Tantalum nitride for photocatalytic water splitting: concept and applications
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Along with many other solar energy conversion processes, research on photocatalytic water splitting to generate hydrogen and oxygen has experienced rapid major development over the past years. Developing an efficient visible-light-responsive photocatalyst has been one of the targets of such research efforts. In this regard, nitride materials, particularly Ta3N5, have been the subject of investigation due to their promising properties. This review focuses on the fundamental parameters involved in the photocatalytic processes targeting overall water splitting using Ta3N5 as a model photocatalyst. The discussion primarily focuses on relevant parameters that are involved in photon absorption, exciton separation, carrier diffusion, carrier transport, catalytic efficiency, and mass transfer of the reactants. An overview of collaborative experimental and theoretical approaches to achieve efficient photocatalytic water splitting using Ta3N5 is discussed.
KeywordsTa3N5 Water splitting Photocatalysis Crystal structure Optoelectronic properties Kinetics Carrier dynamic Interface Oxygen evolution reaction
The development of new clean and inexpensive energy resources as an alternative to conventional fossil fuel energy is a crucial challenge for the scientific community. Solar H2O splitting is one of the most innovative solutions that have emerged over the past years [1, 2, 3, 4, 5, 6, 7, 8]. H2 production can currently be achieved through an artificial photosynthetic way, in the UV domain, using broad bandgap photocatalyst materials such as TiO2  or using photoactive oxides such as α-Fe2O3, WO3 or BiVO4 under visible light irradiation (1.7–2.8 eV) [10, 11, 12, 13, 14, 15]. However, these types of materials are effective only for half of the reaction (water oxidation). For overall water splitting, there is a limited choice of photocatalysts, such as Ta3N5, TaON, or some material from the family of oxynitride perovskites [16, 17, 18, 19, 20, 21, 22, 23, 24]. Achieving one step overall water splitting with one single photocatalyst presents a practical way in term of engineering design of the reactions. The simplicity of using powder semiconductor photocatalysis makes this technique economically feasible for its scalability and capital cost. Thanks to its extended visible absorption (600 nm) and its ability for the redox reaction, Ta3N5 is an attractive photocatalyst that can theoretically achieve a solar energy to hydrogen conversion efficiency of ~17 %. Many studies have addressed improving the photocurrent at the potential for water oxidation [20, 21, 22, 23, 24]. A significant photocurrent can be achieved by increasing the structuration of the photocatalyst and using a high-performance cocatalyst, but understanding the intrinsic properties of Ta3N5 for obtaining a significant improvement in the global photoelectrocatalytic reaction is still a challenge. Knowledge of the material’s intrinsic properties can help to improve the photocatalytic performance.
Photocatalytic water splitting mechanism
From the mechanism for photocatalytic water splitting, it is clear that this reaction involves complex photophysical and chemical processes on different time scales. Our recent review addressed the fundamental parameters involved in photocatalytic overall water splitting . The steps involved in the photocatalysis for water splitting are divided into the following six processes: (1) photon absorption, (2) exciton separation, (3) carrier diffusion, (4) carrier transport, (5) catalytic efficiency, and (6) mass transfer of reactants. All steps are illustrated in Fig. 1.
The current review aims to describe the relevant parameters related to the six processes mentioned above using Ta3N5 as a model photocatalyst. Some physicochemical properties, such as electronic structure, interface development, and electrocatalytic properties, will be described and correlated to achieve a comprehensive understanding of the complex sequential processes of overall water splitting.
For a semiconductor to be considered a good material for photocatalytic water splitting, there are several general requirements. First, to utilize visible light, it needs to have a low bandgap energy [7, 8, 18, 25, 26, 27, 28, 29, 30, 31]. A photocatalyst with a bandgap energy of approximately 2.1 eV, corresponding to a wavelength of 600 nm, is sufficient for achieving the targeted efficiency. The second requirement is that the photocatalyst band positions have to straddle the water redox potential (ECB > 0 vs. RHE, EVB < 1.23 vs. RHE) [7, 8, 18, 25, 26, 27, 28, 29, 30, 31]. Next, the photocatalyst needs to have excellent stability under the photocatalytic reaction conditions, under illumination and in the dark [7, 8, 18, 25, 26, 27, 28, 29, 30, 31]. Some potential low bandgap energy photocatalysts, such as CdS and CdSe, fulfill the first requirement and are very active for hydrogen evolution. However, these photocatalysts suffer from photocorrosion or self-oxidation due to their valence band positions, which lie more negative than the water oxidation potential [32, 33, 34, 35, 36]. WO3, on the other hand, is very good for oxygen evolution, but its CB position lies more positive than the water reduction potential [37, 38]. Hence, compared to photoelectrochemical (PEC) systems, a single photocatalyst system has fewer choices of existing photocatalysts. The fourth requirement is that the semiconductor should possess a high catalytic activity toward the oxidation or reduction of water. Finally, the semiconductor must be economically viable. Considering the scalability of the powder photocatalyst system, the photocatalyst must therefore be composed of inexpensive, abundant materials and have a largely scalable synthesis.
Studies on finding suitable and effective photocatalysts to fulfill all the aforementioned requirements have been conducted using various methods. Investigations have mainly focused on reducing the bandgap of the photocatalyst while maintaining its band position relative to the H+/H2 and O2/H2O redox potentials. The successful synthesis of a visible-light-responsive photocatalyst was demonstrated by the formation of (oxy)nitrides [19, 20, 21, 22, 23, 24, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50]. Particular interest has been focused on Ta3N5 due to its absorption spectrum that can go up to 600 nm: a target for achieving efficient photocatalytic water splitting. It has been reported that Ta3N5 can generate H2 or O2 from water under visible light in the presence of suitable sacrificial reagents [19, 39, 40, 41, 42, 43, 44, 45, 46, 47].
Despite considerable studies as a photocatalyst for many years, information on Ta3N5 is still lacking regarding its thorough characterization to understand its underlying physical and chemical properties. Theoretical studies on this material are also limited. This review addresses the global picture of photocatalytic reactions using Ta3N5 as a model photocatalyst. This material was selected for two reasons. First, the initially reported photocatalytic activities were promising. Second, prior to the start of the work presented in this review, there were conflicting reports on the performance levels of different Ta3N5 materials, i.e., there was limited understanding of the factors that affect its photocatalytic performance.
Different methods for Ta3N5 synthesis
Direct nitridation generally produces Ta3N5 with good photocatalytic activity toward oxygen evolution but low activity for hydrogen evolution, approximately one order of magnitude lower than that for oxygen evolution [19, 39, 40, 41, 42, 43, 44, 45, 46, 47]. The resulting Ta3N5 generally contains some anion defects, as observed from its absorption spectrum after the band edge absorption (~710 nm), which is associated with its low photocatalytic performance due to the recombination reaction of photogenerated electrons-holes . In addition, this method resulted in agglomerated Ta3N5 with a very low surface area [19, 39, 40, 41, 42, 43, 44, 45, 46, 47]. To overcome these problems, several attempts to improve the synthesis method of Ta3N5 have been pursued in various ways.
To improve the surface area and obtain a uniform size distribution, nanoparticle (NP) Ta3N5 has been successfully synthesized using mesoporous graphitic carbon nitride (mpg-C3N4) as a decomposable and reactive template [45, 46]. The particle size of Ta3N5 is controlled to sizes as small as approximately 7 nm by controlling (Fig. 2b) the pore size of the mpg-C3N4 template, and its surface area can reach 60 m2 g−1. The resulting Ta3N5 NPs exhibits improved photocatalytic activity for H2 evolution in the presence of methanol as a sacrificial reagent. The size of semiconductor particles is believed to affect their photocatalytic activity by reducing the distance that the excited electrons and holes must travel to reach the active surface sites. Moreover, the size has also been hypothesized to affect the space-charge layer and band bending that govern the photocatalytic activity .
Improved photocatalytic hydrogen evolution was reported on Ta3N5 synthesized using a sol–gel method. In this method, Ta2O5 was first grown on the surface of SiO2 spheres with a diameter of ~550 nm and subsequently subjected to nitridation . The final Ta3N5/SiO2 sample exhibited a uniform size distribution with high crystallinity and a core–shell structure with a narrow size distribution without aggregation. The photocatalytic activity of the Ta3N5/SiO2 samples toward hydrogen evolution was considerably higher than that of Ta3N5 synthesized via the conventional nitridation of commercial Ta2O5. Interestingly, when no SiO2 was used, the photocatalytic hydrogen evolution of sol–gel Ta3N5 was relatively comparable to that of Ta3N5/SiO2. Hence, attributing the improvement in photocatalytic activity toward hydrogen evolution to only the smaller particle size and higher surface area is questionable. Note that light scattering by SiO2 in such a way to improve the absorption of light by Ta3N5 may occur. A simple physical mixture of SiO2 and Ta3N5 did not improve photocatalytic performance; thus, an additional understanding of this type of supported photocatalyst is required.
Through a simple modification of the synthesis with an alkali metal salt, an improved photocatalytic oxygen evolution has been achieved on Ta3N5 . In this method, the Ta2O5 precursor was first modified with an alkali metal salt (i.e., Na2CO3) prior to nitridation. This alkali metal salt produces a molten salt state at high synthesis temperatures (flux). The presence of the alkali metal salt improved the activity of Ta3N5 by affecting the crystal growth, which further led to higher crystallinity and a smaller particle size (Fig. 2c). Although low valence cation substitution may reduce the number of reduced Ta species by charge compensation, the enhanced physicochemical properties of Na-doped Ta3N5 and their relationship to higher photocatalytic activity remain unclear.
Quantum confinement in semiconductor nanoparticles (NPs) is believed to be capable of tuning the redox properties of the material if the size of the NPs becomes smaller than the exciton radius of an electron–hole pair [56, 57]. The synthesis of colloidal Ta3N5 NPs has been achieved by injecting a variety of reactive tantalum and nitrogen precursors into hot coordinating solvents under an inert atmosphere . Through the use of different organic solvents and reaction times, the size of the NPs can be tuned in the range from 2 to 23 nm. A change in the bandgap energy of colloidal Ta3N5 NPs was observed from the absorption spectrum, where colloidal NPs have absorption onsets greater than that of the bulk powders by ∼0.3 eV. Despite the successful formation of very small Ta3N5 particles, this method suffers from very low Ta3N5 yields along with problems of Ta3N5 oxidation, which suggest that this method is not likely to be effective for the large-scale production of metal nitride nanoparticles .
The controllable synthesis of Ta3N5 with a tailored chemical composition and size has been attempted using urea as the N source rather than NH3 gas . By varying the urea/Ta ratio in the precursor gel, both TaON and Ta3N5 NPs with defined structures and sizes can be achieved. Assisted by SiO2, the production of TaON and Ta3N5 NPs with tailored compositions was achieved through the calcination of Ta–urea gels with suitable urea/Ta ratios (RU/Ta). In this method, urea is first converted into carbon–nitride (CNx) species on the surface of SiO2 at mild temperatures, which further acts as a slow-release N source for controlled nitridation.
Regarding the synthesis of Ta3N5 electrodes, the majority of the films were obtained by Ta foil post-calcination [48, 49, 60], Ta anodization (Fig. 2d) [24, 61], or by sputtering [41, 62]. In our previous work on Ta3N5, we reported thin films with different thicknesses fabricated in a controlled manner using reactive direct current sputtering followed by optimal annealing and nitridation. The preparation of such Ta3N5 photoanodes in the form of dense thin films is a useful approach for obtaining a high crystalline quality that may effectively increase the efficiency of the process and the chemical stability of the PEC system. The thin film configuration also allows for thorough characterization of the photophysical properties of the material, such as the Ta3N5 properties and photocatalytic performance.
Structural and optoelectronic properties of Ta3N5: detailed experiments and theoretical calculations
The crystal and electronic structures are two factors that primarily determine the absorption properties of a powder semiconductor . The electronic properties of a semiconductor define the bandgap and band positions, the nature of direct and indirect light absorption, and the absorption coefficient. Essentially, the main effect resulting from direct and indirect bandgaps is the absorption coefficient, where a direct bandgap provides a high absorption coefficient and an indirect bandgap leads to a lower absorption coefficient. An indirect transition involves both a photon and a phonon because the band edges of the conduction and valence bands are widely separated in k-space. The crystal orbitals at the top of the valence band and at the bottom of the conduction band have the same wave vector in a direct bandgap solid but different wave vectors in an indirect bandgap material.
The crystal structure of Ta3N5 was first reported by Brese and O’Keeffe using time-of-flight neutron diffraction . Ta3N5 has an orthorhombic structure composed of irregular octahedra of N atoms with Ta atoms in the center. Two nitrogen atoms have four Ta atoms as the nearest neighbors, while another single nitrogen atom is coordinated to three Ta atoms. The Ta–N distances range from 1.96 to 2.24 Å and are similar to those in TaON.
In our previous work, Ta3N5 powder was synthesized through the direct nitridation of crystalline Ta2O5 under a NH3 flow at high temperature [43, 44, 64]. XRD characterization was performed to investigate the crystal structures of the synthesized materials. The successive nitridation completely changed the crystal structures of the products from the initial β-Ta2O5 to monoclinic β-TaON and then to orthorhombic Ta3N5, which is consistent with the literature [39, 40, 44]. The XRD patterns of samples heated at 800 and 900 °C for 15 h under different NH3 flow rates were compared. For both temperatures, changing the flow rate clearly changed the TaON and Ta3N5 concentrations. At 900 °C, a mixture of oxynitride (TaON) and nitride (Ta3N5) material phases was observed for lower flow rates (i.e., 50-100 mL min−1 NH3), whereas a higher amount of the Ta3N5 phase was observed at higher flow rates. Complete transformation of Ta2O5 to Ta3N5 was achieved after prolonged nitridation at 900 °C for 15 h under a 200 mL min−1 NH3 flow. No remaining Ta2O5 or partially nitrided phase (e.g., β-TaON) was detected, as can be seen from XRD pattern presented in Fig. 3a. The resulting Ta3N5 has an orthorhombic crystal structure (ICSD card No. 1005006) with Cmcm space group. Similarly, for samples nitrided at 800 °C, the amount of the Ta3N5 phase increased with increasing NH3 flow rate. However, at 800 °C, higher flow rates up to 500 cm3 min −1 did not change the entire Ta2O5 precursor into the pure Ta3N5 phase. Some remaining TaON (~8 wt %) phase was still observed. It is suggested that competitive substitution of O with N between the NH3 feed and H2O occurred, where the chemical potential of oxygen kinetically determines these pseudo-thermodynamically stable phase diagrams .
The fact that a mixture of TaON and Ta3N5 always forms at 800 °C regardless of the flow rates under our conditions suggested that, during nitridation, the oxide was first transformed into oxynitride prior to nitride. This finding is in good agreement with reports in the literature, which state that TaON is the intermediate phase in the formation of Ta3N5 from the Ta2O5 precursor [39, 40, 51, 64]. This also implies that the nitridation proceeds via the successive transformation of Ta2O5 TaON Ta3N5, where a continuous dehydration reaction occurs. In this study, no concrete evidence could be found that the transformation of Ta2O5 to Ta3N5 is initially triggered by the incorporation of N atoms into the Ta2O5 lattice to form N-doped Ta2O5, as observed by Dabirian et al. where in situ XRD was utilized to follow the nitridation of Ta2O5 films .
Comparison of Ta3N5 crystal structure obtained from theoretical calculation and experimental measurement
a, b, c (Å)
3.89, 10.25, 10.27
a, b, c (Å)
3.89, 10.22, 10.27
a, b, c (Å)
3.89, 10.21, 10.26
Neutron diffraction 
a, b, c (Å)
3.87, 10.22, 10.26
PBE (QE) 
a, b, c (Å)
3.90, 10.33, 10.32
PBE (CRYSTAL) 
a, b, c (Å)
3.85, 10.14, 10.16
LDA (QE) 
a, b, c (Å)
3.99, 10.69, 10.69
rev PBE 
a, b, c (Å)
3.87, 10.24, 10.26
a, b, c (Å)
3.91, 10.32, 10.35
a, b, c (Å)
4.00, 10.43, 10.48
CGA + U 
a, b, c (Å)
3.87, 10.25, 10.27
a, b, c (Å)
3.89, 10.22, 10.28
Neutron diffraction 
a, b, c (Å)
Ta1 (4e); x, y, z
0, 0.198, 0.250
0, 0.197, 0.250
DFT-PBE and powder XRD, Rietveld analysis 
Ta2 (8e); x, y, z
0, 0.133, 0.559
0, 0.134, 0.560
N1 (4e); x, y, z
0, 0.764, 0.250
0, 0.762, 0.250
N2 (8e); x, y, z
0, 0.046, 0.120
0, 0.044, 0.116
N3 (8e); x, y, z
0, 0.309, 0.073
0, 0.304, 0.072
Ta1 (4c); x, y, z
0, 0.197, 0.250
Neutron diffraction 
Ta2 (8f); x, y, z
0, 0.135, 0.559
N1 (4c); x, y, z
0, 0.763, 0.250
N2 (8f); x, y, z
0, 0.047, 0.119
N3 (8f); x, y, z
0, 0.309, 0.074
Prior to our report, there was only limited literature on the lattice dynamics of Ta3N5 . In our recent paper, detailed theoretical calculations and experimental results of Ta3N5 were explored . Orthorhombic Ta3N5 with space group Cmcm is predicted to have a total of 24 Raman-active modes, 8Ag + 16Bg, and 21 IR-active modes, 3Au + 18Bu. However, some frequencies predicted by the DFPT/PBE method have not been experimentally observed, most likely due to the low scattering cross-section of these modes and because the peaks are consequently hidden in the background. On the other hand, the experimental Raman spectrum exhibits 19 active modes and a few modes that are not predicted by the theoretical calculation. Nevertheless, the model generally reproduces the experimental data accurately, particularly at low wavenumbers.
The experimentally measured Raman spectra (Fig. 3b) show the most intense peaks at low wavenumbers, 102, 123, 138, 152, 168, 230, 266, 400, and 495 cm−1, which are very well matched with the theoretically calculated peaks within 3–4 cm−1. These modes are assigned to Bg, Bg, Ag, Bg, Bg,Ag, Ag, Ag, and Bg, respectively. However, at higher wavenumbers, the agreement is less evident. The experimental peaks located at 524 and 601 cm−1, which are both assigned to Ag modes, are still quite well reproduced by the computed values within 5 cm−1, whereas for the peaks at 749, 869, and 900 cm−1, which are assigned to the Bg, Bg, and Ag modes, respectively, there is a larger variation in the range of 10 cm−1. This behavior at higher wavenumbers was expected according to several reports in which the low energy part of the experimental spectrum was always better matched by theory than the higher energy part [71, 72]. Two peaks at 658 and 823 cm−1 were observed experimentally but not predicted by theory. These peaks were broadened and enhanced in resonance conditions using λex = 532 nm (2.33 eV) and λex = 473 nm (2.62 eV), and likely due to LO overtones, were too weak to be observed in non-resonance conditions. In particular, the peak at 823 cm−1 observed with λex = 532 nm might be the second overtone (3ωAg) of ωAg = 266 cm−1, and the peak at 523 cm−1, which was not very well matched by the theoretical models, might be the first overtone 2ωAg. In addition, these modes might also originate from different localized non-stoichiometries in the crystal that can be activated by one or various excitation wavelengths .
In addition to Raman’s spectra, we reported calculated and experimentally measured IR spectra (Fig. 3c). There is a good agreement between the theoretical calculations with the experimental frequencies. However, most of the theoretically predicted modes cannot be associated with any experimental peak, probably because they are hidden in the background due to the low intensity. Moreover, a part of the spectrum, namely below 360 cm−1, could not be measured experimentally due to the instrumental limitation.
Absorption coefficient and bandgap energy
One interesting feature observed in the absorption spectrum of Ta3N5 is the extent of sub-bandgap absorption at approximately 715 nm. The enhanced sub-bandgap absorption at wavelengths greater than 600 nm has been previously assigned to reduced Ta5+, which is presumably present at the surface of the Ta3N5 . In contrast, Dabirian and Van de Krol  attributed the enhanced absorption of Ta3N5 at wavelengths greater than 600 nm to its bulk properties rather than to its surface. The highly charged nitrogen vacancy that formed during prolonged nitridation is a likely candidate for the deep donor state responsible for the absorption feature at wavelengths greater than 600 nm. This assignment was rationalized by the fact that oxygen is always present at the 3-coordinated nitrogen sites in the Ta3N5 structure. To elaborate the origin of this sub-bandgap absorption, a theoretical calculation was performed . The 720 nm sub-bandgap optical absorption of Ta3N5 was formed due the appearance of new deep donor metallic states located within the 0.7 eV range just below the original conduction band edge of pure Ta3N5, which corresponds to reduced Ta species (Ta3+) originating from O species substituted at N sites, as in Ta3N4.83O0.17. This finding is in a good agreement with the Raman spectra, where the Ta3N5 crystal is most likely non-stoichiometric with oxygen substituted for nitrogen . Indeed, elemental analysis under a control atmosphere always shows the presence of oxygen even though XRD shows a pure phase of Ta3N5 [43, 44, 64]. In line with this, our group has systematically investigated the possible structures and found that the experimentally prepared tantalum nitride materials are most likely not stoichiometric but strongly enriched in O, closer to Ta(3−x)N(5−5x)O5x (for x ≥ 0.16) rather than Ta3N5 .
Direct/indirect bandgap nature
The DOS of Ta3N5 was calculated to understand the nature of direct/indirect bandgaps [43, 44, 64]. Our calculated DOS is presented in Fig. 5c, in which the valence band is dominated by completely filled N 2p states and the conduction band is primarily composed of Ta 5d states. This compound can be identified as a direct (at the Г or Y point) or an indirect (Г-Y) semiconductor with the same bandgap energy of 2.2 eV. Inconsistent with our work, a previous study on calculating the DOS of Ta3N5 has shown that it is an indirect bandgap semiconductor with the VBM located at the Г point and the CBM located at the Y point. The density of states shows that the top of the valence band is primarily composed of N 2p orbitals, whereas the bottom of the conduction band is mainly composed of Ta 5d orbitals.
In addition to the DOS calculation and DR–UV–Vis absorption, we studied the photoluminescence (PL) spectra of Ta3N5 to investigate its absorption properties . The photoluminescence spectra of Ta3N5 were acquired using 473 and 532 nm lasers as excitation sources. Ta3N5 has a quite narrow PL centered at 580 nm that is considerably more intense when λex = 532 nm is used. This PL is related to the direct bandgap transition at the Γ point (2.2 eV) with slightly lower energy for the absorption onset due to non-radiative recombination.
The flatband potentials of Ta3N5 are typically estimated from Mott-Schottky experiments or ultraviolet photoemission spectroscopy (UPS) measurements . The first study on Ta3N5 flatband potentials was reported by Chun et al. utilizing Mott-Schottky and UPS measurements . Both techniques provided good agreement with a flatband potential of −4.44 eV (0 V vs. NHE at pH = 0). Another work by Cong et al.  found the flatband potential of Ta3N5 to be −0.05 V vs. NHE (pH = 0).
In our work, we utilized a Mott-Schottky experiment to estimate the Ta3N5 flatband potential [43, 44]. The Mott-Schottky analysis at pH 13.5 provided a flatband potential of ~−0.5 V vs. RHE. The experimentally obtained flatband potential is considered to be located close to the conduction band and is consistent with the fact that the contribution of conduction bands is commonly strongly associated with empty Ta d0 orbitals for Ta3N5. Here, the band positions are assumed to follow a typical pH dependence relationship of 0.059 × pH (in V) such that the band positions relative to water redox potentials remain constant at any pH . In addition, the slope of the Mott-Schottky graph exhibited a positive value, characteristic of n-type semiconductors. From the bandgap obtained from the measurement of optical properties, the measured conduction and valence band positions are located at ~−0.5 and 1.6 V vs. SHE (at pH 0). Based on these data, the band positions for Ta3N5 are suitably located for overall water splitting (they straddle the water redox potentials).
The band positions of Ta3N5 were predicted through theoretical calculations . For pure Ta3N5, the HSE06 calculations predict the VB edge position to be 0.3 eV higher in energy than the O2/H2O level. The CB edge position is found to be 1.3 eV higher in energy than the H+/H2 level. Because of its unsuitable VB edge position with respect to the O2/H2O potential, pure Ta3N5 is predicted to be a good candidate only for the hydrogen evolution reaction. It is important to discuss here the potential error bars in the calculation of the band edge positions. General benchmarks are available for the ionization potential and electron affinities of molecular sets, and they indicate a mean absolute error of ~0.2 eV. Regardless, the VB edge position is not below the O2/H2O level, and thus, the holes created upon photon absorption in pure Ta3N5 will have a very limited (if not null) capability to oxidize water. Additionally, the pH value is able to slightly affect the O2/H2O potential. In contrast, the position of the CB band edge is undoubtedly above the H+/H2 level, and the excited electrons thus have a strong capability to reduce H+.
The predicted band edge positions for pure Ta3N5 are found to differ from the experimental results. For the experimental results, the measurements and calculations are based on the Mott-Schottky relationship, where the surface states may present and play a crucial rule in modifying the band positions [19, 24, 43, 44]. This effect, however, was not taken into account for the theoretical calculations. In addition, as mentioned previously, the formation of pure Ta3N5 is not possible. Although the XRD patterns show the pure phase, the elemental analysis shows a considerable amount of remaining oxygen for Ta3N5, which indicates an oxygen-enriched Ta3N5 crystal structure. Our group has systematically predicted the band edge positions of various oxygen-rich Ta3N5 structures through theoretical calculations. The measured band edge positions are found to be highly consistent with the band edge positions for O-enriched Ta3N5 materials, particularly for the non-stoichiometric Ta(3−x)N(5−5x)O5x (for x ≥ 0.16) compounds. Considering the accuracy of bandgap prediction using the HSE06 functional, this result confirms that the experimentally prepared tantalum nitride materials are not stoichiometric but rather strongly enriched in O, as also observed from Raman and DR-UV–Vis absorption spectra.
Ta3N5 optical properties and band positions
Bandgap energy (eV)
Flatband potential (V vs. NHE)
0 (pH 0)
−0.05 (pH 0)
−0.3 (pH 13)
−0.3 (pH 0)
−0.52 (pH 0)
−1.3 (pH 0)
−0.4 (pH 0, defective Ta3N5)
+1.58 (pH 0)
+1.58 (pH 0)
+0.9 (pH 0)
+1.6 (pH 0, defective Ta3N5)
Absorption coefficient (cm−1)
1 × 105
8 × 105–1 × 104
UV-Vis spectroscopy 
6.2 × 104
UV-Vis spectroscopy 
2.5 × 105–3 × 103
Spectroscopic ellipsometry 
1.4 × 105–2.2 × 105
Kinetic and dynamic properties of photoexcited carriers on Ta3N5
Charge carrier separation
Ta3N5 kinetic and dynamic properties
Kinetic and dynamic properties
Reflectance fringe 
Not specified 
Spectroscopic ellipsometry 
Electron Effective massc
0.60 m0–1.94 m0
0.23 m0–2.70 m0
Hole Effective massc
0.85 m0–3.38 m0
0.66 m0–3.56 m0
Charge carrier concentration (cm−3)
5 × 1017–5 × 1020
Hall effect measurement 
3.7 × 1019
Mott-Schottky plot 
Mobility (cm2 V−1 s−1)
Van der Pauw 
Carrier lifetime (ps)
Transient spectroscopy 
Femtosecond DR-spectroscopy 
Diffusion length (nm)
Subtracted from mobility and lifetime 
Clearly, there is a significant difference among the measured and theoretically calculated dielectric constants [41, 64]. For the measured dielectric constant, the experiment was conducted only for high frequency (UV–Vis range), where it contributes only to electronic vibration, whereas for the calculated dielectric constant, both low and high frequencies were taken into account, and therefore, the dielectric constant also considered the ionic and electronic contributions [41, 64, 66].
Charge carrier diffusion
Quantitatively, charge carrier diffusion can be examined from the effective mass property. Acceptable effective masses are considered optimal, at least in one crystallographic direction, when they are less than 0.5 m0 (m0 being the electron mass) for an efficient diffusion of charge carriers in the material [64, 66, 79, 80, 81, 82, 83]. For a measured donor density of 5 × 1020 cm−3, deduced from Mott–Schottky measurements, we estimate the electron effective mass to be 0.36 me* for Ta3N5-50 nm using Fig. 6b. We also computed the charge carrier effective masses (i.e., electron and hole effective masses) and compared them to previous experimental works on semiconductors used in photovoltaic devices. The smallest hole and electron effective masses are both found to be along either the  or  direction with mh* = 0.85 m0 and me* = 0.6 m0, and the highest hole and electron mobilities are expected to be along these two specific directions. These two obtained values are larger than 0.5 m0 (threshold value), and hence, relatively poor charge carrier transport properties are expected along this specific direction. Note that the current data using the DFT/HSE06 level of theory had a subtle discrepancy with the reported values using the DFT/PBE level of theory , as expected from the different calculated energy diagrams. In addition, compared to the calculated electron effective mass obtained from measurements, 0.36 m0, the value is below the threshold, and thus, good electron charge carrier transport is expected. Calculations estimate the electron effective masses to be between (0.2–2.7 me*) [64, 67] depending on the crystallographic direction. Additionally, it appears that the effective masses of holes are larger than those for electrons (0.66–3.6 mh*). This result suggests that low hole mobility may be responsible for the limitation of the generated photocurrent in Ta3N5.
The charge carrier concentration can be calculated from the slopes of the Mott-Schottky plots. Using a geometric area of the films, the obtained values were ~5 × 1019 cm−3 for the thin films and 5 × 1020 cm−3 for the thicker ones. If the relative surface area of the thicker film was used, the carrier concentration of this film was in the same range as the rest of the samples. Clearly, the donor density measurement from the Mott-Schottky analysis is very complex; therefore, the carrier concentrations of the films were also measured using Hall measurements. We found values between 5 × 1017 and 5 × 1020 cm−3, which are within the same range for other reported photoanodes, such as hematite or bismuth vanadate materials [103, 104, 105]. We also reported on the electronic transport properties (carrier mobility) and spectroscopic measurements (carrier recombination). These two parameters determine the diffusion length of the material and can be correlated to the PEC performances. The van der Pauw four-probe measurement provides the resistivity of the material, and the Hall measurements provide the charge carrier concentration. The resistivity of the films decreased from 32.44 to 0.01 O cm with a thickness increase from 160 to 470 nm, which correlates to the crystallinity of the films. For the thick films (960 nm), the resistivity increased to 54.88 O cm. All of these values are within the range of typical semiconductor materials (10−3 – 103 Ω cm). The calculated mobilities were in the range of 1.3–4.4 cm2 V−1 s−1. Thicker films possess better mobility.
The intrinsic defects in Ta3N5 can significantly reduce the carrier lifetime and consequently decrease the photocurrent. It is well known that the carrier lifetime is significantly increased by enhancing the film morphology, grain size, and crystallinity. Femtosecond (fs) transient absorption spectroscopy provides direct information regarding the carrier dynamics and excited-state deactivation pathways, including carrier trapping. We utilized this method to probe the events that occurred due to photoexcitation of the Ta3N5 films. We explored the carrier dynamics of Ta3N5 thin films using broadband transient absorption spectroscopy with 120 fs temporal resolution. We observed a significant increase in the carrier lifetime of the thick films compared with the thinner films. The observed dynamics can be attributed to the decrease in carrier trapping, indicating that there are less defects in the thicker film compared with the thinner one, consistent with the crystalline structure and the surface morphology of the thick films. The measured lifetimes are in the range of 3.1–8.7 ps, consistent with the 12 ps reported for Ta3N5 powder . Using the mobility and lifetime, we can estimate the Ta3N5 diffusion length to be between 3 and 9 nm, which again highlights the difficulty in diffusing charge carriers. Material structuration and cocatalyst addition are mandatory to diffuse charge carriers and subsequently improve the photocatalytic reaction.
The kinetic and dynamic properties from our experimental and theoretical calculation results and those reported in the literature are compared in Table 3.
Surface states related phenomena
The photocatalytic activity of Ta3N5 for oxygen and hydrogen evolution reactions has primarily been attributed to its activity related to its bulk properties, such as crystallinity, particle size, and optical properties [18, 19, 20, 22, 23, 26, 29, 30]. However, as previously described, photocatalytic water splitting is a complex reaction that is affected by many factors associated with the photocatalyst and cocatalyst properties. Although the effect of the surface properties of semiconductors is significant, it is less investigated and correlated with photocatalytic activity.
Our work reported that not only the bulk properties but also the surface properties greatly affected the photocatalytic activity of Ta3N5 . A thin TaN layer on the surface (~2 nm), which formed depending on the synthesis method, was observed to change the energetic profile on the Ta3N5-electrolyte interface, thus changing the photocatalytic activity. The surface layer changes the potential distribution on the Ta3N5 surface-electrolyte interface, as evidenced by the perturbed flatband potential. The flatband potentials shifted as a consequence of Fermi level pinning of the semiconductor-electrolyte interface due to the drastic influence of surface states. However, the surface states were difficult to elucidate in this study, but it can be said that the layer, which can be removed by alkaline piranha solution, shifted the overall flatband potential to be more negative without changing the bandgap, consistent with the enhanced hydrogen evolution while minimizing the oxygen evolution. This study also emphasized the importance of surface modification to improve the photocatalytic performance. Interface modification together with cocatalyst function and intentional perturbation of the surface states by introducing an additional hetero-layer would have the potential to further improve the photocatalytic performance.
One of the requirements of a photocatalyst to undergo photocatalytic/PEC reactions is possessing suitable band positions with respect to the water redox potentials (thermodynamic requirement). To estimate the band positions, Mott-Schottky plots using impedance spectroscopy were obtained in an attempt to retrieve the flatband potential [26, 41]. Note that the Mott–Schottky relation is only applicable to “ideal” semiconductors with a uniform bulk and surface and with preferably known surface areas. Additionally, the surface states present on the semiconductor significantly affect the Mott–Schottky plot because the pH effects follow the Nernstian relationship of −59 mV pH−1 for many semiconductors. Hence, the fabrication of high-quality electrodes using powder semiconductors is indispensable for obtaining reliable results [26, 41].
Several works have reported the flatband potentials of Ta3N5 films synthesized using different methods, where the reported flatband potentials show different values for different film preparations [19, 24, 41]. In addition to films synthesized from Ta3N5 by applying electrophoretic deposition, we applied the sputtering method to synthesize Ta3N5 on Ta foils with different thicknesses [41, 43, 44]. The Mott-Schottky plots for the Ta3N5 films with different thicknesses were determined based on the impedance data. All of the Ta3N5 films exhibited positive slopes, which are characteristic of n-type semiconductors.
To obtain accurate flatband potentials, the selection of a suitable potential window for extrapolation from the Mott-Schottky plots is crucial. For this purpose, the Faradaic current should remain negligible. However, in most cases, the non-flat behavior of the capacitive current causes difficulty in selecting the right potential window for measuring the flatband potential. For example, in our case, in the potential range from −1.2 to +1.0 V vs. RHE, the Mott-Schottky analysis leads to a flatband potential from −0.1 to 0 V vs. RHE, which is consistent with previously reported values. However, in a different potential range at more positive potentials, the flatband potential can be found between +0.8 and +1.1 V vs. RHE, which are more positive compared with those reported. Notably, the flatband potentials mentioned in the literature were taken in different potential ranges, resulting in different flatband potentials. Mott-Schottky analysis is more preferable for studying the flatband potential of a single-crystal semiconductor with a moderate doping content and a good ohmic contact [26, 41, 87]. These ideal conditions are not the case for these samples nor those reported in the literature. Additionally, due to the surface state capacitance and associated double-layer capacitance at the semiconductor-electrolyte interface, it is often observed that the flatband potential depends on the frequency used, which should not be the case.
Upon successful charge transfer, the culmination of all the photophysical processes on the semiconductor surface succeeds with an effective electrocatalytic process [1, 26, 88]. An efficient electrocatalyst is one of the most important parameters for achieving efficient photocatalytic water splitting in visible light. To achieve efficient water splitting under visible light irradiation where there is no significant overpotential for electrocatalysis, the electrocatalysts need to transfer the received electrons and holes to the relevant reactants in the water splitting redox reactions [1, 26].
OER cocatalyst deposition effect on Ta3N5
A comprehensive understanding of the cocatalyst and of its fundamental effect on photocatalytic water splitting has yet to be elucidated. Studies on electrochemical water splitting have obtained some promising results regarding the development of oxygen evolution reaction (OER) cocatalysts. Some new cocatalysts, such as Co3O4, Ni(OH)2, and Mn3O4, have good potential for replacing the employed noble metal catalysts (RuO2) because of their excellent electrocatalytic activity [76, 87, 88, 89, 90]. However, little attention has been focused on investigating these materials as OER cocatalysts in photocatalytic water splitting. Despite the difference in these two systems, this electrochemical approach is applicable for studying the surface reaction of water oxidation.
Physical and chemical properties of CoOx on Ta3N5 surface
Cobalt oxide (CoOx) has been extensively investigated as a cocatalyst, particularly for electrochemical and PEC OER [15, 24, 91, 92, 93, 94, 95, 96, 97, 98]. Although many studies have reported improvement of the electrochemical OER using CoOx, studies on powder suspension systems are limited [42, 93]. An improved photocatalytic OER rate with a quantum efficiency as high as 27 % (at 440 nm) has been reported on LaTiO2N with deposited CoOx . In addition to providing active sites for the electrocatalytic OER, CoOx is also believed to play a vital role in promoting charge separation. However, despite the tremendous efforts toward developing a CoOx catalyst, its chemical state and properties on a photocatalyst have yet to be elucidated.
Kinetics and dynamics of photoexcited carriers on CoOx/Ta3N5 and noble metal co-loaded CoOx/Ta3N5
The effect of cocatalyst modification on improved photocatalytic/PEC performance has been a matter of debate. The role of the cocatalyst has been reported to provide two effects: 1) improvement in electrochemical performance by lowering the overpotentials for the redox reactions, and 2) effectively separating the excited charges utilizing the photocatalyst (semiconductor)–catalyst interface through the differences in Fermi levels between them.
The optimized system exhibited one of the highest quantum efficiencies (QEs) reported over 20 % in visible light range in 0.1 M Na2S2O8 at pH 14. Using time-resolved spectroscopy, we are attempting to identify whether hole transfer kinetics is improved by the addition of noble metal. The initial attempts likely suggest that hole transfer was also improved, and the results are to be published in the near future. In addition, as claimed in the literature, improved electrocatalytic OER by lowering the onset potential also plays a significant role in enhancing the photocatalytic OER .
As previously mentioned, investigating electrocatalysis is one of the best approaches for studying photocatalytic water splitting. In our recent review, we included the mass transfer of reactants, which significantly affects the photocatalytic water splitting performance . The study of photocatalytic water splitting has primarily focused on developing efficient materials, including cocatalysts, without consideration of thermodynamic and kinetic information from the electrocatalysis perspective. Indeed, in a single powder photocatalyst system, there is no pH gradient because the H2 and O2 are generated in the same compartment.
Most of the photocatalytic water splitting is conducted at neutral pH due to the stability issue of most semiconductor photocatalysts. Therefore, it is important to study the electrolysis of water under neutral pH. In acidic or alkaline conditions, pH 0 and 14, respectively, one H+ or OH− ion is present among approximately 55 H2O molecules. In terms of kinetics, the reaction with hydronium ions (protons) or hydroxyl ions that have a very large diffusion coefficient is more facile than that with water molecules for reduction and oxidation, respectively [26, 100]. In neutral conditions, however, a different reaction mechanism has been observed [26, 101]. Insufficient hydronium/hydroxyl ion activities at near-neutral pH induced the limiting diffusion currents of reactions with these ions. In this condition, buffering actions are effective based on the reactant switching over varied pH . In addition, for electrochemical measurements, the supporting electrolyte is an essential component for avoiding solution resistance (iR drop). We recently reported a rigorous study on the effect of a supporting electrolyte for electrochemical hydrogen evolution under neutral conditions in a buffered system . However, there is a lack of information available regarding this pH and the electrolyte effect in photocatalytic water splitting.
Kinetics and activation energy measurement for photocatalytic OER using NiFeOx catalyst
Photocatalytic reaction is essentially electrochemical redox reactions driven by photogenerated electrons and holes in the semiconductor. Our attempt is to study electrochemical reaction apart from semiconductor, and then connect to photophysical process using the semiconductor. For this, we used the NiFeOx electrocatalyst in alkaline solution as highly active, industrially-vial catalyst . As expected, the catalyst achieved 10 mA cm−2 at an overpotential of 260 mV in 1 M of KOH solution and the different temperature measurement resulted in an apparent activation energy of 25 kJ mol−1. The same catalyst was used to decorate Ta3N5 powder photocatalyst by applying identical hydrothermal treatment in the presence of Ta3N5 powder. The NiFeOx/Ta3N5 was used for photocatalytic OER reaction in the presence of 0.1 M Na2S2O8 as a strong electron scavenger, thus likely leading to the OER being kinetically relevant. The incorporation of NiFeOx catalyst leads to fivefold improvement of Ta3N5 photocatalytic OER in the visible range with quantum efficiency up to 24 % at 480 nm. The apparent activation energy for photocatalytic OER was found to be 16 kJ mol−1.
The comparison between electrocatalytic and photocatalytic studies  shows very strong correlation that the improvement in electrocatalysis leads to similar improvement in photocatalysis. Both electrocatalytic and photocatalytic systems have similar dependence on pH change, where high rates were observed for higher pH. The pH dependence is associated with electrocatalytic kinetics that accordingly influenced the photocatalytic rates. Furthermore, the difference in apparent activation energies for both systems is associated with the possible effects of temperature on the individual thermodynamic and kinetic parameters of the reaction process.
Developing efficient visible-light-responsive photocatalysts is essential for making the photocatalytic overall water splitting reaction economically competitive for hydrogen production. To achieve this goal, all the parameters involved in photocatalytic water splitting should be optimized. A collection of theoretical and experimental studies of properties associated with Ta3N5 have been utilized to obtain a comprehensive understanding of this material. The fundamental structural and optoelectronic properties of Ta3N5 have been addressed. The nitridation is confirmed to proceed via the successive transformation of Ta2O5 TaON Ta3N5 on the basis of XRD patterns and Rietveld analysis. From the electronic properties, the dielectric constant and effective masses have been calculated. Because of its high dielectric constant and relatively low effective masses, Ta3N5 is promising for photocatalytic reaction applications. Studies of lattice dynamics, optical properties, and band positions have been able to clearly show that the synthesized Ta3N5 is essentially defective and non-stoichiometric and that a truly pure phase of Ta3N5 has never been achieved, even though XRD has shown a pure phase sample. The photophysical properties of Ta3N5, such as the absorption coefficient, carrier mobility, and carrier lifetime, have been experimentally measured by synthesizing Ta3N5 thin films. Very low kinetic properties with very low transport properties and fast carrier recombination explained why overall water splitting has never been achieved with Ta3N5 as a photocatalyst to date. A surface modification with a cocatalyst and new interface construction are thus suggested to improve the photocatalytic activity. The extent to which the surface states of Ta3N5 photocatalysts affect photocatalytic performance has been investigated. The surface topmost layer is demonstrated to play a critical role in the photocatalytic activity of Ta3N5; further research on the surface properties of Ta3N5 should be conducted to understand and improve charge separation and the resulting photocatalytic activity. Finally, a remarkable improvement in the photocatalytic OER has been achieved with the addition of cobalt as a cocatalyst. There is a trade-off between the optimum contact of hole transfer from bulk Ta3N5 to the surface of the cobalt cocatalyst and providing active sites for the electrochemical reaction. Knowing the characteristics of cobalt on the Ta3N5 surface, further improvement was attempted by adding a noble metal to the CoOx/Ta3N5 photocatalyst system, where a synergetic effect of CoOx and noble metals was observed. Although a solar energy to hydrogen conversion efficiency of greater than 1 % using Ta3N5 has been achieved, substantial bias in the PEC configuration or the use of a sacrificial reagent is still mandatory. As mentioned in the beginning of this part, the main limitation for Ta3N5 is that we cannot make it until now chemically stoichiometric and without crystalline defects. The ammonia treatment necessary for the production of Ta3N5 is an aggressive treatment and induces many defects in the crystalline structure that act as sites of charge carrier recombination in the material. Developing new synthesis techniques or new in situ methods of the growth control for Ta3N5 is predicted to be able to produce better chemical stoichiometry and improve the crystalline structuration of the material (like epitaxial growth). It can be a key to overcome the Ta3N5 limitations and make it efficient for overall water splitting. On the other hand, developing efficient cocatalysts with new deposition techniques for surface functionalization can help to improve the photocatalytic efficiency of Ta3N5. Research should continue as the efficiency has not yet reached the desired level. However, a thorough understanding of all the processes involved in the photocatalytic process would pin down the crucial parameters, which will lead to the design of photocatalysts for solving targeted problems.
- 6.Ismail, A.A., Bahnemann, D.W.: Photochemical splitting of water for hydrogen production by photocatalysis. Solar Energy Mater. Solar Cells. 128, 85–101 (2014). doi: 10.1016/j.solmat.2014.04.037
- 17.Kubota, J., Domen, K.: Photocatalytic water splitting using oxynitride and nitride semiconductor powders for production of solar hydrogen. Electrochem. Soc. Interface Summer, 22(2), 57–62 (2013). doi: 10.1149/2.F07132if
- 21.Li, Y., Zhang, L., Torres-Pardo, A., Gonzalez-Calbet, J.M., Ma, Y., Oleynikov, P., Terasaki, O., Asahina, S., Shima, M., Cha, D., Zhao, L., Takanabe, K., Kubota, J., Domen, K.: Cobalt phosphate-modified barium-doped tantalum nitride nanorod photoanode with 1.5% solar energy conversion efficiency. Nat. Commun. (2013). doi:10.1038/ncomms3566 Google Scholar
- 32.Kakuta, N., Park, K.H., Finlayson, M.F., Ueno, A., Bard, A.J., Campion, A., Fox, M.A., Webber, S.E., White, J.M.: Photoassisted hydrogen production using visible light and coprecipitated ZnS·CdS without a noble metal. J. Phys. Chem. 89, 732–734 (1985). doi:10.1016/j.solmat.2014.04.037
- 33.Reber, J.F., Rusek, M.: Photochemical hydrogen production with platinized suspensions of cadmium sulfide and cadmium zinc sulfide modified by silver sulfide. J. Phys. Chem. 90, 824 (1985). doi:10.1021/j100251a002
- 36.Matsumura, M., Saho, Y., Tsubomura, H.: Photocatalytic hydrogen production from solutions of sulfite using platinized cadmium sulfide powder. J. Phys. Chem. 87(20), 3807–3808 (1983). doi:10.1021/j100243a005
- 43.Nurlaela, E., Ould-Chikh, S., Harb, M., del Gobbo, S., Aouine, M., Puzenat, E., Sautet, P., Domen, K., Basset, J.-M., Takanabe, K.: Critical role of the semiconductor—electrolyte interface in photocatalytic performance for water-splitting reactions using Ta3N5 particles. Chem. Mater. 26, 4812–4825 (2014). doi:10.1021/cm502015q CrossRefGoogle Scholar
- 44.Harb, M., Sautet, P., Nurlaela, E., Raybaud, P., Cavallo, L., Domen, K., Basset, J.-M., Takanabe, K.: Tuning the properties of visible-light-responsive tantalum (oxy)nitride photocatalysts by non-stoichiometric compositions: a first-principle viewpoint. Phys. Chem. Chem. Phy. 16, 20548–20560 (2014). doi:10.1039/c4cp03594a CrossRefGoogle Scholar
- 45.Yuliati, L., Yang, J.H., Wang, X., Maeda, K., Takata, T., Antonietti, M., Domen, K.: Highly active tantalum(V) nitride nanoparticles prepared from a mesoporous carbon nitride template for photocatalytic hydrogen evolution under visible lght irradiation. J. Mater. Chem. 20, 4295–4298 (2010). doi:10.1039/C0JM00341G CrossRefGoogle Scholar
- 46.Fukasawa, Y., Takanabe, K., Shimojima, A., Antonietti, M., Domen, K., Okubo, T.: Synthesis of ordered porous graphitic-C3N4 and regularly arranged Ta3N5 nanoparticles by using self-assembled silica nanospheres as a primary template. Chem. Asian J. 6, 103–109 (2011). doi:10.1002/asia.201000523 CrossRefGoogle Scholar
- 57.Ekimov, A.I., Onushchenko, A.A.: Size quantization of the electron energy spectrum in a microscopic semiconductor crystal. JETP Lett. 40, 1136–1139 (1984)Google Scholar
- 71.Lacomba-Perales, R., Martinez-García, D., Errandonea, D., Le Godec, Y., Philippe, J., Le Marchand, G., Chervin, J.C., Polian, A., Múñoz, A., López-Solano, : Experimental and theoretical investigation of the stability of the monoclinic BaWO4-II phase at high pressure and high temperature. J. Phys. Rev. B 8, 144117 (2010)CrossRefGoogle Scholar
- 81.Young, K.F., Frederikse, H.P.R.: Compilation of static dielectric constant of inorganics solids. J. Phys. Chem. Ref. Data 409, 2313 (1973)Google Scholar
- 86.Furube A, Maeda K, Domen K Transient absorption study on photogenerated carrier dynamics invisible light responsive photocatalysts GaN:ZnO. Proc of SPIE 2011 8109 810904Google Scholar
- 87.Chen, Z., Jaramillo, T.F., Deutsch, T.G., Kleiman-Shwarsctein, A., Forman, A.J., Gaillard, N., Garland, R., Takanabe, K., Heske, C., Sunkara, M., McFarland, E.W., Domen, K., Miller, E.L., Turner, J.A., Dinh, H.N.: Accelerating materials development for photoelectrochemical hydrogen production: standards for methods, definitions, and reporting protocols. J. Mater. Res. 25, 3–16 (2010). doi:10.1557/JMR.2010.0020 CrossRefGoogle Scholar
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