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Zr doped mesoporous Ta3N5 microspheres for efficient photocatalytic water oxidation Yawei Wang, Dazhang Zhu, and Xiaoxiang Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14230 • Publication Date (Web): 07 Dec 2016 Downloaded from http://pubs.acs.org on December 13, 2016

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ACS Applied Materials & Interfaces

Zr doped mesoporous Ta3N5 microspheres for efficient photocatalytic water oxidation Yawei Wang, Dazhang Zhu and Xiaoxiang Xu* *

Shanghai Key Lab of Chemical Assessment and Sustainability, School of Chemical Science and Engineering, Tongji University, 1239 Siping Road, Shanghai, 200092, China Email: [email protected], telephone: +86-21-65986919

Abstract Tantalum nitride (Ta3N5) has been considered as a promising candidate for photocatalytic water splitting due to its strong visible light absorbance as far as 600 nm. However, its catalytic activity is often hampered by various intrinsic/extrinsic defects. Here, we prepared a series of Zr doped mesoporous tantalum nitride (Ta3N5) via a template-free method and carried out a detailed investigation upon the role of Zr doping upon the photocatalytic performance. Various physicochemical properties including crystal structure, optical absorption etc. were systematically explored. Our results show that doping Zr into Ta3N5 induces an enhancement of oxygen content and a suppression of absorption band around 720 nm, indicating an increase of • defects and a decrease of ••• defects in the structure. Introducing Zr significantly boost the photocatalytic oxygen production of Ta3N5. The optimized photocatalytic oxygen production rate approaches 105 µmol h−1 under visible light illumination (λ ≥ 420 nm), corresponding to apparent quantum efficiency as high as 3.2%. Photoelectrochemical analysis and DFT calculation reveal that the superior photocatalytic activity of Zr doped Ta3N5 originates from a high level of • defects concentration which contributes to a high electron mobility and a low

level of ••• defects concentration which often act as charge recombination centers. Keywords: Ta3N5, Zr doped, microspheres, photocatalyst, water oxidation

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1. Introduction Photocatalytic water splitting into H2 and O2 under solar insolation has been proposed as a clean and renewable approach to mitigate global concerns on the deficiency of fossil fuels and degradation of our environment.1-4 The fully photocatalytic water splitting reactions include both photo-reduction reaction (H2 evolution) and photo-oxidation reaction (O2 evolution), which cleave water molecules stoichiometrically. However, the complete water splitting reactions are often hampered by the sluggish oxidation reactions that involve the removal of four electrons and four protons from two H2O molecules, being both energetically and mechanistically difficult.5,6 Semiconductor photocatalysts capable of promoting water oxidation reactions are highly desired. After the pioneering work of Fujishima and Honda in 1972 using TiO2 as a photo-anode for water oxidation,7 various types of photocatalysts have been studied, yet most of them exhibit a low efficiency in solar energy conversions, primarily due to their wide band gaps that are too large to harvest a great portion of solar spectrum.8-11 Recently, a number of metal (oxy)nitrides have gained considerable attention, not only due to their small band gaps (< 3 eV) but also because of their appropriate band edge positions that matches with the thermodynamic requirements for overall water splitting.12-18

Among these metal (oxy)nitrides, tantalum nitride (Ta3N5) has been intensively investigated because of its desirable band gap value (~ 2.1 eV), corresponding to a theoretical solar-to-hydrogen efficiency approaching 16%.19-21 Efficient photocatalytic hydrogen and oxygen production have been realized upon Ta3N5 with the aid of sacrificial agent, suggesting promising prospect for direct water splitting.21,22 Its compositional simplicity, chemical inertness and environmental compliance guarantees wide range of applications in many fields such as


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optoelectronics and solar cells etc.23 Despite these promising properties, Ta3N5 is essentially an anisotropic and hydrophobic compound whose synthesis always accompanied by the generation of various intrinsic/extrinsic defects within the structure, which substantially deteriorates its application in the field of photocatalysis.24,25

Compositional alteration by doping various foreign elements into crystal structure is a useful method to tailor various physicochemical properties of a target compound including structural, electronic, optical or even morphological properties.26-29 For Ta3N5, the activity for photocatalytic water oxidation has been greatly boosted through adding various alkaline-(e.g. Na, K, Rb, and Cs) or alkaline-earth (e.g. Mg and Ba) cations.30-34 It is known that aliovalent doping introduces charged defects into the crystal structure due to a charge compensation phenomenon.35,36 Such an improvement over photocatalytic activity is quite unusual as charged defects are generally considered as charge recombination centers and one of the main causes for a poor activity. Our previous study on Mg doped Ta3N5 demonstrates the positive role of charged defects (i.e. • ) arisen from Mg doping by promoting electron migrations, clarifying the underlying mechanism for an improved activity.33 Whether the same mechanism applies to other doping schemes for Ta3N5 or exclusively belongs to dopants from alkaline/alkaline-earth elements is still unknown. Thereby, it is fundamentally necessary to explore alternative doping schemes for Ta3N5. Zirconium (Zr) forms a diagonal relationship with respect to Tantalum (Ta) in the periodic table. The similar ionic radii between Zr4+ and Ta5+ in octahedral coordination suggest Zr be an ideal dopant for Ta3N5. Recent study on Mg/Zr co-doped Ta3N5 reveals encouraging results on photoelectrochemical water splitting in terms of a lower onset potential and a higher photocurrent.31 However, a single phase compound was not yet achieved in that


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study, let alone a clear picture with regards to individual functionality of these dopants, i.e. Mg and Zr. Therefore, it is highly desired to perform an investigation on Zr doped Ta3N5 for a better understanding of the mechanism for Mg/Zr co-doped Ta3N5.

In this work, a series of Zr doped Ta3N5 were prepared via a simple method without templates. Their crystal structure, optical properties and various physicochemical properties were carefully • studied. Our studies reveal that Zr doping induces an increment of defects in the structure as

well as a simultaneous reduction of ••• defects, both of which contribute to the superior photocatalytic activity. In other words, the activity improvements of Mg and Zr doped Ta3N5 likely originates from the same mechanism and anion defects control is seemingly a dominant factor for a high photocatalytic activity of Ta3N5.

2. Experimental 2.1. Preparation of Zr doped Ta3N5 Zr doped Ta3N5 was synthesized according to the literatures reported.37 In a typical synthesis, 1 g tantalum pentachloride (TaCl5, Aladdin, 99.99%) was added to 5 mL absolute alcohol (Aladdin, 99.9%) to form a transparent solution. 6.75 mL absolute acetic acid (Aladdin, 99.9%), 0.75 mL acetic anhydride (Aladdin, 99.9%) and appropriate amounts of zirconium tera-n-propoxide (70% in n-propanol, Zr(OPr)4) were added to above solution. The resultant mixtures were magnetic stirred for 2 h and stored in an oven at 50 oC for 48 h until white precipitants were formed. These white precipitants were dried in vacuum at 100 oC for 2 h, calcined in N2 at 250 oC for 2 h and then in O2 at 600 oC for 5 h for the removal of organic residues. The white precursor obtained were finally nitridized at 950 oC for 5 h with a NH3 flow rate ~300 mL min−1. The resultant


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samples were named according to mole ratio of Ta/Zr (Ta :Zr = 3−x : x) as TZ-x. Pristine Ta3N5 was also prepared under the same conditions without adding Zr(OPr)4 in the initial step.

2.2. Materials characterization Phase composition and crystal structures were examined by X-ray powder diffraction (XRD) technology using a Bruker Focus D8 Advance diffractometer. CuKα1 radiation (λ = 1.5406 Å) and CuKα2 radiation (λ = 1.5444 Å) were used as the incident radiation. Rietveld refinement was performed on the collected data using General Structure Analysis System (GSAS) software package.38 UV-Vis spectra were collected using a UV-vis spectrophotometer (JASCO-750) equipped with an integrating sphere in diffuse reflectance mode. Data collected was analyzed by JASCO software suite with respect to non-absorbing material BaSO4 as a reference. The freshly prepared samples were also analyzed by a field emission scanning electron microscope (Hitachi S-4800, Japan) and a transmission electron microscope (JEM-2100, Japan) for microstructure study. The total nitrogen content within these samples was investigated by thermogravimetric analysis (TGA) in air ramping from room temperature to 1200 oC at a heating speed of 10 oC min−1 using Labsysevo (SETARAM, France). Nitrogen adsorption-desorption isotherms were determined by NOVA 2200e adsorption apparatus (Quantachrome, USA). The specific surface areas was analyzed by Brunauer-Emmett-Teller (BET) method while Barrett-Joyner-Halenda (BJH) model was used to calculate pore size distribution.39 Element distribution at the surface and their binding energies were determined by X-ray photoelectron spectroscopy techniques (Thermo Escalab 250 with a monochromatic AlKα source). The collected binding energies were referred to adventitious carbon 1s peak centered at 284.7 eV. The XPSPEAKFIT software was


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used to analysis the XPS date with Shirley backgrounds. Gaussian-Lorentian function with Lorentzian weighting of 20% were adopted for peak fit.

2.3. Photocatalytic oxygen production Photocatalytic experiments for all samples were performed in glass reactor with a quartz top window for light illumination. The reactor was connected to a gas-closed circulation and evacuation system (Perfect Light Labsolar-IIIAG, China). For a typical experiment, 100 mg sample were suspended into silver nitrate aqueous solution (100 mL, 0.05 M) containing 0.2 g La2O3. The reactor was then sealed and mounted into the system for evacuation to remove the dissolved air. Silver nitrate serves as a sacrificial agent to promote photo-oxidation half reaction and La2O3 was used for pH control. The cocatalyst (CoOx) was loaded onto sample powders as following procedures: proper amounts of aqueous Co(NO3)2 solution was impregnated into 0.1 g sample powders. Magnetic stirring was applied during impregnation until slurry was formed. The slurry was then dried at 70 oC in vacuum and heated in flowing ammonia at 750 oC for 1 h and then heated again in air at 150 oC for 1 h. Visible light illumination was produced by filtering the output of a Xeon lamp (300 W, Perfect Light, PLX-SXE300) using a UV cut-off filter (λ ≥ 420 nm). The output of the lamp was analyzed by a quantum meter (Apogee MP-300). A photon flux of 840.2 µmol m−2 s−1 was recorded for the lamp under visible light illumination (420 nm ≤ λ ≤ 700 nm). The temperature of the reactor was stabilized at 20 oC using a water jacket. The gas produced during photocatalytic experiment was sampled by the gas chromatograph (TECHCOMP, GC7900, China) connected to the reactor and signals were read by a thermal conductivity detector (TCD). 5 Å molecular sieve columns were used to separate different gases


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using Ar as a carrier gas. The photocatalytic activity was evaluated by calculated the apparent quantum efficiency (AQE) considering the amount of gas produced and incident photon flux:

AQE = 4 × number of oxygen evolved / number of incident photon × 100%

2.4. Photoelectrochemical experiment Photoelectrodes of typical Zr doped Ta3N5 and pristine Ta3N5 were fabricated by using electrophoretic deposition (EPD) method. A suspension for electrophoretic deposition was obtained by dispersing 10 mg iodine and 40 mg sample powders in 50 ml acetone under sonication. Two parallel fluorine doped tin oxide (FTO) glass with dimension 3×1 cm were immersed in the suspension with a 1 cm separation where a stable 10 V voltage bias was charged between the glass using potentiostatic control (Keithley 2450 Source Meter) for 3 min. The coated area of the glass was approximately 1 cm2. The prepared electrodes were then calcined at 400 oC for 1 h for the removal of iodine. The electrodes were then dispersed with a few drops of diluted tantalum chloride (Alfa-Aesar, 99.9%) methanol solution (10 mM) and heated to 350 oC for 15 min. This procedure was used to cover the naked FTO glass during electrophoretic deposition and was repeated for five times. The resultant electrode was then heated in ammonia (flow rate 20 mL min−1) at 673 K for 1 h.40 Photoelectrochemical measurements were carried out in a three-electrode model, utilizing a Pt foil (1×1 cm) and Ag/AgCl electrode as the auxiliary and reference electrode, respectively. The electrolyte and buffer for the experiment is 20 ml K3PO4/K2HPO4 aqueous solution (0.1 M, pH = 11.5). Visible light illumination was produced by filtering the output of a 300 W Xenon lamp (Perfect Light, PLX-SXE 300) using a UV cut-off


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filter (λ ≥ 420 nm). The photon flux illuminated onto the electrode was controlled by an electronic timer and shutter (DAHENG, GCI-73).

2.5. Theoretical calculations Density function theory (DFT) was adopted to perform the theoretical calculations, which is implemented in the Vienna ab initio simulation package (VASP).41 The calculations involve the application of the Perdew, Burke and Ernzerh of (PBE) exchange-correlation functional with the generalized gradient approximation (GGA) method. The pseudo potential considered is the projector augmented-wave type.42,43 An orthorhombic cell (a = 3.9 Å, b = 10.2 Å and c = 10.3 Å) was setup for Ta3N5 calculation (32 atoms). Zr incorporation was simulated by substituting Ta and N atoms in the cell with Zr and O atoms. N vacancy defects in the structure were simulated simply by deleting N atoms randomly. Forces on each atom was adjusted to less than 0.01 eV for the fully relaxation of all geometry structure. A 12×4×4 Monkhorst-Pack k-point grid was used for static calculations.44

3. Results and discussion 3.1. Crystal structure Table 1 Unit cell parameters of as-prepared samplesTa3-xZrxN5-yOy. x

Space group

a (Å)

b (Å)

c (Å)

V(Å3)

0 0.05

Cmcm Cmcm

3.8883(1) 3.8907(1)

10.2200(2) 10.2220(2)

10.2663(2) 10.2750(2)

407.97(2) 408.65(2)

0.1

Cmcm

3.8948(1)

10.2294(2)

10.2813(2)

409.63(1)

0.2

Cmcm

3.8970(1)

10.2320(2)

10.2889(2)

410.26(1)


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0.3

Cmcm

3.9010(1)

10.2349(2)

10.2961(2)

411.08(1)

0.4

Cmcm

3.9057(1)

10.2408(4)

10.3046(4)

412.16(3)

XRD patterns of freshly prepared samples are displayed in Figure 1a. All samples exhibit diffraction patterns consistent with the single phase of Ta3N5 (JCPDS file 89-5200) with an orthorhombic symmetry (Cmcm). There is a slight shift of diffraction peaks toward low angles at high Zr doping level, e.g. (110) peak, implying lattice expansion by Zr incorporation. Considering the relatively larger ionic radius of Zr4+ (72 pm) compared to Ta5+ (64 pm) in octahedral coordination and absence of peaks attributable to Ta2O5, TaON or zirconium (oxy)nitride, we have successfully prepared Zr doped Ta3N5.45 Rietveld refinements were then performed on the diffraction data. The unit cell parameters after refinement are plotted in Figure 1b and also tabulated in Table 1. All unit cell parameters increase linearly with increasing Zr incorporation, following Vegard’s law. This observation is largely different from those in Mg doped Ta3N5 where shrinkage of b parameter is clearly seen. This is probably due to a random substitution of O2− for N3− (i.e. • defects) after Zr doping and is consistent with previous reports.37 Typical refinement was illustrated in Figure 1c for sample TZ-0.1. The typical refined crystal structure of TZ-0.1 is illustrated in Figure 1d.

3.2. UV-Vis spectroscopy and microstructures The UV-Vis absorption spectra of as-prepared powders were converted from their diffuse reflectance spectra. Figure 2a displays the absorption spectra of Zr doped Ta3N5 and pristine Ta3N5. An absorption edge around 600 nm was found for all samples, indicating narrow band gap semiconductors. A slightly blue-shift of the absorption edge was found for the Zr doped


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Ta3N5, resulting in 0.1 eV higher of the band gap value than pristine Ta3N5. This phenomenon is also found in recent literatures.31 Band gaps were then calculated by the Kubelka-Munk transformation (Figure 2b) and are listed in Table 2. The sub-band gap optical absorption lies approximately at 720 nm for un-doped Ta3N5 is significantly depressed after Zr doping, resulting in a flattened absorption curve above absorption edge. Despite of the debate on the origin of this sub-band gap optical absorption,46 recent studies demonstrate a strong correlation between this absorption band and nitrogen vacancies (i.e. ••• defects) and is generally accepted as a hint for low photocatalytic activity.47 Apparently, doping Zr into Ta3N5 can effectively remove nitrogen vacancies, as indicated by the flatten absorption curve around 720 nm.

The microstructures of precursor, pristine Ta3N5 and Zr doped one (TZ-0.1) were investigated by FESEM and TEM. As seen in the Figure 3a and b, the particles of precursor have spherical shape with a relatively smooth surface and a diameter approximately 1 µm. Such a spherical morphology is largely maintained after nitridation except a porous texture at the surface of individual particle. TEM studies reveal that the porous texture is also well developed at the inner part of the particles and all samples are essentially mesoporous with a pore diameter less than 50 nm. This is also confirmed by pore size distribution curves analysis, as discussed in the next section. Interestingly, Zr doped Ta3N5 have a more uniform pore distributions than pristine Ta3N5.


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3.3. Thermogravimetric analysis & BET measurements Table 2 Effective chemical formula determined from TGA, band gap values and BET surface area of as-prepared samples Ta3-xZrxN5-yOy. x

Band gap (eV)

Effective chemical formula

BET surface area (m2 g−1)

0 0.05

2.03(2) 2.04(1)

Ta3N4.01O0.99 Ta2.95Zr0.05N3.74O1.26

19 15

0.1

2.05(2)

Ta2.9Zr0.1N3.43O1.57

17

0.2

2.06(2)

Ta2.8Zr0.2N2.72O2.28

19

0.3

2.08(2)

Ta2.7Zr0.3N2.62O2.38

19

0.4

2.10(2)

Ta2.6Zr0.4N2.32O2.68

22

All as-prepared samples were then analyzed by TGA techniques in air from room temperature up to 1200 oC. Roughly three regions can be identified according to their TGA curves (Figure 4a). At low temperature region (from room temperature to ~400 oC), the TGA curves are characterized by a small mass decrease of ~0.4 wt%, attributable to the removal of adsorbed moistures. In the middle temperature region (between ~400 oC and ~800 oC), the TGA curves of the all samples exhibit a clear mass enhancement, assignable to the oxidation of the sample (oxygen uptake). In the high temperature region (from ~800 oC to ~1200 oC), the TGA curves gradually decrease in mass and eventually approaches a plateau, attributing to an elimination of nitrogen species and a full oxidation of these samples.48 Therefore, the nitrogen content can be estimated by assuming the increase in mass was due to the replacement of N with O in the structure according to following equation: Ta Zr N O +

15 − x − 2y 5−y O → Ta Zr O( )⁄ + N 4 2


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The results are listed in Table 2. It can be seen from Table 2 that the oxygen content of Zr doped Ta3N5 increases significantly with increasing Zr doping levels. Therefore, Zr doping effectively determines the level of • defects concentration in the structure of Ta3N5. It is also interesting to see pristine Ta3N5 also contains some oxygen. This result has been confirmed by previous observations.22

Figure 4b shows nitrogen adsorption-desorption isotherms and pore size distribution curves of all samples. N2 adsorption isotherms of all samples exhibit type-IV isotherms with a type-H2 hysteresis loop at the relative pressure of 0.7~0.9, indicating mesoporous structure. Therefore, we have successfully prepared Zr-doped mesoporous Ta3N5 microspheres without resorting to template method. Nevertheless, the specific surface area of Zr doped Ta3N5 prepared using this method is considerably larger than that of Ta3N5 reported in previous studies.25,33 The pore-size distribution curves exhibit a wide peak around 17 nm (inset in Figure 4b), confirming a mesoporous structure.

3.4. X-ray photoelectron spectroscopy XPS techniques were applied to examine the surface conditions of all samples. Figure 5 shows the Zr 3d, N 1s, O 1s and Ta 4f high solution spectra of Zr doped Ta3N5 (TZ-0.1) and pristine Ta3N5. Signals were referred to the adventitious C 1s peak which was adjusted to 284.7 eV. The dopant Zr species in Ta3N5 is confirmed by the peak lied at approximately 184.2 eV (Figure 5a), corresponding to a lattice Zr4+ 3d state.49,50 For the O 1s state (Figure 5b), two overlapping peaks can be identified at 530.7 eV and 532.8 eV, corresponding to lattice oxygen and surface hydroxide species.12,51 N 1s signal can be easily identified at 396.5 eV, which slightly overlaps


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with Ta 4p3/2 peak at 404 eV.20,52 Nevertheless, the binding energy of N 1s in TZ-0.1 is slightly shifted to a lower value compared with pristine Ta3N5, probably due to the incorporation of Zr which has a smaller eletronegativity than Ta. The Ta 4f state of both samples involves two overlapping peaks at 24.2 and 26.1 eV, assignable to Ta 4f7/2 and Ta 4f5/2 peaks due to spin-orbital interactions.20

3.5. Photocatalytic and photoelectrochemical properties The photocatalytic activity of all samples was examined in the presence of AgNO3 and CoOx, which are used as an electron scavenger and a cocatalyst, respectively. Their oxygen production activities are displayed in Figure 6. Parameters such as Zr doped levels as well as the amounts of cocatalyst loaded were carefully studied to find the optimized conditions. For different Zr doping levels, TZ-0.1 shows the highest oxygen production activity among all samples, which produced nearly twice as much oxygen as pristine Ta3N5 during the 4 h experiment. However, a very low or very high Zr doping level (i.e. TZ-0.05, TZ-0.4) seems to deteriorate the activity, as they exhibit a lower activity than pristine Ta3N5. Further investigations of TZ-0.1 by loading different amounts of CoOx suggested the optimal amount of cocatalyst lies around 1 wt%. Under this optimal conditions, an average oxygen production rate ~105 µmol h−1 was achieved, corresponding to AQE of ~3.2% (λ ≥ 420 nm). Such AQE can be further enhanced by some surface engineering techniques where appropriate surface reaction sites can be identified and significantly increased.46,53


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For better understanding the origin of the superior photocatalytic activity in Zr doped Ta3N5, we then carried out some photoelectrochemical investigations. In the initial experiment, chopped light linear sweep voltammetry (CLLSV) techniques were applied on as-prepared photo-electrodes made of Zr doped Ta3N5 (TZ-0.1) or pristine Ta3N5 under visible light (λ ≥ 420 nm) or AM 1.5 irradiation (Figure 7a and b). Both TZ-0.1 and pristine Ta3N5 display an anodic photocurrent, implying n-type semiconductivity. Apparently, TZ-0.1 shows a much higher photocurrent than pristine Ta3N5, in accord with its higher photocatalytic activity than pristine Ta3N5. More importantly, such a high photocurrent after introducing Zr is more pronounced at high anodic bias and/or under illumination of a large photon flux (i.e. AM 1.5, Figure 7b), highlighting the benefits of Zr doping in charge generation and separation. The mild improvement in photocurrent of Zr doped Ta3N5 observed in previous literature was likely due to the presence of impurity phases such as ZrO2 and Zr2ON2 which may block the surface reaction sites.31 Furthermore, the electrochemical impedance spectra of prepared electrodes (Figure 7c) shows TZ-0.1 has a much smaller interfacial resistance than pristine Ta3N5, indicating a faster charge transfer process after introducing Zr. Figure 7d shows the Mott-Schottky curves of as-prepared photoelectrodes. The flat band potential (Vfb) of electrodes was calculated following Mott-Schottky equation: 1 2 ,- . = ) − *+ − / ! ##$ % &'( & where C and A are capacitance at the interface and electrode area, respectively, V is the bias applied, kB is Boltzmann's constant, ND is the donor concentration, T is the absolute temperature, ɛ is the dielectric constant, and e is the electron charge. Zr doped Ta3N5 demonstrates a clear cathodic shift of Vfb with respect to pristine Ta3N5 as high as 0.23 V, similar to Mg doped Ta3N5.


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However, the slope of Mott-Schottky curve is considerably reduced after doping Zr into Ta3N5, suggesting a much higher donor concentration in Zr doping Ta3N5 with respect to pristine Ta3N5 if dielectric constant is not severely altered between them.

Cyclic voltammograms (CVs) recorded in the dark conditions of freshly prepared electrodes are displayed in Figure 8. The CV curves can be roughly divided into three potential regions. The region with large positively biased potentials (˃1.0 V vs. NHE) has a sharp anodic current increase, assignable to the inversion region of Ta3N5. Thereby, the onset potential for inversion region can be estimated to around +1.1 V (vs. NHE) for TZ-0.1 and pristine Ta3N5 electrodes. Similar situation is also found on the negative potential side (˂−1.0 V vs. NHE) with sharp increase in cathodic current due to the onset of accumulation region of Ta3N5. However, it is difficult to estimate this onset potential as it was covered by the “current hump”. This “current hump” has been attributed to the deposition and withdrawing of electrons at some band gap states. Presumably, energy windows between these two onset potential (≈2.1 V) corresponds to the band gap value of the semiconductor. Interestingly, the “current hump” of Zr doped Ta3N5 is substantially modified when switching experimental atmosphere from Ar to O2, in contrast to pristine Ta3N5 which shows little alteration. It is well-known that O2 is an electron scavenger that can efficient consume those electrons approaching sample surface. A high sensitivity of “current hump” toward O2 implies that electrons within these band gap states have a higher mobility to reach the surface.


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The open-circuit voltage (Voc) measurements were performed by monitoring the voltage alterations with respect to light illumination and dark conditions. The electrode was kept under dark conditions first to reach equilibrium. A stable Voc can be reached due to the Fermi level difference between the photoelectrode and the counter electrode.54 Upon illumination, the Voc undergoes a negative shift due to the consumption of photo-generated holes at the sample surface and subsequent accumulation of photo-generated electrons in the electrode. A new equilibrium Voc will reached once the electron accumulation events compete with various electron drainage processes (e.g. recombination with traped holes or captured by dissolved O2 etc.). Terminating the light irradiation offers a direct evaluation of all electron dissipation pathways and electron lifetime.55 The lifetime of electrons accumulated in the semiconductor can be approximately calculated by the following equation:56 01 =

,- . 234 ( ) 25 &

where 0n is lifetime, kB is Boltzmann’s constant, T is the absolute temperature and e is the elementary charge.

Figure 9 illustrates the temporal Voc profile with respect to light illumination/termination under different conditions (O2 absence or presence). The Voc of both samples is subject to a sharp negative shift upon light illumination and gradual restoration upon light termination, corresponding to electron accumulation and dissipation processes. The restoring process for pristine Ta3N5 is nearly unaffected in the presence or absence of O2 and Voc got recovered to its original value in the dark for approximately 1500 s. On the contrary, O2 plays an important role


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during the restoring process of Zr doped Ta3N5. Rapid Voc restoration was observed in the presence of O2 while extremely slow restoration of Voc was found in the absence of O2 where Voc time profile reached a plateau after 500 s and could not be recovered from its original dark value. The calculated lifetime for photo-generated electrons of both samples in different atmospheres is plotted in Figure 9c. The very short lifetime of photo-generated electrons of Zr doped Ta3N5 in the presence of O2 suggest a fast migration of photo-generated electrons to the surface, which reflects the superior mobility of these electrons. This result is consistent with previous CVs measurement. In addition, the very long lifetime of these electrons in absence of O2 explains the high photocatalytic activity of Zr doped Ta3N5 where charge recombination is considerably prohibited. However, the comparable lifetime of photo-generated electrons in pristine Ta3N5 in different atmospheres suggest some intrinsic charge recombination processes dominate the lifetime of electrons. Apparently, photo-generated charge carriers have a longer lifetime in Zr doped Ta3N5 than pristine Ta3N5 and are less prone to recombine with each other. These results are all in accord with the CLLSV study, CVs analysis and photocatalytic activity.

3.6. Theoretical calculations We then carried out some theoretical calculations to explore how Zr incorporation helps to enhance the electron transport of Ta3N5. We already know that • defects concentration is significantly enhanced after incorporation of Zr in Ta3N5 while ••• defects concentration is considerably reduced. Thereby, structure models with • defects or ••• defectswere built by randomly substituting N atoms with O atoms or simply removing N atoms within Ta3N5 structure (chemical formula Ta11Zr1N14O6 and Ta12N19). The results obtained from calculations are illustrated in Figure 10. In accord to literatures, some localized deep states and some shallow


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donor states will be induced in the presence of ••• defects.57,58 It is believed that these localized deep states may trap photo-generated charges and promote charge recombination therefore deteriorate photocatalytic activity. On the contrary, • defects mainly induce shallow donor states which couple with those electronic states around conduction band minimum. Electrons at these shallow donor states presumably will have a high mobility. This expectation is confirmed by the previous experimental observations such as increased band gap states in CV and negative shift of flat band potential in Mott-Schottky analysis after Zr doping. These results are also consistent with previous theoretical calculations which point out that oxygen defects in Ta3N5 will induce some adjustment in band edge positions and are helpful for the photocatalytic water splitting reactions from thermodynamic considerations.59

In light of above results, we then tentatively explain the observed improvement on photocatalytic activity over Zr doped Ta3N5 by the following mechanism (as shown in Figure 11). Introducing Zr in the Ta3N5 induces an increment of • defects and reduction of ••• defects. • defects substantially modify the electronic structure of Ta3N5 by forming delocalized shallow donor states below the conduction band minimum (CBM). These shallow donor states strongly couple with electronic states at CBM therefore contribute to high electron mobility. High electron mobility is beneficial for the separation of photo-generated charges, as recombination of electrons and holes is less likely to occur. On the contrary, ••• defects induce localized deep states which can trap photo-generated charges and deteriorate the photocatalytic activity. Thereby, the superior activity of Zr-doped Ta3N5 originates from the proper defect control that considerably extend the lifetime of photo-generated charges.

4. Conclusions


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We have synthesized Zr doped mesoporous Ta3N5 microspheres using a simple method without any templates. Their crystal structure and various physicochemical properties were investigated. Doping Zr into Ta3N5 induces a blue shift of the absorption edge. More importantly, the absorption band centred at 720 nm was significantly depressed after Zr incorporation, indicative of low ••• defects. FESEM and TEM images revealed that the morphology of all samples were microspheres with a diameter approximately 1 µm. BET analysis suggests that mesopores are distributed not only at the surface of the sample but also penetrate thoroughly across the inner part. TGA and XPS analysis indicating a clear increase in oxygen content after introducing Zr into Ta3N5 structure. Significant enhancement on photocatalytic activity as well as photocurrent were observed in these Zr doped Ta3N5. The highest activity was found for TZ-0.1, which exhibit an oxygen production rate ~105 µmol h−1 under visible light irradiation (λ ≥ 420 nm), corresponding to AQE of ~3.2%. Photoelctrochemical analysis revealed higher electron mobility in the Zr doped Ta3N5 compared to pristine one. Theoretical calculations suggest that the high electron mobility originates from • defects which form shallow donor states below CBM and facilitate charge migrations. On the other hand, the disadvantages of ••• defects were also realized as they would induce deep donor state within the band gap of Ta3N5, which often act as charge recombination centres. Thereby, the superior photocatalytic activity of Zr doped Ta3N5 • can be understood as a reduction of ••• defects and an increment of defects in the

structure.

Acknowledgements


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This work was financially supported by the National Natural Science Foundation of China (Grant No. 21401142, 21405114 and 21573160) and 1000 plan. This work is also supported by the Science & Technology Commission of Shanghai Municipality (14DZ2261100) and the Fundamental Research Funds for the Central Universities.

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Figure 1. (a) XRD patterns of as-prepared samples, (b) the unit cell parameters of individual sample with different Zr doping level, (c) observed and calculated X-ray powder diffraction patterns of TZ-0.1. The refinements converged with good R-factors and χ2 (Rwp = 4.09%, Rp = 3.20% and reduced χ2 = 1.431), Iobs is observed diffraction data and Icalc is calculated data by the refinement and (d) schematic representation of the refined crystal structure.

Figure 2. (a) UV-Vis light absorption spectra of the as-prepared samples, and (b) Kubelka-Munk transformations of the diffuse reflectance data.

Figure 3. SEM and TEM images of (a-c) precursor, (d-f) pristine Ta3N5 and (g-i) Zr doped Ta3N5 (TZ-0.1) at different magnifications.

Figure 4. (a) TGA curves of all samples in air with heating rate of 10 oC min−1. (b) Nitrogen adsorption-desorption isotherms of all samples, the inset shows the pore size distribution curves.

Figure 5. XPS spectra of constituent elements of Zr doped Ta3N5 (TZ-0.1) and pristine Ta3N5: (a) Zr 3d; (b) O 1s; (c) N 1s; (d) Ta 4f.

Figure 6. (a) Photocatalytic oxygen production of as-prepared samples under visible light irradiation (λ ≥ 420 nm) in AgNO3 aqueous solution (0.05 M); 1 wt% CoOx was used as a


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cocatalyst; (b) average photocatalytic oxygen production rate of as-prepared samples under visible light irradiation (λ ≥ 420 nm); (c) different amounts of CoOx cocatalyst loading at TZ-0.1.

Figure 7. CLLSV of the TZ-0.1 and pristine Ta3N5 photoelectrodes in an aqueous solution of K3PO4/K2HPO4 (0.1 M, pH = 11.5) under (a) visible light irradiation (λ ≥ 420 nm) and (b) AM 1.5 irradiation. The potential was swept at 20 mV s−1 from negative potential to positive potential. (c) Nyquist plot of TZ-0.1 and pristine Ta3N5 photoelectrodes at Voc under visible light irradiation or dark conditions with frequency range from 100 kHz to 0.1 Hz at 10 mV and (d) Mott-Schottky plots, the capacitance was extracted from impedance analysis at a fixed frequency of 1000 Hz at amplitude of 10 mV.

Figure 8. (a) CVs of the as-prepared photoelectrodes at a potential range of −1.5 V to +1.5 V, scan rate of 500 mV s−1; (b) CVs of the pristine Ta3N5 and (c) CVs of the Zr doped Ta3N5 (TZ-0.1) photoelectrode at a potential range of −1.5 V to 0 V, scan rate of 500 mV s−1, 0.1 M K3PO4/K2HPO4 working solution purged with Ar or O2.

Figure 9. Voc time profile of (a) pristine Ta3N5 and (b) Zr doped Ta3N5 (TZ-0.1) under O2 absence or presence, irradiation (λ ≥ 420 nm) started after electrodes attained a steady Voc in the dark and was terminated after 100 s; and (c) the electron lifetime of pristine Ta3N5 and Zr doped


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Ta3N5 derived from the above Voc time profile. Vocdark and Voclight are the open circuit voltage in the dark and under irradiation, respectively.

Figure 10. Theoretical calculations of (a) Ta3N5 with ••• defects and (b) Zr doped Ta3N5 with • defects. Different spin states are marked by arrows (↑↓). The Fermi level is set at zero and is indicated by a dashed line.

Figure 11. The proposed mechanism of the photocatalytic processes in pristine Ta3N5 and Zr doped Ta3N5. Photogenerated holes and electrons are expressed as open and filled circles.

Table 1. Unit cell parameters of as-prepared samples Ta3-xZrxN5-yOy.

Table 2. Effective chemical formula determined from TGA, band gap values and BET surface area of as-prepared samples Ta3-xZrxN5-yOy.


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Table of Contents (TOC)


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Figure 1. (a) XRD patterns of as-prepared samples, (b) the unit cell parameters of individual sample with different Zr doping level, (c) observed and calculated X-ray powder diffraction patterns of TZ-0.1. The refinements converged with good R-factors and χ2 (Rwp = 4.09%, Rp = 3.20% and reduced χ2 = 1.431), Iobs is observed diffraction data and Icalc is calculated data by the refinement and (d) schematic representation of the refined crystal structure.


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Figure 2. (a) UV-Vis light absorption spectra of the as-prepared samples, and (b) Kubelka-Munk transformations of the diffuse reflectance data.



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Figure 3. SEM and TEM images of (a-c) precursor, (d-f) pristine Ta3N5 and (g-i) Zr doped Ta3N5 (TZ-0.1) at different magnifications.


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Figure 4. (a) TGA curves of all samples in air with heating rate of 10 oC min−1. (b) Nitrogen adsorption-desorption isotherms of all samples, the inset shows the pore size distribution curves.



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Figure 5. XPS spectra of constituent elements of Zr doped Ta3N5 (TZ-0.1) and pristine Ta3N5: (a) Zr 3d; (b) O 1s; (c) N 1s; (d) Ta 4f.


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Figure 6. (a) Photocatalytic oxygen production of as-prepared samples under visible light irradiation (λ ≥ 420 nm) in AgNO3 aqueous solution (0.05 M); 1 wt% CoOx was used as a cocatalyst; (b) average photocatalytic oxygen production rate of as-prepared samples under visible light irradiation (λ ≥ 420 nm); (c) different amounts of CoOx cocatalyst loading at TZ-0.1.



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Figure 7. CLLSV of the TZ-0.1 and pristine Ta3N5 photoelectrodes in an aqueous solution of K3PO4/K2HPO4 (0.1 M, pH = 11.5) under (a) visible light irradiation (λ ≥ 420 nm) and (b) AM 1.5 irradiation. The potential was swept at 20 mV s−1 from negative potential to positive potential. (c) Nyquist plot of TZ-0.1 and pristine Ta3N5 photoelectrodes at Voc under visible light irradiation or dark conditions with frequency range from 100 kHz to 0.1 Hz at 10 mV and (d) Mott-Schottky plots, the capacitance was extracted from impedance analysis at a fixed frequency of 1000 Hz at amplitude of 10 mV.


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Figure 8. (a) CVs of the as-prepared photoelectrodes at a potential range of −1.5 V to +1.5 V, scan rate of 500 mV s−1; (b) CVs of the pristine Ta3N5 and (c) CVs of the Zr doped Ta3N5 (TZ-0.1) photoelectrode at a potential range of −1.5 V to 0 V, scan rate of 500 mV s−1, 0.1 M K3PO4/K2HPO4 working solution purged with Ar or O2.



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Figure 9. Voc time profile of (a) pristine Ta3N5 and (b) Zr doped Ta3N5 (TZ-0.1) under O2 absence or presence, irradiation (λ ≥ 420 nm) started after electrodes attained a steady Voc in the dark and was terminated after 100 s; and (c) the electron lifetime of pristine Ta3N5 and Zr doped Ta3N5 derived from the above Voc time profile. Vocdark and Voclight are the open circuit voltage in the dark and under irradiation, respectively.


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Figure 10. Theoretical calculations of (a) Ta3N5 with 𝑉𝑁••• defects and (b) Zr doped Ta3N5 with 𝑂𝑁• defects. Different spin states are marked by arrows (↑↓). The Fermi level is set at zero and is indicated by a dashed line.



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Figure 11. The proposed mechanism of the photocatalytic processes in pristine Ta3N5 and Zr doped Ta3N5. Photogenerated holes and electrons are expressed as open and filled circles.


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Zr-Doped Mesoporous Ta3N5 Microspheres for ... - ACS Publications

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