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Converting the Conducting Behavior of Graphene Oxides from N-type to P-type via Electron-Beam Irradiation Ali Mirzaei, Yong Jung Kwon, Ping Wu, Sang Sub Kim, and Hyoun Woo Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16458 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 13, 2018

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ACS Applied Materials & Interfaces

Converting the Conducting Behavior of Graphene Oxides from N-type to P-type via Electron-Beam Irradiation Ali Mirzaei§, Yong Jung KwonϨ, Ping Wu†,* ,Sang Sub Kim‡, Hyoun Woo Kim§,Ϩ*

§The

Research Institute of Industrial Science, Hanyang University, Seoul 04763, Republic of Korea

ϨDivision

of Materials Science and Engineering, Hanyang University, Seoul 04763, Republic of Korea

†

Entropic Interface Group, Singapore University of Technology & Design, Singapore 138682, Singapore

‡Department

of Materials Science and Engineering, Inha University, Incheon 22212, Republic of Korea

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT

We studied the effects of electron-beam irradiation (EBI) on the structural and gas sensing properties of graphene oxide (GO). To understand the effects of EBI on the structure and gassensing behavior of irradiated GO, the treated GO was compared with non-irradiated GO. Characterization results indicated an enhancement in the number of oxygen functional groups occurs with EBI exposure at 100 kGy, and then decreases with doses in the range of 100 to 500 kGy. Data from Raman spectra indicated that EBI could generate defects, and NO2 sensing results at room temperature showed a decreased NO2 response after exposure to EBI at 100 kGy; further increasing the dose to 500 kGy resulted in p-type semiconducting conductivity. The conversion of GO from n-type to p-type via EBI is explained not only through the generation of holes, but also the variation in the amount of residual functional groups, including carboxyl (COOH) and hydroxyl groups (C-OH). The obtained results suggest that EBI can be a useful tool to convert GO into a diverse range of sensing devices.

KEYWORDS: Graphene oxide, Electron-beam irradiation, Oxygen functional group, Gas sensor, Conducting behavior


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1. INTRODUCTION Graphene was successfully isolated in 2004 for the first time.1 The unique plane structure and geometry of monolayer graphene (0.35-1.6 nm in thickness),2 contributes to its amazing properties, which include a high fracture strength, outstanding electrical and thermal (~5000 W/mK) conductivity, high charge carrier mobility (~104 cm2/V.s), and large specific surface area (up to 2630 m2/g).3 Furthermore, graphene has a low density, a relatively low price, and chemical inertness.4 These remarkable properties make graphene ideal for usage in different areas such as electronics,5 energy storage and conversion,4 biotechnology,6 water treatment,7 photo catalyst8 and sensing.9 Graphene is particularly popular for realization of gas sensors due to a combination of excellent metallic conductivity, a huge surface area, and few defects due to crystalline integrity, resulting in a low noise.10 Nevertheless, the gas sensors fabricated from graphene have relatively low sensing capabilities. Since there are few dangling bonds on the surfaces of graphene; target gases cannot effectively chemisorbed on the graphene`s surfaces.11 Moreover, it has no band gap.12 Therefore, functionalization is necessary to increase the adsorption properties of graphene. Graphene oxide (GO) is a good alternative to pristine graphene. In 2008, Liu et al.,13 overcame the aggregation issue of graphene using the oxidized form of graphene, namely GO. Due to presence of hydroxyl and epoxy functional groups on sp3 hybridized carbon and carbonyl and carboxyl functional groups on sp2 hybridized carbon, GO has plenty of functional groups, which provide adsorption sites for target gases. Additionally, GO is cheaper than graphene and can be easily synthesized, and like pristine graphene, its surface area is very high.14 The first report of 3 ACS Paragon Plus Environment


GO for sensing applications was published by Chung et al.,15 which was related NO2 sensing. While GO has several advantages over pristine graphene, its conductance is extremely low due to the conjugated electronic structure being disturbed by oxygen containing groups. This hinders its application in practical electronic measurement devices for gas sensing. Conductivity of GO can be partially restored by chemical or thermal reduction of GO, where because of incomplete reduction, oxygen groups still are present in the reduced GO. Another approach to restoring conductivity is the modification of GO using electron-beam irradiation (EBI). Recent approaches to modifying the physio-chemical properties of GO use irradiation techniques such as ion16 and laser17 irradiation. Among irradiation techniques, EBI is increasingly used to modify the dielectric, electrical, structural, chemical, mechanical, and thermal properties of materials. The modification degree depends upon the radiation dose rate and material characteristics. Whenever EB energy interacts with material, it induces changes in the molecular structural arrangement such as ionization, displaced atoms, carbonization, and free radical production. The radiation can change the chemical structure of the material and enhances the number of trapped charges or creates defects,18 for example, generating nanopores, slits,19 hillocks,20 and vacancies.21 These alterations affect the properties of irradiated materials; EBI can influence the structure and properties of GO via lattice defects or oxidation from broken carbon bonds. In fact, the reduction time of GO can be significantly decreased using EBI.22 Chen et al.,23 studied structural changes in GO caused by EBI with an absorbed dose of 500 kGy. He reported changes in the number of functional groups and the reduction effect, which ultimately led to reduced interlayer spacing in GO. Kim et al. studied the effects of EBI on the properties of graphene as compared to GO synthesized via the modified Hummer’s method, demonstrating 4 ACS Paragon Plus Environment

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high thermal stability due to lower defects.24 Kwon et al.,25 showed that EBI can be a useful tool in the optimization of reduced graphene oxide (RGO) for sensing applications. Few published works have studied the modification of GO via EBI for sensing applications. To the best of our knowledge, NO2 room temperature sensing properties of EBI GO has not been reported yet. Here, we use 2 MeV electron beams to irradiate GO in doses of 100 kGy and 500 kGy, and investigate the relationship between EBI and sensor behavior. Structural changes before and after irradiation were evaluated via XRD and Raman spectroscopy and chemical composition was examined using XPS. The effects of EBI were investigated by studies of the changes in NO2 sensing properties. It was found that the oxygen functional groups generated by EBI can significantly change the adsorption of NO2 molecules.

2. EXPERIMENTAL Synthesis of GO and EBI We synthesized GO from graphite powders using a modified Hummers’ method.26, 27 Graphite powder, H2SO4 (98%), H3PO4 (98%), KMnO4 (98%), H2O2 (30 wt%), were obtained from Sigma-Aldrich and used as received. GO was synthesized from graphite powder via a modification of Hummer`s method. In a typical reaction, 1 g of graphite powder, 12 mL of H3PO4, 12 g KMnO4 and 46 mL of H2SO4 were stirred together for 3h. Next, 90 mL deionized water and 10 mL H2O2 were added to the obtained solution and the resultant solution was stirred for 5h at 90°C. Afterwards, the obtained products were washed by HCl (5%) for five times and were cleaned with deionized water. By sonication for 5 h and subsequent centrifugation, GO nanosheets were obtained.



A colloidal preparation of GO was produced by dispersing GOs in ethanol and subjecting the solution to sonication for 60 min. By spraying the GOs colloid using spray gun onto the Al 2O3 substrates located on a hot-plate (∼150°C) GO films with uniform thickness were produced. The thickness of deposited films can be controlled by the time of spraying. However it should be noted that because of the deposition was not performed by advanced physical methods, naturally the deposited films had not perfect uniformity. However, for sensing application, an acceptable uniformity can be obtained by this procedure. EBI irradiation of the as-synthesized GO films was done using an ELV-8 electron accelerator (EBTech, Daejeon, Korea) at room temperature. The accelerated voltage, electron dose, beam current, and pulse duration were set to 2 MV, 100 and 500 kGy, 1 mA, and 400 ps, respectively.

Characterization The morphology of the GO sheets was studied via scanning electron microscopy (SEM, JEOL, JSM 5900 LV, Japan), and high-resolution transmission electron microscopy (HR-TEMTECNAI 20 microscope). For TEM measurements, the synthesized samples were put in ethanol, sonicated and then deposited on the carbon holy grids. The preparation procedure of TEM samples was identical, whether the sample was irradiated or not. The structural features were studied via X-ray diffraction (XRD) using Cu Kα radiation (λ=1.548 Å). Raman spectroscopy was used for structural characterization using an excitation laser at a λ=532 nm and power density of 2.9 mW.cm-2 (Jasco Laser Raman Spectrophotometer NRS-3000 Series). X-ray photoelectron spectroscopy (XPS, VG Multilab ESCA 2000 system, UK) was used to study the elemental composition of GO.


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Gas Sensing Tests For making sensor, Ni and Au (~200 nm and 50 nm thickness respectively) bilayer interdigitated electrodes were sequentially deposited on the substrate, having GO thin films by sputtering as described in our previous report (Figure S1).28 For sensing measurements, the fabricated sensor was put in a tube furnace, and electrically was connected to an electrical measuring system (Keithley 2400) interfaced with a computer. By changing the mixing ratio of the NO2 gas and dry air (total flow rate =100 sccm) through mass flow controllers, the gas concentration (10 ppm) was controlled. The partial pressure of NO2 was 10-5 atm. The electrical conductivity was measured in the chamber at atmospheric pressure and room temperature. The gas response was determined as a ratio of (Rg-Ra)/Ra, where Rg and Ra are the resistance in the presence of NO2 gas and air, respectively. The response and recovery times were defined as the times need for the resistance to change by 90% upon exposure to NO2 and air, respectively.

3. RESULTS AND DISCUSSION

Morphological Studies Figure 1a shows a typical SEM image of GO that shows a general morphology GO. Figure 1b shows a TEM image of single-layer GO, demonstrating the highly transparency of a single layer of GO. The inset of Figure 1b presents a corresponding selected area electron diffraction (SAED) pattern. Figure 1c shows another TEM image of a single layered GO nanosheet, confirming the existence of GO in a layered form. Figure 1d shows a lattice-resolved TEM image, where the relatively good crystalline nature of GO can be observed. 7 ACS Paragon Plus Environment


Figure 1. (a) SEM micrograph of few-layered GO and (b,c) TEM micrographs of a singlelayered GO at two different magnifications (Inset shows the corresponding SAED pattern). (d) lattice-resolved TEM image. The GO was as-synthesized and unirradiated.

XRD Studies Figures 2a-c show XRD patterns of un-irradiated, 100 kGy-irradiated, and 500 kGy-irradiated GO, respectively. The Bragg angles of the (001) diffraction peaks are located at 12.79°, 11.41°, and 12.25°, respectively, which are characteristic of GO;29 therefore, all GOs display a strong XRD peak at Bragg angles of 11.41-12.79° and the position of the (001) peak was slightly shifted, indicating no significant change of the crystal lattice structure of GO after EBI. Pristine 8 ACS Paragon Plus Environment

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graphite shows a sharp XRD peak at 26.5° belong to (002) crystal plane.30 As shown in Table S1, the position of (001) peaks in GO and graphite oxide are very close together. The location of (001) peak of GO depends on the preparation procedure and degree of oxidation. Accordingly, many researchers have reported different Brag angle values for (001) peak of GO. It suggests that different amounts of oxygen containing functional groups were inserted into the sheets of graphene at different oxidizing conditions.31 In addition, increases in interlayer spacing were due to the introduction of oxygen functional groups in the layered structure which shifted the peaks to the lower Bragg angles. It was also determined that the d-spacing was increased after exposure to EBI at a dose of 100 kGy. The existence of a few number of residual oxygen functional groups led to this increased dspacing in EBI-treated GO.25 In contrast, the d-spacing decreased when the dose was increased from 100 kGy to 500 kGy. Decreases in inter-planar spacing following EBI with high dose maybe owing to the removal of some oxygen functional groups.32



Figure 2. XRD patterns of (a) unirradiated, (b) 100 kGy-irradiated, and (c) 500 kGy-irradiated GO samples.

Raman Features

Raman spectroscopy is a nondestructive and fast technique which provides significant structural and electronic information.33 Figures 3a-c show the Raman spectra of the unirradiated GO, GO sheets irradiated at a dose of 100 kGy, and 500 kGy, respectively. It is well-accepted that the most notable features in the Raman spectra of carbon are the so-called G and D peaks, which are located ~1560 and 1360 cm−1, respectively.33 The G peak is owing to the bond stretching of all pairs of sp2 atoms in both rings and chains in the G band, which is assigned to the first-order scattering of the E2g phonon (in-plane optical mode) close to the Γ point observed for sp2 carbon domains. Also the D peak is owing to the breathing modes of sp2 atoms in rings and the broad D band is caused by sp3-hybridized carbon and the presence of defects/disordered structures in the graphite layer. 26,33,34 The empirical Tuinstra-Koenig formula; I= ID/IG, is the ratio of the intensity of the D and G peaks and can show the degree of order across graphitic plans.35 In general, the I value reveals the reduction and defect levels; the smaller the I value, the greater the regularity, and a larger I value indicates presence of higher defects and disorders in the carbon structure.36 The I value in the Raman spectra of unirradiated, 100 kGy-irradiated, and 500 kGy-irradiated GO sheets are estimated to be 0.89, 0.96, and 0.96, respectively, indicating increased defects as a result of EBI. By the way, by considering error bars in the inset of Fig. 3, we expect that the I value monotonically depends on the defect level, although the amount of the increase of the I value by 10 ACS Paragon Plus Environment

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increasing the electron dose from 100 to 500 kGy is small. The increase of defects can to related to the destruction of sp2-hybridized carbon.30

Figure 3. Raman spectra of (a) unirradiated, (b) 100 kGy-irradiated, and (c) 500 kGy-irradiated GO sheets.

The size of in-plane crystallites (La) after EBI was calculated by the Tuinstra-Koenig (TK) formula:37



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(1)

ID La = (2.4 × 10−10 )λ4( )−1 IG

Where La (nm) is the crystallite size and λ is the beam wavelength in nm (excitation laser wavelength of 532 nm). The calculated La for un-irradiated, 100 kGy- and 500 kGy-irradiated samples are 22.61, 19.81, and 19.60 nm, respectively.

XPS Studies XPS is a well-known and useful method of determining the chemical compositions and chemical states of material surfaces. In GO, different functional groups such as carboxyl, hydroxyl, or epoxy groups exist, therefore, XPS can be a valuable tools to obtain some information about existence of functional groups on the surface of GO.38 Figure 4 provides the XPS survey spectra of unirradiated, 100 kGy-irradiated, and 500 kGy-irradiated samples. The data were calibrated by refereeing to binding energy of 284.6 eV belong to carbon to compensate for the surface charge effect.39 The XPS spectra of all samples reveal two peaks at 284.6 and 534.0 eV, corresponding to the C 1s and O 1s core-levels, respectively. The atomic percentages (at %) of each element are also shown in Figure 4.


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Figure 4. XPS survey spectra of (a) unirradiated, (b)100 kGy-irradiated, and (c) 500 kGyirradiated samples.

For un-irradiated GO, the atomic percentages of C and O were 66.68 and 33.32%, respectively. These values were changed to 55.70 and 44.30%, respectively, after EBI at a dose of 100 kGy, and to 58.60 and 41.40%, respectively, after EBI at a dose of 500 kGy. It is interesting to note that after 100 kGy-irradiation, the amount of oxygen increased from 33.32% to 44.30%, while 13 ACS Paragon Plus Environment


the amount of carbon decreased from 66.68% to 55.7%. With 500 kGy-irradiation, the amount of oxygen increased from 33.32% to 41.4%, whereas the amount of carbon decreased from 66.68% to 58.60%. Accordingly, it is surmised that the EBI excites the ambient oxygen to high-energy forms, like O3 and they may oxidize GO nanosheets.15 Previously, Xu et al. demonstrated that EBI induced partial oxidation of graphene.40 Our XPS results show that the O/C ratio became slightly higher (from 41.54% to 43.45%) by EBI, which may be due to oxygen adsorption from air.23 Figures 5a-c, show XPS C1s spectra of the un-irradiated, 100 kGy-irradiated, and 500 kGyirradiated samples, respectively. Curve fitting of the C1s spectra was done using a GaussianLorentzian peak shape. The deconvolution of XPS spectra shows an intense peak at 284.6 eV assigned to the sp2 carbon framework. In addition, the deconvolution shows that carbon-oxygen bonds (C-O-C (286.8 eV), C=O (288.6 eV)) are present at the surface of all samples, whereas O=C-OH (290.1eV) is present only in the irradiated samples. The high intensity of the C-O-C peak in unirradiated GO demonstrates that the unirradiated GO has many hydroxyl functional groups; however, after EBI the intensity of hydroxyl functional groups significantly decreased. Similar behavior was observed in thermally-treated RGO samples.41 The peak ratios of C-O-C bonding of unirradiated, 100 kGy-irradiated, and 500 kGy-irradiated GO sheets are approximately 46.8, 31.5, and 29.4, respectively. Accordingly, it can be supposed that the C-O-C groups are removed in the form of CO2 gas, which eventually can lead to formation of different defects, like carbon single vacancies and 5-8-5 defects.42


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Figure 5. XPS C1s spectra of (a) unirradiated, (b) 100 kGy-irradiated, and (c) 500 kGy-irradiated GOs.

Figures 6a-c, shows XPS O1s spectra of the un-irradiated, 100 kGy-irradiated, and 500 kGyirradiated samples. In all cases, peaks related to C=O (531.5 eV), O-C-O (532.5 eV) and C-OH (533.5 eV) bonds can be observed; however, the amounts of these bonds differ among samples, a clear indication of the effect of EBI on the amount and type of oxygen-containing bonds. In 15 ACS Paragon Plus Environment


particular, the number of C=O bonds (carbonyl group) increases after irradiation at a dose of 100 kGy and then decreases at 500 kGy. It seems that 100 kGy is not sufficient to break C=O bonds, as these bonds need high energy for the separation of C and O atoms. Instead, it may break other bonds such as C=OH, and then C will form additional C=O bonds with oxygen from the air, increasing the number of C=O bonds. In contrast, the energy in the 500 kGy-irradiated sample is sufficient to break high-energy C=O bonds, resulting fewer of these bonds relative to the unirradiated sample. For epoxide bonds (O-C-O), there exists an inverse relationship between the irradiation dose and the number of bonds, whereby increased irradiation leads to more broken bonds and consequently, the amounts of epoxy bonds drops from 26.09% for un-irradiated samples to 22.07% and 18.20% for 100 kGy- and 500 kGy-irradiated samples, respectively. Hydroxyl groups can easily form on the surface of GO through -OH bonding between water vapor and carbon atoms on the surface. Since the number of C-OH bonds increases in conjunction with the irradiation dose, it can be concluded that high-energy broken carbon bonds can easily form bonds with free -OH from the atmosphere. These results show that oxygen exists on the surface of GO primarily in the form of hydroxyl groups.


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Figure 6. XPS O1s spectra of (a) unirradiated, (b)100 kGy-irradiated, and (c) 500 kGy-irradiated GO samples.



UV- Vis Studies The absorbance spectra of GO are shown in Figure 7. The peaks located at 224-240 nm corresponds to π-π* transitions of C=C bonds.43 After EBI, a red shift of the maximum is observed due to the increased number of electron-donating groups.44 The blue shift in GO materials can be related to the strong coupling effect between graphene sheets and the semiconductor nanoparticles or loss of sheets stacking and disordering.45 Accordingly, the observed blue shift in UV spectrum of GO sample irradiated with 500 kGy can be attributed to a loss of GO stacking and conformational disordering, caused by EBI.

Figure 7. UV-Vis spectroscopy of (a) unirradiated, (b) 100 kGy-irradiated, and (c) 500 kGyirradiated samples. 18 ACS Paragon Plus Environment

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Gas Sensing Studies It is commonly accepted that pure GO sensors exhibits a relatively weak and long response and recovery times when used for NO2 detection at room temperature.46 Therefore, we tested our sample to determine the effects of EBI on the response, and response/recovery times of GO sensor. The dynamic response curves of three GO gas sensors towards NO2 gas are shown in Figures 8a-c. Since the resistance of unirradiated and 100 kGy-irradiated GO increased and decreased upon the injection and removal of NO2 gas, respectively, these samples exhibited ntype behavior. Note the decrease in sensitivity after irradiation at 100 kGy. The oxygen functional groups on GO may bind tightly with NO2 molecules, ultimately reducing resistance upon exposure to NO2 gas. The opposite behavior was observed in 500 kGy-irradiated GO, where the resistance decreased and increased upon the injection and removal, respectively, of NO2 gas; this behavior is demonstrating p-type semiconducting conductivity. The introduction of NO2 gas results in the abstraction of electrons from the GO lattice, which increases the number of holes and induces the p-type sensing behavior. Graphene is regarding as a semiconductor with bonding π and antibonding (π*) orbitals which form the valence-band maximum and conduction-band minimum, respectively. Structural changes to graphene can withdraw electrons, resulting in p-type semiconducting behavior;47 therefore, in the case of GO, the origin of p-type behavior can be investigated in light of structural changes after EBI. According to Prades et al.,48 EBI can generate holes via two mechanisms. In the first mechanism, holes are generated via the anion interstitial model:

(2)

𝐸𝐵𝐼 1 𝑂2 (𝑔) → 𝑂𝑖𝑛− + 𝑛ℎ+ 2



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+ Where 𝑶𝒏− 𝒊 is an interstitial oxygen ion, n is the degree of ionization (=1 or 2), and h is a hole.

In the second mechanism, a cation vacancy is created in which holes are created by the following relation:

(3)

𝐸𝐵𝐼

𝐶𝐶 → 𝑉𝐶𝑛− + 𝑛ℎ+ + 𝐶𝑔𝑎𝑠

Where 𝑽𝒏− 𝑪 is a C vacancy and n = 1-4. Because the interstitial sites in the GO lattice are significantly smaller than an oxygen ion, it can be surmised that the creation of cation vacancies must be the primary source of holes.49 In fact, a strong electron beam (such as 500 kGy) can directly reject C atoms. Subsequently, the rejected C is sublimated into the air through the surface, because if it occupies an interstitial site, it will make the GO an n-type semiconductor according to the following reaction:

𝐶𝑐 → 𝐶𝑖𝑛+ + 𝑛𝑒 −

(4)

Therefore, the created holes may play more important role relative to electrons, leading to ptype behavior in GO irradiated at a dose of 500 kGy. It is possible that a relatively small number of holes will be generated via irradiation at a low dose (100 kGy). By help of density functional theory (DFT) calculations, Kim et al.,49 reported that the formation energy of acceptor-type VSn defects in SnO2 nanostructures decreases with an increased electric field. Similarly, in our study, it is likely that 𝑽𝒏− 𝑪 is more readily formed at an irradiation dose of 500 kGy.


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Figure 8c also shows a significantly longer recovery time of GO irradiated with 500 kGy when compared to un-irradiated and 100 kGy-irradiated sensors. The recovery time depends on the mobility of the major carriers, which must return to their original states. In the 500 kGyirradiated GO sensor, carbon vacancies diffuse more slowly than oxygen vacancies, leading to a prolonged recovery time. Recovery times are shorter in the case of un-irradiated and 100 kGyirradiated GO, as the electrons have high mobility.

Figure 8. Dynamic normalized resistance curves towards 10 ppm NO2 at room temperature for sensors fabricated from samples that are (a) unirradiated, (b) irradiated at a dose of 100 kGy, and (c) irradiated at a dose of 500 kGy.



GO exhibits n-type behavior before irradiation (Figure 9a), and upon irradiation with the electron beam, oxygen vacancies, electrons, carbon vacancies, and positive holes are created. The holes are generated due to the newly-formed acceptors. When a low irradiation dose (100 kGy) is applied to GO, holes are created as a result of the creation of carbon vacancies. Oxygen vacancies are also generated; however, the n-type conduction is still dominant owing to an insufficient amount of holes to neutralize the increased oxygen vacancies. In contrast, when a high irradiation dose (500 kGy) is applied to GO, a sufficient number of holes are created and positive holes are prevalent in the GO lattice, therefore GO exhibits p-type behavior. Similar behavior was observed in case of CO gas, which is a reducing gas (Figure S2), demonstrating again the conversion of conductivity of GO from n-type to p-type by EBI.


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Figure 9. (a) Variation in the normalized resistance of sensors that are unirradiated, 100 kGyirradiated, and 500 kGy-irradiated with respect to the introduction and removal of 100 ppm NO2 gas at room temperature. (b) Sensitivities of unirradiated, 100 kGy-irradiated, and 500 kGyirradiated GO sensors.

The conversion between n- and p-type RGO via changes in annealing temperature is determined by the type of residual functional groups.50 The electron-withdrawing groups are comprised of carboxyl, carbonyl, and sp3-bonded hydroxyl, ether, and epoxide groups, whereas the electron-donating groups include sp2-bonded hydroxyl, ether, and epoxide groups. The O/C ratio is reduced by increasing the annealing temperature.50 In addition, the amount of carboxyl and carbonyl groups does not change significantly with variations in annealing temperature. In the present work, the ratio of carbonyl groups (C=O) in the C1s peak increased via EBI at 100 kGy from 9.2 to 27.7%. Upon increasing the irradiation dose from 100 to 500 kGy, the ratio of carbonyl groups (C=O) in the C1s peak decreased from 27.7 to 24.3%. After EBI at 100 kGy, the ratio of carboxyl groups (COOH) in the C1s peak increased from 0 to 0.7%, and upon increasing the irradiation dose from 100 to 500 kGy, the ratio of carboxyl groups in the C1s peak increased from 0.7 to 3.9%. The XPS data in the present work agree well with the XPS data in work by Tu et al.,50 in terms of carbonyl and carboxyl groups. In addition, the number of hydroxyl groups (COH) decreases with higher annealing temperatures. In the present work, the ratio of hydroxyl groups (C-OH) in the O1s peak increased from 68.86 to 70.45% after EBI at 100 kGy, and upon increasing the irradiation dose from 100 to 500 kGy, the ratio of hydroxyl groups in the O1s peak increased from 70.45 to 78.51%. We determine that the hydroxyl groups in the present work are close to the sp3-bonded groups, corresponding to the electron-withdrawing group. As shown in



the Raman spectra in Figure 3, the ID/IG has been increased via EBI, suggesting an enhancement in the number of sp3 groups. We ascribe the decreased sensitivity after irradiation at 100 kGy to a reduced concentration of conduction electrons (Figure 9b). When there is a low concentration of electrons, the initial resistance of the sensor will be relatively high. The resistance increases with the adsorption of oxidizing gas. In case of a sensor with high initial resistance (Ra), the increase of the same amount of resistance (Rg-Ra) by the introduction of NO2 gas will result in reduced sensor response when compared to the case of sensor with low initial resistance. The initial resistances of the un-irradiated, 100 kGy irradiated, and 500 kGy-irradiated sensors are 1160, 1460, and 1080 MΩ, respectively. These are in agreement with XPS results, where oxygen functional groups were increased via EBI at 100 kGy and then decreased upon exposure to EBI at 500 kGy. For 10 ppm NO2, the sensitivity of (Rg-Ra)/Ra of the unirradiated, 100 kGyirradiated, and 500 kGy-irradiated sensors were 0.3, 0.0055, and -0.15, respectively. Note that the negative value of the sensitivity shows the p-type behavior of the sensor. Accordingly, the sensitivity of the GO sensors decreased initially, and then increased with the higher EBI dose. From structural analysis and gas sensing tests, we demonstrated that structural defects and oxygen functional groups have significant roles on the electrical and sensing properties of GO after EBI. The variations in response, and response/recovery times in the presence of NO2 molecules are due to the generation of defects and oxygen functional groups. The sites with high enough energy may increase the effective adsorption of NO2 gas (Figure 10), decreasing the response time, while at the same time, delaying the desorption of those molecules, which result in longer recovery time.


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Figure 10. Effect of defects on the NO2 sensing behavior of EBI GO.

The increased d-spacing in the XRD data of 100 kGy-irradiated samples indicates the formation of oxygen functional groups between the sheets of GO. Accordingly, the sensing behavior of 100 kGy-EBI-treated GO sensor is mainly due to the generation of oxygen functional groups; however, the d-spacing and oxygen content decreased at the higher dose of 500 kGy, resulting in fewer generated oxygen functional groups. Therefore, in this case, carbon defects played a major role sensing behavior. Additional TEM study indicates that noticeable structural 25 ACS Paragon Plus Environment


defects are observed in the GO sensor irradiated with a dose of 500 kGy (Figure 11). Since the unirradiated GO did not show the surface defects (Figure 1), it is possible that the defects have been generated. These findings further enhance the graphene-based detection of chemical species using an optimized dose of EBI.

Figure. 11. (a) Low-magnification TEM micrograph and (b) lattice-resolved TEM micrograph of GO sensor irradiated with 500 kGy dose. The main goal of this study was not to demonstrate the advantage of EBI for NO2 sensing at room temperature. In this paper, we demonstrated the possibility of n- to p-type conversion of GO by EBI. However further experiments with a variety of sensing materials are needed to fully explore the advantages of EBI in regard to gas sensing.


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4. CONCLUSIONS We studied the effects of EBI on the properties and NO2 sensing capability of GO gas sensors. XRD studies revealed that the use of EBI at 100 kGy led to increased d-spacing, while a higher dose of 500 kGy resulted in decreased d-spacing. The results showed that although the use of EBI at a dose rate of 100 kGy led to a greater number of oxygen functional groups, this effect was reversed and resulted in fewer functional groups at the higher dose (500 kGy). Raman spectroscopy results showed occurrence of defects after EBI. Oxygen functional groups played the key role in the sensing behavior of GO exposed to 100 kGy dose, while carbon defects played a major effect in increasing the sensitivity of GO to NO2 gas, exposed to 500 kGy dose. The present study demonstrates the effects of EBI on the structural and electrical properties of GO-based sensors, including inducing p-type conduction behavior in initially n-type GO via irradiation with a high irradiation dose (500 kGy). The behavior of the GO is converted from ntype to p-type using EBI, as a result of hole generation and changes in the amounts of residual functional groups that include carboxyl and hydroxyl groups. These results provide new avenues for the improved control of the sensing behavior of GO-based sensors through EBI.



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ASSOCIATED CONTENT

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b02396. Figure showing schematic of sensor device, Table showing position of (001) peaks in GO and graphite oxide, Figure showing normalized resistance curves of GO,GO irradiated at 100 kGy and GO irradiated at 500 kGy towards 50 ppm CO gas at room temperature.

AUTHOR INFORMATION

Corresponding Authors * P. Wu. E-mail: [email protected] *S. S. Kim. E-mail: [email protected] *H. W. Kim. E-mail: [email protected] Author Contributions S.S.K. and H.W.K. conceived the study, and designed the experiments. ‡S.S.K., §A.

§, Ϩ

H.W.K.,

M and †P.W prepared the manuscript. ϨY. J. K. performed the experiments. All authors have

given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENTS

This research was supported by the Basic Science Research Program through the National Research

Foundation

of

Korea

(NRF)

funded

by

(2016R1A6A1A03013422).


the

Ministry

of

Education


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