J Mater Sci E N E R G Y Mmaterials ATERIALS Energy
3D flowerlike TiO2/GO and TiO2/MoS2 heterostructures with enhanced photoelectrochemical water splitting Hongxia Li1,*, Wei Dong1, Junhua Xi1, Gang Du1, and Zhenguo Ji1,* 1
College of Materials and Environmental Engineering, Hangzhou Dianzi University, Hangzhou 310018, China
Received: 24 November 2017
ABSTRACT
Accepted: 17 January 2018
TiO2 nanoflowers modified with MoS2 and GO nanosheets are prepared by a one-step hydrothermal method followed by a dip-coating process. The morphology, structure and composition of the samples are investigated by XRD, SEM and XPS, respectively. The photoelectrochemical (PEC) measurement reveals that the modified samples exhibit obvious improved photocurrent density. The enhancement can be attributed to the higher absorption in visible light region and faster charge transmission as analyzed by the UV–Vis absorption, photoluminescence spectra and electrochemical impedance analysis. This facile method provides a promising low-cost way to enhance the PEC performance of TiO2-based photoanodes.
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Springer Science+Business
Media, LLC, part of Springer Nature 2018
Introduction Photoelectrochemical (PEC) water splitting is a sustainable and clean approach for producing hydrogen by converting the solar energy to chemical energy [1, 2]. To obtain high solar-to-hydrogen (STH) efficiency, many semiconductors have been investigated for PEC application [3, 4]. Among them, TiO2 has been widely studied as one of promising semiconductor photoanodes due to the suitable band structure, low cost, excellent chemical and physical stability, non-toxicity [5, 6]. However, there still exist some barriers hindering the pure TiO2 in large-scale PEC application [7–10]. One side, the wide band gap (3.2 eV) of bare TiO2 limits the absorption of the solar light that only the photons in ultraviolet region can be absorbed. Moreover, the high recombination loss of photogenerated electron–hole pairs decreases the
STH efficiency. Therefore, effective approaches are essential for narrowing the band gap and restraining the recombination loss of photogenerated carriers. In previous works, 1D TiO2 nanorod arrays, nanowires and nanotubes have been extensively studied [11–13]. Recently, 3D TiO2 nanostructures like TiO2 nanoflowers (TiO2 NFs) or branched TiO2 nanowires have attracted much attention with lager specific area and enhanced electrode/electrolyte interface compared to 1D nanowires [14–16]. To date, various strategies have been studied to improve the PEC performance of TiO2-based photoelectrodes such as doping [15, 17, 18], surface decoration [19, 20], forming heterojunction [21–24] and controlling the morphology of nanostructure [14, 20, 25]. Graphene oxide (GO) is considered to be a promising carbon material, and it is hydrophilic [26–28]. In addition, numerous oxygen-containing
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groups can be contributed to the combination with TiO2, which is superior to be applied in photoanodes for PEC water splitting compared with grapheme [29, 30]. During the past few years, many studies have reported the GO sheets could serve as the electron sink, which is helpful to store the photogenerated electrons and facilitate the separation of photoexcited electrons and holes [31–33]. Therefore, many studies focused on the nanostructured photoelectrodes decorated with GO sheets were conducted and the enhancement of PEC performance in visible light region was achieved [29, 34, 35]. On the other hand, MoS2 has been extensively investigated as a layered transitional metal sulfide with a narrow band gap and ideal morphologies [36–38]. In particular, TiO2/MoS2 nanocomposites have excellent photoresponse in visible region compared with pure TiO2 [10, 39–41]. However, the efficiency of TiO2/MoS2 is still low which may be owning to the unoptimized geometrical configuration of MoS2 on TiO2 surface [42, 43]. Herein, we report a 3D hierarchical flowerlike TiO2 nanostructure decorated with GO (TiO2 NFs/GO) and MoS2 (TiO2 NFs/MoS2) by a simple one-step hydrothermal method followed with a dip-coating process, respectively. To our knowledge, the 3D TiO2 NFs/GO and TiO2 NFs/MoS2 nanocomposites fabricated with this facile preparing process are rarely reported. The PEC performance is greatly improved compared with the bare TiO2 nanoflowers. The modified PEC mechanism of the nanocomposites is also proposed.
Experimental details Synthesis of the TiO2 nanoflowers Fluorine-doped tin oxide (FTO)-coated glasses (2 cm 9 1.5 cm) were ultrasonically washed and dried in air at 60 °C. 10 mL ethanol, 20 lL hydrochloric acid (HCl, wt = 37%) and 0.7 mL titanium tetraisopropanolate (TTIP) were stirring continuously for 20 min to form the gel, and then the gel was spin-coated onto the FTO substrates. Immediately, the substrates were dried at 80 °C for 1 h. Subsequently, the TiO2 seed films were annealed in air at 500 °C for 30 min with the temperature ramp of 15 °C/min during heating.
The TiO2 NFs were grown onto the seed substrates via a facile hydrothermal method. In detail, 10 mL HCl, 10 mL deionized water and 0.15 mL TTIP were mixed and ultrasonically dispersed for 20 min and then transferred into the 100-mL Teflon-lined stainless steel autoclave. The substrates were immersed in it with the seeds facing up. Then, the autoclave was heated to 170 °C and maintained for 3.5 h. After cooling down to room temperature, the obtained TiO2 NFs were washed with deionized water and dried in air. Finally, the TiO2 NFs were annealed at 500 °C for 1 h in air with the temperature ramp of 15 °C/min during heating.
Preparation of MoS2 nanosheets The MoS2 nanosheets were synthesized via a simple hydrothermal process. In particular, 300 mg two-hydrated sodium molybdate (Na2MoO42H2O) and 600 mg thiourea (CN2H4S) were dissolved in 60 mL deionized water for 20-min stirring, then the mixed solution was transferred into 100-mL autoclave and kept at 200 °C for 24 h. Eventually, the obtained MoS2 powder were washed with deionized water and dried in air at 80 °C for 24 h.
Fabrication of TiO2 NFs/GO and TiO2 NFs/ MoS2 by dip-coating process TiO2 nanoflowers were decorated with GO and MoS2 nanosheets by a facile dip-coating method. The GO solution is purchased without further purification (Tangushangxi limited company, Hangzhou, China). Typically, 0.1, 0.3, 0.5, 0.8 g GO solution and 1.0 g MoS2 powder were dispersed in 10 mL deionized water for 30-min ultrasonic oscillation, respectively. The as-prepared TiO2 NFs were dipped in the solution and kept for 1 h. Then, the samples were taken out and dried at 80 °C for 30 min in air. The TiO2 NFs/GO and TiO2 NFs/MoS2 nanocomposites were obtained and denoted as TiO2 NFs/GO-x (where x = 0.1, 0.3, 0.5, 0.8 g representing the amount of GO solution dispersed in deionized water) and TiO2 NFs/MoS2-1.0.
The characterization and the PEC measurements The structural properties were determined by X-ray diffractometer (XRD, TD3500, TongDa instrument)
J Mater Sci
over 2h range from 10° to 70°. X-ray photoelectron spectroscopy (XPS) measurements were made on a Kratos AXIS Ultra DLD (Kratos, Japan) spectrometer with Al target to gain information on the chemical binding energy of the photoanodes. The C 1s peak at 284.8 eV of the adventitious carbon was referenced to rectify the binding energies. The microstructure and morphology of the as-prepared samples were characterized by scanning electron microscopy (FESEM, ULTRA 55) and high-resolution transmission electron microscopy (HRTEM, GATAN 832). The diffused reflectance spectrum (DRS) was collected by UV–Vis spectrophotometer (Shimadzu, MPC-3100, Japan) with an integration sphere. Photoluminescence (PL) spectra of the samples were detected with a fluorescence spectrophotometer (Shimadzu, RF-530TPC, Japan) from 200 to 600 nm using the 300-nm line of a Xe lamp as the excitation source at room temperature. PEC measurement was conducted in a three-electrode mode with an electrochemical analyzer (CHI760E, CH instrument, China). In detail, the asprepared samples were used as working electrodes, a Pt foil as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. 1 M NaOH aqueous solution was used as the electrolyte. All photoelectrodes were illuminated under a 300-W Xe lamp with the light intensity of 100mW/cm2. The electrochemical impedance spectrum (EIS) was measured by applying a bias of the open-circuit potential over the frequency from 105 to 1 Hz under AM1.5G illumination. Mott–Schottky plots were recorded from the impedance-potential (IMPE) test from - 1 to 0.2 V at 1000 Hz without light irradiation.
Results and discussion XRD patterns of all the prepared samples are shown in Fig. 1 to exhibit the phase structures. The peaks at 27.4°, 36.1°, 41.2° and 54.3° correspond to (110), (101), (111) and (211) faces of rutile TiO2 (JCPDS #21-1276), respectively, while the peak at 62.6° is well matched with the (204) face of anatase TiO2 (JCPDS #21-1272). The TiO2 NFs modified with different content of GO solution from 0.1 to 0.8 g show no peak ascribed to GO due to comparatively low loading amount of GO. For the sample of TiO2 NFs/MoS2-1.0, two peaks of MoS2 appeared at 14.1° and 33.6°, corresponding to (002) and (101) planes of MoS2 (JCPDS #73-1508). XPS was employed to analyze the chemical states and the bonding configuration for the as-prepared samples as shown in Fig. 2. In Fig. 2a, the binding energies located at 464.3, 458.8 eV are assigned to the Ti 2p1/2, Ti 2p3/2 spin–orbital splitting photoelectrons in Ti4? state [30], respectively. Figure 2b shows the C 1s XPS spectra of TiO2 NFs/GO composites deconvoluted into three peaks. The peak at 284.0 eV is attributed to adventitious carbon (C–C/C=C), while the peaks at 286.5 and 289.0 eV are assigned to oxygenated carbon species of C–O and C=O from GO, respectively [44, 45]. The existence of oxygen-containing carbon in GO can provide active sites for directed connection with TiO2 NFs. The XPS spectra of Ti 2p of TiO2 NFs/MoS2 composites are shown in Fig. 2c, which locate at 463.8 and 458.1 eV corresponding to Ti 2p1/2 and Ti 2p3/2 of TiO2, respectively. As shown in Fig. 2d, the peaks located at 232.7, 229.4 eV are ascribed to Mo 3d3/2, Mo 3d5/2,
Hydrogen and oxygen production measurement The photoelectrocatalytic H2 and O2 evolution experiments were performed on gas chromatography (GC-7806) tests combined with a Labsolar 6A online photoelectrocatalytic analysis system (Perfect Light). The production amount of gases was measured by using a gas chromatograph with high pure Ar as the carrier gas. 300-W Xe lamp with the light intensity of 100 mW/cm2 was used as the light source with AM1.5G filter. The electrolyte was 1 M NaOH solution, and the photoanode of TiO2, TiO2/GO and TiO2/MoS2 was applied a bias of 0.5 V versus SCE. Figure 1 XRD patterns of all the TiO2 NF-based samples.
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Ti 2p3/2
Ti 2p1/2
450
455
460
465
C C C=O
470
280
Binding energy (eV)
(c)
Ti 2p3/2
(d)
460
465
295
Mo 3d5/2
470
475
Mo 3d3/2
222
Binding energy (eV)
(e)
290
Intensity (a.u.)
Intensity (a.u.)
455
285
Binding energy (eV)
Ti 2p1/2
450
C O
(b) Intensity (a.u.)
(a) Intensity (a.u.)
Figure 2 XPS spectra of a Ti 2p of TiO2 NFs/GO-0.3, b C 1s of TiO2 NFs/GO-0.3, c Ti 2p of TiO2 NFs/MoS2-1.0 and d Mo 3d of TiO2 NFs/MoS21.0, e C 1s of TiO2 NFs/MoS21.0 sample and f Mo 3d of TiO2 NFs/MoS2-1.0 after 1 h J–V curves test under AM1.5G illumination in 1 M NaOH.
225
228
231
234
237
Binding energy (eV)
(f)
C-C
Mo 3d3/2
Intensity (a.u.)
Intensity (a.u.)
Mo 3d5/2
270
275
280
285
290
Binding Energy (eV)
respectively. The spin–orbit separation of Mo is 3.3 eV, which suggests that the Mo in the nanocomposites is present as Mo4? [25]. The C 1s peak of TiO2 NFs/MoS2 sample is shown in Fig. 2e. It can be seen that there is only C–C bonds located at 284 eV and no other oxidized peaks located at higher binding energy can be found. So it is reasonable that the C–O or C=O peaks of TiO2 NFs/GO come from GO. In order to testify the stability of MoS2, we studied the Mo 3d XPS peak of TiO2 NFs/MoS2-1.0 after 1 h J– V curves test under AM1.5G illumination in 1 M NaOH as is shown in Fig. 2f. It can be found that the Mo 3d3/2 and Mo 3d5/2 peaks located at 232.68 and 229.38 eV, respectively, after 1-h PEC test. The spin– orbit separation doesn’t change at all, which suggests that the MoS2 isn’t oxidized at the present condition. As for the GO (graphene oxide), it has many oxygen-
295
300
225
228
231
234
237
240
Binding energy (eV)
containing functional groups so it can be hardly oxidized during the water splitting. Figure 3 displays the SEM images of various samples with 3.5-h hydrothermal reaction time. The topview and the cross-sectional SEM images of pure TiO2 NFs (Fig. 3a, b) suggest the dense 3D hierarchical flowerlike TiO2 nanostructures uniformly grown onto the TiO2 nanorod arrays. The 3D TiO2 nanoflowers enlarge the surface roughness and the specific area. The inset of Fig. 3a shows the diameter of single tetragonal nanorod of the TiO2 NFs is about 200 nm. From Fig. 3b, the thickness of the nanoflowers layer is about 30 lm, and the length of nanorods composing the nanoflowers is estimated to be 4 lm. The morphology of TiO2 NFs/MoS2-1.0 is presented in Fig. 3c, d. It is obvious that the MoS2 nanosheets are well embedded between the nanorods
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Figure 3 SEM images of bare TiO2 NFs (a, b), TiO2 NFs/ MoS2 (c, d), TiO2 NFs/GO (e, f) and HRTEM images of TiO2 NFs/GO (g, h).
of TiO2 NFs. Moreover, the nanocomposite architecture is well preserved by the dip-coating process as shown in Fig. 3d. Figure 3e, f displays the images of TiO2 NFs/GO-0.3. From Fig. 3e, some TiO2 nanorods are muffled by the curved GO sheets distributed onto the TiO2 NFs. In detail, the high magnification SEM is shown in Fig. 3f. The TiO2 nanorods are partly or totally wrapped by the GO sheets. Further, it is clear that the GO sheets have a flake-like structure with wrinkles and folds, which is corresponding to the previous works [46, 47]. It is a feature of GO sheets
when they are not conformally coated on the surface of TiO2 NFs [34, 48] . Furthermore, stacked GO sheets with an interlayer spacing of 0.48 nm were observed. TiO2 NFs showed the lattice fringes with lattice spacing of 0.32 nm, which was corresponding to the (110) plane of rutile TiO2 [29]. To investigate the light absorption properties of the as-prepared samples, the UV–Vis diffuse reflectance spectra (DRS) are shown in Fig. 4a. A sharp absorption edge is observed at about 395 nm which corresponds to the band edge of TiO2. After coupling with
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(a)
(b)
(c)
Figure 4 a Absorption spectra of different samples, b Tauc plots of different samples and c photoluminescence emission spectra of different samples.
GO or MoS2 sheets by dip-coating process, the absorption edge is red shifted. Moreover, the surface modification improves the light absorption compared to bare TiO2 NFs, and the TiO2 NFs/MoS2-1.0 shows the best enhancement in absorbance. Notably, the visible light absorption of the modified TiO2 NFs samples is obviously enhanced with GO content increasing from 0.1 to 0.5 g. The light absorption decreases when the GO content further increases to 0.8 g, which is caused by the excess of GO sheets decoration obstructing the light absorption. The band gaps (Eg) of TiO2 NFs, TiO2 NFs/GO-x and TiO2 NFs/MoS2-1.0 are estimated from the following equation and presented in Tauc plots [49] (Fig. 4b). aðhmÞ ¼ CðEg hmÞm=2
ð1Þ
where C is a constant, a is the absorption coefficient, hm is the photon energy and m = 1 for the direct transition between bands. The band gap is about
3.02 eV for pure TiO2 NFs. This value decreases to 2.93, 2.86 and 2.81 eV for TiO2 NFs/GO-0.1, TiO2 NFs/GO-0.3 and TiO2 NFs/GO-0.5, respectively, which further confirms the red shift and visible light response for the modified TiO2 NFs. However, the band gap increases to 2.85 eV for the TiO2 NFs/GO0.8. The photoluminescence (PL) measurement is performed to understand the efficiency of charge carrier trapping, migration and separation in the photoanodes since the PL emission spectra are obtained from the recombination of free electrons and holes [50, 51]. From Fig. 4c, the trend of PL emission intensity is observed in the following order: TiO2 NFs [ TiO2 NFs/GO-0.1 [ TiO2 NFs/GO-0.3 [ TiO2 NFs/GO0.8 [ TiO2 NFs/GO-0.5 [ TiO2 NFs/MoS2-1.0. It is obvious that the TiO2 NFs decorated with GO or MoS2 sheets show apparent decreased PL peak intensities compared with the bare TiO2 NFs, which
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indicates lower recombination rate of electrons and holes under UV light irradiation. In other words, it implies that the surface modification facilitates the carrier transfer and separation dramatically and leads to the PL intensities decreasing as a result, which may also improve the conductivity and carrier density of the photoelectrode/electrolyte interface. Generally, lower recombination of the photogenerated electrons and holes is prerequisite for higher PEC performance. Figure 5a displays the J–V curves obtained from linear voltammetry sweeps (LSV) of pure TiO2 NFs, TiO2 NFs/GO-x and TiO2 NFs/MoS2-1.0 under the simulated AM1.5G illumination in 1 M NaOH (ph = 13.6). The dark scan of bare TiO2 NFs only exhibits a photocurrent density of lA cm-2 scale (the dotted line in Fig. 5a). The highest photocurrent density of 0.58 mA/cm2 at 0 V versus SCE is observed for TiO2 NFs/MoS2-1.0 among all samples. In addition, TiO2 NFs/GO-0.5 shows the photocurrent density of 0.24 mA/cm-2 at 0 V versus SCE, which are much higher than that of other TiO2 NFs/ GO samples. The enhancement of photocurrent density can be ascribed to the improved light absorption and more effective carrier transmission by MoS2 and moderate GO decoration. Moreover, the photocurrent density increases with increasing the content of GO from 0.1 to 0.5 g. However, the photocurrent density lowers when it further increases to 0.8 g, which is consistent with the UV–Vis absorption spectrum. The excess of GO sheets on the surface of TiO2 NFs may obstruct the light absorption and depress electrolyte contact areas. The J–t curves of the samples are shown in Fig. 5b with the applied bias of 0.5 V versus SCE. It is obvious that the photocurrent
density of all samples is stable which demonstrates the good anti-photocorrosion and lifetime of the TiO2 NF-based photoanodes. The electrochemical impedance spectra (EIS) measurement is measured at the open-circuit potential condition under AM1.5G illumination to investigate the charge transfer process. The corresponding Nyquist plot is shown in Fig. 6a. The smaller size of the impedance arc reflects the smaller charge transfer resistance at the electrode/electrolyte interface, indicating more effective separation of the photogenerated electron–hole pairs and faster charge transfer process. Compared to other samples, TiO2 NFs/ MoS2-1.0 shows the smallest charge transfer resistance, which implies favorable charge separation and carrier transportation process. Moreover, the TiO2 NFs/GO-0.5 has the lowest charge transfer resistance among all TiO2 NFs/GO-x samples, which indicates the best loading amount of GO sheets. In addition, all the samples modified with GO or MoS2 sheets show decreased charge transfer resistance compared to the bare TiO2 NFs, which can be inferred that the loaded MoS2 and GO sheets play a significant role in increasing the light absorption, accelerating the charge separation and transfer at the interface and thus resulting in better PEC performance. The Mott–Schottky (M–S) plots were recorded at 1000 Hz without illumination as shown in Fig. 6b, c. The positive slopes confirm all the photoanodes are n-type semiconductors. The donor density can be calculated from the following equation [52]: 1 Nd ¼ ð2=e0 ee0 Þ d 1=C2 =dV ð2Þ
Figure 5 a J–V curves of the samples collected under AM1.5G illumination in 1 M NaOH, b J–t Curves measured in 1 M NaOH under AM1.5G illumination.
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Figure 6 a Nyquist plots of samples collected at open-circuit potential in 1 M NaOH under AM1.5G illumination, b Mott–Schottky plots of samples collected at 1000 Hz and c Mott–Schottky plots partly magnified from (b).
where Nd, e0, e, e0 and d(1/C2)/dV represent the donor density, the electron charge, the dielectric constant of TiO2 (Taking 70 for rutile), the permittivity of vacuum and the straight slope, respectively. The calculated carrier densities of TiO2 NFs, TiO2 NFs/GO-0.1, TiO2 NFs/GO-0.3, TiO2 NFs/GO-0.5, TiO2 NFs/GO-0.8 and TiO2 NFs/MoS2-1.0 are 6.13 9 1016, 2.1 9 1017, 3.68 9 1017, 1.63 9 1018, 5.67 9 1017 and 4.48 9 1018 cm-3, respectively. The increased donor density of TiO2 NFs/GO-x and TiO2 NFs/MoS2-1.0 is owing to the enhanced electrical conductivity as well as the charge transfer process, which leads to the improved PEC performance. In addition, the upward shift of Fermi level caused by the increased electron density makes a larger degree of band bending between the surface of TiO2 NFs and GO or MoS2 sheets, which can promote the charge separation at the interface of the photoanodes and electrolyte [11, 53].
Figure 7a presents the H2 evolution curves of pure TiO2 NFs, TiO2 NFs/GO-0.5 and TiO2 NFs/MoS2-1.0, respectively. TiO2 NFs/GO-0.5 and TiO2 NFs/MoS21.0 show a significant increase in the production of H2. The H2 evolution curves of TiO2 NFs/GO-0.5 and TiO2 NFs/MoS2-1.0 nearly shows a linear relation with the irradiation time, which indicates a good stability of the prepared samples during the period of 3 h. The highest production of H2 with the value of 33.52 lmol is obtained for TiO2 NFs/MoS2-1.0 after 3-h measurement. Figure 7b shows the rate of H2 evolution on different samples for comparison. The pure TiO2 NFs show very low H2 evolution activity of 0.574 lmol/h because of fast recombination of photogenerated electron–hole pairs. The TiO2 NFs/GO0.5 sample displays enhanced H2 evolution activity of 5.202 lmol/h, which indicates that a suitable content of GO is crucial to improve the PEC performance. H2 evolution activity of the TiO2 NFs/MoS2-1.0 can
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Figure 7 Measured H2 production as a function of time (a) and H2 evolution rate (b) over different sample measured at 0.5 V versus SCE in 1 M NaOH solution under AM 1.5G illumination.
reach up to 11.17 lmol/h, which is much higher than the pure TiO2 NFs. From the J–t curve in Fig. 5b, the area that the curve covered represents the number of charges which took part in the water oxidation in the scan range. Because :
J ðt Þ ¼
I ðtÞ 1 dQ ¼ A A dt
ð3Þ
where J is the measured current density, A is the exposed active area of photoanode with the value of 1.03 cm2 and Q is the amount of charges transferred through the active surface. Thus, the amount of transferred charges through the photoanode surface for water oxidation is: Q1
t1
Q0
t0
Q ¼ A r dQ ¼ A r J ðtÞdt
ð4Þ
where the Q0 and Q1 are the amount of charges transferring through the active surface corresponding to t0 and t1, respectively. Here, t0 = 0, and t1 = 300 s. Through the integration calculation of the corresponding J–t curves, we have got the values of Q of pure TiO2, TiO2/GO-0.5 and TiO2/MoS2-1.0 during the period of 300 s, which is the quantity of electric charges passed through the circuit under the applied bias of 0.5 V versus SCE. While the actual quantity of electric charge calculated from the amount of produced hydrogen can be calculated with the following equation: Qactual ¼ 2nNA e
ð5Þ
where n stands for the amount of the produced hydrogen under AM 1.5G illumination during the
period of 3 h, NA is the Avogadro constant with the value of 6.023 9 1023/mol and e is the charge amount of one electron with the value of 1.602 9 10-19 C. So the Faradic efficiency ¼ Qactual =36Q
ð6Þ
According to the above equations, the Faradic efficiency is calculated and listed in Table 1. All the photoelectrodes exhibited a Faradic efficiency above 90%, suggesting high PEC activity and stability of these photoanodes. The slight deviation from 100% Faradic efficiency may be caused by some unavoidable gas leakage or dissolution in the electrolyte solution. Based on the above analysis, the possible improved PEC mechanism of the 3D TiO2 nanoflowers decorated with MoS2 or GO sheets is proposed and illustrated in Fig. 8. The conduction band (CB) of TiO2 is located between conduction band and valence band (VB) of MoS2 (GO), forming the type II band alignment. When light irradiates the photoelectrode, the TiO2 NFs and MoS2 nanosheets absorb the photons and generate electron–hole pairs. The photoexcited electrons transfer from the CB of MoS2 to CB of TiO2, then migrate to the conductive FTO substrate and reach the Pt counter electrode to reduce water, while the photoexcited holes transfer to the VB of MoS2 (GO) from that of TiO2. As a result, the photogenerated electron–hole pairs will be efficiently separated at the interface of the TiO2 NFs and MoS2 (GO). Moreover, the TiO2 nanorod arrays between FTO substrates and TiO2 nanoflowers provide direct charge transport paths, which would also reduce the electron–holes recombination loss.
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Table 1 The Faradic efficiency and corresponding values of different samples
Values
n (lmol)
Q (mC)
Qactual (mC)
Faradic efficiency (%)
Sample TiO2 TiO2/GO-0.5 TiO2/MoS2-1.0
1.723676 15.60622 33.52054
10.046 88.372 188.681
332.46409 3010.14 6465.468
91.9 94.61 95.18
Pt
H+
H2 e-1 0
hν e-
e-
Ef
1 2
e-
CB
CB
h+
VB
4
CB
VB
VB
h+
3
hν
O2 H2O
h+ TiO2
MoS2
GO
Figure 8 Schematics illustration of the 3D TiO2 NF/MoS2 (GO) nanosheet photoelectrode configuration and energy diagram of TiO2/ MoS2 (GO) nanocomposite.
Conclusions In summary, the 3D TiO2 nanoflowers modified with GO or MoS2 nanosheets as the photoanode for PEC water splitting have been successfully fabricated by a one-step hydrothermal method followed by a dipcoating process. Compared with pure TiO2 nanoflowers, the decoration with GO and MoS2 nanosheets enlarge the photoresponse especially in visible light region. In addition, the introduction of MoS2 (GO) on TiO2 improves the charge separation and reduces the recombination of photogenerated electron–hole pairs. So, the TiO2/MoS2 (GO) heterostructures show improved PEC performance.
Acknowledgements The financial aids of Zhejiang Provincial Natural Science Foundation of China (Grant No.
LGG18E020004), Chinese National Natural Science Foundation (Grant No. 61704042) and Science and Technology Project of Zhejiang Province (Grant No. 2015C37037) are gratefully acknowledged.
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