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 3 6 ( 2 0 1 1 ) 6 5 6 0e6 5 6 8

Avai lab le at www.sc iencedi rect .com

journa l homepage : www.e lsev ie r . com/ loca te /he

Highly efficient CuO incorporated TiO2 nanotubephotocatalyst for hydrogen production from water

Shiping Xu, Alan Jianhong Du, Jincheng Liu, Jiawei Ng, Darren Delai Sun*

School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore

a r t i c l e i n f o

Article history:

Received 27 December 2010

Received in revised form

17 February 2011

Accepted 20 February 2011

Available online 24 March 2011

Keywords:

TiO2 nanotube

CuO

Hydrogen production

* Corresponding author. Tel.: þ65 67906273; fE-mail address: DDSun@ntu.edu.sg (D.D.

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.02.103

a b s t r a c t

Highly dispersed CuO was introduced into TiO2 nanotube (TNT) made by hydrothermal

method via adsorptionecalcination process or wet impregnation process, to fabricate CuO

incorporated TNT photocatalysts (CuO-TNT) for hydrogen production. It was found that

CuO-TNT possessed excellent hydrogen generation activity, which was constantly vigorous

throughout 5 h reaction. Depending on the preparation method, hydrogen evolution rates

over CuO-TNT were founded in the range of 64.2e71.6 mmol h�1 g�1catalyst, which was

much higher than the benchmark P25 based photocatalysts, and even superior to some

Pt/Ni incorporated TNT. This high photocatalytic activity of CuO-TNT was mainly attrib-

uted to the unique 1-D tubular structure, large BET surface area and high dispersion of

copper component. Compared to wet impregnation, adsorptionecalcination process was

superior to produce active photocatalyst, since it was prone to produce photocatalyst with

more highly dispersed CuO.

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction hindrance, many effective modification techniques to TiO2

Depletion of non-renewable fossil fuels and global warming

problem have made hydrogen an ideal future energy source,

due to its high energy content and zero emission of green-

house gases. Currently, themajority of hydrogen demands are

sourced from steam reforming of natural gas; however, this

process does not mitigate dependence on fossil fuels, and

generates large amount of carbon dioxide. Alternatively,

photocatalytic water splitting has been considered as one of

the most promising approach for renewable hydrogen

production, ever since the discovery of photoelectrochemical

water splitting at a TiO2 electrode by Fujishima and Honda [1].

TiO2, as a representative photocatalyst, is usedwidely because

of its high activity, chemical stability, cheap and environ-

ment-friendly properties. However, hydrogen generation

efficiency over bare TiO2 is low, mainly due to the fast

recombination of electron/hole pairs [2]. To overcome this

ax: þ65 67910676.Sun).2011, Hydrogen Energy P

have been identified, such as, noble metal loading [3,4], metal

ion doping [5e7], and semiconductor composition [8].

Recently, some researchers found that cost-effective copper

incorporated TiO2 (copper-TiO2) possessed satisfactory pho-

tocatalytic activity for the reduction of water under sacrificial

conditions to produce hydrogen [9e14], since copper

compounds facilitated the charge separation and provided

reduction sites for hydrogen formation. In our previous study

[15], CuO incorporated TiO2 (CuO-TiO2) even exhibited supe-

rior activity to some noble metals loaded TiO2. Moreover,

several copper compounds (CuO/Cu2O/Cu) have been proved

to be active [16e19], which further affirms the huge potential

of copper-TiO2 in photocatalytic hydrogen generation.

TiO2 nanotube (TNT) has been widely studied since its

inceptionanddiscoverybyKasugaetal. in1998 [20], byasimple

hydrothermal treatment of TiO2 powder in NaOH aqueous

solution. Many studies have been devoted to understand the

ublications, LLC. Published by Elsevier 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 3 6 ( 2 0 1 1 ) 6 5 6 0e6 5 6 8 6561

formationmechanismsof TNTduringhydrothermal synthesis

[21], and to optimize fabrication conditions [22], as well as to

study the properties of TNT. It has been reported that TNT

possesses a unique combination of morphological and physi-

cochemical properties, such as large specific surface area,

mesoporous structure, high aspect ratio, and efficient electron

conductivity [23]. TNT has exhibited immense potential in

a wide variety of applications, ranging from environmental

purification [24,25], energystorage [26] andgas sensing [27], but

little is known about its performance in photocatalytic

hydrogen production. In the limited studies about TNT for

hydrogen generation, TNT with Pt [28e30], Ir and Co nano-

particles [31], or hydrated Ni complex [32] was proved to be

active; however, no performance about cost-effective copper

incorporated TNThas been investigated yet. Herewe reported,

for the first time, that CuO-TNT possessed excellent hydrogen

generation activity, even superior to some Pt/Ni incorporated

TNT. This may pave way for the large application of highly

efficient CuO-TNT photocatalyst in hydrogen generation.

In this paper, 1-D TNT was fabricated via hydrothermal

route, and then incorporated with CuO by adsorptionecalci-

nation (AeC) or wet impregnation (WI) method. After struc-

ture and component characterizations, the fabricated CuO-

TNT was evaluated in photocatalytic hydrogen generation. It

was found that CuO-TNT fabricated by AeC method was

highly efficient for hydrogen generation, with activity superior

to sample fabricated byWImethod, andmuch better than P25

based photocatalysts. Factors contributing to high activity

were also discussed in this paper.

2. Experimental

2.1. Preparation of TNT and CuO-TNT

TNT was fabricated by a typical hydrothermal method fol-

lowed by calcination process. Degussa P25 (Germany) powder

was added to 10 M NaOH (Merck, AR) aqueous solution, fol-

lowed by an ultrasonic process and thorough mixing. The

homogeneous suspensionwas then hydrothermally treated in

a Teflon-lined autoclave at 150 �C for 48 h. The obtained

precipitate was first washed with de-ionized water (Milli-Q,

ultrapure), and then dispersed into 0.1 M HCl (Merck, AR)

under continuous stirring for 24 h. The acid treatment was

repeated for 3 times, and the product was washed thoroughly

with de-ionizedwater until the resulting pHwas neutral. After

drying in a freeze dryer, the product was calcined at 400 �C to

obtain the final TNT photocatalyst.

Copper compound was introduced into TNT by AeC or WI

method, with Cu(NO3)2$3H2O (Merck, GR) as precursor. With

reference to our previous study [15], during fabrication, Cu/Ti

ratio was controlled at 10 atom% by adjusting Cu(NO3)2concentration.

Adsorption of Cu2þ on TNT in AeCmethod was carried out

in Cu(NO3)2 solution with aqueous ammonia, since high pH

promoted cation adsorption on TNT, and Cu2þ could be stabi-

lized by complexation with ammonia. During preparation,

aqueous ammonia solution was added into Cu(NO3)2 solution

dropwise to forma clear solution. TNTwas then dispersed into

the respective solution and stirred for 20 h to ensure

homogeneous dispersion and diffusion. After carefully

washing with de-ionized water to remove excess Cu2þ, theproduct was dried at 60 �C overnight and calcined in air at

400 �C for 2 h. The obtained sample was denoted as TNT-AeC.

In WI method, TNT was added into Cu(NO3)2 solution at

a ratio of 1g/3 ml, followed by ultrasonication and thorough

mixing to obtain a homogeneous slurry. After wet impregna-

tion, the sample underwent same dry and calcination

processes as TNT-AeC, to yield TNT-WI photocatalyst.

As comparison, P25 based CuO-TiO2 photocatalysts (CuO-

P25) were also fabricated by AeC and WI methods, and were

denoted as P25-AeC, and P25-WI, respectively.

2.2. Characterization of TNT and CuO-TNT

X-ray diffraction (XRD) pattern was obtained using a Bruker

D8 Advance X-ray diffractometer with monochromated high-

intensity Cu Ka radiation (l ¼ 1.5418 A). Elemental composi-

tions of photocatalyst were detected using an energy disper-

sive X-ray spectrometer (EDS) attached to a scanning electron

microscope (Jeol JSM-6360). Samplemorphology was observed

by a high-resolution transmission electron microscope

(HRTEM, Jeol JEM-2010). BET surface area and pore size

distribution were determined at liquid nitrogen temperature

(77 K) using a Micromeritics ASAP 2010 system. X-ray photo-

electron spectroscopy (XPS) analysis was carried out in an

ultrahigh vacuum chamber with a base pressure below

2.66 � 10�7 Pa at room temperature. Photoemission spectra

were recorded by a Kratos Axis Ultra spectrometer equipped

with standard monochromatic Al Ka excitation source

(hn ¼ 1486.71 eV). All binding energies were referenced to C 1s

at 284.8 eV. Absorption spectra of photocatalysts were recor-

ded using a Thermo Scientific Evolution 300 UVevisible

spectrophotometer equipped with an integration sphere.

2.3. Photocatalytic hydrogen production

The photocatalytic reaction was performed in an inner-irra-

diation type Pyrex reactor (volume: 270 ml) with a 400 W high

pressure Hg lamp (Riko, UVL-400HA) as the light source. The

peak wavelength of the Hg lamp is centred at 365 nm. To

maintain a constant reactor temperature of 25 �C, a quartz

water jacket, whichwas cooled by recycledwater, was utilized

to cover the lamp. Powdered photocatalyst was suspended in

10 volume% methanol water mixture at a concentration of

1 g L�1. A magnetic stirrer was placed at the bottom of the

reactor to ensure homogeneity of the suspension during

reaction. Prior to irradiation, the photocatalyst suspension

was de-aerated thoroughly for 30min by nitrogen gas purging.

Gas produced via the photocatalytic reaction was analyzed

using an off-line TCD-type gas chromatography (Agilent

7890A, HP-PLOT MoleSieve/5A).

3. Results and discussion

3.1. Characterization of photocatalysts

Fig. 1 shows theXRDpatternsof theas-preparedTNTandCuO-

TNT photocatalysts, with P25 based photocatalysts as

Fig. 2 e EDS spectra of photocatalysts (a: TNT; b: TNT-AeC;

c: TNT-WI).

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 3 6 ( 2 0 1 1 ) 6 5 6 0e6 5 6 86562

comparison. It can be seen that the fabricated TNT and CuO-

TNT exhibited characteristic features of anatase TiO2 (JCPDS

21-1272), but no obvious copper components were detected in

CuO-TNT samples, although EDS analysis of CuO-TNT

confirmed the existence of copper. Xu et al. reported the

instances where diffraction peaks of copper species dis-

appeared when copper component was highly dispersed in

TiO2 [33]. Therefore, it is believed that the copper components

in CuO-TNT were highly dispersed in the samples, with small

dimensions below XRD detection limit. Different from CuO-

TNT samples, in XRD pattern of P25-WI, diffraction peaks of

tenoriteCuO (JCPDS48-1548)wereclearlyobserved, evidencing

relativelypoorerdispersionof coppercomponents inP25based

samples. In addition, diffraction pattern of pure TNT indicated

that crystallinity of TiO2 in TNT was poorer than P25. The

calcination process carried out during copper incorporation

improved crystallinity and enhanced the intensity of TiO2

peaks in TNT-WI sample; whereas TNT-AeC possessed better

thermalstability, andmaintainedsimilardiffractionpatternas

TNT. Thiswas related to the exchange of Cu2þ into TNT lattice,

and would be discussed in the XPS analysis later.

EDS was employed to detect elemental compositions of

photocatalysts, with spectra showed in Fig. 2 and data

summarized in Table 1. In EDS spectrum of TNT, peaks of C, O,

Au, Ti were clearly observed, where C and Au were attributed

to the carbon tape and gold coating required by the EDS test.

Therefore, the as-prepared TNT sample was supposed to

contain only O and Ti elements. Na was expected to be

introduced into the fabricated TNT during the alkaline

hydrothermal process [34]; however, it was not detected in

EDS analysis. It indicated that Na concentration in the sample

was reduced by repeated washing in dilute HCl solution, and

less than the lower detection limit of EDS. In the spectra of

CuO-TNT samples, Cu peakswere clearly observed, alongwith

elements presented in TNT. Comparing Cu/Ti ratio in all TNT

and P25 based CuO-TiO2 samples (Table 1), it is interesting to

find that TNT-AeC contained nearly same amount of copper

Fig. 1 e XRD patterns of photocatalysts (a: TNT; b: TNT-

AeC; c: TNT-WI; d: P25; e: P25-AeC; f: P25-WI).

with TNT-WI, whereas copper content in P25-AeC was much

lower than P25-WI. Considering the preparation procedure, it

indicated that, TNT possessed great Cu2þ adsorption ability,

which was much higher than P25.

Microstructures of the fabricated TNT and CuO-TNT were

studied by HRTEM. Fig. 3a revealed that TNT possessed

a multilayered nanotubular structure, and the majority of

nanotubes were open at both ends. The inner and outer

diameters of the nanotubes were c.a. 5e7 nm and 10e12 nm,

respectively. With copper incorporation, TNT-AeC sample

still maintained similar tubular structure with TNT, while

some aggregation of nanotubes occurred. Surprisingly, no

obvious copper component could be identified from HRTEM

image of TNT-AeC. On the contrary, in TNT-WI sample,

particles of copper componentwith diameter of c.a. 3 nmwere

clearly observed. Larger cluster of copper component in TNT-

WI than TNT-AeC might be due to characteristics of WI

method: continuously variation of liquid phase composition

during drying, and less adsorption of Cu2þ on TNT surface. In

addition, for TNT-WI sample, 1-D structure of TNT was still

maintained; nevertheless, the inner tubes became much

narrower, even disappeared, and relatively serious aggrega-

tion of nanotubes occurred.

Table 1 e Summary of elemental compositions detectedby EDS and BET surface area of photocatalysts.

Catalyst Elementalcomposition (atom %)

Cu/Ti (%) BET surfacearea

(m2 g�1)O Ti Cu

TNT 58.2 41.8 0.0 0.0 279.9

TNT-AeC 49.0 46.8 4.2 9.0 152.4

TNT-WI 54.5 41.6 4.0 9.6 96.3

P25-AeC 45.5 51.4 3.1 6.0 41.0

P25-WI 65.9 30.7 3.5 11.3 36.1

Fig. 3 e HRTEM images of photocatalysts (a: TNT; b: TNT-AeC; c: TNT-WI).

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 3 6 ( 2 0 1 1 ) 6 5 6 0e6 5 6 8 6563

BET surface area of photocatalyst was determined by N2

adsorption/desorption, with data summarized in Table 1. N2

adsorption/desorption isotherm of the as-prepared TNT

exhibiteda typical type IVcurvewithahysteresis loop (datanot

shown), indicating mesoporosity of the photocatalyst [35].

Mesoporous nanotubular structure endowed the fabricated

TNT with large BET surface area of 279.9 m2 g�1, which could

account for the superior Cu2þ adsorption ability of TNT

compare to P25. However, copper incorporation depressed BET

surface area of TNT to a certain extent; surface areas of TNT-

AeC and TNT-WI reduced to 152.4 and 96.3 m2 g�1, respec-

tively. Nevertheless, compared to CuO-P25, TNT based photo-

catalysts still possessed relatively large surface area, which is

attractive and critical towards enhancing photoactivity.

Pore size distribution curves of the photocatalysts (Fig. 4)

supported the observation of HRTEM images and BET surface

area changes. The nanotubular TNT exhibited distinct pore

size distribution curve, which comprised two portions with

peak values of c.a. 2e3 nmand 20e30 nm, respectively. Taking

into account the morphology of TNT, the smaller pores could

be attributed to the inner wall of the nanotubes, with its size

similar to the inner diameter of the nanotubes; while, the

larger pores should refer to the pores between the nanotubes

[22]. With incorporation of copper, TNT-AeC sample main-

tained similar pore size distribution as TNT, but pore volumes

of both portions were observed to decrease. It seems that both

inner and outer surfaces of TNT were occupied by copper

species. Considering pore size distribution curvewith EDS and

HRTEM results, it is believed that the copper component in

TNT-AeC sample was attached and highly dispersed on both

inner and outer surfaces of TNT, without forming any visible

clusters in the HRTEM image. For TNT-WI sample, small pores

corresponding to pores inside of TNT almost disappeared;

meanwhile, larger pores, associated with pores between the

nanotubes, shrank to smaller size, with diameters of

10e20 nm. This is consistent with the observation of HRTEM

image: narrowing of inner tube pores and aggregation of

nanotubes. Both phenomena attributed to the large particles

of copper component prepared viaWImethod, where plugged

copper particles narrowed even blocked the inner hollow

Fig. 4 e Pore size distributions of photocatalysts.

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 3 6 ( 2 0 1 1 ) 6 5 6 0e6 5 6 86564

channels of TNT, while agglomerated copper particles on the

exterior promoted aggregation of nanotubes. These resulted

in the decline of BET surface area of TNT-WI compared to its

TNT precursor. It is noteworthy that TNT-AeC experienced

a decline in surface area from these factors as well, but to

a much lesser extent than TNT-WI sample.

Quantitative XPS analyses of TNT and CuO-TNT photo-

catalysts were preformed, with survey spectra and high-

resolution scans shown in Fig. 5. In the survey spectra, along

with peaks of C, O, Ti and Cu, small Na peaks were also

observed, confirming Na residue remained in TNT after HCl

washing reached the XPS detection limit.

Surface elemental compositions of TNT and CuO-TNT

detected by XPS are summarized in Table 2. Surface Cu/Ti

ratios of both CuO-TNT samples were smaller than values

detected by EDS analysis. This might be due to the depth

limitation of XPS detection, where X-ray beam could not reach

inner surface of TiO2 nanotubes, resulting in detection of

copper only attached on outer surface of tubes, rather than

overall copper component. Surface Cu/Ti ratio of TNT-AeC

was higher than that of TNT-WI, as separated particles of

copper component in TNT-WI sample was not evenly

distributed on TiO2 surfaces like that in TNT-AeC. In addition,

Na content in TNT decreased after copper incorporation,

indicating that Naþ underwent ion-exchange during prepa-

ration of CuO-TNT photocatalysts. Hþ was available when

TNT was dispersed in aqueous solution, and ion-exchange

between Naþ and Hþ might occur [36]. According to Zhang’s

report that ion-exchange between Naþ and Hþ was pH sensi-

tive [37], less Naþ should be exchanged during preparation of

TNT-AeC, due to higher solution pH (TNT-AeC, pH ¼ 9-10 vs.

TNT-WI, pH ¼ 3e4). However, less Na remained in final TNT-

AeC than TNT-WI sample. It is supposed that Cu2þ com-

plemented the ion-exchange with Naþ, and more occurred

during TNT-AeC preparation. XRD results supported this

inference, since Cu2þ exchanged into lattice of TiO2 benefited

thermal stability of photocatalyst [38], while TNT-AeC did

exhibit better thermal stability compared to TNT-WI. More

exchange with Naþ was beneficial for copper component to be

highly dispersed in TNT-AeC sample.

High-resolution scans of photocatalyst show detailed

chemical environment of elements. In this study, we applied

peak fitting to the spectra using GaussianeLorentzian peak

shape after subtraction of Shirley background. In Cu 2p

spectra of TNT-AeC and TNT-WI (Fig. 5d), obvious shake-up

satellite peaks were observed, which is typical characteristic

of CuO [39]. Existence of CuO was also confirmed by the

binding energies of Cu 2p3/2 and Cu 2p1/2 located at 933.5 and

953.5 eV, respectively. However, to give a good fit to the

experimental total peaks, besides the predominant CuO

peaks, a minor species of copper was found, with binding

energies of Cu 2p3/2 and Cu 2p1/2 located at 932.2 and 952.2 eV,

respectively. This is assigned to reduced copper species (Cu2O

or metallic Cu). According to Chusuei’s study [40], CuO would

be reduced to Cu2O during XPS analysis, and smaller CuO

particle was more susceptible to the reduction. Since calci-

nation process employed in preparation of CuO-TNT favoured

formation of CuO [39], and amount of reduced copper species

in XPS spectra was minuscule, it seems reasonable to assume

the reduced copper species to be Cu2O, and it was reduced

from CuO by X-ray. In addition, content of Cu2O in TNT-AeC

was larger than TNT-WI, indicating that CuO was smaller in

size and more highly dispersed in TNT-AeC sample [40]. Ti 2p

spectra of TNT and CuO-TNT (shown in Fig. 5e) were similar;

binding energies of Ti 2p3/2 and Ti 2p1/2 were found at 458.4

and 464.0 eV, respectively, which was assigned to typical Ti4þ

in TiO2 [11]. In O 1s spectra of TNT and CuO-TNT (Fig. 5f),

prominent O 1s peak at about 529.6 eV was associated with

oxygen in TiO2 and CuO. In addition, a broad shoulder at

a higher binding energy region was evident in all the samples,

indicating the presence of surface hydroxides [39]. TNT-AeC

possessed more surface hydroxides, which is advantageous

for high hydrogen generation activity [32].

UVevisible absorption spectra of TNT and CuO-TNT are

shown in Fig. 6, with P25 based CuO-TiO2 as comparison. All

photocatalysts exhibited strong absorption band at around

350 nm, which is associated with excitation of O 2p electron

to Ti 3d level. Compared to P25, absorption onset of TNT

shifted slightly towards the shorter wavelength range, giving

rise to a small increase in band gap to 3.25 eV (P25: 3.15 eV), as

calculated by KubelkaeMunk function. All copper incorpo-

rated photocatalysts showed extra large absorption band

between 400 and 800 nm, which were attributed to the

presence of copper compounds. However, profiles of the

absorption spectra were different, indicating different

combination modes of copper components with TiO2. For P25

based CuO-TiO2 photocatalysts, the obvious two-phase

absorption in UV and visible region signified two-phase

materials presented in the samples, referring to TiO2 and

copper compounds. Nevertheless, for both CuO-TNT

samples, only one absorption peak in UV region was recor-

ded, suggesting relatively high dispersion of copper compo-

nents, and good contact between copper compounds and

TiO2 in CuO-TNT samples [41].

Characterization of photocatalysts revealed that CuO-TNT

samples maintained 1-D tubular structure, and possessed

larger surface area and higher copper components dispersion

than P25 based CuO-TiO2. In addition, copper component,

present in the form of CuO, was smaller in size and more

highly dispersed in TNT-AeC sample, than TNT-WI sample.

Fig. 5 e XPS survey spectra of photocatalysts (a: TNT; b: TNT-AeC; c: TNT-WI), and high-resolution spectra of (d) Cu 2p, (e) Ti

2p, and (f) O 1s.

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 3 6 ( 2 0 1 1 ) 6 5 6 0e6 5 6 8 6565

3.2. Photocatalytic hydrogen production

Fig. 7 shows time courses of hydrogen evolution over CuO-

TNT photocatalysts, with CuO-P25 as comparison; average

hydrogen generation rates over 5 h reactions are summarized

in the inset. It was observed that negligible hydrogen was

evolved over bare TiO2, although bare TNT possessed large

BET surface area. As compared to bare TiO2, presence of CuO

significantly promoted hydrogen generation activity. This can

be credited to effective charge separation in CuO-TiO2 pho-

tocatalysts, which is crucial for efficient hydrogen generation.

Take CuO-TNT as example, charge transfer and separation in

CuO-TiO2 is illustrated in Fig. 8. Since conduction band of TiO2

is more negative than that of CuO, excited electrons in TiO2

would easily transfer to CuO, resulting in accumulation of

excess electrons in CuO, and negative shift in Fermi level of

CuO [9]. This endowed CuO with required overvoltage for

proton reduction, and serving as hydrogen formation site.

Efficient charge separation in CuO-TiO2 mitigated the elec-

tron/hole recombination, and gave rise to high activity of

hydrogen generation.

From Fig. 7, it also can be found that TNT based CuO-TiO2

photocatalysts exhibited excellent hydrogen generation activity;

hydrogen evolution rate over TNT-AeC was 7.9 ml min�1

Table 2 e Summary of surface elemental compositions ofphotocatalysts detected by XPS.

Catalyst Surface elementalcomposition(atom %)

Cu/Ti (%) Na/Ti (%)

O Na Ti Cu

TNT 71.0 2.7 26.3 0.0 0.0 10.2

TNT-AeC 72.2 1.2 25.1 1.5 6.1 4.7

TNT-WI 73.4 1.6 23.6 1.3 5.7 6.8

Fig. 7 e Time courses of hydrogen evolution over

photocatalysts under irradiation. Inset: average hydrogen

evolution rates in 5 h reactions.

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 3 6 ( 2 0 1 1 ) 6 5 6 0e6 5 6 86566

(71.6 mmol h�1 g�1catalyst), which was c.a. 70% higher than P25

based CuO-TiO2, and even superior to some Pt/Ni incorporated

TNT [28,32]. Nevertheless, compared to TNT-AeC, TNT-WI with

relatively larger CuO particles showed inferior activity, with

hydrogen evolution rate recorded at 7.1 ml min�1

(64.2 mmol h�1 g�1catalyst).

Different activity of CuO-TiO2 photocatalysts implied that,

in addition to efficient charge separation, there were still

several factors that influencedactivityofCuO-TiO2.Combining

hydrogen generation activity with photocatalyst character-

ization, it suggested that 1-D tubular structure, large surface

area, and high copper component dispersion were advanta-

geous for CuO-TiO2 for high hydrogen generation activity.

Compared with CuO-P25 photocatalysts, CuO-TNT exhibi-

ted greater hydrogen generation activity. This was primarily

attributed to the unique 1-D nanotubular structure, rather

than pure anatase crystal structure, since pure anatase gran-

ular TiO2 was proved to possess similar hydrogen generation

activity with anatase/rutile mixture of P25 [19]. 1-D tubular

structure facilitated fast electrons transfer due to reduced

grain boundary, and was expected to improve charge sepa-

ration in photocatalytic reactions [31]. In addition, large

surface area inherent with nanotubular structure provided

more active sites in CuO-TNT photocatalysts, which increased

probability for protons in the aqueous phase to contact the

reduction sites, and accelerated hydrogen formation further.

Comparing activity of TNT-AeC and TNT-WI, it can be

induced that higher CuO dispersion in CuO-TiO2 was

Fig. 6 e UVevisible absorption spectra of photocatalysts

(a:P25-WI;b:P25-AeC;c:TNT-WI;d:TNT-AeC;e:P25; f:TNT).

beneficial for high hydrogen generation activity. Both TNT-

AeC and TNT-WI samples possessed similar tubular struc-

ture, copper content, and copper chemical state; but CuO in

TNT-AeC was more highly dispersed than that in TNT-WI.

Therefore, it is reasonable to attribute superior hydrogen

generation activity of TNT-AeC to higher copper component

dispersion. High CuO dispersion improved cohesion between

copper component with TiO2, facilitated charge transfer from

TiO2 to copper compound, and then enhanced proton reduc-

tion; whereas, relatively larger CuO clusters in TNT-WI might

act as the recombination centres, and depressed hydrogen

generation activity [31]. Advantage of high CuO dispersion

was also manifested within CuO-P25 photocatalysts, where

P25-AeC with higher copper component dispersion but infe-

rior copper content, exhibited relatively higher hydrogen

generation activity, in comparison with P25-WI. In addition,

CuO-TNT possessed higher CuO dispersion than CuO-P25,

which should partly contribute to higher activity that CuO-

TNT exhibited.

It is worthy to note that hydrogen evolution rates over

CuO-TNT photocatalysts remained vigorous during 5 h reac-

tion, with no deactivation occurred. Excellent activity and

stability ensured CuO-TNT a potential candidate for intensive

photocatalytic hydrogen production.

Fig. 8 e Schematic diagram of charge transfer in CuO-TNT

photocatalyst under irradiation.

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 3 6 ( 2 0 1 1 ) 6 5 6 0e6 5 6 8 6567

4. Conclusion

CuO incorporated TNT photocatalyst fabricated via adsorp-

tionecalcination or wet impregnation was proved to be highly

efficient for photocatalytic hydrogen production. Hydrogen

evolution rates over both CuO-TNT during 5 h reaction were

recorded at 7.9 and 7.1 ml min�1 respectively, which were

much higher than P25 based CuO-TiO2, even superior to some

Pt/Ni incorporated TNT. Both CuO-TNT photocatalysts were

characterized by 1-D tubular structure, large specific surface

area, and high copper component dispersion, all of which

were proved to be advantageous for high hydrogen generation

activity. CuO-TiO2 fabricated by adsorptionecalcination

process showed superior activity to that prepared by wet

impregnation, due to better CuO dispersion. During 5 h reac-

tion, hydrogen generation rate over CuO-TNT remained

vigorous, and no deactivation occurred. High efficiency and

excellent stability highlight the great potential of CuO-TNT for

large applications in photocatalytic hydrogen production.

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] Ni M, Leung MKH, Leung DYC, Sumathy K. A review andrecent developments in photocatalytic water-splitting usingTiO2 for hydrogen production. Renew Sust Energ Rev 2007;11:401e25.

[3] Rosseler O, Shankar MV, Du MKL, Schmidlin L, Keller N,Keller V. Solar light photocatalytic hydrogen production fromwater over Pt and Au/TiO2 (anatase/rutile) photocatalysts:influence of noble metal and porogen promotion. J Catal2010;269:179e90.

[4] Zhang YX, Gao B, Puma GL, Ray AK, Zeng HC. Self-assembledAu/TiO2/CNTs ternary nanocomposites for photocatalyticapplications. Sci Adv Mater 2010;2:503e13.

[5] Dholam R, Patel N, Adami M, Miotello A. Hydrogenproduction by photocatalytic water-splitting using Cr- or Fe-doped TiO2 composite thin films photocatalyst. Int JHydrogen Energy 2009;34:5337e46.

[6] Lucky RA, Charpentier PA. Synthesis of Fe doped TiO2

nanostructures in supercritical CO2: morphology andphotocatalytic properties. Sci Adv Mater 2009;1:167e74.

[7] Manivel A, Naveenraj S, Kumar PSS, Anandan S. CuO-TiO2

nanocatalyst for photodegradation of acid red 88 in aqueoussolution. Sci Adv Mater 2010;2:51e7.

[8] So WW, Kim KJ, Moon SJ. Photo-production of hydrogen overthe CdS-TiO2 nano-composite particulate films treated withTiCl4. Int J Hydrogen Energy 2004;29:229e34.

[9] Bandara J, Udawatta CPK, Rajapakse CSK. Highly stable CuOincorporated TiO2 catalyst for photocatalytic hydrogenproduction from H2O. Photochem Photobiol Sci 2005;4:857e61.

[10] Sreethawong T, Yoshikawa S. Comparative investigation onphotocatalytic hydrogen evolution over Cu-, Pd-, and Au-loaded mesoporous TiO2 photocatalysts. Catal Commun2005;6:661e8.

[11] Choi HJ, Kang M. Hydrogen production frommethanol/waterdecomposition in a liquid photosystem using the anatasestructure of Cu loaded TiO2. Int J Hydrog Energy 2007;32:3841e8.

[12] Yoong LS, Chong FK, Dutta BK. Development of copper-doped TiO2 photocatalyst for hydrogen production undervisible light. Energy 2009;34:1652e61.

[13] Gombac V, Sordelli L, Montini T, Delgado JJ, Adamski A,Adami G, et al. CuOx-TiO2 photocatalysts for H2 productionfrom ethanol and glycerol solutions. J Phys Chem A 2010;114:3916e25.

[14] Miwa T, Kaneco S, Katsumata H, Suzuki T, Ohta K, Verma SC,et al. Photocatalytic hydrogen production from aqueousmethanol solution with CuO/Al2O3/TiO2 nanocomposite. Int JHydrogen Energy 2010;35:6554e60.

[15] Xu SP, Sun DD. Significant improvement of photocatalytichydrogen generation rate over TiO2 with deposited CuO. Int JHydrogen Energy 2009;34:6096e104.

[16] Sakata Y, Yamamoto T, Okazaki T, Imamura H, Tsuchiya S.Generation of visible light response on the photocatalyst ofa copper ion containing TiO2. Chem Lett; 1998:1253e4.

[17] Wu NL, Lee MS. Enhanced TiO2 photocatalysis by Cu inhydrogen production from aqueous methanol solution. Int JHydrogen Energy 2004;29:1601e5.

[18] Wu YQ, Lu GX, Li SB. The role of Cu(I) species forphotocatalytic hydrogen generation over CuOx/TiO2. CatalLett 2009;133:97e105.

[19] Xu SP, Ng JW, Zhang XW, Bai HW, Sun DD. Fabrication andcomparison of highly efficient Cu incorporated TiO2

photocatalyst for hydrogen generation from water. Int JHydrogen Energy 2010;35:5254e61.

[20] Kasuga T, Hiramatsu M, Hoson A, Sekino T, Niihara K.Formation of titanium oxide nanotube. Langmuir 1998;14:3160e3.

[21] Chen Q, Zhou WZ, Du GH, Peng LM. Trititanate nanotubesmade via a single alkali treatment. Adv Mater 2002;14:1208e11.

[22] Bavykin DV, Parmon VN, Lapkin AA, Walsh FC. The effect ofhydrothermal conditions on the mesoporous structure ofTiO2 nanotubes. J Mater Chem 2004;14:3370e7.

[23] Bavykin DV, Walsh FC. Elongated titanate nanostructuresand their applications. Eur J Inorg Chem; 2009:977e97.

[24] Zhang X, Pan JH, Fu W, Du AJ, Sun DD. TiO2 nanotubephotocatalytic oxidation for water treatment. Water SciTechnol Water Supply 2009;9:45e9.

[25] Ng J, Zhang X, Zhang T, Pan JH, Du JHA, Sun DD. Constructionof self-organized free-standing TiO2 nanotube arrays foreffective disinfection of drinking water. J Chem TechnolBiotechnol 2010;85:1061e6.

[26] Armstrong G, Armstrong AR, Canales J, Bruce PG. Nanotubeswith the TiO2-B structure. Chem Commun; 2005:2454e6.

[27] Sennik E, Colak Z, Kilinc N, Ozturk ZZ. Synthesis of highly-ordered TiO2 nanotubes for a hydrogen sensor. Int JHydrogen Energy 2010;35:4420e7.

[28] Lin CH, Lee CH, Chao JH, Kuo CY, Cheng YC, Huang WN, et al.Photocatalytic generation of H2 gas from neat ethanol overPt/TiO2 nanotube catalysts. Catal Lett 2004;98:61e6.

[29] Khan MA, Akhtar MS, Woo SI, Yang OB. Enhancedphotoresponse under visible light in Pt ionized TiO2

nanotube for the photocatalytic splitting of water. CatalCommun 2008;10:1e5.

[30] Li CL, Yuan JA, Han BY, Jiang L, Shangguan WF. TiO2

nanotubes incorporated with CdS for photocatalytichydrogen production from splitting water under visible lightirradiation. Int J Hydrogen Energy 2010;35:7073e9.

[31] Khan MA, Yang OB. Photocatalytic water splitting forhydrogen production under visible light on Ir and Co ionizedtitania nanotube. Catal Today 2009;146:177e82.

[32] Jang JS, Choi SH, Kim DH, Jang JW, Lee KS, Lee JS. Enhancedphotocatalytic hydrogen production from water-methanolsolution by nickel intercalated into titanate nanotube. J PhysChem C 2009;113:8990e6.

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 3 6 ( 2 0 1 1 ) 6 5 6 0e6 5 6 86568

[33] Xu B, Dong L, Chen Y. Influence of CuO loading on dispersionand reduction behavior of CuO/TiO2 (anatase) system. JChem Soc Faraday Trans 1998;94:1905e9.

[34] Thorne A, Kruth A, Tunstall D, Irvine JTS, Zhou WZ.Formation, structure, and stability of titanate nanotubes andtheir proton conductivity. J Phys Chem B 2005;109:5439e44.

[35] Gregg SJ, Sing KSW. Adsorption, surface area and porosity.2nd ed. London: Academic Press; 1982.

[36] Xing CJ, Jing DW, Liu MC, Guo LJ. Photocatalytic hydrogenproduction over Na2Ti2O4(OH)2 nanotube sensitized by CdSnanoparticles. Mater Res Bull 2009;44:442e5.

[37] Zhang SL, Zhou JF, Zhang ZJ, Du ZL, Vorontsov AV, Jin ZS.Morphological structure and physicochemical properties ofnanotube TiO2. Chin Sci Bull 2000;45:1533e6.

[38] Sun XM, Li YD. Synthesis and characterization of ion-exchangeable titanate nanotubes. ChemeEur J 2003;9:2229e38.

[39] Poulston S, Parlett PM, Stone P, Bowker M. Surface oxidationand reduction of CuO and Cu2O studied using XPS and XAES.Surf Interface Anal 1996;24:811e20.

[40] Chusuei CC, Brookshier MA, Goodman DW. Correlation ofrelative X-ray photoelectron spectroscopy shake-upintensity with CuO particle size. Langmuir 1999;15:2806e8.

[41] Bokhimi X, Morales A, Novaro O, Lopez T, Chimal O,Asomoza M, et al. Effect of copper precursor on thestabilization of titania phases, and the optical properties ofCu/TiO2 prepared with the sol-gel technique. Chem Mater1997;9:2616e20.