Effective photodegradation of tetracycline by narrow-energy band gap photocatalysts La 2-x Sr x NiMnO 6 (x ¼ 0, 0.05, 0.10, and 0.125)

Effective photodegradation of tetracycline by narrow-energy band gap photocatalysts La _2-x Sr _x NiMnO ₆ (x ¼ 0, 0.05, 0.10, and 0.125)

Xiaoyan Yu

^a,b

, Junfeng He

^a

, Yimin Zhang

^a

, Jiamin Hu

^a

, Fuming Chen

^a

, Yinzhen Wang

^a

, Guannan He

^a

, Junming Liu

^a

, Qinyu He

^a,*

aGuangdong Engineering Technology Research Center of Efficient Green Energy and Environmental Protection Materials, Institute for Advanced Materials, Guangdong Provincial Key Laboratory of Quantum Engineering and Quantum Materials, School of Physics and Telecommunication of South China Normal University, Guangzhou, 510006, China

bSchool of Physics and Telecommunication Engineering, Yulin Normal University, Yulin, 537000, China

a r t i c l e i n f o

Article history:

Received 18 March 2019 Received in revised form 18 July 2019

Accepted 19 July 2019 Available online 20 July 2019

Keywords:

La2-xSrxNiMnO6

Photodegradation Tetracycline Narrow-Eg

Oxygen vacancy High-potentialEVB

a b s t r a c t

Owing to the adverse effects of tetracycline (TC) on human health, it should be completely degraded by the active species that are usually generated by photocatalysts under ultraviolet (UV) radiation. Such radiation which accounts for about only 5% energy of the sunlight. In general, obtaining the active species of the photocatalysts upon irradiation with light is challenging because of the wide spectrum of light. To address this, novel photocatalysts, La2-xSrxNiMnO6(x¼0, 0.05, 0.10, and 0.125, denoted by LNMO, LSNMO005, LSNMO010, and LSNMO0125, respectively), with high-potential valence band edge (EVB), abundant oxygen vacancies (OV-s), and narrow-energy band gap (Eg) were designed and prepared via a solid-state reaction.

The synthesized powders were characterized by several techniques. Sr^2þdoping was found to significantly raise theEVBof LNMO and slightly increase itsOV-s. The highestEVB, maximumOV-s, longest lifetime, and fastest transferring speed of the photogenerated carriers were observed for LSNMO010. It was found that LSNMO010 could effectively photodegrade TC in 240 min. TC could also undergo thermocatalytic degra- dation at 100C in 90 min in the presence of LSNMO010. However, TC could not be effectively degraded either by photolysis or thermolysis alone, in the absence of LSNMO010. It was concluded that the photo- generated holes (h^þ-s) and hydroxyl radicals (OH) played dominant roles in the photodegradation of TC.

Measurement of the total organic carbon (TOC) revealed that only 18.28% TOC was removed by direct photolysis, and about 83.90% TOC was mineralised by LSNMO010. The outcomes of this work suggest that introduction of dopants to increaseOVand to obtain high-potentialEVBin double perovskites could be a novel methodology for designing a class of narrow-Egphotocatalysts that can effectively degrade TC.

©2019 Elsevier B.V. All rights reserved.

1. Introduction

Tetracycline (TC), the second most widely used antibiotic in the world, exhibits broad-spectrum antimicrobial activity against a variety of diseases and is often used in human therapy and livestock industry [1]. However, a major portion of the utilised TC is dis- charged directly into water by faeces and urine [1]. Consequently, a large amount of TC has been detected in surface water and groundwater because of the lack of methods for their effective removal from water [2,3]. The misuse of TC render this situation even more serious.

Photocatalytic semiconductors are often used to photodegrade

the dyes and antibiotics in waste water. Some photocatalysts, such as TiO₂based photocatalysts and ZnO based photocatalysts, have been employed to photodegrade TC [4e10]. The energy band gap (Eg) of these materials are usually higher than 3 eV. The large-Eg

photocatalysts can be excited only under ultraviolet (UV) irradia- tion. However, UV light comprises only about 5% of the sunlight, resulting in low quantum efficiency of photocatalysis [11]. At the same time, it also seems that only the large-Egphotocatalysts can completely degrade TC into nontoxic basic matters such as H₂O, CO2, or inorganic products [12,13]. Therefore, obtaining a photo- catalyst that responds over a wider range of the light spectrum and completely photodegrades TC remains a challenge.

Recently, two novel routes have been utilised to solve the above- mentioned problems. Thefirst route involves the incorporation of oxygen vacancies (O_V-s) in narrow-E_g photocatalysts. This is because OV-s can enhance the photocatalytic activity [14,15] by

*Corresponding author.

E-mail address:gracylady@163.com(Q. He).

Contents lists available atScienceDirect

Journal of Alloys and Compounds

j o u rn a l h o m e p a g e :h t t p : / / w w w . e l s e v i e r . c o m / l o c a t e / j a l c o m

https://doi.org/10.1016/j.jallcom.2019.07.233 0925-8388/©2019 Elsevier B.V. All rights reserved.

capturing electron (e^-), which improves the separation efficiency of photogenerated holes (h^þ) ande^-in the photodegradation reaction.

Such separation is beneficial for the photocatalytic activity. TheOV- s can make the transfer of the photogenerated carriers easier [16]

and also decrease the binding energy in organics [17]. The other route utilies theh^þin higher-potentialEVBore^-with more negative potential of conduction band edge (ECB), as in some oxides such as a-Bi₂O₃[15], Bi₂₄O₃₁Br₁₀[18],b-Ga₂O₃[19], BiOBr [20], Fe₂(MoO₄)₃ [21], and CaCu3Ti4O12[22], since high-potentialh^þpossesses suf- ficient oxidizing power for oxidizing the organic pollutants.

Recently, perovskites (ABO₃) or double perovskite oxides (A2BʹBʺO6) as new photocatalyst semiconductors have received much attentions because all ABO3-type perovskites possess both high visible-light-driven photocatalytic and temperature-induced thermocatalytic activities towards organic pollutants [23e26].

Generally, perovskites or double perovskites have the formulae ABO3or A2B^’B⁰⁰O6, wherein A is the larger cation and B is the smaller cation [27,28]. Ion substitution in A or B site can result in cation deficiencies, therefore altering their physicochemical properties according to different applications [29,30]. Besides, cation de- ficiencies at A or B in perovskites often generateO_V-s to balance the charge of the compound. In recent years, the double perovskite La2NiMnO6 has been studied widely as a ferromagnetic semi- conductor with narrowE_gRefs. [31e34]. K^þdoped La_2-xK_xNiMnO₆ [29] was used as soot combustion catalysts, while K^þ doped La1.7K0.3NiMnO6eCuCl2/g-Al2O3 supported perovskite promoter catalysts [35] were used for ethane oxychlorination. The improved catalytic ability of the above two La2NiMnO6based catalysts can be attributed to the monovalent K^þion doping at La^3þsites to induce OV-s. TheEVBof La2NiMnO6is far higher than the redox potential of OH/H₂O (2.72 eV) andOH/OH(2.38 eV) [36].

In this work, we propose a method to design a photocatalyst with narrow-Egbut with the ability to effectively degrade TC. We choose a narrow-E_gmaterial with high-potentialE_VBas a candidate photocatalyst, and then try to introduceOV-s in this photocatalyst to prolong the lifetime of the photogenerated carriers. A dopant is employed to further raise the E_VB to a higher positive value to render the degradation of TC easier. Based on this concept, the double perovskite La2NiMnO6with narrow-Egwas chosen as the photocatalyst for the photodegradation of TC, and Sr^2þwas doped for substituting La^3þin La2NiMnO6in order to introduceOV-s and raise the EVB. The La2-xSrxNiMnO6(LSNMO, x¼0, 0.05, 0.10, and 0.125, denoted by LNMO, LSNMO005, LSNMO010, and LSNMO0125, respectively) was prepared via a solid-sate reaction. To testify the above proposal and the corresponding possibility of the effective degradation of TC, photolysis of TC in absence of LSNMO, photo- catalytic and thermocatalytic degradation of TC in presence of LSNMO had been undergone. As expected, there was abundantOV-s and high-potentialEVBin LSNMO, especially in LSNMO010. There- fore, an effective strategy for the degradation of TC by LSNMO010 through photocatalysis and thermocatalysis could be developed, thereby testifying the above proposal.

2. Material and methods

2.1. Preparation of La_2-xSr_xNiMnO₆(x¼0, 0.05, 0.10, and 0.125)

La2-xSrxNiMnO6samples were fabricated via a solid-state reac- tion. Stoichiometric mixtures of La2O3 (99.5%, Alfa Aesar), NiO (99.5%, Alfa Aesar), MnO₂(99.5%, Alfa Aesar), SrO (97%, Alfa Aesar), and absolute ethyl alcohol (Qidonghongchun Company, China) were milled for 10 h in a planetary miller (QM-SP1, Nanjing Uni- versity, China). The obtained slurries were dried at 50C for 12 h.

Then, the dried powders were pressed into pellets in a metal cru- cible under 40 MPa for 5 min. The pellets were calcined at 1000C

for 12 h, ground, and then pressed into pellets again. The pellets were calcined again at 1200C for 12 h. Thereafter, the calcined pellets were ground in an agate mortar for 0.5 h, producing La2- xSrxNiMnO6(x¼0, 0.05, 0.10, and 0.125) powder.

2.2. Characterisation of the prepared samples

X-Ray diffraction (XRD) data were collected using an X-ray diffractometer (Bruker D8 Advance, Germany) at room temperature with Cu Karadiation (l¼1.5406 Å) at 40 kV and 40 mA with a scan rate of 8min¹from 15 to 80. The morphologies of samples were characterized by scanning electron microscopy (SEM, Zeiss Ultra 55, Germany). The detailed microstructures were observed by high- resolution transmission electron microscopy (HRTEM, JEM- 2100HR, JEOL). X-ray photoelectron spectroscopy (XPS) measure- ments were performed to investigate elemental states in LSMNO as well as theE_VB-s of LSMNO (VB-XPS) using a ESCALAB 250 X-ray photoelectron (America) with an excitation source of Al Ka_¼1486.6 eV. The binding energies obtained in the XPS analysis were corrected for specimen charging referencing the C 1s to 284.8 eV as well as the VB-XPS. UVeViseNIR diffusive reflectance spectra (UVeViseNIR DRS) were acquired using a Shimadzu 2550 PC spectrophotometer (Japan). The photoluminescence (PL) spectra were collected using a Hitachi F-4500fluorescence spec- trophotometer (Japan) to observe the combination rates of the photogeneratede^--h^þpairs. Total organic carbon (TOC) was esti- mated with a TOC analyzer (Model TOC-L, Shimadzu).

2.3. Measurement of photocatalytic activity

The photocatalytic performances of the prepared photocatalysts were evaluated from the degradation of TC (AR, Macklin Industrial Corporation, China) under simulated sunlight. The photocatalytic degradation of TC (10 mg/L) was carried out in a quartz reactor with a magnetic stirrer and a circulating water jacket to cool the reaction solution. A 350 W Xe lamp (Shanghai Lansheng Company, China) was used as the light source. Briefly, 0.1 g photocatalyst was added into a 100 mL TC solution (10 mg/L). Prior to irradiation, the sus- pension was magnetically stirred in the dark for 60 min to establish an adsorption-desorption equilibrium between TC and the photo- catalyst. During dark adsorption and light irradiation, 1 mL of the TC solution was withdrawn from the reactor at regula intervals and centrifuged at 8000 rpm for 20 min to remove any suspended solid for the determination of TC concentration. The TC concentration during degradation was analysed by monitoring the absorbance at 358 nm on a UVeVis spectrometer [37,38]. The adsorption or photocatalytic degradation efficiency of TC over the photocatalysts was evaluated using the following equation [39]:

Degradation efficiency¼

C₁C C₁

100% (1) Here,C_IandCare the initial TC concentration and TC concen- tration during the adsorption/photodegradation at time t (min), respectively.

2.4. Trapping of active species

To explore the active species of the prepared photocatalyst during the photodegradation of TC, trapping experiments were performed in a manner similar to the photocatalytic activity mea- surements. Before the photodegradation of TC, the following scavengers were added into the TC solution: NaHCO₃(0.01 mol/L) as a quencher ofh^þ[40,41], AgNO3(0.01 mol/L) as a quencher ofe [42,43], isopropyl alcohol (IPA, 0.01 mol/L) as a quencher ofOH,

and N2(40 m L/min) as a quencher ofO2[44,45].

2.5. Photoelectrochemical measurements

Photocurrent (PC) was measured on a CHI660E electrochemistry workstation using high pressure mercury lamp as the light source.

Electrochemical impedance spectroscopy (EIS) measurements were carried out by applying an AC voltage of 10 mV with frequency ranging from 0.1 to 100 kHz. All the experiments were carried out in a standard three electrode cell employing the photocatalytic material depositedfluoride tin oxide (FTO) electrode as the work- ing electrode, a standard Ag/AgCl in saturated KCl as the reference electrode, and a platinum wire (0.5 mm37 mm) as the counter electrode. An aqueous solution of 0.5 mol/L Na₂SO₄was used as the supporting electrolyte. To prepare the working electrode FTO glass plates (2 cm2 cm) were ultrasonically cleaned for 30 min suc- cessively with acetone, ethyl alcohol, and deionized water, and then dried at 60C. Then, 0.05 g of the analysed sample and 1 mL 1- methyl-2-pyrrolidinone fluoride were mixed. The mixture was sonicated to obtain a suspension, which was then coated onto the FTO glass substrate. Finally, the coated FTO glass was dried at 60C to obtain the working electrode.

3. Results and discussion

3.1. Phases, microscopic morphology and elemental states

The crystal structures of the prepared samples have been refined by Rietveld method implemented in the GSAS program [46]. The refinement results are based on the rhombohedral structure (space group: R-3). As shown inFig. 1(aed), the acquired Rietveld refinements are in good agreement with the experimental profiles (10.47%<Rwp <13.43%, and 1.058<c²_<1.386), which confirms that LNMO, LSNMO005, and LSNMO010 are in a pure phase while LSNMO0125 is not so. LSNMO0125 contains about 5%

weight fraction NiO as an impurity, as evident from the Rietveld analysis. Thus, x¼0.125 corresponds to overdopping.

The structural parameters (including atomic coordinates and occupation parameters) of different samples obtained from Riet- veld analysis are listed inTable S1in order to investigate the in- fluence of Sr^2þdoping. It is evident fromTable S1that Sr^2þis doped at La sites, and it mainly affects the atomic coordinates of La and O.

The atomic coordinates of Sr alsofluctuate. It should be mentioned that the occupation of O is less than 1, which indicates the existence ofOVin samples.

The main diffraction peaks are labelled inFig. 1(e). In order to understand the influence of Sr^2þdoping on the peak intensities and peak positions, the variation in peak intensities and peaks shift of (2e1 0) and (1 0 4) are presented inFig. 1(f). It is clear that with increasing Sr^2þcontent, the peak intensities of (2e1 0) and (1 0 4) decrease, and the peaks of all the compounds, except LSNMO0125, shift to right.

The cell volume was extracted from the Rietveld analysis Fig. 2(a). It should be noted that the cell volume (left Y axis) initially decreases with relatively smaller amount of Sr^2þ. When a minimum is reached at x¼0.10, the cell volume starts increasing with increasing Sr^2þcontent. The right Y axis inFig. 2(a) is used to plot the strain caused by the Sr^2þdoping in each cell volume [47]:

Sr doping strain in each cell volume¼

V_undopedV_doped

V_undoped 100%

(2) As shown in the right Y axis inFig. 2(a), the strain in cell volume

increases with increasing Sr^2þcontent. The crystallite size is esti- mated by full-spectrum fitting with constant full width at half maximum in Jade 6. Consistent with the cell volume, the crystallite size also decreases with relatively smaller amount of Sr^2þ(shown inFig. 2(b)). When a minimum is reached at x¼0.10, the crystallite size starts increasing with increasing Sr^2þcontent. This can explain the peak shift of (2e1 0) and (1 0 4) inFig. 1(f).

To gather further information on the microscopic structure, SEM images of LNMO, LSNMO005, LSNMO010 and LSNMO0125 were captured and are shown inFig. 3(a) and (b), (c) and (d), respectively.

The particles in all the four samples are agglomerated. In order to evaluate the chemical uniformity within particles, Fig. 3(e) was selected for energy-dispersive X-ray spectroscopy elemental map- ping.Fig. 3(f) clearly confirmed that Sr was uniformly distributed on the particle surfaces in LSNMO010. Detailed structural infor- mation for LNMO and LSNMO010 as representative samples was obtained by HRTEM analysis (Fig. 3(g) and (f)) [48]. The lattice spacings of 2.672 and 2.679 Å matched the characteristic data of the (1 0 4) lattice planes in LNMO and LSNMO010. The spacing of (1 0 4) lattice plane in LSNMO010 was higher than that in LNMO because of Sr^2þdoping.

The elemental states of the prepared samples were investigated by XPS (Fig. 4). The survey spectrum of LSNMO010 (Fig. 4(a)) in- dicates La, Sr, Ni, Mn, and O were present on the particle surface.

The four peaks (Fig. 4(b)) at binding energies of around 835.1, 838.1, 851.9, and 855.0 eVcan be ascribed to La 3d5/2, La 3d5/2satellite, La 3d_3/2, and La 3d_3/2satellite, indicating that La^3þexists in the pre- pared samples [49]. Furthermore, the binding energies of La 3d in different samples were different owing to the incorporation of Sr^2þ into the LNMO lattice.

As shown inFig. 4(c), Sr 3d peak can be deconvoluted into two peaks, one at around 132 eV (Sr^2þ3d5/2) and the other at around 133.7 eV (Sr^2þ3d3/2) [50]. The varying binding energy of Sr 3d is ascribed to the diverse chemical environment of Sr^2þ.

Fig. 4(d) shows that the core-level spectra of Mn 2p can be separated intofive peaks at around 641.5, 643.3, 653.17, 654.54, and 646.0 eV, corresponding to Mn^3þ2p_3/2, Mn^4þ 2p_3/2, Mn^3þ 2p_1/2, Mn^4þ 2p1/2, and satellite of Mn^3þ 2p3/2 [51], respectively. The atomic ratios of elements are equal to the area ratios of the corre- spondingfitting peaks [29,51]. Based on this theory, the calculated atomic ratios of Mn^4þ/Mn^3þfor LNMO, LSNMO005, LSNMO010, and LSNMO0125 are 0.64, 0.65, 1.00, and 1.12 (Table 1), respectively.

This trend suggests that the introduction of Sr^2þincreases the Mn^4þ content.

It is well known that the Ni 2p3/2core-level spectra of both Ni^2þ and Ni^3þstrongly overlap with the existing La 3d3/2satellite, while the spectrum of Ni 3p does not interfere with the other spectra [52].

Thus the Ni 3p spectrum is selected for analysing the chemical states of Ni. The peaks at around 66.9, 68.9, 71.0, and 73.0 eV (Fig. 4(e)) are assigned to Ni^2þ3p_3/2, Ni^2þ3p_1/2, Ni^3þ3p_3/2, and Ni^3þ 3p1/2, respectively, and these are consistent with a previous report [35]. The evaluated atomic ratios of Ni^2þ/Ni^3þin LNMO, LSNMO005, LSNMO010, and LSNMO0125 are 1.58, 2.07, 2.44, and 1.80, respec- tively (Table 1).

The O 1s spectra of the prepared samples (Fig. 4(f)) show asymmetric peaks close to 529.7 eV. Each asymmetric peak of O 1s can be deconvoluted into two peaks at around 529.7 and 531.7 eV, which are close to those reported previously [31]. The peak at lower binding energy (529.7 eV) corresponds to the lattice oxygen (OL) [53], while that at higher binding energy corresponds toO_V[53].

The atom ratios of OV/OL on the surface of LNMO, LSNMO005, LSNMO010, and LSNMO0125 are 0.78, 0.80, 0.87, and 0.56, respectively (Table 1), thereby suggesting that theO_Vcontent on the surface of LSNMO010 is the maximum among the prepared samples.

Overall, the substitution of La^3þby Sr^2þlead to the appearance of Mn^4þ, Ni^2þ, andO_V-s, in order to compensate for the charge neutrality of the samples. The ratios of O_V/O_L increase with increasing amount of doped Sr^2þ in LNMO, LSNMO005, and LSNMO010. It is speculated that the substitution of La^3þby Sr^2þand the transformation of Mn^3þto Mn^4þand Ni^3þto Ni^2þgenerated abundantOVin the prepared samples. It was reported thatOV-s act

as active sites for capturing e with surface-adsorbed oxygen molecules (O₂), thereby leading to the formation of doubly ionised (O22) or singly ionised (O2) species on the surface of the photo- catalysts and improving the transfer of photogenerated carriers [14].

3.2. Optical absorption and band structure

It is well known that the photocatalytic activity is closely related to the propensity of optical absorption and the band structure of photocatalysts [54e56]. Optical absorption spectra of the prepared samples in the range 200e2000 nm are shown inFig. 5(a) It is clear that LNMO and LSNMO005 respond to visible light, while LSNMO010 can respond even to infrared light around 1500 nm.

However, the optical response from LSNMO0125 is limited in the region 260e400 nm. It seems that the introduction of Sr2þin all the samples, except LSNMO0125, broadens the range of optical absorbance from visible to near infrared light.

Eg-s of the prepared samples can explain this UVeViseNIR absorbance.Eg-s of the prepared samples were evaluated using the following equation and Tauc plots [57,58]:ahn_¼AðhnE_gÞⁿ⁼².

a

^h

n

¼A h

n

E_gn=2

(3)

Fig. 1.Rietveldfit for (a) LNMO, (b) LSNMO005, (c) LSNMO010 and (d) LSNMO0125; (e) Main diffraction peaks of the prepared samples and (f) Intensities variations and shifts of (21 0) and (1 0 4) peaks.

Fig. 2.Cell volumes, lattice strain and crystal sizes of the prepared samplesvsSr^2þ content.

Here,a^,h,n^{, andA}represent the absorption coefficient, Plank's constant, frequency of light, and a constant, respectively. In the formula, the value ofnis determined by the type of optical tran- sition in the semiconductor (i.e., n¼1 for direct transition and n¼4 for indirect transition). Since LNMO undergoes direct transition,n was assumed to be 1 [57]. The opticalEgcan be deduced from the intersection of the tangent and X-axis.Fig. 5(b) shows the Tauc plots for the prepared samples. Eg-s of LNMO, LSNMO005, LSNMO010, and LSNMO0125 are 2.30, 2.20, 1.71, and 2.58 eV, respectively. Obviously, Sr^2þdoping reduced theE_gof LNMO, except

for LSNMO0125. The narrowEgof LSNMO010 led to the Vis-NIR absorbance (Fig. 5(a)).

In order to understand the band structures of the Sr-doped La₂NiMnO₆ samples, VB-XPS was performed. VB-XPS of the Sr- doped La2NiMnO6 samples are shown in Fig. 6.EVB-s of LNMO, LSNMO005, LSNMO010, and LSNMO0125 are estimated to be 4.46, 6.76, 7.59, and 7.22 eV versus NHE, respectively, from the VB-XPS [59]. The following equation was used [60]:

E_CB¼E_VB-E_g (4)

Fig. 3.SEM images of (a) LNMO, (b) LSNMO005, (c) LSNMO010, and (d) LSNMO0125; (e) Selected LSNMO010 SEM image for Sr mapping; (f) Elemental mapping image of Sr in LSNMO010; HRTEM images of (g) LNMO and (f) LSNMO010.

Fig. 4.(a) XPS survey of LSNMO010; High resolution XPS spectra of (b) La 3d, (c) Sr 3d, (d) Mn 2p, (e) Ni 3p, and (f) O 1s of the prepared samples.

Table 1

Mn^4þ/Mn^3þ, Ni^2þ/Ni^3þ, andOV/OLratios of the prepared samples.

Atomic ratio LNMO LSNMO005 LSNMO010 LSNMO0125

Mn^4þ/Mn^3þ 0.64 0.65 1.00 1.12

Ni^2þ/Ni^3þ 4.67 4.81 4.89 3.91

OVOV/OL 0.78 0.80 0.87 0.56

Fig. 5. (a) UVeViseNIR adsorption spectra and (b) Tauc plots of the prepared samples.

Fig. 6.VB-XPS data of (a) LNMO, (b) LSNMO005, (c) LSNMO010, and (d) LSNMO0125.

The estimated ECB-s of LNMO, LSNMO005, LSNMO010, and LSNMO0125 were calculated to be 2.16, 4.56, 5.88, and 4.64 eV, respectively, using theE_VBandE_gobtained in UVeviseNIR DRS and VB-XPS. The estimatedECB-s of the prepared samples are listed in Table 2. Moreover,Eg-s (Fig. S1),EVB-s (Fig. S2), andECB-s of com- mercial NiO were also measured and calculated and are listed in Table 2for comparison.

3.3. Dark absorption and photocatalytic performances

The photocatalytic abilities of the prepared samples were investigated by studying the degradation of TC upon irradiation

with simulated sunlight. Prior to photodegradation, the mixture of TC solution and the prepared sample was stirred and the TC con- centration was monitored to ensure that the TC solution reached the dark adsorption-desorption equilibrium. The dark adsorption experiment demonstrated the mixture of TC solution and LSNMO reached the adsorption-desorption equilibrium that after 60 min.

Fig. 7(a) shows that the degradation efficiency of TC through dark adsorption over the prepared samples is relatively low (less than 25%).

The photolytic efficiency and photodegradation efficiency of TC in the presence of LNMO, LSNMO005, LSNMO010, and LSNMO0125 were evaluated in triplicates. The mean values (with error bars) of the photolytic degradation efficiency and photodegradation effi- ciency for TC are presented inFig. 7(b). It can be found that the photodegradation efficiency of TC in the presence of LSNMO is higher than the photolytic degradation efficiency, suggesting that the prepared photocatalysts, especially LSNMO010, have enhanced the degradation of TC.

Fig. 7(c) shows the UVeVis absorption spectrum of TC upon photolysis. There are two major absorption peaks around 270 and 358 nm in the initial UVeVis absorption spectrum of TC (Fig. 7(c)).

TC consists of two chromophores with four rings, namely, A, B, C, and D [61]. The absorption peak around 270 nm corresponds to ring A while that around 358 nm corresponds to the aromatic rings B, C, Table 2

RelativeEg,EVBandECBvalues EVBof the prepared samples and commercial NiO using VB-XPS data.

Photocatalyst Band-gap from VB-XPS

Eg(eV) EVBEVBt (eV) ECBECBb(eV)

LNMO 2.30 4.46 2.16

LSNMO005 2.20 6.76 4.56

LSNMO010 1.71 7.59 5.88

LSNMO0125 2.58 7.22 4.64

NiO 2.69 0.75 1.94

Fig. 7.(a) Degradation efficiency of tetracycline (TC) via adsorption in the dark over the prepared samples; (b) Photolytic and photocatalytic degradation efficiencies of TC over LNMO, LSNMO005, LSNMO010, and LSNMO0125; (c) and (d) UVeVis absorption spectra of TC solution during photolysis and photocatalytic degradation using LSNMO010, respectively, at different times under simulated sunlight irradiation; (e) Kinetics behaviour of photolysis and photocatalytic degradation of TC over the prepared samples.

and D and the developing chromophores. The decay of absorbance around 270 nm is ascribed to the release of acylamino and hydroxyl groups from ring A. The decay of absorbance around 358 nm is attributed to the fragmentation of phenolic groups connected to aromatic rings B, C, and D [62]. The blue shift of the absorption peak at 270 nm could be because of the formation of intermediates, which need further investigation. It is clear fromFig. 7(c) that the phenolic groups and acylamino and hydroxyl groups in TC could not be completely degraded even after 300 min of irradiation by simulated light; i.e., photolysis could not degrade these groups in TC completely. The results of the photolytic degradation are in good agreement with those reported previously [63], which suggested that photolysis only involved the photo-deamination of TC and a partial mineralisation, and the naphthol ring of TC remained intact during photolysis.

However, TC could be effectively photodegraded after 240 min in the presence of LSNMO010, when irradiated with simulated light (Fig. 7(d)). This means thatTC can be effectively degraded in the presence of LSNMO010, although LSNMO010 is a narrow-Eg

photocatalyst. This is thefirst report of the effective degradation of TC by a narrow-E_gphotocatalyst. The mechanismis discussed later.

It should also be noted that the UVeVis absorption peak of TC at 358 nm was red-shifted to 390 nm after dark adsorption in the presence of LSNMO010 (Fig. 7(d)). This may be due to the chelation of TC with Sr^2þ in LSNMO010, because alkali metal ions easily chelate TC [61]. The chelation causes a conformational trans- formation in TC to form a complex. This leads to the red shift of the peak at 358 nme390 nm [61].

Fig. 7(e) shows that the photolytic and photocatalytic degrada- tion of TC in the presence of the prepared samples can be described by simplefirst-order kinetics (Eq.(5)) [64]:

lnC₀

C ¼kt (5)

Here,kis the apparentfirst-order rate constant, andtis the time of light irradiation.C0andCrepresent the concentrations of TC at time t¼0 andt, respectively. The value ofk for photolysis and photodegradation by LNMO, LSNMO005, LSNMO010, and LSNMO0125 are listed inTable 3. It is evident that thekvalues of photodegradation with the prepared samples are higher than that those for photolysis, indicating that the degradation rate of TC in all the samples (except LSNMO0125) increased with increasing Sr^2þ content. It can also be seen that the rate of degradation of TC by LSNMO010 is the highest. Perhaps, Ov-s and EVB dominantly affected the photodegradation rate. It is also clear fromTable 3that the correlation coefficients of all the prepared samples were greater than 0.99 [65,66], which indicated a goodfit tofirst-order kinetics for the photocatalytic degradation in the presence of LNMO, LSNMO005, LSNMO010 and LSNMO0125.

In order to further verify that the degradation of TC (especially the degradation of ring A in TC) over the Sr-doped LNMO is easier than that over other photocatalysts, the degradation of TC by LSNMO010 was carried out under absolute darkness at different temperatures of 30, 40, 60, 80, and 100C (Fig. 8(a), hereafter referred to as thermocatalysis). Fig. 8(b) shows the UVeVis

absorption spectra of TC solution upon degradation via thermoca- talysis with LSNMO010 under absolute darkness at 100C.Fig. 8(a) and (b) shows that TC can be effectively degraded by LSNMO010 at 100C in 90 min. To further confirm the roles of LSNMO010 and temperature (100C) during the degradation in dark TC was sub- jected to degradation without LSNMO010 (Fig. 8(c)). It was found that thermolysis only involved the photodeamination of TC and a partial mineralisation, while the naphthol ring of TC remained intact after 120 min. This was similar to that observed during photolysis [63]. Moreover, comparing the degradation efficiency of thermolysis without LSNMO010 and thermocatalysis with LSNMO010 in absolute darkness at 100C (Fig. 8(d)) clearly pre- dicted that LSNMO010 played a major role in the complete degra- dation of ring A in TC without light irradiation. These results imply that the prepared samples with narrowEg-s can degrade TC under hot and dark conditions, indicting theirhigh potential in treating TC using waste heat.

3.4. Carrier separation efficiency evaluated by PL, PC, and EIS

PL spectra can reflect the synergistic effects of charge carrier trapping and recombination in photocatalysts, since PL emission originates from the recombination of charge carriers [67]. In a PL spectrum, the weaker emission intensity corresponds to lower recombination efficiency of the photogenerated carriers and longer lifetime of the carriers.Fig. 9shows the PL spectra of the prepared samples upon excitation at 530 nm, following which the strongest PL intensity (at 790 nm) can be acquired. With increasing Sr^2þ content, the PL intensity at 790 nm decreases for all the samples, except LSNMO0125. LSNMO010 exhibits the lowest emission in- tensity, indicating that LSNMO010 has the lowest recombination efficiency or the longest lifetime of the photogenerated carriers.

These results agree well with the variation ofOVwith Sr^2þcontent in the prepared samples. This can be explained as follows. Ac- cording to the XPS results, the ratios ofOV/OLin all the samples (except LSNMO0125) increase with increasing Sr^2þcontent, and LSNMO010 possesses the highest O_V/O_L, i.e., the maximum O_V. BecauseOVcan trap the photogeneratede, the recombination of the photogenerated carriers would be suppressed and can effec- tively prolong the lifetime of photogenerated carriers [22,68,69].

Higher theOVin the prepared samples, longer is the lifetime of the photogeneratedh^þ. This is why the PL intensity of the samples (expect LSNMO0125) decreased with increasing Sr^2þ content.

LSNMO010 has the lowest carriers recombination efficiency. This is also precisely why LSNMO010 exhibited the lowest PL intensity at 790 nm. These results confirmed the suppression of the recombi- nation of photogenerated e-h^þ pairs in the samples (except LSNMO0125) with increasing Sr^2þ content. Consequently, LSNMO010 has highest photocatalytic performance.

To further investigate the separation of the photogenerated carriers, the transient PC responses of the prepared samples were recorded for several on-off cycles of irradiation. The photocurrent was generated from the separation and transportation of the photogeneratede^-to the working electrodes. Higher photocurrent intensity indicates higher separation efficiency and longer lifetime of the photogenerated e^--h^þ pairs, which are beneficial for enhancing the photocatalytic performance [70].Fig. 10shows that the transient PC responses of all the samples are very stable in cycles. After turning off the light, a gradual decay in the PC was observed until it reached a constant value. The PC decays mainly because of the recombination of the photogenerated carriers. The decay could be also attributed to the oxidization ofe^- by some active species in the electrolyte [70]. Importantly, with the increasing Sr^2þ content, the PC intensity increased for all the samples, except LSNMO0125. LSNMO010 had the highest PC Table 3

kandR²of photolysis and photodegradation reactions of TC over the prepared samples.

Reaction Photolysis Photodegradation over

LNMO LSNMO005 LSNMO010 LSNMO0125

k(10³min¹) 4.5 7.2 7.6 10.1 6.7

R² 0.999 0.999 0.999 0.999 0.999

intensity, which was nearly 4 times higher than that of LSNMO0125, thus confirming that (i) the lifetime of photo- generated carriers increased with increasing Sr^2þcontent, and (ii) LSNMO010 has the longest lifetime of the photogenerated carriers.

This behaviour was consistent with the trend ofO_VvsSr^2þcontent in the prepared samples. Therefore, it may be concluded thatOVin the prepared samples is a dominant factor for improving the sep- aration efficiency of the photogeneratede^--h^þpairs, as concluded in Refs. [64,71].

EIS was also performed to investigate the transferring speed of the photogenerated carriers in the prepared photocatalyst.Fig. 11 gives the EIS Nyquist plots of the prepared samples. It is well known that the radius of curvature of a curve in the Nyquist plots corresponds to the impedance of the electrode. A smaller radius corresponds to smaller impedance and a higher transferring speed of the photogenerated carriers [72]. It is evident fromFig. 11that the radii of curvature of the EIS Nyquist plots of the prepared samples (except LSNMO0125) decrease with increasing Sr^2þcon- tent, indicating that the photogenerated carriers can transfer more easily in the prepared photocatalysts (except LSNMO0125) with increasing Sr^2þcontent (orO_Vcontent). The photogenerated car- riers in LSNMO010 possess the fastest transferring speed, which partly accounts for its best photocatalytic performance among the prepared samples. Due to the presence of NiO impurity in LSNMO0125, its resistance increases, and the radius of curvature becomes larger than that of LSNMO005 and LSNMO010.

The above results demonstrated that the separation and migration of the photogenerated carriers was significantly improved byOV, indirectly suggesting that the introduction ofOV

can prolong the lifetime of the photogenerated e-h^þ pairs and improve the photocatalytic performance.

3.5. Active species trapping experiments

To investigate the mechanism of photocatalysis, a trapping Fig. 8.(a) Degradation efficiency of thermocatalysis using LSNMO010 in absolute darkness at different temperatures, i.e., 30, 40, 60, 80, and 100C; UVeVis absorption spectra of TC solution during degradation via (b) thermocatalysis (with LSNMO010) and (c) thermolysis (without LSNMO010) in absolute darkness at 100C; (d) Comparison of degradation efficiencies of thermolysis (without LSNMO010) and thermocatalysis (with LSNMO010) in absolute darkness at 100C.

Fig. 9.PL spectra of the prepared samples excited using 530 nm light.

Fig. 10.PC responses of the prepared samples.

experiment was conducted to detect the main active species during photocatalysis over LSNMO010, as an example. The active species generated in the photocatalytic process can be determined through the addition of scavenger into the photocatalytic testing solution.

Different scavengers were added to detect the corresponding active species: 0.01 mol/L of AgNO3fore^-, 0.01 mol/L of NaHCO3forh^þ, 0.01 mol/L of IPA for OH, and 40 ml/min of N₂ forO₂.Fig. 12 shows the degradation efficiency of TC by LSNMO010 in the trap- ping experiments. The degradation efficiency of TC decreased upon the addition of NaHCO₃and IPA. The extent of decrease in degra- dation efficiency of TC after the addition of NaHCO3was more than that after the addition of IPA. These results imply thath^þis thefirst efficient active species, while OH is the second efficient active species during the photodegradation of TC over LSNMO010. The addition of AgNO3accelerated the rate of degradation of TC. This was because AgNO3suppressed the recombination ofe^--h^þpairs and facilitated the production of moreh^þfor participating in the redox reaction [40]. After charging N2into the reaction system, it was believed that there were less O2and moreOVon the surface of the testing samples, resulting in longer lifetime of the photo- generated h^þ and acceleration of the degradation rate of TC.

Furthermore, after irradiating for 300 min, the degradation effi- ciencies of TC with AgNO₃ and N₂ were close to those without scavengers, suggesting that the active speciese^-andO2contribute little to the photocatalytic reaction [38].

3.6. Are the prepared samples photocatalysts or absorber?

To investigate whether the reduction in TC concentration had contribution from photodegradation or the absorption on the sur- face of catalysts, XPS of LSNMO010 was performed after the pho- todegradation of TC. XPS survey and N 1s high spectra of LSNMO010 after photodegradation of TC are presented inFig. 13. The peak of N 1s is located around 400.0 eV in the survey spectrum (Fig. 13(a)). N

1s high resolution spectrum can be fitted with two peaks (Fig. 13(b)). The peak at 400.7 eV may be attributed to N₂that was adsorbed on the surface of LSNMO010 [73], and the peak at 399.29 eV indicates the presence of TC (Fig. 13(b)). Peak area calculation reveals that the area ratio of N 1s of TC and the survey is about 0.032%. This demonstrates that little TC was absorbed on the surface of LSNMO010, and most of the TC was photodegraded by LSNMO010. That is to say, TC was predominantly degraded by photodegradation and not by adsorption over the catalysts.

3.7. Evaluation of mineralisation activity towards TC degradation

The mineralisation activity of the prepared photocatalysts is also worth investigating, in order to evaluate their photocatalytic degradation ability.Fig. 14shows that only 18.28% TOC is removed by direct photolysis. LSNMO010 displays the highest mineralisation activity (83.90%), which is 4.59 times and 1.52 times higher than those of pure direct photolysis and photocatalysis by pure LNMO in 240 min, respectively. These results indicate that Sr^2þdoping at La sites can improve the mineralisation activity of LNMO towards TC degradation. Thus, in the present work, La_2-xSr_xNiMnO₆ (x¼0, 0.05, 0.10, and 0.125) samples can effectively degrade TC by either photodegradation or thermocatalysis.

3.8. Possible mechanism of enhanced photocatalytic activity

In order to elucidate the possible mechanism of the enhanced photocatalytic activity, the parameters of the energy band struc- tures (Eg,EVB, andECB) andOVcontent were analysed comprehen- sively. Based on the above results, it can be concluded that increasing Sr^2þ contents decreasesE_g-s of the prepared samples, resulting in broad optical absorption from UV to NIR;EVB-s is raised to higher positive potential, because of which the photogenerated h^þpossesses stronger oxidation capacity to degrade TC. Owing to this,OH would also be formed during the photodegradation of TC [36]. LSNMO010 has the highest valance band edge; thus, it has the best photocatalytic performance. Increasing the Sr^2þcontent also increases theOVcontent, hence improving the separation ofeand h^þto prolong the lifetime of the carriers. Due to the above ad- vantages of doping Sr^2þat La sites, the photocatalytic activity of LNMO is enhanced.

The energy band structures of LSNMO010 and commercial NiO were also examined for elucidating the mechanism (Scheme 1).

Because of the narrow E_g, a large quantity e-h^þ pairs were photoexcited in LSNMO010 upon irradiation with simulated light.

Since the potential of photogeneratedh^þin LSNMO010 is far higher than the redox potential of OH/H₂O(2.72 eV) and OH/OH^. (2.38 eV),OH would be formed during the photodegradation of TC [36]. Consequently, h^þ and OH should be accounted for the effective degradation of TC both during photocatalysis and ther- mocatalysis. The photogeneratedewould be trapped byOVduring the photodegradation of TC, resulting in a longer lifetime of the photogeneratedh^þto degrade TC.

Furthermore, the complexation between TC and the prepared samples, especially LSNMO010, is for the effective photo- degradation of TC [68]. There might be other possible mechanisms for the effective degradation of TC by narrow-E_gphotocatalyst; they will be revealed in the future works.

4. Conclusions

In summary, La2-xSrxNiMnO6(x¼0, 0.05, 0.10, and 0.125) with narrowE_g-s, high-potentialE_VB, and abundantO_V-s was designed and successfully prepared via a solid-state reaction. Sr^2þdoping in the double perovskite La2NiMnO6 conferred several advantages, Fig. 11.EIS Nyquist plots of the prepared samples.

Fig. 12.Degradation efficiency of TC using LSNMO010 during trapping experiments.

thereby improving the effective degradation of TC. First, the addi- tion of Sr^2þreducedEgand resulted in broad optical absorption extending from UV to NIR. Second, the addition of Sr^2þinducedOV, which could trap photogeneratede, improve the separation ofe- h^þpairs, and transfer the photogenerated carriers, thus prolonging the lifetime of photogeneratedh^þ. The photogeneratedh^þandOH played a dominant role in photodegradation of TC. Third, the doped Sr^2þraised theE_VBof the prepared samples, resulting in the pho- togeneratedh^þpossessing stronger oxidation capacity to degrade TC. Fourth, the complexation between TC and Sr^2þin the prepared samples could be beneficial for the effective photodegradation of TC by LSNMO010. This work presents a class of photocatalysts, with narrowEg, high-potentialEVB, and abundantOV.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant No. 51672090, 51372092, and 51561031) and Technological Plan of Guangdong Province (2018A050506078).

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.07.233.

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Fig. 14.TOC removal ratio of TC (initial TC concentration, CI¼10 mg/L) during direct photolysis and photodegradation over the prepared samples under simulated sunlight irradiation.

Scheme 1.Energy band structures of LSNMO010 and NiO under simulated sunlight irradiation.