Graphite Oxide-TiO2 Nanocomposite Type Photocatalyst for Methanol Photocatalytic Reforming Reaction

TLDR: In this article, a graphite-oxide/TiO2 composite material was prepared by heterocoagulation method from Brodie's graphiteoxide (GO) in order to test them as catalysts in the methanol photocatalytic reforming reaction in liquid phase.

Abstract: Graphite-oxide/TiO2 (GO/TiO2) composite materials were prepared by heterocoagulation method from Brodie’s graphite-oxide (GO) in order to test them as catalysts in the methanol photocatalytic reforming reaction in liquid phase. The preparation of the composite itself resulted in only little changes in the structure of GO as it was indicated by attenuated total reflection infrared (ATR-IR) and 13C magic-angle spinning nuclear magnetic resonance (13C MAS NMR) spectroscopic measurements. However, during the photocatalytic reaction, all of the GO/TiO2 samples darkened strongly indicating structural changes of GO. X-ray photoelectron spectroscopy along with NMR confirmed the loss of oxygen functionalities and emergence of graphitic species in the samples recovered from the photocatalytic reaction. Model experiments were designed to identify the key factors determining the activity of the GO/TiO2 derived photocatalysts. It was found that the emergence of a pronounced coupling between TiO2 and the graphite-like carbonaceous material is the most important contribution to get active and stable photocatalysts.

Figures

Fig. 4 C 1s XPS spectra of GO in its initial state (a) and after reduction at 300 °C in hydrogen for 1 h (b)

Table 1 A quantitative evaluation of the composition of the samples based on XPS data

Fig. 5 C 1s XPS spectra of the 2 wt% (A) and the 10 wt% GO/TiO2 (B) samples in the fresh (curve a) and recovered (curve b) state. C 1s region of the bare P25 TiO2 is also shown indicating the presence of adventitious hydrocarbon contamination (curve c, intensity is scaled according to the C/Ti ratio)

Fig. 6 Rate of H2 formation in model experiments with specially treated catalysts after 240  min reaction time. For abbreviations see the experimental part

Fig. 7 ATR-IR spectra of model catalysts. a Fresh UV-GO/TiO2, b fresh HT-GO/TiO2, c recovered A-GO/TiO2, d recovered AHT-GO/ TiO2, e recovered UV-GO/TiO2. For abbreviations see the experimental part

Fig. 1 Rate of H2 formation after 240 min reaction time

Full text

Vol.:(0123456789)
1 3
Topics in Catalysis
https://doi.org/10.1007/s11244-018-0989-z
ORIGINAL PAPER
Graphite Oxide-TiO
2
Nanocomposite Type Photocatalyst forMethanol
Photocatalytic Reforming Reaction
KatalinMajrik
2
· ÁrpádTurcsányi
1
· ZoltánPászti
2
· TamásSzabó
1
· AttilaDomján
3
· JudithMihály
2
·
AndrásTompos
2
· ImreDékány
1
· EmíliaTálas
2
© Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
Graphite-oxide/TiO
2
(GO/TiO
2
) composite materials were prepared by heterocoagulation method from Brodie’s graphite-
oxide (GO) in order to test them as catalysts in the methanol photocatalytic reforming reaction in liquid phase. The prepa-
ration of the composite itself resulted in only little changes in the structure of GO as it was indicated by attenuated total
reflection infrared (ATR-IR) and
13
C magic-angle spinning nuclear magnetic resonance (
13
C MAS NMR) spectroscopic
measurements. However, during the photocatalytic reaction, all of the GO/TiO
2
samples darkened strongly indicating struc-
tural changes of GO. X-ray photoelectron spectroscopy along with NMR confirmed the loss of oxygen functionalities and
emergence of graphitic species in the samples recovered from the photocatalytic reaction. Model experiments were designed
to identify the key factors determining the activity of the GO/TiO
2
derived photocatalysts. It was found that the emergence
of a pronounced coupling between TiO
2
and the graphite-like carbonaceous material is the most important contribution to
get active and stable photocatalysts.
Keywords Graphite oxide· TiO
2
· Methanol· Hydrogen· XPS· MAS NMR
1 Introduction
Nowadays, hydrogen is regarded as one of the emerging
sources of clean energy [1–3]. Photocatalytic hydrogen
production is a promising approach for transforming solar
energy into chemical energy for storage and transport.
Methanol photocatalytic reforming (1) is a potential reac-
tion for large-scale H
2
production [4].
Since Fujishima’s and Honda’s pioneering work in 1972
[5], there has been a continuously growing interest in TiO
2
as a photocatalyst because of its efficiency, long term sta-
bility, cheapness and low toxicity [6–9]. Although TiO
2
is
a good candidate for photocatalyst in general, especially in
degradation of organic pollutants both in gas phase [7, 10]
and in liquid phase [7, 11–13] bare TiO
2
(in the absence
of metal co-catalyst) results only in poor H
2
evolution in
reaction (1) [14, 15]. The activity of TiO
2
in the hydrogen
producing reaction can be increased at least by an order of
magnitude when a proper co-catalyst is involved [16–18].
The advantages of the co-catalysts are attributed to the sup-
pressed charge recombination; promoted charge separation
and transport driven by junctions/interfaces [19–21]. Pt is
one of the most effective co-catalysts for H
2
production [16,
20] however it is rather expensive and is available in limited
quantities.
(1)
CH
3
OH + H
2
O
Photocatalyst,h
ν
���������������������������������������������������→ 3H
2
+ CO
2
Electronic supplementary material The online version of this
article (https ://doi.org/10.1007/s1124 4-018-0989-z) contains
supplementary material, which is available to authorized users.
* Emília Tálas
talas.emilia@ttk.mta.hu
1
Department ofPhysical Chemistry andMaterials Science,
University ofSzeged, Aradi vértanúk tere 1, Szeged6720,
Hungary
2
Institute ofMaterials andEnvironmental Chemistry,
Research Centre forNatural Sciences, Hungarian Academy
ofSciences, Magyar tudósok körútja 2, Budapest1117,
Hungary
3
NMR Research Group, Research Centre forNatural Sciences,
Hungarian Academy ofSciences, Magyar tudósok körútja 2,
Budapest1117, Hungary

Topics in Catalysis
1 3
Combination of carbon materials with a semiconductor
has been reported to enhance the photocatalytic activity [22,
23]. The enhancement mechanism can be related to carbon
materials, which (i) offer more active and larger number
of adsorption sites and photocatalytic reaction centers, (ii)
contribute to the suppression of the recombination of the
photogenerated electron/hole pairs, (iii) prolong the lifetime
of electrons and holes, (iv) narrow the band gap of photo-
catalyst, and (v) act as photosensitizer for catalytic reaction
[24, 25].
Graphite oxide (GO) is an excellent supporting matrix
in nanocomposite materials due to its high specific surface
area. GO is a layer-structured graphite compound which is
relatively cheap, easily available and suitable for mass pro-
duction. It can be prepared from graphite by electrochemical
oxidation [26], most frequently by strong oxidizing agents
according to the two main preparation procedures and their
more elaborated versions (e.g. NaClO
3
/HNO
3
in Brodie’s
method [27]; KMnO
4
, H
2
SO
4
, NaNO
3
in Hummers-Offeman
method [28]). GO is a non-stoichiometric compound with
an empirical chemical formula of C
4
O
2
H for well-oxidized
samples [29], although the structure of GO depends on the
type of the oxidation method [30, 31]. Dynamic changes of
its structure have also been suggested [32]. GO has weakly
acidic character [33] owing to O and H containing func-
tional groups which are covalently attached to the carbon
skeleton [29, 34]. Typical functional groups of GO are –OH,
cyclic ether, C=O and COOH. Nevertheless, non-oxidized
aromatic regions and/or isolated C=C double bonds are also
characteristic for the structure of GO. However, O-contain-
ing functional groups provide relatively hydrophilic char-
acter of GO [35] so unlike graphene it is easy to use in wet-
chemistry [36]. The GO is built up from hydrophilic, stacked
graphene-based sheets, exfoliated GO is often added to TiO
2
in aqueous dispersions [37, 38] or used as a “base board”
for TiO
2
preparation from different Ti precursors [39–41]
in order to get reduced graphene oxide (R-GO) containing
composites after reduction processes [42–45]. Hydrothermal
treatment of the GO-TiO
2
(i.e. treatment with the mixture
of water and ethanol at elevated temperature in autoclave)
also leads to the reduction of the GO, with the formation
of R-GO/TiO
2
composite materials [38, 46]. During the
hydrothermal treatment removal of O containing functional
groups of GO has been observed [47] which has led to re-
establishment of conjugated graphene network [48] along
with the possible appearance of C–O–Ti bonds [46, 47, 49,
50]. Composites prepared from GO and TiO
2
have been
found beneficial for hydrogen producing reactions [42, 49].
Several works have reported a synergism between reduced
graphene oxide (R-GO), TiO
2
and Pt [26, 51].
The key point to taking the benefits of GO is in its exfo-
liation. Different methods, such as sonication, thermal treat-
ment, microwave treatment, etc., can be used for the above
purpose [36]. Sonication seems to be a clean and comfort-
able method to separate the GO sheets from each other in
case of Hummers’ GO, however sonication itself is not sat-
isfactory for exfoliation of Brodie’s GO [25, 30] and slightly
basic conditions are needed to achieve larger degrees of
dispersion [33]. However, the application of graphite oxide
samples synthesized via Brodie protocol may be superior
to Hummers’ GO in terms of purity (metal and sulfur-con-
taining species are absent in Brodie-GO which can have a
detrimental effect related to the activity of catalytic reac-
tions) [30]. Recently, GO/TiO
2
exfoliated nanocomposite
has been prepared from Brodie’s GO by heterocoagulation
[52]. In this method the negative charges on the GO sheets
formed in mild alkaline solution contribute to the exfolia-
tion of GO and to the nanocomposite formation driven by
the emergence of electrostatic interaction between GO and
TiO
2
colloidal particles, which possess a net positive surface
charge density in aqueous electrolyte solutions below the pH
of the point of zero charge (pH
p.z.c
. = 6.3) [53].
Our goal was to prepare GO/TiO
2
composite type materi-
als from Brodie’s GO by heterocoagulation in order to test
them as catalysts in the methanol photocatalytic reforming
reaction in liquid phase. To our knowledge, it is the first use
of this type of composite in the above reaction. Correla-
tions between the photocatalytic behavior and the interac-
tion of the carbonaceous material and the semiconductor
were sought. Our recent results revealed that the working
conditions of the methanol photocatalytic reforming reaction
may result in significant changes of the structure of certain
metal oxide–semiconductor catalyst systems involving both
the semiconductor [54] and co-catalyst [15] compared to
the fresh state. Therefore we characterized both the fresh
and recovered samples by bulk and surface characteriza-
tion methods such as diffuse reflectance UV–Vis,
13
C MAS
NMR, ATR-IR spectroscopy and XPS.
2 Experimental
2.1 Materials
Nanocomposites were prepared using commercially avail-
able P25 TiO
2
(Evonik, Germany). This solid contains 75%
anatase and 25% rutile with a specific surface area (a
S
BET
) of
50m
2
/g and its average primary particle size is 30nm (manu-
facturer data). GO was synthesized from natural flaky graph-
ite (Graphitwerk Kropfmühl AG, Germany) by the Brodie
method. This sample is highly oxidized (C
2
O
0.98
H
0.40
), and
it is identical with that codenamed as GO-2 in an earlier
publication [29]. Sodium hydroxide, hydrogen chloride
used for pH setting and methanol, absolute ethanol solvent
were products of Reanal (Hungary). Double distilled water
(18MΩ) was used in every experiment.

Topics in Catalysis
1 3
2.2 Preparation ofPhotocatalysts
A series of GO/TiO
2
nanocomposite type photocatalysts
with various GO content (1, 2, 10wt%) were prepared
from aqueous dispersions of P25 TiO
2
and exfoliated GO
by heterocoagulation as described before [52]. Briefly,
40–400mg of GO samples was dispersed in 1.6L of
water. In each case, the pH was adjusted to 8.5 ± 0.3.
After 15min of sonication and one day of continuous stir-
ring in the dark the pH was reset to 8.5 ± 0.3. Next, these
suspensions were poured into 0.4L of 9–9.9g/L aque-
ous colloid dispersions of TiO
2
(pH 5) upon which the
system coagulated in several seconds. However, we must
note that the composites, unlike to the reported cases in
ref [52], settled completely only after the first washing
step and the supernatants remained slightly turbid even at
larger GO concentrations when they were progressively
washed from the electrolyte contaminations. The reason
might have been the ageing of the GO samples, resulting
in incomplete exfoliation. After decantation or centrifuga-
tion the wet sediment was dried at 50°C and then crushed
into powder. Blank TiO
2
was obtained by the same proce-
dure in the absence of GO.
In order to simulate the effect of each assumed pro-
cess which can contribute to the structural changes of the
heterocoagulated GO/TiO
2
composites under the condi-
tions of the photocatalytic reaction, samples for model
experiments were prepared by certain treatments of the
fresh GO/TiO
2
composite. In order to simulate the physi-
cal removal of GO sheets from the composites and their
rupture, the fresh sample of 2wt% GO/TiO
2
was stirred
for a week at room temperature in the aqueous metha-
nol reaction mixture in dark. This agitated sample was
denominated as A-GO/TiO
2
and it was used in the same
reaction mixture in which it was prepared. Removal of
functional groups and reduction of carbon skeleton can
also occur in the methanol solution during the photoin-
duced reaction. In order to get a model sample represent-
ing this process the fresh composite of 2wt% GO/TiO
2
underwent a hydrothermal treatment at 120°C for 24h
in an N
2
rinsed autoclave in ethanol/water 1:1 mixture
similarly to the procedure used for composite preparation
from Hummer’s GO [46]. This hydrothermally treated
sample was denominated as HT-GO/TiO
2
. Model sam-
ple AHT-GO/TiO
2
was obtained by agitation of HT-GO/
TiO
2
and it also was used in the same reaction mixture
in that it was stirred. In order to show the sole effect of
UV irradiation during the photoinduced reaction the dry
sample of the fresh 2wt% GO/TiO
2
was exposed to UV
irradiation (430nm) for 2h. The UV irradiated model
sample was denominated as UV-GO/TiO
2
.
2.3 Characterization ofthePhotocatalysts
ATR-IR spectra were recorded by the means of a Varian
2000 (Scimitar Series) FT-IR spectrometer (Varian Inc, US)
equipped with an MCT (Mercury–Cadmium–Telluride)
detector and with a ‘Golden Gate’ diamond single reflec-
tion ATR unit (Specac Ltd, UK). 64 scans were collected
at a spectral resolution of 4cm
−1
. Baseline correction was
performed using the GRAMS/AI (7.02) software (Ther-
moGalactic Inc., US).
Solid state NMR magic angle spinning (MAS) spectra of
the fresh and recovered samples were recorded on a Varian
NMR System (Varian Inc., Palo Alto, CA, U.S.A.) operat-
ing at
1
H frequency of 400MHz with a Chemagnetics T3
4.0mm narrow bore double resonance probe. The
1
H MAS
spectra were measured with a rotor spinning rate of 12kHz.
The
1
H π/2 pulse was 3µs and a repetition delay of 30s was
used. For the
13
C CP MAS (cross-polarization magic angle
spinning) [55] spectra 20,000 transients were recorded with
2ms of contact time with SPINAL-64 decoupling [56] and
5s of recycle delay. The direct polarization
13
C MAS spec-
tra were recorded with 60s of relaxation delay and 20,000
transients were collected. For the
13
C spectra adamantane
was used as external chemical shift reference (38.55 and
29.50ppm), π/2 pulse lengths were 3.0µs for carbon and
3.0µs for the proton channel. For the
13
C measurements
a rotor spinning rate of 8kHz were used. The measuring
temperature was 20°C in all cases.
X-ray photoelectron spectroscopy (XPS) measure-
ments were carried out using an EA125 electron spectrom-
eter manufactured by OMICRON Nanotechnology GmbH
(Germany). The photoelectrons were excited by MgKα
1253.6eV radiation. Spectra were recorded in the Constant
Analyzer Energy mode of the energy analyzer with 30eV
pass energy resulting in a spectral resolution of around 1eV.
For XPS experiments the samples in the form of fine powder
were suspended in isopropyl alcohol. Drops of this suspen-
sion were placed on standard OMICRON sample plates;
after evaporation of the solvent catalyst coatings with suf-
ficient adhesion and electric conductivity were obtained.
Effects of possible electric charging were compensated by
adjusting the binding energy of the Ti 2p
3/2
peak to 458.8eV
(consensual value for TiO
2
[42]). Chemical states of the ele-
ments were deduced from high resolution spectra using XPS
databases [57, 58]. Quantification was performed using com-
bination of CasaXPS [59] and XPS MultiQuant [60].
2.4 Photocatalytic Hydrogen Generation
The photocatalytic reaction was carried out in liquid phase
in a reactor system of 10 quartz glass units equipped with
magnetic stirrers, gas inputs and outputs as described before
[15]. The size of the cylindrical glass units were: 60mm in

Topics in Catalysis
1 3
height and 140mm in diameter. Nitrogen gas with 20mL/
min flow rate was continuously bubbled through all reactor
units in parallel. Gas outlets were connected to the gas chro-
matograph (GC) via a ten position selector valve. According
to blank experiments all of the reactor units were equivalent
in terms of the catalytic activity. In case of kinetic measure-
ments one channel mode was used. The initial concentra-
tion of methanol was 6 v % in distilled water. It has been
known that the rate of hydrogen generation versus methanol
concentration relationship gives a saturation curve and use
of diluted solution is favorable [16]. The reaction was car-
ried out at room temperature. The amount of catalyst and
the reaction volume in every unit was 0.100g and 280mL,
respectively. Osram HQL de luxe 125W lamps were used
as light sources operated in UV–Visible region. The reaction
was monitored for 4h. Hydrogen formation was followed
by GC analysis of the outlet gas upon using SUPELCO
Carboxen 1010 column, TCD and FID detection and argon
internal standard. The calculation of the H
2
formation rate
was based on the results obtained by TCD. FID was used
to monitor the methanol signal, sudden decrease of which
indicated if clogging from the water appeared in the gas
tube system.
After the photocatalytic reaction, the samples were recov-
ered from the aqueous methanol solution by centrifugation,
washing with 3 × 50mL absolute ethanol followed by drying
under N
2
flow.
3 Results andDiscussion
3.1 Behavior oftheNanocomposites Obtained
byHeterocoagulation
As products, only H
2
and CO
2
could be detected by TCD in
the outlet gas. Although the liquid phase was not analyzed, it
is known that methanol is photooxidized to CO
2
via the for-
mation of the stable intermediates [4]. Consequently, pres-
ence of formaldehyde, formic acid, methyl formate, and CO
2
can be supposed in the diluted aqueous solution besides the
starting MeOH. Regarding the photocatalytic behavior in the
methanol reforming reaction, the introduction of GO into
the composite increased the catalytic activity about three-
to-fourfold compared to the blank TiO
2
while unsupported
GO led to H
2
formation at the detection limit (see Fig.1).
This increase is slightly less than that reported by El-Bery
etal. [61]. The H
2
evolution showed a maximum as a func-
tion of the GO content, similarly as reported before [42,
49]. For comparison, significantly smaller hydrogen forma-
tion rates were observed with 1.0wt% Ag/TiO
2
reference
catalysts prepared by sodium borohydride reduction, while a
0.5wt% Pt/TiO
2
reference catalyst obtained by impregnation
and subsequent reduction in H
2
gave outstandingly better
results (see data of H
2
formation and details of preparation
of reference catalysts in the Electronic Supplementary Mate-
rials). We must note here also that, for the fair comparison
of activities expressed in mmolg
−1
h
−1
dimension, the spe-
cific surface area of the catalysts needs to be similar to each
other. Here, this criterion is fulfilled because the composite
particles are characterized by a large mass excess of TiO
2
.
Although graphite oxide particles have a large specific sur-
face area at high degrees of exfoliation, the surface area of
the composites must fall close to that of the TiO
2
particles
(a
S
BET
= 50m
2
/g) that completely cover the surface of the
delaminated carbon sheets at low GO loadings, as shown
in our previous publication [52]. Moreover, although the
total surface area may be slightly higher than for the bare
particles, the reactive surface area is only represented by the
TiO
2
particles, which is exactly the same for both the “TiO
2
blank” and for the nanocomposite samples irrespectively of
their phase ratios.
During the photocatalytic reaction significant color
change (blackening) of the GO/TiO
2
composites was
observed. The UV–Visible spectroscopic measurements
of fresh and recovered composite samples confirmed the
darkening (see Fig.1SA in the Electronic Supplementary
Materials). The absorbance of the fresh GO/TiO
2
samples
was proportional to the carbon content (see Fig.1SA in the
Electronic Supplementary Materials) as has been described
in the literature [26, 49]. At the same time the color change
of pristine TiO
2
and blank TiO
2
(see diffuse reflectance
UV–Vis spectra in Fig.1SB in the Electronic Supplemen-
tary Materials) during the irradiation in methanol solution
was negligible. The slight grey color can be explained by the
organic deposits picked up from the solution. According to
photoelectron spectroscopy measurements, no reduction of
the TiO
2
particles is evident after the photocatalytic reaction,
thus the significant blackening of the recovered composite
type samples indicates that structural changes of the GO/
TiO
2
composites occurred during the reaction and these
changes must be related to the GO part.
In order to map the supposed structural/chemical changes
of GO, ATR-IR spectra of fresh (Fig.2A) and recovered
Fig. 1 Rate of H
2
formation after 240min reaction time

Topics in Catalysis
1 3
samples (Fig.2B) were recorded. It has to be noted that
unsupported GO and 10wt% GO/TiO
2
had rather dark color,
resulting in a heavy baseline shift so all the discussed spectra
underwent baseline correction. It can be safely reported that
composite preparation by heterocoagulation method resulted
in only little changes in the structure of GO, as all major
spectral features are observable after the procedure (cf. lines
a and b in Fig.2A). In details, pure GO exhibits several
bands corresponding to oxygen containing surface species
like –OH groups (v-OH stretchings around 3735, 3600 and
3460cm
−1
), carbonyl group (νC=O at 1711cm
−1
) and car-
boxylates (νCOO at 1641 and 1367cm
−1
). The latter may
be overlapped with C=C stretching of aromatic rings. The
bands between 1000 and 800cm
−1
belong to C–O stretch-
ing vibrations of C–OH and C–O–C species. The spectrum
resembles that of pure TiO
2
; only very small bands referring
to oxygen-containing surface species can be witnessed (sur-
face –OH and surface carbonates/carboxylates at 3734cm
−1
and at 1419, 1123 and 1061cm
−1
, respectively).
As far as the recovered samples are concerned, lit-
tle changes are seen in the case of the pure GO. On the
other hand, in the case of the used GO/TiO
2
catalysts, sev-
eral bands indicate species derived from adsorbed MeOH/
organic compounds on TiO
2
(Fig.2B, lines b, c and d) which
resulted in difficulty in the interpretation of the spectra.
Accordingly, ATR-IR analysis of the recovered samples
was not really informative for the changes of the supported
GO but it is important to mention that newly formed OH
groups (at 3736 and 3685cm
−1
) in the GO containing recov-
ered samples were well-marked see lines b and c in Fig.2B.
It seems reasonable that the GO activates the TiO
2
surface.
In order to get further information exclusively about
the carbonaceous part of the fresh and recovered samples,
13
C solid state MAS NMR technique was chosen. Because
of the low carbon content of the samples only the com-
posite sample of 10wt% GO/TiO
2
and unsupported GO
were investigated in detail. Direct polarization spectra of
fresh GO/TiO
2
composite and bulk GO are very similar
to each other (cf. spectrum a and b in Fig.3A) confirm-
ing the finding with ATR-IR, i.e. no significant structure
change of GO appeared during the composite preparation
by heterocoagulation. Characteristic functional groups on
Fig. 2 ATR-IR spectra of fresh (A) and recovered samples (B). a GO;
b 10wt% GO/TiO
2
; c 2wt% GO/TiO
2
; d TiO
2
Fig. 3 MAS NMR spectra of GO and GO/TiO
2
composite samples.
A
13
C MAS NMR spectra; B
13
C MAS NMR cross polarization spec-
tra; C
1
H MAS NMR. a GO; b fresh 10wt% GO/TiO
2
; c recovered
10wt% GO/TiO
2

Frequently asked questions

Q1. What are the contributions in "Graphite oxide-tio2 nanocomposite type photocatalyst for methanol photocatalytic reforming reaction" ?

In this paper, Tálas et al. used graphite-oxide/TiO2 composite materials to test catalysts in the methanol photocatalytic reforming reaction in liquid phase. 

Q2. What are the advantages of the co-catalysts?

The advantages of the co-catalysts are attributed to the suppressed charge recombination; promoted charge separation and transport driven by junctions/interfaces [19–21]. 

Q3. What is the promising approach for transforming solar energy into chemical energy for storage and transport?

Photocatalytic hydrogen production is a promising approach for transforming solar energy into chemical energy for storage and transport. 

Q4. What is the key factor for the beneficial co-catalytic effect of the GO?

Model experiments with specially pre-treated photocatalysts suggested that in situ development of a strong chemical coupling between the reduced graphene oxide sheets and TiO2 may be the key factor for the beneficial co-catalytic effect of the GO. 

Q5. What is the method for reducing graphene oxide?

In addition, the UV-assisted photocatalytic reduction of graphene oxide for preparation of R-GO/TiO2 composites is a well-known method [66], during which TiO2 and GO is irradiated in a slurry in a reductive medium i.e. ethanol. 

Q6. What is the criterion for the GO/TiO2 composites?

According to photoelectron spectroscopy measurements, no reduction of the TiO2 particles is evident after the photocatalytic reaction, thus the significant blackening of the recovered composite type samples indicates that structural changes of the GO/ TiO2 composites occurred during the reaction and these changes must be related to the GO part. 

Q7. What is the important contribution of graphitic carbon to the methanol?

In the spectra of the hydrothermally treated catalysts both in the fresh and the recovered state the dominant contribution arises from graphitic carbon; its binding energy around 284.0 eV suggests the pronounced coupling of the carbon skeleton to TiO2 during the treatment. 

Q8. How many eV were recorded in the energy analyzer?

Spectra were recorded in the Constant Analyzer Energy mode of the energy analyzer with 30 eV pass energy resulting in a spectral resolution of around 1 eV. 

Q9. What is the effect of the presence of alcohol on the structure of the electric double layer?

The presence of the alcohol changes both the dielectric and solvation properties of the medium and causes a significant effect on the structure of the electric double layer developed at the surface of charged colloidal particles. 

Q10. What is the reason why the GO is an important material?

the GO may become an important material which could make the photocatalyst cheaper; that’s why understanding its behavior in composites is essential. 

Q11. What is the main contribution to the spectra of the fresh samples?

This analysis suggests that the main contribution to the spectra of the fresh samples in both the 2 and 10 wt% cases comes from a double-peaked feature with maxima at 284.3 and 286.3 eV, which corresponds to the C–C and C–O peaks from the spectrum of GO. 

Q12. What was the effect of the functional groups in the organic substances?

The factor of photocatalytic activity enhancement was strongly affected by the various functional groups appearing in the organic substances. 

Q13. How was the effect of possible electric charging compensated?

Effects of possible electric charging were compensated by adjusting the binding energy of the Ti 2p3/2 peak to 458.8 eV (consensual value for TiO2 [42]). 

Q14. Why did the GO/TiO2 samples have a heavy baseline shift?

Because of the low carbon content of the samples only the composite sample of 10 wt% GO/TiO2 and unsupported GO were investigated in detail. 

Q15. What is the effect of the agitation on the activity of the composite?

Photocatalytic behavior of the model system prepared by agitation in the non-illuminated reaction mixture [A-GO/ TiO2 (Fig. 6)] indicated that long term stirring of the slurry in dark had significant negative effect on the activity of H2 formation in accordance with the supposed mechanical disintegration of the composite. 

Q16. What is the method to get more information about the carbonaceous part of the fresh and?

In order to get further information exclusively about the carbonaceous part of the fresh and recovered samples, 13C solid state MAS NMR technique was chosen.