Challenges in Co-Alloyed Titanium Oxynitrides, a Promising Class of

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Challenges in Co-Alloyed Titanium Oxynitrides, a Promising Class of Photochemically Active Materials† James J. Brancho and Bart M. Bartlett* Department of Chemistry, University of Michigan, 930 North University, Ann Arbor, Michigan 48109, United States ABSTRACT: We present a perspective on recent developments in modified TiO2 photocatalysts for visible light-driven photochemistry with an emphasis on water splitting. We focus on doped and alloyed TiO2 and in particular address the synergistic effects observed in materials with both transition metal cations and nonmetal anions. Several reports have demonstrated absorption of longer wavelengths (λ = 500−600 nm) by codoped materials compared to the absorption edge of TiO2. We review these advances against the backdrop of well-established doped TiO2 research, suggesting on the basis of compositional analysis and wavelength-resolved measurements of photon conversion efficiency that the increase in visible light absorption is likely due to absorption between defect states rather than true band gap narrowing. We draw a distinction between codoped and co-alloyed materials, stressing the attractive electronic structure of the latter. In highlighting recent literature, data examining the rate of photochemical water splitting or magnitude of anodic current as they depend on the wavelength of incident light are emphasized. Finally, areas for further research are highlighted, particularly in the synthesis of co-alloyed compositions of TiO2.

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exhibit (1) a band gap suitable for visible light absorption, (2) valence and conduction band edges that straddle the standard potentials for proton reduction and water oxidation, (3) a kinetically active surface that is (4) stability over thousands of cycles of illumination and resting, and finally (5) minimal cost, as dictated by the abundance of constituent elements and also by processing costs. A photoelectrochemical system relaxes some of these requirements but adds other considerations. In photoelectrochemical water splitting, water oxidation and reduction take place on separate electrodes, which increases the variety of materials that can be used. N-type metal oxides, for example, are not typically fast catalysts for hydrogen evolution but are the most effective materials available for oxygen evolution. Nanostructured photoelectrodes must exhibit high conductivity for majority carriers (electrons for n-type or holes for p-type) to maximize their efficiency. Minority carriers can be rapidly extracted when charges are generated near the electrode−electrolyte interface, but majority carriers must traverse the thickness of the electrode before being collected at the back contact. This requirement is relaxed in nanoparticle catalysts because the particle size can be tuned below the carrier diffusion length. Both photochemical and photoelectrochemical water splitting remain active areas of research, and both will be discussed in the current perspective.

olar energy storage stands as one of the most important research topics for today’s scientists, particularly inorganic and materials chemists. Generally, the purpose of a solar energy storage device is to harness incident solar radiation to carry out a chemical reaction, thereby storing that energy in a chemical fuel. The archetypal solar energy storage reaction is water splitting, in which water is decomposed into its constituent element gases, H2 and O2.1−4 Producing hydrogen via water splitting is an exciting alternative to the existing standard of methane reformation, which uses nonrenewable fossil fuels and produces CO2 as a byproduct. Combusting H2 regenerates water and releases the stored energy. While gaseous hydrogen’s low volumetric energy density limits its potential as a fuel for personal vehicles, the market for hydrogen as a chemical feedstock remains large. Worldwide hydrogen production, mainly for use as a chemical feedstock, stands at around 50 million tons/year.5,6 Photochemical1,7−9 and photoelectrochemical1,3,10 schemes for water splitting have been studied extensively. In such systems, a catalyst is exposed to sunlight, generating reactive electron−hole pairs that are used to carry out chemical reactions. The energetic photogenerated electron is stored in a molecular producta stable, portable solar fueland the hole oxidizes an electron donor such as water. Photochemical water splitting is attractive due to its simplicity. A typical reactor is comprised only of an aqueous suspension of nanoparticulate catalyst material; the catalyst carries out both the oxidation and reduction reactions. However, the design criteria for the catalyst material are demanding. A single-phase material must at once

Received: June 19, 2015 Revised: October 7, 2015

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This Perspective is part of the Up-and-Coming series. © XXXX American Chemical Society

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DOI: 10.1021/acs.chemmater.5b02357 Chem. Mater. XXXX, XXX, XXX−XXX

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1. MODIFYING TITANIUM DIOXIDE BY SINGLE-ION DOPING To date, no single-phase material or material tandem has shown the requisite activity for implementation as a commercial energy solution. Transition metal oxides, particularly TiO2, have been studied since the earliest days of water splitting research due to their oxidative stability, abundance, and relative ease of synthesis.11,12 With few exceptions, these oxides are hindered either by wide band gaps that limit solar absorption to ultraviolet light or by partially filled d-shells that effect low mobility of charge carriers.4,12,13 TiO2 is an attractive industrial material due to its abundance, durability, and an impressive versatility of available syntheses.14−16 Its energetic band positions enable it to drive a variety of desirable reactions photochemically, including water splitting. The main limitation in TiO2 is a band gap energy of 3.2 eV (anatase) or 3.0 eV (rutile), restricting TiO2 to an ultraviolet photocatalyst. Attempts to produce a visible lightactive titania began with singly doping the material, either with a cation on a Ti4+ site or with an anion on an O2− site. Cationic doping of TiO2 has largely focused on first-row transition metals in various oxidation states.17,18 Transition metal-doped TiO2 materials have not been widely successful as visible light photocatalysts. For example, Serpone et al. prepared TiO2 doped with Cr3+, Fe3+, and V5+ and measured their photoconductivity and photochemistry relative to pure TiO2.19 Although the compounds displayed high activity for more kinetically facile reactions (oxalic acid splitting), their ability to generate hydrogen via photochemical water splitting was diminished (for Cr3+) or eliminated entirely (for Fe3+ and V5+). The visible light excitations were attributed to dopant metal crystal-field transitions. It is thought that metals with partially filled d-shells, whether incorporated as dopants or stoichiometrically, facilitate the recombination of photogenerated charge carriers by acting as trap sites. However, a number of cation-doped systems have demonstrated increased photochemical activity under ultraviolet excitation relative to TiO2, particularly when doped with Fe3+. It has been suggested that Fe3+-doped catalysts are uniquely effective among cationdoped TiO2 due to the ability of Fe3+ to serve as a shallow trap for both electrons and holes.17 Also, an important relationship exists between the Fe3+ loading in the photocatalyst and the size of the host particle. Zhang et al. prepared several Fe3+doped TiO2 materials of various particle sizes and measured their photoactivity for chloroform oxidation, finding a different optimum Fe3+ loading for each particle size.20 In smaller particles, recombination of charge carriers at the particle surface becomes a dominant loss pathway; the Fe3+ doping slows the migration of excited charge carriers toward the surface, thereby reducing the rate of surface recombination. However, if Fe3+ is doped in beyond the optimum concentration, the mobility of charge carriers decreases critically as they are trapped more often en route to the surface. Importantly, Zhang et al. found optimum Fe3+ doping levels of 420 nm λ > 400 nm UV + visible λ > 400 nm UV + visible λ > 400 nm UV + visible

62 63 74 74 76 76 76 76 77 77

Compositional analysis determined by XPS and does not represent bulk concentration unless otherwise noted. Variable subscripts or “nd” imply undetermined values. bIncident light intensities not reported. “UV + visible” refers to an unfiltered metal halide lamp (Philips HPA 400/30S).

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Tables 1 and 2. In most cases, incorporation of impurity species (N, in particular) remains low, creating electronic structures depicted in Figure 2c,d. In order to realize band gap narrowing and maximize absorption of visible light, unit mole ratios of substituent ions are necessary, and mole fraction must be high.

4. CHALLENGES FOR PREPARING THE ELUSIVE CO-ALLOYED MATERIALS Synthetic preparations for co-alloyed materials remain limited. A useful synthetic method must be generalizable, tunable, and scalable. Many of the methods presented in this perspective and elsewhere rely on the preparation of a TiO2 or cation-doped TiO2 precursor followed by heat treatment in a reactive atmosphere to achieve anion codoping. Heterogeneous reactions are often confined to particle surfaces, which is a likely explanation for the 2−7 mol % ceiling observed in most N-doped TiO2 studies, even with a cationic dopant present to increase the solubility of the nitride anion. With only a narrow range of anion contents available by these methods, tunability is limited. Conversely, electrosynthetic methods based on TiO2 anodization have not yet demonstrated complete control over cation contents over their solubility range, shown by Cottineau et al.,65 for example. Additionally, dopants introduced in gas-phase surface reactions do not necessarily diffuse throughout the material bulk, as evidenced by secondary-ion mass spectrometry (SIMS) mapping analysis by Cho et al. in TiO2:(W,C) nanowires (Figure 7).68 In codoping schemes, absorption between impurity pairs is the primary driver of visible light absorption. However, with indirect band gap materials such as anatase

Figure 7. SIMS profile of a single TiO2:(W,C) nanowire showing dopant concentration across the nanowire thickness. Adapted with permission from ref 68. Copyright 2013 Nature Publishing Group.

TiO2, complete photon absorption occurs deep within the material bulk. If impurity pairs segregate closer to the surface F

DOI: 10.1021/acs.chemmater.5b02357 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials than the penetration depth of incident light, absorption is not likely to be efficient. Ruddy et al. presented an interesting synthesis for attempts at co-alloyed TiO2:(Mo,N) which proceeded via high-temperature, high-pressure thermolysis of a molybdenum nitride complex in the presence of titanium tertbutoxide in benzene solution.71 Compositional analysis of the resulting materials shows superior nitrogen incorporation, with N:Mo ratios of 0.73−0.77 in the co-alloyed products with Ti:Mo ranging from 3.05 to 21.5. This clever synthetic approach releases cationic and anionic dopant species simultaneously in a charge compensating stoichiometry. However, nitrido complexes are not accessible for all transition metals, so the scope of this approach may be limited to dopant candidates for which metal nitrido complexes are isolable. Another promising body of synthetic work has been the development of the urea-glass synthesis, which has been used to prepare a variety of phase-pure metal nitride nanoparticles with low carbon content.80−83 This attractive synthetic pathway combines readily available metal chlorides with urea as an abundant nitrogen source in alcohol solution. It has been suggested that the synthesis is amenable to preparing thin film electrodes, but to date electrodes have been prepared (when necessary) by suspending nanoparticles in Nafion. The nitride materials are promising electrocatalysts.84,85 However, only one report of synthesizing oxynitrides via the urea-glass route has been published,86 and no reports of mixed-phase or ternary metal nitrides have been published using this synthesis. Extending the urea-glass synthesis to co-alloyed materials could be a promising research area. Moving forward with co-alloyed materials, determining composition remains difficult. Chemical analysis is typically performed by X-ray photoelectron spectroscopy (XPS) and less often by energy dispersive X-ray spectroscopy (EDX). Chemical analysis by XPS is confined to the 5−10 nm surface layer of a particle and is not representative of the bulk composition. Particularly with respect to anionic dopants such as nitrogen introduced by heterogeneous reactions with reactive gaseswhere surface segregation is likelyXPS does not give an accurate picture of the bulk material composition. EDX analysis is more favorable for metallic elements but is difficult for light elements due to overlapping lines and ease of contamination. Reliable, direct quantification of nitrogen content remains a challenge in the field. Rutherford backscattering spectrometry (RBS) has been used to probe compositions in TiO2 and N-doped TiO2 materials.87−91 RBS, often coupled with angle-resolved XPS, has been well-utilized in materials science to characterize thin films and assemblies of a variety of compositions.92−96 Depth profiling is also possible. However, for titanium oxynitride species, compositional analysis is complicated by partially overlapping signals for N and O. Brijs et al. compared RBS to another depth profiling technique, low-energy heavy ion elastic recoil detection (ERDA), and concluded that both techniques suffered from loss of resolution as compositions were probed in deeper regions of the materials under study.92 Often, nitrogen content in terms of atomic percentage is not reported directly by RBS, but rather using a combination of RBS and XPS or RBS and nuclear reaction analysis (NRA).87,91 We conclude that SIMS is perhaps the most promising technique currently available for examining depth-dependent compositions. SIMS has already been used to measure sensitive phenomena such as nitrogen dopant diffusion through rutile crystals.97,98

Many exciting reports have been published about the photochemical activity of codoped and co-incorporated materials, and wavelength-dependent quantifications of reactivity have been very useful for establishing a picture of their electronic structure. Generally, modified titania photocatalysts have exhibited poor quantum efficiencies in the visible range of the spectrum despite, in some cases, strong visible light absorption. Additionally, activity in the ultraviolet range is decreased relative to parent TiO2 prepared using similar methods as charges generated by band-to-band excitations are trapped by localized dopant states residing in the midgap. Improved spectral response in both the UV and visible regions has been indicative of improved charge compensation and reduced defect structure in the material bulk, as discussed above. However, the inherent difficulty of preparing wellcontacted thin films in many cases precludes highly resolved wavelength-dependent measurements for powder photocatalysts. More general methods of interrogating both thin films and powder materials are needed. Photoluminescence (PL) spectroscopy is a standard technique for the characterization of defect structure in inorganic solids. It has been applied to both rutile and anatase TiO2 of various morphologies99−102 and also to doped titania systems.103−105 PL lifetimes have informed on the nature of defect states and how various defects affect carrier transport and recombination. However, measured lifetimes are typically in the μs to ms regime. To investigate faster processes that can have a profound effect on photocatalysis, more sophisticated techniques are required. Transient absorption spectroscopy (TAS) has become an extremely useful technique for measuring fast photodynamical processes in reactive semiconductors. The technique has been applied to several TiO2-based systems.30,106−111 Perhaps the greatest strength of TAS is its adaptability to in situ and in operando measurements, revealing carrier dynamics under the most relevant experimental conditions. TAS has also revealed kinetic information such as surface reaction kinetics and activation energies for heterogeneous reactions.112−114 Although doped TiO2 has been investigated with TAS, codoped TiO2 has not received the same attention. Understanding the effects of codoping, coincorporating, and potentially co-alloying on charge carrier dynamics will be critical to understanding observed differences in reactivity; TAS provides such information with great detail and unparalleled time resolution. Ruddy et al. present a similarly useful transient photoconductance study on their coincorporated TiO2:(Mo,N) materials and find that the TiO2: (Mo,N) samples exhibit long-lived (ms time scale) photoconductance at both UV and visible excitation wavelengths. The photoconducance signal is unexpectedly low, pointing to potential picosecond-time scale charge trapping activity or to low generation yield.71 Detailed experimental probes of electronic structure through ultraviolet photoelectron spectroscopy (UPS) and valence band XPS will further augment this understanding, allowing a complete band structure for codoped and co-alloyed materials to be assembled. Quantifications of the band edge energies and defect structures of potential photocatalysts will guide their application toward chemical reactions; however, we must stress that these measurements only realize their full impact when conducted in full knowledge of the chemical composition and homogeneity of the materials in question. Therefore, together with new synthetic approaches, we place priority on the improvement/further application of methods for characterizing G

DOI: 10.1021/acs.chemmater.5b02357 Chem. Mater. XXXX, XXX, XXX−XXX

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efficiency; ERDA, energy recoil detection analysis; IPCE, incident photon-to-current efficiency; IQE, internal quantum efficiency; NTs, nanotubes; NWs, nanowires; PL, photoluminescence spectroscopy; SIMS, secondary-ion mass spectrometry; RBS, Rutherford backscattering spectrometry; TAS, transient absorption spectroscopy; UPS, ultraviolet photoelectron spectroscopy; XPS, X-ray photoelectron spectroscopy

both the bulk composition and the depth-dependent composition of photocatalysts and photoelectrodes.

5. CONCLUSION In this perspective, we offer a review of current research in the area of codoped, co-incorporated, and co-alloyed titanium dioxides. We cover some of the expansive literature on aniondoped titania to bring important points about the electronic structure of doped materials to the forefront and explain how codoping or, more preferably, co-alloying, stands to build upon the previous discoveries. Finally, we point out areas for fruitful research on these materials. Most importantly, versatile synthetic methods for incorporating multiple dopants into a host structure homogeneously and in precisely controlled ratios remain critically deficient. We believe that co-alloyed titania, although experimentally evasive at present, will remain an exciting and important area of photocatalysis research moving forward due to the potential scalability of their syntheses, earthabundance, and relatively low cost.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions from both authors. Both authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. Biographies James J. Brancho, a native of Canonsburg, PA, earned a B.S. in Chemistry from Duquesne University in Pittsburgh, PA, in 2011, after which he entered the chemistry graduate program at the University of Michigan. He is excited by solar energy’s potential to alleviate environmental and geopolitical stresses, and his research project focuses on solution-based syntheses of co-alloyed titanium−niobium oxynitrides. Jimmy hopes to leverage his research experience in a career in science communication, and he regularly blogs chemistry and student issues at Tree Town Chemistry (http://treetownchem. blogspot.com). Bart M. Bartlett is the Seyhan N. Eǧe Associate Professor of Chemistry at the University of Michigan. He entered scientific research in 1995 during his junior year at Metro High School (St. Louis, MO), where he worked in a worm genetics lab at the Washington University School of Medicine. More fascinated by the lab’s buffer solutions than by the worms, Bart went on to complete his professional training in chemistry at Washington University, MIT, and UC Berkeley. Now, he and his talented research team relentlessly pursue preparing new compositions, phases, and morphologies of matter for solar energy conversion and electrical energy storage. Bart is still an avid fan of buffers, as they are often essential for synthesizing and/or testing materials for their desired applications.

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ACKNOWLEDGMENTS J.J.B. and the research presented from our group have been supported by the United States Department of Energy DEF602-11ER16262 and DE-SC0006587.

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ABBREVIATIONS APCE, absorbed photon-to-current efficiency; EDX, energydispersive X-ray spectroscopy; EQE, external quantum H

DOI: 10.1021/acs.chemmater.5b02357 Chem. Mater. XXXX, XXX, XXX−XXX

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