Photocatalytic Hydrogen Production from Water Using N-Doped

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Photocatalytic Hydrogen Production from Water Using N-Doped Ba5Ta4O15 under Solar Irradiation Aniruddh Mukherji,† Chenghua Sun,‡ Sean C Smith,‡ Gao Qing Lu,†,* and Lianzhou Wang*,† †

ARC Centre of Excellence for Functional Nanomaterials, School of Chemical Engineering and Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, St Lucia, Brisbane, QLD 4072 Australia ‡ Centre for Computational Molecular Science, Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, St Lucia, Brisbane, QLD 4072 Australia ABSTRACT: Solar light induced water splitting on photocatalysts is a very important area of research. Anion doping of photocatalysts normally active only under ultraviolet (UV) light has been reported to be a possible way of increasing visible light photocatalytic performance. Here we report a (111) layered perovskite material Ba5Ta4O15 that was doped with nitrogen. The resulting Ba5Ta4O15 xNx compound exhibited an extraordinary increase in visible light absorbance. The uniform distribution of the nitrogen dopant was attributed to the unique layered (111) structure, which provides intergallery spacings between the perovskite layers for the dopant to diffuse easily in the compound particles during the doping process. It was further verified by density of states that the N 2p states mixed with pre-existing O 2p states that moved the valence band maximum upward without effecting the conduction band, which was composed of the Ta 4d orbital. The doped photocatalysts exhibited not only increased visible light absorbance but increased photocatalytic hydrogen production of ∼50% under simulated solar irradiation, in comparison to that of undoped parent compound.

1. INTRODUCTION Photocatalytic water splitting could potentially be one of the most efficient and carbon neutral ways of utilizing solar energy to produce hydrogen.1,2 So far a large number of photocatalysts have been reported to effectively split water.3 7 However most of these materials only function as a photocatalyst under UV light irradiation because of their wider band gap of ∼3.2 4 eV. The percentage of UV light is less than 5% of the total solar spectrum incident on earth. Thus, in the past few years it has been a research challenge to explore photocatalyst materials that can utilize visible light.7 This implies that such materials should function in the visible light region (420 nm < λ < 800 nm) with a band gap of 2 eV. Band gap engineering of photocatalysts to induce absorption into the wide visible light region seems to be a possible solution to this problem. This would enable the photocatalyst to utilize a much larger proportion of visible-light energy. The early approaches to band gap engineering mainly focused on doping with transition metals.8 Doping of transition metal cations into the wide-band gap energy semiconductors was however reported to be detrimental to the overall performance because metal ions could act electron hole recombination centers.9,10 This process also had issues related to thermal stability and was deemed expensive. Another approach has been the doping of anions such as N,11,12 S,13,14 P,15 and C15 into known wide band gap semiconductors mainly TiO2. This approach was first reported by Asahi et al.,11,16 and since then the nitrogen doping has been considered as one of mostly commonly used ways in reducing the overall wide band gap of r 2011 American Chemical Society

semiconductor photocatalysts. Domen et al. have made significant progress in recent years by developing TaON and many other tantalum oxynitrides including perovskite oxynitrides.1,7,17,18 However it must be considered that a number of nitrogen-doped semiconductors including oxynitrides show oxygen evolution in the presence of an electron scavenger but show very low or no hydrogen production. It is now generally recognized that efficacy of anion doping can be related to structural shortcomings of most photocatalysts. Some of our previous studies in this field have demonstrated that the structural characteristics can play a key role in ensuring effective doping.19,20 Nonhomogenous doping and long diffusion lengths within the bulk of the photocatalysts before reaching surface active sites could play a crucial role in determining efficiency. We recently reported a new strategy to realize homogeneous nitrogen doping which exhibited significantly improved visible light absorption and better photocatalytic properties. The layered structure was of great importance in achieving homogeneous doping.19 A similar strategy was also employed to a tunnelled pyrochlore structure and the material showed a remarkable 2-fold increase in photocatalytic performance.20 This idea forms a good basis for achieving visible light activity, but it must be considered that a number of parameters need to be considered and optimized in order to achieve higher yields. Received: March 25, 2011 Revised: June 21, 2011 Published: June 23, 2011 15674

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The Journal of Physical Chemistry C In this paper in continuation of our previous efforts we report photocatalytic reduction of water methanol mixtures using nitrogen-doped Ba5Ta4O15 under solar light irradiation. A highly donor doped (111) Ba5Ta4O15 is hereby reported to work as an excellent photocatalyst for photocatalytic splitting of water for hydrogen evolution in the presence of a sacrificial agent methanol. This material was earlier reported to be a UV-active photocatalyst by Otsuka et al.21 The nitrogen doping of the material resulted in a remarkable red shifting of the material’s UV visible absorption spectra into the visible region and also enhanced the water splitting capability of the photocatalyst.

’ RESULTS AND DISCUSSION Synthesis and Characterization of Nitrogen-Doped Ba5Ta4O15. Figure 1 shows X-ray diffraction (XRD) patterns of

Ba5Ta4O15 and its nitrogen-doped analogue. Nitridation was carried out at 1173 K in the tube furnace under ammonia flow using the parent oxide which was synthesized by solid state reaction (SSR) following a reported procedure.22 The parent oxide was in good agreement with the reported structure attributed to crystalline Ba5Ta4O15, based on standard diffraction patterns of Ba5Ta4O15 (JCPDS 72-0631). Ba5Ta4O15 has a 111 layered perovskite structure. Figure 2 shows the structure of Ba5Ta4O15. Ba5Ta4O15 materials can be regarded as perovskites that have been cut along the [110] and [111] directions, respectively, into slabs which are separated by Barium cations

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(green spheres) as shown in Figure 2. Both structures have two perovskite layers in each slab. The uniform distribution of dopants can intrinsically determine the electronic structure and consequently play a key role in the absorption threshold and charge carrier mobility of the semiconductor photocatalysts. The layered structure as shown in the figure is thus of great importance in ensuring the feasibility of uniform anion doping resulting in visible light activity of wide band gap photocatalysts. The functional unit of these photocatalysts the TaO6 octahedron has also been shown on the extreme right-hand side. The active sites for photocatalysis are these units. The presence of an intergallery allows the diffusion of active nitrogen molecules and occupies oxygen vacancies in the octahedron. The nature of the nitrogen dopant and its chemical state has been explained in greater detail in the sections ahead. Photoabsorption and Identification of Electronic Structure. Figure 3 compares the UV-light visible spectra of the pristine layered crystalline Ba5Ta4O15 to its nitrogen-doped analogue. The absorbance threshold of the nitrogen species showed an extraordinary red shifting of the spectrum from 305 to 700 nm. This was accompanied by a distinct color change from white to bright orange. This red shift is very significant in light of the commonly reported small absorption shoulders usually obtained as a result of nitrogen doping of TiO2.11 The band gaps of the materials were determined by the Kubelka Munk function. The parent precursor had a band gap of 4.06 eV, which was reduced to 1.78 eV after the successful nitrogen doping of the precursor. A drastic change of greater than 2 eV is very significant, and such a large band gap shift has never been reported for this or any other 111 layered perovskite material. The relative energy levels of the conduction and valence bands have known to be of crucial importance in determining the redox potentials of a photocatalyst and the power of photoinduced charge carriers in photocatalytic reactions. To investigate the reasons responsible for such a drastic red shift of the UV visible light absorption spectrum and the effect of nitrogen doping on the positions of the valence and conduction bands the total densities of states of the valence bands was measured. Density of States. The effect of nitrogen doping was investigated using density of states (DOS) as illustrated in Figure 4. The valence band maxima in nitrogen-doped Ba5Ta4O15 in contrast to the undoped species shifted from 4.06 to 1.78 eV.

Figure 1. XRD patterns of Ba5Ta4O15 and N-doped Ba5Ta4O15.

Figure 2. Structural models for undoped (left) and N-Ba5Ta4O15 (middle). The basic Ta O unit is shown (right) with one Ta atom in the center.

Figure 3. UV visible absorbance spectra of Ba5Ta4O15 and N-doped Ba5Ta4O15. 15675

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Figure 4. Calculated DOS for (a) undoped and (b) N-doped Ba5Ta4O15. Local DOS of nitrogen atom (N-DOS) (c). Given net spin has been introduced, spin-polarization is considered, and the spin-up and spin-down DOS profiles have been shown in (a c). The highest-occupied state (dotted line) has been shifted to zero energy as an energy reference.

Figure 5. XPS spectra for (A) Ba5Ta4O15 and (B) N-doped Ba5Ta4O15.

The substitutional doping of nitrogen as shown in Figure 4b significantly narrowed the overall band gap instead of some isolated states in the band gap. The local DOS of only the nitrogen atom in the lattice has also been shown separately in Figure 4c to highlight this fundamental issue. This shift of greater than 2 eV was a result of the mixing of the N 2p and O 2p orbitals. Increasing the doping concentration can introduce more unoccupied states above the top of intrinsic VB, as indicated in parts b and c of Figure 4. The calculated band gap is slightly lower than the experimentally obtained value of 1.76 eV. Pure DFT usually underestimates band gaps because of the limitation of semilocal exchange-correlation functionals. But the predicted narrowing of band gap above agrees well with the experimental observations from the UV vis absorbance spectroscopy. It has been suggested earlier that tantalates in general are better performing materials for applications in hydrogen production through water splitting due to their greater reduction potentials. Our findings suggested that both the parent precursor and the nitrogen species of Ba5Ta4O15 have more negative energy levels ( 0.9 V vs NHE (normal hydrogen electrode)) than any of the other transition metal oxide materials, i.e., Nb ( 0.7 V vs NHE) or Ti ( 0.1 V vs NHE). It is thus clear that the reduction in the overall band gap resulted from the shifting of the valence band maximum (VBM) after nitrogen was doped into the crystal lattice resulting in O 2p and N 2p orbital mixing. This has been schematically depicted later in Figure 6. Chemical State of Nitrogen. The states of the N dopant were investigated by using X-ray photoelectron spectroscopy (XPS).

N 1s core levels were measured using XPS. As shown in Figure 5 no signal for nitrogen species is observed. A strong peak centered at 403 eV was evident in both scans. This could be attributed to the Ta (4p 3/2). It is known that tantalum was abundant in the sample. After the nitrogen doping however an additional peak in the vicinity of the Ta 4f peak centered at 395 eV was observed. Nitrogen doping and the chemical states related to the nitrogen have been widely investigated especially for TiO2 many reports are divergent with binding energy values reported from 395 to 401 eV. In our previously reported results we observed that N1s peaks centered at 396 eV could be attributed to β-N. A similar value has also been reported by Hara et al for tantalum oxynitrides.6 This suggests that in the bilayers, which are mainly consisted of TaO6 octahedral units, a small portion now consisted of Ta N linkages formed as a result of nitrogen doping. The doping of nitrogen to such an extent may be attributed to the interlayer gallery spacing in this 111 layered perovskite material. The presence of intergallery spacing facilitates the nitrogen precursor to diffuse into the inner parts of the inter gallery layer spacings where the active sites for photocatalysis are known to exist. It is also important to understand that unlike three-dimensional anatase particles which only get doped mostly on the outer surface these layered materials were doped homogenously. Inductively coupled plasma (ICP) revealed an amount of ca. 0.23 wt % of N in the N-doped Ba5Ta4O15, while elemental analysis using a flash elemental analyzer revealed that the nitrogen content was ca. 0.30 wt % in the same sample, which is in good agreement with the ICP and also with the quantitative XPS analysis results. Photocatalytic Activity for Hydrogen Production. Photocatalytic water splitting yields of hydrogen for Ba5Ta4O15 and the nitrogen-doped Ba5Ta4O15 in aqueous suspension with ethanol as a sacrificial agent (electron donor) were realized. The experiments were conducted under simulated sunlight (AM1.5, 1000 W m 2), containing small amount of UV light ( 420 nm a cut off filter was used to establish if photons with lesser energy could be utilized or not. After the incorporation of the cut off filter a yield of 42 μM/h was 15676

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Figure 6. Schematic figure showing band structures of Ba5Ta4O15 and N-doped Ba5Ta4O15. The conduction band and valence band levels have been applied based on the DOS studies carried out. Ba5Ta4O15 is considered to be a flat band semiconductor here due to its nature as an n-type semiconductor.

Figure 7. H2 production rate comparison of pristine Ba5Ta4O15 and N-doped Ba5Ta4O15 under simulated solar irradiation.

observed. However the parent precursor did not show any activity under similar conditions which further confirmed that the nitrogen doping had rendered the Ba5Ta4O15 visible light active (see Figure 7). The above-mentioned reactions were carried out in an aqueous phase in a 100-mL quartz cell. The top of the quartz cell was circular in shape with an area of 25 cm2. Photocatalyst (0.1 g) was suspended in 10 mL of 20 vol % methanol solution. The procedure relied on analyzing the actual number of hydrogen molecules generated as a result of photocatalytic water splitting under simulated solar irradiation. It must be emphasized that unless identical reactors, catalyst loadings, reaction conditions, and light sources, the significance of these results lies in the fact that under identical conditions the nitrogen-doped Ba5Ta4O15 performed much better compared to the undoped precursor, and the increase in performance was 166%. This increase was attributed to the 0.2 wt % nitrogen doping. The evolved gases were sampled continuously using an online quadropole mass spectrometer and the photocatalyst exhibited reasonably good stability under the same experimental conditions over 15 h of experimental period. Longer term photocatalytic tests (>30 h)

showed slight drop in hydrogen production rate (E0H+/H2) is unaltered after the N doping, which again confirms our experimental findings of a higher yield of solar hydrogen, which is also in good agreement with the band structure calculations. This process of successful homogeneous anion doping of layered perovskite materials was of particular importance in two respects: first, it offers a feasible way of inducing significant visible light absorption as a result of the extraordinary band to band shift in many transition metal oxides based wide band gap semiconductors with layered perovskite structures, which were usually active only under UV irradiation; second, the process of N doping is very simple from the viewpoint of scale-up production of photocatalysts and is of low cost.

’ CONCLUSIONS We developed a new route to successfully dope nitrogen into layered (111) perovskite tantalate material. The nitrogen doping leads to an extraordinary increase in absorption of visible light accompanied by enhanced visible light photocatlytic activity. The doping resulted in a significant narrowing of the band gap from 4.06 eV to ca. 1.76 eV, and first-principle DFT studies were used to confirm the lowering of the band gap for the nitrogen-doped Ba5Ta4O15. The nitrogen-doped materials exhibited a considerably increased photocatalytic hydrogen production rate compared to the undoped Ba5Ta4O15. 15677

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’ EXPERIMENTAL SECTION Synthesis. Ba5Ta4O15 was synthesized by SSR. Stoichiometric mixtures of BaCO3 and Ta2O5 powders at 1150 °C for 36 h with one intermediate grinding after 18 h were calcined. Nitrogen doping of this Ba5Ta4O15 powder was carried out by annealing these powders in flowing ammonia atmosphere at 900 °C. The time period for annealing was 150 min which was optimum. Doping procedure resulted in N doping level of 0.3 wt %. Characterization. XRD patterns of al samples were collected in the range of 10 90° (2θ) using a Rigaku Miniflex X-ray diffractometer (Co KR radiation, λ = 1.7902 Å). The UV vis absorbance spectra were obtained for the samples using a scan UV vis spectrometer (Shimadzu UV-2450). The spectra were recorded at room temperature in air within the range of 200 800 nm. XPS was carried out using a Kratos axis ULTRA X-ray photoelectron spectrometer. The binding energies were charge corrected using adventitious carbon as reference. The surface states of the photocatalysts before and after 18 h water splitting reaction cycles were also examined using XPS. Elemental analysis was carried out using a flash elemental analyzer. The products of combustion were sent through a packed chromatographic column. The column converted the products into NO2. These simple compounds were then measured using a thermal conductivity detector. These elemental composition results were also verified by using ICP analysis. Computational Details. All calculations were carried out using DFT within the generalized-gradient approximation,23 with the exchange-correlation functional of Perdew Burke Ernzerhof,24,25 based on the numerical double-numerical polarization (DNP) basis set, which has been implemented in the Dmol3 modules. The N/O ratios of 1/17 was used for these calculations. More tests and the discussion regarding the efficiency and reliability of the DNP basis set can be found in Delley’s work, in which the estimated errors from the PBE functional with the DNP basis set was supposed to be lower than that with hybrid B3LYP/6-31G functional. Photocatalyst Evaluation. Photocatalysis reactions were performed in an air free closed gas circulation system with a quartz reaction cell using a 300-W Xe lamp (Newport, Oriel 91160) solar simulator with integrated AM1.5 filter. Hydrogen evolution was detected online. Argon with a flow rate of 100 mL/min was used as a carrier gas, controlled by a Brooks 5850E mass flow controller, and was passed through a quartz glass cell containing aqueous suspensions composed of 100 mg of active photocatalyst and 100 cm3 of 20 vol % methanol solution (electron donor) in water. The parameters used for the analysis were similar to our earlier reported work.20

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Smart Future Fellowship for C.H.S.). Computational resources used in this work were provided by Centre for Computational Molecular Science and Australian Partnership for Advanced Computing National Facility.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The Australian Research Council is acknowledged for its financial support through DPs and Centre of Excellence to this project and Dr. Barry Wood from the Centre for Microanalysis and Microscopy (CMM), the University of Queensland is appreciated for his help with the XPS experiments. This work is supported by The University of Queensland (Research Excellence Award for C.H.S.) and Queensland Government (through 15678

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