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Photocatalytic hydrogen evolution over theisostructural titanates: Ba3Li2Ti8O20 and Na2Ti6O13

modified with metal oxide nanoparticles

Luis F. Garay-Rodrı́guez a, Ali M. Huerta-Flores a,b,Leticia M. Torres-Martı́nez a,*, Edgar Moctezuma b

a Universidad Aut�onoma de Nuevo Le�on, UANL, Facultad de Ingenierı́a Civil, Departamento de Ecomateriales y

Energı́a, Av. Universidad S/N Ciudad Universitaria, San Nicol�as de los Garza, Nuevo Le�on, C.P. 66455, Mexicob Facultad de Ciencias Quı́micas, Universidad Aut�onoma de San Luis Potosı́, Av. Manuel Nava #6, San Luis Potosı́,

S.L.P 78290, Mexico

a r t i c l e i n f o

Article history:

Received 27 July 2017

Received in revised form

10 November 2017

Accepted 1 December 2017

Available online 23 December 2017

Keywords:

Tunnel structure

Titanates

Hydrogen evolution

Cocatalysts

* Corresponding author.E-mail address: lettorresg@yahoo.com (L.

https://doi.org/10.1016/j.ijhydene.2017.12.0040360-3199/© 2017 Hydrogen Energy Publicati

a b s t r a c t

Isostructural titanates exhibiting rectangular tunnel structure with the general formula

Na2Ti6O13 and Ba3Li2Ti8O20, were synthesized by solid state and sol-gel methods. The

structural, morphological, optical, textural and electrical properties of the materials were

characterized by XRD, SEM, UV-vis, BET and EIS. The photocatalytic activity for hydrogen

evolution of the materials was evaluated under UV light. Na2Ti6O13 exhibited higher ac-

tivity (120 mmol/gh) than Ba3Li2Ti8O20 (30 mmol/gh), and it was attributed to the distortion of

the octahedrons in Ba3Li2Ti8O20, which increases the recombination of charges in the

material. Additionally, the 1D dimension obtained in Na2Ti6O13 promotes a better charge

separation, transport and utilization in this phase. The activity of the materials was

enhanced with the incorporation of metal oxide nanoparticles MO (M ¼ Cu, Ni) as co-

catalysts. The activity of Ba3Li2Ti8O20 increased 10 times with the addition of CuO

(240 mmol/gh), while the activity of Na2Ti6O13 increased 3.5 times (416 mmol/gh). This

improvement in the photocatalytic activity of the isostructural titanates was attributed to

the formation of a p-n heterostructure between n-type titanates and p-type CuO, which

promoted an enhanced charge separation, transference and utilization.

© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction

Since the Fujishima and Honda study in 1972 [1,2], heteroge-

neous photocatalysis has attracted attention for different

novel applications. One of these includes its use for hydrogen

generation from water splitting. This process involves an

endothermic reaction where 2 electrons are transferred,

M. Torres-Martı́nez).

ons LLC. Published by Els

requiring 1.23 eV for every electron. The energy required for

this process can be achieved through heterogeneous photo-

catalysis [3]. For this reason, traditional and novel photo-

catalysts based on semiconductor materials have been

developed. Simple metal oxides, such as TiO2 [4e7], ZnO

[8e10], ZrO2 [11,12], WO3 [13,14], and other oxides belonging to

the families of perovskites [15e17], pyrochlores [18e20],

evier Ltd. All rights reserved.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 3 ( 2 0 1 8 ) 2 1 4 8e2 1 5 9 2149

ilmenites [21e23], tunnel structured materials [24e26], among

others, have exhibited suitable properties for the hydrogen

evolution process. The recombination of the photogenerated

charges in the bulk and the surface of thematerials represent a

problem that reduces significantly the photocatalytic activity.

In order to reduce the recombination of charges, several

strategies have been employed. This includes the develop-

ment of novel morphologies [2,24], the increase in the surface

area and the number of active sites for the catalytic reactions,

and the control of the conditions of synthesis for obtaining

suitable optical and electronic properties to perform the

reduction of water. Other strategy consist in the use of metal

oxides as cocatalysts (such as CuO, FeO, NiO, RuO2) [27e29], to

promote the efficient charge separation, transport and utili-

zation. Cocatalysts increase the lifetime of the

photogenerated charges and enhance the overall yield of the

photocatalytic reaction. Tunnel structured titanates have

attracted the interest of researchers in the photocatalytic field

due to their suitability to obtain 1D morphologies that pro-

motes a more efficient separation of the photogenerated

charges [30,31]. In the tunnel structuredmaterials, the heavily

distorted TiO6 octahedrons allows a higher efficiency of the

photoexcited charge separation process [32e34]. For this

reason, considerable number of research works about these

materials have been reported recently for photocatalytic ap-

plications [35e37].

Na2Ti6O13 is a material with tunnel structure that has been

synthesized for different methods and applied for photo-

catalytic hydrogen production and degradation of organic

compounds [38e40]. In recent reports, the activity of this

material has been improved with the addition of CuO [40] and

RuO2 [41] as cocatalysts. Another group of tunnel structured

titanates is the solid solution with general formula Ba2-x(Li2/

3þx-4yTi16/3þx/4þy)O13, with 0.32 � x � 0.42 and 0 � y � 0.2,

present in the system BaO-Li2O-TiO2. The isothermal phase

diagram of this system was studied by Suckut et-al. [42],

reporting the crystalline structure of the composition Ba2Li2/

3Ti16/3O13. This phase has been applied for catalytic combus-

tion of methane [43] and photocatalytic degradation of 2, 4-

dinitroaniline [44]. Ba2Li2/3Ti16/3O13 is an isostructural mate-

rial to Na2Ti6O13, and its conduction band is suitable to

perform the reduction of water. To this date, there are no re-

ports of the photocatalytic activity for hydrogen evolution

using this phase. For this reason, in thisworkwe prepared this

material by two different methods and evaluated the activity

for photocatalytic hydrogen generation. Also, we present a

comparative and integral study of the structural, optical,

textural and photocatalytic activity of Ba3Li2Ti8O20 and the

isostructural phase Na2Ti6O13, and the improvement in their

photocatalytic activity with the addition of copper and nickel

oxides as co-catalysts.

Experimental

Synthesis of Na2Ti6O13 and Ba3Li2Ti8O20 by solid statereaction and sol-gel method

For the solid state synthesis of Na2Ti6O13, Na2CO3 anhydrous

(99.9% DEQ) and TiO2 (Degussa P25) were mixed in equimolar

proportions according to the phase diagram [42] in an agate

mortar using acetone as dispersant. The same methodology

was used for the preparation of Ba3Li2Ti8O20, where BaCO3

(>99% Sigma Aldrich), LiCO3 (99.9% Fermont) and TiO2

(Degussa P25), where mixed in a relation 32-16-78 mol%

respectively, to obtain the previously reported composition

[44]. In order to reduce lithium loss by volatilization, tablets

were fabricated in a Carver hydraulic press at 6 metric tons.

The materials were transferred into a platinum crucible and

thermally treated in air at 1100 �C for 12 h for the Ba3Li2Ti8O20

and 800 �C for the Na2Ti6O13, using a heating rate of

3 �C min�1.

Sol-gel synthesis of Na2Ti6O13 was performed following the

methodology reported previously [35]. A stoichiometric

amount of titanium butoxide (97% Sigma Aldrich) was dis-

solved in ethanol anhydrous. Separately, sodium acetate

anhydrous (99.9% Fermont) was dissolved in water and

ethanol and added drop by drop into the flask containing ti-

tanium butoxide. Similarly, Ba3Li2Ti8O20 was prepared using

the methodology reported by Hernandez et. al in 2002 [44],

where barium acetate (99.8% Fermont) and titanium butoxide

were dissolved in ethanol and glacial acetic acid (Fermont). In

other vessel, lithium acetate (99.9 Sigma Aldrich) were dis-

solved in water and then added drop to drop to the first

solution.

Both flasks were refluxed at 70 �C during 72 h under con-

stant stirring and the resulting gel was dried until complete

evaporation of solvents. Finally, the obtained gels were ther-

mally treated at 900 �C to ensure the complete crystallization.

Deposit of MO (M ¼ Cu, Ni) as co-catalysts by wetimpregnation method

Metal oxide particles were deposited on Ba3Li2Ti8O20 and

Na2Ti6O13 by the method of wet impregnation [40]. Metallic

acetates (cupric acetate, Fermont, and nickel acetate, DEQ) in

different amounts (0.5e5% in weight) were dissolved in

ethanol. The respective mass fraction of the photocatalyst

was added and the suspension was kept under continuous

stirring for 1 h. After this time, the temperature was kept at

80 �C until complete evaporation. Finally, the samples were

thermally treated at 400 �C for 2 h to obtain the metal oxides.

Characterization

Phase formation of the materials was studied by X-ray

diffraction using a Bruker D8 Advance diffractometer with a

wavelength of l ¼ 1.5406�A of Cu-Ka radiation. Oxidation state

of MO particles impregnated was analyzed in a Perkin Elmer

Phi 560 XPS/Auger System doing the data treatment with the

AAnalyzer software. The morphological and elemental anal-

ysis of the samples was explored with a Scanning Electron

Microscope JEOL 6490LV in the secondary electron mode

under high vacuum at 20 kV coupled to an Energy Dispersive

X-ray Spectroscopy analyzer.

Specific surface area was measured by N2 physisorption

through the BET method using a Belsorp II mini (Bel Japan)

equipment and the optical properties were analyzed in a

range of 200e800 nm in a UV-vis NIR spectrophotometer Cary

5000 coupled to an integration sphere for diffuse reflectance.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 3 ( 2 0 1 8 ) 2 1 4 8e2 1 5 92150

Photoluminescence measurements were evaluated at

room temperature in a fluorescence spectrophotometer Agi-

lent Cary Eclipse, using a scan speed of 1000 nm/min and a

wavelength of excitation of 380 nm for Ba3Li2Ti8O20 and

350 nm for Na2Ti6O13.

Additionally, the electrochemical characterization of the

materials was performed in a potentiostat-galvanostat Met-

rohm Autolab with a three electrode quartz cell. Ag/AgCl was

used as reference electrode, platinum as counter electrode,

and the working electrode was prepared using a slurry of the

sample of interest (0.2 g), terpineol (0.05 g) and ethanol (2 mL).

The slurry was screen printed on an ITO substrate (1 cm2 of

active working area); and then, dried and calcined at 400 �C for

30 min. The three electrodes were immersed on an electrolyte

solution of 0.5 M Na2SO4 (pH 7) and bubbled with nitrogen for

removing the oxygen. Mott-Schottky curves were performed

under dark conditions through an impedance potential spec-

troscopy study at 1000 Hz.

Photocatalytic hydrogen evolution

Hydrogen evolution was tested using a 250 mL slurry Pyrex

reactor containing 200 mL of deionized water and 0.1 g of

catalyst. An UV pen-lamp (lmax ¼ 254 nm, Imax ¼ 2.2 mW/cm2)

was immersed in the suspension into a quartz tube.

Hydrogen productionwasmeasured in-situ taking samples

every 30 min in a Trace GC Ultra Thermo Scientific gas chro-

matograph equipped with a Thermal Conductivity Detector

(TCD), a fused silica capillary column (30 m � 0.53 mm) and

nitrogen (ultra high purity, Infra) as carrier gas.

Results and discussion

In this section, we present the results of the characterization

of the titanate phases synthesized by solid state and sol-gel

method pure and modified with the optimal loading (2%) of

CuO and NiO nanoparticles. For facility, in some of the Figures

the identification of the phases Ba3Li2Ti8O20 and Na2Ti6O13 is

presented as BLTO and NTO, respectively, and the solid state

and sol-gel methods as SS and SG.

X-ray diffraction

Fig. 1 (a) and (b) shows the XRD patterns of Ba3Li2Ti8O20 and

Na2Ti6O13 synthesized by solid state reaction and sol-gel

method. The materials exhibited crystalline patterns, with

well-defined peaks. Both materials crystallized in a mono-

clinic structure with space group C2/m, according to the JPDS

cards 049-0189 and 01-073-1398, respectively.

The crystal structure of Ba3Li2Ti8O20 is isostructural to

Na2Ti6O13 (Fig. 2). In this structure, three TiO6 octahedral form

edge-sharing trimers, which share corners with other trimers

to form infinite ribbons in a zig-zag arrangement. The Ba and

Na atoms are located inside the tunnels. Lithium partially

substitutes titanium in one of the three octahedral sites in the

structure [45,46]. The catalytic activity is promoted by this

tunnel structure, due to the octahedral distortion, which

produces dipole moments directed to the center of the tunnel,

enhancing the separation of the photogenerated charges.

In the Rietveld refinement of these materials previously

reported by Torres-Martı́nez et al. [35,46] the atomic fractional

coordinates and the sites for Na, Ti, O, Ba and Li are discussed.

In Na2Ti6O13, one of the oxygen atoms is refined in the site

2a, while in Ba3Li2Ti8O20, in the site 2b.

Additionally, differences in the cations nature and size

cause small variations in the bond lengths, the lattice pa-

rameters and the reflections observed in the patterns pre-

sented in Fig. 1 (a) and (b).

In Fig. 1 (a), it is observed that the patterns of the powders

of Ba3Li2Ti8O20 prepared by solid state and sol-gel are identical

and exhibit a strongest peak at 31.6 q, corresponding to the

plane (112). The crystallite size of Ba3Li2Ti8O20 samples was

calculated through the Scherrer equation. The values calcu-

latedwere 39.6 and 38.7 nm for thematerials obtained by solid

state and sol-gel. In both synthesis, the materials were ob-

tained at 1100 �C, promoting similar crystallite size.

The XRD patterns of Na2Ti6O13, presented in Fig. 1 (b), are

also similar, and the strongest peak is located at 11.8 q, which

corresponds to the plane (200). The crystallite sizes calculated

for Na2Ti6O13 obtained by solid state and sol-gel were 132 and

137 nm. The temperature used for the obtention of the pure

phase by the two methods was 800 �C. The differences in the

crystallite size calculated in the phases obtained by different

methods was no significant.

In Fig. 1 (a) and (b) it is also included the XRD patterns of

titanate phasesmodifiedwith 2% ofmetal oxide nanoparticles

(MO ¼ Ni, Cu). Due to the low concentration of the metal ox-

ides, no additional peaks corresponding to copper or nickel

oxide were observed in the XRD patterns of Na2Ti6O13 and

Ba3Li2Ti8O20. Further surface analysis of the phases by SEM,

EDS and XPS were performed in order to corroborate the

presence of themetal oxide nanoparticles in the surface of the

titanate photocatalysts.

Scanning electron microscopy

Fig. 3 shows the SEM images of pure andmodified Ba3Li2Ti8O20

and Na2Ti6O13 samples. Ba3Li2Ti8O20 exhibited grains of

irregular shape with particle size smaller than 1 mm, while

Na2Ti6O13 showed the formation of well-defined nanobelts

with a regular size of around 1 mm. This is a common feature

of Na2Ti6O13 powders and a variety of 1D structures have been

prepared from this material [32]. In both methods of synthe-

sis, the phases Ba3Li2Ti8O20 and Na2Ti6O13 exhibited similar

morphologies, due to the temperature needed for the phase

formation by the two methods was equal (Tformation

Ba3Li2Ti8O20 ¼ 1100 �C, and Tformation Na2Ti6O13 ¼ 800 �C).Fig. 3 also presents the morphology of the phases modified

with 2% of NiO and CuO. From these images, we assume the

NiO and CuO particles deposited on the surface of the iso-

structural titanates by the impregnation method are in the

nanometric scale. In some of the images, NiO and CuO

nanoparticles can be appreciated deposited on the surface of

the isostructural titanates.

In order to corroborate the presence of MO (M ¼ Cu and Ni)

particles on the surface of the photocatalysts, an elemental

analysis by EDS was carried out. Fig. 4 presents the distribu-

tion of copper and nickel on Ba3Li2Ti8O20 and Na2Ti6O13

modified with 2% of NiO and CuO. As can be seen in this

1 0 2 0 3 0 4 0 5 0 6 0 7 0

B a 3 L i2 T i8 O 2 0 S G 2 % N iO

B a 3 L i2 T i8 O 2 0 S S 2 % C u O

B a 3 L i2 T i8 O 2 0 S G

B a 3 L i2 T i8 O 2 0 S S

M o n o c lin ic

Inte

nsity

(a.u

.)

2 th e ta (d e g re e s )

B a 3 L i2 T i8 O 2 0J C P D S : 0 4 9 -0 1 8 9

(1 1

2)

10 20 30 40 50 60 70

Na2Ti6O13 SG 2% NiO(200

)

JCPDS: 01-073-1398

2 theta (degrees)

Base-centered monoclinic

Na2Ti6O13

Na2Ti6O13 SS

Inte

nsity

(a.u

.)

Na2Ti6O13 SG

Na2Ti6O13 SS 2% CuO

b)

a)

Fig. 1 e XRD patterns of the tunnel structured titanates: (a) Ba3Li2Ti8O20 and (b) Na2Ti6O13, and their modification with 2% of

metal oxide nanoparticles (MO ¼ Ni, Cu).

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 3 ( 2 0 1 8 ) 2 1 4 8e2 1 5 9 2151

Figure, the dispersion of NiO and CuOparticles on the surfaces

of the tunnel structured titanates is homogeneous. A quanti-

tative analysis of the content of the metal oxides deposited is

presented in Table 1. In this table, the weight percent of MO

Fig. 2 e Crystalline structure of (a)

deposited theoretically is also included. The obtained values

of metal oxides in the samples is slightly smaller to the

theoretical amount, due to weight losses during the impreg-

nation process.

Ba3Li2Ti8O20 and (b) Na2Ti6O13.

Fig. 3 e SEM images of pure and modified tunnel structured titanates: Ba3Li2Ti8O20 and Na2Ti6O13.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 3 ( 2 0 1 8 ) 2 1 4 8e2 1 5 92152

X-ray photoelectron spectroscopy

XPS analysis were carried out in order to obtain information of

the chemical species present on the surface of the samples

and to corroborate the deposition of the metal oxide nano-

particles over the tunnel structured titanates. Two represen-

tative samples were analyzed: Ba3Li2Ti8O20 SS 2% CuO and

Na2Ti6O13 SG 2% NiO.

Fig. 5 (a) and (c) shows the XPS survey of the samples,

where is confirmed the presence of Ba, Li, Ti, O and Cu in the

first sample, and Na, Ti, O and Ni in the second.

In Fig. 5 (b) is presented the Cu 2p spectra of the first

sample, where it was possible to identify two signals at 933

and 953 eV, corresponding to Cu 2p 3/2 and Cu 2p 1/2,

respectively. Two strong satellite peaks were observed at 943

and 963 eV. These signals are characteristic of Cu2þ and

confirmed the presence of CuO in the sample [47,48].

Fig. 5 (d) shows the Ni 2p spectrum of the second sample

with two main peaks at 855 eV and 856 eV. Also, a satellite

peak is observed at 862 eV. The signals were assigned to nickel

oxidized species (NiO) in the sample [49,50].

UV-vis diffuse reflectance

The diffuse reflectance spectra of Ba3Li2Ti8O20 and Na2Ti6O13

modified with NiO and CuO nanoparticles are presented in

Fig. 6. In these figures, the absorption range of the materials is

observed, and it begins at wavelengths below 350 nm. This

Fig. 4 e Element mapping of tunnel structured titanates modified with metal oxide nanoparticles: (a) Ba3Li2Ti8O20 SS 2%

CuO, (b) Ba3Li2Ti8O20 SG 2% NiO, (c) Na2Ti6O13 SS 2% CuO and (d) Na2Ti6O13SG 2% NiO.

Table 1 e Quantitative analysis by EDS of the metal oxidecontent deposited on the photocatalysts.

MO Theoreticalweight (%)

Weight determined byEDS (%)

Ba3Li2Ti8O20 Na2Ti6O13

CuO 0.5 0.4 0.3

1 0.9 1.1

2 2.0 1.9

5 6.1 4.7

NiO 0.5 0.9 0.4

1 0.9 0.8

2 1.3 1.8

5 4.5 4.9

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 3 ( 2 0 1 8 ) 2 1 4 8e2 1 5 9 2153

corresponds to an UV absorption, characteristic of the studied

isostructural titanates [40].

The deposition of the metal oxide nanoparticles on the

surface of titanates modified the absorption properties of the

materials. Transition metal oxides promoted an absorption in

the visible range due to the nature of their electronic structure

and their characteristic ded transitions. Due to this, the

modified titanates exhibited an increased absorption, re-

flected in smaller band gaps.

Table 2 summarizes the calculated band gaps. Band gap of

the materials were found in the range of 3.0e3.5 eV.

Physisorption analysis

To obtain the surface area of the photocatalysts, BET analysis

were performed. The results are presented in Table 2. All

materials exhibited small surface areas (lower than 10 m2/g).

The materials synthesized by sol-gel method exhibited

slightly higher surface area compared to the materials ob-

tained by solid state reaction, however, the difference was no

significant. After the process of impregnation with metal ox-

ides, no change was observed on this parameter. Based on

this, we assume that the pore structure of the photocatalysts

was not modified during the impregnation process.

Photoluminescence spectroscopy

Photoluminescence analysis were carried out in order to study

the efficiency of separation and transference of the photo-

generated charges, and to understand the effect of the metal

oxide nanoparticles deposited on the surface of the photo-

catalysts, associating the emission with the recombination of

electrons and holes. Fig. 7 shows the photoluminescence

spectra of pure andmodified (a) Ba3Li2Ti8O20 and (b) Na2Ti6O13.

In the emission spectras of the structured titanate materials,

Ba3Li2Ti8O20 synthesized by sol gel exhibited higher emission

intensity compared to the material obtained by solid state,

while in Na2Ti6O13, the opposite behavior was observed. The

highest photoluminescence emission was associated with a

highest recombination rate in the samples. In both titanates,

the photoluminescence emission was reduced with the

incorporation of metal oxide particles as cocatalysts. This

implies that the CuO and NiO nanoparticles are promoting a

better charge separation and transport in the interface of ti-

tanates with metal oxides. The lowest photoluminescence

emissions were observed in the samples modified with CuO,

due to the band alignment and p-type nature of this oxide,

which allows a suitable coupling, charge separation and

transference in the interface of the heterostructure with n-

type barium and sodium titanates. The formed n-p hetero-

structure improves the kinetic of the photogenerated charges

Fig. 5 e XPS spectra of (a, b) Ba3Li2Ti8O20 SS 2% CuO and (c, d) Na2Ti6O13 SG 2% NiO.

200 300 400 500 600 700 800

0.0

0.2

0.4

0.6

0.8

1.0

Abs

orba

nce

(a.u

.)

Wavelength (nm)

NTO SS NTO SS 2% CuO NTO SS 2% NiO NTO SG NTO SG 2% CuO NTO SG 2% NiO

200 300 400 500 600 700 8000.0

0.2

0.4

0.6

0.8

1.0

Abs

orba

nce

(a.u

.)

Wavelength (nm)

BLTO SS BLTO SG BLTO SS 2% CuO BLTO SS 2% NiO BLTO SG 2% NiO BLTO SG 2% NiO

)b()a(

Fig. 6 e UV-vis diffuse reflectance spectra of (a) Ba3Li2Ti8O20 and (b) Na2Ti6O13 modified with NiO and CuO nanoparticles.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 3 ( 2 0 1 8 ) 2 1 4 8e2 1 5 92154

and reduce the recombination process, enhancing the effi-

ciency in the use of the electrons and holes in the photo-

catalytic reactions.

Electrochemical characterization

As it is well known, the flat band potential influences the

photoresponse of a semiconductor. Amore negative character

of this parameter promotes a better charge separation in the

material. For this reason, in this work an electrochemical

impedance spectroscopy study of representative samples

(Ba3Li2Ti8O20 SS and Na2Ti6O13 SG pure and with 2% CuO) was

performed. These samples correspond to the materials that

exhibited the lowest photoluminescence emission in the

previous study. Fig. 8 shows the Mott-Schottky plots of these

bare and modified titanates. The flat band potential (EFB) was

calculated from these plots. The intercept at X of the plot of

C�2 versus E is the value of the flat band potential. Both tita-

nates exhibited positive slopes, characteristic of n-type

semiconductors. As it is shown in this Figure, the conduction

band of the titanates becomemore negative with the addition

of metal oxides. A more negative conduction band in a

Table 2 e Summary of the optical, textural andphotocatalytic properties of the tunnel structuredtitanates studied in this work.

Tunnelstructuredmaterial

Metaloxide

Surfacearea (m2/g)

Bandgap (eV)

H2 evolution(mmol/gh)

Ba3Li2Ti8O20 SS Bare <10 3.1 25

2% CuO 3.0 240

2% NiO 3.0 196

Ba3Li2Ti8O20 SG Bare 3.0 30

2% CuO 3.0 41

2% NiO 3.0 54

Na2Ti6O13 SS Bare 3.5 90

2% CuO 3.3 400

2% NiO 3.4 250

Na2Ti6O13 SG Bare 3.5 120

2% CuO 3.3 416

2% NiO 3.4 263

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 3 ( 2 0 1 8 ) 2 1 4 8e2 1 5 9 2155

material is more favorable for the hydrogen evolution. Addi-

tionally, the slope in the samples modified with metal oxides

increased, which implies that the charge carrier density is

higher in the modified samples compared to the bare

titanates.

The flat band potential calculated for Ba3Li2Ti8O20 SS 2%

CuOwas�0.5 V, while for Na2Ti6O13 SG 2% CuO,�0.6 V. These

results imply that the sample Na2Ti6O13 SG 2% CuO exhibits a

higher photoresponse and that the processes of charge sepa-

ration and transport in this sample are improved, which is

beneficial for its photocatalytic performance.

Photocatalytic activity for hydrogen evolution

The evaluation of the photocatalytic activity for hydrogen

evolution of pure and modified Ba3Li2Ti8O20 and Na2Ti6O13

under UV light is discussed in this section. Fig. 9 shows the

hydrogen production exhibited by the samples during 3 h. As

it is shown in these figures, all samples exhibited stable ac-

tivity under UV light. From these curves, the average hydrogen

production rate of the samples was calculated and summa-

rized in Table 2.

400 405 410 415 420 425 430 435

100

200

300

400

500

600

700

800

900

Inte

nsity

(a. u

.)

Wavelength (nm)

BLTO SS BLTO SS 2% NiO BLTO SS 2% CuO BLTO SG BLTO SG 2% NiO BLTO SG 2% CuO

(a)

Fig. 7 e Photoluminescence spectra of bare and mod

First, we evaluated the effect of the method of synthesis

(solid state and sol-gel) on the photocatalytic activity of the

tunnel structured titanates. In Ba3Li2Ti8O20, the samples pre-

pared by solid state and sol-gel exhibited similar activity:

25 mmol/gh and 30 mmol/gh, respectively. In Na2Ti6O13, the use

of the sol-gel method allowed the obtention of a higher ac-

tivity (120 mmol/gh) compared to the traditional solid state

route (90 mmol/gh). This difference in the activity could be

attributed to the soft conditions employed in the sol-gel syn-

thesis, which allows a controlled nucleation and growth of the

material. Due to this, the crystallite size of the sample ob-

tained by sol-gel (137 nm) was slightly higher to the obtained

by the sample prepared by solid state (132 nm). The higher

crystallinity obtained in the sol-gel sample promoted a higher

photocatalytic activity.

Ba3Li2Ti8O20 samples exhibited lower photocatalytic ac-

tivity compared to Na2Ti6O13. Though these materials are

isostructural, the bond lengths in Ba3Li2Ti8O20 [36] are higher

compared to the calculated in Na2Ti6O13 [37]. This generates a

higher distortion in Ba3Li2Ti8O20 octahedrons, promoting a

higher recombination of charges in this material and a lower

photocatalytic activity. Additionally, the powders obtained of

the phaseNa2Ti6O13 exhibited a characteristic 1Dmorphology,

in contrast to the grains observed in Ba3Li2Ti8O20. Materials

with morphologies in 1D exhibit an improved charge separa-

tion, transport and utilization, leading to higher photo-

catalytic efficiencies.

The incorporation of metal oxide nanoparticles on the

surface of the tunnel structured titanates improved the pho-

tocatalytic performance of the materials.

In both phases, the materials modified with CuO nano-

particles exhibited the highest activities. Ba3Li2Ti8O20 syn-

thesized by solid state exhibited an activity of 30 mmol/gh and

it increased 10 times with the addition of CuO (240 mmol/gh).

In the case of the phase Na2Ti6O13 prepared by sol-gel, the

pure material showed an activity of 120 mmol/gh, and it

increased 3.5 times with the incorporation of copper oxide

(416 mmol/gh). This is the sample with the highest photo-

catalytic efficiency, and corresponds to the sample with the

lower emission exhibited in the photoluminescence analysis

450 460 470 480 490 500 510 520 530 540 550

50

100

150

200

250

Inte

nsity

(a.u

.)

Wavelength (nm)

NTO SS NTO SS 2% NiO NTO SS 2% CuO NTO SG NTO SG 2% NiO NTO SG 2% CuO

(b)

ified (a) Ba3Li2Ti8O20 and (b) Na2Ti6O13 phases.

-1.0 -0.8 -0.6 -0.4 -0.2 0.0

0.0

0.2

0.4

0.6

0.8

1.0

Na2Ti6O13 SG Na2Ti6O13 SG 2% CuO

1/C

2 x 1

012 (C

2 cm

4 )

Potential (V versus Ag/AgCl)

(a)

-1.0 -0.8 -0.6 -0.4 -0.2 0.0

0.0

0.2

0.4

0.6

0.8

1.0 BLTO SS BLTO SS 2% CuO

1/C

2 x 1

012 (C

2 cm

4 )

Potential (V versus Ag/AgCl)

(b)

Fig. 8 e Mott-Schottky plots of (a) Na2Ti6O13 and (b) Ba3Li2Ti8O20.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 3 ( 2 0 1 8 ) 2 1 4 8e2 1 5 92156

(lower recombination of charges) and themore negative value

of the flat band potential.

The major photocatalytic activity of this sample is attrib-

uted to the formation of a n-p heterostructure between n-type

0 30 60 90 120 150 1800

200

400

600

800

1000

1200 BLTO SS NTO SS

H2

evol

utio

n ( µ

mol

/g)

Time (h)0 30 60 9

BLTO SS 2% NTO SS 2%

Ti

0 30 60 90 120 150 1800

200

400

600

800

1000

1200

1400

H2 ev

olut

ion

(µm

ol/g

)

BLTO SG NTO SG

Time (h)0 30 60 9

BLTO SG 2% NTO SG 2%

Ti

Time Time

Time Tim

Fig. 9 e Photocatalytic performance of the tunnel structured t

Na2Ti6O13 and p-type CuO, which allows an efficient separa-

tion and transference of the photogenerated charges in the

interface of the semiconductors, promoting an improved

photocatalytic activity. This mechanism is represented in

0 120 150 180

NiONiO

me (h)

0 30 60 90 120 150 180

BLTO SS 2% CuO NTO SS 2% CuO

Time (h)

0 120 150 180

NiO NiO

me (h)

0 30 60 90 120 150 180

BLTO SG 2% CuO NTO SG 2% CuO

Time (h)

Time

e Time

itanates synthesized by solid state and sol gel synthesis.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 3 ( 2 0 1 8 ) 2 1 4 8e2 1 5 9 2157

Fig. 10. Based on this, the irradiation of the tunnel structured

titanate with UV light, promotes the generation of electron

and hole pairs. The holes in the valence band of the titanate

are transferred to the valence band of CuO, where the water

oxidation reaction is performed. Meanwhile, in the conduc-

tion band of titanate, the electrons reduce protons to complete

the hydrogen evolution.

Representative samples were photocatalytic tested under

simulated solar radiation (see Fig. S2). In all cases, the activity

of thematerials decreased at least in a 60%. This correspond to

the expected, due to the low portion (4%) of UV light present in

the simulated solar radiation, and considering that the tita-

nate photocatalysts are active only under UV light.

For the best of our knowledge, the activities obtained by the

photocatalysts developed in this work are competitive with

similar materials reported in the literature.

Stability of the photocatalysts

To analyze the stability, tests of representative samples were

performed (See Fig. S3). As shown in this figure, the activity of

the materials remains stable after 3 cycles of reaction. This is

indicative that there is an appropriate coupling between the

titanate and the metal oxides, and that the materials are not

being corroded, reduced or lost in the solution. To corroborate

this, the as prepared and post-reaction samples were char-

acterized by X-ray diffraction, SEM and XPS.

a) XPS

Fig. S4 (a) shows the XPS spectra of Na2Ti6O13 SS modified

with 2% of CuO. Two signals were identified at 933 eV and

953 eV, corresponding to Cu 2p 3/2 and Cu 2p 1/2 (Cu2þ) sig-nals. In the Ba3Li2Ti8O20 SG 2% NiO sample, two peaks at

855 eV and 856 eV representative of Ni 2p 3/2 and Ni 2p 1/2

(Ni2þ) were observed. These peaks are similar to the observed

Fig. 10 e Schematic diagram of photocatalytic hydrogen evolutio

oxide nanoparticles.

in the pre-reaction samples. This indicates that CuO and NiO

particles deposited over titanates do not undergo corrosion or

reduction during the hydrogen evolution reactions. All of this

information gives support about the stability of the samples

and is similar with the previously reported in our group about

composites prepared by solvocombustion and impregnated

with CuO and NiO used for hydrogen evolution [40] where is

shown the presence of nickel and copper oxidized species (as

CuO and NiO) after the photocatalytic reaction.

b) X-ray diffraction

As it is shown in Fig. S5, XRD patterns of Ba3Li2Ti8O20 and

Na2Ti6O13 loaded with 2 w% of CuO exhibit similar reflections

than the observed in the original samples; indicating no

changes in the structure of the photocatalysts. Due to the low

amount of CuO and NiO, no peaks attributed to these phases

were identified in the diffractograms obtained of the samples

before and after the reaction.

c) SEM

Fig. S6 shows the SEM images of the samples after the

photocatalytic reaction. As can be observed in these images,

the characteristic morphology of the materials was kept

after the reaction, and the small particles of CuO and NiO

remain coupled to the surface of titanates. A quantitative

analysis through EDS was performed (Table S1), confirming

that the load of co-catalyst was not affected during the re-

action, and there were no losses of metal oxides into the

solution.

Finally, it is important to remark that the materials pre-

pared in this work only exhibited the evolution of hydrogen.

Even when they have appropriate potential of the valence

band to perform the oxidation of water, the experimental re-

sults did not show the evolution of oxygen. Theoretically, the

n over the tunnel structured titanates modified with metal

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 3 ( 2 0 1 8 ) 2 1 4 8e2 1 5 92158

splitting of water should generate stoichiometric amounts of

hydrogen and oxygen, however, the kinetics of the oxygen

evolution is slower and more difficult compared to the

hydrogen evolution, due to it needs the transference of 4

electrons, instead of 2, as it is in the case of hydrogen pro-

duction. Additionally, due to its solubility, it is possible that a

considerable amount of the evolved oxygen generated in the

reaction remains dissolved in the water. Also, oxygen can be

adsorbed in the surface of the photocatalyst. Due to the fac-

tors discussed, the detection of the photocatalytic oxygen

evolved is particularly difficult.

Conclusions

Ba3Li2Ti8O20 and Na2Ti6O13 isostructural phases were prepared

by solid state and sol-gel methods. The photocatalysts showed

stable activity under UV light for hydrogen evolution. Na2Ti6O13

exhibited higher activity (120 mmol/gh) compared to Ba3Li2-Ti8O20 (30 mmol/gh). The lower activity of Ba3Li2Ti8O20 was

attributed to the distortion in the octahedrons due to the larger

bond lengths in this phase, which increases the recombination

rate in the material. Also, the characteristic 1D morphology

obtained in the sodium hexatitanate phases played an impor-

tant role in the photocatalytic activity, enhancing the charge

separation of the charges through the 1D structure. Further, the

activity of the tunnel structured titanateswas improved 10 and

3.5 times in the case of Ba3Li2Ti8O20 andNa2Ti6O13, respectively,

with the incorporation of CuO nanoparticles as cocatalyst. The

enhancement in the photocatalytic activity of the studied tita-

nateswaspossiblewith the formationofann-pheterostructure

in the interface of n-type titanates and p-type CuO, promoting

an improved separation, transference and utilization of the

photogenerated charges in the photocatalytic reaction. In

summary, the photocatalysts developed in this work exhibited

competitive activities for hydrogen evolution compared to

similar phases reported in literature.

Acknowledgments

Authors would like to thank CONACYT (CB-2014-237049, CB-

256795-2016, INFRA-2015-252753, PN-2015-01-105, PN-2015-

01-487, NRF-2016-278729 and PhD Scholarship 386267 and

635249), SEP (PROFOCIE-2014-19-MSU0011T-1, PRODEP-103.5/

15/14156), UANL (PAICYT 2015) and FIC-UANL (PAIFIC 2015-5).

Appendix A. Supplementary data

Supplementary data related to this article can be found at

https://doi.org/10.1016/j.ijhydene.2017.12.004

r e f e r e n c e s

[1] Fujishima A, Honda K. Electrochemical photolysis of water ata semiconductor electrode. Nature 1972;238:37e8.

[2] Ho C, Li CV, Chan K. Scalable template-free synthesis ofNa2Ti3O7/Na2Ti6O13 nanorods with composition tunable forsynergistic performance in sodium-ion batteries. Ind EngChem Res 2016;55(38):10065e72.

[3] Wang Y, Li L, Zhang Y, Zhang N, Fang S, Li G. Crystalline-to-amorphous transformation of tantalum-containing oxidesfor a superior performance in unassisted photocatalyticwater splitting. Int J Hydrogen Energy 2017;42(33):21006e15.

[4] Ni M, Leung MKH, Leung DYC, Sumathy K. A review andrecent developments in photocatalytic water-splitting usingTiO2 for hydrogen production. Renew Sustain Energy Rev2007;11(3):401e25.

[5] Wang C, Cai X, Chen Y, Cheng Z, Luo X, Mo S, et al. Efficienthydrogen production from glycerol photoreforming overAg2O-TiO2 synthesized by a sol-gel method. Int J HydrogenEnergy 2017;42(27):17063e74.

[6] Xu Z, Fu H, Wen T, Zhang L, Zhang J. Improving thephotocatalytic H2 evolution activities of TiO2 by modulatingthe stabilizing ligands of the nanoscale Ti8-O8-clusterprecursors. Int J Hydrogen Energy 2017;42(39):24737e43.

[7] Han C, Yan L, Zhao W, Liu Z. TiO2/CeO2 core/shellheterojunction nanoarrays for highly efficientphotoelectrochemical water splitting. J Hydrogen Energy2017;42(17):12276e83.

[8] Kanade KG, Kale BB, Baeg JO, Lee SM, Lee CW, Moon SJ, et al.Self-assembled aligned Cu doped ZnO nanoparticles forphotocatalytic hydrogen production under visible lightirradiation. Mater Chem Phys 2007;102:98e104.

[9] Wang X, Liu G, Lu GQ, Cheng HM. Stable photocatalytichydrogen evolution from water over ZnO-CdS core-shellnanorods. Int J Hydrogen Energy 2010;35:8199e205.

[10] Martha S, Reddy KH, Parida KM. Fabrication of In2O3

modified ZnO for enhancing stability, optical behaviour,electronic properties and photocatalytic activity forhydrogen production under visible light. J Mater Chem A2014;2:3621e31.

[11] Kazuhiko M, Hiroaki T, Kentaro K, Masanobu H, Masashi T,Kazunari D. Surface modification of TaON with monoclinicZrO2 to produce a composite photocatalyst with enhancedhydrogen evolution activity under visible light. Bull ChemSoc Jpn 2008;81:927e37.

[12] Kokporka L, Onsuratoom S, Puangpetch T, Chavadej S. Sol-gel-synthesized mesoporous-assembled TiO2-ZrO2 mixedoxide nanocrystals and their photocatalytic sensitized H2

production activity under visible light irradiation. Mater SciSemicond Process 2013;16:667e78.

[13] Ho GW, Chua KJ, Siow DR. Metal loaded WO3 particles forcomparative studies of photocatalysis and electrolysis solarhydrogen production. Chem Eng J 2012;181:661e6.

[14] Mohamed AM, Shaban SA, El Sayed HA, Alanadouli BE,Allam NK. Morphology-photoactivity relationship: WO3

nanostructured films for solar hydrogen production. JHydrogen Energy 2016;41(2):866e72.

[15] Ouyang SX, Tong H, Umezawa N, Cao J, Li P, Bi YP, et al.Surface-alkalinization-induced enhancement ofphotocatalytic H2 evolution over SrTiO3-basedphotocatalysts. J Am Chem Soc 2012;134:1974e7.

[16] Lu L, Ni S, Liu G, Xu X. Structural dependence ofphotocatalytic hydrogen production over La/Cr co-dopedperovskite compound ATiO3 (A ¼ Ca, Sr and Ba). J HydrogenEnergy 2017;42(37):23539e47.

[17] Jana P, de la Pena O'Shea VA, Mata Montero C, Pizarro P,Coronado JM, Fresno F, et al. Factors influencing thephotocatalytic activity of alkali Nb-Ta perovskites forhydrogen production from aqueous methanol solutions. JHydrogen Energy 2016;41(44):19921e8.

[18] Zou ZG, Ye JH, Arakawa H. Photocatalytic water splitting intoH2 and/or O2 under UV and visible light irradiation with a

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 4 3 ( 2 0 1 8 ) 2 1 4 8e2 1 5 9 2159

semiconductor photocatalysts. Int J Hydrogen Energy2003;28:663e9.

[19] Ravi G, Palla S, Veldurthi NK, Reddy JR, Padmasri H, Vithal M.Solar water-splitting with the defect pyrochlore type ofoxides KFe0.33W1.67O6 and Sn0.5Fe0.33W1.67O6xH2O. J HydrogenEnergy 2014;39(28):15352e61.

[20] Ruiz-Gomez MA, Torres-Martinez LM, Figueroa-Torres MZ,Moctezuma E, Juarez-Ramirez I. Hydrogen evolution frompure water over a new advanced photocatalyst Sm2GaTaO7. JHydrogen Energy 2013;38(28):12554e61.

[21] Singh J, Uma S. Efficient photocatalytic degradation oforganic compounds by ilmenite AgSbO3 under visible and UVlight irradiation. J Phys Chem C 2009;113:12483e8.

[22] Han T, Chen Y, Tian G, Fu H. Hierarchical FeTiO3-TiO2 hollowspheres for efficient simulated sunlight-driven wateroxidation. Nanoscale 2015;7(38):15924e34.

[23] Ma Z, Wu K, Sun B, He C. Band engineering of AgSb1-xBixO3

for photocatalytic water oxidation under visible light. J MaterChem A 2015;3:8466e74.

[24] Pu YC, Che YC, Hsu YJ. Au-decorated NaxH2-xTi3O7 nanobeltsexhibiting remarkable photocatalytic properties undervisible-light illumination. Appl Catal B Environ2010;97:389e97.

[25] Najafpour MM, Allakhverdiev SI. Manganese compounds aswater oxidizing catalysts for hydrogen production via watersplitting: from manganese complexes to nano-sizedmanganese oxides. J Hydrogen Energy 2012;37(10):8753e8.

[26] Shangguan W. Hydrogen evolution from water splitting onnanocomposite photocatalysts. Sci Technol Adv Mater2007;8:76e81.

[27] Hisatomi T, Kubota J, Domen K. Recent advances insemiconductors for photocatalytic andphotoelectrochemical water splitting. Chem Soc Rev2014;43:7520e35.

[28] Ran J, Zhang J, Yu J, Jaroniec M, Qiao SZ. Earth-abundantcocatalysts for semiconductor-based photocatalytic watersplitting. Chem Soc Rev 2014;43:7787e812.

[29] Bai S, Yin W, Wang L, Li Z, Xiong Y. Surface and interfacedesign in cocatalysts for photocatalytic water splitting andCO2 reduction. RSC Adv 2016:57446e63.

[30] Kment S, Riboni F, Pausova S, Wang L, Han H, Hubicka Z,et al. Photoanodes based on TiO2 and a-Fe2O3 for solar watersplittingesuperior role of 1D nanoarchitectures and ofcombined heterostructures. Chem Soc Rev 2017;46:3716e69.

[31] Zhang Y, Jiang Z, Huang J, Lim LY, Li W, Deng J, et al. Titanateand titania nanostructured materials for environmental andenergy applications: a review. RSC Adv 2015;5:79479e510.

[32] Teshima K, Lee S, Murakoshi S, Suzuki S, Yubuta K,Shishido T, et al. Highly crystalline, idiomorphic Na2Ti6O13

whiskers grown from a NaCl flux at a relatively lowtemperature. Eur J Inorg Chem 2010;19:2936e40.

[33] Kohno M, Ogura S, Sato K, Inoue Y. Reduction and oxidationof BaTi4O9 with a pentagonal prism tunnel structure effectson radical formation upon UV irradiation and on the activityof RuO2/BaTi4O9 photocatalyst for water decomposition. JChem Soc Faraday Trans 1997;93:2433e7.

[34] Kohno M, Kaneko T, Ogura S, Sato K, Inoue Y. Dispersion ofruthenium oxide on barium titanates (Ba6Ti17O40, Ba4Ti13O30,

BaTi4O9 and Ba2Ti9O20) and photocatalytic activity for waterdecomposition. J Chem Soc Faraday Trans 1998;94:89e94.

[35] Torres-Martinez LM, Juarez-Ramirez I, Del Angel-Sanchez K,Garza-Tovar L, Cruz-Lopez A, Del Angel G. Rietveldrefinement of sol-gel Na2Ti6O13 and its photocatalyticperformance on the degradation of methylene blue. J Sol GelSci Technol 2008;47:158e64.

[36] Cao KZ, Jiao LF, Pang WK, Liu HQ, Zhou TF, Guo ZP, et al.Na2Ti6O13 nanorods with dominant large interlayer spacingexposed facet for high-performance Na-ion batteries. Small2016;12:2991e7.

[37] Tao YR, Zhang YL, Wen LL, Wu XC. Synthesis,characterization and gas-sensing properties of sodiumtitanate nanobelts. Chin J Inorg Chem 2008;24:1570e5.

[38] Inoue Y, Niiyama T, Sato K. Photocatalysts using hexa- andocta-titanates with different tunnel space for waterdecomposition. Top Cat 1994;1:137e44.

[39] Inoue Y, Kubokawa T, Sato K. Photocatalytic activity ofsodium hexatitanate, Na2Ti6O13, with a tunnel structure fordecomposition of water. J Chem Soc Chem Commun1990;0:1298e9.

[40] Huerta-Flores AM, Torres-Martı́nez LM, Moctezuma E.Overall photocatalytic water splitting on Na2ZrxTi6�xO13

(x ¼ 0, 1) nanobelts modified with metal oxide nanoparticlesas cocatalysts. Int J Hydrogen Energy 2017;42:14547e59.

[41] V�azquez-Cuchillo O, G�omez R, Cruz-L�opez A, Torres-Martı́nez LM, Zanella R, Alejandre Sandoval FJ, et al.Improving water splitting using RuO2-Zr/Na2Ti6O13 as aphotocatalyst. J Photochem Photobiol A 2013;266:6e11.

[42] Torres-Martinez LM, Suckut C, Jimenez R, West AR. Phaseformation and electrical properties in the system BaO-Li2O-TiO2. J Mater Chem 1994;4:5e8.

[43] Lopez T, Hernandez A, Bokhimi X, Torres-Martinez LM,Garcia A, Pecchi G. Sol-gel titania modified with Ba and Liatoms for catalytic combustion. J Mater Sci 2004;39:565e70.

[44] Hernandez A, Torres-Martinez LM, Sanchez-Mora E, Lopez T,Tzompantzi F. Photocatalytic properties of Ba3Li2Ti8O20 sol-gel. J Mater Chem 2002;12:2820e4.

[45] Dussarrat C, Howie RA, Mather GC, TorresMartinez LM,West AR. Crystal structure of Ba2Li2/3Ti16/3O13. J Mater Chem1997;7:2103e6.

[46] Hernandez A, Torres-Martinez LM, Lopez T. Preparation ofternary compound Ba3Li2Ti8O20 by the sol-gel process. MaterLett 2000;45:340e4.

[47] Huerta-Flores A, Torres-Martı́nez LM, Moctezuma E,Ceballos-Sanchez O. Enhanced photocatalytic activity forhydrogen evolution of SrZrO3 modified with earth abundantmetal oxides (MO, M ¼ Cu, Ni, Fe, Co). Fuel 2016;181:670e9.

[48] Akhavan O, Azimirad R, Safa S, Hasani E. CuO/Cu(OH)2hierarchical nanostructures as bactericidal photocatalysts. JMater Chem 2011;21:9634e40.

[49] Tomellini M. A comment on “Final states after Ni2pphotoemission in NiO”. J Electron Spectrosc Relat Phenom1992;58:75e8.

[50] Uhlenbrock S, Scharfschwerdt C, Neumann M, Illing G,Freund HJ. The influence of defects on the Ni 2p and O 1s XPSof NiO. J Phys Condens Matter 1992;4:7973e8.