Pyrolysis of Titanicone Molecular Layer Deposition Films as

Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215, United States. ‡ Department of Mechanical Engineerin...
17 downloads 3 Views 4MB Size
Download PDF
Article pubs.acs.org/JPCC

Pyrolysis of Titanicone Molecular Layer Deposition Films as Precursors for Conducting TiO2/Carbon Composite Films Aziz I. Abdulagatov,† Kalvis E. Terauds,‡ Jonathan J. Travis,† Andrew S. Cavanagh,† Rishi Raj,‡ and Steven M. George*,†,‡ †

Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215, United States Department of Mechanical Engineering, University of Colorado, Boulder, Colorado 80309-0427, United States

‡

ABSTRACT: Titanium alkoxide films known as “titanicones” were grown using molecular layer deposition (MLD) techniques using the sequential exposure of TiCl4 and glycerol. These titanicone MLD films were then pyrolyzed under argon to yield conducting TiO2/carbon composite films. The Raman spectra of the pyrolyzed titanicone films revealed the characteristic “D” and “G” peaks associated with sp2-graphitic carbon. X-ray diffraction analysis of the pyrolyzed titanicone films displayed the signatures for anatase and rutile TiO2 after heating to 600 °C and then only rutile TiO2 after heating to 900 °C. X-ray photoelectron depth profiling of the pyrolyzed titanicone films showed that the carbon was distributed throughout the film and began to segregate to the surface after heating to 900 °C. The sheet resistance of the pyrolyzed titanicone films dropped dramatically versus pyrolysis temperature and reached a minimum sheet resistance of 2.2 × 104 Ω/□ after heating to 800 °C. On the basis of the measured film thickness of 88 nm, the resistivity of the pyrolyzed titanicone film after heating to 800 °C was ρ = 0.19 Ω cm. Segregation of other hybrid organic− inorganic films into sp2-graphitic carbon and metal oxide domains after pyrolysis under argon was also observed for alucone films and various metalcone films based on Zn, Zr, Hf, and Mn. The conducting TiO2/carbon composite films and other metal oxide/ carbon composite films could have important electrochemical applications as electrodes for Li ion batteries or pseudocapacitance supercapacitors. varied over a wide range.10,11 The properties were consistent with “rule of mixtures” approximations.10,11 The hybrid organic−inorganic MLD films can also be used as precursors for metal oxide/carbon composite or porous metal oxide films. Pyrolysis under argon can produce metal oxide/ carbon composite films. Annealing in O2 can produce porous metal oxide films.12,13 This general strategy is shown in Figure 1. This strategy is similar to the fabrication of ceramics from hybrid organic−inorganic polymers in polymer-derived ceramic processing.14 For the hybrid organic−inorganic MLD films, the resulting ceramics can be derived from the wide range of elements that can form the metalcone MLD films.15 In the field of polymer-derived ceramics (PDC), the resulting ceramics are predominantly based on SiO2.16 An important advantage of using metalcone MLD films as precursors to ceramic films is that they are deposited conformally on the starting substrate.3 Consequently, the resulting metal oxide/carbon composite after pyrolysis under argon will also be conformal to the initial substrate. The result is a high surface area metal oxide/carbon composite film when the starting material has a high surface area. These high surface

I. INTRODUCTION Many hybrid organic−inorganic films can be deposited using molecular layer deposition (MLD) techniques.1,2 These films utilize sequential, self-limiting surface chemistry to deposit films with atomic layer control that are conformal to the initial substrate. The “alucones” are one type of hybrid organic− inorganic film that can be formed by the reaction of trimethylaluminum (TMA) and various organic alcohols such as ethylene glycol (EG) to produce aluminum alkoxide polymer films.3 The “zincones” and “titanicones” are other types of hybrid organic−inorganic films that are formed by the reaction of diethylzinc (DEZ) or titanium tetrachloride with various organic alcohols such as EG, glycerol (GL), or hydroquinone (HQ).4−7 These reactions produce zinc alkoxide and titanium alkoxide polymer films, respectively. The hybrid organic−inorganic films have properties that are intermediate between organic and inorganic films. The elastic modulus of Al2O3 ALD films is E = 195 GPa.8 The elastic modulus of alucone MLD films is much lower at 37 GPa.9 Organic polymers typically have elastic moduli in the range from 1 to 4 GPa. Al2O3 ALD and alucone MLD can also be used to form ALD:MLD alloys that have properties that vary depending on the fraction of organic constituent in the hybrid film.10 Recent work showed that the density, refractive index, elastic modulus, and hardness of ALD:MLD alloys could be © 2013 American Chemical Society

Received: May 26, 2013 Revised: July 27, 2013 Published: July 30, 2013 17442

dx.doi.org/10.1021/jp4051947 | J. Phys. Chem. C 2013, 117, 17442−17450

The Journal of Physical Chemistry C

Article

Aldrich), an organic diol.4 Ultrahigh purity grade nitrogen (99.999%, Airgas, Colorado Springs, CO) was used as the purge and carrier gas in the reactor. The reactor base pressure was ∼1 Torr at a N2 gas flow rate of 195 sccm. Titanicone films grown using TiCl4 and GL are denoted as TiGL. The titanicone films grown using TiCl4 and EG are denoted as TiEG. The TiGL and TiEG films were deposited at 150 and 115 °C, respectively.4 The reactant timing sequences for growth of TiGL and TiEG films were (t1, t2, t3, t4) where t1 is the TiCl4 exposure, t2 is the N2 purge time, t3 is the organic alcohol exposure time, and t4 is the N2 purge time. TiGL and TiEG were grown using timing sequences of (1, 35, 1, 35) and (5, 35, 8, 35), respectively, where the times are in seconds. Silicon wafers with dimensions of 2.5 cm × 2.5 cm were used as the substrates. The growth rates of the titanicone MLD films were examined using in situ quartz crystal microbalance (QCM) measurements and ex situ X-ray reflectivity analysis in previous studies.4 The QCM measurements observed growth rates for TiGL that varied slightly with temperature from 49 ng/(cm2 cycle) at 130 °C to 34 ng/(cm2 cycle) at 210 °C. The growth rates for TiEG were ∼83 ng/(cm2 cycle) from 90 to 115 °C before decreasing significantly at 135 °C. The QCM measurements also revealed that the surface chemistry for titanicone MLD was self-limiting. Dose times of 1 s for GL and 5 s for EG were sufficient for selflimiting surface reactions. X-ray reflectivity (XRR) studies also demonstrated that TiGL has a growth rate of 2.2 Å/cycle at 150 °C with a film density of 1.8 g/cm3. TiEG has a growth rate of 4.5 Å/cycle at 115 °C with a film density of 1.84 g/cm3. Pyrolysis of the titanicone MLD films was performed at temperatures from 500 to 900 °C in a high-temperature furnace (Centorr M-60, Centorr Vacuum Industries, Nashua, NH). Prior to pyrolysis, the furnace was pumped down with a diffusion pump and then backfilled with ultrahigh-purity argon (Airgas, Colorado Springs, CO) to a pressure slightly higher than atmospheric pressure. The concentration of O2, H2O, CO, and CO2 impurities in the argon was below 3 ppm. The heating and cooling rate of the samples was 5 °C/min. The samples were held at the pyrolysis temperatures for 1 h. Electrical measurements were performed on pyrolyzed titanicone MLD films deposited on Si wafers containing a 450 nm thick thermal SiO2 layer on the Si wafer. This thermal oxide was insulating and prevented any conductivity from the underlying Si wafer. The resistance was measured using fourpoint probe techniques. The four-point probe setup consisted of mounting stand (Signatone model: S-301-6, Signatone Corp., Gilroy, CA) and probe head (Signatone model: SP4− 40045TFS, Signatone Corp., Gilroy, CA) equipped with a current source and measuring instrumentation (Keithley 2400 SourceMeter, Keithley Instruments, Solon, OH). The sheet resistance obtained by the four-point probe measurements and the film thickness were used to determine the film resistivity. Raman spectra were recorded on a Micro-Raman spectrometer (LabRAM HR Evolution, HORIBA Jobin Yvon, Japan). The spectra were recorded at room temperature in ambient atmosphere. For excitation at 532 nm, an Ar+ laser was used with a focused laser spot size of ∼5 μm. The samples were measured with an incident laser power of 1 or 50 mW. All of the scanning electron microscopy (SEM) images shown in this work were obtained with a beam voltage and current of 3.0−5.0 keV and 1−10 A, respectively. Film crystallinity was determined by grazing incidence X-ray diffraction (GIXRD) scans performed using an X-ray

Figure 1. Strategy to obtain either metal oxide/carbon composite films or porous metal oxide films from hybrid organic−inorganic MLD films by pyrolyzing under argon or by annealing in O2, respectively.

area films may have key applications in electrochemical reactions and electrochemical energy storage. In this paper, the titanicone MLD films are grown primarily using TiCl4 and GL as the reactants as illustrated in Figure 2.4

Figure 2. Surface chemistry for titanicone MLD using TiCl4 and glycerol as the reactants.

These titanicone MLD films were then pyrolyzed under argon to yield TiO2/carbon composite films. Metal oxides like TiO2 typically have high band gaps and low electrical conductivity that limit their electrochemical ability. In contrast, the metal oxide/carbon composites can have high electrical conductivity because of the electron conductivity of sp2-carbon. Metal oxide/carbon composites will enable the electrochemical activity of metal oxides. This paper characterizes the formation and properties of TiO2/carbon composite films formed from titanicone MLD films by pyrolysis under argon. The formation of other metal oxide/carbon composite films is also demonstrated by pyrolysis under argon of various hybrid organic−inorganic films.

II. EXPERIMENTAL SECTION Metalcone MLD was performed in a viscous-flow, hot-wall-type reactor described in detail elsewhere.4,17 The films were deposited using titanium tetrachloride (TiCl4, 98% pure, Strem Chemicals Inc.). The TiCl4 was handled in a glovebag due to its air and moisture sensitivity. Titanicone MLD films were grown using glycerol (GL) (C3H8O3, ≥99.5%, SigmaAldrich), an organic triol.4 Other titanicone MLD films were grown using ethylene glycol (EG) (C2H6O2, ≥99.8%, Sigma17443

dx.doi.org/10.1021/jp4051947 | J. Phys. Chem. C 2013, 117, 17442−17450

The Journal of Physical Chemistry C

Article

diffractometer (Bede D1, Bede Scientific Inc., Englewood, CO). The X-ray diffractometer was equipped with a Cu X-ray tube and monochromator for Cu Kα radiation at λ = 1.54 Å. The filament current was 40 mA, and the voltage was 40 kV. For the GIXRD, the incident angle was ω = 0.5° and the detector (2θ) was scanned from 20° to 80°. X-ray photoelectron spectroscopy (XPS) survey scans were performed using a Physical Electronics PHI Model 5600 spectrometer with a monochromatic Al Kα X-ray source with an energy of 1486.6 eV. Film analysis was conducted in an ultrahigh-vacuum chamber with a base pressure of 1 × 10−10 Torr equipped with an Ar ion sputtering source for depth profiling. The scans were performed with an electron pass energy of 58.7 eV and a step size of 0.25 eV. The photoelectrons were collected using a hemispherical analyzer. XPS depth profile experiments measured the relative concentration of titanium, oxygen, and carbon atoms in the films. Depth profiles were performed with a Physical Electronics PHI Model 04-303A differentially pumped ion gun. During sputtering, the pressure in the ion gun was 1 × 10−4 Torr and the pressure in the vacuum chamber was 2 × 10−8 Torr. The Ar ion beam was rastered over the sample to yield a sputtered region of 5 × 5 mm2. The ion gun was operated at a beam voltage of 3.5 kV and an emission current of 15 mA. Survey experiments were also conducted on a variety of other hybrid organic−inorganic MLD films. The metal and organic precursors used to grow these films were obtained from SigmaAldrich. The metal precursors were trimethylaluminum (Al(CH3)3, 97%), diethylzinc (52.0 wt %), bis(ethylcyclopentadienyl)manganese(II) (Mn(C5 H 4 C 2 H 5 ) 2 , ≥98%), zirconium tetra-tert-butoxide (Zr(OC(CH 3 ) 3 ) 4 , 99.999%), and tetrakis(diethylamino)halfnium (Hf(N(CH3)2)4, 99.99%). These hybrid organic−inorganic MLD films were grown using glycerol, ethylene glycol, hydroquinone (C6H6O2, ≥99%), or tetrafluorohydroquinone (C6F6O2, ≥98%). The hydroquinone and tetrafluorohydroquinone films are denoted using HQ and FHQ, respectively. The AlEG, AlGL, AlHQ, AlFHQ, ZnEG, ZnHQ, ZrEG, and HfEG hybrid organic− inorganic films were deposited at 150 °C using conditions reported previously.6,11,15

Figure 3. Raman spectra of titanicone MLD films grown using TiCl4 and glycerol with initial thickness of 500 nm after pyrolysis under argon at 600, 700, 800, and 900 °C for 1 h.

due to double resonant Raman scattering.20,21 Raman spectra of various carbonaceous materials such as pyrolytic graphite, activated charcoal, or glassy carbon also exhibit peaks at around 1594 and 1324 cm−1.22 A second pair of peaks located at 2646 and 2906 cm−1 are referred to as the G′ band overtone of the D band and a D + G combination mode, respectively.23 The intensity of the G peak is higher than the D peak at 600 °C. The G and D peak intensities are almost equal at 700 °C. In the past, a similar trend in Raman spectra was observed with increase in pyrolysis temperature for organic thin films such as photoresist, polyparaphenylene, phenylcarbyne polymers, or hydrogenated carbon prepared by plasma chemical vapor deposition. 24−27 This temperature region 600−700 °C coincides with dehydrogenation of organics and hydrogenated carbon during the pyrolysis process.28−30 Higher pyrolysis temperatures resulted in the G band position shifting slightly from 1583 cm−1 at 600 °C to a higher frequency of 1598 cm−1 at 900 °C. Concurrently, the D band shifted from 1332 cm−1 at 600 °C toward a lower frequency of 1324 cm−1 at 900 °C. The frequency shift and decrease in the full width at half-maximum (fwhm) of the G peak are similar to previous observations for hydrogenated amorphous carbon pyrolyzed from 400 to 1000 °C.20 A narrowing and upward frequency shift of the G peak is associated with the growth in size and/or number of graphite crystallites.31 B. X-ray Diffraction of Pyrolyzed Titanicone Films. The GIXRD scans are sensitive to the long-range order in the pyrolyzed titanicone MLD films. Figure 4 shows GIXRD diffractograms of the as-deposited TiGL film and the TiGL film pyrolyzed under argon at various temperatures. The titanicone MLD films had an initial thickness of 500 nm. The as-deposited TiGL film had an amorphous structure. Weak diffraction peaks began to appear at 2θ values of 24.6° and 55.4° at 600 °C, corresponding to (101) and (211) diffraction peaks from the anatase form of TiO2.32 The crystallization temperature of 600 °C is higher than the temperature of 300−400 °C that has been reported for the appearance of anatase TiO2 peaks from annealed TiO2 gel and amorphous TiO2 thin films.33−36 Inhibition of the crystallization temperature for TiO2 was also observed during

III. RESULTS AND DISCUSSION A. Raman Spectroscopy of Pyrolyzed Titanicone Films. Raman spectroscopy was used to monitor the state of the carbon versus pyrolysis temperature. Raman spectra of TiGL films pyrolyzed at 600, 700, 800, and 900 °C under argon are shown in Figure 3. These titanicone films had an initial thickness of 500 nm. The Raman spectra were recorded between 1050 and 3000 cm−1 and baseline-corrected at 2000 cm−1 to compensate for background photoluminescence. The Raman spectra show a distinct pair of peaks at 1600 and 1330 cm−1 that appeared as result of pyrolysis and that increased in intensity with pyrolysis temperature. These prominent peaks at 1600 and 1330 cm−1 are usually designated G (graphitic) and D (disordered), respectively.18 These peaks are commonly observed in the spectra of amorphous carbon materials containing nanocrystalline graphitic domains. The G and D peaks can be attributed to sp2carbon. The G peak is believed to be derived from band stretching of all pairs of sp2-carbon in both rings and chains.19 The D peak is associated with the breathing modes of sp2carbon atoms in rings within disordered carbonaceous structure 17444

dx.doi.org/10.1021/jp4051947 | J. Phys. Chem. C 2013, 117, 17442−17450

The Journal of Physical Chemistry C

Article

Figure 4. Grazing incidence X-ray diffraction scans for titanicone MLD films grown using TiCl4 and glycerol with initial thickness of 500 nm after pyrolysis under argon at 600, 700, 800, and 900 °C for 1 h.

pyrolysis of titanium alkoxide gels.37 This effect was explained by hindrance of crystallization by organic functional groups that constrain amorphous TiO2 in a carbon matrix. Further increase of the pyrolysis temperature to 700 °C produced small peaks corresponding to rutile TiO2. By 900 °C, the anatase phase disappeared and the sample showed only well-defined peaks at 2Θ values of 27.28°, 35.9°, 38.7°, 41.09°, 43.7°, and 68.5°, corresponding to the (110), (101), (200), (111), (210), and (301) diffraction peaks from rutile TiO2.38 For samples treated at 600 and 700 °C, the diffraction peaks are very broad, indicating titanium oxide crystallites of nanometer size. At 800 °C, the most pronounced rutile TiO2 peak at 2θ = 27.15° is consistent with a TiO2 nanocrystallite size of ∼6 nm estimated from the Scherrer equation. At 900 °C, the rutile TiO2 peak at 2θ = 27.28° became narrower and was consistent with TiO2 nanocrystallite diameters of ∼12 nm. Sharpening of the peaks with temperature indicates an increase in the TiO2 crystallite size. The identification of the initial nanocrystals growing in the amorphous bulk material may not be possible because of the detection limits of XRD analysis. GIXRD analysis was also performed on thinner TiGL films with an initial thickness of 115 nm that were pyrolyzed at the same temperatures and under the same conditions as the 500 nm films. These titanicone MLD films showed diffraction peaks starting at 700 °C and the presence of only rutile TiO2. The absence of the anatase TiO2 phase suggests that the anatase TiO2 crystallites are below the detection limit in these thinner TiGL films. C. Optical and SEM Imaging of Pyrolyzed Titanicone Films. The surfaces of the TiGL films pyrolyzed at all temperatures were smooth, uniform, and free of cracks. A slight change in color was noticed compared with the color of the deposited films. This change was attributed to a change in film thickness and density. An optical image showing the surface of a TiGL film after pyrolysis under argon at 800 °C is shown in Figure 5a. This film had an initial film thickness of 500 nm. Figure 5a shows that the pyrolyzed film has no cracks or defects. Formation of cracks is a common problem for liquid phase coated polymer-derived ceramics.16 The surface morphology of a pyrolyzed TiGL film at 800 °C was also examined in detail using SEM. The SEM image of this film is shown in Figure 5b. This high-resolution SEM image

Figure 5. Images of the surface of a titanicone MLD films grown using TiCl4 and glycerol with initial thickness of 500 nm after pyrolysis under argon at 800 °C for 1 h: (a) optical image and (b) scanning electron microscope image.

shows that the film is uniformly covered with nanoparticles having an average diameter of ∼30 nm. XPS analysis shows that these nanoparticles are composed of TiO2. This particle size is larger than the estimated TiO2 crystallite size of ∼6 nm from GIXRD measurements. The TiO2 nanoparticles on the surface may form because of the presence of a trace amount of oxygen in the argon during the heat treatment. This oxygen may react with carbon in the TiO2/C composite film and leave behind the TiO2 nanoparticles. D. X-ray Photoelectron Spectroscopy of Pyrolyzed Titanicone Films. XPS depth profiles were performed on the pyrolyzed titanicone MLD films. The XPS depth profile for a TiGL film pyrolyzed under argon at 800 °C is shown in Figure 6a. This TiGL film had an initial thickness of 500 nm. On the surface, the XPS monitors high % atomic concentrations for Ti and O. These XPS signals are consistent with TiO2 particles on the surface of the film observed by SEM in Figure 5b. Carbon is distributed throughout the bulk of the film. There is some increase in the carbon concentration at the film surface after pyrolysis at 800 °C. The results of XPS depth profile measurements of a TiGL film pyrolyzed under argon at 900 °C are shown in Figure 6b. This TiGL film also had an initial thickness of 500 nm and a 17445

dx.doi.org/10.1021/jp4051947 | J. Phys. Chem. C 2013, 117, 17442−17450

The Journal of Physical Chemistry C

Article

Gibbs free energy change for this reaction is ΔG ≤ 0 only at temperatures T ≥ 1400 °C.42 The XPS depth profile results are also inconsistent with additional oxygen loss resulting from carbothermal reduction at higher pyrolysis temperatures. Figure 6b displays a lower Ti:O ratio of ∼3:4 after the pyrolysis of TiGL films at 900 °C. E. Electrical Properties of Pyrolyzed TiGL Films. The electrical properties of the pyrolyzed TiGL films were measured using four point probe measurements. Figure 7 shows the sheet

Figure 7. Sheet resistance versus pyrolysis temperature for titanicone MLD films with an initial thickness of 115 nm that were grown using TiCl4 and glycerol.

resistance versus pyrolysis temperature. The initial TiGL films had a thickness of 115 nm. These TiGL films were deposited on silicon wafers that had a SiO2 thermal oxide with a thickness of 450 nm. The TiGL films were pyrolyzed under argon at temperatures from 500 to 900 °C. Figure 7 reveals that the sheet resistance decreases dramatically from 500 to 700 °C. The as-deposited TiGL sample was too resistive, and the sheet resistance could not be measured using the four-point probe technique. The lower sheet resistance versus pyrolysis temperature in Figure 7 is attributed to dehydrogenation and formation of sp2carbon in the pyrolyzed film. The carbon segregates and forms a TiO2/C composite film. A similar evolution in electrical resistance has been monitored for the pyrolysis of silica-based organic−inorganic hybrids. In these studies, a 5 orders of magnitude decrease in resistivity upon heat treatment in argon is observed from 500 to 1050 °C.43 Carbon content and interconnectivity of the carbon network were identified as the origin of the electrical properties.43 In addition, the electrical resistance also drops dramatically during the pyrolysis of pure organic polymers.14,27 The lower resistance versus pyrolysis temperature in Figure 7 is attributed to facile electron transport by percolation through the conducting sp2-carbon network in the TiO2/C composite film. For pyrolysis temperatures above 700 °C in Figure 7, the sheet resistance continues to decrease and reaches a minimum of 2.2 × 104 Ω/□ after heating to 800 °C. The sheet resistance then increases slightly after pyrolysis at 850 and 900 °C. The film thickness of the pyrolyzed titanicone film was measured after heating to 800 °C. This measurement was performed

Figure 6. X-ray photoelectron spectroscopy depth profiles of titanicone MLD films after pyrolysis under argon for 1 h at (a) 800 and (b) 900 °C. The initial titanicone MLD films had a thickness of 500 nm and were grown using TiCl4 and glycerol.

thickness after pyrolysis of 280 nm. The thicknesses were determined using SEM. Based on the sputtering time and the film thickness, the estimated sputtering rate for the pyrolyzed TiGL film was 0.024 nm/s. The XPS depth profile results show that carbon is still distributed throughout the film. However, the carbon is not distributed uniformly in the film. An elevated concentration of carbon was found near the surface of the film. This behavior suggests that there is a phase separation between TiO2 and carbon at higher temperatures that leaves carbon segregated at the surface. The XPS depth profile in Figure 6a shows a carbon atomic concentration of ∼45−55% and a titanium atomic concentration of ∼20−25%. These atomic concentrations yield a carbon to titanium ratio of approximately 2:1. This ratio is close to expectations given the initial composition of the TiGL film from the proposed growth mechanism shown in Figure 2. For the TiGL film pyrolyzed at 800 °C, the Ti:O ratio is ∼1:1. This ratio is higher than the Ti:O ratio of 1:2 expected for TiO2. However, Figure 4 shows crystalline anatase and rutile TiO2 after pyrolysis at 800 °C. The Ti:O ratio of ∼1:1 is believed to be an artifact of preferential sputtering of oxygen.39−41 The Ti:O ratio of ∼1:1 could also be attributed to the carbothermal reduction of TiO2. However, this possibility is not consistent with thermochemical calculations for the reaction TiO2 + C → TiO + CO(g). These calculations show that the 17446

dx.doi.org/10.1021/jp4051947 | J. Phys. Chem. C 2013, 117, 17442−17450

The Journal of Physical Chemistry C

Article

using the cross-sectional SEM image. The pyrolyzed titanicone film was very smooth and conformal to the underlying SiO2 substrate. Based on the measured film thickness of 88 nm, the resistivity of the pyrolyzed titanicone film after heating to 800 °C was ρ = 0.19 Ω cm. In comparison, carbon deposited by thermal decomposition of acetylene (C2H2) at 950 °C shows a lower resistivity of 4.5 × 10−3 Ω cm.44 Highly oriented pyrolytic graphite (HOPG) has a two-dimensional resistivity of (1−3) × 10−1 Ω cm in the c direction and (4−5) × 10−4 Ω cm in the ab (in-plane) direction.45 On the other hand, fully oxidized single crystal rutile TiO2 is an insulator with a resistivity on the order of 1 × 1013 Ω cm at room temperature.46 The resistivity of the TiO2/ C composite produced by pyrolyzing titanicone films is more than 13 orders of magnitude lower than the resistivity of rutile TiO2. The low electrical resistivity of the pyrolyzed TiGL film could also be associated with oxygen vacancies in anatase or rutile titanium oxide. Semiconducting n-type TiO2−x can be formed by high temperature annealing in vacuum and by deposition in vacuum.47,48 Hydrogen can also contribute to an increase in the electrical conductivity of TiO2−x.49,50 Oxygendeficient TiO2 has been shown to improve substantially the photoelectrochemical water oxidation efficiency51 and the high rate capability of Li ion batteries.52 The TiO2/C composites formed from TiGL pyrolysis may have two potential mechanisms of electrical conductivity: tunneling or percolation. These mechanisms have been previously identified for SiO2/C polymer-derived ceramics.43 More research into the microstructure of the TiO2/C composite is necessary to identify which of these two mechanisms is operative. The conductivity of polymer-derived ceramic films can be manipulated over a wide range, depending on the polymeric precursor, added dopants, pyrolysis temperature, and atmosphere.14 F. Pyrolysis of Other Hybrid Organic−Inorganic Films. A survey of the pyrolysis of other hybrid organic−inorganic films under argon was performed to determine if the formation of metal oxide/carbon composites is a general phenomena. The organic diols and triols used to deposit the hybrid organic− inorganic films were ethylene glycol (EG), glycerol (GL), hydroquinone (HQ), and tetrafluorohydroquinone (FHQ). The metal alkoxide films that were examined were composed of titanium (TiEG), aluminum (AlGL, AlEG, AlHQ, AlFHQ), zinc (ZnGL, ZnHQ), zirconium (ZrEG), hafnium (HfEG), and manganese (MnEG). All films were deposited at 150 °C, except for TiEG that was deposited at 115 °C, using conditions reported previously.4,6,11,15 All of these hybrid organic− inorganic films were pyrolyzed under argon at 900 °C. The Raman spectra of TiEG and TiGL after pyrolysis at 900 °C are shown in Figure 8. Both of these samples exhibit the “D” and “G” peaks of graphitic carbon. The TiEG sample displayed lower intensity D and G signals and a stronger D peak compared with the G peak. In contrast, TiGL samples exhibited higher intensity and narrower D and G peaks, with a stronger G peak compared with the D peak. Greater intensity for the D and G peaks for TiGL compared with TiEG suggests that the pyrolyzed TiGL film has higher sp2-carbon content. Greater D peak intensity compared with G peak intensity for the pyrolyzed TiEG film is indicative of increased disorder in the carbon phase compared with the pyrolyzed TiGL film.19 The Raman spectra for the pyrolyzed TiEG sample also shows distinct peaks at lower frequencies. Figure 8 reveals

Figure 8. Raman spectra of titanicone MLD films grown using TiCl4 and either glycerol or ethylene glycol after pyrolysis under argon at 900 °C for 1 h. The Raman spectrum of the initial Si ⟨100⟩ wafer is shown for comparison.

peaks at 142, 252, 420, and 601 cm−1 that are consistent with vibrations from rutile TiO2.53 The appearance of these frequencies indicates the presence of TiO2 crystallites in the pyrolyzed TiEG film. The rutile TiO2 peaks also appear in the Raman spectrum for the pyrolyzed TiGL film. However, these rutile TiO2 peaks are much lower in intensity. The Raman spectra of AlGL, AlEG, AlHQ, and AlFHQ alucone films pyrolyzed under argon at 900 °C are presented in Figure 9. The D and G peaks are very pronounced for all the

Figure 9. Raman spectra of alucone MLD films grown using Al(CH3)3 (trimethylaluminum) and either ethylene glycol, hydroquinone, glycerol, or fluorinated-hydroquinone after pyrolysis under argon at 900 °C for 1 h.

alucone films after pyrolysis at 900 °C. This behavior indicates that alucone pyrolysis leads to the formation of Al2O3 and carbon in sp2-graphitic domains. There is also a high background photoluminescence for the pyrolyzed AlHQ and AlEG films. The photoluminescence is much less for the pyrolyzed AlGL and AlFHQ films. Photoluminescent properties have been observed for amorphous SiO2/C composite thin films.14 The photoluminescence was attributed to the formation 17447

dx.doi.org/10.1021/jp4051947 | J. Phys. Chem. C 2013, 117, 17442−17450

The Journal of Physical Chemistry C

Article

or pseudocapacitance supercapacitors.55−57 Many metal oxides, such as TiO2, are electrochemically active but are not electrically conducting or have only limited electrical conductivity.46 These new conducting metal oxide/carbon composite films derived from the pyrolysis of hybrid organic− inorganic MLD films under argon may facilitate the use of a variety of metal oxides for many electrochemical applications by providing the needed electrical conductivity. The metal oxide/carbon composites may also be more resistant to corrosion than carbon by itself. Metal oxide/carbon composite coatings on carbon electrodes may help to protect the carbon electrodes from corrosion in polymer electrolyte membrane (PEM) fuel cells.58 Ultrathin TiO2-coated multiwalled carbon nanotubes have displayed corrosion resistance in PEM fuel cells.59 Similar behavior may be expected from TiO2/ carbon composite coatings with very little sacrifice in electrical conductivity.

of sp2-carbon clusters in the amorphous SiO2 matrix after pyrolysis.14,54 The origins of photoluminescence observed in this work for Al2O3/C may be similar to SiO2/C. The Raman spectra of ZnGL, ZnHQ, ZrEG, HfEG, and MnEG films pyrolyzed under argon at 900 °C are presented in Figure 10. The D and G peaks are again very pronounced. The

IV. CONCLUSIONS Titanium alkoxide hybrid organic−inorganic films were grown using the sequential exposure of TiCl4 and glycerol. These titanicone MLD films were then pyrolyzed under argon and produced conducting TiO2/carbon composite films. The Raman spectra of the pyrolyzed titanicone films were consistent with the presence of sp2-graphitic carbon. The X-ray diffraction analysis of the pyrolyzed titanicone films was consistent with the presence of anatase and rutile TiO2 after heating to 600 °C and then only rutile TiO2 after heating to 900 °C. X-ray photoelectron depth-profiling showed that the carbon was distributed throughout the pyrolyzed titanicone films. The carbon began to segregate to the surface after pyrolysis at 900 °C. The sheet resistance of the pyrolyzed titanicone films displayed a dramatic reduction versus pyrolysis temperature and obtained a minimum sheet resistance of 2.2 × 104 Ω/□ after heating to 800 °C. The resistivity of the pyrolyzed titanicone film after pyrolysis at 800 °C was ρ = 0.19 Ω cm. Other hybrid organic−inorganic films also displayed segregation into metal oxide and sp2-graphitic carbon domains and may display low electrical resistivity. These TiO2/carbon composite films and other metal oxide/carbon composite films could have important applications in electrochemical reactions and electrochemical energy storage.

Figure 10. Raman spectra of a variety of hybrid organic−inorganic MLD films grown using various metal precursors and organic diols and triols after pyrolysis under argon at 900 °C for 1 h.

results in Figures 9 and 10 suggest that carbon segregates into sp2-graphitic domains for a variety of hybrid organic−inorganic films. These metal oxide/carbon composites may all have low electrical resistivity. Preliminary experiments for the AlGL film are consistent with this expectation. MnEG films after pyrolysis under argon at 900 °C did not show D and G peaks in Figure 10. The absence of these peaks in the Raman spectra was observed for several MnEG samples of different thicknesses. XPS depth profile analysis of the MnEG films after pyrolysis at 900° observed Mn and O but no carbon in the film. The absence of carbon is consistent with a carbon concentration below the XPS detection limit of ∼0.1%. The loss of carbon in the pyrolyzed MnEG film can be attributed to carbothermal reduction of manganese oxide by graphitic carbon at the pyrolysis temperature of 900 °C. Thermochemical calculations are consistent with carbothermal reduction of manganese oxide at much lower temperatures than TiO2, ZnO, ZrO2, or HfO2.42 Lower pyrolysis temperatures may avoid carbothermal reduction. Preliminary experiments have observed D and G peaks for MnEG samples pyrolyzed under argon at 600 °C. G. Electrochemical Applications of Metal Oxide/ Carbon Composite Films. The hybrid organic−inorganic metalcone films deposited using MLD techniques yield conformal films with Ångstrom-level control of the film thickness. These hybrid organic−inorganic films can be deposited conformally on very high surface area conducting electrode materials such as graphene. After pyrolysis under argon, a conformal metal oxide/carbon composite film remains on the high surface area electrode. These coated electrodes may have a variety of electrochemical applications.55 Metal oxide/carbon composite films on high surface area electrode materials can serve as electrodes for Li ion batteries

■

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (S.M.G.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS A.I.A., J.J.T., A.S.C., and S.M.G. acknowledge funding from the National Science Foundation (CHE-1012116). Additional funding was provided by the Department of Energy through the DOE-BATT program. K.T. and R.R. acknowledge the Air Force Office of Scientific Research and NASA (FA9550-090477) for partial support of this research. The authors acknowledge Byounghoon Lee, Byunghoon Yoon, Robert Hall and Younghee Lee for supplying MLD films for the survey pyrolysis experiments. The authors also thank Virginia R. Anderson and Jaime Dumont from University of Colorado at Boulder for their assistance in GIXRD measurements. 17448

dx.doi.org/10.1021/jp4051947 | J. Phys. Chem. C 2013, 117, 17442−17450

The Journal of Physical Chemistry C

■

Article

(20) Ferrari, A. C.; Robertson, J. Resonant Raman Spectroscopy of Disordered, Amorphous, and Diamondlike Carbon. Phys. Rev. B 2001, 64, 075414. (21) Maultzsch, J.; Reich, S.; Thomsen, C. Double-Resonant Raman Scattering in Graphite: Interference Effects, Selection Rules, and Phonon Dispersion. Phys. Rev. B 2004, 70, 155403. (22) Chu, P. K.; Li, L. H. Characterization of Amorphous and Nanocrystalline Carbon Films. Mater. Chem. Phys. 2006, 96, 253−277. (23) Pimenta, M. A.; Dresselhaus, G.; Dresselhaus, M. S.; Cancado, L. G.; Jorio, A.; Saito, R. Studying Disorder in Graphite-Based Systems by Raman Spectroscopy. Phys. Chem. Chem. Phys. 2007, 9, 1276−1291. (24) Kostecki, R.; Schnyder, B.; Alliata, D.; Song, X.; Kinoshita, K.; Kotz, R. Surface Studies of Carbon Films from Pyrolyzed Photoresist. Thin Solid Films 2001, 396, 36−43. (25) Kumar, S. Unhydrogenated Diamond-Like Carbon Films Prepared by DC Plasma Chemical Vapor Deposition at Room Temperature. Appl. Phys. Lett. 1991, 58, 1836−1838. (26) Matthews, M. J.; Bi, X. X.; Dresselhaus, M. S.; Endo, M.; Takahashi, T. Raman Spectra of Polyparaphenylene-Based Carbon Prepared at Low Heat-Treatment Temperatures. Appl. Phys. Lett. 1996, 68, 1078−1080. (27) Sun, Z.; Shi, X.; Wang, X.; Sun, Y. Structure and Properties of Hard Carbon Films Depending on Heat Treatment Temperatures via Polymer Precursor. Diamond Relat. Mater. 1999, 8, 1107−1113. (28) Conway, N. M. J.; Ferrari, A. C.; Flewitt, A. J.; Robertson, J.; Milne, W. I.; Tagliaferro, A.; Beyer, W. Defect and Disorder Reduction by Annealing in Hydrogenated Tetrahedral Amorphous Carbon. Diamond Relat. Mater. 2000, 9, 765−770. (29) Xiang, H. Q.; Fang, S. B.; Jiang, Y. Y. Carbonaceous Anodes for Lithium-Ion Batteries Prepared from Phenolic Resins with Different Cross-Linking Densities. J. Electrochem. Soc. 1997, 144, L187−L190. (30) Zheng, T.; Zhong, Q.; Dahn, J. R. High-Capacity Carbons Prepared from Phenolic Resin for Anodes of Lithium-Ion Batteries. J. Electrochem. Soc. 1995, 142, L211−L214. (31) Dillon, R. O.; Woollam, J. A.; Katkanant, V. Use of RamanScattering to Investigate Disorder and Crystallite Formation in AsDeposited and Annealed Carbon Films. Phys. Rev. B 1984, 29, 3482− 3489. (32) Joint Committee for Powder Diffraction Standard (JCPDS) 861156. (33) Gnanasekar, K. I.; Subramanian, V.; Robinson, J.; Jiang, J. C.; Posey, F. E.; Rambabu, B. Direct Conversion of TiO2 Sol to Nanocrystalline Anatase at 85 Degrees C. J. Mater. Res. 2002, 17, 1507−1512. (34) Hou, Y. Q.; Zhuang, D. M.; Zhang, G.; Zhao, M.; Wu, M. S. Influence of Annealing Temperature on the Properties of Titanium Oxide Thin Film. Appl. Surf. Sci. 2003, 218, 97−105. (35) Li, G. H.; Yang, L.; Jin, Y. X.; Zhang, L. D. Structural and Optical Properties of TiO2 Thin Film and TiO2 + 2 wt.% ZnFe2O4 Composite Film Prepared by R.F. Sputtering. Thin Solid Films 2000, 368, 163−167. (36) Wang, X. D.; Shen, J.; Pan, Q. Raman Spectroscopy of Sol-Gel Derived Titanium Oxide Thin Films. J. Raman Spectrosc. 2011, 42, 1578−1582. (37) Preiss, H.; Berger, L. M.; Schultze, D. Studies on the Carbothermal Preparation of Titanium Carbide from Different Gel Precursors. J. Eur. Ceram. Soc. 1999, 19, 195−206. (38) J. Joint Committee for Powder Diffraction Standard (JCPDS) 86-0148. (39) Babelon, P.; Dequiedt, A. S.; Mostefa-Sba, H.; Bourgeois, S.; Sibillot, P.; Sacilotti, M. SEM and XPS Studies of Titanium Dioxide Thin Films Grown by MOCVD. Thin Solid Films 1998, 322, 63−67. (40) Baer, D. R.; Engelhard, M. H.; Lea, A. S.; Nachimuthu, P.; Droubay, T. C.; Kim, J.; Lee, B.; Mathews, C.; Opila, R. L.; Saraf, L. V.; Stickle, W. F.; Wallace, R. M.; Wright, B. S. Comparison of the Sputter Rates of Oxide Films Relative to the Sputter Rate of SiO2. J. Vac. Sci. Technol., A 2010, 28, 1060−1072.

REFERENCES

(1) George, S. M. Atomic Layer Deposition: An Overview. Chem. Rev. 2010, 110, 111−131. (2) George, S. M.; Yoon, B.; Dameron, A. A. Surface Chemistry for Molecular Layer Deposition of Organic and Hybrid Organic-Inorganic Polymers. Acc. Chem. Res. 2009, 42, 498−508. (3) Dameron, A. A.; Seghete, D.; Burton, B. B.; Davidson, S. D.; Cavanagh, A. S.; Bertrand, J. A.; George, S. M. Molecular Layer Deposition of Alucone Polymer Films Using Trimethylaluminum and Ethylene Glycol. Chem. Mater. 2008, 20, 3315−3326. (4) Abdulagatov, A. I.; Hall, R. A.; Sutherland, J. L.; Lee, B. H.; Cavanagh, A. S.; George, S. M. Molecular Layer Deposition of Titanicone Films Using TiCl4 and Ethylene Glycol or Glycerol: Growth and Properties. Chem. Mater. 2012, 24, 2854−2863. (5) Peng, Q.; Gong, B.; VanGundy, R. M.; Parsons, G. N. “Zincone” Zinc Oxide-Organic Hybrid Polymer Thin Films Formed by Molecular Layer Deposition. Chem. Mater. 2009, 21, 820−830. (6) Yoon, B.; Lee, B. H.; George, S. M. Highly Conductive and Transparent Hybrid Organic-Inorganic Zincone Thin Films Using Atomic and Molecular Layer Deposition. J. Phys. Chem. C 2012, 116, 24784−24791. (7) Yoon, B.; O’Patchen, J. L.; Seghete, D.; Cavanagh, A. S.; George, S. M. Molecular Layer Deposition of Hybrid Organic-Inorganic Polymer Films Using Diethylzinc and Ethylene Glycol. Chem. Vap. Deposition 2009, 15, 112−121. (8) Miller, D. C.; Foster, R. R.; Jen, S. H.; Bertrand, J. A.; Cunningham, S. J.; Morris, A. S.; Lee, Y. C.; George, S. M.; Dunn, M. L. Thermo-Mechanical Properties of Alumina Films Created Using the Atomic Layer Deposition Technique. Sens. Actuators, A 2010, 164, 58−67. (9) Miller, D. C.; Foster, R. R.; Jen, S. H.; Bertrand, J. A.; Seghete, D.; Yoon, B.; Lee, Y. C.; George, S. M.; Dunn, M. L. Thermomechanical Properties of Aluminum Alkoxide (Alucone) Films Created Using Molecular Layer Deposition. Acta Mater. 2009, 57, 5083−5092. (10) Lee, B. H.; Yoon, B.; Anderson, V. R.; George, S. M. Alucone Alloys with Tunable Properties Using Alucone Molecular Layer Deposition and Al2O3 Atomic Layer Deposition. J. Phys. Chem. C 2012, 116, 3250−3257. (11) Lee, B. H.; Yoon, B.; Abdulagatov, A. I.; Hall, R. A.; George, S. M. Growth and Properties of Hybrid Organic-Inorganic Metalcone Films Using Molecular Layer Deposition Techniques. Adv. Funct. Mater. 2013, 23, 532−546. (12) Liang, X.; King, D. M.; Li, P.; George, S. M.; Weimer, A. W. Nanocoating Hybrid Polymer Films on Large Quantities of Cohesive Nanoparticles by Molecular Layer Deposition. AIChE J. 2009, 55, 1030−1039. (13) Liang, X. H.; Yu, M.; Li, J. H.; Jiang, Y. B.; Weimer, A. W. UltraThin Microporous-Mesoporous Metal Oxide Films Prepared by Molecular Layer Deposition (MLD). Chem. Commun. 2009, 7140− 7142. (14) Colombo, P.; Mera, G.; Riedel, R.; Soraru, G. D. PolymerDerived Ceramics: 40 Years of Research and Innovation in Advanced Ceramics. J. Am. Ceram. Soc. 2010, 93, 1805−1837. (15) George, S. M.; Lee, B. H.; Yoon, B.; Abdulagatov, A. I.; Hall, R. A. Metalcones: Hybrid Organic-Inorganic Films Fabricated Using Atomic and Molecular Layer Deposition Techniques. J. Nanosci. Nanotechnol. 2011, 11, 7948−7955. (16) Riedel, R.; Mera, G.; Hauser, R.; Klonczynski, A. Silicon-Based Polymer-Derived Ceramics: Synthesis Properties and Applications - A Review. J. Ceram. Soc. Jpn. 2006, 114, 425−444. (17) Elam, J. W.; Groner, M. D.; George, S. M. Viscous Flow Reactor with Quartz Crystal Microbalance for Thin Film Growth by Atomic Layer Deposition. Rev. Sci. Instrum. 2002, 73, 2981−2987. (18) Knight, D. S.; White, W. B. Characterization of Diamond Films by Raman Spectroscopy. J. Mater. Res. 1989, 4, 385−393. (19) Ferrari, A. C.; Robertson, J. Interpretation of Raman Spectra of Disordered and Amorphous Carbon. Phys. Rev. B 2000, 61, 14095− 14107. 17449

dx.doi.org/10.1021/jp4051947 | J. Phys. Chem. C 2013, 117, 17442−17450

The Journal of Physical Chemistry C

Article

(41) Kubart, T.; Nyberg, T.; Berg, S. Modelling of Low Energy Ion Sputtering from Oxide Surfaces. J. Phys. D: Appl. Phys. 2010, 43, 205204. (42) HSC Chemistry, 5.11 ed.; Outokumpu Research Oy: Pori, Finland. Values are given at 0 °C. (43) Cordelair, J.; Greil, P. Electrical Conductivity Measurements as a Microprobe for Structure Transitions in Polysiloxane Derived Si-O-C Ceramics. J. Eur. Ceram. Soc. 2000, 20, 1947−1957. (44) Graham, A. P.; Schindler, G.; Duesberg, G. S.; Lutz, T.; Weber, W. An Investigation of the Electrical Properties of Pyrolytic Carbon in Reduced Dimensions: Vias and Wires. J. Appl. Phys. 2010, 107, 114316. (45) Pierson, H. O. Handbook of Carbon, Graphite, Diamonds and Fullerenes: Processing, Properties and Applications; Noyes Publications: Park Ridge, NJ, 1993. (46) Breckenridge, R. G.; Hosler, W. R. Electrical Properties of Titanium Dioxide Semiconductors. Phys. Rev. 1953, 91, 793−802. (47) Diebold, U. The Surface Science of Titanium Dioxide. Surf. Sci. Rep. 2003, 48, 53−229. (48) Pore, V.; Rahtu, A.; Leskela, M.; Ritala, M.; Sajavaara, D.; Keinonen, J. Atomic Layer Deposition of Photocatalytic TiO2 Thin Films from Titanium Tetramethoxide and Water. Chem. Vap. Deposition 2004, 10, 143−148. (49) Chen, W. P.; Wang, Y.; Chan, H. L. W. Hydrogen: A Metastable Donor in TiO2 Single Crystals. Appl. Phys. Lett. 2008, 92, 112907. (50) Nowotny, J. Effect of Hydrogen on Semiconducting Properties of TiO2 Single Crystal. Jonker Analysis. J. Phys. Chem. C 2011, 115, 18316−18326. (51) Wang, G. M.; Ling, Y. C.; Li, Y. Oxygen-Deficient Metal Oxide Nanostructures for Photoelectrochemical Water Oxidation and Other Applications. Nanoscale 2012, 4, 6682−6691. (52) Shin, J. Y.; Joo, J. H.; Samuelis, D.; Maier, J. Oxygen-Deficient TiO2-Delta Nanoparticles via Hydrogen Reduction for High Rate Capability Lithium Batteries. Chem. Mater. 2012, 24, 543−551. (53) Chang, H.; Huang, P. J. Thermo-Raman Studies on Anatase and Rutile. J. Raman Spectrosc. 1998, 29, 97−102. (54) Karakuscu, A.; Guider, R.; Pavesi, L.; Soraru, G. D. White Luminescence from Sol-Gel-Derived SiOC Thin Films. J. Am. Ceram. Soc. 2009, 92, 2969−2974. (55) Wu, Z. S.; Zhou, G. M.; Yin, L. C.; Ren, W. C.; Li, F.; Cheng, H. M. Graphene/Metal Oxide Composite Electrode Materials for Energy Storage. Nano Energy 2012, 1, 107−131. (56) Ban, C. M.; Xie, M.; Sun, X.; Travis, J. J.; Wang, G. K.; Sun, H. T.; Dillon, A. C.; Lian, J.; George, S. M. Atomic Layer Deposition of Amorphous TiO2 on an Al2O3 Adhesion Layer on Graphene as an Anode for Li-Ion Batteries. Nanotechnology 2013, in press. (57) Sun, X.; Xie, M.; Wang, G. K.; Sun, H. T.; Cavanagh, A. S.; Travis, J. J.; George, S. M.; Lian, J. Atomic Layer Deposition of TiO2 on Graphene for Supercapacitors. J. Electrochem. Soc. 2012, 159, A364−A369. (58) Maass, S.; Finsterwalder, F.; Frank, G.; Hartmann, R.; Merten, C. Carbon Support Oxidation in PEM Fuel Cell Cathodes. J. Power Sources 2008, 176, 444−451. (59) Akalework, N. G.; Pan, C. J.; Su, W. N.; Rick, J.; Tsai, M. C.; Lee, J. F.; Lin, J. M.; Tsai, L. D.; Hwang, B. J. Ultrathin TiO2-Coated MWCNTs with Excellent Conductivity and SMSI Nature as Pt Catalyst Support for Oxygen Reduction Reaction in PEMFCs. J. Mater. Chem. 2012, 22, 20977−20985.

17450

dx.doi.org/10.1021/jp4051947 | J. Phys. Chem. C 2013, 117, 17442−17450