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with a higher valence band (VB) potential (Fig. S17, ESI†).
Meanwhile, electrons produced in the conduction band (CB)
of ZnO2 tend to inject into facets {210} and {101} of the
nanorods with a lower CB potential, which is energetically
favourable.13,18 Therefore, photo-induced charge carriers could
be efficiently transported and separated in the present unique
heterojunction of ZnO2–brookite-PD, while the recombination
process of the electron–hole pairs could be remarkably
impeded, which contributes crucially to the dramatic enhance-
ment of activity when compared to the components ZnO2 or
brookite. The holes accumulated on the ZnO2 surfaces would
subsequently participate in the oxidation of OHÀ and the
physically adsorbed water molecules to form ꢀOH radicals, while
the electrons trapped on facets {210} and {101} of the brookite
nanorods are probably consumed in the reduction of dissolved
O2 for the generation of ꢀO2À radicals. These radicals could
eventually have a vital effect in directly oxidizing organic con-
taminants to produce CO2 and H2O, thus constructing excellent
photo-reactivity.
Fig. 3 Photocatalytic degradation of MO over the given catalysts (a) and phenol
degradation (b) under UV-light irradiation.
degrade MO in 90 min. Interestingly, when ZnO2 was selectively
deposited on the {201} facets of the brookite nanorods, the
reactivity of ZnO2–brookite-PD was remarkably improved, as
evidenced by a much shortened degradation time of 40 min,
and there was no noticeable change within 5 cycles of catalytic
reactions (Fig. S10, ESI†). However, in the case of ZnO2 deposi-
ted brookite nanorods illuminated for 12 h, the activity
enhancement was not noticeable (Fig. S13, ESI†), because ZnO2
was likely slightly unevenly deposited on the brookite surface,
which reduces the accumulation of electrons on these surfaces
and thus feebly suppresses the useless recombination of photo-
induced electron–hole pairs. If ZnO2 was randomly precipitated
onto the brookite nanorod surfaces, the photocatalytic activity of
ZnO2–brookite-PRE was inhibited instead. Therefore, the loading
sites of co-catalysts are important for an effectual photo-reactivity
boost, as documented previously.13 In agreement with the photo-
reactivity data, the heterostructure possessed a better perfor-
mance as represented by a lower electron–hole recombination
rate (as indicated by weaker PL emission in Fig. S15a, ESI†) and
production of more ꢀOH radicals (as indicated by stronger
fluorescence intensity in Fig. S15b, ESI†).
To exclude the impact of parallel self-sensitized paths such
as that for MO, colorless phenol was also selected to study the
catalytic activity of ZnO2–brookite-PD. It is seen that phenol was
completely removed in 3 h under UV irradiation (Fig. 3b). The
significant difference in activities for the samples before and
after deposition by various methods should not be attributed to
surface area or light absorption (Table S1 and Fig. S16, ESI†).
Therefore, the special photocatalytic activity of brookite-TiO2
heterostructures is highly dependent on the surface structure,
and deposition of co-catalyst onto specific oxidation facets would
be highly beneficial for efficient improvement of reactivities.
Under UV irradiation, photo-generated electrons of the
valence bands of both ZnO2 and TiO2 were excited and trans-
This work was financially supported by NSFC (No: 21025104,
21271171, 91022018), the NBRPC (2011CBA00501, 2013CB632405),
and NSF of Fujian (No. 2013J01067).
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c
11754 Chem. Commun., 2013, 49, 11752--11754
This journal is The Royal Society of Chemistry 2013