Gaseous Reactions in Adsorbed Water Present on Transition Metal

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Gaseous Reactions in Adsorbed Water Present on Transition Metal Oxides Qi Wang, Ajinkya Puntambekar, and Vidhya Chakrapani J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 24 May 2017 Downloaded from http://pubs.acs.org on May 27, 2017

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The Journal of Physical Chemistry

Gaseous Reactions in Adsorbed Water Present on Transition Metal Oxides

Qi Wang,a Ajinkya Puntambekar,a and Vidhya Chakrapani,a,$,* a

Howard P. Isermann Department of Chemical and Biological Engineering $

Department of Physics, Applied Physics, and Astronomy Rensselaer Polytechnic Institute, Troy, NY-12180 *

Email: [email protected]

1 ACS Paragon Plus Environment


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ABSTRACT Water from ambient atmosphere is known to adsorb naturally on the surfaces of many transition metal oxides and this adsorbed water is known to affect electrical, optical and many functional properties such as catalytic activities. However, the nature of this underlying interaction remains unknown. In this work, we report a new type of semiconductor-adsorbed water interaction in metal oxides that is known as “electrochemical surface transfer doping,” a phenomenon that has previously been observed in hydrogen-terminated diamond. Herein, the adsorbed water on the metal oxide allows dissolution of gaseous species and induces charge transfer processes at the oxide/adsorbed-water interface. We further elucidate, using in-situ infrared photoluminescence, electrical resistance, and X-ray photoelectron spectroscopy, the role of vacancy defects in catalytic process by directly monitoring the charge transfer process between gaseous species and vacancies in defective metal oxides such as p-type nickel oxide, and n-type tungsten oxide at room temperature. We show that adsorbed water and vacancy defects are necessary ingredients in affecting catalytic, electronic, electrical, and optical changes, such as metal-to-insulator transitions and radiative emissions. The electrochemical structure-property-function correlation shown here has important implications for fields as diverse as surface chemistry, catalysis, optoelectronics, corrosion, sensors, and geochemistry.


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INTRODUCTION Water is one of the most prevalent chemicals on Earth, and thus it is no surprise that it is an adsorbate important in diverse fields such as catalysis, electrochemistry, tribology, geochemistry, atmospheric chemistry, and many others.1,2 Transition metal oxides (TMOs) are an important class of materials that are valued for their catalytic, ferroelectric, and optical properties. Many TMOs are highly sensitive to gases and hence they are used as heterogeneous catalysts for several gaseous reactions,3,4,5 sensors, and as active material in fast-switching optoelectronic devices that show metal–insulator phase transitions (MIT) as a result of gaseous interactions.6,7 Water adsorbs naturally on most TMOs at ambient conditions,8,9 and is known to impact diverse properties. For example, adsorbed water is known to affect electronic properties such as work function and electron affinity of oxides, which has been the subject of two major reviews.1,2 Water is also known to enhance catalytic rate of several gaseous reactions, although the nature of this interaction is not known.10,11,12,13 Despite their similar lattice structure, the difference in the nature of water adsorption on α-Fe2O3 and α-Al2O3 leads to marked differences in the reactivity of these oxides towards geochemical processes, such as chemical speciation and heavy metal adsorption.14,15 Finally, water in the presence of certain gases, such as H2, is also known to affect optical properties of metal oxide, a phenomenon referred to as gasochromism.16 These examples clearly illustrate the importance of adsorbed water in enabling many functional properties of oxides, but the fundamental nature of the dynamics of interaction between adsorbed water and the semiconductor surface is still not clear.17,10,18 A type of adsorbed water-substrate interaction and its role in affecting electronic properties has previously been demonstrated in hydrogen-terminated diamond that is known as “electrochemical surface transfer doping”.19-20,21 It has been shown that exposure of undoped,



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hydrogen-terminated diamond to humid air can induce the formation of a highly unusual p-type surface conductive layer on an otherwise insulating surface. This hole accumulation layer is a result of electron exchange between the valence band (VB) of diamond and an gaseous reaction occurring in the adsorbed water film that acts as a source or sink for electrons. 20,22,19,23,24 Here, the water film facilitates the dissolution and solvation of ambient gases such as O2 and CO2, and thus enables electron exchange with the solid through electrochemical reactions between the dissolved species that otherwise would have not been possible with direct gas phase interaction. This surface transfer doping process occurs due to the difference in the Fermi level of the semiconductor and the electrochemical redox potential of the adsorbed water film (set by the redox reaction), which leads to nearly seven orders of magnitude change in conductivity in diamond. In this work, we report a similar process in hydrated transition metal oxides at room temperature and show the various manifestations of this phenomenon, such as its effect on metalto-insulator electronic phase transitions, optical emission, and defect equilibrium processes. The proposed structure-property-function correlation provides a new electrochemical framework for understanding the adsorbed-water/oxide interaction and its effect on the catalytic and electronic properties. To illustrate the effects of surface transfer doping phenomenon, nominally undoped oxides of nickel (NiO) and tungsten (WO3) served as our model oxide systems. Nominally undoped NiO is a p-type, strongly-correlated semiconductor with propensity to form Ni (cationic) vacancy,25 VNi, and it serves as a good reference for comparison with WO3, which is ntype due to the presence of O (anionic) vacancy, VO.26 WO3 was chosen because its properties have been well understood in terms of gasochromic and electrochromic effects.27,28

Both

semiconductors have markedly different band line-ups with respect to the redox couples of


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various gases, as illustrated later in the text. Changes in the optical, electronic and electrical properties of these oxides as a result of gaseous reactions with H2, O2, and CO at room temperature were monitored with and without the presence of water vapor. The mechanism of H2 dissociation on metal oxide has been long disputed,29,30,31 as illustrated in a recent review article,29 hence the study of hydrogen dissociation is alevant reaction for this study. Hydrogen dissociation and spill-over is known to occur in ‘reducible’ oxides such as Fe2O3, WO3, MoO3 and is prevented in oxides such as MgO, SiO2, Al2O3 and ZrO2, which are refered to as ‘nonreducible’ oxides.29 The relationship between selectivity and electronic properties of the oxide that enables selective dissociation of H2 is not clearly understood. A suite of complementary insitu measurements techniques such as two-probe electrical resistance and near infrared (IR) photoluminescence (PL), and X-ray photoelectron spectroscopic (XPS) measurements were performed in order to monitor the interaction of gaseous species with the vacancy defects and understand the charge transfer processes at the adsorbed-water/oxide interface. EXPERIMENT Nanowires of tungsten and nickel oxide were grown by hot filament chemical vapor deposition (HFCVD) technique, which involved resistively heating W or Ni metal wire close to its melting point in the presence of either pure oxygen or a mixture of argon, oxygen and water vapor at pressure ranging from 0.1 Torr to 10 Torr. The reaction of oxygen with the hot metal filament results in the formation of the respective metal oxide on the substrates placed underneath the filament. By tuning the filament power and the partial pressure of oxygen in the chamber, both morphology and stoichiometry of the deposited sample could be modulated. High-purity nanowires with different metal-to-oxygen ratios were synthesized using this technique without introducing extraneous reducing agents that may get incorporated into the 5 ACS Paragon Plus Environment


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lattice during growth and produce additional lattice defects.32,33 In addition to the nanostructures prepared using HFCVD, non-stoichiometric, single crystal nickel oxide nanowire and nanopowder were also purchased from Sigma-Aldrich. No significant differences in the results of electrical and optical testing were observed between these sets of samples. Representative results are shown in the main text. Nickel oxide nanowires used in this study had a diameter of ~100 nm and were 1−2 µm in length, as seen in the transmission electron microscopy (TEM) and the scanning electron micrographs (SEM) images shown in Fig.S1 in the Supporting Information (SI).

X-ray diffraction (XRD) patterns of both stoichiometric nickel oxide (NiO) and

nonstoichiometric nickel oxide (Ni1-xO), as shown in Fig.S2 in SI, conform to the JCPDS pattern of cubic NiO.

SEM images of both stoichiometric and non-stoichiometric tungsten oxide

nanowires show them to be aligned nanowires of 5−6 µm in length with a diameter of 60-80 nm. Representative images are shown in Fig.S3 in SI. The XRD pattern of the non-stoichiometric phase shows the crystal structure to be monoclinic W18O49 (WO2.72), as shown in Fig.S4 in SI. High resolution TEM (HRTEM) images of W18O49 show that the nanowires are single crystalline in nature (See Fig.S3C in SI). Distinct in-plane vacancy ordering in the direction parallel to the growth direction, as seen by the presence of streak lines in the selected area electron diffraction pattern shown in Fig.S3D in SI, confirmed the presence of VO defects in the (600) plane as the predominant defect in non-stoichiometric tungsten oxide.34,35 Experiments were done in a custom-built, multiport high vacuum (~10-7 torr) chamber that allowed in-situ optical, and electrical measurements under controlled gaseous environments. Since gas phase reactions are surface-mediated, the observed effects are expected to be amplified in high surface area nanowires. Nanostructured films were coated with Pt or Au catalysts of 1-5 nm thickness through e-beam evaporation of the respective metals. Platinum metal was used as


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catalyst for most experiments with WO3 and NiO, and is denoted in the text as ‘Pt-WO3 or PtNiO’. Gold catalyst was used for the reaction of CO only. As-synthesized samples were transferred to inert-gas glove box to minimize water adsorption from the ambient atmosphere. Changes occurring with both “air-exposed” and dry samples were recorded upon exposure to different gases of H2, CO, O2, and H2O vapor. All optical and electrical measurements were done under dark conditions at ambient temperature.

Two-probe electrical resistance

measurements were done between ohmic contacts formed by placing two silver clips to the nanostructured films using Ag conductive epoxy. Resistance of the film was measured using a Keithley 2701 data acquisition system. Measurements were done in-situ in the presence of various gases, such as H2, CO, O2. For some experiments water vapor was introduced into the chamber to probe its effect on the gas-phase catalytic reaction. PL measurements in the range of 330 nm to 1700 nm were obtained using a HORIBA Scientific LabRAM HR Evolution spectrometer equipped with Si and InGaAs CCD detectors using an excitation wavelength of 325 nm (He-Cd laser) and 633 nm (He-Ne laser). All PL spectra were obtained at room temperature in a reflective backside configuration in-situ during exposure to various gases.

XPS

measurements were done using a Physical Electronics PHI 5000 Versa Probe system comprising of a monochromatic Al(Κα) (1486.7 eV) X-ray source and a 150 mm radius hemispherical electron energy analyzer. The analyzer was operated at pass energies between 11.75 and 29.35 eV, and at a chamber base pressure of ~10-10 Torr. RESULTS Role of Adsorbed Water during Charge Transfer and Phase Transitions: The changes in the electrical resistivity during gas phase reactions indicate the direction of electron transfer between the metal oxide and the adsorbates, and involvement of any electron exchange. In-situ changes 7 ACS Paragon Plus Environment


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in the two-probe electrical resistivity of n-type WO3, and p-type Ni1-xO was measured under different gases, such as H2, CO, and O2, with and without the presence of water to determine the role of water in catalyzing reactions. The results are summarized in Fig.1A and 1B. As can be seen, seven orders of magnitude change in the resistivity is observed in Pt-WO3 upon exposure to wet H2 versus the two order of magnitude change seen in dry H2, leading to an insulator-to-metal phase transition (MIT). Similarly, MIT transition can also be seen in Pt-WO3 when exposed to wet CO. This strongly points to the importance of water in catalyzing the reactions and confirms the electrons transfer from H2 or CO to n-type WO3. The effect is reversed, i.e. a metal-toinsulator transition can be observed when Pt-WO3 that was previously exposed to wet H2 is exposed to O2, which indicates that e− in WO3 are scavenged by O2. Very little change in the resistivity can be observed with exposure to wet N2, which confirms that the change in the electrical resistance seen with wet H2 is not due to ionic conduction facilitated by adsorbed water. On WO3 surfaces without Pt, changes were very small and quite slow, which further supports our theory that dissociation of gaseous H2 is very slow on bare metal oxide surface, and the role of the catalyst is to enable dissociation of gases, which is discussed in detail in the Discussion section. The changes in the electrical resistance of Ni1-xO and NiO with wet H2 were markedly different from that observed in WO3. Unlike the insulator-to-metal transition seen in n-type WO3 with H2 exposure, a semiconductor-to-insulator behavior, as characterized by an increase in the electrical resistance, is observed in p-type Ni1-xO with exposure to wet H2. Stoichiometric NiO showed very high resistivity even in pristine state and showed no measurable change in the resistance with H2 exposure, which indicates that cationic defects are needed for the reaction of H2 with NiO. Similar formation of insulating phases in the presence of H2 has


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been observed in certain doped nickelates, such as SmNiO3 by Shi and co-workers,7 which the authors attributed to electron doping. Role of Vacancy Defects in Inducing Charge Transfer: To understand the origin of the electrical changes, PL measurements were done in-situ in the presence of various gases to probe the nature of vacancy defects and their interactions with gas phase species. Many properties of metal oxides are believed to be due to the presence of high density of lattice defects that significantly perturb the electronic structure and profoundly impact the functional properties of the oxides.36 Prior studies have shown that defects, such as vacancies, play an important role in enhancing the catalytic activity by acting as “hot-spots” for adsorptive binding of gaseous species such as O2 and H2O molecules.37,38,39,40 Reaction of gaseous species with metal oxides can lead to the formation of new vacancy defects that have electronic states within the band gap, or can lead to changes in the density of existing defect states. In either case, one needs a spectroscopic technique that allows in-situ monitoring of these changes. Although vacancy defects in oxides have been studied using techniques such as scanning tunneling microscopy (STM),41,42 ultraviolet photoelectron spectroscopy,43,38 electron energy loss spectroscopy (EELS),44,45 they are difficult to adapt to catalytic studies, which typically involves reactive gas environment, presence of metal catalysts and sometimes samples with high density of existing defects.

In our recent studies,35,

46-47

we showed the usefulness of near-IR PL for in-situ

electrochemical studies of defects in oxides and selenides. Here we adapt this technique for gasphase catalytic studies.

Photoluminescence spectra of stoichiometric and nonstoichiometric

tungsten and nickel oxides in the ultraviolet (UV) to near-IR spectral regions are shown in Fig.S5A and S5B respectively in SI. Both the excitonic and free-carrier emission in nickel oxides occur at ~3.8 eV. The emission peak in the visible range at energy of ~2.5 eV is a result



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of d-d transitions that occurs between the electronic levels of incompletely filled “d” orbitals in nickel oxide. Nonstoichiometry in NiO (Ni1-xO) predominately results in a high density of VNi defects that give rise to a near-IR PL emission peak at ~0.8 eV. In contrast, the stoichiometric oxide (NiO) shows no near-IR emission. Similarly, the excitonic transitions in stoichiometric (WO3) and nonstoichiometric tungsten oxide (WO3-x) gives rise to emission peaks at energies of 2.2 and 3.2 eV respectively. The predominant defects in WO3-x are VO defects that also give rise to mid-gap electronic states within the band gap43 and a PL emission peak centered at ~0.76 eV, while the stoichiometric oxide (WO3) shows no near-IR emission. These defect-induced near-IR emissions in Ni1-xO and WO3-x provide a direct tool for probing vacancy interactions with gaseous species during catalytic reactions. Further, the technique is surface-sensitive and nondestructive that requires minimal surface preparation, and most importantly, it can be adapted for in-situ studies in the presence of metal catalysts ( µe (WO3 ) , which leads to the dissociation of H2 and injection of electrons and protons into WO3, thereby decreasing the pH of the solution. On the other hand, reaction of WO3 that was previously exposed to H2 with O2 leads to the consumption of protons and electrons to form water, as µe (WO3 ) > µe (O2 / H 2O) , thereby increasing the pH of the solution; B) Diffuse reflectance infrared spectra of WO3 taken in air, after exposure to wet H2 followed by bleaching in air.

The shaded region in the curve denotes the integrated peak area of the

vibrational stretches of H3O+ and free H2O.


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TOC Graphic


Gaseous Reactions in Adsorbed Water Present on Transition Metal

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