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Doubling of photocatalytic H2 evolution from g-C3N4 viaits nanocomposite formation with multiwall carbonnanotubes: Electronic and morphological effects

Anil Suryawanshi a,c, P. Dhanasekaran b, Dattakumar Mhamane a, Sarika Kelkar a,*,Shankar Patil c, Narendra Gupta b,*, Satishchandra Ogale a,*aNational Chemical Laboratory (CSIR-NCL), Physical and Materials Chemistry Division, Dr. Homi Bhabha Road, Pashan,

Pune 411008, IndiabNational Chemical Laboratory (CSIR-NCL), Catalysis and Inorganic Chemistry Division, Dr. Homi Bhabha Road, Pashan,

Pune 411008, IndiacDepartment of Physics, University of Pune, Pune 411008, India

a r t i c l e i n f o

Article history:

Received 15 November 2011

Received in revised form

20 February 2012

Accepted 23 March 2012

Available online 20 April 2012

Keywords:

g-Carbon nitride

MWCNT

Organic photocatalyst

Visible-light water splitting

* Corresponding authors. Tel.: þ91 20 259022E-mail addresses: sarika.phadke@gmail.c

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2012.03.123

a b s t r a c t

Nanocomposite of graphitic carbon nitride (g-C3N4) with optimum concentration of

multiwall carbon nanotubes (MWCNTs) is shown to render 100% improvement in its

photocatalytic activity for visible-light induced water splitting. Our study reveals that in

addition to the charge transfer effects, morphological changes in g-C3N4 introduced by mild

MWCNT reinforcement play a vital role in the photocatalytic activity. We present results of

structural and opto-electronic characterizations in support of these inferences.

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction to the stringent electronic, optical and microstructural

The design and fabrication of a photocatalyst with requisite

quantum efficiency and a long life is a vital requirement for

viable implementation of water dissociation process for the

generation of hydrogen to serve as a renewable, green fuel

of the future [1e3]. Although the theoretical efficiency of

the photocatalytic H2 evolution process is as high as 30.7%

[4] the practical efficiencies have been quite low. This is due

60, þ91 9822628242; fax:om (S. Kelkar), nm.gupta2012, Hydrogen Energy P

requirements necessary to be met by the semiconducting

material, leaving only a few candidates allowing the desir-

able high efficiency of the photocatalytic activity [5]. In view

of the UV-sensitivity and rather poor quantum efficiency

displayed by most of the mixed metal oxide photocatalysts

discovered during the last two decades [6], efforts are now

shifted to certain metal free, organic semiconducting

materials.

þ91 20 25902636.@ncl.res.in (N. Gupta), sb.ogale@ncl.res.in (S. Ogale).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 7 ( 2 0 1 2 ) 9 5 8 4e9 5 8 9 9585

In most semiconductoremetal composite photocatalyst

systems, it is the metallic counterpart which acts as

a strong and dominant co-catalyst improving the charge

separation efficiency at the semiconductoremetal interface

[7e9]. The discovery of photocatalytic hydrogen generation

activity in the case of metal-free polymer graphitic carbon

nitride (g-C3N4) has created new hopes in this context [10].

In a typical photocatalytic hydrogen generation process,

a semiconductor with suitable band structure and band gap

acts as a photocatalyst. When it is illuminated, photons of

energy higher than the band gap create electrons and holes

in the conduction (CB) and valance band (VB), respectively.

The photogenerated electrons reduce water to form

hydrogen while photogenerated holes oxidize water to

evolve oxygen, thus decomposing water. However, for this

redox reaction to occur, it is very important for the photo-

catalyst to have the bottom level of the CB to be more

negative than the redox potential of Hþ/H2 (0 V vs. NHE),

and the top level of the VB to be more positive than the

redox potential of O2/H2O (1.23 eV), hence also requiring the

band gap to be more than 1.23 eV. Graphitic carbon nitride

has a very favorable electronic structure in this context for

visible-light photocatalysis. Its favorable energy band

alignment, band gap and stability in water make it a highly

promising organic material for this application. So far the

photocatalytic activity of bulk g-C3N4 has been moderate

mainly due to its very low surface area and high recombi-

nation rate [11]. Several attempts have been made to

improve the photocatalytic performance by increasing its

effective surface area by using different templating tech-

niques [12,13] as well as through energy band engineering

by doping g-C3N4 with different elements such as phos-

phorous, sulfur, iron and zinc [14e17]. There have also been

efforts in synthesizing g-C3N4 based composites with

different metals (Au, Ag, Pt) [18], metal oxides (TiO2, ZnO,

SrTiO3) [19e21], organic materials such as poly(3-

hexylthiophene) [22] and graphene [23]. In all these

composite systems the enhanced photocatalytic H2 evolu-

tion activity has been associated with the charge transfer

process taking place between g-C3N4 and the other

component of the composite material, which reduces the

unfavorable recombination of photogenerated electro-

nehole pairs. It is important to note that in all these works

platinum has been incorporated as a co-catalyst which

plays a very crucial role in improving the overall photo-

catalytic performance [24]. In this sense, these systems do

not qualify as totally metal-free systems. In the present

work, we have synthesized a nanocomposite of function-

alized multiwall carbon nanotubes (MWCNTs) with g-C3N4

to form an all-organic metal-free composite and have

examined its activity for photocatalytic H2 evolution under

visible light without addition of any metal such as Pt. We

have found a dramatic 100% improvement for an optimum

of 0.5% MWNCT/g-C3N4 nanocomposite. The structural,

optical and electronic properties of the composite have

been examined and analyzed. Interestingly, it has been

observed that morphological changes induced by MWCNT

are the primary influence in the improvement in the overall

photocatalytic performance of the composite, although

charge transfer can also assist the process.

2. Material and methods

2.1. Materials synthesis

Bulk g-C3N4 was synthesized using cyanamide (Aldrich 99%

purity) as precursor [10]. Cyanamide was heated in

a programmable furnace at 550 �C in air for 4 h with a ramp

rate of 2.3 �C min�1. The MWCNT/g-C3N4 composite was

synthesized by adding functionalized MWCNT in situ to 1.5 M

aqueous solution of cyanamide, following the same heat

treatment protocol as mentioned above. A systematic varia-

tion of MWCNT loading (0, 0.2, 0.5, 1, 1.5, 2, 5 wt%) was

employed. Before the heating procedure, the aqueousmixture

ofMWCNTand cyanamidewas sonicated for 2 h to formawell

dispersed solution and then heated in an oil bath at 100 �C to

remove water and to obtain a semisolid product. Few repre-

sentative samples for comparative study were synthesized by

physically mixing g-C3N4 with MWCNT.

2.2. Methods

The structure and morphology of the bulk and composite

powders were characterized by X-ray powder diffraction

(Philips X’Pert PRO) and High resolution TEM (FEI Tecnai 300).

The optical properties were studied by diffuse reflectance

spectroscopy (JASCO Instruments), UVeVisible spectroscopy

(JASCO V-670 spectrophotometer) and photoluminescence

spectroscopy. The surface area of g-C3N4 and all the

composites was determined by BrunauereEmmetteTeller

(BET) adsorption method.

The photocatalytic H2 evolution measurements were

carried out at normal pressure using a quartz photo-reactor

of w80 ml capacity, equipped with a port for periodic

withdrawal of the gas samples. For each experiment,

100 mg of fresh catalyst was dispersed sonically in 10 ml of

water and 3 ml of methanol. The cell was purged with

nitrogen before each measurement. A 300 W Xenon arc

lamp (M. Watanabe & Co, Japan, model XDS-301S, irradi-

ance 100,000 Lx), coupled with a >395 nm cut-off filter of

2 mm thickness (CVI Meller Griot CG-GG-395-50) was

employed as a visible-light source. The incident radiation

energy in these set of experiments measured on a Newport

(model 842 PE) visible-light energy meter was estimated at

209 mW cm2. The reaction products were analyzed on a gas

chromatograph (Nucon-5765, India, 2.5 m molecular sieve-

5A packed column, thermal conductivity detector at 360 K,

Argon for carrier gas).

3. Results and discussion

The XRD data of bulk g-C3N4 and its composites with MWCNT

in increasing concentration from 0.2% to 2% are shown in the

Supporting information (SI-I). With the addition ofMWCNTno

significant deviation from the XRD pattern of the bulk g-C3N4

was observed, although a substantial broadening of the main

peak could be noted testifying to the nanocomposite forma-

tion. Strains induced at the interfaces between two integrated

materials can lead to such broadening.

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 7 ( 2 0 1 2 ) 9 5 8 4e9 5 8 99586

The diffuse reflectance spectra (DRS) of g-C3N4 and its

various composites with MWCNT are shown in the

Supplementary information (SI-2) and the results for some

specific cases of interest are shown in Fig. 1. In Fig. 1 we also

show the data for two cases of simple physical mixtures (PM)

of g-C3N4 and MWCNT for the same specific composition

ratios as the data shown for the composites. It can be seen

from Fig. 1 and SI-2 that bulk g-C3N4 shows an absorption edge

at 460 nm corresponding to its reported optical band gap at

around 2.7 eV. With increasing concentration of MWCNT in

the composite, the absorption in the visible region is seen to

increase significantly, retaining the absorption edge at almost

the same wavelength as that for bulk g-C3N4. The cases of

simple physical mixtures (PM) also show the same behavior

although the absorption is suppressed even more in this case.

The difference in absorption below the band gap of g-C3N4 for

the cases of 0.5% MWCNT composite and 0.5% PM is much

higher than that for the corresponding 2% MWCNT cases.

Also, as shown in the inset of Fig. 1, the feature in theUV range

which is noted in the g-C3N4 sample is also present in PM

samples, as expected, but is suppressed in the case of the

composite samples. These changes speak for changes in the

electronic states upon composite formation.

Fig. 1 e (a) Diffuse reflectance spectra and (b) UVeVisible

absorption spectra (with integrated sphere) of bulk g-C3N4

and its composites with MWCNT.

We also recorded the UVeVis transmittance spectra (using

integrating sphere geometry) for these cases of interest,

namely g-C3N4, 0.5% and 2% MWCNT/g-C3N4 nanocomposites

and simple physical mixtures with the same compositions.

The data for pure MWCNT case are also included for

comparison. Use of integrating sphere is found to be of critical

importance in the case of particulate matter because scat-

tering effects in such samples can otherwise obscure the true

systematic. The data for g-C3N4 are completely consistent

with the results reported by Li et al. [25]. Two clear absorption

signatures are observed at about 250 and 390 nm, respectively.

As discussed by Li et al. the relatively weaker peak at 250 nm

has been attributed in the literature to pep* electronic tran-

sition in the aromatic 1,3,5-triazine compounds, while the

stronger peak near 390 nm may be attributed to the nep*

electronic transitions involving lone pairs of nitrogen atoms.

In the data for pure MWCNT a clear signature is seen at about

215 nm, which has been attributed to pep* electronic transi-

tion in the said system. It may be seen that this feature also

develops in the case of the composite samples in addition to

the two signatures corresponding to g-C3N4 mentioned above.

Distinct differences are seen between these results and the

spectra representing simple physical mixtures of g-C3N4 and

MWCNT, suggesting again that the composite represents real

electronic state changes. It is useful to point out here that the

absorbance near and above 390 nm is nearly the same for the

compared cases of composite and physical mixture because

the value is dominated by the quantity of g-C3N4 present in the

sample. As discussed next however the photocatalytic effects

differ distinctly between the cases of composites and physical

mixtures.

Fig. 2 compares the normalized photocatalytic H2 evolution

activity of bulk g-C3N4 for composites with increasing wt% of

MWCNT. The photocatalytic activity of MWCNT/g-C3N4

composites of up to 1% MWCNT loading was higher as

compared to the bulk g-C3N4. The highest, almost double, the

activity is observed for the 0.5% MWCNT/g-C3N4 composite.

The increase in the photocatalytic activity of g-C3N4 could be

due to two effects; effective charge transfer from g-C3N4 to

Fig. 2 e Photocatalytic H2 evolution activity of g-C3N4 and

its composites with MWCNT.

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MWCNT resulting into longer lifetimes of photogenerated

charges and favorablemorphological changes in g-C3N4 due to

reinforcement of MWCNT making more active area available

for the photocatalysis. It is important tomention here that the

photocatalytic performance of simple physical mixtures of g-

C3N4 and MWCNT in the same proportions as used in the

composites was found to be even worse than that of pure g-

C3N4. The results and discussion pertaining to the related

mechanisms are given below in accordance with Fig. 3.

The photoluminescence spectra of the bulk g-C3N4 and

0.5% MWCNT composite, shown in Fig. 3(a), reveal a single

peak at 460 nm corresponding to the recombination

between electrons from conduction band and holes from

the valance band of bulk g-C3N4. The fact that the MWCNT

composite shows quenching in the PL intensity signifies

that the photogenerated conduction band electrons of g-

C3N4 are indeed transferred to MWCNT thereby reducing

the recombination effect. It is not obvious however whether

doubling of the hydrogen output can be solely attributed to

this effect.

Fig. 3 e (a) Photoluminescence data of g-C3N4 and the 0.5%

MWCNT composite showing quenching effect. (b) Plot

showing data of BET surface area and absorbance at

425 nm of g-C3N4 and MWCNT composites with

schematics representing attachment of MWCNT and

corresponding morphological changes with increasing

concentration.

The surface area values of MWCNT/g-C3N4 composites,

as measured by the BET method, are shown in Fig. 3(b). It

can be seen that up to 1% MWCNT case the increase in area

with CNT concentration is gradual, but it begins to increase

quite rapidly above this concentration. Given the large

measured surface area of MWCNT itself (w208 m2 g�1) the

trend shown by the composite indicates that the concen-

tration of loose or non-integrated MWCNTs begins to grow

above 1%. These results suggest that at low concentrations

such as 0.5%, the MWCNTs reinforce the g-C3N4 sheets,

enabling exposure of their higher surface area for catalytic

activity as compared to only g-C3N4. At higher concentra-

tions of MWCNT however one only has catalytically inef-

fective and optically hindering addition of loose MWCNTs.

The particle size measurements performed on the bulk and

composite samples using dynamic light scattering also

revealed similar information. The basic feature size of

g-C3N4 reduced from 5 mm to 1 mm to less than micron as

the MWCNT concentration increased, suggesting that more

and more active sites of g-C3N4 were available at higher

concentration of MWCNT. However, as mentioned above in

the context of the DRS and UVeVisible measurements, an

optimum concentration of MWCNT was observed for the

photocatalytic activity of the composite due to the delirious

effect of light absorption posed by MWCNT at higher

concentration. In the same figure we have shown the

absorbance at 425 nm (a typical wavelength value in the

absorption range of g-C3N4, although the general systematic

are no different for other choices of wavelength in this

regime) for two typical cases of composites and corre-

sponding physical mixtures, along with the data for g-C3N4

and MWCNT. It can be seen that the absorbance is higher

for the physical mixture for the 0.5% case than the corre-

sponding composite but the photocatalytic activity of the

composite is far superior (42 mmol/g for the composite vs.

only 6 mmol/g for the physical mixture). This clearly brings

out the important role of charge transfer in the process,

which cannot occur in the physical mixture. In fact the

performance of the physical mixture is weaker than that of

pure g-C3N4 because of the availability of light for absorp-

tion in the presence of absorbing MWCNT.

The HRTEM (micrographs aef) and FESEM (micrographs

gei) images of bulk and MWCNT/g-C3N4 composites repre-

senting different length scales are shown in Fig. 4. The

micrographs a, d and g for bulk g-C3N4 clearly brings out

flaky, sheet-like morphology of this compound. From the

micrographs b, e and h representing the case of 0.5% it can

be seen that the MWCNTs are very well incorporated and

attached to the g-C3N4 sheets for this concentration.

However, in the 2% case represented by micrographs c, f

and i the concentration of unattached and independently

coiled MWCNTs is seen to have grown and the bulk g-C3N4

flakes are seen to get crumpled into such MWCNT mesh.

The same is also expected for composites with higher

MWCNT concentration. Thus, increasing MWCNT loading

beyond an optimum value inflicts a negative influence on

the photocatalytic performance (as observed) via undesir-

able changes in morphology. This effect can also be corre-

lated to the more gradual increase in the surface area of

the composites for <1% MWCNT, above which the CNTs

Fig. 4 e HRTEM images of g-C3N4 (a, d), 0.5% MWCNT/g-C3N4 (b, e), 2% MWCNT/g-C3N4(c, f) and FESEM images g-C3N4 (g),

0.5% MWCNT/g-C3N4 (h), 2% MWCNT/g-C3N4 (i). In Fig. 4(c, i) red arrows indicate the reduced size of g-C3N4 flakes and in

Fig. 4(h) blue lines indicate the positions of MWCNT. (For interpretation of the references to color in this figure legend, the

reader is referred to the web version of this article.)

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 7 ( 2 0 1 2 ) 9 5 8 4e9 5 8 99588

are loosely bound and exhibit a rapid increase in the

surface area.

Therefore from the optical absorption, photoluminescence,

surface area andmicrostructural characterization datawe can

confirm that morphological effects play a primary role in

improving the photocatalytic performance of the MWCNT/g-

C3N4 composites, while the charge transfer effect does also

contribute.

4. Conclusion

A metal-free composite photocatalyst containing g-C3N4

and MWCNT is synthesized and its efficacy for visible-light

photocatalytic water splitting is reported. Following

a systematic variation of MWCNT loading with g-C3N4 an

optimum composition giving maximum (doubled as

compared to g-C3N4) photocatalytic activity is identified. It

is observed that the catalyst morphology and the charge

transfer mechanism are optimally favorable for the 0.5%

MWCNT loading. We believe that this paper brings out

the key and interesting role of morphology in photo-

catalytic activity enhancement of an engineered composite

catalyst.

Acknowledgments

We would like to sincerely thank Dr. Rahul Banarjee and Mr.

Pradip Pachfule from NCL for their help in BET surface area

measurements.

Appendix A. Supplementary data

Supplementary data associated with this article can be

found, in the online version, at doi:10.1016/j.ijhydene.2012.

03.123.

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