Improving the photo-cathodic properties of TiO2 nano-structures with graphdiynes

Article abstract of DOI:10.1039/c9nj02351h

Graphdiyne (GD) nanoscale films were initially synthesised on copper foils and its oxide form, GD oxide (GDO) obtained by subsequent oxidation of purified GD. Hybrid nanocomposites of both GD and GDO with hydrothermally grown TiO₂ nanorods were

Full text of DOI:10.1039/c9nj02351h

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Improving the photo-cathodic properties of TiO2
nano-structures with graphdiynes†
Cite this:DOI: 10.1039/c9nj02351h
Vivek Ramakrishnan, ‡ Hyun Kim and Beelyong Yang
*
Received 7th May 2019,
Accepted 3rd August 2019
DOI: 10.1039/c9nj02351h
rsc.li/njc
Graphdiyne (GD) nanoscale films were initially synthesised on copper materials was introduced into the research world, namely,
7
–10
foils and its oxide form, GD oxide (GDO) obtained by subsequent graphyne, graphdiyne (GD), graphone, and graphane.
oxidation of purified GD. Hybrid nanocomposites of both GD and GD is one of the most stable among various human made
GDO with hydrothermally grown TiO nanorods were prepared by diacetylenic carbon allotropes studied to date and therefore
2
various methods, and their photo-electrochemical (PEC) properties one of the most synthetically approachable. Similar to GR, GD
were studied. Both the systems were found to perform as an is also a two-dimensional planar structured material which has
improved photo-cathodic material with GDO found to have superior several properties to improve the photocatalytic performance
PEC properties.
2
of TiO , including large surface area and high electron
8
,11
mobility.
Low bandgap combined with high charge carrier
Metal oxide nanostructures and their hybrid materials have mobility are very desirable characteristics which make them
been widely used for energy production mainly in the area of impending candidates in photocatalytic water splitting.
electro- and photo-catalysts or both combined for energy power
GD is found to be a semiconductor and is highly stable
1
–6
conversion.
Researchers are engineering inorganic–organic at elevated temperatures even up to 800 1C. These materials
hybrid systems, in such a way to achieve maximum efficiency by can be interconverted to other nanomeric forms possessing
combining two or more systems to get superior properties. superior properties such as increased electron mobility and
One of the potential candidates in such systems is Titanium conductivity. After a hydrothermal reaction, the diacetylenic
2
dioxide (TiO ) based nanostructures which have been well linkage of GD can partly transform into a two-dimensional
studied in the field of photocatalysis. It’s chemical stability, p-conjugated structure favourable for electronic transmission.
low cost, abundance and favourable band edge alignment with In addition, these materials can be used to expand the light
water redox potentials make them a desirable contender as photo- absorption range and suppress electron–hole recombination
1
2,13
anodes for water oxidation. Among the scientific community, when linked with TiO like metal oxides.
2
there have been numerous reports for enhancing the photo-
Recently, GD and GD oxide (GDO) based photocatalysts have
cathodic activity of metal oxide nanostructures with metal-free been reported in combination with semiconductors having a
systems (polymers, nitrides etc.) and also for improving the larger bandgap by several research groups. The various forms of
photocathodic activity of TiO
2
nanostructures (Table S1, ESI†). GD like nanoparticle composites, nanoflakes and nanowires
It’s always desirable to make a tandem nanostructure which have been reported to show superior photocatalytic properties
1
4–18
can be active for photo-cathodic and anodic activation of by depositing on semiconducting metal oxides.
water by coupling with a suitable material and it is highly In this work, we have synthesised GD and GDO and nano-
advantageous to have if it’s a metal-free system like graphene or structure composites with TiO (Fig. 1) were further prepared
2
its derivatives. Following enormous ‘activity’ of graphene (GR), and photo-electrochemical (PEC) studies were performed. GD films
the pathway for a unique series of two-dimensional layered on copper foils were synthesised by in situ growth according to the
11
reported method by G. Li et al. (ESI†) with hexaethynylbenzene
monomer as the starting material. The as-prepared GD on copper
foil was transformed into GD powder by immersing in trichloro-
butane followed by sonication after purifying with various solvent
mixtures (ESI†).
School of Advanced Materials and System Engineering, Kumoh National of Institute
of Technology, Yangho-dong, Gumi-si, Gyeongsangbuk-do, Korea.
E-mail: blyang@kumoh.ac.kr
†
‡
Electronic supplementary information (ESI) available. See DOI: 10.1039/c9nj02351h
Present address: National Solar Energy Centre, Ben-Gurion University of the
The as-prepared GD films were characterised by Raman
spectroscopy. As shown in Fig. 2a, spectra display four peaks
Negev, Sede Boqer Campus, 84990 Israel.
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Fig. 1 (a) A schematic diagram of the GDBGDO/TiO
2
nanostructure and
(b) band diagram electron–hole movement.
2 2 2
Fig. 3 FESEM images of (a) TiO , (b) GD/TiO and (c) GDO/TiO and (d–f)
À1
À1
À1
À1
at 1380 cm , 1570 cm , B1940 cm (broad) and B2190 cm
their respective vertical image analysis prepared by one step synthesis.
1
9,20
À1
(
1
broad) respectively.
570 cm corresponds to the vibration of sp carbon domains in
aromatic rings and in-phase stretching vibration of sp carbon
domains in aromatic rings. Both the peaks are deviated from the
graphitic peaks, where the former one (D band) is blue shifted and
the latter one (D band) is red shifted compared to the G band of
The prominent peak at 1380 cm and
À1
2
2
The surface morphology of the hybrid GD and GDO with
TiO
Horizontal images outline the gradual increase in the size of
the nanorods from bare TiO to GD and then to GDO incorpo-
2 2
is shown in Fig. 3 and compared with bare TiO nanorods.
2
À1
rated hybrids. The vertical FESEM (Fig. 3d–f) analysis clearly
outlines the growth of GD and its oxide on the nanorod
structure.
graphite (1575 cm ). The intensity ratio of the D to G band is
calculated to be B0.70, indicating the multilayer nature of the GD
films with high order and low defects. The broad and comparatively
weaker peaks observed at a higher wavenumber region are attri-
HRTEM measurements of the GD/TiO2 composite in situ
À1
(
1 step method) also evidently mark the GD grafted TiO
nanorods (Fig. 4). The high-resolution image, Fig. 4c identifies
both the GD and TiO nanostructures marking the corres-
2
buted to the acetylenic linkages (B1940 and B2190 cm ).
To further prove the GD formation on Cu foil and crystalline
behaviour, X-ray diffraction (XRD) analysis was performed, as
shown in Fig. 2b. The peak at 44.71 represents the GD whereas
the rest of the peaks arise due to Cu foil matching very well,
with and without GD.
by the high-resolution transmission electron microscopy
HRTEM). Fig. S1a in the ESI† depicts the SAED (Fig. S1b–d,
ESI†) and lattice fringe corresponding to GD consistent with the
XRD measurements. The chemical nature of the GD films
synthesised was further confirmed by Fourier transform infrared
spectroscopy (FT-IR) analysis as well (Fig. S1e, ESI†). The charac-
2
ponding crystal planes of both components as can be inter-
preted from Fig. 4c. The SAED pattern also confirms the
1
1,17,19
attachments of GD on the TiO nanostructure as depicted in
The above fact is further supported
2
Fig. 4c. The formation of the hybrids prepared was further
probed by XRD and FT-IR methods (Fig. S2b and c, ESI†).
To further confirm the formation of the hybrid GD/TiO and
2
(
to understand the chemical and valence state, X-ray photo-
electron spectroscopic measurements (XPS) were performed
1
1,17,19,20
and compared with the reported data.
The survey
À1
teristic peaks (1590 and 2100 cm ) were identified attributed to
aromatic and acetylenic systems.
The GDO powder samples were prepared by strong acid
1
6
oxidation by a mixture of H SO /HNO with KMnO (ESI†).
2
4
3
4
We have tried to engineer hybrid samples of GD and GDO with
TiO nanorods by several processes (Table S2, ESI†), among
which the hydrothermal route was found to be effective for PEC
activity. Anchoring of GD/GDO on TiO nanorods was carried
out by one step (in situ) synthesis, i.e. the GD or GDO sample
2
2
2
will be added along with standard growth of TiO nanorods on
FTO with varying weight percentages (ESI†).
Fig. 4 (a) Low resolution and (b) high-resolution TEM analysis and
(d) SAED pattern of GD/TiO prepared by one step synthesis.
Fig. 2 (a) Raman spectra and (b) XRD analysis of synthesized GD.
2
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spectrum clearly shows the presence of C, Ti and O (Fig. S2d, particular case, the GD or GDO sample would be grown in a
ESI†). The C 1s peak in Fig. S2e (ESI†) is analysed and found second step following the growth of TiO nanorods on FTO by a
to be composed of 4 states such as C–C (sp , 284.6 eV), C–C hydrothermal method. The PEC activity was not high enough
sp, 285.9 eV), C–O (287.3 eV), and CQO (288.8 eV). The compared to that of the in situ prepared nanostructures (Fig. S7,
occurrence of characteristic peaks of Ti at 458.5 (Ti 2p3/2) and ESI†). But the photocurrent density of the GD incorporated
2
2
(
4
+
4
64.3 eV (Ti 2p1/2) clearly suggests that Ti is the major valence nanostructure was heavily increased compared to the in situ
state in the hybrid (Fig. S2f, ESI†). Before executing the PEC process. In contrast, the GDO integrated TiO nanocomposite
2
activity, preliminary photo-physical properties have been explored, showed fluctuating behaviour and showed decreased activity
such as UV-Visible spectral measurements. From Fig. S3 (ESI†), (Fig. S8, ESI†) compared to its in situ counterpart. In any case,
we can depict a small, distinct improvement towards the higher the amount of GD was not found to have enough influence on
wavelength for the hybrids of GDO compared to bare TiO2.
Photocurrent densities of the as-prepared TiO , GD/TiO and
the PEC activity (Fig. S9, ESI†).
The improved photo-cathodic activity is attributed to the
2
2
GDO/TiO by a 1 step process are measured and compared to combined effect of GD and its oxide on TiO , unlike the
2
2
the PEC activity among each other in a 0.1 M Na
solution under visible light (in addition, the PEC activity of activity.
2
SO
4
aqueous graphene or its derivatives where it may improve the anodic
2
1,22
Recently, researchers have tried to improve the
hybrids prepared by other methods stated in Table S2 is shown photo-cathodic activity coupling with GD and our system shows
in Fig. S4, ESI†). A photocurrent density of B58 mA can be comparable or even better photo-current density reported so far
1
5,23
obtained at an applied potential of 0 V which was more than to the best of our knowledge.
In the 1 step in situ growth,
1
0 times larger than bare TiO (B5 mA), improving the photo- there will be an efficient interaction between TiO and GDO.
2
2
cathodic property of the nanostructure remarkably. Interest- With the introduction of lattice oxygen, the interlinkage
ingly with the in situ preparative method, the PEC activity of the between the metal oxide and oxidic GD enhances, causing
GD composite was not improved very much from that of bare better charge transfer leading to improved PEC activity. Such
2 2
TiO . As we prepared the GDO/TiO composite by an in situ kind of interaction is less for GD where it has less polarity and
method, the effect of intercalation amount also has been taken minor interaction with the titanium precursors in the polar
into account as can be seen from Fig. 5b. A gradual increase in reaction medium. The charge transfer phenomenon could be
the PEC activity was shown when the weight percentage of the very effective by in situ synthesis where there can be direct Ti–C
2
4
GDO is gradually increased from 1 to 10 to 100. The PEC activity linkage. Nevertheless, the corresponding interaction will
was found to decrease with further increase in the loading be less prominent for ex situ derived hybrids. This kind of
amount of GDO to TiO (Fig. S5a, ESI†). A photocurrent density advantage for in situ methods has been explored for hybrid
2
2
4–27
of 0.24 mA is observed for a cathodic applied potential of 1 V. nanostructure preparation with enhanced photoactivity.
At 1 V, the enhancement factor was calculated to be B30 times In summary, a metal-free carbon material GD/GDO has been
that of the bare TiO . The effect of varying light intensities was infused with TiO nanostructures with enhanced photo-cathodic
also studied on the most efficient sample (100 wt% GDO/TiO activity for the first time. In situ and ex situ introduction of GD and
2
2
2
–
in situ). As shown in Fig. S5b (ESI†), we can descry a linear its oxide caused improvement in the PEC activity, with the former
increase in the net photocurrent with an increase in the light favouring the GDO hybrid and the latter favouring the GD. Hole
intensity.
transportation and improved photocurrent was established by the
The stability of the GDO/TiO hybrid sample was tested by strong p–p interactions between GD/GDO and TiO . GDO/TiO2
2
2
applying a constant potential of À0.8 V under illumination in photocathodes prepared by the in situ method showed an enhance-
.1 M Na SO aqueous solution (Fig. S6a, ESI†). The PEC ment factor of 10 times the photocurrent density even for a bias-
stability of GDO/TiO is further proved by characterisation free state (0 V) compared to bare TiO . This study paves a new way
0
2
4
2
2
before and after PEC measurements (Fig. S6b, ESI†). To optimise to introduce GD and its oxides as a promising candidate in
and obtain maximum PEC activity, ex situ (2 step) synthesis of improving the photo-cathodic activity of well-known photo-
GD/GDO on TiO2 nanorods was carried out (ESI†). In this anodes by which tandem nanostructures could be developed for
efficient solar-to-fuel conversion. Systems with a tandem nano-
structure of GD/GDO for PEC as well as gas evolution studies are in
progress for a better understanding of the activity of the catalyst.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Fig. 5 (a) Comparison of the photocurrent density–voltage diagram of
GD/TiO
synthesis and TiO
with visible light and (b) with varying weight percentage of GDO with TiO
2
and GDO/TiO
2
nanostructures prepared by 1 step hydrothermal
This research was supported by the Space Core Technology
Development Programme through the National Research
2
nanorods on FTO in 0.1 M Na
2
SO aqueous solution
4
2
.
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