Metal-to-insulator transition near room temperature in graphene oxide and graphene oxide + TiO₂ thin films

Abstract

Thin films of graphene oxide (GO) and a composite of graphene oxide with titanium oxide (GO + TiO₂) were prepared via an alternative chemical route based on Hummer's method. The morphology and crystalline structure of the GO and GO + TiO₂ thin films were investigated by X-ray power diffraction (XRD) and scanning electron microscopy (SEM). Anatase TiO₂ nanostructures were incorporated into the bulk and surface of GO. Ultraviolet-visible spectroscopy (UV-vis) and Fourier transform infrared spectroscopy (FTIR) were also performed in order to corroborate with the results obtained by XRD, SEM and TEM. The electrical characterization of GO and GO + TiO₂ TFs was performed using the four-probe van der Pauw method from 400 K down to 77 K. The resistivity, carrier density and carrier mobility in the TFs were determined as a function of temperature. It was found that the conductivity and mobility increased with the incorporation of TiO₂ on GO; however, the concentration of free-carriers decreased. The analysis of the temperature dependence of the resistivity also showed that both films present a metal-to-insulator transition (MIT) near room temperature, at 280 K and 260 K for GO and GO + TiO₂, respectively. Thus, for temperatures near 300 K the carrier density, thermally induced in GO and GO + TiO₂ thin films, is so high that both the systems suffer a Mott MIT, which increases the prospects of using these films in a novel class of electronics, spintronics and photonics.

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Introduction

In order to explore the features of a material and to provide better materials for technological applications, an understanding of the electron transfer behavior is very important for developing electronic devices.

Graphene oxide (GO) and reduced graphene oxide (rGO) have been widely studied in recent years because of their promising applications as energy storage devices,^1–4 transistors^5,6 sensors,^7–10 and solar cells^11–13 and for photocatalytical and photoelectrochemical processes.^14–19 They are also relevant in the field of nanomedicine, for the degradation of estrogens²⁰ and as a material for cellular imaging and drug delivery.²¹

The possible chemical exfoliation routes allow the large-scale production of GO, which is advantageous with respect to the usual graphene production methods.^22–25

The conduction mechanism of GO is strongly dependent on its oxidation level; therefore, it can be considered as an insulator with tunable properties, or as a semiconductor, particularly in the case of rGO.^5,26 Its structure contains hydroxyl and epoxide functional groups on the top and bottom of the sheet and carboxyl and carbonyl groups on the edges.^5,24,26 The electronic properties of rGO are usually associated with the variable-range hopping (VRH) mechanism. However, due to the complexity of the rGO structure, some deviations from the VRH mechanism are found.

Titanium dioxide (TiO₂) is a semiconductor that has been widely studied for several years due to its interesting electrical and optical properties,^2,10,27–29 such as high refractive index, high dielectric constant, and excellent transmittance in the visible range.^27–31 These properties make TiO₂ an important material for optoelectronic devices such as gas sensors^10,27,29,30 and solar cells.^27,30,31

In this study, TiO₂ was incorporated into GO during the chemical exfoliation of graphite, via a modified Hummer's method. Hybrid thin films were produced and the effect of TiO₂ incorporation on GO was evaluated. TiO₂ can promote an enhancement in the reduction of GO to rGO, thus improving its electrical conductivity.

Recently, hybrid materials made of carbon-based materials, due to their inherent high thermal conductivity,^36–38 have been studied and developed for applications as electromagnetic wave absorbers.^32–35

Experimental

Synthesis of GO and GO + TiO₂

In order to evaluate the properties of GO, we added TiO₂ to the synthesis of GO via the modified Hummer's method.^39,40

In a beaker placed in a water bath at 5 °C, 1000 mg of graphite powder (Synth®) were mixed with 750 mg of sodium nitrate NaNO₃ (Aldrich) and 64 g of sulfuric acid H₂SO₄ (Vetec®). After 30 min of magnetic stirring, 4500 mg of potassium permanganate KMnO₄ (Aldrich®) were gradually added. The mixture was allowed to stir for 3 days, and then 100 mL of a 10% aqueous H₂SO₄ : H₂O solution were added and allowed to stir for 2 hours. Subsequently, 250 mL of a 10% aqueous solution of hydrogen peroxide H₂O₂ (Vetec®) were added and left to stir for 3 days to give a yellowish mass. The solution was filtered and the obtained product was dried in a vacuum oven for 24 h at 60 °C. The product was washed with 1 L of a 20% aqueous HCl solution (Vetec®), filtered, then washed with 1 L of deionized water and subsequently placed to dry in a vacuum oven for 24 hours at 60 °C. Finally graphene oxide (GO) was obtained as a powder.

For the GO + TiO₂ nanocomposite the route was the same, other than the composition of the initial mixture. In a beaker placed in a water bath at 5 °C, 1000 mg of graphite powder (Synth®) were mixed with 1000 mg of titanium dioxide TiO₂ (Aldrich), 1000 mg of sodium nitrate NaNO₃ (Aldrich®) and 70 g of sulfuric acid (Vetec®). The further steps were identical to the ones previously described for GO. Finally, graphene oxide (GO) with TiO₂ incorporated in the bulk and in the surface was obtained as a powder.

Sample characterization

The morphology of the samples was studied by scanning electron microscopy (SEM – Zeiss EVO® MA15) and transmission electron microscopy (TEM – JEOL JEM® 1200EX-II). Fig. 1 shows the SEM images for GO (Fig. 1(A)) and GO + TiO₂ (Fig. 1(B)) films. Fig. 2 shows the TEM images for GO (Fig. 2(A)) and GO + TiO₂ (Fig. 2(B)) films. Fig. 2(B) displays the incorporation of TiO₂ into GO, seen as white crystals onto the surface of GO. TEM images were also observed in order to show that the incorporation of TiO₂ into GO is not only on the surface, but also in the bulk, seen as black particles on top and inside of GO. The incorporation of TiO₂ into GO is so intense/stable that even in an aqueous solution stirred for several days, or in an ultrasonic bath for several hours, no isolated particles of TiO₂ were observed in the TEM grid, displayed only those attached to GO.

Fig. 1 SEM images of GO (A) and GO + TiO₂ (B) thin films.

Fig. 2 TEM images of GO (A) and GO + TiO₂ (B) thin films.

For the structural characterization, X-ray diffraction (XRD – Shimadzu XRD-700®) was performed to identify the crystallographic phases present in the samples. UV-vis absorbance (UV-vis) with a FEMTO 800-XI® spectrophotometer and Fourier transform infrared spectroscopy (FTIR) with a Varian 640-IR® spectrophotometer were also carried out.

Electrical characterization

In the present study, the electrical DC transport was measured using the van der Pauw method⁴¹ in order to reduce contact effects. The electrical transport properties of the samples were studied by measuring the Hall effect (with a Keithley® 7100 Hall effect card), and the electrical resistivity (Keithley® 7100 Hall effect card, Keithley® 196, Keithley® 220), in the dark and as a function of the temperature in the range of 77–400 K. The samples were measured inside a CF104 cryostat equipped with an ITC 503 Oxford® temperature controller.

Results and discussion

XRD

Fig. 3 shows the XRD spectra of GO (black line) and GO + TiO₂ (blue line) thin films. The GO spectra exhibits two peaks at 2θ ≈ 10.5° and 20°. The first peak is correlated to an interlayer spacing of 0.85 nm in the layer-like GO. The peak at 20° is broad and weak and is attributed to reduced graphene oxide, rGO.¹¹ For GO + TiO₂, zone axes (101), (103), (004), (112), (200), (105), (211), (204), (116), (301) and (215) were observed, corresponding to the anatase phase of TiO₂.¹¹ In the inset of Fig. 3, the UV-vis spectra of GO (black line) and GO + TiO₂ (blue line) is shown. The GO spectrum exhibits a peak at 233 nm due to the π → π* transitions of aromatic C–C and C=C bonds, and a shoulder at 300 nm due to the n → π* transitions of C=O bonds.⁴² GO–TiO₂ shows absorbance intensity in the range of 240–450 nm with a peak at 350 nm, attributed to the strong absorption of TiO₂, and a shoulder at 250 nm, indicating the interaction between TiO₂ and GO.⁴²

Fig. 3 XRD patterns of GO (black) and GO + TiO₂ (blue). In the inset, UV-vis absorption spectra of GO (black) and GO + TiO₂ (blue) are displayed.

FTIR

Fig. 4 shows the spectra for GO and GO + TiO₂, obtained from the Fourier transform infrared spectroscopy (FTIR) studies. The GO spectrum shows a broad and intense band at 3000–3500 cm⁻¹ (corresponding to the O–H stretching vibrations from hydroxyl groups⁴³), 2984 cm⁻¹ (C–H stretching vibrations⁴⁴), 1729 cm⁻¹ (C=O stretching vibrations from carbonyl and carboxyl groups⁴⁵), 1629 cm⁻¹ (C=C stretching vibrations, skeletal vibrations from non-oxidized graphene domains),⁴⁶ 1380 cm⁻¹ (O–H stretching vibrations from hydroxyl groups),⁴⁷ 1227 cm⁻¹ (C–OH stretching vibrations, breathing vibrations from epoxy groups⁴⁸), and 1037 cm⁻¹ (C–O stretching vibrations⁴⁹). The GO + TiO₂ spectrum shows the corresponding small displacements and a broad band between 600 and 1000 cm⁻¹, characteristic of Ti–O stretching.⁵⁰

Fig. 4 FTIR spectra of GO (black) and GO + TiO₂ (blue).

Variable range hopping (VRH) conduction

Fig. 5 shows the electrical resistivity ρ(T) of GO and GO + TiO₂ TFs as a function of the temperature, between 77 K and 400 K. Kaiser⁴⁹ and Joung⁵¹ already reported that electrical conduction in rGO can be explained by Mott variable-range hopping (Mott-VRH) between localized states. In the 2D Mott-VRH model the temperature dependence of the conductivity can be described as follows:^52,53where T_M is the Mott temperature, e is the elementary charge, R_o is the optimum hopping distance, ν_ph is the frequency of the phonons associated with the hopping process, k_B is the Boltzmann constant, α is the wave function decay constant, and N(E_F) is the density of states near the Fermi level.

Fig. 5 Temperature dependence of the electrical resistivity ρ(T) for (A) GO, and (B) GO + TiO₂ thin films. In the inset, Arrhenius plot showing the metal to insulator transition at 280 K and 260 K for GO and GO + TiO₂, respectively is displayed. The red lines correspond to the fitting of the experimental data using eqn (1).

In the inset of Fig. 5, we show the VRH plot of ln(σ) vs. T^−1/3 in the temperature range of 280–100 K for GO (Fig. 5(A)) and 260–140 K for GO + TiO₂ (Fig. 5(B)). The red lines correspond to the fitting of the experimental data using eqn (1). A summary with the most important obtained hopping parameters is presented in Table 1.

Table 1 Summary of hopping parameters: Mott conductivity (σ_0,M), Mott temperature (T_M), density of states near the Fermi level (N(E_F)), optimized hopping distance (R_o) and critical carrier density (n_MIT)

Sample	σ0,M (10^−5 Ω cm^−1)	TM (10^3 K)	N(EF) (10^13 cm^−2/eV)	Ro (nm)	nMIT (10^19 cm^−3)
GO	1.695	4.607	18.8	0.164	2.831
GO + TiO2	1.855	1.454	60.0	0.172	2.474

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Metal to insulator transition – MIT

The Bohr radius can be assigned as an estimate for the length of the localized states, and can be calculated by:

a_Bohr = (4πεh_c²/m*e²) (3)

where ε is the dielectric constant of the material (for GO ε = 3ε_o),⁵⁴ h_c is Planck's constant, m* is the effective mass (for GO we consider m* = 1 × me) and e is the elementary charge.

The value obtained for the Bohr radius (0.159 nm) is very close to the optimum hopping distance obtained for GO and GO + TiO₂, which is 0.164 and 0.172 nm, respectively. This result can indicate that both GO and GO + TiO₂ could have very similar MIT temperatures, which experimentally can be observed at 280 K for GO and at 260 K for GO + TiO₂.

Mott⁵⁵ pointed out that a system suffers a metal to insulator transition when the electrons are in a narrow band, for which the orbitals can be described by the tight-binding method as consisting of atomic wave functions. The critical carrier density (n_MIT) at which a MIT should take place can be determined by the following equation:⁵⁵

(n_MIT)^1/3a_Bohr = const. (4)

where a_Bohr is the Bohr radius defined by eqn (3), and the constant is estimated to be 0.05.⁵⁵

If we consider the optimum hopping distance as the localization length of the defects, we can estimate the critical density of defects n_MIT for each sample, using a_Bohr = R_o in eqn (4). For GO we expected a MIT transition of approximately 2.8 × 10¹⁹ cm⁻³, and for GO + TiO₂ at 2.5 × 10¹⁹ cm⁻³. Experimentally, the MIT transition occurs at 280 K and 260 K for GO and GO + TiO₂, or at carrier densities of 7.52 × 10²⁰ cm⁻³ or 5.6 × 10²⁰ cm⁻³, respectively.

Due to the existence of a near room temperature metal to insulator transition, GO and GO + TiO₂ are potential candidates for a novel class of electronics and photonics applications, such as field effect switches, memristive devices, electronic oscillators and spintronics.⁵⁶

The presence of a Mott MIT is also an indication that at a lower temperature range (above 77 K) it is expected that GO and GO + TiO₂ would undergo a transition from Mott-VRH to Efros–Shklovskii VRH conduction, when the Coulomb gap (soft gap) that cannot be neglected appears in the density of states, at the Fermi level.⁵⁷

In order to better understand the transport properties of GO and GO + TiO₂, Hall effect measurements were performed at the same temperature range. Fig. 6(A) shows the free carrier density n_Hall as a function of the temperature. The n_Hall is of the order of 10²⁰ cm⁻³ for both materials, although throughout the temperature range the carrier density in GO was higher than in GO + TiO₂. The free carrier mobility μ_Hall is given by the following equation:

σ(T) = n_Hallμ_Halle → μ_Hall = [q_oρn_Hall]⁻¹ (5)

where ρ is the resistivity, n_Hall is the free carrier density and q_o is the elementary charge.

Fig. 6 The temperature dependence of the (A) free carrier density n_Hall(T) and (B) free carrier mobility μ_Hall(T), for GO (black dots) and for GO + TiO₂ (blue dots).

Fig. 6(B) shows the free carrier mobility n_Hall as a function of the temperature μ_Hall(T). Throughout the temperature range the carrier mobility in GO + TiO₂ was higher than in GO. For highly doped semiconductors (metal-like), the mobility increases as the temperature drops due to a reduction in electron–electron scattering. However, GO exhibits the opposite trend, due to a reduction in electron-lattice scattering, and we expected a decrease of the mobility as the temperature dropped.⁵⁸ In Table 2 there is a summary of the electrical parameters for the GO and GO + TiO₂ thin films.

Table 2 Summary of the thickness (t), crossover MIT temperature (T_MIT) and the highest value of free carrier density (n_Hall) and free carrier mobility (μ), obtained at 400 K and 200 K, respectively

Sample	t (nm)	TMIT (K)	pmáx,400 K (10^20 cm^−3)	μmáx,200 K (cm^2 V^−1 s^−1)
GO	723	180	11.4	153
GO + TiO2	980	160	8.6	318

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Conclusions

In conclusion, we show a comparative study between graphene oxide (GO) and graphene oxide–titanium oxide (GO + TiO₂) composite thin films prepared by a modified Hummer's method. XRD, FTIR and UV-vis absorption showed that the incorporation of TiO₂ on GO was successful. Electrical characterization of the materials was carried out and the temperature dependence of the resistivity ρ(T), carrier density n_Hall(T) and carrier mobility μ_Hall(T) was determined for the films. The GO film presented a higher carrier density and resistivity, and consequently a lower mobility than that of the GO + TiO₂ film. The analysis of the temperature dependence of the resistivity also showed that both films present a near room temperature metal-to-insulator transition (MIT), at 280 K and 260 K for GO and GO + TiO₂, respectively. Thus, for temperatures near 300 K the thermally induced carrier density in the films is so high that both systems suffer a Mott MIT transition. The presence of a near room temperature MIT makes GO and GO + TiO₂ potential candidates for a novel class of electronics and photonics applications, such as field effect switches, memristive devices, electronic oscillators and spintronics.

Acknowledgements

The authors would like to acknowledge the “Centro Multi-usuário de Caracterização de Materiais – CM² (UTFPR)”, “Centro de Microscopia Eletrônica – CME (UFPR)”, Prof. Dr Julio Cesar Klein das Neves, Prof. Dr Joao Batista Floriano and Alexandre José Gonçalves of Universidade Tecnológica Federal do Paraná for providing the equipment and technical support for experiments involving SEM, TEM microscopy and XRD, UV-vis and FTIR measurements. The financial support of the Brazilian funding agencies CAPES, CNPq, FAPEMIG and Fundação Araucária is also acknowledged.

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