材料化學(xué)專業(yè)外文翻譯--合成β-mo2c薄膜（英文）

1、Published: January 20, 2011r 2011 American Chemical Society 517 dx.doi.org/10.1021/am101095h | ACS Appl. Mater. Interfaces 2011, 3, 517–521RESEARCH ARTICLEwww.acsami.orgSynthesis of β-Mo2C Thin FilmsColin A. Wolden,*,? A

2、nna Pickerell,? Trupti Gawai,? Sterling Parks,? Jesse Hensley,? and J. Douglas Way??Department of Chemical Engineering, Colorado School of Mines, Golden, Colorado 80401, United States?National Renewable Energy Laboratory

3、, 1617 Cole Boulevard, Golden, Colorado 80401, United StatesABSTRACT: Thin films of stoichiometric β-Mo2C were fabricated using a two-step synthesis process. Dense molybdenum oxide films were first depo- sited by plasma-

4、enhanced chemical vapor deposition using mixtures of MoF6, H2, and O2. The dependence of operating parameters with respect to deposition rate and quality is reviewed. Oxide films 100-500 nm in thickness were then convert

5、ed into molybdenum carbide using temperature-programmed reaction using mixtures of H2 and CH4. X-ray diffraction confirmed that molybdenum oxide is completely transformed into the β-Mo2C phase when heated to 700 ?C in mi

6、xtures of 20% CH4 in H2. The films remained well-adhered to the underlying silicon substrate after carburization. X-ray photoelectron spectroscopy detected no impurities in the films, and Mo was found to exist in a singl

7、e oxidation state. Microscopy revealed that the as-deposited oxide films were featureless, whereas the carbide films display a complex nanostructure.KEYWORDS: thin film, carbide, oxide, plasma-enhanced chemical vapor dep

8、osition, catalyst’INTRODUCTIONMolybdenum carbide is a versatile material with potential applications in a variety of areas. Molybdenum carbide exhibits catalytic properties analogous to platinum group metals, and in the

9、last few decades, efforts have been made to exploit this trait in a number of chemical processes including ammonia synthesis, hydrocarbon reforming, water gas shift, H2 production, and alcohol synthesis.1-7 As a thin fil

10、m, the high hardness and ther- mal stability of the transition metal carbides make them useful as wear-resistant materials.8,9 More recent studies have focused on the optoelectronic properties of molybdenum carbide for a

11、 range of applications including mirrors,10 diffusion barriers,11 interc- onnects,12 and electron field emission.13 Our interest is in the creation of inexpensive alternatives to platinum group metals for surface dissoci

12、ation of H2. Formation of this material as a thin film would facilitate fundamental studies of catalyst performance. Numerous techniques have been used to deposit molybde- num carbide films including chemical vapor depos

13、ition (CVD),8,14 physical vapor deposition (PVD),9,11 and electrode- position.15 The Mo-C system is quite complex, with numerous stable and metastable compounds and crystal phases that have been observed.14,16 Control of

14、 phase purity has been problematic for both CVD and PVD approaches, and films often contain a mixture of carbon-rich products that display a complex depen- dence on the specific operating conditions employed.8,11,14 In c

15、ontrast, for catalysis applications it is relatively straightforward to achieve the desired β-Mo2C phase in powder form.1-4 The key development came from Boudart and coworkers1,2 who developed a method to convert dense M

16、oO3 powders into high- surface-area Mo2C by temperature-programmed reaction or TPR, using mixtures of hydrocarbons diluted in H2. The mecha- nism for the MoO3 to Mo2C transformation involves the substitution of carbon fo

17、r oxygen in the MoO3 lattice, with little displacement of the Mo atoms during the reaction. Because themolar volume of Mo2C is smaller than the molar volume of MoO3, micropores form as the oxide transforms into the carbi

18、de. Under proper conditions the MoO3 is converted into Mo2C without forming metallic Mo as a reaction intermediate. Metal sintering is avoided using this method, and unsupported catalysts can be prepared with very high s

19、urface areas (50-90 m2/g). Rebrov et al.5 used this strategy to form catalyst coatings through the carburization of pre-oxidized molybdenum sheets, and applied them to the water gas shift reaction. The goal of this work

20、was to produce phase pure β-Mo2C films for future study as model catalyst layers. Below we describe a two- step synthesis approach. First, dense molybdenum oxide films are deposited on silicon using plasma-enhanced chemi

21、cal vapor deposition (PECVD) using mixtures of MoF6/H2/O2. The use of MoF6 and this reaction chemistry is somewhat unique, as previous CVD of Mo-containing compounds have employed either Mo(CO)6 17,18 or MoCl5 14,19 as t

22、he molybdenum precur- sor. The dependence of oxide growth rate and quality on PECVD parameters is described. The use of PECVD to form the oxide allows these films to be produced on a broader set of substrates. The films

23、were then transformed into the β-Mo2C phase by applying TPR conditions developed by the catalysis community. The evolution of film composition and structure throughout these processes is quantified using a suite of analy

24、tical techniques.’EXPERIMENTAL SECTIONOxide Synthesis. Molybdenum oxide films were deposited using mixtures of MoF6/H2/O2 in a capacitively-coupled PECVD system. This PECVD chemistry adopts a similar approach that has be

25、enReceived: November 11, 2010Accepted: December 31, 2010519 dx.doi.org/10.1021/am101095h |ACS Appl. Mater. Interfaces 2011, 3, 517–521ACS Applied Materials & Interfaces RESEARCH ARTICLEThe observed peak position of t

26、he as-deposited film at a binding energy (BE) of 232.8 eV is in good agreement with the Moþ6state of fully oxidized MoO3. The d orbital peaks are separated by 3.1 eV and appear in the theoretically expected 3:2 rati

27、o,24providing further support that the Mo is present in a single oxidation state. Of course hydrogen cannot be detected by XPS, but the position of Mo spectra in its fully oxidized state supports its absence. XPS analysi

28、s of molybdenum oxide films intercalated with H display the presence of Moþ5 and Moþ4 oxidation states.21Moreover, the transparent nature of the film provide further support for its chemical purity, since MoO3H

29、x has a grey color that is exploited in electrochromic applications.21,27Carburization. Molybdenum oxide films with thickness of 100-500 nm were deposited on pieces of (100) silicon wafers as described above. These sampl

30、es were then carburized using stan- dard TPR conditions described above. Films remained well- adhered to the underlying silicon substrate after carburization,and were characterized using the techniques described below. T

31、he changes in crystal structure that were observed in conjunction with TPR are summarized in Figure 4, which compares results from both powder samples and a 200 nm thick film with literature standards. The stable phase o

32、f molybdenum oxide has an orth- orhombic structure,27 and the as-received powders were in good agreement with the literature standards (JCPDS 76-1003). The as-deposited oxide films are XRD amorphous, however they crystal

33、lize when annealed in air at temperatures >150 ?C. During the carburization process, the oxide films are transformed during the calcining step, and an example of a XRD pattern obtai- ned from a film after this treatme

34、nt is shown in Figure 4. The pattern obtained from the thin film sample is noisy relative to the powder samples, but nevertheless the film is clearly identified as polycrystalline molybdenum oxide with a preferential ori

35、entation in the (110) direction located at 2θ = 23.33?. The same materials were examined after the completion of the carburization process. Again it is observed that the powder samples are in perfect agreement with liter

36、ature expectations for the β-Mo2C phase. Note that the β-Mo2C phase has an orth- orhombic crystal structure (JCPDS 79-0744), though its lattice positions are nominally identical to a slightly strained hexagonal closed pa

37、cked structure (JCPDS 35-0787), and it has often been described as such in the catalyst literature.16,28 Both carbide pow- ders and films display the three significant peaks at 2θ of 34.4, 37.9, and 39.4 which are indexe

38、d as the (100), (002), and (101) planes of β-Mo2C, respectively. There is no evidence of residual oxide, molybdenum metal, or any other crystalline phases of molybdenum carbide. The XRD pattern obtained from a carbur- iz

39、ed film is nominally identical to both the literature values and the powder sample, confirming its transformation to the β-Mo2C phase. XPS was used to examine the composition of the carburized films. Figure 3 contains th

40、e high resolution spectra of Mo 3d region. The position of the Mo peaks in the carbide was shifted significantly to lower binding energy relative to the oxide film, and the peak position at BE = 228.2 eV is in perfect ag

41、reement with literature values for Moþ2 (Table I). The compositional purity of this material is again supported by both the spacing (3.2 eV) and relative intensity of the two peaks, which suggests that the Mo is pre

42、dominantly in a single oxidation state. Figure 5 compares high resolution spectra of the O 1s region obtained from an oxide film and after carburization. The oxide film displays a prominent peak at BE = 530.8 eV, which i

43、s in good agreement with literature values.29 The surface of the carbide is partially oxidized during the passivation step. Although the signal is attenuated substantially relative to the oxide film, there are twoFigure

44、2. High-resolution XPS spectra of the F 1s region obtained from an as-deposited MoO3 films and after sputter cleaning.Figure 3. High-resolution XPS spectra of the Mo 3d region obtained from an as-deposited MoO3 film, and

45、 after carburization.Table I. Summary of the Binding Energy Positions Reported in the Literature (( 0.2 eV) for the Mo 3d5/2 and C 1s Core Levels in Oxidation States of Interest to Mo2C Formationstate BE (eV)Mo6þ 23