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Cu2O precipitation-assisted with ultrasound andmicrowave radiation for photocatalytic hydrogenproduction

E. Lu�evano-Hip�olito a, L.M. Torres-Martınez b,*, D. S�anchez-Martınez b,M.R. Alfaro Cruz a

a CONACYT - Universidad Aut�onoma de Nuevo Le�on, Facultad de Ingenierıa Civil-Departamento de Ecomateriales y

Energıa, Cd. Universitaria, C.P. 66455, San Nicol�as de los Garza, NL, Mexicob Universidad Aut�onoma de Nuevo Le�on, Facultad de Ingenierıa Civil-Departamento de Ecomateriales y Energıa, Cd.

Universitaria, C.P. 66455, San Nicol�as de los Garza, NL, Mexico

a r t i c l e i n f o

Article history:

Received 2 December 2016

Received in revised form

24 March 2017

Accepted 27 March 2017

Available online 25 April 2017

Keywords:

Hydrogen production

Copper oxide

Microwaves

Sonochemical

Cu2O

Photocatalysis

* Corresponding author.E-mail addresses: lettorresg@yahoo.com,

http://dx.doi.org/10.1016/j.ijhydene.2017.03.10360-3199/© 2017 Hydrogen Energy Publicati

a b s t r a c t

Copper oxides are considered efficient photocatalysts for H2 generation. In addition, due to

their interesting properties such as surface plasmon resonance, they are applied in photo-

induced reactions. In heterogeneous photocatalysis, CuO and Cu2O are the main oxides

based in copper that are used as catalysts in water splitting. In this work, Cu2O is prepared

by precipitation method assisted with ultrasound and microwave radiation at 80 �C. For the

Cu2O synthesis, the use of glucose is proposed as a reducing agent due to its abundance in

nature, non-toxicity, and low cost. According to the results obtained, the highest glucose

concentration and the suspension exposure to microwave irradiation promote the for-

mation of Cu2O particles with low and homogeneous particle size and a convenient posi-

tion of their conduction and valence band to produce H2. The highest H2 generation using

Cu2O under the aforementioned experimental conditions is 78 mmol gcat�1 . Additionally, the

effect of adding glucose in the photocatalytic reaction is studied in order to provide more

electrons to the reaction due to its effect as a hole scavenger, which inhibits the recom-

bination of the electron and hole, promoting a higher H2 production (400 mmol gcat�1 ).

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

Introduction

Recently, the pollution levels in the environment have been

increasing at alarming rates, somuch so that it is necessary to

act and propose alternative technologies to mitigate pollution

levels. The use of hydrogen as fuel is one of the best alterna-

tives that have been proposed to replace fossil fuels. There are

leticia.torresgr@uanl.edu92ons LLC. Published by Els

several advantages from the use of hydrogen as a fuel, for

example; it is a potential emission-free fuel and it can be

produced using renewable energy which can reverse the high

pollution levels recorded daily. Among the options for H2

production, photocatalytic water splitting is an attractive re-

action due to its low-cost, clean, and sustainable process that

requires solar light and a semiconductor with a conduction

band edge more negative than the required potential for

.mx (L.M. Torres-Martınez).

evier Ltd. All rights reserved.

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 2 ( 2 0 1 7 ) 1 2 9 9 7e1 3 0 1 012998

Hþ / H2. When a semiconductor material is irradiated with

energy equal or higher than its band gap, electrons and holes

are generated in the conduction and valence bands, respec-

tively. Photogenerated electrons can reduce hydrogen ions to

form hydrogen and photogenerated holes participate in oxy-

gen generation [1,2]. Several semiconductor compounds have

been proposed as photocatalysts in order to produce hydrogen

fromwater splitting. The photocatalysts proposed in this area

are classified into four groups: 1. d0 metal (Ti4þ, Zr4þ, Nb5þ,Ta5þ, W6þ, and Mo6þ) oxide, 2. d10 metal (Cuþ1, In3þ, Ga3þ,Ge4þ, Sn4þ, and Sb5þ) oxide, 3. f0 metal (Ce4þ) oxide, and 4. An

additional small group of non-oxide photocatalyst (ZnS, CdSe,

GaN, G3N4, AgBr, C3N4) has been proposed [3]. Some materials

can be used as co-catalysts (Cu, Ag, Pt, Pd, RuO2, Ni, etc) to

avoid fast recombination between photogenerated charges,

since these materials can act as electrons and hole traps [1].

Copper oxide (d10 metal) represents an alternative photo-

catalyst which can be activated with visible light to be used in

the photo-induced process [4]. Copper (Cu) is one of the most

abundant metals on Earth, it is cheap, and it has high elec-

trical conductivity. In addition, copper exhibits plasmonic

properties related to an enhancement in the photocatalytic

activity due to the Schottky junction and its surface plasmon

resonance (SPR) [5]. It is reported that this effect forces the

electrons and holes moving in different directions to mini-

mize their recombination.

In heterogeneous photocatalysis, CuO and Cu2O are the

main oxides based in copper that have been used as a catalyst

in water splitting [6,7]. CuO (black) and Cu2O (reddish) oxides

are p-type semiconductors with direct band gaps (<2 eV) [8]. In

addition, the potential of the conduction band (CB) of Cu2O is

negative enough, to carry out the reduction of Hþ to H2. On the

other hand, Montini and collaborators have made an exten-

sive study in the application of CuxOy (x, y ¼ 0, 1, 2) thin films,

specially as CuO photocatalysts for H2 production [9,10].

Particularly, they have found a direct correlation between the

morphology and a possible upward shift in the conduction

band of CuO for photocatalytic H2 production [9], and with the

exposition of the crystallographic plane (�111) of CuO [10],

which confirms the possibility for the application of CuO

particles.

In literature, there are several reports of Cu2O obtained by

different synthesis methods as powders and thin films, which

are summarized in Table 1.

In Cu2O synthesis it is necessary to add a reducing agent,

which generally is an organic molecule to reduce the pre-

cursor of copper (Cu2þ / Cu1þ) [11e36]. The most common

reducing agents are ascorbic acid (C6H8O6), ethylene glycol,

and sodium tartrate (Na2C4H4O6). However, the latter com-

pound has a strong chelating effect between copper and

tartrate ions, which limits the diffusion of the copper to the

reaction medium. In addition, the use of other molecules as a

reducing agent has been proposed, such as TMEDA, hy-

droxylamine hydrochloride, related to high costs and a long

procedure to remove them from the final product. As an

alternative of these compounds, glucose (C6H12O6) is an

excellent reducing agent, which has numerous advantages;

for example, it is ecofriendly, abundant as biomass residue,

and is related to low costs [38,39]. Regarding the synthesis

method, there are several reports which proposed the use of

a sonochemical method to produce homogeneous particles

with a narrow distribution size for photocatalytic, catalytic,

and adsorptive applications. Also, the use of microwaves to

assist precipitation is of high interest due to the rapid

transfer of energy, heating homogeneity, rapid phase for-

mation and a small particle size being able to obtain mate-

rials with unique properties [40,41]. The use of microwaves

has been recently employed in the synthesis of Cu2O for its

application as a photocatalyst and gas sensor [28,29]. How-

ever, the synthesis of Cu2O by microwave and its application

as a photocatalyst for H2 generation has not been reported so

far. Regarding the sonochemical synthesis of Cu2O, there are

two reports in which the material has been employed as a

photocatalyst and photoelectrocatalyst for H2 production

[11,25].

In the paper herein, the preparation of Cu2O by precipita-

tion assisted by ultrasound and microwave methods is pro-

posed to promote homogeneous heating and the formation of

particles with both a low and closed distribution size. Cu2O

samples were tested as catalyst in the photocatalytic

hydrogen production.

Experimental

Synthesis of Cu2O

Cu2O was prepared by precipitation assisted with ultrasound

and microwave radiation. This method consists in the prep-

aration of two solutions. In the first one, 0.003 mol of copper

acetate (Cu(CO2CH3)2) (98% Aldrich) was dissolved in 40 mL of

deionizedwater at 50 �C. A second solution of 0.6mol of NaOH

(99% Fermont) was prepared and added to the copper solution

with vigorous stirring at 80 �C. The resulting brown suspen-

sion was maintained at 80 �C for 2 h and then different

amounts of glucose (C6H6O12) (99% Aldrich) were added to

obtain amolar ratio of 1:0, 1:0.5, 1:1, and 1:1.5 regarding copper

acetate. The resulting mixture was stirring vigorously for 2 h

and, after this time, we obtained a red solution. On the other

hand, this mixture was exposed to different energy sources

(ultrasound and microwave) to study their effect on the

physical properties developed by Cu2O. The mixture was

exposed to ultrasound radiation employing a cavitation field

generated by a 150 W Hielscher's UP200Ht ultrasonic proces-

sor for 1 h. The temperature at the end of the ultrasound

treatment was 80 �C ± 5 �C. Alternatively, the mixture was

exposed to microwave radiation, using a 150 W MARS-6 pro-

grammable microwave and a temperature of 80 �C for the

same time (1 h). The resulting mixtures obtained were

centrifuged and washed with deionized water and ethanol in

order to remove the by-products generated during the syn-

thesis process. Finally, the powders were dried at 80 �C. Fig. 1shows a scheme of the Cu2O synthesis by precipitation

assisted by different energy sources. The samples obtained

will be hereinafter referred to as Cu-x/y, where x is the molar

relation of copper acetate and glucose and y refers to the

synthesis method P ¼ precipitation, US ¼ ultrasound, and

MW ¼ microwave.

Table 1 e Summary of the reported synthesis and applications for Cu2O particles.

Synthesis method Precursors Morphology Studied reaction Ref.

Precipitation

and hydrothermal

1. Copper sulfate.

2. Copper acetate

3. Sodium tartrate

4. Pyrrole

5. Polyvinyl

pyrrolidone (PVP)

6. Hydrazine

7. Ethanediamine

8. NaOH

Branched Wires

and spheres.

Photocatalytic H2

production

(263.8 mmol/h)

[11]

Precipitation 1. Copper chloride

2. PVP

3. Ascorbic acid

Spheres H2 production from

butyric acid degradation

(30 mL)

[12]

Precipitation 1. Copper sulfate

2. Sodium hydroxide

3. Glucose

Irregular shape Photocatalytic H2

production

(2250 mmol/g) with

20% ethanol

[13]

Precipitation 1. Copper nitrate

2. Ethanol

Flakes Photocatalytic H2

production

(16,000 mmol/g) in glycol

[14]

Precipitation 1. Copper acetate.

2. NaOH

Irregular spheres Photocatalytic H2

production

(12,000 mmol/g) in Na2S,

NaSO3 and CdS

[15]

Precipitation 1.Copper acetate

2. TMEDA1

3. Ascorbic acid

4. Polyethylenglycol (PEG)

Spheres and microcubes e [16]

Precipitation 1. Copper sulfate

2. Glucose

3. Sodium hydroxide

Cubic, spheres, and

octahedral

e [17]

Hydrothermal 1.Copper chloride

2. PVP

3.Potassium carbonate

4. Trisodium citrate

Shuriken Non enzymatic

biosensors

[18]

Hydrothermal 1. Copper nitrate

2. Ethanol

3. Formic acid

Cuboid and octahedral CO2 photoreduction [19]

Sonochemical 1.Copper acetate

2. Hydroxylamine

hydrochloride.

Spheres Adsorption of methyl

orange

[20]

Sonochemical 1. Copper nitrate

2. NaNO3

3. Ethanol

4. Propanol

Sheets Photocatalytic

degradation and

adsorption of methyl

orange (MO).

[21]

Sonochemical 1.Copper acetate

2. Ascorbic acid

3. Sodium dodecyl

sulfate (SDS).

Hollow spheres Photocatalytic

degradation of MO.

[22]

Sonochemical 1.Copper acetate

2. SDS

3. Octanol

Urchins Photocatalytic

degradation of MO.

[23]

Sonochemical 1. Copper acetate

2. PVP

3. Ascorbic acid

Chrysanthemum Photocatalytic

degradation of phenol.

[24]

Sonochemical 1. Copper chloride

2. SDS

Cubes and rods Photoelectrochemical

H2 production

[25]

Sonochemical 1. Copper acetate

2. Glycerol

Spheres Catalytic activity in N-

arylation of imidazole

[26]

Sonochemical 1. Copper sulfate.

2. Sodium hydroxide.

3. Hydroxylamine

hydrochloride.

4. PVP

Cubes Photocatalytic

degradation of MO

[27]

(continued on next page)

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 2 ( 2 0 1 7 ) 1 2 9 9 7e1 3 0 1 0 12999

Table 1 e (continued )

Synthesis method Precursors Morphology Studied reaction Ref.

Microwave 1. Copper nitrate

2. PVP

3. Ethylenglycol (EG)

Etched cubes Photocatalytic

degradation of MO

[28]

Microwave 1. Copper acetate

2. Glucose

Quasi spherical Gas sensor [29]

CVD films 1. Cu(hfa)2 Faceted agglomerates

and wires

Photocatalytic H2

production (40 l h�1 m�2)

in methanol

[9]

Sputtering films 1. Copper target Rods Photocatalytic H2

production

(3 mmol cm�2) in

methanol

[10]

Sputtering and

Electrodeposition films

1. Copper

2. FTO substrates

3. Copper sulphate

4. Lactic acid

5. Potassium hydroxide

Rods Photoelectrochemical

H2 production

[30]

Electrodeposition films 1. Copper sulfate

2. Lactic acid

3. Sodium hydroxide

Irregular Photoelectrochemical

H2 production

[31]

Electrodeposition films 1. Copper nitrate

2. Ethanol

3. TiO2

Spheres Photocatalytic H2

production (75 mmol) in

Na2S and Na2SO3

[32]

Electrodeposition films 1. Copper sulfate

2. Lactic acid

Cubic Photoelectrochemical

H2 production

(17% efficiency)

[33]

Chemical bath films 1. Copper

2. Sodium hydroxide

3. Ammonium persulfate

Wires Photoelectrochemical

H2 production

[34]

Spray pyrolysis films 1. Copper acetate

2. Glucose

3. 2-Propanol

Irregular Photoelectrochemical

H2 production

(2.44 mA cm�2)

[35]

Impregnation Commercial Irregular and wires Photocatalytic

production of H2

(160 mmol in 6 h)

[36]

Anodizing 1. Copper mesh

2. Sodium hydroxide

e Photoelectrochemical

H2 production

(86.8 mmol cm�2)

[37]

Microwave and

sonochemical

1. Copper sulfate

2. Glucose

3. Sodium hydroxide

Flakes, octahedral

and seed particles

Photocatalytic

production of H2

(78 mmol/g and with

400 mmol/g glucose

in 3 h)

This work

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 2 ( 2 0 1 7 ) 1 2 9 9 7e1 3 0 1 013000

Characterization

The structural characterization was carried out by X-ray

powder diffraction using a Bruker D8 Advance diffractometer

with Cu Ka radiation (40 kV, 30 mA, l ¼ 1.5418 �A) equipped

with a high-speed Vantec detector. A typical run was made

with a scan rate of 0.05� for 0.5 s. The crystallite size was

calculated by Debye's-Scherrer equation (1):

D ¼ Klbhklcosq

(1)

where D is the crystallite size, K is the shape factor (0.94), l is

the wavelength of Cuka radiation, and bhkl is the instrument

broadening, which was calculated using equation (2). In this

case, we used the Al2O3 corundum as reference.

bhkl ¼hðbhklÞ2measured � ðbhklÞ2instrumental

i1=2(2)

In addition, the strain due to crystal imperfections and

distortions was estimated using equation (3) [42].

ε ¼ bhkl

4 tan q(3)

The morphology of the samples was analyzed by scanning

electron microscopy using a JEOL 6490 LV. The optical prop-

erties of the samples were analyzed between 200 and 1500 nm

using a UVeVis NIR (Cary 5000) spectrophotometer coupled

with an integration sphere for diffuse reflectance measure-

ments. The band gap energy (Eg) was calculated using the

KubelkaeMunk function taking a direct charge transfer into

consideration in Cu2O and CuO samples. The energy band

positions of the samples were studied in order to know the

thermodynamic feasibility to produce H2 and they were

calculated by equations (4) and (5) [43].

Fig. 1 e Experimental procedure to prepare Cu2O powders.

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 2 ( 2 0 1 7 ) 1 2 9 9 7e1 3 0 1 0 13001

EVB ¼ X� Ee þ 0:5Eg (4)

ECB ¼ X� Ee � 0:5Eg (5)

where X is the absolute electronegativity, Ee is the energy of

free electrons on a hydrogen scale (4.5 eV), and Eg is the energy

band gap.

The surface study was determined using X-ray photoelec-

tron spectroscopy (XPS) with a monochromated Al Ka

(1486.7 eV) and an X-ray source with a 0.20 eV line width in a

dedicated analysis chamber at a base pressure of <4.3 � 10-

10 mbar. The photoelectrons were separated with a semi-

hemispherical analyzer with a pass energy of 20 eV (Thermo

scientific, Escalab 250 xi, Al anode, 1486.68 eV).

Photocatalytic activity

The photocatalytic reactions were carried out in a 250 mL

cylindrical Pyrex batch reactor at 25 �C. The photocatalyst'smass used was 0.1 g and the volume of deionized water used

was 200mL. The suspensionwas bubbledwithN2 for 15min in

order to remove the dissolve oxygen from the reaction me-

dium. Once the oxygen was removed from the suspension, it

was irradiated in the center with a 254 nm UV Pen Ray Lamp

with 4400 mW cm�2 of irradiance The hydrogen produced was

monitored every 30 min using a gas Shimadzu GC-2014

chromatograph equipped with a thermal conductivity detec-

tor (TCD).

Additional photocatalytic experiments were performed

adding glucose as a sacrificial agent in order to increase H2

production. For this purpose, different amounts of glucose

were added to the photocatalyst þ water suspension until

the formation of the following glucose concentrations: 0.01,

0.03, 0.05, 0.10, and 0.15 M. In these cases, the reaction time

was maintained constant as previous experiments, which

was 3 h.

Results and discussion

Synthesis and characterization of Cu2O

Cu2O synthesis involves a consecutive color changes from

blue to dark brown and finally a reddish color. The first step in

the synthesis is the dissolution of blue color copper acetate in

deionizedwater. After that, the formation of copper hydroxide

[Cu(OH)2] is promoted when NaOH is added into the reaction

medium (equation (6)), which is accompanied of a color

change from blue to dark brown. Then, glucose is added into

the medium to reduce Cu(OH)2 to Cu2O (equation (7)) and a

color change from brown to a reddish color. The reduction is

related to the loss of an electron of the glucosemolecule due to

its oxidation into gluconic acid [44]. During the washing of the

precipitate, sub-products are removed and pure Cu2O is ob-

tained. On the other hand, the time required for Cu2O for-

mation using an external energy source (MW or US) is 1 h,

which is lower than the required time in the conventional

precipitation (3 h).

CuðCH3COOÞ2 þ 2NaOH/CuðOHÞ2 þ 2NaCH3COO (6)

2CuðOHÞ2 þ CH2OHðCHOHÞ4CHO/Cu2OY

þ CH2OHðCHOHÞ4COOHþ 2H2O (7)

X-ray diffraction

The X-ray diffraction pattern of the Cu-1:0/y samples obtained

in absence of glucose are shown in Fig. 2, where only the

presence of CuO oxide was observed. All the CuO samples

exhibited the X-ray diffraction pattern of the CuO monoclinic

structure according to JCPDS card No. 05-0661 (Fig. 2). The

crystallinity of CuO samples increased in the following order:

Cu-1:0/P > Cu-1:0-US > Cu-1:0-MW,which suggest the effect of

microwave radiation in the efficient energy absorption of the

reaction medium to induce the formation of CuO. Addition-

ally, some differences in the ratio of the intensities of the

planes (111) and (002) were observed, suggesting a preferential

growth of the (111) plane when the precipitation was assisted

with an external energy source (see Onset in Fig. 2).

X-ray diffraction patterns of the samples obtained by

adding different amounts of glucose are shown in Fig. 3. The

addition of glucose in the reaction medium promotes a

change in the crystal structure of copper oxide from mono-

clinic to cubic structure according to JCPDS card 05-0667 in all

the range of glucose concentrations studied in this work. The

diffractograms of the samples prepared under equimolar

amounts of copper precursor and glucose (Cu-1:1/y) show an

additional reflection, which corresponds with elemental

copper (Cu0) (JCPDS card No. 70-3038). However, at higher

concentrations of glucose, only Cu2O was obtained. This fact

can be related to the amounts of electrons available in the

reaction medium to participate in the reduction of Cuþ2 to

Cuþ, and Cu0. In this sense, the standard half-cell potential

for the redox equilibrium of glucose is given in equation (8)

[45].

C6H12O7 þ 2Hþ þ 2e�4C6H12O6 þH2O E0 ¼ þ0:05 V (8)

Fig. 2 e XRD patterns of CuO samples prepared (Cu-1:0/y).

Fig. 3 e XRD patterns of Cu2O samples prepared (Cu-x/y).

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 2 ( 2 0 1 7 ) 1 2 9 9 7e1 3 0 1 013002

As can be seen in equation (8), the oxidation of glucose

releases two electrons per molecule, which confirms that,

whenever the concentration of glucose in the medium was

half the concentration of copper (Cu-1:0.5/y), Cu(OH)2 was

effectively reduced into Cu2O as the process only need one

electron to reduce Cu2þ into Cuþ. On the other hand, when an

equimolar relation was used (Cu-1:1/y), twice as many elec-

tronswere available to participate in the reduction of Cu(OH)2,

which can further reduce Cuþ1 into Cu0 as it was shown in

Fig. 3. However, at higher glucose concentrations, only Cu2O

appears. This fact may be related to Le Chatelier principle,

which states that the system readjusts itself to counteract the

initial effect. Thus, the effect of reducing Cuþ1 / Cu0 is

minimized. Therefore, the electrons released from glucose

oxidation tend to be captured by other available species in the

medium and only Cu2O is formed.

An additional analysis of the X-ray diffraction patterns

shows that the ratio of the intensities related to the Cu2O

planes (111) and (200) increasedwhen its preparation included

heating with microwaves, which can be related to the high

dipole moment of glucose (8.6) in comparison to the corre-

sponding value of H2O (3.8) [46]. Due to its high dipole

moment, the suspension attempts to align itself with the field

of radiation and, once it's aligned with the field, its direction

reverses and the molecules of the suspensions tend to realign

[47]. This aligning and realigning of the Glucose and Cu(OH)2molecules produces heating through friction, which provides

the energy required for Cu2O formation. In addition, the syn-

thesis of Cu-x/MW involves the use of a closed vessel which

causes the generation of positive pressure, which increases

the solubility and growth of the crystals formed. The growth is

reflected in an increase of the crystallinity of the Cu2O-x/MW

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 2 ( 2 0 1 7 ) 1 2 9 9 7e1 3 0 1 0 13003

samples, specifically along plane (111). In particular, plane

(111) in the cubic structure of Cu2O has the lowest surface

energy related to a low number of dangling bonds (or defects)

on Cu2O surface.

Regarding the crystallite size, Cu2O samples did not show

any clear tendency related to the growth of their crystals (see

Table 2). In general, higher amounts of glucose in the reaction

medium promote lower crystallite sizes when Cu2O was

synthesized under microwave radiation, whose values

decrease from 201 to 40 nm for Cu-1:0.5/MW and Cu-1:1.5/MW

samples, respectively.

On the other hand, lattice strain (%) related to crystals

imperfections was calculated and it is shown in Table 2. The

tendency was to develop a lower lattice strain when the re-

action medium was exposed to microwave irradiation, which

is consistent with the increase in crystallinity considering the

intensity of the plane (111).

Scanning electron microscopy

The morphology and particle size of the Cu2O particles were

analyzed by SEM (Fig. 4). CuO (Cu-1:0/P) reference sample has

a flake-like morphology with an average particle size of

220 nm. When this sample was exposed to ultrasound, the

flake-like particles grew in the direction of plane (111) and

decreased their average length to 180 nm. When the mixture

was exposed to microwave radiation (Cu-1:0/MW),

morphology changed from flakes to seeds that grow along

plane (111) with an average size of 320 nm.

Cu2O samples preparedwith the lowest amount of glucose,

Cu-1:0.5/P, without any external treatment, developed an

octahedral particle morphology with an average size of

780 nm, and some particles forming four-point stars were

observed. The morphology evolution of Cu-1:0.5/y samples

when US was applied in order to assist the precipitation

resulted in flakes with a wide range of sizes, from 50 to

200 nm. In comparison, when the same suspension was

exposed to MW radiation, the resulting morphology was

similar to the sample without any treatment, with a homo-

geneous and bigger average particle size (1 mm).

The sample prepared under an equimolar relation of cop-

per and glucose (Cu-1:1/P) shows heterogeneity in the distri-

bution of their particles with small particles on the surface of

Table 2 e Physical properties of the Cu-x/y samples.

Sample Crystal phase(s) Crystallitesize (nm)

Latticestrain (%

Cu-1:0/P CuO 57 0.109

Cu-1:0/US CuO 41 0.139

Cu-1:0/MW CuO 38 0.140

Cu-1:0.5/P Cu2O 403 0.085

Cu-1:0.5/US Cu2O 80 0.075

Cu-1:0.5/MW Cu2O 201 0.030

Cu-1:1/P Cu2O þ Cu 33 0.180

Cu-1:1/US Cu2O þ Cu 57 0.105

Cu-1:1/MW Cu2O þ Cu 67 0.030

Cu-1:1.5/P Cu2O 56 0.378

Cu-1:1.5/US Cu2O 50 0.221

Cu-1:1.5/MW Cu2O 40 0.151

big 10 mm agglomerates. In the case of sample Cu-1:1/US, the

agglomerates tend to brake and separate, however their size is

still high (~5 mm) with some small (220 nm) particles on their

surface. In comparison, sample Cu-1:1/MW has seeds with an

average length of 200 nm. On the other hand, an increase in

the amount of glucose during the synthesis of Cu2O causes the

formation of spheres of different sizes depending on the

synthesis method. In these samples, the reference (Cu-1:1.5/P)

has the highest spheres diameter with 720 nm, which is

double the diameter when the precipitation was assisted with

ultrasound. In this case, the ultrasound caused the dispersion

of the spheres. Sample Cu.1:1.5/MW has spheres with an

average diameter of 800 nm, but has some small particles

(150 nm) deposited in their surface. The morphology of these

small particles was that of flakes.

Fig. 5 shows the formation of CuO and Cu2O with the

addition of different amounts of glucose.

UVeVis diffuse reflectance spectroscopy

The optical properties of the Cu2O samples were analyzed by

UVeVis spectroscopy. The band gap values (Eg) of copper

oxide samples were estimated using the KubelkaeMunk

remission function taking a direct transition for both CuO and

Cu2O oxides into consideration. The band gap values corre-

spondingwith the three CuO sampleswere around 1.4e1.7 eV,

which agree with the reported value in scientific literature

[48]. On the other hand, the band gap values for Cu2O samples

are shown in Table 2. These values vary from 1.2 to 2.1 eV,

which are lower than the values reported in literature

(2e2.5 eV) [49]. Cu2O samples with Cu0 in their composition

showed band gaps less than 1.4 eV due to the presence of

metal over the oxide.

This fact can be related to the presence of Cu0 in Cu2O

samples although it only appeared in diffractograms from

Cu:1-1/y samples. In the other samples, the composition of

this residual metal was probably less than 5%. We analyzed

the absorption spectra of the Cu2O samples in order to find

evidence of Surface Plasmon Resonance, which is character-

istic of Cu0 nanoparticles. The sample that showed a SPR band

between 600 and 800 nm was Cu:1:1/US, which has Cu0 in its

composition according to its diffractogram (see

Supplementary Figure S1). Samples Cu-1:1/P and Cu-1:1/MW

)Relation betweenplanes (111)/(200)

Eg (eV) Average particlesize (nm)

e 1.5 220

e 1.4 180

e 1.7 320

2.8 1.9 780

3.0 1.9 80

3.4 2.0 1000

2.9 1.3 500

3.0 1.2 220

3.6 1.4 200

3.1 2.0 720

3.3 2.0 350

3.7 2.1 150

Fig. 4 e SEM images of the Cu-x/y samples.

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 2 ( 2 0 1 7 ) 1 2 9 9 7e1 3 0 1 013004

did not show any absorption characteristic of SPR probably

due to the agglomeration of the Cu0 particles identified by XRD

in Fig. 3.

Band structure calculations

The position of the conduction and valence band of the copper

oxide samples is shown in Fig. 6. As can be seen in Fig. 5, the

conduction band (CB) of the CuO samples (Cu-1:0/y) is not

sufficiently negative to carry out the reduction of Hþ into H2.

On the other hand, the Cu2O samples (Cu-1:1/y) that show Cu0

in its composition by XRD do not have the sufficient potential

to perform the water reduction. Nevertheless, the samples

with only Cu2O in its composition have the CB sufficient

negative to reduce Hþ into H2, particularly, sample Cu-1:1.5/

MW.

Hydrogen production

The hydrogen production using the Cu-x/y samples as a

photocatalyst after 3 h of continuous irradiation is shown in

Fig. 7. CuO samples did not show any photocatalytic activity

due to their insufficient CB potentials to produce H2. In addi-

tion, we did not observe any color change in CuO associated

with its reduction by the photogenerated electrons during the

photocatalytic reaction as was previously reported by other

researchers [10].

Conversely, when Cu2O samples were applied as photo-

catalyst, different activities were observed. The most active

photocatalysts were the samples prepared under the highest

amount of glucose, whose H2 generation values were 78, 62,

and 42 mmol/g for Cu-1:1.5/MW, Cu-1:1.5/US, and Cu-1:1.5/P,

respectively. On the other hand, the Cu2O samples with the

lowest photocatalytic activity in each case were the ones

prepared with equimolar relation of copper and glucose.

These results can be related to the positive position of the CB

in these samples (Cu-1:1/y), which has no sufficient potential

to carry out the reduction of Hþ into H2. Contrary to what is

reported in literature, the presence of Cu0 as co-catalyst did

not have a positive efficiency in this case, probably due to its

relative high amount in Cu2O samples (>5%). There are several

reports that propose the use of less than 1%wt. of Cu0 to

produce a higher amount of H2 [50,51]. Despite the fact that Cu

can act as an electron collector, higher amounts of Cu0 can

Fig. 5 e Scheme of the formation mechanism of Cu-x/y.

Fig. 6 e Band diagram of the Cu-x/y samples prepared by different methods.

Fig. 7 e Hydrogen production using the Cu-x/y samples as photocatalysts.

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 2 ( 2 0 1 7 ) 1 2 9 9 7e1 3 0 1 0 13005

block the active sites of Cu2O samples, which further reduces

H2 production.

For comparative purposes, the commercial photocatalyst

TiO2 Degussa P-25 was tested under the same experimental

conditions, obtaining only 1.2 mmol of H2/g after 3 h of reac-

tion, which is significantly lower than the obtained values

using Cu2O as the photocatalyst. This can be related to TiO2

insufficient potential of the conduction band to split water. In

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 2 ( 2 0 1 7 ) 1 2 9 9 7e1 3 0 1 013006

this sense, the Cu2O sample has shown much better perfor-

mance in this reaction.

In summary, the high photocatalytic activity obtainedwith

the Cu-1:1.5/MW sample can be correlated to several factors.

The first one is its high crystallinity, which is related to the

intensity obtained in the crystal plane (111). This is associated

with low surface energy and less amount of crystal defects

[52]. In heterogeneous photocatalysis, the surface defects on

Fig. 8 e XPS of: a. Cu-1:1.5/U

the semiconductor play an important role in the recombina-

tion of the photogenerated electron and hole, which eventu-

ally reduces photocatalytic activity, in particular H2

generation [53]. Sample Cu-1:1.5 had the lowest particle size of

all samples, which causes electrons and holes to reach the

surface of the photocatalyst and reduce Hþ to produce H2.

Also, the position of its conduction band is more favorable to

carry out the reduction of water than with other samples.

S and b. Cu-1:1.5/MW.

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 2 ( 2 0 1 7 ) 1 2 9 9 7e1 3 0 1 0 13007

In order to differentiate the activity of the samples that

causes the highest H2 production, the surface of Cu2O was

studied with XPS. Fig. 8 shows the main and satellite peaks of

Cu2p for Cu-1:1.5/US and Cu-1:1.5/MW. The Cu2p core level

XPS spectrum for Cu-1:1.5/US (Fig. 8a) shows two peaks cor-

responding to Cu2p1/2 and Cu2p3/2 at 934.32 and 954.26 eV.

These peaks have been deconvoluted into two peaks related to

Cuþ2 [54,55]. Fig. 8b shows the Cu2p core level, which corre-

spond to Cu2p1/2 and Cu2p3/2 at 933.4 and 952.85 eV, respec-

tively. In accordance with literature, these peaks correspond

to Cuþ2 and Cuþ1 [56,57]. In summary, these results are very

important in order to differentiate the photocatalytic activity

of these two samples. Sample Cu-1:1.5/US has entirely

covered its surface with CuO, which has no photocatalytic

activity, as was previously demonstrated. This can block the

Cu2O active sites to carry out the photocatalytic H2 produc-

tion. On the other hand, sample Cu-1:1.5/MWhas only a slight

amount of CuO over its surface that can promote a synergistic

effect between CuO and Cu2O. This fact can improve charge

transfer during the photocatalytic reaction as the CB of Cu2O is

more negative than the CB of CuO. Thus, the electrons

generated in Cu2O can migrate to the CuO promoting the

charge separation and thus increasing the photocatalytic

activity.

Hydrogen production employing glucose as sacrificial agent

The glucose molecule in the solution is oxidized into gluconic

acid, giving an electron to the reaction medium to produce

hydrogen. In addition, the ability of glucose to act as a hole

scavenger has been reported, which inhibit the recombination

of the photogenerated pair [58]. For these reasons, this mole-

cule was chosen as a sacrificial agent to increase the hydrogen

production through heterogeneous photocatalysis.

Fig. 9 e Effect of the initial glucose concentratio

With the purpose of studying the effect of the initial con-

centration of glucose on H2 generation, we took the produc-

tion after 3 h of continuous irradiation into consideration. As

it is shown in Fig. 9, as the concentration of glucose increases

in the reactionmedium, the H2 production rate increased. The

experiments shown in Fig. 9 were fitted using the Langmuir-

type model (equation (9)) in order to determine the rate and

the adsorption constants.

r ¼ dCH2

dt¼ kKdC0

1þ KdC0(9)

where r is the rate of H2 production, k is the reaction rate

constant, Kd is the adsorption constant of glucose on the Cu2O

surface, and C0 is the initial concentration of glucose. The

obtained values from a fitting of equation (9) was

k ¼ 8.7 � 10�7 mol/min and Kd ¼ 2.8/M. Different values of

reaction rate constants have been reported in literature using

other photocatalysts in the presence of sacrificial agents. For

example, Li et al. [59] studied the effect of adding oxalic acid

during the hydrogen production and determined a constant

rate reaction of 6.4 � 10�7 mol/min using the commercial

Degussa TiO2 P-25 coated with Pt and using a 250 W lamp. On

the other hand, Chowdhury et al. [60] reported a reaction rate

constant of 6.77 � 10�6 mol/min when TiO2 sensitized with Pt/

EosinY was used as photocatalyst, under solar irradiation.

Both reaction rates are comparable with our obtained results.

According to the results obtained, we suggest the reaction

mechanism for H2 production that is shown in Fig. 10, when

the sample Cu-1:1.5/MW was used as photocatalyst. Accord-

ing to the XPS analysis, this sample has Cu2O and CuO in its

surface which improves the photocatalytic activity by

enhancing the charge transfer process as follows: first, the CB

of CuO is lower than the CB of Cu2O, which promotes the

transfer of the electrons in the CB of Cu2O to CuO; second, the

n on the H2 photocatalytic production rate.

Fig. 10 e Reaction mechanism for photocatalytic H2

generation.

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 2 ( 2 0 1 7 ) 1 2 9 9 7e1 3 0 1 013008

photocatalytic H2 production increases as long as the glucose

concentration increases (Fig. 9), which can be related to the

oxidation of glucose by the holes that remains available in the

VB of Cu2O. The oxidation of glucose could be carried out by a

direct oxidation of the photogenerated holes or by a reaction

involving the radical hydroxyl produced by the oxidation of

H2O / OH�þHþ.With the purpose of comparing the photocatalytic activity

of the Cu2O powders prepared in this paper with others re-

ports in literature, the solar-to-hydrogen efficiency (STH) was

calculated [61]. When sample Cu-1:1.5/MW was used as a

photocatalyst, the STH efficiency was 0.16%. This value was

increased to 0.81% when glucose was added into the reaction

medium as a sacrificial agent. The theoretical solar-to-

hydrogen (STH) maximum efficiency for Cu2O is 18% under

AM 1.5 G solar illumination [62]. In this context, Morales et al.

estimated an efficiency of 7.7% for H2 production for Cu2O,

however, this relatively high efficiency was obtained using a

photoelectrochemical process and by the addition of Ni-Mo

and MoSx catalysts to the Cu2O surface. On the other hand,

Artioli et al. found a maximum efficiency of 0.07% when CuO

was used as a photocatalyst and in the presence of methanol

as a sacrificial agent [10].

Our findings suggest that hydrogen production using Cu2O

as a photocatalyst is a feasible process, which represents a

sustainable process at a low cost, as the activation of Cu2O can

be carry out by solar radiation and its production is derived

from available raw materials at a low cost, since they are ob-

tained without the need of a later thermal treatment.

Conclusions

H2 production was carry out by heterogeneous photocatalysis

using Cu2O and CuO powders as catalysts. The powders ob-

tainedwere prepared by precipitation of copper acetate salts in

a basic medium using glucose as a reducing agent. The pre-

cipitation was assisted with ultrasonic and microwave radia-

tion at 80 �C without the requirement of any additional

thermal treatments to obtain the Cu2O powders in pure form.

The samples prepared with microwaves at the highest con-

centration of glucose showed the best photocatalytic efficiency

for H2 production. The high photocatalytic activity obtained

with this sample can be correlated to its high crystallinity and

homogeneous low particle size. Also, the favorable position of

its conduction band promotes a feasible hydrogen production.

From XPS results, we propose a synergistic effect between the

Cu2O and CuO sample, which promotes the charge transfer

from Cu2O to CuO that eventually increases H2 production.

Glucose was used as a sacrificial agent and resulted in an in-

crease of the H2 production rate, which is related to an inhi-

bition of the recombination of the charges due to the oxidation

of glucose by a direct reaction with the photogenerated holes

or by the action of hydroxyl radical. Based on the results seen,

we propose the use of Cu2O obtained by a synthesis method

that involves a low synthesis temperature and a short reaction

time to be used as an efficient photocatalyst to carry out H2

production through light energy and biomass residue (glucose)

associated to a low cost process.

Acknowledgments

The authors wish to thank CONACYT for the financial support

of this research through the following projects: CB-2014-

237049, INFR-2015-251936, CB-2013-220802, Problemas Nacio-

nales (project number 2015-01-487), PAICYT 2015, SEP PIFI

2014, SEP Integraci�on de redes tem�aticas de colaboraci�on

acad�emica 103.5/15/14156, and C�atedras CONACYT.

Appendix A. Supplementary data

Supplementary data related to this article can be found at

http://dx.doi.org/10.1016/j.ijhydene.2017.03.192.

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