Article pubs.acs.org/cm
Controlling the Structural and Optical Properties of Ta3N5 Films through Nitridation Temperature and the Nature of the Ta Metal Blaise A. Pinaud,† Arturas Vailionis,‡,§ and Thomas F. Jaramillo*,† †
Department of Chemical Engineering, Stanford University, 381 North-South Axis, Stanford, California 94305, United States Geballe Laboratory for Advanced Materials, Stanford University, 476 Lomita Mall, Stanford, California 94305, United States § Stanford Synchrotron Radiation Laboratory, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States ‡
S Supporting Information *
ABSTRACT: The development of a reliable synthetic route to produce high performance Ta3N5 photoanodes has been complicated by the large number of synthetic parameters, notably nitridation conditions. A systematic study of nitridation from 850 °C−1000 °C reveals that, contrary to common knowledge, nitridation temperature has little effect on the quality of the Ta3N5 produced. Rather, it is the nature of the tantalum starting material and substrate that play a key role. Ta3N5 films synthesized by thermal oxidation and subsequent nitridation of Ta thin films on inert fused silica substrates exhibit identical structural and optical properties, regardless of preparation temperature. The optical spectra collected on these samples reveal clear, distinct features that give insight into the electronic band structure. Films grown in the same manner on Ta foils, however, reveal that textured Ta2N is formed at the Ta3N5/Ta interface even at low temperature, as shown by grazing incidence X-ray scattering. Ta3N5 on Ta foils is converted to bulk Ta5N6 at 1000 °C, and the possible mechanisms for these phase transitions are discussed.
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INTRODUCTION Tantalum nitride (Ta3N5) is an n-type semiconductor that has received increasing attention recently as a photoanode for solar water splitting.1 Its band gap of 2.1 eV yields absorption of a large fraction of the solar spectrum, and its band edge positions straddle the hydrogen and oxygen evolution redox potentials.2 Despite the near-optimal band structure, several challenges remain including stabilizing the surface against photooxidation and improving surface reaction kinetics by the addition of cocatalysts as well as optimizing charge transport and light absorption via nanostructuring.3−6 Interestingly, there exists no widespread standard method to prepare Ta3N5. The range of synthetic routes explored to date includes thermal nitridation of tantalum oxide films7,8 or powders,9,10 electrochemical anodization followed by thermal nitridation to form nanorods or nanotubes,4,6,11 reactive sputtering,12 vapor phase hydrothermal processes,5 reverse homogeneous precipitation to form an oxide precursor,13 and atomic layer deposition.14,15 Wellcontrolled growth of high quality but often insulating materials can also be achieved via chemical vapor deposition using metal−organic precursors or other variations of this method.16−19 A key step in nearly all of the above methods is a high temperature heat treatment in ammonia (NH3) to convert a precursor oxide to the nitride. The goal of the work presented herein is to use well-defined sample architectures to systematically study this key synthetic step in order to gain deeper insights as to how the structural and optical properties of Ta3N5 films are affected by the nitridation temperature. We find that © 2014 American Chemical Society
while temperature has little effect on the crystal grain size or absorption properties of Ta3N5, the presence of metallic Ta in the underlying substrate results in the formation of interfacial TaxNy phases even at low temperatures which extend into the bulk as the temperature increases. At a temperature of 1000 °C, the reducing atmosphere triggers the formation of reduced Ta species leading to sub-band gap optical absorption. Thus the nature of the Ta starting material and substrate play key roles in Ta3N5 film quality, more so than nitridation temperature.
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EXPERIMENTAL SECTION
Sample Preparation. Two types of samples were utilized to probe the properties of interest as a function of nitridation temperature. The first type consists of Ta3N5 supported on fused silica (referred to as Ta3N5/fused silica), and the second more conventional type was Ta3N5 grown on Ta foils (referred to as Ta3N5/Ta). Photoanode devices must be synthesized on a conductive back contact such as Ta for effective collection of the majority charge carriers. However, samples on the transparent fused silica substrates were used for accurate measurement of the absorption properties. Both fused silica and polished Ta foil substrates were cleaned by sequential sonication for 30 min in acetone, isopropyl alcohol, and Milli-Q water and then dried under a stream of Ar. All heat treatments were carried out in a tube furnace (Mellen Company SC12.5R, three zones) with a ramp rate of 10 °C/min. Ta3N5/fused silica samples were synthesized by eReceived: October 22, 2013 Revised: January 17, 2014 Published: February 4, 2014 1576
dx.doi.org/10.1021/cm403482s | Chem. Mater. 2014, 26, 1576−1582
Chemistry of Materials
Article
beam evaporation (Innotec ES26C) of 10−100 nm Ta metal onto clean fused silica substrates (Chemglass, 75 × 25 × 1 mm slides diced into 12.5 × 12.5 mm pieces). The thickness of the as-deposited metal and the resulting nitride films were measured by two methods: (i) profilometry (Veeco Dektak 150) on fused silica samples masked with thin strips of aluminum foil and (ii) cross-sectional scanning electron microscopy (FEI Magellan 400 XHR, 5 kV) of sister samples deposited on silicon. Figure S1 reveals excellent agreement between the two methods. The measured thicknesses of the as-deposited Ta metal, as determined by profilometry, were 16, 30, 49, 72, and 92 nm. The oxidation and nitridation of the Ta metal were carried out in a single run as shown graphically in Figure S2(a). First, the temperature was ramped to 700 °C in 20 sccm O2 + 80 sccm Ar and held for 1 h to oxidize the Ta metal to Ta2O5. The furnace was then purged with 100 sccm Ar and ramped to 850 °C, 900 °C, 950 °C, or 1000 °C and held for 8 h in 50 sccm NH3 before cooling to room temperature in 100 sccm Ar. The resulting nitrides prepared at 900 °C had thicknesses of 17, 54, 96, 150, and 199 nm (Figure S1). The samples used in the temperature study all had a Ta3N5 thickness of approximately 200 nm. Ta3N5/Ta samples were synthesized from Ta foils (20 × 10 × 0.5 mm, Alfa Aesar, 99.95% metals basis excluding Nb) mechanically polished to a mirror finish. To produce Ta3N5 films of similar thickness (
