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Activating layered perovskite compound Sr2TiO4 via La/ N co-doping for visible light photocatalytic water splitting Xiaoqin Sun, Yongli Mi, Feng Jiao, and Xiaoxiang Xu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00369 • Publication Date (Web): 09 Mar 2018 Downloaded from http://pubs.acs.org on March 9, 2018

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ACS Catalysis

Activating layered perovskite compound Sr2TiO4 via La/N co-doping for visible light photocatalytic water splitting

Xiaoqin Suna, Yongli Mia,b,*, Feng Jiaoc,* and Xiaoxiang Xua,*

a

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 b

The Hong Kong University of Science and Technology, Department of Chemical and Biomolecular Engineering, Kowloon, Hong Kong, China c

Center of Catalytic Science and Technology, Department and Biomolecular

Engineering, University of Delaware, Newark, Delaware 19716, United States

Abstract Solar water splitting into H2 and O2 upon a particulate photocatalyst relies on significant advance in material engineering where a number of important properties such as optical absorption, charge transportation and defects level etc. can be manipulated. In this work, we have gained control over these properties for wide band gap semiconductor Sr2TiO4 and successfully actualized water splitting under visible light illumination. This has been realized by co-doping La/N into the laminated perovskite structure of Sr2TiO4. Strong visible light absorption as far as 650 nm can be tailored by varying La/N content in Sr2-xLaxTiO4-yNy (0 ≤ x ≤ 0.5). Optimal

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photocatalytic H2 and O2 production has been achieved at x = 0.2 and x = 0.3 and outweighs a number of typical perovskite oxynitrides. These activities seem to be a function of several important parameters for photo-generated charges (e.g. concentration, mobility and lifetime) which are all linked to La/N co-doping levels. More strikingly, overall water splitting has been achieved at x = 0.2 with Rh/Cr2O3 as a cocatalyst. Defects like Ti3+ species play a negative role to the photocatalytic activity as they strongly promote charge recombination and shorten the electron lifetime. Theoretical calculations reveal the crucial role of N in uplifting the valence band maximum of Sr2TiO4 by hybridization with O 2p orbitals. La therefore, serves to balance the charge discrepancies induced during N/O replacements which would otherwise be unfeasible for substantial doping. Our calculations also suggest that Sr2TiO4 has a 2D charge transportation character which is extremely useful for charge separations.

Keywords: Photocatalyst; Water splitting; La/N co-doping; visible light photocatalyst; Sr2TiO4; DFT calculation

1. Introduction The utilization of fossil fuels emits an extremely large amount of greenhouse gases to the atmosphere, which inevitably caused numerous environmental problems, such as climate change and extreme weather1-6. Many efforts have been devoted into the development of renewable energy technologies that can substantially lower down our


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reliance on fossil fuel7. Among them, splitting water molecules into H2 and O2 stoichiometrically on a particulate photocatalyst, driven solely by solar insolation, warrants an intriguing scenario upon which a clean and sustainable energy infrastructure can be built8-12. Because solar energy is infinite and globally accessible, solar-driven water splitting is scalable to meet the global energy demand13-14. Nevertheless, implementation of this appealing technique depends on tangible advancements on photocatalytic materials or systems whereby solar energy conversion efficiency stands high enough to win commercial interest15-16. Very few compounds in previous reports attain such stringent criteria and most stable semiconductors, such as TiO2, own a band gap too large to be usable for solar energy conversions17-19. How to modify these wide band gap semiconductors so that they acquire sensitivity towards longer wavelength photons (λ ≥ 400 nm) is an intriguing topic. Previous attempts by doping these semiconductors have witnessed many successes in extending light absorbance. For instance, primary perovskite SrTiO3 (Eg ~ 3.2 eV) has continuously been a research target as it becomes visible-light active by performing a number of doping schemes including single doping (Fe, Ni, Cr etc.) or co-doping (Ag/Nb, Cr/Ta, Rh/F etc.) 20-32. The latter strategy has gained more interest as overall charges in the system can be properly balanced. This is of particular importance as charged defects often serve as trapping centers that are detrimental to photocatalytic activity33. Apart from primary perovskites, a number of layered perovskite compounds have demonstrated superior photocatalytic activities under UV light irradiation, being correlated with their interlayer framework that promotes


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charge separations and reduction/oxidation reactions34-40. Recent studies have indicated that primary perovskites often contains layered perovskite structures (Ruddlesden-Popper type (RP)) at their upper surface41-43. The photocatalytic performance of primary perovskites would be mostly controlled by these layer perovskite counterparts as all catalytic reactions proceed at the surface. It would be of great interest to apply co-doping strategy to these layered perovskites not only to extend their photocatalytic activities into visible light region but also to gain a better understand on the photocatalytic processes of primary perovskites. In this work, we performed an investigation on RP type compound Sr2TiO4 and co-doped La/N into this RP type layered perovskite. Co-doping La/N effectively expands the light absorption of Sr2TiO4 as far as 650 nm and induces superior visible light photocatalytic activity for water splitting.

2. Experimental 2.1. Materials preparation Thermal ammonolysis from amorphous oxide precursors was used for the preparation of all the samples and the method was adapted from previous reports44. The oxide precursors were first prepared using a polymerized complex (PC) method. In a typical synthesis, 25 mL ethylene glycol was mixed with appropriate amounts of citric acid (Aladdin, 99.5%) and titanium isopropoxide (Aladdin, 95%). Subsequently, aqueous solution containing suitable amounts of strontium nitrate (SCR, ≥99.5%) and lanthanum nitrate hexahydrate (Aladdin, 99%) were added dropwisely. The resulting


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solution was magnetically stirred at 423 K for water evaporation and polymerization and a brown gel was formed. The gel was collected and was heated at 400 °C for 5 h and 550 °C for another 5 h to burn out organic species. The resultant white powders were generally amorphous according to XRD analysis. Ammonolysis were then performed by calcining the white powders under the flow of ultrapure ammonia gas (Jiaya Chemicals, 99.999%) in a tube furnace at 1373 K. The gas flow rate was controlled to be ~ 300 mL/min and the calcination time varied from 5 h to 10 h. For comparison purpose, precursors produced by conventional solid-state reactions were also used for ammonolysis: mixture of calculated amounts of SrCO3 (Aladdin, 99.9%), La2O3 (Aladdin, 99.99%) and TiO2 (Aladdin, 99%) powders were ground thoroughly using agate mortar and pestle. The powder mixtures were pressed into pellets which were then calcined in a muffle furnace at 1573 K for 20 h. Pristine Sr2TiO4 and SrTiO3 were also prepared using the same annealing temperature mentioned previously. In addition, La/Rh co-doped Sr2TiO4 (Sr1.97La0.03Ti0.97Rh0.03O4) was also synthesized using the PC method adapted from a previous literature report45.

2.2. Materials Characterization Purity and crystal structure of sample powders were analyzed by X-ray powder diffraction (XRD) techniques using a Bruker D8 Focus diffractometer. Incident radiations are Cu Kα1 (λ = 1.5406 Å) and Cu Kα2 (λ = 1.5444 Å). Step size and duration for data collection is 0.01 ° and 0.1 s. Rietveld refinement was performed on the data collected using The General Structure Analysis System (GSAS) software


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package46. The morphologies of sample powders were examined using a field emission scanning electron microscope (Hitachi S4800). A UV/Vis spectrometer (JASCO-V750) was used to collect diffuse reflectance spectra of sample powders where BaSO4 was adopted as a reference material47. The spectra were analyzed using the JASCO software suite. Thermogravimetric analysis (TGA) (SETARAM, France) was used to determine the nitrogen content for all samples. Micro-meritics instrument TriStar 3000 was used to analyze surface areas of all sample powders using the BET model.

X-ray

photoelectron

spectroscopy

(Thermo

Escalab

250

with

a

monochromatic Al Kα source) was used to analyze element distribution at the sample surface and the binding energies of their core level electrons were also determined. Adventitious carbon 1s peak centered at 284.7 eV was used as a reference to all collected data48. The XPS PEAKFIT software was used to analyze the XPS data. Gaussian-Lorentian function with Lorentzian weighting of 20% were adopted for peak fit and the background was assumed to be Shirley type.

2.3. Photocatalytic water splitting A top-irradiation-type reactor linked to a gas-closed circulation and evacuation system (Perfect Light, Labsolar-IIIAG) was used to perform all photocatalytic experiments. Typically, sample powders (100 mg) were dispersed ultrasonically into 100 mL aqueous solution which was then sealed in the reactor. The reactor was subjected to evacuation for 30 min in order to remove air dissolved in the solution. The temperature of the reactor was maintained at 293 K using a water jacket. Sodium


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ACS Catalysis

sulfite (0.05M) or AgNO3 (0.05M) were added to the solution as sacrificial agents to promote photo-reduction or photo-oxidation half-reactions, respectively. Pt was used as a co-catalyst to further promote photo-reduction reactions (H2 evolution). Pt was loaded onto sample powders according to previous literature:49 sample powders were immersed into appropriate amounts of H2PtCl6 aqueous solution to form a slurry. The slurry was calcined on a hot-plate at 363 K until dry and further calcined at 453 K for 2 hours. Likewise, for photo-oxidation reactions (O2 evolution), CoOx was loaded onto sample powders according to method reported50-51: sample powders were immersed into proper amounts of cobalt nitrate aqueous solution to form a slurry. The slurry was calcined sequentially in air at 353 K until dry, in flowing H2 at 723 K for 1 h and in air again at 423 K for 1 h. Light illumination was generated using a 300 W Xenon lamp (Perfect light, PLX-SXE300). A UV cutoff filter (λ ≥ 400 nm) was mounted to the output of the lamp to generate visible light illumination. An on-line gas chromatography (TECHCOMP, GC7900) with a TCD detector and Ar carrier gas was applied to monitor the gas composition within the reactor. Direct water splitting was also tested for sample powders and was performed under AM 1.5 illumination without adding sacrificial agent to the reactor. A AM 1.5 filter (Perfect light) was used to filter the output of the Xenon lamp to generate AM 1.5 illumination. 0.5 wt% Rh/Cr2O3 was loaded onto sample powders as a cocatalyst according to the literature52: sample powders were immersed into appropriate amounts of rhodium chloride trihydrate (HWRK, 98%) and Cr(NO3)3·9H2O aqueous solution and a slurry was formed. The slurry was dried on a hot water bath and the resultant powders were


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calcined at 623 K for 1 h under 100 mL min-1 of N2 flow.

2.4. Photoelectrochemical analysis For photoelectrochemical measurements, proper photo-electrodes were fabricated by an electrophoretic deposition method45, 53-56: two pieces of fluorine-doped tin oxide (FTO) glass were cut into proper size (30 × 10 mm) as electrodes for deposition. The glass was rinsed by anhydrous alcohol and deionized water under ultrasonic conditions before use. 40 mg sample powders and 10 mg iodine was dispersed into 50 ml acetone ultrasonically to form a suspension. The glass was inserted in parallel into the suspension with conductive sides facing inward. A potentiostatic control (Keithley 2450 Source Meter) was used to control the potential bias between the two pieces of glass. In a typical deposition, constant bias (15 V) was applied for 3 min. Absorbed iodine on the electrode was removed by heating the electrode at 673 K for 1 h. To improve the connectivity between sample particles and to lower down the level of naked FTO, a post-treatment on these electrodes were performed. The electrodes were dropped with diluted titanium chloride (Alfa Aesar, 99.9%) methanol solution (10 mM) and were dried in air. This procedure was done for six times before the electrodes were calcined in ammonia flow (20 ml min-1) at 673 K for 0.5 h. A three-electrode setup using a Zahner electrochemical workstation was configured for photoelectrochemical measurements. Working, counter, and reference electrodes are the as-prepared photo-electrode, Pt foil (10 × 10 mm), and Ag/AgCl electrode, respectively. Typical electrolyte is K3PO4/K2HPO4 aqueous solution (0.1 M, pH = 7.9)


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ACS Catalysis

which also serves as a buffer. Visible light illumination was generated by filtering the output of a 300W Xenon lamp (Perfect Light, PLX-SXE300) using a UV cutoff filter (λ ≥ 420 nm). An electronic timer and shutter (DAHENG, GCI-73) were used to rectify the incident light.

2.5. Theoretical calculations We have used using density functional theory (DFT) for theoretical calculations. The calculations were performed using commercial Vienna Ab initio Simulation Package (VASP)

57

. During the calculations, the Perdew, Burke and Ernzerhof (PBE)

exchange-correlation functional within the generalized gradient approxiamtion (GGA) were applied58. The projector augmented-wave pseudopotential was adopted59. Structure built for simulation of La/N co-doped Sr2TiO4 is a tetragonal unit cell (a = b = 3.9 Å, c = 12.6 Å, α = β = γ = 90 °). 1 Sr and 1 O in the structure were replaced with 1 La and 1 N randomly for the simulation of La/N co-doping. Forces on each atom less than 0.01 eV·Å-1 was used as a criteria for fully relaxation of all geometry structures. 13 × 13 × 5 Monkhorst-Pack k-point grid was used for static calculations60.

3. Results and Discussions 3.1. Phase purity and crystal structure


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Figure 1. X-ray powder diffraction patterns of as-prepared samples Sr2-xLaxTiO4-yNy (0 ≤ x ≤ 0.5). Standard patterns for Sr2TiO4 (JCPDS 00-039-1471) are illustrated as vertical bars at the bottom for comparisons. Main peaks around 32 ° are enlarged on the right, red dotted line are guide to the eye.


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ACS Catalysis

Figure 2. Rietveld refinement of XRD patterns for Sr1.8La0.2TiO4-yNy (x = 0.2). The refinement converged with good R and χ2 factor (Rp= 9.70%, Rwp = 7.63%, χ2 = 2.651). Refined crystal structure is schematically shown as inserted image, unit cell is marked by blue lines.

Table 1. Parameters for the unit cell, band gap values as well as BET surface area of freshly prepared sample powders Sr2-xLaxTiO4-yNy (0 ≤ x ≤ 0.5), standard deviation in parentheses V/Å3

x

space group

a/Å

0.0

I4/mmm

3.8861(1)

12.5922(3)

190.17(1)

0.1

I4/mmm

3.8915(1)

12.5895(4)

0.2

I4/mmm

3.8965(1)

12.6137(2)

0.3

I4/mmm

3.9017(1)

0.4

I4/mmm

0.5

I4/mmm

c/Å

Band gap / eV

BET surface area (m2/g)

2.54(2)

0.4

190.65(1)

2.32(3)

1.3

191.51(1)

2.22 (1)

3.2

12.5935(8)

191.56(2)

2.16(1)

3.4

3.9022(1)

12.6189(8)

192.16(1)

2.10(1)

3.8

3.9044(2)

12.6092(9)

192.23(3)

2.08(1)

4.2


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The synthesis of metal oxynitrides generally involves ammonolysis of crystallized oxide precursors such as La2Ti2O7 and LaTaO4 for the preparation of LaTiO2N and LaTaON261-62. The lack of single crystallized oxide as a precursor for RP type metal oxynitrides has been the major obstacle for the synthesis of La/N co-doped Sr2TiO4. Long ammonolysis time is often needed for the purification of samples when use multiple crystallized phases as the precursor63. This is indeed the case here when we ammonolyze precursors prepared by solid state reactions. The secondary phase La2O3 persist even after 25 h ammonolysis at merely 5% doping level (x = 0.1) (Figure S1). This is attributable to the slow ion diffusion kinetics among different crystallized phases64-65. In light of these difficulties, we have developed amorphous oxide precursors for the synthesis of RP type metal oxynitrides through polymerized complex (PC) method. All ions are considered to be homogeneously distributed within amorphous precursors. This essentially avoids long-distance cation diffusions needed for phase formation, which in turn, greatly reduces the reaction time during ammonolysis step. More importantly, amorphous precursors also provide us additional freedoms in designing cation ratios, i.e. doping levels, which would otherwise be impossible in single crystallized precursor. Single phase samples Sr2-xLaxTiO4-yNy with doping level as high as 25% (x = 0.5) have been successfully obtained by this method within 10 h. Figure 1 illustrates the XRD patterns of all samples Sr2-xLaxTiO4-yNy (0 ≤ x ≤ 0.5) from PC method. All sharp peaks can be indexed by a tetragonal symmetry. These peaks are all consistent with standard


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ACS Catalysis

patterns of pristine Sr2TiO4 (JCPDS 00-039-1471). There is a clear shift of all diffraction peaks towards low angles at high doping levels, suggesting a gradual expansion of crystal lattice in response to La/N co-doping (Figure 1 right image). Rietveld refinement has also been carried out using the collected XRD data and the refined unit cell parameters are tabulated in Table 1. A typical refined XRD patterns are displayed in Figure 2. Goodness-of-fit factors (Rp, Rwp and χ2) for the refinements are generally reasonable by setting up the constraints that La and N occupy the same crystallographic positions as Sr and O, respectively. The gradual expansion of unit cell volume can be understood by the replacements of small O2- by large N3anions66-67. Therefore, the uptake of N in these samples is directly linked to La content i.e. x in Sr2-xLaxTiO4-yNy which is also confirmed by thermogravimetric analysis (Table S1). Further increasing La level (x > 0.5) in the sample fails to give a single phase compound even with extended ammonolysis time (20 h), implying that the doping limit is probably around 25% (x = 0.5). The role of La can be understood as a charge balancer to compensate charge imbalance induced by nitrogen doping, i.e.

La∙ + N . This is confirmed by the fact that single phase compound is not formed if La is doped alone (Figure S2).

3.2. Microstructures


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Figure 3. Field emission scanning electron microscopy images of freshly prepared samples Sr2-xLaxTiO4-yNy (0 ≤ x ≤ 0.5): (a) Sr2TiO4 oxide, (b) x = 0.0, (c) x = 0.1, (d) x = 0.2, (e) x = 0.3, (f) x = 0.4, (g) x = 0.5, (h) digital photographs of sample powders, value of x is label on the cap.

Electron microscopy was used to analyze the morphology of as-prepared sample powders. Figure 3 displays typical field emission scanning electron microscopic (FESEM) images. Pristine Sr2TiO4 and N doped Sr2TiO4 (x = 0.0) are composed of irregular particles with size up to several microns (Figure 3a and 3b). Doping La clearly reduces the particle size down to submicron level, highlighting the effect of La in controlling the growth of sample crystals. Similar phenomena have also been noticed in other material systems involving La doping68-69. La is believed to act as


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ACS Catalysis

textural promoter which greatly enhances surface area of sample powders68. This is consistent with the trend of BET surface areas among samples (Table 1). Transmission electron microscopy (TEM) further reveals that the submicron particles of La/N co-doped sample are aggregates of small granules about a few hundred nanometers (Figure 4a). Their laminated crystal structures have been verified under high resolution TEM conditions where lattice fringes with separation as large as 0.63 nm can be identified, corresponding to (002) planes of the tetragonal unit cell (Figure 4b).

Figure 4. (a) Transmission electron microscopy (TEM) image of Sr1.8La0.2TiO4-yNy (x = 0.2) and (b) high resolution TEM image of Sr1.8La0.2TiO4-yNy (x = 0.2) showing (002) lattice fringe.

3.3. UV-vis spectroscopy


ACS Catalysis

(a) 1.0

(b)

0.6

Kubelka-Munk (a. u.)

Oxide x=0 x = 0.1 x = 0.2 x = 0.3 x = 0.4 x = 0.5

0.8

Absorbance (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.4

0.2

0.0 200

300

400

500

600

Wavelength (nm)

700

800

2

3

4

5

Photon energy (eV)

Figure 5. (a) UV-vis light absorption spectra (converted from diffuse reflectance spectra) of as-prepared sample powders Sr2-xLaxTiO4-yNy (0 ≤ x ≤ 0.5) and (b) Kubelka-Munk transformation of diffuse reflectance data, band gap values are determined by extrapolating the linear part down to the energy axis.

Pristine Sr2TiO4 has a wide band gap as large as 3.5 eV40, 45. The white color of Sr2TiO4 powders clearly suggests negligible absorbance to the photons in the visible light region. However, distinct colors appear in La/N co-doped Sr2TiO4. The color of sample powders varies from light green, yellow, orange to red when doping level increases, implying enhanced visible light absorption. Their UV-Vis absorption spectra are illustrated in Figure 5a. It can be seen from the figure that the absorption edge for pristine Sr2TiO4 lies below 400 nm. This is consistent with their wide band gap nature that is inert to visible light photons (400nm ≤ λ ≤ 800 nm). Nevertheless, the absorption edge of all La/N co-doped Sr2TiO4 is significantly red-shifted, approaching almost 650 nm at the highest doping level (x = 0.5). Clearly, the level of shift is directly associated with La content in the sample with a saturation


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phenomenon occurs around x = 0.4. Further increase La has trivial impact on the absorption edge except a slight uplift of absorption tail above 650 nm. The band gap values are then extracted from the diffuse reflectance data (Figure 5b) by Kubelka-Munk transformation. The results are tabulated in Table 1. Almost 1.5 eV decrease in band gap has been achieved at x = 0.5 compared to pristine Sr2TiO4, signifying the effectiveness of La/N dopant in tailoring the optical properties of Sr2TiO4.

3.4. Fourier transformed infrared (FTIR) and Raman spectroscopy

(a)

x = 0.5

x = 0.5

(b)

x = 0.3 x = 0.2 x = 0.1 x = 0.0 -NH3

-NH3

x = 0.4

Raman intensity (a. u.)

x = 0.4

Transmittance (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

x = 0.3 x = 0.2 x = 0.1

x = 0.0

Oxide

1800

1600

1400

Oxide

CO23

CO23 1200

1000

800

600

400

200 300 400 500 600 700 800 900 1000 1100 1200

-1

Wavenumber (cm )

-1

Energy shift (cm )

Figure 6. (a) Fourier transformed infrared (FTIR) spectra and (b) Raman spectra of as-prepared sample powders Sr2-xLaxTiO4-yNy (0 ≤ x ≤ 0.5), spectra for Sr2TiO4 oxide are also shown for comparisons.

IR spectroscopy has been widely used to monitor the chemical and structural changes in materials. Figure 6a exhibits FTIR spectra of all samples in the range from 2000


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cm−1 to 400 cm−1.Spectra of non-doped Sr2TiO4 were also shown as a reference. A broad absorption band around 570 cm-1 appears in non-doped Sr2TiO4, which is assignable to Ti–O stretching vibration (TO4 modes) according to the literature70. A distinct shift of this band to higher wavenumber can be seen with increasing La/N co-doping levels. This observation can be attributed to a change in the bond strength (i.e. force constant) as O2- is replaced by N3- in TiO6 octahedrons which increase the bond order (Ti-N vs. Ti-O). Moreover, there exists CO32- groups in the non-doped Sr2TiO4 (bands around 850 cm-1 and 1460 cm-1)71. This has been attributed to SrCO3 impurity on the surface of Sr2TiO4 due to the instability of Sr at a low coordination number (CN = 9)40. It is worth noting that co-doping La/N removes carbonate impurity. Instead, new peaks around 930 cm-1 and 1380 cm-1 emerge and are assignable to NH3 groups72. These NH3 groups are likely due to the unreacted ammonia species in the process of ammonolysis. We have further collected Raman spectra for these samples (Figure 6b). La/N co-doped Sr2TiO4 roughly shows a similar Raman signals to pristine Sr2TiO4. However, a gradual degrading and broadening of all Raman peaks have been noticed when x increases, suggesting that the level of ion disordering are enhanced and solid solutions between Sr2TiO4 and LaTiO2N starts to prevail63. Interestingly, there are new Raman peaks around 950 cm-1 in La/N co-doped Sr2TiO4 compared with pristine Sr2TiO4. Considering the spectral region, these peaks are likely due to the stretching mode of apical Ti-N in TiO(N)6 octahedrons73-74.

3.5. X-ray photoelectron spectroscopy (XPS)


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(a)

Ti 2p 2p1/2

2p3/2

(b)

Ti3+

x = 0.4

-

OH

O 1s

2-

O

Intensity (a. u.)

Intensity (a. u.)

x = 0.5

Ti4+

x = 0.3

x = 0.2 x = 0.1

x = 0.0 468

466

464

462

460

458

456

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(c)

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*

3d3/2

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860

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N 1s

528

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Intensity (a. u.)

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ACS Catalysis

835

830

-NHx

404

402

Binding energy (eV)

400

398

396

394

Binding energy (eV)

Figure 7. X-ray photoelectron spectra of freshly prepared samples Sr2-xLaxTiO4-yNy (0 ≤ x ≤ 0.5): (a) Ti 2p state, (b) O 1s state, (c) La 3d state and (d) N 1s state.

XPS techniques provide useful information on the nature of sample surface and were applied to as-prepared sample powders. Figure 7 illustrates the XPS spectra of constituent elements Ti, O, La, and N. Overlapping peaks were separated and fitted for better inspections. Two strong peaks around 457.7 eV and 463.4 eV appear for Ti 2p level for all samples which can be assigned to Ti 2p3/2 and Ti 2p1/2 states of Ti4+ species75. Nevertheless, for samples heavily doped (x ≥ 0.3), additional shoulders


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appear at lower energy side, indicating the presence of Ti3+ species in these samples54. Similar observation was also found in La/N co-doped SrTiO376. For the O 1s state, there are always two peaks for all samples. The one at lower energy side ( about 528.9 eV) belongs to lattice oxygen whilst the other around 531.0 eV is normally assignable to surface OH- groups77. It can be inferred from the strong signal for surface OHgroups that all samples are highly hydrophilic. La 3d signals gradually emerged for doped samples and involved two peaks centered on 834.3 and 851.1 eV for 3d3/2 and 3d5/2 states because of spin orbital splitting54. Charge transfer shake-up satellites can be clearly seen at high bonding energy (indicated by asterisks)78. In addition, N 1s state generally contains a single peak around 394.8 eV, assignable to lattice N3species79. Along with the increase of x, this signal is clearly enhanced, indicative of the increased amounts of N in these samples. Nevertheless, additional peak around 401 eV appears for samples with low doping level (x = 0.1 and 0.2), corresponding to surface NHx group80. In additional, the gradual band gap narrowing at high doping levels stems mainly from the uplift of valence band. This can be confirmed by XPS valence band scan (Figure S3).

3.6. Photocatalytic water splitting


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(b) 30

Oxide x = 0.0 x = 0.1 x = 0.2 x = 0.3 x = 0.4 x = 0.5

30

O2 evolution (µmol)

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H2 evolution / µ mol

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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30

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Figure 8. (a) Temporal photocatalytic hydrogen evolution of sample powders Sr2-xLaxTiO4-yNy (0 ≤ x ≤ 0.5) under visible light illumination (λ ≥ 400 nm), sodium sulfite aqueous solution (0.05 M) and 1 wt% Pt was used as sacrificial agent and co-catalyst; (b) temporal photocatalytic oxygen evolution of sample powders Sr2-xLaxTiO4-yNy (0 ≤ x ≤ 0.5) under visible light illumination (λ ≥ 400 nm), silver nitrate aqueous solution (0.05 M) and 1 wt% CoOx was used as sacrificial agent and co-catalyst. Data for Sr2TiO4 oxide are also shown for comparisons; (c) temporal photocatalytic hydrogen evolution under AM 1.5 illumination for sample Sr1.8La0.2TiO4-yNy (x = 0.2), Sr1.97La0.03Ti0.97Rh0.03O4, Sr2TiO4 and SrTiO3, 1 wt% Pt


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was loaded as cocatalyst; (d) direct water splitting reactions for sample Sr1.8La0.2TiO4-yNy (x = 0.2) under AM 1.5 illumination, 1 wt% Rh/Cr2O3 was loaded as cocatalyst, evacuation is performed at every 6 hours.

Photocatalytic performance of all samples has been evaluated by monitoring H2 or O2 production from water under visible light illumination (λ ≥ 400 nm). Sacrificial agent such as sodium sulfite or silver nitrate has been used to promote photo-reduction or photo-oxidation reactions. Co-catalyst such as Pt (1 wt%) or CoOx (1 wt%) has been loaded under proper conditions to further facilitate H2 or O2 evolution reactions. Control experiments in the dark were performed and no gas evolution was detected for 3 h. Therefore, we can rule out any side reactions that could lead to spontaneous hydrogen evolution or oxygen evolution. All results under illumination are summarized in Figure 8. Pristine Sr2TiO4 exhibits no signals for H2 and O2 evolution for 2.5 h, presumably due to its wide band gap nature that is not sensitive to visible light photons. However, clear H2 and O2 signals have been immediately detected when illuminating samples co-doped with La/N. Continuous H2 and O2 evolution have been recorded for the entire illumination period, confirming real photocatalytic processes. For photocatalytic H2 production reactions, the best performance belongs to sample Sr1.8La0.2TiO4-yNy (x = 0.2) in which an average H2 production rate ~ 16 µmol/h was achieved. The activity decreases appreciably at either below (x = 0.1) or above (x ≥ 0.3) this level. Nevertheless, all La/N co-doped samples demonstrated a better activity than N doped Sr2TiO4 (x = 0.0), highlighting the benefit of co-doping.


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In case of photocatalytic O2 production, however, the optimal doping level shifts to sample Sr1.7La0.3TiO4-yNy (x = 0.3) which displays an average O2 production rate ~ 13 µmol/h. Similar to the situation in H2 production reactions, all La/N co-doped samples own a much better or at least comparable performance than N doped Sr2TiO4 (x = 0.0). The photocatalytic activity was further examined by varying the level of co-catalysts loaded (Figure S4). 1 wt% Pt and 1 wt% CoOx seem to be the optimal loading point. In addition, under AM 1.5 illumination conditions, La/N co-doped Sr2TiO4 also demonstrate much better photocatalytic activities than La/Rh co-doped Sr2TiO4, SrTiO3 and pristine Sr2TiO4 (Figure 8c), confirming the usefulness of La/N co-doping strategy. More strikingly, overall water splitting with stoichiometric H2 and O2 evolution was achieved on Sr1.8La0.2TiO4-yNy (x = 0.2) under AM 1.5 illumination with 0.5 wt% Rh/Cr2O3 as a cocatalyst (Figure 8d). XRD analysis reveals identical patterns for sample powders (Figure S5) before and after experiment and XPS analysis also confirms no discernable changes in surface compositions (Figure S6 and Table S2), suggesting good stability of this material. The photocatalytic activities of perovskite oxynitrides are generally poor under ordinary conditions despite their strong visible light absorbance. The photocatalytic activities we achieved here are much better than a number of typical perovskite oxynitrides such as AMO2N (A = Ca, Sr and Ba; M = Nb and Ta)81-82, ATaON2 (A = La and Pr)83-84, Sr1-xLaxTiO3-xNx63, Sr5Ta4O15-xNx85 etc. Therefore, by introducing La/N dopants, we have successfully gained superior visible light photocatalytic activities over Sr2TiO4.


ACS Catalysis

(a)

30

x = 0.0 x = 0.2 x = 0.3 x = 0.4 x = 0.5

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light off

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3.7. Photoelectrochemical (PEC) analysis

Current density (µ µ A /cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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20

x = 0.0 x = 0.2 x = 0.3 x = 0.4 x = 0.5

ΑΜ 1.5 10 light on

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Figure 9. Linear sweep voltammetry (LSV) of photo-electrodes prepared from sample powders Sr2-xLaxTiO4-yNy (0 ≤ x ≤ 0.5) under chopped light illumination: (a) visible light (λ ≥ 400 nm) and (b) AM 1.5, 1 wt% CoOx was loaded as a co-catalyst.

For better understanding the origin of such large variations in photocatalytic activities among different samples, we have performed PEC analysis based on photo-electrodes fabricated by these sample powders. First, under chopped light conditions for both visible light and AM 1.5 illumination, linear sweep voltammetry (LSV) measurements were carried out (Figure 9a and b). Anodic photocurrent is clearly seen for all investigated samples, suggesting that they are n-type semiconductors with visible light response. There is however, only small improvement on photocurrent when switching illumination from visible light to AM 1.5, implying that these samples are more sensitive to visible light photons. Close examination of their LSV curves


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suggests that Sr1.8La0.2TiO4-yNy (x = 0.2) owns the most negative onset potential (~ -0.4 V vs NHE) for photocurrent among these samples. This onset potential corresponds well with Mott-Schottky analysis that Sr1.8La0.2TiO4-yNy (x = 0.2) has a flat band potential ~ - 0.48 V vs NHE (Figure S7). Nevertheless, its photocurrent remains rather low even under a high anodic bias (> 0.6 V vs NHE). This is likely due to its relatively weak visible light absorption that greatly limits the production of photo-generated charges. On the contrary, much larger photocurrent has been reached for sample Sr1.7La0.3TiO4-yNy (x = 0.3) albeit it has a less negative onset potential (~ -0.2 V vs NHE). Such a large discrepancy between onset potential and flat band potential for samples other than Sr1.8La0.2TiO4-yNy (x = 0.2) is probably due to improper surface conditions that are unfavorable for charge transfer. Moreover, there seems a clear correlation between photocurrent and oxygen evolution. A high photocurrent is critical to photo-oxidation reactions (O2 production) as they are more sensitive to charge concentrations (four-holes-four-protons reactions). This deduction is consistent with previous results where the highest activity for photocatalytic O2 evolution is shifted from Sr1.8La0.2TiO4-yNy (x = 0.2) to Sr1.7La0.3TiO4-yNy (x = 0.3). However, further increasing the doping level considerable decreases the activity although light absorption is somewhat enhanced. Recalling the fact that defects like Ti3+ species have been substantially enriched at high doping levels (x ≥ 0.3), fast charge trapping phenomenon and subsequent charge recombination events are expected which will indubitably dissipate appreciable amounts of photo-generated charges and lower the activity63.


ACS Catalysis

(a) 0.2

Light on

O2

x = 0.3

(b)

1000

x= 0.2 in O2 x= 0.2 in Ar x= 0.3 in O2

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τ n (s)

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Figure 10. (a) open-circuit voltage (Voc) decay time profile of photo-electrodes prepared from sample powders Sr2-xLaxTiO4-yNy (x = 0.2 and 0.3), illumination started after a steady Voc achieved in the dark and was terminated after 100 s; (b) the electron lifetime (τn) derived according to equation 1 as a function of normalized Voc.

Second, to gain a better insight onto the role of defects towards photo-generated charges, we have performed open-circuit voltage decay (OCVD) experiments on samples Sr1.8La0.2TiO4-yNy (x = 0.2) and Sr1.7La0.3TiO4-yNy (x = 0.3) where XPS analysis suggests large difference in defect concentrations (i.e. Ti3+ species) (Figure 7a). The accumulation of electrons (for n-type semiconductors) in samples upon light illumination and their subsequent dissipation after light termination can be easily monitored by recording open-circuit voltage (Voc) of the photo-electrode which is the potential difference between the Fermi level of sample and reference electrode86-87. This provides us an easy and reliable estimate on the behavior of photo-generated


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electrons such as their dissipation pathways and lifetime. Figure 10a illustrates the Voc time profile in response to light on and off under different atmospheres. Both samples display a sharp decrease in Voc upon light illumination, presumably due to accumulation of electrons (water oxidation reactions occur at the surface consume holes). Thereby, these samples are indeed n-type semiconductors where downward bending of band edges is expected in equilibrium with the electrolyte88-89. A steady Voc will be reached after extended light illumination once accumulation of electrons is balanced by electron dissipations (e.g. charge recombination). Voc decay occurs once light illumination is turned-off which is governed by electron disposal processes. Presumably, the rate of Voc decay directly reflects the lifetime of accumulated electrons and gives useful clues to charge recombination events. Clearly, Sr1.8La0.2TiO4-yNy (x = 0.2) has a much slower Voc decay rate compared to Sr1.7La0.3TiO4-yNy (x = 0.3) in both O2 and Ar atmospheres, indicating that it has a much longer electron lifetime. In addition, O2 has often been used as an efficient electron scavenger to capture photo-generated electrons provided that they transport to the sample surface51, 87. Thereby, switching the experimental atmosphere between O2 and Ar offers a comparative study on the mobility of electrons. A pronounced response in Voc decay towards the change of testing atmosphere (e.g. from Ar to O2) implies fast electron migration to the surface (i.e. high electron mobility) and vice versa51. Apparently, Sr1.8La0.2TiO4-yNy (x = 0.2) is more sensitive to testing atmosphere where Voc decay is considerably accelerated in the presence of O2. The lifetime of electrons accumulated in the semiconductor can be approximately


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calculated by the following equation86-87: τ

(1)

where kB, T and e are Boltzmann’s constant, absolute temperature and elementary charge, respectively. The results are illustrated against normalized Voc in Figure 10b. Sr1.8La0.2TiO4-yNy (x = 0.2) has electron lifetime substantially longer (one order of magnitude) than that of Sr1.7La0.3TiO4-yNy (x = 0.3) which well explains its superior photocatalytic activity for H2 production. Considering the fact that large amounts of Ti3+ species emerge at higher doping levels (x ≥ 0.3), the activity decrease beyond x = 0.2 therefore most likely stems from severe charge recombination events induced by Ti3+ species which greatly shortens the electron lifetime.

3.8. Theoretical calculations

Figure 11. Calculated band structure, total density of states (DOS) and projected density of states (PDOS) of constituent elements for La/N co-doped Sr2TiO4. Spin direction is indicated by arrows (↑↓) and the Fermi level is marked by dotted orange


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lines.

To understand the role of La/N dopants and photocatalytic performance, we have carried out theoretical calculations using DFT theory to determine the electronic structure of La/N co-doped Sr2TiO4. Figure 11 illustrates the calculated band structures and the density of states (DOS) for individual element is shown on the right. It is clear from the figure that La/N co-doped Sr2TiO4 has an indirect band gap. An indirect band gap value ~ 0.95 eV can be read between Γ and M point. The severe underestimation of real band gap values is a common result for the generalized gradient approximation (GGA) method90. Nevertheless, qualitative predictions can be used for these calculations. It is interesting to see anisotropic behavior for charge migrations according to the band structure. For instance, from point Г to point X in the conduction band (CB), band dispersion suggests electron behavior along [100] direction and a large spread in energy range (~ 2 eV) can be seen. Opposite situation occurs from point M to point A (i.e. [001] direction) which reveals negligible band dispersion in energy range (flat band). As effective mass (m*) of charge carriers is governed by the second derivatives of E versus k curve 91: ∗ ℏ

(2)

Thereby, the m* for electrons along [001] is enormously larger than that along [100]. Similar situations can be noticed in the valence band (VB). For both electrons and holes, directions perpendicular to [001] generally have small m*. Further inspecting the DOS data suggests that Ti 3d orbitals dominates CB whilst the top of the valence


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band (VB) mainly contains hybridized O 2p and N 2p orbitals. Charge migrations, therefore, are confined within individual TiO(N)6 octahedron layer (i.e. intralayer migration). In other words, interlayer migration across neighboring layers is not allowed. Such a 2D charge transportation property has strong implications to the photocatalytic reactions since charge recombination between different layers is now removed. However, La has contributions neither to conduction band minimum (CBM) nor to valence band maximum (VBM) closed to the Fermi level and therefore serves as an equilibrator to balance the charge discrepancies induced by O/N replacements. Similar phenomenon has been seen in other co-doping strategies such as B/N co-doped TiO292. The photocatalytic reactions for these La/N co-doped Sr2TiO4 under visible light illumination thereby involves charge excitations from O 2p/N 2p orbitals to Ti 3d orbitals and each layer of TiO(N)6 octahedron acts as an independent photocatalyst.

4. Conclusions A series of La/N co-doped Sr2TiO4 are synthesized by thermal ammonolysis from amorphous oxide precursors. Co-doping La/N works as an effective means in tailoring the optical absorption of this wide band gap semiconductor where its light absorption edge can be shifted from 350 nm to as far as 650 nm without breaking down the laminated architectures. Various characterizations confirm these dopants in the structure of Sr2TiO4 which substantially modifies a number of important parameters such as particle size, BET surface area and defects level etc. Efficient photocatalytic


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activities for H2 and O2 production under visible light illumination (λ ≥ 400 nm) have been achieved over these co-doped samples. Their activities strongly depend on La/N co-doping levels. For photocatalytic H2 production reactions, the best performance belongs to sample Sr1.8La0.2TiO4-yNy (x = 0.2) which an average H2 production rate ~ 16 µmol/h was achieved. Nevertheless, the optimal doping level for O2 production shifts to sample Sr1.7La0.3TiO4-yNy (x = 0.3) which displays an average O2 production rate ~ 13 µmol/h. The large variation in photocatalytic activities among samples probably stems from some important parameters such as light absorption, charge mobility and defects level etc. which are directly correlated with La/N co-doping levels. A more negative onset potential for photocurrent as well as a low Ti3+ level are likely the reason for Sr1.8La0.2TiO4-yNy (x = 0.2) to be optimal for photocatalytic H2 production. The situation probably changes for photocatalytic O2 production which is more sensitive to the concentration of photo-generated charges and prefers high doping level. The enrichment of Ti3+ species in the sample considerably promotes charge recombination and shortens the electron lifetime which is responsible for the decreased activity at high doping level. DFT calculations suggested that the band gap narrowing of the La/N co-doping Sr2TiO4 origins from the hybridization of N 2p and O 2p orbitals which uplifts the VBM. La has negligible contribution to both CBM and VBM and acts as a charge balancer. DFT results also point out that these RP compound has 2D charge transportation properties which are beneficial to the photocatalytic activity.


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Supporting Information XRD patterns for samples prepared by solid state reactions, XRD patterns for La doped Sr2TiO4, XPS valence band scan, photocatalytic activity of La/N co-doped Sr2TiO4 loaded with different amounts of cocatalyst, XRD patterns and XPS data before and after photocatalytic experiments, Mott-Schottky analysis, nitrogen content and surface composition of La/N co-doped Sr2TiO4 before and after photocatalytic reactions.

5. Acknowledgements We would acknowledge the National Natural Science Foundation of China (Grant No. 21401142) for funding. The work was also supported by Shanghai Science and Technology Commission (14DZ2261100) and the Fundamental Research Funds for the Central Universities. Xu also thanks Recruitment Program of Global Youth Experts (1000 plan).

Reference 1. Lewis, N. S.; Nocera, D. G., Powering the Planet: Chemical Challenges in Solar Energy Utilization. P. Natl. Acad. Sci. USA 2006, 103, 15729-15735. 2. Annual Energy Outlook 2017; U. S. Energy Information Administration: 2017. 3. Dudley, B. BP Statistical Review of World Energy June 2016; 2016. 4. Meinshausen, M.; Meinshausen, N.; Hare, W.; Raper, S. C. B.; Frieler, K.; Knutti, R.; Frame, D. J.; Allen, M. R., Greenhouse-Gas Emission Targets for Limiting Global Warming to 2 Degrees C. Nature 2009, 458, 1158-1162. 5. Zou, Z. G.; Ye, J. H.; Sayama, K.; Arakawa, H., Direct Splitting of Water Under Visible Light Irradiation with An Oxide Semiconductor Photocatalyst. Nature 2001, 414, 625-627. 6. Nocera, D. G., Living Healthy on A Dying Planet. Chem. Soc. Rev. 2009, 38, 13-15. 7. Turner, J. A., A Realizable Renewable Energy Future. Science 1999, 285, 687-689. 8. Meyer, T. J., Catalysis - The Art of Splitting Water. Nature 2008, 451, 778-779.


Page 32 of 41

Page 33 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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9. Maeda, K.; Teramura, K.; Lu, D. L.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K., Photocatalyst Releasing Hydrogen from Water - Enhancing Catalytic Performance Holds Promise for Hydrogen Production by Water Splitting in Sunlight. Nature 2006, 440, 295-295. 10. Wang, X. C.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen, K.; Antonietti, M., A Metal-Free Polymeric Photocatalyst for Hydrogen Production from Water under Visible Light. Nat. Mater. 2009, 8, 76-80. 11. Fujishima, A.; Honda, K., Electrochemical Photolysis of Water at A Semiconductor Electrode. Nature 1972, 238, 37-38. 12. Gratzel, M., Photoelectrochemical Cells. Nature 2001, 414, 338-344. 13. Turner, J. A., Sustainable Hydrogen Production. Science 2004, 305, 972-974. 14. Dennis, C.; Davey, R.; Stein, W., Solar Energy: Radiation Nation. Nature 2006, 443, 23-24. 15. Bolton, J. R.; Strickler, S. J.; Connolly, J. S., Limiting and Realizable Efficiencies of Solar Photolysis of Water. Nature 1985, 316, 495-500. 16. Chen, Z. B.; 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. 2010, 25, 3-16. 17. Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y., Visible-Light Photocatalysis in Nitrogen-Doped Titanium Oxides. Science 2001, 293, 269-271. 18. Kudo, A.; Miseki, Y., Heterogeneous Photocatalyst Materials for Water Splitting. Chem. Soc. Rev. 2009, 38, 253-278. 19. Chen, X. B.; Shen, S. H.; Guo, L. J.; Mao, S. S., Semiconductor-Based Photocatalytic Hydrogen Generation. Chem. Rev. 2010, 110, 6503-6570. 20. Xu, X. X.; Lv, M. L.; Sun, X. Q.; Liu, G., Role of Surface Composition upon the Photocatalytic Hydrogen Production of Cr-Doped and La/Cr-Codoped SrTiO3. J. Mater. Sci. 2016, 51, 6464-6473. 21. Lv, M. L.; Xie, Y. H.; Wang, Y. W.; Sun, X. Q.; Wu, F. F.; Chen, H. M.; Wang, S. W.; Shen, C.; Chen, Z. F.; Ni, S.; Liu, G.; Xu, X. X., Bismuth and Chromium Co-Doped Strontium Titanates and Their Photocatalytic Properties under Visible Light Irradiation. Phys. Chem. Chem. Phys. 2015, 17, 26320-26329. 22. Lu, L. W.; Ni, S.; Liu, G.; Xu, X. X., Structural Dependence of Photocatalytic Hydrogen Production over La/Cr Co-Doped Perovskite Compound ATiO3 (A = Ca, Sr and Ba). Int. J. Hydrogen Energ. 2017, 42, 23539-23547. 23. Ishii, T.; Kato, H.; Kudo, A., H-2 Evolution from An Aqueous Methanol Solution on SrTiO3 Photocatalysts Codoped with Chromium and Tantalum Ions under Visible Light Irradiation. J. Photoch. Photobio. A 2004, 163, 181-186. 24. Sulaeman, U.; Yin, S.; Sato, T., Solvothermal Synthesis and Photocatalytic Properties of Chromium-Doped SrTiO3 Nanoparticles. Appl. Catal. B-Environ. 2011, 105, 206-210. 25. Li, H. H.; Yin, S.; Wang, Y. H.; Sekino, T.; Lee, S. W.; Sato, T., Roles of Cr3+ Doping and Oxygen Vacancies in SrTiO3 Photocatalysts with High Visible Light Activity for NO Removal. J. Catal. 2013, 297, 65-69. 26. Wang, D. F.; Ye, J. H.; Kako, T.; Kimura, T., Photophysical and Photocatalytic Properties of SrTiO3 Doped with Cr Cations on Different Sites. J. Phys. Chem. B 2006, 110, 15824-15830. 27. Niishiro, R.; Kato, H.; Kudo, A., Nickel and Either Tantalum or Niobium-Codoped TiO2 and


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SrTiO3 Photocatalysts with Visible-Light Response for H2 or O2 Evolution from Aqueous Solutions. Phys. Chem. Chem. Phys. 2005, 7, 2241-2245. 28. Zhou, X.; Shi, J. Y.; Li, C., Effect of Metal Doping on Electronic Structure and Visible Light Absorption of SrTiO3 and NaTaO3 (Metal = Mn, Fe, and Co). J. Phys. Chem. C 2011, 115, 8305-8311. 29. Wu, G. L.; Li, P.; Xu, D. B.; Luo, B. F.; Hong, Y. Z.; Shi, W. D.; Liu, C. B., Hydrothermal Synthesis and Visible-Light-Driven Photocatalytic Degradation for Tetracycline of Mn-Doped SrTiO3 Nanocubes. Appl. Surf. Sci. 2015, 333, 39-47. 30. Wang, D. F.; Kako, T.; Ye, J. H., Efficient Photocatalytic Decomposition of Acetaldehyde over A Solid-Solution Perovskite (Ag(0.75)Sr(0.25))(Nb(0.75)Ti(0.25))O(3) under Visible-Light Irradiation. J. Am. Chem. Soc. 2008, 130, 2724-2725. 31. Modak, B.; Ghosh, S. K., Role of F in Improving the Photocatalytic Activity of Rh-Doped SrTiO3. J. Phys. Chem. C 2015, 119, 7215-7224. 32. Lu, L. W.; Lv, M. L.; Wang, D.; Liu, G.; Xu, X. X., Efficient Photocatalytic Hydrogen Production over Solid Solutions Sr1-xBixTi1-xFexO3 (0

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