collaborative effect of these unique properties endow an extremely high sensitivity and low detection limit in metal oxide semiconductor based photoelectrochemical probing systems. In summary, we have fabricated mesostructured singlecrystal-like ZnO NWs that can simultaneously deliver a high surface area of 50.7 m2/g, comparable to that of NPs, and over a 100
times faster electron transport rate than that in NP films. The collaborative effect between these two properties lead to the effective generation and collection of charge carrier, and an ultra-sensitive photoelectronic probing system. In light of these results, we believe that ZnO NWs with high surface areas and rapid charge transport will be of great interest in other photoelectrochemical systems such as artificial photosynthesis, solar cells and water splitting. Moreover, this work establishes a basic science foundation to approach better photoelectronic system-by-materials design.
ASSOCIATED CONTENT Supporting Information Detailed description of experimental methods, SEM, TEM images and XRD pattern of Zn(OH)F NWs (Figure S-1), SEM images and XRD pattern of ZnO NRs (Figure S-2), SEM images of the ZnO NWs covered photoelectrode (Figure S-3), Current-time curve, relative response vs. HCHO concentration for commercial ZnO NPs based gas sensor (Figure S-4) and ZnO NRs based gas sensor (Figure S-5), photocurrent response of ZnO NWs based gas sensor in different atmosphere (Figure S-6), current response of ZnO NWs based sensor without UV light illumination (Figure S-7). (PDF)
AUTHOR INFORMATION Corresponding Author *X. Feng, Email:
[email protected] Author Contributions L. Chen, J. Cui and X. Sheng contributed equally to this work.
Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT This research was financially supported by the National Natural Science Foundation of China (21371178, 51772198, 21501193) and the Jiangsu Province Science Foundation for Distinguished Young Scholars (BK20150032).
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