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Enhanced visible-light-driven photocatalytic H2-production activity of CdS-loaded TiO2 microspheres with exposed (001) facets
Journal of Physics and Chemistry of Solids 87 (2015) 171–176
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Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Enhanced visible-light-driven photocatalytic H2-production activity of CdS-loaded TiO2 microspheres with exposed (001) facets Bifen Gao n, Xia Yuan, Penghui Lu, Bizhou Lin, Yilin Chen Department of Applied Chemistry, College of Materials Science & Engineering, Huaqiao University, Xiamen 361021, China
art ic l e i nf o
a b s t r a c t
Article history: Received 25 April 2015 Received in revised form 24 July 2015 Accepted 26 August 2015 Available online 28 August 2015
CdS-loaded TiO2 microspheres with highly exposed (001) facets were prepared by hydrothermal treatment of a TiF4–HCl–H2O mixed solution followed by a chemical bath deposition of CdS onto TiO2 microspheres. The crystal structure, surficial micro-structure and photo-absorption property of the samples were characterized by XRD, FE-SEM, TEM and UV–vis diffuse reflectance spectroscopy, etc. The as-prepared samples exhibited superior visible-light-driven photocatalytic H2-production activity from lactic acid aqueous solution in comparison with CdS-sensitized TiO2 nanoparticles, whose surface was dominated by (101) facets. Photoelectrochemical measurement confirmed that (001) facet is beneficial for the transfer of photo-generated electron from CdS to TiO2 microsphere, which led to the unexpected high photocatalytic activity of CdS-loaded TiO2 microspheres. & 2015 Elsevier Ltd. All rights reserved.
Keywords: A. oxides B. chemical synthesis D. crystal structure D. microstructure
1. Introduction Anatase TiO2 crystals with exposed (001) facets have attracted a lot of attention due to their higher reactivity in photocatalytic water splitting [1,2], photocatalytic degradation of pollutants [3,4], and dye-sensitized solar cells (DSSCs) [5,6]. Since the breakthrough in synthesizing anatase TiO2 with exposed reactive (001) facets in 2008 [7], many efforts have been devoted to developing new routes for fine-tuning the TiO2 surface with desirable facets and exploring their applications [8–12]. Han et al. [13] have successfully synthesized anatase TiO2 sheet with the percentage of exposed (001) facets up to 89%, which showed higher photocatalytic activity than the anatase TiO2 with (101) facets as basal surfaces. Yu et al. [10] investigated the mechanistic role of hydrofluoric acid on the crystal facet growth and compared the photocatalytic activity of TiO2 with different exposed facets. Sutradhar et al. [14] synthesized highly truncated bi-pyramidal anatase TiO2 nanocrystals with exposed (001) facet by preferential adsorption of CO2− 3 ions. Our recent work has synthesized TiO2 microspheres with high surface coverage of reactive (001) facets by hydrothermal treatment of TiF4. The TiO2 microspheres had excellent efficiency for the degradation of Acid Red dye under UV irradiation [15]. Anatase TiO2 with highly exposed (001) facets facilitated light harvest and the separation of photogenerated charge carriers. However, the large band gap (3.2 eV) limits the light absorption in the ultraviolet region, which accounts for ca. 4% of the solar energy. n
Corresponding author. Fax: þ 86 592 6162221. E-mail address: [email protected] (B. Gao).
http://dx.doi.org/10.1016/j.jpcs.2015.08.018 0022-3697/& 2015 Elsevier Ltd. All rights reserved.
To extend the photo-response of TiO2 to visible-light region, considerable efforts have been taken on coupling TiO2 with narrow band-gap semiconductors, such as CdS [16–21], CdSe [22,23], PbS [24], Bi2S3 [25], Cu2O [26], and CdTe [27]. Among these sensitizers, CdS as an n-type semiconductor is more promising due to its narrow band gap (2.25 eV for the bulk material) and high absorption in the visible-light region, which has been widely studied as sensitizer for photocatalytic degradation of organic pollutants and photocatalytic water splitting [28,29]. The efficient coupling of TiO2 nanocrystals and CdS nanoparticles can provide a suitable interface for the transfer of charge carriers from CdS to TiO2 and improve the photocatalytic H2-evolution activity from water splitting [30–32]. In this study, TiO2 microspheres with highly exposed (001) facets were prepared by a simple hydrothermal treatment of titanium tetrafluoride and was subsequently loaded with CdS by chemical bath deposition. The photocatalytic H2-production performance of the as-obtained CdS-sensitized TiO2 microspheres in lactic acid aqueous solutions under visible light (λ Z420) irradiation were investigated. To the best of our knowledge, this is the first report showing that CdS-sensitized TiO2 microspheres with exposed (001) facets exhibit excellent visible-light-driven photocatalytic activity for water splitting.
2. Experimental 2.1. Synthesis of the catalysts The CdS loaded TiO2 microspheres (CdS/TiO2 microspheres) were prepared by a two-step procedure. Firstly, TiO2 microspheres
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were synthesized by the hydrothermal method as described in our previous work [15]. Typically, 0.297 g TiF4 was dissolved in HCl aqueous solution under vigorous stirring. The transparent solution was then transferred to Teflon-lined stainless steel autoclave and kept at 160 °C for 20 h. After reaction, the white precipitates were filtered, washed with distilled water and dried at 80 °C overnight. Second, the as-obtained TiO2 microspheres were dispersed in an aqueous solution of cadmium nitrate. Subsequently, sodium sulfide solution was added dropwise to the above suspension. After stirring for one hour, the precipitates were collected, washed with distilled water and dried at 80 °C overnight. The molar ratio of CdS to TiO2 was controlled to be 0.2, 0.4, 0.8, 1, 1.5 and the as-synthesized CdS/TiO2 microspheres were marked as ST1, ST2, ST3, ST4 and ST5, respectively. Pure CdS was prepared by the same procedure as described in the second step without the addition of TiO2 microspheres. The composite of CdS/TiO2 nanoparticles was also synthesized for comparison with CdS/TiO2 microspheres. Most of the procedures for the synthesis of CdS/TiO2 nanoparticles were the same as those of CdS/TiO2 microspheres except that the precursor TiF4 was displaced by tetrabutyl titanate. This composite of CdS/TiO2 nanoparticles was marked as STN, in which the molar ratio of CdS to TiO2 was 1:1. 2.2. Characterization The crystal structure of the as-prepared samples was characterized by XRD on a Rigaku D/MAX-RB diffractometer using Cu Kα radiation (λ ¼1.54056 Å). The morphology and micro-structure were observed using a field emission scanning electron microscope (FE-SEM, Hitachi S4800) and a transmission electron microscope (TEM, JEOL JEM-2100 operated at 200 kV). UV–vis diffuse reflectance spectra were acquired on a Shimadzu UV-2550 spectrometer using BaSO4 as the reference. XPS measurements were carried out on an SECA Lab 220i-XL spectrometer by using an unmonochromated Al Kα (1486.6 eV) X-ray source. All the spectra were calibrated to the binding energy of the adventitious C1s peak at 284.6 eV. 2.3. Photoelectrochemical measurement The working electrode was prepared as follow: The indium-tin oxide (ITO) glasses were cleaned by sonicating in the cleanout fluid of acetone, ethanol and distilled water for 15 min, respectively. 10 mg photocatalyst powder was dispersed in 2 mL distilled water under sonication for 15 min to get slurry. The as-obtained slurry was spread on the conductive surface of ITO glass to form a photocatalyst film with the area of 2 cm2. Photocurrent measurements were carried out in the conventional three-electrode electrochemical cell using a CHI630D electrochemical workstation. The CdS/TiO2 microspheres dispersed on ITO, Pt plate and Ag/AgCl were used as the working, counter and reference electrodes, respectively. The working electrode was immersed in 0.35 M/0.25 M Na2SO3–Na2S-aqueous solution, irradiated by a 300 W xenon lamp equipped with a 420 nm cut-off filter. The bias was 0.6 V versus Ag/AgCl electrode. 2.4. Photocatalytic H2-production efficiency Water splitting reaction was carried out on a CEL-SPH2N photocatalytic system (Aulight Tech Co. Ltd., Beijing) with a top-irradiation vessel. The light source was a 300 W xenon lamp with a UV-cutoff filter (λ Z420 nm). In a typical photocatalytic reaction, 100 mg photocatalyst was suspended in 100 mL aqueous solution containing lactic acid (10 vol%) served as the sacrificial agent for scavenging holes. Prior to irradiation, the reactor was sealed and
degassed thoroughly to ensure vacuum ambient condition. During the reaction, the temperature was maintained at 7 °C by a flow of cooling water. The amount of evolved H2 was analyzed with an online 7890II gas chromatograph (TCD, 5 Å molecular sieve columns, argon gas). The apparent quantum efficiency (QE) of the samples was also measured under the same photocatalytic reaction conditions. The light intensity was ca. 67 mW/cm2. The QE was calculated according to Eq. (1):
QE[%] = =
number of reacted electrons × 100 number of incident photons number of evolved H2 molecules × 2 × 100 number of incident photons
(1)
3. Results and discussion 3.1. Morphology characterization The morphologies of pure TiO2 mircrosphere and the CdS-loaded TiO2 mircrospheres were observed by SEM and TEM. As shown in Fig. 1, the pure TiO2 consists of microspheres with diameter of 1–2 μm. The surface of the microspheres is covered by square nanoflakes with side length of 500–700 nm and thickness of 20–50 nm. Flaws in the nanoflakes can be observed due to the erosion by HF generated through the hydrolysis of TiF4. After the loading of CdS nanoparticles, the whole microsphere structure does not change, whereas the edge of the nanoflakes becomes blurred (Fig. 1b). The surface of the microsphere turns to be smoother due to the coverage of CdS nanoparticles. The low magnification TEM image (Fig. 1d and e) shows that TiO2 and the CdS modified TiO2 microspheres are solid with underdeveloped porous structure. The inset of Fig. 1d exhibits the lattice fringes of the lateral side of nanoflake. The lattice spacing is about 0.235 nm, in agreement with the d spacing of (004) plane in anatase TiO2, which indicates that the top and bottom surfaces of the nanoflake can be identified as the (001) facet. The percentage of exposed (001) facet is about 90%, as calculated by using the previously reported method [33]. In Fig. 1e, CdS nanoparticles are clearly observed on the surface of TiO2 microspheres. The interplanar spacing of 0.336 nm (inset of Fig. 1e) is consistent with the (111) plane of cubic CdS. The EDS spectrum (Fig. 1g) obtained on the single CdS modified TiO2 microsphere (shown in Fig. 1e) exhibits the coexistence of Cd and S elements with Ti and O. The quantitative analysis gives a molar ratio of Cd:S to be 1:1, confirming the stoichiometric formation of CdS. The SEM and TEM images of STN are also shown in Fig. 1c and f for comparison. This sample is mainly composed of TiO2 nanoparticles with irregular shapes, whose surface is generally dominated by the (101) facet [7–12]. The loading of CdS nanoparticles on the TiO2 particles can be confirmed from the HRTEM images (inset of Fig. 1f). 3.2. Crystal structure Fig. 2 shows the XRD patterns of the pure CdS, TiO2 microspheres and the composites of CdS/TiO2 microspheres with different compositions. The characteristic peaks at 2θ of 25.3°, 37.9°, 48.04°, 53.9°, and 54.98° correspond to the (101), (004), (200), (105), and (211) crystal planes of anatase TiO2 (JCPDS 21-1272). The pure CdS sample exhibits three diffraction peaks at 2θ of 26.65°, 43.87°, and 51.96°, which are attributed to the (111), (220), and (311) crystal plane of cubic phase (JCPDS65-2887). In the samples of CdS modified TiO2 microspheres, the peaks of anatase become weak gradually due to the coverage of CdS on TiO2,
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Fig. 1. SEM and TEM images of the pure TiO2 microspheres (a, d), CdS-loaded TiO2 microspheres (b, e) and CdS modified TiO2 particles (c, f). The insets in (d), (e) and (f) present the HRTEM images of TiO2 nanoflake, CdS nanoparticle and TiO2 particles, respectively. EDS spectrum of CdS/TiO2 microspheres (g).
♣ ♣
whereas the intensity of CdS diffraction peaks increases with the increment of CdS content. The diffraction peaks of TiO2 and CdS particles can also be clearly observed in the STN sample.
♣ CdS
♣
CdS STN
3.3. XPS analysis
Intensity (a.u.)
ST5 ST4 ST3 ST2 ST1 TiO
20
30
40
50
60
70
2-Theta (degree) Fig. 2. XRD patterns of the CdS/TiO2 microspheres.
80
Fig. 3 presents the XPS spectra of the CdS/TiO2 microspheres. All the samples exhibit symmetric doublets with Ti 2p3/2 at about 458.6 eV and Ti 2p1/2 at 464.4 eV, which are typical of Ti4 þ species. The O 1s spectrum shows an asymmetric peak at around 530.0 eV, which can be fitted into two peaks centered at 529.8 and 531.2 eV. The former is due to the lattice oxygen of TiO2, and the latter can be assigned to surface oxygen species [20]. The peaks of Cd 3d5/2 at 405.1 eV and 3d3/2 at 411.8 eV are in good agreement with the reported values of Cd2 þ on CdS. A doublet structure located at
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S 2p
Counts
Counts
Cd 3d
402
404
406
408
410
412
414
158
416
160
Binding Energy (eV)
162
164
166
Binding Energy (eV)
O 1s
Counts
Counts
Ti 2p
454
456
458
460
462
464
466
468
470
526
528
530
532
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538
Binding Energy (eV)
Binding Energy (eV)
Fig. 3. XPS spectra of the CdS/TiO2 microspheres.
1.2
Absorbance (a.u.)
0.8
CdS ST5 ST4 ST2 ST1 TiO2
0.6 0.4 0.2 0.0 300
400
500
600
700
800
Wavelength (nm)
Rate of H 2 production (µmol/h)
80 1.0
70 60 50 40 30 20 10 0
TiO2 ST1 ST2 ST3 ST4 ST5 CdS STN Fig. 4. Diffuse reflectance spectra of the pure CdS, TiO2 microspheres, and CdS/TiO2 microspheres.
161.2 and 162.4 eV is observed for S 2p core level spectrum, corresponding to the S 2p3/2 and 2p1/2 of S2 ions [18]. 3.4. Photo-absorption property The UV–vis diffuse reflectance spectra of the CdS/TiO2 microspheres are shown in Fig. 4. Due to its large band gap of 3.2 eV, pure TiO2 microspheres only exhibit light response in the ultraviolet region, whereas CdS reveals strong visible light absorption up to 600 nm. Hence, the CdS/TiO2 composites exhibit significantly high absorption in the visible region, which was further enhanced with increasing CdS content. The strong absorption of the CdS/TiO2 composites in the visible region implies efficient utilization of solar light for the photocatalytic reaction.
Fig. 5. Photocatalytic H2 evolution under visible light irradiation (λ Z 420 nm) over pure CdS and the CdS-loaded TiO2 composites.
3.5. Photocatalytic activity Photocatalytic reduction of H2O into H2 was performed to evaluate the activity of CdS/TiO2 microspheres. Lactic acid was used as the scavenger for photo-generated holes. As shown in Fig. 5, TiO2 microsphere with exposed (001) facet is inactive to generate H2 under visible light irradiation since it only exhibits UV light response. Pure CdS has detectable visible-light-driven photocatalytic H2-evolution activity from water splitting. About 11.2 μmol H2 is generated by CdS per hour. CdS loaded TiO2 microsphere presents higher photocatalytic efficiency than pure CdS or TiO2 microspheres. The maximum activity is achieved over the ST4 sample with a H2-production rate of 76.55 μmol/h, which corresponds to an apparent quantum efficiency (QE) of 21.7%. The
B. Gao et al. / Journal of Physics and Chemistry of Solids 87 (2015) 171–176
off
Current density (mA/cm)
1.0
on
0.8
c
0.6
400 350
Produced H 2 (µmol)
activity of ST4 is nearly 6.8 times that of pure CdS. However, the H2-evolution activity of CdS/TiO2 microspheres decreases once the CdS/TiO2 molar ratio exceeds 1, because the overloading of CdS results in serious agglomeration of CdS nanoparticles so that the large agglomerated CdS clumps would hinder the transfer of photoexcited electrons from CdS to TiO2 microspheres and increase the recombination of photogenerated charge carriers in CdS particles. In addition, excess CdS particles cover on the surface of TiO2 would result in the reduced contact area between TiO2 and the electrolyte. As a result, the photocatalytic activity decreases. To explore the effect of exposed (001) facet on the photocatalytic H2-production, a comparison of the H2-production rate of the ST4 and STN samples under visible light irradiation was carried out. The molar ratio of CdS to TiO2 was the same in both samples. The hydrogen production rate of STN is 18.06 μmol/h. The activity of ST4 is more than four times that of STN. The unexpected high H2-evolution efficiency of the ST4 sample should result from the high exposure of (001) facet on the surface of TiO2 microspheres. It has been reported that the (001) faceted surface of anatase TiO2 is much more reactive than the thermodynamically more stable (101) surface, since the (001) facet has a strong ability to dissociatively adsorb water to form hydrogen peroxide and peroxide radicals due to the low atomic coordination numbers of exposed atoms and the wide bond angle of Ti–O–Ti [1,4,34]. In the present case, the surface of TiO2 microspheres mainly consists of the (001) facet, whereas the (101) facet dominates the surface of TiO2 nanoparticles. The different surface structures of TiO2 result in the significantly distinct photocatalytic efficiencies. On the other hand, the presence of (001) facet on the TiO2 microsphere is beneficial for the transfer of photo-generated electron from CdS to TiO2, as can be confirmed by the photoelectrochemical responses of CdS/TiO2 microspheres (ST4) and CdS/TiO2 nanoparticles (STN). As shown in Fig. 6, the transient photocurrent of the catalysts was investigated by several on-off cycles of visible light irradiation. As expected, TiO2 microsphere has no response to illumination. Both CdS/ TiO2 composites show a transient photocurrent upon irradiation. However, the differences between the photocurrent and dark current (ΔI) of the two samples are quite different. The average ΔI value of ST4 is ca. 5.3 times higher than that of STN. The higher photocurrent of ST4 indicates the more efficient separation of photo-generated electrons and holes, which accounts for the high activity for hydrogen production [35]. As described above, the photocatalytic H2-production efficiency of the CdS/TiO2 microspheres is significantly enhanced compared to pure TiO2, CdS, and CdS/TiO2 nanoparticles. The excellent visible-light-driven photocatalytic activity of the CdS/TiO2 microspheres is attributed to the synergetic effect of good visible-light response, efficient charge carriers separation, and high reactivity of exposed (001) facet. CdS is a relatively smaller band gap semiconductor, having strong absorption in the region of 400–600 nm.
175
300 250 200 150 100 50 0 0
5
10
15
Time (h) Fig. 7. Stability test of the ST4 sample for photocatalytic H2 production.
The conduction band of CdS is much more negative than that of TiO2, which fit the basic requirement to form a “staggered” type II heterojunction at the interface of the CdS/TiO2 hybrid semiconductor [36–38]. Under visible light irradiation, the electrons are excited from the valence band to the conduction band of CdS, and then transfer to the conduction band of TiO2. This electron injection improves the separation of charge carriers and is greatly promoted by the exposed (001) facet as the (001) facet possesses a considerable amount of defects originating from the low-coordinated atoms, which would capture electron efficiently and facilitate the electron transfer from CdS to TiO2. In addition, the (001) facet exhibits superior photocatalytic activity in comparison with other crystal planes due to the efficient formation of hydrogen peroxide and peroxide radicals. Cadmium sulfide always shows declination in its photocatalytic activity due to photocorrosion. The stability and recycling performance of the CdS/TiO2 microsphere composite was investigated. The time dependence of the photocatalytic production of H2 over the ST4 sample is shown in Fig. 7. The reaction system was evacuated every 5 h. Obviously, the ST4 sample possesses good stability and maintains high photocatalytic performance after three reaction cycles.
4. Conclusions CdS-loaded TiO2 microspheres with various CdS:TiO2 molar ratios have been successfully synthesized via a hydrothermal treatment followed by chemical bath deposition. The exposed surface of the microspheres was confirmed to be the reactive (001) facet of anatase TiO2, which was in close contact with CdS nanoparticles. The CdS/TiO2 microsphere composites exhibited excellent visiblelight-driven photocatalytic H2-production efficiency, among which the sample with the CdS:TiO2 molar ratio of 1 showed the highest activities. The presence of surficial (001) facet was beneficial for the transfer of photo-generated electrons from CdS to TiO2, and promoted the dissociative adsorption of water to produce hydroxide radicals, which led to the unexpected high photocatalytic efficiency of the CdS/TiO2 microsphere composites.
0.4
Acknowledgment 0.2
b 0.0
a 0
20
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100
120
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Time (s) Fig. 6. Transient photocurrent of the catalysts irradiated by visible light (λZ 420 nm). (a)TiO2; (b)STN; (c)ST4.
The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Nos. 21103054, 21003055), Natural Science Foundation of Fujian Province of China (No. 2012J05024), and the Promotion Program for Young and Middle-aged Teachers in Science and Technology Research of Huaqiao University (No. ZQN-PY206).
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Enhanced visible-light-driven photocatalytic H2-production activity of CdS-loaded TiO2 microspheres with exposed (001) facets Bifen Gao n, Xia Yuan, Penghui Lu, Bizhou Lin, Yilin Chen Department of Applied Chemistry, College of Materials Science & Engineering, Huaqiao University, Xiamen 361021, China
art ic l e i nf o
a b s t r a c t
Article history: Received 25 April 2015 Received in revised form 24 July 2015 Accepted 26 August 2015 Available online 28 August 2015
CdS-loaded TiO2 microspheres with highly exposed (001) facets were prepared by hydrothermal treatment of a TiF4–HCl–H2O mixed solution followed by a chemical bath deposition of CdS onto TiO2 microspheres. The crystal structure, surficial micro-structure and photo-absorption property of the samples were characterized by XRD, FE-SEM, TEM and UV–vis diffuse reflectance spectroscopy, etc. The as-prepared samples exhibited superior visible-light-driven photocatalytic H2-production activity from lactic acid aqueous solution in comparison with CdS-sensitized TiO2 nanoparticles, whose surface was dominated by (101) facets. Photoelectrochemical measurement confirmed that (001) facet is beneficial for the transfer of photo-generated electron from CdS to TiO2 microsphere, which led to the unexpected high photocatalytic activity of CdS-loaded TiO2 microspheres. & 2015 Elsevier Ltd. All rights reserved.
Keywords: A. oxides B. chemical synthesis D. crystal structure D. microstructure
1. Introduction Anatase TiO2 crystals with exposed (001) facets have attracted a lot of attention due to their higher reactivity in photocatalytic water splitting [1,2], photocatalytic degradation of pollutants [3,4], and dye-sensitized solar cells (DSSCs) [5,6]. Since the breakthrough in synthesizing anatase TiO2 with exposed reactive (001) facets in 2008 [7], many efforts have been devoted to developing new routes for fine-tuning the TiO2 surface with desirable facets and exploring their applications [8–12]. Han et al. [13] have successfully synthesized anatase TiO2 sheet with the percentage of exposed (001) facets up to 89%, which showed higher photocatalytic activity than the anatase TiO2 with (101) facets as basal surfaces. Yu et al. [10] investigated the mechanistic role of hydrofluoric acid on the crystal facet growth and compared the photocatalytic activity of TiO2 with different exposed facets. Sutradhar et al. [14] synthesized highly truncated bi-pyramidal anatase TiO2 nanocrystals with exposed (001) facet by preferential adsorption of CO2− 3 ions. Our recent work has synthesized TiO2 microspheres with high surface coverage of reactive (001) facets by hydrothermal treatment of TiF4. The TiO2 microspheres had excellent efficiency for the degradation of Acid Red dye under UV irradiation [15]. Anatase TiO2 with highly exposed (001) facets facilitated light harvest and the separation of photogenerated charge carriers. However, the large band gap (3.2 eV) limits the light absorption in the ultraviolet region, which accounts for ca. 4% of the solar energy. n
Corresponding author. Fax: þ 86 592 6162221. E-mail address: [email protected] (B. Gao).
http://dx.doi.org/10.1016/j.jpcs.2015.08.018 0022-3697/& 2015 Elsevier Ltd. All rights reserved.
To extend the photo-response of TiO2 to visible-light region, considerable efforts have been taken on coupling TiO2 with narrow band-gap semiconductors, such as CdS [16–21], CdSe [22,23], PbS [24], Bi2S3 [25], Cu2O [26], and CdTe [27]. Among these sensitizers, CdS as an n-type semiconductor is more promising due to its narrow band gap (2.25 eV for the bulk material) and high absorption in the visible-light region, which has been widely studied as sensitizer for photocatalytic degradation of organic pollutants and photocatalytic water splitting [28,29]. The efficient coupling of TiO2 nanocrystals and CdS nanoparticles can provide a suitable interface for the transfer of charge carriers from CdS to TiO2 and improve the photocatalytic H2-evolution activity from water splitting [30–32]. In this study, TiO2 microspheres with highly exposed (001) facets were prepared by a simple hydrothermal treatment of titanium tetrafluoride and was subsequently loaded with CdS by chemical bath deposition. The photocatalytic H2-production performance of the as-obtained CdS-sensitized TiO2 microspheres in lactic acid aqueous solutions under visible light (λ Z420) irradiation were investigated. To the best of our knowledge, this is the first report showing that CdS-sensitized TiO2 microspheres with exposed (001) facets exhibit excellent visible-light-driven photocatalytic activity for water splitting.
2. Experimental 2.1. Synthesis of the catalysts The CdS loaded TiO2 microspheres (CdS/TiO2 microspheres) were prepared by a two-step procedure. Firstly, TiO2 microspheres
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were synthesized by the hydrothermal method as described in our previous work [15]. Typically, 0.297 g TiF4 was dissolved in HCl aqueous solution under vigorous stirring. The transparent solution was then transferred to Teflon-lined stainless steel autoclave and kept at 160 °C for 20 h. After reaction, the white precipitates were filtered, washed with distilled water and dried at 80 °C overnight. Second, the as-obtained TiO2 microspheres were dispersed in an aqueous solution of cadmium nitrate. Subsequently, sodium sulfide solution was added dropwise to the above suspension. After stirring for one hour, the precipitates were collected, washed with distilled water and dried at 80 °C overnight. The molar ratio of CdS to TiO2 was controlled to be 0.2, 0.4, 0.8, 1, 1.5 and the as-synthesized CdS/TiO2 microspheres were marked as ST1, ST2, ST3, ST4 and ST5, respectively. Pure CdS was prepared by the same procedure as described in the second step without the addition of TiO2 microspheres. The composite of CdS/TiO2 nanoparticles was also synthesized for comparison with CdS/TiO2 microspheres. Most of the procedures for the synthesis of CdS/TiO2 nanoparticles were the same as those of CdS/TiO2 microspheres except that the precursor TiF4 was displaced by tetrabutyl titanate. This composite of CdS/TiO2 nanoparticles was marked as STN, in which the molar ratio of CdS to TiO2 was 1:1. 2.2. Characterization The crystal structure of the as-prepared samples was characterized by XRD on a Rigaku D/MAX-RB diffractometer using Cu Kα radiation (λ ¼1.54056 Å). The morphology and micro-structure were observed using a field emission scanning electron microscope (FE-SEM, Hitachi S4800) and a transmission electron microscope (TEM, JEOL JEM-2100 operated at 200 kV). UV–vis diffuse reflectance spectra were acquired on a Shimadzu UV-2550 spectrometer using BaSO4 as the reference. XPS measurements were carried out on an SECA Lab 220i-XL spectrometer by using an unmonochromated Al Kα (1486.6 eV) X-ray source. All the spectra were calibrated to the binding energy of the adventitious C1s peak at 284.6 eV. 2.3. Photoelectrochemical measurement The working electrode was prepared as follow: The indium-tin oxide (ITO) glasses were cleaned by sonicating in the cleanout fluid of acetone, ethanol and distilled water for 15 min, respectively. 10 mg photocatalyst powder was dispersed in 2 mL distilled water under sonication for 15 min to get slurry. The as-obtained slurry was spread on the conductive surface of ITO glass to form a photocatalyst film with the area of 2 cm2. Photocurrent measurements were carried out in the conventional three-electrode electrochemical cell using a CHI630D electrochemical workstation. The CdS/TiO2 microspheres dispersed on ITO, Pt plate and Ag/AgCl were used as the working, counter and reference electrodes, respectively. The working electrode was immersed in 0.35 M/0.25 M Na2SO3–Na2S-aqueous solution, irradiated by a 300 W xenon lamp equipped with a 420 nm cut-off filter. The bias was 0.6 V versus Ag/AgCl electrode. 2.4. Photocatalytic H2-production efficiency Water splitting reaction was carried out on a CEL-SPH2N photocatalytic system (Aulight Tech Co. Ltd., Beijing) with a top-irradiation vessel. The light source was a 300 W xenon lamp with a UV-cutoff filter (λ Z420 nm). In a typical photocatalytic reaction, 100 mg photocatalyst was suspended in 100 mL aqueous solution containing lactic acid (10 vol%) served as the sacrificial agent for scavenging holes. Prior to irradiation, the reactor was sealed and
degassed thoroughly to ensure vacuum ambient condition. During the reaction, the temperature was maintained at 7 °C by a flow of cooling water. The amount of evolved H2 was analyzed with an online 7890II gas chromatograph (TCD, 5 Å molecular sieve columns, argon gas). The apparent quantum efficiency (QE) of the samples was also measured under the same photocatalytic reaction conditions. The light intensity was ca. 67 mW/cm2. The QE was calculated according to Eq. (1):
QE[%] = =
number of reacted electrons × 100 number of incident photons number of evolved H2 molecules × 2 × 100 number of incident photons
(1)
3. Results and discussion 3.1. Morphology characterization The morphologies of pure TiO2 mircrosphere and the CdS-loaded TiO2 mircrospheres were observed by SEM and TEM. As shown in Fig. 1, the pure TiO2 consists of microspheres with diameter of 1–2 μm. The surface of the microspheres is covered by square nanoflakes with side length of 500–700 nm and thickness of 20–50 nm. Flaws in the nanoflakes can be observed due to the erosion by HF generated through the hydrolysis of TiF4. After the loading of CdS nanoparticles, the whole microsphere structure does not change, whereas the edge of the nanoflakes becomes blurred (Fig. 1b). The surface of the microsphere turns to be smoother due to the coverage of CdS nanoparticles. The low magnification TEM image (Fig. 1d and e) shows that TiO2 and the CdS modified TiO2 microspheres are solid with underdeveloped porous structure. The inset of Fig. 1d exhibits the lattice fringes of the lateral side of nanoflake. The lattice spacing is about 0.235 nm, in agreement with the d spacing of (004) plane in anatase TiO2, which indicates that the top and bottom surfaces of the nanoflake can be identified as the (001) facet. The percentage of exposed (001) facet is about 90%, as calculated by using the previously reported method [33]. In Fig. 1e, CdS nanoparticles are clearly observed on the surface of TiO2 microspheres. The interplanar spacing of 0.336 nm (inset of Fig. 1e) is consistent with the (111) plane of cubic CdS. The EDS spectrum (Fig. 1g) obtained on the single CdS modified TiO2 microsphere (shown in Fig. 1e) exhibits the coexistence of Cd and S elements with Ti and O. The quantitative analysis gives a molar ratio of Cd:S to be 1:1, confirming the stoichiometric formation of CdS. The SEM and TEM images of STN are also shown in Fig. 1c and f for comparison. This sample is mainly composed of TiO2 nanoparticles with irregular shapes, whose surface is generally dominated by the (101) facet [7–12]. The loading of CdS nanoparticles on the TiO2 particles can be confirmed from the HRTEM images (inset of Fig. 1f). 3.2. Crystal structure Fig. 2 shows the XRD patterns of the pure CdS, TiO2 microspheres and the composites of CdS/TiO2 microspheres with different compositions. The characteristic peaks at 2θ of 25.3°, 37.9°, 48.04°, 53.9°, and 54.98° correspond to the (101), (004), (200), (105), and (211) crystal planes of anatase TiO2 (JCPDS 21-1272). The pure CdS sample exhibits three diffraction peaks at 2θ of 26.65°, 43.87°, and 51.96°, which are attributed to the (111), (220), and (311) crystal plane of cubic phase (JCPDS65-2887). In the samples of CdS modified TiO2 microspheres, the peaks of anatase become weak gradually due to the coverage of CdS on TiO2,
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Fig. 1. SEM and TEM images of the pure TiO2 microspheres (a, d), CdS-loaded TiO2 microspheres (b, e) and CdS modified TiO2 particles (c, f). The insets in (d), (e) and (f) present the HRTEM images of TiO2 nanoflake, CdS nanoparticle and TiO2 particles, respectively. EDS spectrum of CdS/TiO2 microspheres (g).
♣ ♣
whereas the intensity of CdS diffraction peaks increases with the increment of CdS content. The diffraction peaks of TiO2 and CdS particles can also be clearly observed in the STN sample.
♣ CdS
♣
CdS STN
3.3. XPS analysis
Intensity (a.u.)
ST5 ST4 ST3 ST2 ST1 TiO
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2-Theta (degree) Fig. 2. XRD patterns of the CdS/TiO2 microspheres.
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Fig. 3 presents the XPS spectra of the CdS/TiO2 microspheres. All the samples exhibit symmetric doublets with Ti 2p3/2 at about 458.6 eV and Ti 2p1/2 at 464.4 eV, which are typical of Ti4 þ species. The O 1s spectrum shows an asymmetric peak at around 530.0 eV, which can be fitted into two peaks centered at 529.8 and 531.2 eV. The former is due to the lattice oxygen of TiO2, and the latter can be assigned to surface oxygen species [20]. The peaks of Cd 3d5/2 at 405.1 eV and 3d3/2 at 411.8 eV are in good agreement with the reported values of Cd2 þ on CdS. A doublet structure located at
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S 2p
Counts
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Fig. 3. XPS spectra of the CdS/TiO2 microspheres.
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Absorbance (a.u.)
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0.6 0.4 0.2 0.0 300
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Rate of H 2 production (µmol/h)
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TiO2 ST1 ST2 ST3 ST4 ST5 CdS STN Fig. 4. Diffuse reflectance spectra of the pure CdS, TiO2 microspheres, and CdS/TiO2 microspheres.
161.2 and 162.4 eV is observed for S 2p core level spectrum, corresponding to the S 2p3/2 and 2p1/2 of S2 ions [18]. 3.4. Photo-absorption property The UV–vis diffuse reflectance spectra of the CdS/TiO2 microspheres are shown in Fig. 4. Due to its large band gap of 3.2 eV, pure TiO2 microspheres only exhibit light response in the ultraviolet region, whereas CdS reveals strong visible light absorption up to 600 nm. Hence, the CdS/TiO2 composites exhibit significantly high absorption in the visible region, which was further enhanced with increasing CdS content. The strong absorption of the CdS/TiO2 composites in the visible region implies efficient utilization of solar light for the photocatalytic reaction.
Fig. 5. Photocatalytic H2 evolution under visible light irradiation (λ Z 420 nm) over pure CdS and the CdS-loaded TiO2 composites.
3.5. Photocatalytic activity Photocatalytic reduction of H2O into H2 was performed to evaluate the activity of CdS/TiO2 microspheres. Lactic acid was used as the scavenger for photo-generated holes. As shown in Fig. 5, TiO2 microsphere with exposed (001) facet is inactive to generate H2 under visible light irradiation since it only exhibits UV light response. Pure CdS has detectable visible-light-driven photocatalytic H2-evolution activity from water splitting. About 11.2 μmol H2 is generated by CdS per hour. CdS loaded TiO2 microsphere presents higher photocatalytic efficiency than pure CdS or TiO2 microspheres. The maximum activity is achieved over the ST4 sample with a H2-production rate of 76.55 μmol/h, which corresponds to an apparent quantum efficiency (QE) of 21.7%. The
B. Gao et al. / Journal of Physics and Chemistry of Solids 87 (2015) 171–176
off
Current density (mA/cm)
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on
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c
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Produced H 2 (µmol)
activity of ST4 is nearly 6.8 times that of pure CdS. However, the H2-evolution activity of CdS/TiO2 microspheres decreases once the CdS/TiO2 molar ratio exceeds 1, because the overloading of CdS results in serious agglomeration of CdS nanoparticles so that the large agglomerated CdS clumps would hinder the transfer of photoexcited electrons from CdS to TiO2 microspheres and increase the recombination of photogenerated charge carriers in CdS particles. In addition, excess CdS particles cover on the surface of TiO2 would result in the reduced contact area between TiO2 and the electrolyte. As a result, the photocatalytic activity decreases. To explore the effect of exposed (001) facet on the photocatalytic H2-production, a comparison of the H2-production rate of the ST4 and STN samples under visible light irradiation was carried out. The molar ratio of CdS to TiO2 was the same in both samples. The hydrogen production rate of STN is 18.06 μmol/h. The activity of ST4 is more than four times that of STN. The unexpected high H2-evolution efficiency of the ST4 sample should result from the high exposure of (001) facet on the surface of TiO2 microspheres. It has been reported that the (001) faceted surface of anatase TiO2 is much more reactive than the thermodynamically more stable (101) surface, since the (001) facet has a strong ability to dissociatively adsorb water to form hydrogen peroxide and peroxide radicals due to the low atomic coordination numbers of exposed atoms and the wide bond angle of Ti–O–Ti [1,4,34]. In the present case, the surface of TiO2 microspheres mainly consists of the (001) facet, whereas the (101) facet dominates the surface of TiO2 nanoparticles. The different surface structures of TiO2 result in the significantly distinct photocatalytic efficiencies. On the other hand, the presence of (001) facet on the TiO2 microsphere is beneficial for the transfer of photo-generated electron from CdS to TiO2, as can be confirmed by the photoelectrochemical responses of CdS/TiO2 microspheres (ST4) and CdS/TiO2 nanoparticles (STN). As shown in Fig. 6, the transient photocurrent of the catalysts was investigated by several on-off cycles of visible light irradiation. As expected, TiO2 microsphere has no response to illumination. Both CdS/ TiO2 composites show a transient photocurrent upon irradiation. However, the differences between the photocurrent and dark current (ΔI) of the two samples are quite different. The average ΔI value of ST4 is ca. 5.3 times higher than that of STN. The higher photocurrent of ST4 indicates the more efficient separation of photo-generated electrons and holes, which accounts for the high activity for hydrogen production [35]. As described above, the photocatalytic H2-production efficiency of the CdS/TiO2 microspheres is significantly enhanced compared to pure TiO2, CdS, and CdS/TiO2 nanoparticles. The excellent visible-light-driven photocatalytic activity of the CdS/TiO2 microspheres is attributed to the synergetic effect of good visible-light response, efficient charge carriers separation, and high reactivity of exposed (001) facet. CdS is a relatively smaller band gap semiconductor, having strong absorption in the region of 400–600 nm.
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300 250 200 150 100 50 0 0
5
10
15
Time (h) Fig. 7. Stability test of the ST4 sample for photocatalytic H2 production.
The conduction band of CdS is much more negative than that of TiO2, which fit the basic requirement to form a “staggered” type II heterojunction at the interface of the CdS/TiO2 hybrid semiconductor [36–38]. Under visible light irradiation, the electrons are excited from the valence band to the conduction band of CdS, and then transfer to the conduction band of TiO2. This electron injection improves the separation of charge carriers and is greatly promoted by the exposed (001) facet as the (001) facet possesses a considerable amount of defects originating from the low-coordinated atoms, which would capture electron efficiently and facilitate the electron transfer from CdS to TiO2. In addition, the (001) facet exhibits superior photocatalytic activity in comparison with other crystal planes due to the efficient formation of hydrogen peroxide and peroxide radicals. Cadmium sulfide always shows declination in its photocatalytic activity due to photocorrosion. The stability and recycling performance of the CdS/TiO2 microsphere composite was investigated. The time dependence of the photocatalytic production of H2 over the ST4 sample is shown in Fig. 7. The reaction system was evacuated every 5 h. Obviously, the ST4 sample possesses good stability and maintains high photocatalytic performance after three reaction cycles.
4. Conclusions CdS-loaded TiO2 microspheres with various CdS:TiO2 molar ratios have been successfully synthesized via a hydrothermal treatment followed by chemical bath deposition. The exposed surface of the microspheres was confirmed to be the reactive (001) facet of anatase TiO2, which was in close contact with CdS nanoparticles. The CdS/TiO2 microsphere composites exhibited excellent visiblelight-driven photocatalytic H2-production efficiency, among which the sample with the CdS:TiO2 molar ratio of 1 showed the highest activities. The presence of surficial (001) facet was beneficial for the transfer of photo-generated electrons from CdS to TiO2, and promoted the dissociative adsorption of water to produce hydroxide radicals, which led to the unexpected high photocatalytic efficiency of the CdS/TiO2 microsphere composites.
0.4
Acknowledgment 0.2
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Time (s) Fig. 6. Transient photocurrent of the catalysts irradiated by visible light (λZ 420 nm). (a)TiO2; (b)STN; (c)ST4.
The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (Nos. 21103054, 21003055), Natural Science Foundation of Fujian Province of China (No. 2012J05024), and the Promotion Program for Young and Middle-aged Teachers in Science and Technology Research of Huaqiao University (No. ZQN-PY206).
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