The photo-electrochemical performance of the samples were investigated as described earlier in Chapter 2 on experimental studies by forming photoelectrodes from the nanocomposite powder on conducting substrates. The photocurrent response at applied potential in a three-electrode PEC cell with the TiO2 and TiO2-MoS2 nanocomposite
photoanodes was investigated. Fig. 4.9 shows the current-voltage characteristics of the pristine TiO2 and TiO2-MoS2 nanocomposite samples having different concentrations
of MoS2 nanoflakes measured under the visible-light conditions. As already
mentioned,1M NaOH solution was used as electrolyte. During photo-
0 10 20 30 40 50 60 In (Co /C ) of R hB
Irradiation Time (min)
10.0% TiO2-MOS 2 k= 3.5 E-2 min -1 7.5% TiO2-MOS 2 k= 3.0 E-2 min -1 5.0% TiO2-MOS 2 k= 1.68 E-2 min -1 2.5% TiO2-MOS 2 k= 1.01 E-2 min -1
87 electrochemical reaction, the photo-generated holes in the material reaches on the surface of the photoanode and the photo-generated electrons migrates to the Pt counter electrode through external circuit and participate in chemical reactions at the electrode- electrolyte interface resulting in water splitting reactions. The photocurrent densities for pristine TiO2 and TiO2-MoS2 nanocomposite samples were measured with respect
to applied potential under dark and visible light conditions. The difference in the current under light and dark conditions represents the photocurrent and the values are shown in Fig. 4.9. The photocurrent density for pristine TiO2 was obtained at 0.04 mA/cm2,
which is quite low. However, the TiO2-MoS2 nanocomposite samples exhibits
significantly higher photocurrent density as compared to pristine TiO2 which are 0.22,
0.91, 2.27 and 1.21 mA/cm2, (at 1.0 V) for the samples of MoS
2-TiO2 nanocomposite
samples with 2.5, 5.0, 7.5 and 10.0 wt.% MoS2 in TiO2, respectively. The considerably
low photocurrent density in pristine TiO2 photo-electrode is due to large band bap of
TiO2 which responds only to UV (Ultra-violet) light. The enhancement in photocurrent
density of TiO2-MoS2 nanocomposite photo-electrodes can be understood through two
aspects of (i) enhanced visible light absorption and (ii) accelerated photo-excited charge separation. As shown in schematic diagram in Fig. 4.10, TiO2-MoS2 nanocomposites
exhibits efficient light absorption in the visible region. Also, the TiO2-MoS2
nanocomposites could trap more incident light within the nanocomposite material as MoS2 nanoflakes in TiO2 nanoparticles may cause multiple reflections due to dispersion
of two phase in the nanocomposite sample. This can further increase photon absorption and thus PEC response. In addition, the band alignment between MoS2 and TiO2 is
favourable for the electron transfer from the conduction band (CB) of MoS2 to the CB
of TiO2 which suppresses the photo-generated carrier recombination in TiO2
88 particles seem to connected with the neighbouring MoS2 nanoflakes and act as bridge
routes which benefit the electron transfer along the TiO2 channel to the conductive
substrate. Mixing of two phases as seen in the SEM and TEM micrographs of the nanocomposite samples indicate this type of morphology. At higher concentration, MoS2 nanoflakes may not be uniformly distributed due to segregation and thus screen
the TiO2 nanoparticle surface from photo-electro-chemical reaction. This may be
responsible for decrease in photo-electro-chemical (PEC) activity at higher concentration of MoS2 nanoflakes, thus making 7.5 wt. % concentration as the optimum
concentration of MoS2 nanoflakes for highest photo-electro-chemical activity.
Figure 4.9: Photocurrent density vs. applied potential (V vs. Ag/AgCl) curves for TiO2
and MoS2-TiO2 measured in 1M NaOH solution under visible light
illumination.
There are two factors which inhibit the performance of as photocatalyst in water splitting applications. The first limitation is its relatively large optical band gap, about 3.2 eV, which limits its photo response only in UV light; the other being rapid recombination of photogenerated electron-hole pairs, which results in a low quantum efficiency and exhibit low photocatalytic activity under visible light illumination184. In the present studies, use of lower band gap of 2D MoS2 nanoflakes and choosing the
optimum concentration of 7.5% results in increased visible light absorption in the
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.5 1.0 1.5 2.0 2.5 J p ( mA/cm 2 ) Vapp (V vs Ag/AgCl) TiO2 5.0% MoS2/TiO2 2.5% MoS2/TiO2 7.5% MoS2/TiO2 10.0% MoS2/TiO2
89 nanoparticle samples. The present study shows that 7.5% MoS2 is the optimum
concentration in terms of increase in the PEC response. In addition to the visible light absorption, the large interface between TiO2 and MoS2 increasing the charge separation
by forming the junction with is also important for improved performance. This is derived from the recent literature which shows that the layered two-dimensional transition metal dichalcogenides (TMDs), such as molybdenum disulphide (MoS2)
based TiO2 nanocomposite (TiO2-MoS2), satisfy the above-mentioned
requirements185-192. In TiO
2-MoS2 nanocomposite, MoS2 may also play the role of an
effective photosensitizer to enhance the photoactivity of TiO2193-195. For a given
nanocomposite system, interface band alignment is important for the electron–hole separation, which is desired to be tuned to enhance the charge separation efficiency. It has been observed that type II band alignment is formed between MoS2 and TiO2, which
is beneficial for the separation of photogenerated electrons and holes196. Since MoS 2
2D layers are one atomic layer to few atomically thin layers, interfacial interactions are expected to be pronounced with a strong effect on their charge transport properties196. At higher concentration, MoS2 nanoflakes which are not an integral part of the
nanocomposite topography seem to screen the TiO2-electrolyte interface resulting in
90
Figure 4.10: Energy band diagram of MoS2-TiO2 nanocomposite shows absorption of
low energy photons and separation of photogenerated electrons and holes due to favorable band alignment.
4.3 Conclusions
In summary, MoS2-TiO2 nanocomposites having various concentration (2.5, 5.0, 7.5,
10.0 wt. %) of MoS2 nanoflakes have been prepared by two step hydrothermal method.
A reduction in absorption edge was observed in MoS2-TiO2 nanocomposites. An
important result of the present study is that there is an optimum value of MoS2
concentration at which the PEC response is maximum and further increase in concentration results in degradation in response. The MoS2-TiO2 nanocomposite
sample with an optimum concentration of 7.5 wt. % possesses highest photo-catalytic and photo-electro-chemical activity which is due to the appropriate amount of MoS2
which increases visible light absorption and prohibits the recombination of photo- generated charge carriers in TiO2-MoS2 nanocomposite samples without screening
TiO2 surface from photo-electro-chemical reaction. The present study also shows that
there is electronic interaction at the TiO2-MoS2 interface. In addition, highly apparent
photocatalytic reaction rate constant was observed in 7.5 wt. % MoS2-TiO2
nanocomposite sample which is about 17 times than that observed for pristine TiO2
91