Mg modified BaTaO2N as an efficient visible-light-active photocatalyst for water oxidation

Journal of Catalysis 383 (2020) 135–143

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Mg modified BaTaO2N as an efficient visible-light-active photocatalyst for water oxidation Hong Zhang 1, Shunhang Wei 1, Xiaoxiang Xu ⇑ Clinical and Central Lab, Putuo People’s Hospital, Tongji University, 1291 Jiangning Road, Shanghai 200060, China Shanghai Key Lab of Chemical Assessment and Sustainability, School of Chemical Science and Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China

a r t i c l e

i n f o

Article history: Received 24 October 2019 Revised 6 January 2020 Accepted 7 January 2020

Keywords: BaTaO2N Mg modification Photocatalyst Water oxidation Water splitting

a b s t r a c t BaTaO2N owns a promising band gap of 1.8 eV and suitable band edge positions but generally exhibits poor photocatalytic activity for water splitting reactions. In this work, we have modified BaTaO2N by introducing Mg to the B site of perovskite lattice to form a new compound, i.e. BaTa0.95Mg0.05O2+xN1
y. This modification results in an alteration to a number of important parameters such as unit cell size, nitrogen content, optical absorption and microstructures etc. More importantly, Mg modifications are responsible for stronger Ta-O/N bonds, lower Ta4+ defects concentration and positively shift of band edge positions, which in turn contribute to high photocatalytic activities for water oxidation reactions. Apparent quantum efficiency as high as 2.59% has been realized at 420 ± 20 nm, being the highest for BaTaO2N reported to date under similar conditions. Further photoelectrochemical analysis confirms better charge utilizations in Mg modified BaTaO2N. Ó 2020 Elsevier Inc. All rights reserved.

1. Introduction The rapid depleting of fossil fuel reservoir on this planet as well as numerous environmental issues caused by fossil fuel usage has posed strong incentives for us to search and develop alterative green and renewable energy resources [1]. As a flagship example for this purpose, photocatalytic water splitting on particulate semiconductors holds the promise to build a clean and sustainable energy feedstock [2–4]. This is not only because its product hydrogen is a green energy vector but also because it is driven by solar insolation which is inexhaustible and widely accessible across the world [5–7]. Despite these appealing merits, real application of this technique needs substantial advancements on photocatalytic materials to achieve high solar energy conversion efficiency [8–10]. Conventional semiconductors are suffered from either limited solar photon absorption and/or poor charge separation conditions, thereby are not potentially feasible to gain high conversion efficiency [11]. Recently, oxynitrides with perovskite structure, i.e. AM(O, N)3 (A = La, Ca, Sr and Ba; M = Ti, Nb and Ta) have been investigated as potential water splitting photocatalysts [12–20]. This is at least partially due to their strong visible light sensitivity as well as suitable conduction/valence band edge ⇑ Corresponding author at: Clinical and Central Lab, Putuo People’s Hospital, Tongji University, 1291 Jiangning Road, Shanghai 200060, China. E-mail address: [email protected] (X. Xu). 1 These authors contribute equally. https://doi.org/10.1016/j.jcat.2020.01.005 0021-9517/Ó 2020 Elsevier Inc. All rights reserved.

positions that satisfy thermodynamically requirements for water redox reactions [21,22]. More interestingly, extremely high dielectric constant has been found in compounds such as BaTaO2N (e ~ 4000) [23] and SrNbO2N (e ~ 25000) [24,25] which is two or three orders of magnitude higher than conventional semiconductors like anatase TiO2 (e ~ 45) [26] and ZnO (e ~ 8) [27]. This indicates a much easier separation of electrons and holes within these compounds as attraction force between them is inversely proportional to the dielectric constant of these compounds [27], i.e. F ¼
4pee0 er, where e is electron charge, e0 is dielectric constant in 2

vacuum, e is the dielectric constant of the material and r is the distance between electron and hole. Nevertheless, poor photocatalytic activities that are incommensurate to their visible light absorption are generally found for these compounds [12–20]. High defects concentration and/or inappropriate surface conditions for water reactions are likely the major causes [28,29]. How to inhibit detrimental defects as well as to build proper surface conditions for water redox reactions remain a challenge. In this work, using BaTaO2N as an example, we demonstrate that photocatalytic performance for water oxidation can be significantly improved by Mg modifications, i.e. introducing Mg into B site of BaTaO2N (BaTa0.95Mg0.05O2+xN1
y). Water oxidation is a four-proton-fourelectron reaction that is considered to be the rate-limiting step for water splitting [30]. The successful realization of efficient water oxidation reactions therefore is critical for overall water splitting reactions. The presence of Mg within perovskite lattice reduces

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Ta4+ defects concentration and ameliorates interplay between BaTaO2N and cocatalyst CoOx, all of which contribute to efficient photocatalytic water oxidation reactions. 2. Experimental 2.1. Materials synthesis BaTaO2N and Mg modified BaTaO2N (i.e. BaTa0.95Mg0.05O2+xN1
y) were prepared by ammonolysis of proper amorphous precursors. These precursors are produced by a polymerized complex (PC) method which enables homogeneous mixture of all cations. For precursor synthesis of BaTa0.95Mg0.05O2+xN1
y, proper amounts of tantalum pentachloride (TaCl5, Aladdin, 99.99%), Ba(NO3)2 (Aladdin, 99.5%), Mg(NO3)25H2O (SCR, 98.5%) and citric acid (Aladdin, 99.5%) were added into ethylene glycol (Aladdin, GC grade) under magnetic stirring to form a transparent solution. The solution was allowed to polymerize by heating on a hot plate under magnetic stirring at 423 K. After 3 h, the solution became a viscous gel and the temperature was raised to 573 K until a brown resin was formed. The resin was then calcined in a muffle furnace at 823 K for 15 h to burn out organic component. The resultant white powders were amorphous according to XRD analysis and were used for further ammonolysis. Ammonolysis was performed in a tube furnace where precursors were mounted in the center using an alumina boat. Ultrapure ammonia gas (Jiaya Chemicals, 99.999%) with a flow rate ~300 mL min
1 was directed into the tube furnace during the whole experiment. Typical calcination temperature for ammonolysis was 1273 K and duration time was 8 h. Pristine BaTaO2N was synthesized using the same procedure without using Mg(NO3)25H2O in the initial step. 2.2. Materials characterizations The as-prepared sample powders were analyzed by a number of analytic techniques to gain their physicochemical information. Xray powder diffraction (XRD) analysis was performed on a Bruker D8 Focus diffractometer (Bruker, Germany) with Cu Ka1 radiation (k = 1.5405 Å) and Cu Ka2 radiation (k = 1.5444 Å) as incident radiation. Data collection was carried out in a reflection mode using a step size of 0.01° and duration of 0.1 s per step. For structural analysis, high resolution XRD data was adopted to perform Rietveld refinement. The refinement was carried out by using General Structure Analysis System (GSAS) software package. UV–vis light absorption of sample powders was analyzed on a UV–vis spectrophotometer (JASCO-750, Japan). The spectrophotometer was operated in the diffuse reflectance mode using an integrating sphere for data collection. The data was referenced to BaSO4 as a non-absorbing material [31]. A field emission scanning electron microscope (FESEM, Hitachi S4800, Japan) and a transmission electron microscopy (TEM, JEOL JEM-2100, Japan) was used to examine the microstructures of sample powders. Nitrogen content was evaluated by thermogravimetric analysis (TGA) in air using a Labsysevo system (SETRAM, France). Surface areas of sample powders were determined by NOVA 2200e adsorption instrument (Quantachrome, U.S.A.). This was done by using Brunauer-EmmettTeller (BET) model for calculation. The sample powders were pretreated in vacuum at 423 K for 2 h before surface area analysis. BaTaO2N before and after Mg modification were also analyzed by X-ray photoelectron spectroscopy (AXIS Ultra DLD, a monochromatic Al Ka X-ray source). Adventitious C 1s peak at 284.7 eV was used as the reference for data adjustment. The spectra collected were fitted using XPSPEAKFIT software. GaussianLorentzian function (Lorentzian weighting of 20%) and Shirley backgrounds were adopted for the fit.

2.3. Photocatalytic water splitting The photocatalytic performance of BaTaO2N before and after Mg modifications is evaluated by water oxidation reactions in the presence of proper sacrificial agent under visible light illumination (k  420 nm). A top-irradiation-type glass reactor was used to perform the experiments. The reactor was connected to a gas-closed circulation and evacuation system (Perfect Light, Labsolar-IIIAG, China). CoOx was applied as a cocatalyst and was loaded onto sample powders according to previous literatures [32,33]: slurry was formed by impregnating sample powders into proper amounts of cobalt nitrate aqueous solution. The slurry was heated consecutively at 353 K in air for 1 h, 973 K in flowing NH3 (flow rate ~200 mL min
1) for 1 h, and 423 K in air for another 1 h. For photocatalytic experiment, 0.1 g sample powders were ultrasonically dispersed into 100 mL silver nitrate aqueous solution containing 0.2 g La2O3. Silver nitrate was used as a sacrificial agent to promote water oxidation reactions whilst La2O3 was used to control pH around 8.5 [16,34]. The above suspensions were sealed in the reactor and were subjected to evacuation for 45 min to remove air dissolved. The temperature of the reactor was maintained at 293 K by using a water jacket. Photocatalytic water reduction into hydrogen was also evaluated in similar conditions except using sodium sulfite aqueous solution (0.05 M) and 1 wt% Pt as a sacrificial agent and a cocatalyst. Pt was loading onto sample powders according to previous report [35]. Visible light illumination (k  420 nm) were used for the experiment and were produced by filtering the output of a 300 W Xeon lamp (Perfect Light, PLX-SXE300, China) with a UV-cutoff filter. Apparent quantum efficiency for the photocatalytic activity was determined under monochromic light illumination. Monochromic light was generated by filtering the output of a 300 W Xeon lamp (Perfect Light, PLX-SXE300, China) using band pass filters (Perfect light, China) at 420 nm, 450 nm, 500 nm, 550 nm, 600 nm and 700 nm, respectively. Photon flux at individual wavelength was gauged by a quantum meter (Apogee MP-300, China). Gas component within the reactor was analyzed by an online gas chromatograph (TECHCOMP, GC7900, China) equipped with 5 Å molecular sieve columns and a thermal conductivity detector. The carrier gas for gas chromatograph is ultrapure Ar (Jiaya Chemicals, 99.99%). Apparent quantum efficiency (AQE) at individual wavelength was then calculated based on following equation:

AQE ¼

4  moles of oxygen production per hour  100% moles of photon flux per hour

ð1Þ

2.4. Photoelectrochemical analysis Sample powders of BaTaO2N and BaTa0.95Mg0.05O2+xN1
y were deposited onto fluorine-doped tin oxide (FTO) glass (1  3 cm) as the photo-electrodes. This was done by electrophoretic deposition (EPD) method according to previous report [35]. Suspensions formed by dispersing sample powders (40 mg) and iodine (10 mg) into 50 mL acetone were used for EPD. Two FTO glasses in parallel of 1 cm distance with their conductive side facing inward were inserted into above suspensions. EPD was performed by applying 10 V bias between FTO glasses using a potentiostatic control (Keithley 2450 Source Meter, USA). FTO glass at anode side was quickly deposited with sample powders and the deposition time was kept for 10 min. The deposited FTO glass was heated in N2 atmosphere at 673 K for 1 h to remove absorbed iodine and was used as the photo-electrode. Diluted TiCl4 (Alfa-Aesar, 99.9%) (10 mM) was dropped onto the photo-electrode which was then calcined at 623 K for 15 min. This treatment was repeated for five times to strengthen particle interconnections and to minimize the

H. Zhang et al. / Journal of Catalysis 383 (2020) 135–143

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exposure of naked FTO. PEC experiment was carried out in a threeelectrode configuration. Photo-electrode, Ag/AgCl electrode (in saturated KCl solution) and Pt foil (1  1 cm) were used as working, reference and counter electrodes, respectively. These electrodes were connected to a CHI660E electrochemical workstation (CH Instruments, USA). K3PO4/K2HPO4 (0.1 M) was used as electrolyte and buffer (pH = 12.66). A 300 W Xeon lamp (Perfect Light, PLXSXE300, China) equipped with a UV cutoff filter (k  420 nm) or AM1.5 filter was used as the light source. The spectra of light sources are provided in supporting information (Figure S1). Chopped light beam was generated by an electronic timer and shutter (DAHENG, GCI-73, China). Impedance spectra were collected under proper bias with an AC signal from 105 Hz to 10
1 Hz with 10 mV amplitude. Impedance data at 1000 Hz was used to perform Mott-Schottky (MS) analysis. 3. Results and discussion 3.1. Phase composition and structure analysis The amorphous precursors produced by polymeric complex method have been very helpful for producing single phase compound during ammonolysis. This can be rationalized by the homogeneous distribution of all cations at atomic levels in the precursors as well as large freedoms in altering cation ratios, which significantly reduce the threshold for ion diffusion and rearrangement during phase formation [36]. Fig. 1 illustrates the XRD patterns for pristine BaTaO2N and BaTa0.95Mg0.05O2+xN1
y. Both patterns show the same reflections with respect to standard ones for BaTaO2N, therefore singe phase compound has been produced. There is, however, slight shift of all reflections toward high angles for BaTa0.95Mg0.05O2+xN1
y compared with BaTaO2N, indicative of shrinkage of perovskite lattice in response to Mg uptake (Fig. 1, right side plot). This trend is verified by further increasing Mg content in the structure (Figure S2). We then performed Rietveld refinement on XRD data on both compounds. Typical refined XRD patterns for BaTa0.95Mg0.05O2+xN1
y are illustrated in Fig. 2 and refined crystal structures is schematically shown as an inserted image. These refinements converge with reasonable goodness-offit parameters assuming that Mg is accommodated in the same crystallographic positions as Ta. Considering the absence of any super-lattice reflections in XRD patterns, Mg has most probably random distributions at the B site of perovskite lattice. The refined unit cell parameters are tabulated in Table 1. The unit cell parameters are indeed decreased slightly, being consistent with previous expectations. This can be rationalized by the replacements of large N anions with small O anions along with Mg modifications [37,38]. The decrease of nitrogen content after Mg modification is confirmed by thermogravimetric analysis (TGA) (Figure S3 and Table S1). Shrinkage of unit cell has implications of a shorter Ta-O/N bond as well as better orbital hybridizations, both of which favor charge migrations in the perovskite lattice [39–41]. 3.2. UV–vis spectra The as-prepared BaTaO2N powders have a burgundy red color and were turned into brownish red upon Mg modifications (Fig. 3, digital photographs). Such a color alteration in response to Mg uptake implies changes on their visible light absorption. This can be seen from their UV–vis absorption spectra (Fig. 3a). Although both samples own strong absorption in the visible light region, the absorption edge of BaTaO2N is clearly blue-shifted after Mg modifications. Substantial blue-shift can be seen if further increase Mg content in the structures (Figure S4) which is not

Fig. 1. XRD patterns of synthesized BaTaO2N and BaTa0.95Mg0.05O2+xN1
y powders, main reflection around 30° are enlarged on the right side for better observation. Standard reference XRD patterns of BaTaO2N (JCPDS: 00-040-0566) are also included. Dotted orange line is a guide for the eye.

Fig. 2. Observed and calculated XRD patterns of BaTa0.95Mg0.05O2+xN1
y; our Rietveld refinement owns good R and v2 factor (Rp = 5.30%, Rwp = 4.35%, v2 = 1.566). A refined crystal structures are schematically illustrated as the inserted images.

favorable for solar photon harvest. We therefore controlled the Mg content at 5% at B site. The recession of light absorption probably originates from a decrement of nitrogen content in BaTa0.95Mg0.05O2+xN1
y compared with BaTaO2N according to TGA analysis (Table S1). Besides, the absorption tail at long wavelength (700 nm) is also damped after Mg modifications (Fig. 3a and Figure S4). Previous investigations suggest that this absorption tail is generally associated with various types of defects within samples and can be served as an indicator for poor photocatalytic activities [42,43]. It can be realized from Fig. 3a that Mg modifications help to inhibit defects concentration which is advantageous for photocatalytic performance. It is also worth noting that introducing Mg slightly improves absorption in the short wavelength side (500 nm), presumably due to the shortened Ta-O/N bonds or stronger crystal field for Ta(O, N)6 octahedrons. We then use Kubelka-Munk transformations to determine band gap values for BaTaO2N and BaTa0.95Mg0.05O2+xN1
y. This is performed by extrapolating the linear part of absorption curves down to energy axis and is read to be 1.80 eV and 1.88 eV for BaTaO2N and BaTa0.95Mg0.05O2+xN1
y, respectively.

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Table 1 Unit cell parameters from Rietveld refinement, BET surface area and band gap values of BaTaO2N and BaTa0.95Mg0.05O2+xN1
y, numbers in the parenthesis are standard deviations. Sample

a (Å)

V (Å3)

BET surface area (m2g
1)

Band gap (eV)




4.1105(1)

69.452(4)

1.5(1)

1.80(2)




4.1089(1)

69.372(4)

3.4(1)

1.88(2)

Space group

BaTaO2N

Pm 3 m

BaTa0.95Mg0.05O2+xN1
y

Pm 3 m

Fig. 3. (a) UV–vis absorption spectra of BaTaO2N and BaTa0.95Mg0.05O2+xN1
y; the color of sample powders can be visually inspected according to the digital photographs inserted; (b) Kubelka-Munk transformation of UV–vis absorption data for BaTaO2N and BaTa0.95Mg0.05O2+xN1
y.

3.3. Microstructures The microstructures of BaTaO2N before and after Mg modifications were then inspected under scanning electron microscopy (SEM) conditions. Fig. 4 displays typical SEM images for BaTaO2N and BaTa0.95Mg0.05O2+xN1
y. Sample powders of BaTaO2N are characterized by agglomerations of irregular shaped particles ranging from hundreds of nanometers to a few microns. However, BaTa0.95Mg0.05O2+xN1
y powders are composed by particles less than 1 mm. Unlike BaTaO2N whose particles have smooth surface, sharp surface is found for particles of BaTa0.95Mg0.05O2+xN1
y. Thereby, Mg plays an important role during grain growth of BaTaO2N. 3.4. X-ray photoelectron spectra BaTaO2N and BaTa0.95Mg0.05O2+xN1
y powders were then examined by X-ray photoelectron spectroscopy. XPS spectra for core level electrons of Ta 4f, O 1s, N 1s and Mg 1s are displayed in Fig. 5. All data have been adjusted according to adventitious C 1s state at 284.7 eV [44]. Ta 4f state for both samples reveals a broad hump which can be fitted using four overlapping peaks centered at 24.4 eV, 26.5 eV, 25.6 eV and 27.6 eV, respectively. The former two peaks can be assigned to 4f5/2 and 4f7/2 signals of Ta4+ species while the latter ones belong to Ta5+ species [45,46]. The presence of Ta4+ signals indicate that the surface of both samples have defects which may originate from reduction of Ta5+ during synthetic procedures (high temperature and ammonia atmosphere). Nevertheless, Ta4+ species are somewhat decreased upon Mg modifications as can be seen from the Ta5+/Ta4+ ratio by fitting these peaks (Table S2), confirming that Mg helps to inhibit Ta4+defects at the surface. This is consistent with the low absorption tail of BaTa0.95Mg0.05O2+xN1
y in UV–vis absorption spectra (Fig. 3a). The inhibition of Ta4+ species in the presence of Mg can be rationalized by the inductive effect of Mg towards Ta-O/N bonds [47]: Mg2+

is more electron-donating than Ta5+ thereby partially substituting Ta with Mg increases the level of covalence for neighboring Ta-O/N bonds. A higher covalence of Ta-O/N bonds or stronger crystal field for Ta(O,N)6 octahedrons help to stabilize Ta in a high oxidation state [48]. The O 1s state can also be decomposed into two overlapping peaks centered at 529.6 eV and 531.4 eV. These two peaks are attributed to lattice oxygen (O2–) and surface hydroxyl group (OH–) [49]. It is clear from Fig. 5b that peak for hydroxyl group is increased after Mg modifications, indicating a more hydrophilic surface. The N 1s state contains a single peak centered at 395.3 eV, assignable to lattice N3– species [30]. The presence of Mg in BaTa0.95Mg0.05O2+xN1
y is ascertained by the peak centered around 1302.5 eV [50]. The enhanced hydrophilicity might be associated with Mg presence at the surface which serves as a hydrophilicity modifier [32] or additional oxygen vacancies for 000

charge balance (i.e.2MgTa þ 3VO ) which can be anchorage points for water molecules. 3.5. Photocatalytic water splitting The photocatalytic properties of BaTaO2N before and after Mg modifications were assessed by monitoring their O2 production in the presence of proper sacrificial agent under visible light illumination (k  420 nm). Silver nitrate aqueous solution (0.05 M) was used as a sacrificial agent and cocatalyst CoOx were applied to facilitate water oxidation reactions in accord to previous reports [20,32]. Control experiments were carried out first in absence of one of following components such as sample powders, light illumination and water etc. No O2 production was detected under these conditions therefore precluding any spontaneous reactions that lead to O2 generation. The temporal O2 evolution for BaTaO2N and BaTa0.95Mg0.05O2+xN1
y under visible light illumination (k  420 nm) was displayed in Fig. 6a. Steady O2 evolution was realized for both samples along with illumination time, confirming real photocatalytic reactions. However, BaTa0.95Mg0.05O2+xN1
y

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H. Zhang et al. / Journal of Catalysis 383 (2020) 135–143

(a)

BaTa0.95Mg0.05O2+xN1-y

BaTaO2N (b)

1 μm

1 μm

Fig. 4. Field emission scanning electron microscopy images of freshly prepared sample powders: (a) BaTaO2N and (b) BaTa0.95Mg0.05O2+xN1
y.

(a)

Ta 4f

BaTaO2N

(b)

O 1s 2-

O

4+

OH

BaTa0.95Mg0.05O2+xN1-y

-

Counts (a. u.)

Counts (a. u.)

Ta

5+

Ta

30

28

26

24

534

22

Binding Energy (eV)

(c)

532

530

528

Binding Energy (eV)

(d)

N 1s Ta 4p2/3

Counts (a. u.)

Counts (a. u.)

N 1s

BaTa0.95Mg0.05O2+xN1-y

BaTaO2N

408

404

400

396

392

Binding Energy (eV)

1312

1308

1304

1300

1296

Binding energy (eV)

Fig. 5. X-ray photoelectron spectra of sample BaTaO2N and BaTa0.95Mg0.05O2+xN1
y: (a) Ta 4f state; (b) O 1s state; (c) N 1s state and (d) Mg 1s state.

produced almost four fold amounts of O2 as BaTaO2N for the same period of time, indicating much higher photocatalytic performance. The performance of BaTa0.95Mg0.05O2+xN1
y was further investigated by changing the amounts of cocatalyst CoOx loaded. The results are compiled in Fig. 6b. An optimal loading point at 2 wt% was found to give the highest photocatalytic activities and was adopted for action spectra analysis. The action spectra, i.e.

apparent quantum efficiency (AQE) vs excitation wavelength were illustrated in Fig. 6c in which UV–vis absorption spectra were also included. The AQE of BaTa0.95Mg0.05O2+xN1
y shows a clear wavelength dependence that matches well with its UV–vis absorption spectra, confirming a real photon driven process. AQE as high as 2.59% at 420 ± 20 nm was achieved, which stands as the highest one reported to date for BaTaO2N under similar conditions. Mg

H. Zhang et al. / Journal of Catalysis 383 (2020) 135–143

120

(b)

BaTaO2N

120

80

80

O2 (µmol)

100

O2 (µmol)

1.0

1.0 wt% CoOx

BaTa0.95Mg0.05O2+xN1-y 100

60

40

20

20

0

2.5

2.0 wt% CoOx 3.0 wt% CoOx

60

40

1

2

3

4

5

2.0

0.9

1.5 0.8 1.0 0.7

0 0

3.0

(c)

0.0 wt% CoOx

Absorbance (a. u.)

(a)

0

1

2

Time (h)

3

Time (h)

4

5

AQE (%)

140

0.5

0.0 0.6 300 350 400 450 500 550 600 650 700 750 800

Wavelength (nm)

Fig. 6. (a) Photocatalytic oxygen production of BaTaO2N and BaTa0.95Mg0.05O2+xN1
y under visible light illumination (k  420 nm), silver nitrate aqueous solution (0.05 M) and 2 wt% CoOx was used as a sacrificial agent and a cocatalyst, respectively; (b) photocatalytic oxygen production of BaTa0.95Mg0.05O2+xN1
y loaded with different amounts of CoOx under visible light illumination (k  420 nm); (c) action spectra of BaTa0.95Mg0.05O2+xN1
y for O2 production, i.e. apparent quantum efficiency vs excitation wavelength, silver nitrate aqueous solution (0.05 M) and 2 wt% CoOx was used as sacrificial agent and a cocatalyst, respectively.

modifications, thereby serves as a useful strategy for turning on efficient photocatalytic water oxidation over BaTaO2N. In additional, photocatalytic water reduction into hydrogen was also investigated for these two compounds. It is quite interesting that BaTa0.95Mg0.05O2+xN1
y also has a better activity than BaTaO2N under the same conditions (Figure S5). 3.6. Photoelectrochemical analysis To get a deeper insight into the improved photocatalytic activities of Mg modified BaTaO2N, we then performed photoelectrochemical analysis (PEC) based on photo-electrodes fabricated from powders of BaTaO2N and BaTa0.95Mg0.05O2+xN1
y. The results are summarized in Fig. 7. Firstly, linear sweep voltammetry (LSV) under chopped illumination of visible light (k  420 nm) and AM1.5 are shown in Fig. 7a and 7b. LSV of both samples without loading cocatalyst CoOx can be found in Figure S6. In all cases, anodic photocurrent was observed for both samples, indicating n-type semiconductivity. Despite a similar onset potential before and after Mg modifications, BaTa0.95Mg0.05O2+xN1
y owns clearly a much higher photocurrent than BaTaO2N both under visible light (k  420 nm) and AM1.5 illumination. Incident photon to current efficiency (IPCE) measurements also suggest a much higher IPCE values in Mg modified BaTaO2N (Figure S7). Recalling the stronger visible light absorption in case of BaTaO2N, it can be envisaged that Mg modified BaTaO2N owns better charge utilizations so that most photo-generated charges are pulled apart rather than recombined. This observation at least partially explains the improved photocatalytic activities for Mg modified BaTaO2N. In addition, impedance analysis at open-circuit voltage for both samples is shown in Fig. 7c. It is clear from impedance data that BaTa0.95Mg0.05O2+xN1
y has a smaller interfacial charge transfer resistance than BaTaO2N, likely being another factor for its high photocatalytic performance. Further Mott-Schottky (MS) analysis suggest a slightly more positive flat band potential for BaTa0.95Mg0.05O2+xN1
y (Efb ~ -0.94 V vs NHE) than BaTaO2N (Efb ~ -1.05 V vs NHE) at pH = 12.66. Schematic illustration of their band structures using MS data and band gap values are plotting in Figure S8. Both conduction band minimum (CBM) and valence band maximum (VBM) of BaTa0.95Mg0.05O2+xN1
y are positively shifted compared to BaTaO2N. This might be associated with unit cell shrinkage as well as decrement of N/O ratio in the structure of BaTa0.95Mg0.05O2+xN1
y. Unit cell shrinkage corresponds to shorter Ta-(O, N) bonds therefore better p-d orbital hybridizations and more conduction band dispersions. Decrement of N/O ratio corresponds to less contribution of nitrogen to VBM,

resulting in more positive VBM [22]. Therefore, the better charge utilizations in BaTa0.95Mg0.05O2+xN1
y is more likely an intrinsic property rather than a stronger electric field in the depletion layer [51]. Mg modifications lead to a shorter Ta-O/N bonds or stronger covalence bonding networks between Ta and O/N which is also favorable for fast charge migrations. This is mainly because hybridizations between Ta 5d and O/N 2p orbital have major contributions to the conduction and valence band of BaTaO2N near Fermi level [52]. In addition, the MS slope of BaTa0.95Mg0.05O2+xN1
y is much larger than BaTaO2N, indicating a much lower donor concentration in the former compound. This is consistent with previous analysis that Ta4+ species are decreased in BaTa0.95Mg0.05 O2+xN1
y. For better understanding the charge utilizations in BaTaO2N and BaTa0.95Mg0.05O2+xN1
y, we applied open-circuit voltage decay (OCVD) analysis on both photo-electrodes. This refers to pumping/storing photo-generated electrons in sample powders and subsequently tracking the dissipation processes of these electrons after terminating light beams [30,53,54]. Fig. 8a illustrates the typical OCVD profiles of BaTaO2N and BaTa0.95Mg0.05O2+xN1
y in Ar without loading cocatalyst CoOx. The instantaneous negative shift of their open-circuit voltage (Voc) upon light illumination confirms pumping/storing electrons in these compounds as holes are driven to the surface for water oxidation reactions [51]. Removing light illumination results in decay of their Voc back to the ones in the dark as long as the stored electrons are consumed by various dissipation pathways, e.g. recombination with trapped holes etc. The Voc decay profile therefore conveys information on the lifetime of photo-generated charges and can be used to evaluate the charge separation conditions within these samples. It is clear from Fig. 8a that BaTa0.95Mg0.05O2+xN1
y has a slower Voc decay process than BaTaO2N and takes much longer time for a complete restoration of Voc back to its dark value, affirming longer electron lifetime as well as better charge separation conditions. It is interesting to note that Voc decay profile changes substantially when 2 wt% CoOx was loaded onto BaTa0.95Mg0.05O2+xN1
y (Fig. 8b). In this case, Voc decay occurs instantaneously after light termination and is reversed to a more positive value than the one in the dark. It is known that CoOx serves to collect photo-generated holes from sample powders. The sudden reversion of Voc upon light removal can be rationalized by back-donating large amounts of holes from CoOx to BaTa0.95Mg0.05O2+xN1
y [33]. However, this phenomenon is not observed in BaTaO2N. In other words, CoOx works more efficiently to collect and store holes from BaTa0.95Mg0.05O2+xN1
y than BaTaO2N. It can also be inferred that BaTa0.95Mg0.05O2+xN1
y have stronger interac-

H. Zhang et al. / Journal of Catalysis 383 (2020) 135–143

141

Fig. 7. PEC analysis of BaTaO2N and BaTa0.95Mg0.05O2+xN1
y: (a) linear sweep voltammetry (LSV) under chopped visible light illumination (k  420 nm), (b) LSV under chopped simulated solar insolation AM1.5, the sweep start from positive voltage to negative one at a sweep rate of 20 mV s
1; (c) Nyquist plot of impedance spectra at opencircuit voltage (OCV) under light illumination (k  420 nm) or in dark, OCV for BaTaO2N is +0.12 V vs NHE in dark and
0.18 V vs NHE under light illumination; OCV for BaTa0.95Mg0.05O2+xN1
y is + 0.13 V vs NHE in dark and
0.13 V vs NHE under light illumination; (d) Mott-Schottky (MS) plot at pH = 12.66, capacitance was extracted from impedance data at a fixed frequency of 1000 Hz and an amplitude of 10 mV, flat band potential was determined by extrapolating the linear part of MS curves down to potential axis.

(a)

(b)

Fig. 8. Open-circuit voltage decay (OCVD) profile at Ar atmosphere for BaTaO2N and BaTa0.95Mg0.05O2+xN1
y loaded with or without 2 wt% CoOx, visible light illumination started once open-circuit voltage Voc is steady in the dark and was terminated after 100 s: (a) bare sample powders; (b) sample powders loaded with 2 wt% CoOx.

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tions with CoOx as hole transfer back and forth occurs easily. This may be associated with surface reconstruction of BaTaO2N in response to Mg modifications so that firm anchorage of CoOx at the surface of BaTaO2N is realized. As already revealed by SEM analysis, BaTa0.95Mg0.05O2+xN1
y owns a sharper particle surface than BaTaO2N, being supportive for this deduction. A more direct evaluation of such interactions is seen in its TEM image which shows intimate contact between CoOx and BaTa0.95Mg0.05O2+xN1
y while a loose contact occurs between CoOx and BaTaO2N (Figure S9). This might be due to the large curvature of BaTaO2N grains that is not favorable for CoOx attachment.

[9] [10] [11]

[12]

[13]

[14]

4. Conclusions We have successfully applied Mg modifications to BaTaO2N to form a new compound BaTa0.95Mg0.05O2+xN1
y. The presence of Mg at the B site of BaTaO2N considerably tailors a number of importance parameters such as unit cell size, nitrogen content, optical absorption and microstructures etc. Mg modifications are found to be useful to strengthen Ta-O/N bonds and inhibit Ta4+ defects concentration etc., being critical for improved photocatalytic activities for water oxidation reactions. Apparent quantum efficiency as high as 2.59% at 420 ± 20 nm was achieved in Mg modification BaTaO2N, being the highest one reported to date under similar conditions. Further photoelectrochemical analysis suggests ameliorated charge utilizations and facile interfacial charge transfer in Mg modified BaTaO2N. The interaction between BaTaO2N and cocatalyst CoOx is also strengthened after Mg modifications, being favorable for hole collections. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Acknowledgements

[24]

We are deeply appreciated of following funding sources to support this work: National Natural Science Foundation of China (Grant No. 21401142, 51972233), Natural Science Foundation of Shanghai (Grant No. 19ZR1459200), Shanghai Science and Technology Commission (14DZ2261100) and the Fundamental Research Funds for the Central Universities.

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