Hydrogen production by photocatalytic water-splittingusing nitrogen and metal co-doped TiO2 powderphotocatalyst

Munevver Zeynep Selcuk • Mehtap Safak Boroglu •

Ismail Boz

Received: 13 September 2011 / Accepted: 29 February 2012 / Published online: 18 March 2012

� Akademiai Kiado, Budapest, Hungary 2012

Abstract Nitrogen-doped titanium dioxide (N–TiO2) powders were synthesized

by hydrolysis and used as a support for doping with various metals, such as, Fe, Cr,

Ni, and Pt. Aqueous solutions of metal salts were used as a metal source and metals

were deposited on N–TiO2 powders. Ni–N–TiO2 catalysts with various nickel

concentrations were studied in detail. X-ray diffraction and diffuse reflectance

spectrophotometry were used for the characterization of the photocatalysts. The

Ni–N–TiO2 photocatalysts were then tested in photocatalytic water splitting under

visible light. The optimum dopant concentration was found to be 10 lmol Ni/g

N–TiO2 for Ni–N–TiO2. The photocatalyst, Ni–N–TiO2, has shown a stable and

high activity, 490 lmol of H2 gcat-1 h-1 for the first 6 h of operation.

Keywords Photocatalysis � Nitrogen-doped TiO2 � Metal loading �Ni-doping � H2 evolution

Introduction

Non-renewable energy resources, such as coal, petroleum products, etc., supply

the most of the world’s energy requirements but these energy resources are

rapidly depleting. Hydrogen is an alternative clean energy carrier, which satisfies

the environmental and energy requirements. Since Fujishima and Honda [1] first

reported the photocatalytic activity of TiO2 to generate hydrogen by

M. Z. Selcuk � M. S. Boroglu � I. Boz (&)

Department of Chemical Engineering, Faculty of Engineering,

Istanbul University, Avcilar, 34320 Istanbul, Turkey

e-mail: ismailb@istanbul.edu.tr

123

Reac Kinet Mech Cat (2012) 106:313–324

DOI 10.1007/s11144-012-0434-4

photocatalytic water splitting in 1972, numerous studies scrutinized the potential

applications of TiO2 for solar energy conversion. Titanium dioxide is extensively

studied as a photocatalyst for water splitting because of its high photocatalytic

activity, robust chemical stability, relatively low production costs, availability and

nontoxicity.

The principle of photocatalytic water splitting is based on the conversion of light

energy on electricity or chemicals. A photocatalyst absorbs Ultraviolet (UV) and/or

Visible (Vis) light irradiation from sunlight or a light source. The electrons in the

valence band of the photocatalyst are excited to the conduction band, while the

holes are left in the valence band. This exposure to light source creates the negative-

electron (e-) and positive-hole (h?) pairs in ‘‘photo-excited’’ state. After

photoexcitation, the excited electrons and holes separate and migrate to the surface

of the photocatalyst; they act as reducing agent and oxidizing agent to produce H2

and O2, respectively. Water splitting into H2 and O2 requires the standard Gibbs free

energy change DG� of 237 kJ/mol or 1.23 eV.

H2O(l)þ 2hþ ! 1

2O2ðgÞ þ 2Hþ

2Hþ þ 2e� ! H2ðgÞ

2hvþ H2O(l)! 1

2O2ðgÞ þ H2ðgÞ

The band gap energy (Eg) of the photocatalyst should be greater than 1.23 eV to

achieve water splitting. However, to use visible light effectively, the band gap

energy should be smaller than 3.0 eV. TiO2 is favorable for water splitting but

active only under ultraviolet irradiation due to its wide band gap energy of 3.2 eV.

Therefore, significant efforts are made to develop modified TiO2 particles that are

active under visible-light irradiation (k [ 400 nm). Various strategies have been

pursued including doping with metal ions (e.g., iron [2, 3], copper [4], nickel [5],

chromium [6], cobalt [2, 6], vanadium [2]) or non-metallic elements (e.g., nitrogen

[7–10], sulfur [11], and carbon [12]).

The photocatalytic synthesis of hydrogen and oxygen from water on a TiO2

photocatalyst is not efficient enough for an industrial usage. The candidate catalysts

lose their activity via several mechanisms. One of the important activity loss

mechanisms is that induced electrons and holes recombine rapidly; therefore, the

recombination leads to the loss of activity. In one strategy to lessen the

recombination effects, photocatalytic hydrogen production has been extended to

the photodecomposition of aqueous solutions of sacrificial reagents, such as

alcohols or waste products. These alcohols in water-alcohol mixtures act as hole

scavengers.

In this study, nitrogen doped titanium dioxide photocatalysts were synthesized.

Then, the N–TiO2 photocatalyst was doped with Fe, Cr, Ni and Pt metals to

enhance water splitting activity. We examined the photocatalytic activities of these

doped photocatalysts in water splitting under visible light. Among the photocat-

alysts, Ni doped N–TiO2 photocatalysts were chosen due to their high initial

activity.

314 M. Z. Selcuk et al.

123

Experimental

Materials

Tetraisopropyl orthotitanate (C12H28O4Ti, Merck) was used as a titanium precursor

and aqueous ammonia solution (%25, Lachema) was used as nitrogen source.

Iron(III) nitrate nonahydrate (Fe(NO3)3�9H2O, Riedel–de Haen, %96), Chro-

mium(III) nitrate nonahydrate (Cr(NO3)3�9H2O, Merck, %98), Nickel(II) nitrate

hexahydrate (Ni(NO3)2�6H2O, Sigma Aldrich, %99,999) and chloroplatinic acid

hexahydrate (H2PtCl6�6H2O, Alfa Aesar) were used for metal doping on nitrogen

doped titanium dioxide photocatalysts. Hydrazine hydrate ((NH2)2�H2O, Sigma

Aldrich, %98) was used as a reducing agent.

Preparation techniques

Nitrogen doped titanium dioxide photocatalyst were prepared by hydrolysis.

Ti(OH)4 precursor was obtained by the hydrolysis of tetraisopropyl orthotitanate

with aqueous urea solution. Urea solution (3 M) was added to tetraisopropyl

orthotitanate drop by drop under strong mixing. The final hydrolysis mixture was

stirred at room temperature for further 30 min after the end of urea solution addition

and then dried completely in an oven at 100 �C for 12 h. The white fluffy powder

was further treated (ammonized) under ammonia solution (25 %, w/w aqueous

solution) vapor flow at 450 �C for 2 h. The temperature was increased to 450 �C at

a rate of 5 �C/min. At the end of nitrogen doping, the final pale canary yellow

powder was labeled as N–TiO2. Parallel to the synthesis of N doped TiO2, a portion

of the white powder was also calcined at 450 �C for 2 h under flowing air and was

used as undoped TiO2. Metals were loaded onto N–TiO2 photocatalyst. Metal salt

(Fe(NO3)3�9H2O, Cr(NO3)3�9H2O, Ni(NO3)2�6H2O, H2PtCl6�6H2O) loaded support,

N–TiO2, was treated with hydrazine hydrate at 80 �C for 1 h under magnetic stirring

to reduce the metal salts to corresponding metals. Metal loaded N–TiO2 materials

were washed with distilled water, then dried at 80 �C. The samples were labeled as

Fe–N–TiO2, Cr–N–TiO2, Ni–N–TiO2, and Pt–N–TiO2. Among these photocata-

lysts, the nickel doped photocatalyst was prepared in various ratios, 10, 20, 50, 100,

500, 5000 lmol Ni/(g N–TiO2) due to its high activity and these photocatalysts

were labeled as Ni–N–TiO2 (10), Ni–N–TiO2 (20), Ni–N–TiO2 (50), Ni–N–TiO2

(100), Ni–N–TiO2 (500) and Ni–N–TiO2 (5,000).

Experimental procedures and techniques

The crystallographic structure of metals and support TiO2 particles were analyzed

using a Rigaku D/max-2200 Ultima X-ray diffractometer with CuKa radiation at a

scan rate of 2h = 0.01�/s. The accelerating voltage and applied current were 40 kV

and 30 mA, respectively. The mean crystallite diameters were estimated by the

application of the Scherrer equation [19]. The morphologies of representative

samples were also analyzed by scanning electron microscopy (SEM). The SEM

Hydrogen production by photocatalytic water-splitting 315

123

study was performed with a field-emission SEM (FEI-QUANTA FEG 450) attached

with a METEK Energy dispersive X-ray analyzer of silicon drift detector type.

UV–Vis absorption spectra were obtained by a UV–Vis spectrophotometer

equipped with a diffuse reflectance attachment (HR 4000 UV–Vis Spectrometer,

Ocean Optics). BaSO4 was used as a reference for the absorbance measurements.

The photocatalytic water splitting reaction was carried out in an outer irradiation

Quartz reaction system under the visible light irradiation by a 400-W (OSRAM,

HQL 400 Standard) mercury lamp. The light intensity outside the quartz reactor

was measured by an ocean optics USB4000XR (covering all wavelengths from

*200–1,025 nm) spectroradiometer in terms of the absolute irradiance (lW/cm2/nm).

The total output of the lamp was 230 lW/cm2 and less than 15 % of incoming

irradiance was below 400 nm. Hence, the lamp’s illumination can be regarded

mainly in the visible region. Typically, 0.15 g of catalyst powder was dispersed in

a quartz reaction cell containing 300 mL aqueous methanol solution

(H2O:CH3OH = 10:1 v/v). The reaction cell was purged for an hour by bubbling

with ultra-pure N2 gas until the dissolved oxygen was fully removed. The reaction

cell was placed 15 cm away from the center of bulb. The reaction was performed

under magnetic stirring to keep the catalyst particles in suspension. The amount of

H2 and O2 produced was analyzed in situ by a gas chromatograph (Agilent

6890 ? GC) using a TCD detector, Ar as carrier gas and molecular sieve 5A

column. Blank experiments revealed that no hydrogen was produced without

catalyst and light irradiation under detection limits.

Results and discussion

X-ray diffraction patterns of TiO2, N–TiO2, Fe–N–TiO2, Cr–N–TiO2, Ni–N–

TiO2and Pt–N–TiO2 photocatalysts are presented in Fig. 1. In Fig. 1, in all samples,

only the anatase phase of TiO2 was detected and no peaks of the rutile and brookite

phases were found. In Fig. 1c–f, there were not any detectable peaks of metals,

either. Major peaks of Me–N–TiO2 photocatalysts observed at 2h = 25.3�, 48.1�,

37.8� correspond to TiO2 crystallographic anatase structure (JCPDS 21-1272). The

average crystallite sizes of the photocatalysts were determined according to the

Scherrer equation. The average crystallite sizes of TiO2 and N–TiO2 were estimated

to be 19 nm, and nitrogen inclusion into TiO2 in this range of concentration did not

cause any structural change in TiO2. The mean crystallite sizes of Fe, Cr, Ni, Pt

loaded N–TiO2 were in the range of 19 nm. The relative error of crystallite size

calculations by the Scherrer equation was around 4 nm (depending on what

distribution function was fitted to the raw data and the goodness of fit). Differences

in crystallite sizes, caused by nitrogen inclusion and metal loading, were

insignificant. This finding was also supported by Arias et al. [13].

X-ray diffraction patterns of Ni–N–TiO2 photocatalysts of various nickel

loadings are presented in Fig. 2. In the Fig. 2a–e, there were no detectable peaks

of nickel metal. X-ray diffraction (XRD) analysis of Me–N–TiO2 nanocomposites

did not allow the identification of metal related phases probably because

of low concentration and/or high dispersions. In one instance, the XRD pattern of

316 M. Z. Selcuk et al.

123

Ni–N–TiO2 (5,000) had peaks of NiO and metallic Ni besides the peaks of

anatase form of TiO2. Peaks of NiO were observed at 2h = 41.9� (200), 37.1�(111) and 62.9� (220) (JCPDS 47-1049) and peaks of metallic nickel were

observed at 2h = 45.8� (111), 51.3� (200), 76.2� (220) (JCPDS 87-0712). The

mean crystallite diameter of nickel metal in Ni–N–TiO2 (5,000) was approxi-

mately in the range of 15–17 nm.

0 10 20 30 40 50 60 70 80 900

1000

2000

3000

4000

Inte

nsit

y (a

.u.)

2 Theta (deg)

TiO 2 N-TiO2 Fe-N-TiO2 Cr-N-TiO2 Ni-N-TiO2 Pt-N-TiO2

abcdef

a

b

c

d

e

f

Fig. 1 X-ray diffraction patterns of (a) TiO2, (b) N–TiO2, (c) Fe–N–TiO2, (d) Cr–N–TiO2, (e) Ni–N–TiO2 and (f) Pt–N–TiO2 photocatalysts

0 20 40 60 80 1000

1000

2000

3000

4000

Inte

nsit

y (a

.u.)

2 Theta (deg.)

Ni-N-TiO2 (10)

Ni-N-TiO2 (20)

Ni-N-TiO2 (50)

Ni-N-TiO2 (100)

Ni-N-TiO2 (500)

Ni-N-TiO2 (5000)

abcde

f

ab

cd

e

f

Fig. 2 X-ray diffraction patterns of (a) Ni–N–TiO2 (10), (b) Ni–N–TiO2 (20), (c) Ni–N–TiO2 (50),(d) Ni–N–TiO2 (100), (e) Ni–N–TiO2 (500) and (f) Ni–N–TiO2 (5,000)

Hydrogen production by photocatalytic water-splitting 317

123

The UV–Vis absorption spectra of TiO2, N–TiO2, Fe–N–TiO2, Cr–N–TiO2,

Ni–N–TiO2 and Pt–N–TiO2 photocatalyst samples are presented in Fig. 3. The

spectra of all photocatalysts showed a major absorption peak ending at around

380 nm in the UV region. When doped with metal ontoN-TiO2, slight upright and

albeit red shift of the peak towards the visible range at around 400–800 nm occurred

for all the samples. Because of doping by nitrogen and metal, visible light

absorption above 380 nm was created as indicated by shoulders in the absorption

spectra. The visible light absorption was due to excitations of electrons in 3d bands

to the conduction band of N–TiO2. Appreciable shift was not anticipated under

visible light because of wide band gap nature of N–TiO2.

It was found that, after being treated under ammonia solution vapor atmosphere,

the TiO2 powder changed its color from white to pale canary yellow, and after

doping with metals, the Me–N–TiO2 powder changed its color from pale canary

yellow to dirty yellow. In general, the color of a solid was determined by the

position of its absorption edge; a slight shift of the absorption edge towards a higher

wavelength could result in slight absorption in the visible range of spectra [14]. In

Fig. 3, small absorption shoulders (at 430 and 450 nm) were observed in the visible

region too. This was a characteristic feature of nitrogen doping [15]. The energy

band gap (Eg; eV) is determined by extrapolating of the onset of the rising part to x-

axis (k, nm) of the plots and calculation Eg = 1,240/k and band gaps are tabulated

in Table 1. The calculated band gap values also did not point out an appreciable

change due to metal doping.

UV–Vis absorption spectra of Ni–N–TiO2 with different nickel doping concen-

tration are presented in Fig. 4. It was apparent that all samples doped with nitrogen

and/or Ni have shown small shoulders (at 430 and/or 450 nm) and slightly increased

300 400 500 600 7000.0

0.5

1.0

1.5

2.0

2.5

Abs

orba

nce

(a.u

.)

Wavelength (nm)

TiO2

N-TiO2

Fe-N-TiO2

Cr-N-TiO2

Ni-N-TiO2

Pt-N-TiO2

Fig. 3 The UV–Vis absorption spectra of TiO2, N–TiO2, Fe–N–TiO2, Cr–N–TiO2, Ni–N–TiO2 and Pt–N–TiO2 photocatalyst samples

318 M. Z. Selcuk et al.

123

absorbance in the visible range. It has been reported that metal doping could form a

dopant energy level within the band gap of TiO2. The electronic transitions from the

valence band to dopant level or from the dopant level to the conduction band can

effectively show changes in the band edge adsorption threshold [16]. Band gaps

calculated from the extension of the imaginary slopes onto the x-axis are tabulated

in Table 2.

SEM images (Fig. 5a, b) of the Ni–N–TiO2 (10) photocatalyst demonstrated that

the calcination of the solid precursor at 450 �C in ammonia solution vapor

atmosphere resulted in rather homogeneous powder, while the energy dispersive

X-ray (EDX) analysis (Fig. 5c) revealed that nickel particles existed on Ni–N–TiO2

(10) photocatalyst. By using EDX, the presence of nitrogen cannot be observed.

However, there was indirect evidence that proved the presence of the nitrogen on

TiO2. The proof was the slight red shift and increase in absorbance observed on the

absorption spectrum (Figs. 3, 4) of N–TiO2and Me–N–TiO2 photocatalysts.

The photocatalytic activity of the TiO2, N–TiO2 and metal–N–TiO2 was

quantitatively followed in an aqueous methanol solution for a typical six hours of

Table 1 Band gaps of TiO2, N–

TiO2, Fe–N–TiO2, Cr–N–TiO2,

Ni–N–TiO2 and Pt–N–TiO2

photocatalyst samples

Catalysts x-intercept (nm) Band Gap (eV)

TiO2 379.0 3.27

N–TiO2 401.0 3.09

Fe–N–TiO2 401.3 3.09

Cr–N–TiO2 402.4 3.08

Ni–N–TiO2 403.0 3.08

Pt–N–TiO2 409.4 3.03

300 400 500 600 7000

1

2

Abs

orba

nce

(a.u

.)

Wavelength (nm)

Ni-N-TiO2 (10)

Ni-N-TiO2 (20)

Ni-N-TiO2 (50)

Ni-N-TiO2 (100)

Ni-N-TiO2 (500)

Ni-N-TiO2 (5000)

Fig. 4 The UV–Vis absorption spectra of Ni–N–TiO2 with various nickel doping levels

Hydrogen production by photocatalytic water-splitting 319

123

operation under visible light irradiation. As shown in Figs. 6 and 7, the amounts of

H2 produced were constant with the irradiation times up to 6 h. The slope of straight

line showed the rate of hydrogen evolution and, in the first 6 h of operation, all the

catalysts exhibited a constant rate of hydrogen production. In Fig. 6, the activities of

Table 2 Band gaps of Ni–N–

TiO2 (10), Ni–N–TiO2 (20), Ni–

N–TiO2 (50), Ni–N–TiO2 (100),

Ni–N–TiO2 (500) and Ni–N–

TiO2 (5,000) photocatalyst

samples

Photocatalysts x-intercept (nm) Band Gap (eV)

Ni–N–TiO2 (10) 406.2 3.05

Ni–N–TiO2 (20) 405.0 3.06

Ni–N–TiO2 (50) 403.0 3.08

Ni–N–TiO2 (100) 401.5 3.09

Ni–N–TiO2 (500) 400.0 3.10

Ni–N–TiO2 (5,000) 399.0 3.11

Fig. 5 SEM images and EDX analysis of Ni–N–TiO2 (10) photocatalyst

320 M. Z. Selcuk et al.

123

Fe–N–TiO2 and Cr–N–TiO2 were considerably lower than those of TiO2 and

N–TiO2. However, the activity of Pt–N–TiO2and Ni–N–TiO2 (Fig. 7) were much

higher than that of N–TiO2 in the hydrogen generation [17, 18]. While cumulative

hydrogen production of Pt–N–TiO2 catalyst was 11,620 lmol, hydrogen production

of Ni–N–TiO2 catalyst was 2,946 lmol for the first 6 h of irradiation. Durability was

an important parameter in the selection of photocatalysts. The hydrogen production

reaction was an activated process and the photons absorbed on the surface of catalyst

1 2 3 4 5 6

0

60

120

180

240

300

360

Cum

ulat

ive

Hyd

roge

n P

rodu

ctio

n (µ

mol

)

Irradiation Time (h)

TiO2

N-TiO2

Fe-N-TiO2

Cr-N-TiO2

Fig. 6 Cumulative hydrogen evolution profiles of TiO2, N–TiO2, Fe–N–TiO2 and Cr–N–TiO2 (thephotocatalytic water splitting in 10 % methanol aqueous solution under the irradiation of visible light)

1 2 3 4 5 6

0

2000

4000

6000

8000

10000

12000

Irradiation Time (h)

TiO2 N-TiO2 Ni-N-TiO2 Pt-N-TiO

2

Cum

ulat

ive

Hyd

roge

n P

rodu

ctio

n (µ

mol

)

Fig. 7 Cumulative hydrogen evolution of TiO2, N–TiO2, Ni–N–TiO2 and Pt–N–TiO2. (thephotocatalytic water splitting in 10 % methanol aqueous solution under the irradiation of visible light)

Hydrogen production by photocatalytic water-splitting 321

123

increased with an increase in the irradiation time, which in turn helped in the

photodecomposition process. The activity results proved that Ni–N–TiO2 would be a

reliable alternative photocatalyst to a costly Pt–N–TiO2 for the methanol/water

photocatalytic decomposition, hence Ni–N–TiO2 was chosen for further study with

varying concentrations of nickel loadings.

The Ni–N–TiO2 photocatalysts with following loading levels (10, 20, 50, 100, 500,

5000 lmol Ni/g N–TiO2) were also tested. Fig. 8 showed the effect of various Ni

dopant concentrations. The deposition of Ni on N–TiO2 resulted in a substantial

improvement in the H2 evolution. In Fig. 8, Ni–N–TiO2 (10, 20, 50, 100 lmol of Ni)

samples showed much higher photocatalytic activity compared with undoped N–TiO2.

When the Ni doping concentration increased above 100 lmol, the photocatalytic

activities of Ni–N–TiO2 decreased abruptly. This was probably due to the surface

coverage by nickel particles so that the light absorption by N–TiO2 decreased too. It was

concluded that the H2 evolution rate depended on the Ni loading. The higher

photocatalytic activity of Ni-doped samples with increasing Ni loading up to 100 lmol

could be due to the improved dispersion of Ni nanoparticles over N–TiO2 photocatalyst.

Ni nanoparticles behaved as an electron sink to promote the interfacial charge transfer

process, subsequently enhancing the charge separation in/on the photocatalyst [6].

The Ni–N–TiO2 (10) sample exhibited the highest hydrogen synthesis activity.

Therefore, the time-on-stream behavior was studied on Ni–N–TiO2 (10) photocat-

alyst. Fig. 9 exhibits H2 production rates with consecutive and repeated use of the

same Ni–N–TiO2 (10) photocatalyst. As shown in Fig. 9, the catalytic activity

decreased slightly after 12 h. The concentration of nickel dissolved in the aqueous

medium after filtration of photocatalysts particles showed that nickel dissolution in

appreciable ratios occurred by time-on-stream after 24 h of operation. The

dissolution of nickel dopant was considered to be the major deactivation process.

1 2 3 4 5 6

0

500

1000

1500

2000

2500

3000

3500

4000

4500

Irradiation Time (h)

N-TiO2 Ni-N-TiO2 (10)

Ni-N-TiO2 (20)

Ni-N-TiO2 (50)

Ni-N-TiO2 (100)

Ni-N-TiO2 (500)

Ni-N-TiO2 (5000)

Cum

ulat

ive

Hyd

roge

n P

rodu

ctio

n (µ

mol

)

Fig. 8 Cumulative hydrogen evolution of N–TiO2 and Ni–N–TiO2, (10, 20, 50,100, 500, 5000) in thephotocatalytic water splitting in 10 % methanol aqueous solution under the irradiation of visible light

322 M. Z. Selcuk et al.

123

Conclusion

This study was focused on using metal and nitrogen-doped TiO2 synthesized via

post nitrogenation for efficient H2 evolution from the photocatalytic cleavage of

water/methanol. Fe, Cr, Ni, and Pt metal dopants for photocatalytic hydrogen

evolution were tested. Among them, the 10 lmol Ni/g N–TiO2 catalyst was found

to show the highest initial activity. XRD results demonstrated that there were no

changes in the structure and crystallinity of N–TiO2 catalysts after impregnation of

the metal ions. UV–Vis absorption spectra of the doped samples presented a slight

shift to the visible light region. The photocatalyst, Ni–N–TiO2 (10 lmol Ni/g

N–TiO2), showed one of the highest photocatalytic activity (490 micromoles H2

gcat-1 h-1) under visible irradiation for 6 h of operation. The loss of activity of

Ni–N–TiO2 (10 lmol Ni/g N–TiO2) photocatalyst was found to be due to the

dissolution of nickel.

Acknowledgments This study was supported by TUBITAK under contract number 109M458 and

Istanbul University BAP Project No. 9075.

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