Enhanced Photocurrent Responses and Antiphotocorrosion

May 23, 2012 - Recent advances in the TiO 2 /CdS nanocomposite used for photocatalytic hydrogen production and quantum-dot-sensitized solar cells. Dan...
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Enhanced Photocurrent Responses and Antiphotocorrosion Performance of CdS Hybrid Derived from Triple Heterojunction Zhiyuan Wu, Guohua Zhao,* Ya-nan Zhang, Hongyi Tian, and Dongming Li Department of Chemistry, Tongji University, Siping Road 1239, Shanghai 200092, China ABSTRACT: The enhanced photocurrent responses and dramatic antiphotocorrosion performance of CdS hybrid were obtained by formation of triple heterojunction (TH), i.e., n-CdS/n-TiO2/p-BDD:n-TiO2 cube tubes were vertically grown on p-type boron-doped diamond (BDD) film. Then nCdS nanoparticles (NPs) were randomly assembled onto the surface of TiO2 and BDD, forming a film of CdS NPs. This triple heterojunction CdS hybrid reveals 36.5% improvement of initial photocurrent and 78% reduction of photocorrosion rate in contrast with that of the coupled CdS/TiO2 hybrid, which has only single heterojunctions (SH). The mechanisms of TH on charge separation and transport during the photocatalytic reaction have been emphasized. Except for the CdS/TiO2 (n−n) heterojunction, another two p−n heterojunctions, i.e., p-BDD/n-CdS and p-BDD/n-TiO2 exist in the TH. The conduction band (CB) position of TiO2 is lower than the corresponding band positions of CdS and BDD, so TiO2 can act as an acceptor for the photogenerated electrons. Also, the valence band (VB) position of BDD is higher than those of CdS and TiO2, so BDD can act as a sink for the photogenerated holes. Under simulated solarlight, the photogenerated electrons on the conduction band (CB) of CdS flow to the CB of TiO2, whereas the photogenerated holes on valence band (VB) CdS and TiO2 inject to the VB of BDD. It promotes charge separation and leaves not enough holes on CdS to cause photoanodic corrosion, leading to the enhancement of photocurrent responses and the remarkable inhibited photocorrosion. separation.18 Combining n-CdS with 1D n-type TiO2 nanotubes (NTs) used to form type II heterojunctions and make use of solar light more efficiently.14,15,19,20 However, the formation of this heterojunction will simultaneously cause more serious photocorrosion on CdS,21 as the photogenerated holes on the valence band of TiO2 may flow toward the valence band of adjacent CdS. It results in gathering of holes and direct oxidation of CdS by the excess of holes.22−26 The photocatalytic activity of CdS would be significantly affected as a result. When the single heterojunction system was adopted, the work to optimize its performance was investigated. It is very interesting to explore the effect of the multiheterojunction on photoelectrochemical behaviors. The operation of a single heterojunction is different from this mode, in which at least three layers of different semiconductors meet at one interface.1,27 To the best of our knowledge, such an advanced approach of a triple heterojunction reported in the photocatalytic applications is rarely studied.19 In this study, the triple heterojunction structure is applied for constructing a CdS hybrid to obtain a type of efficient and stable photocatalyst. A novel idea is proposed: the photogenerated holes in n-type CdS and TiO2 might be swept into another p-type semiconductor by forming a triple heterojunction. Therefore, we

1. INTRODUCTION A heterojunction is the interface that occurs between two layers or regions of different coupled semiconductors and is of great importance in modern electronics.1,2 These semiconducting materials have attracted particular attention because it is often advantageous to engineer the electronic energy bands in many solid state device applications including electron devices,3 photocatalytic water splitting,4 transistors,5,6 etc. In recent years, coupled semiconducting photocatalysts with a single heterojunction are widely applied in the fields of energy conversion7,8 and the environment,9−11 because of their highly efficient photocatalytic activity or photoelectric conversion. Coupling between different semiconductors in photocatalytic systems is expected to alleviate the charge carrier recombination in individual photoelectrodes. A good matching of their conduction band (CB) and valence band (VB) levels can produce an energy gap between corresponding band levels, driving the charge carriers from one particle to its neighbor, leading to a spatial separation between electrons and holes.12 However, the single heterojunction in coupling of semiconductors is not perfect in every way, even if its band-edge positions are suitable for charge separation. The design of a coupled photoelectrode relies on its components. Take CdS/ TiO2 for example. CdS (∼2.5 eV) is one of the very typical ntype semiconductors13 in the fields of solar cells,14,15 water purification,12 water splitting for hydrogen evolution,4,16 etc. TiO2 is a UV response n-type semiconductor17 and the bandedge positions of CdS and TiO2 are suitable for charge © 2012 American Chemical Society

Received: January 12, 2012 Revised: May 16, 2012 Published: May 23, 2012 12829

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spectroscope (Renishaw Invia) was calibrated and checked using a silicon sample before and after the spectra were recorded. A field-emission scanning electron microscope (FESEM, Hitachi S4800) was used for morphological characterization of the samples. The electrical properties of hybrids were tested on a micromanipulator manual probe station in the dark. The electrochemical experiments were carried out with a CHI660c electrochemical workstation (CH Instruments, U.S.A.). The photoelectrochemical behavior was investigated under open circuit potential using a three-electrode cell system. Samples were used as the working electrode (photoanode), SCE as the reference electrode, and a platinum foil as a counter electrode. The experiments were carried out under simulated solar light illumination provided by a UV−vis light source, 150 W, 350−700 nm (HAYASHI LA-410UV-3 XE4030). All experiments were performed at the room temperature (20 ± 2 °C).

turn our sights to the boron doped diamond (BDD). p-type BDD could be easily and cheaply grown on Si substrate by chemical vapor deposition method. Many advantages can be compared to other known materials due to its low and stable background current and stable surface microstructure over a wide potential range.28−30 When p-BDD is introduced into the CdS/TiO2 system, p-BDD/n-CdS and p-BDD/n-TiO2 are established, and the edge of the valence band of BDD is more cathodic than the corresponding bands of both CdS and TiO2.18,31−33 It may bring about a more powerful force to drive the photogenerated holes on CdS and TiO2 to transfer to BDD rather than accumulate on CdS. The photocorrosion of CdS by holes could be mitigated greatly as a result.

2. EXPERIMENTAL SECTION 2.1. Chemical Reagents. All chemical reagents were used without further purification. Ammonium sulfate ((NH4)2SO4, 99.0%), sulfuric acid (H2SO4, 95−98%), sodium sulfide (Na2S, 98.0%), cadmium chloride (Cd(NO3)2, 99.0%), and tetrabutyl titanate (Ti(OCH2CH2CH2CH3)4, 99.0%) were purchased from Aldrich. Transparent conductive glass (FTO; F:SnO2, 14 Ω/square) was purchased from Nippon Sheet Glass Group (Japan). BDD substrate was purchased from CSEM (Switzerland) and synthesized on single crystal p-type Si (100) wafers, with resistivity of 0.1 Ω cm, by the microwave plasma chemical vapor deposition technique. 2.2. Pretreatment of BDD Substrate. A piece of BDD substrate (1 × 2 cm2) was immerged in a mixed solution of 20 mL of sulfuric acid with 5 g of ammonium sulfate dissolved in it and heated for 1 h. The substrate was cleaned by sonicating the samples in acetone and ethanol, followed by rinsing with deionized water and drying in a nitrogen stream. Then anodization was carried out in a three-photocatalyst configuration with platinum foil as the counter electrode, BDD substrate as the working electrode, and a saturated calomel electrode (SCE) as the reference electrode, with a voltage of 3.2 V for 30 min. 2.3. Growth of n-TiO2 Nanorods (NRs) on p-BDD or FTO Substrate via Hydrothermal Method. TiO2 nanorods formed on BDD or FTO were prepared in 30 mL of deionized water, 30 mL of 36% hydrochloric acid, and 1 mL of titanium butoxide with the interested side facing downward at 150 °C for 16 h, in the sealed Teflon autoclave of 80 mL volume. 2.4. Fabrication of Cubic TiO2 NTs by Anisotropic Chemical Corrosion in Secondary Hydrothermal Process. The prepared TiO2 nanorods were immerged in a mixed solution of 30 mL of 36% hydrochloric acid and 30 mL of deionized water, and hydrothermal corrosion was carried out at 150 °C for 10 h in the sealed autoclave. 2.5. Deposition of p-CdS NPs by Sequential Chemical Bath Deposition Method. The BDD/TiO2 NTs was immerged in a solution of Cd(NO3)2 (0.05 M) for 2 min, then rinsed with deionized water, and immerged in a Na2S solution (0.05 M) for another 2 min, followed by rinsing with deionized water. Such an immersion cycle was repeated several times until the desired deposition of CdS NPs was achieved. 2.6. Characterization of Photocurrent Response and Antiphotocorrosion of Triple Heterojunction CdS Hybrid. UV−vis diffuse reflectance spectra of the samples were acquired by using a UV−vis diffuse reflectance spectrometer (UV−vis DRS, AvaLight-DHS, Avantes). X-ray diffraction analysis (XRD, Bruker Co., Ltd., Germany) was used for determining the crystal structure of samples. The Raman

3. RESULTS AND DISCUSSION 3.1. Optimization and Characterization of CdS Hybrid. Herein, the nanostructures of the CdS hybrid were optimized (shown in Scheme 1). The 1D rutile TiO2 nanotubes were Scheme 1. Schematic Illustration for the TH CdS Hybrid Formation

fabricated by the hydrolysis of titanium butoxide on a BDD plate in hydrochloric acid hydrothermal conditions. CdS films were then deposited onto the TiO2 NTs and BDD film by sequential chemical bath deposition (SCBD) method. The obtained hybrid was called triple heterojunction (TH) CdS. Transparent conductive glass (FTO) substrate was employed as a BDD replacement for the preparation of single heterojunction (SH) CdS. The morphologies of the materials were examined using a field-emission scanning electron microscope (FE-SEM). Figure 1a is a typical SEM image of the BDD film, which reveals a regularly arranged crystallite structure with a crystallite dimension distribution around 1 μm. Figure 1b shows the assynthesized TiO2 nanorods (NRs) that are well-aligned on the substrate with special rectangular cross sections of about 500 nm. Such a large tube diameter can avoid TiO2 tubes gathering in the edges and boundaries of grains if there is a large lattice mismatch between TiO2 and BDD.33 It provides uniform heterogeneous nucleation sites effectively for growing relatively ordered and vertical tubes. The TiO2 NTs obtained after anisotropic corrosion are shown in Figure 1c. The structure of the rectangular cross-section is remained and the well ordered 12830

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The X-ray diffraction (XRD) patterns and Raman plot of the as-prepared CdS hybrid (Figure 3a,b) further confirm that the TiO2 NTs are monocrystalline rutile, and the CdS film deposited is hexagonal crystal. 3.2. Mott−Schottky and Electrical Properties Measurement. To understand the semiconducting properties of CdS, TiO2, and BDD, Mott−Schottky plots were conducted and shown in Figure 4. Mott−Schottky relationships on n-type and p-type semiconductors are expressed according to the following equations:34,35

Figure 1. FE-SEM images of the as-prepared (a−d) materials: (a) BDD film; (b) top view of TiO2 NRs/BDD photocatalyst; (c) top view of hollow TiO2 NTs before CdS sensitized; (d) and the inset are the top views of CdS/TiO2 NTs/BDD hybrid with 15 and 20 cycles, respectively.

⎛ 2 ⎞⎡ 1 KT ⎤ =⎜ ⎥ , for n type ⎟⎢(E − Efb) − 2 e0 ⎦ C ⎝ e0εε0Nd ⎠⎣

(1a)

⎛ 2 ⎞⎡ 1 KT ⎤ =⎜ ⎥ , for p type ⎟⎢( −E + Efb) − 2 e0 ⎦ C ⎝ e0εε0Na ⎠⎣

(1b)

where C is the depletion-layer capacitance per unit surface area, Nd and Na are the donor and acceptor densities, respectively, ε0 is the permittivity of vacuum, ε is the dielectric constant of the semiconductor, E is the electrode potential, Efb is the flat-band potential, and KT/e0 is the temperature dependent term in the Mott−Schottky equation. Figure 4a,b shows the Mott− Schottky plot of the TiO2 film and CdS film, respectively. There is a positive slope in the linear region of the plot, indicating an n-type semiconductor according to eq 1a. Figure 4c shows the Mott−Schottky plot of the BDD film, which had a negative slope indicating p-type behavior in compliance with eq 1b. The electrical properties of the BDD/TiO2 film and BDD/ CdS film were tested on a micromanipulator manual probe station in the dark as displayed in Figure 4d. It can be seen from the black plot (BDD/TiO2 film) that under positive bias (BDD positive with respect to TiO2) the current increases exponentially with the positive voltage. Whereas in the negative region (TiO2 positive with respect to BDD), the reverse current remains very limited. This distinct asymmetry indicates that a p−n junction is established between BDD and TiO2. For the red plot (BDD/CdS film) in Figure 4d, a similar nonlinear and asymmetric I−V curve as mentioned above is clearly demonstrated. In addition, a relatively great reverse leakage current is observed in the negative direction. The reason for this imperfection of the junction is probably that the BDD film used here is relatively heavily boron doped and rather conductive.36

1D structure still exists. The length of TiO2 nanotube after corrosion is about 2−3 μm. The high surface area and dispersibility of the as-prepared tubes can promote the heterojunction efficiency between TiO2 NTs and the CdS film. The SCBD method for CdS deposition was further optimized in changing immersion cycles. Figure 1d depicts the image of the CdS deposited nanotube array after CdS immersion of 15 cycles. It reveals the CdS NPs are continuously dispersed. The tubes and the substrate are almost completely covered by the CdS film. The inset of Figure 2d shows the image of tubes immerged for 20 cycles. Some particle aggregates can be found on the top of the tube, suggesting CdS film is not well dispersed. As shown in Figure 2, the hybrid immerged for 15 cycles has the highest photocurrent responses. It is speculated that the more cycles during immerging, the more thick the CdS film obtained. The aggregation of CdS particles on the top of the tubes may prevent light from arriving at the CdS particles inside the tubes, which leads to the reduced photocurrent. The aggregation of CdS particles may also increase transport distance and transport time, so charge recombination may become more likely.15 Thus 15-cycle immersion was chosen for further experiments.

Figure 2. UV−vis diffuse reflectance absorption spectra (a) and photocurrent responses (vs SCE) in a light on−off process under open circuit potential (b) of TH CdS photocatalyst with different NPs immersion cycles by sequential chemical bath deposition method. 12831

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Figure 3. (a) XRD pattern and (b) Raman plot of as prepared TH CdS, respectively.

Figure 4. Mott−Schottky plots of (a) TiO2, (b) CdS, and (c) BDD films in 1 M K2SO4 solution. (d) I−V characteristics for BDD/TiO2 (black) and BDD/CdS (red).

3.3. Antiphotocorrosion Performance Analysis. The amperometric i−t curve was introduced to compare the simulated solarlight response of as prepared SH CdS and TH CdS hybrids (showed in Figure 5). The initial current density of the SH CdS is 1.45 mA cm−2, whereas the value of TH CdS is 1.98 mA cm−2, which is 36.5% higher than that of the former. However, with the illumination progressing, the photocurrent density of the SH CdS is 1.04 mA cm−2 after 400 s, decreased by 28.3%. By contrast, the photocurrent density of PH CdS is more stable under illumination and maintains at 1.91 mA cm−2 after 400 s, only decreased by 3.5%. Thus the decrement rate of current is reduced by 88%. The in situ UV−vis diffuse reflectance spectra changes of SH CdS and TH CdS with time were also compared (Figure 6), in order to directly study the discrepancy of photostability between these two hybrids. In Figure 6a and the red plot in panel c, it is found that, at around 450 nm of CdS typical wavelength, the relative initial absorbance intensity of SH CdS is about 0.84. With the irradiation going on, the absorbance of CdS continuously decreases. After 120 min, the absorbance at

Figure 5. Photocurrent responses (vs SCE) in light on−off process for SH CdS (black) and TH CdS (red).

450 nm asymptotically approaches zero. The attenuation curve indicates that CdS in the SH system is suffering severe 12832

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TiO2 is atomic d character and that of CdS is s character, so the electron in TiO2 has larger effective mass than that in CdS.37−39 The smaller effective mass is consistent with the high mobility,40 and the electron separation and transport is more difficult in TiO2 as a result. To the latter, without TiO2 as a framework or glue to hold CdS, the BDD film was comparative smooth, so CdS NPs were hardly deposited onto the BDD surface. Furthermore, TiO2 is one of the best UV response semiconductors. It has been proved that the system of n-CdS/ n-TiO2 can expand absorption spectrum for the full use of solar energy. Thus the photo responses of this hybrid are also lower than other types of CdS hybrids. 3.4. Mechanism of Enhanced Photoactivity and Photocorrosion Inhibition. It has been shown that the SH system of CdS/TiO2 is effective in the case of the photocatalytic application. However, it is well-known that CdS will be oxidized by the photogenerated holes in water when exposed to light, according to21 hv

CdS + 2H 2O → H 2 + Cd2 + + S + 2OH−

(2)

Furthermore, the corrosion may be more serious when TiO2 exists. The energy band structure of the SH CdS is shown in Figure 7a. When the coupled CdS/TiO2 is activated by the UV−vis irradiation, according to23 CdS(e−+ h+) + TiO2(e−+ h+) → CdS(h+) + TiO2(e−)

(3)

Charge transfer is directional under the heterojunction effect and the inverse motions are thermodynamically unallowed. The photogenerated electrons rapidly leave the conduction band

Figure 6. In situ changes of UV−vis diffuse reflectance absorption spectra of SH CdS (a) and TH CdS (b) with time under UV−vis illumination of 100 mW cm−2 in 0.1 M Na2SO4 solution. (c) The changes of absorbance at typical wavelength (λ = 450 nm) for SH CdS (red) and TH CdS (black).

photocorrosion with little reserve. Figure 6b and the black plot in panel c show the changes of absorbance for TH CdS. Under irradiation for 120 min, the absorbance of CdS at 450 nm decreases from 0.90 to 0.71 with a low decay rate, which is only 22% of the former. It is revealed that the photoresponses and corrosion resistance of TH CdS is significantly increased after TH construction within other two p−n junctions. In addition, another two SH structures, i.e., n-TiO2/p-BDD and n-CdS/p-BDD, have been fabricated. To the former, the hybrid of TiO2 NTs/BDD film, much poorer photo responses are found (not shown here) under simulated solar irradiation than both SH CdS and TH CdS hybrids. There are two main reasons for TiO2 NTs/BDD behaving poorly without CdS. First, TiO2 cannot absorb dominant visible light in simulated solar illumination; second, the conduction band minimum of

Figure 7. Schematic illustration of the charge transfer, p−n junction, and band gap energy of SH CdS (a) and TH CdS (b). 12833

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initial photocurrent and 78% reduction in photocorrosion rate in contrast with SH CdS hybrid. It is speculated that heterojunction effect generates a field force for the separation of different charges, as well as the holes collection of BDD. Thus the accumulation of holes on CdS is reduced and the CdS photocorrosion is greatly restrained as well. This work opens the door to explore the physical properties of the triple heterojunction and their applications in the fields of energy source and environment, as well as new device technologies. Photocatalytic efficiency and stability are both important. The photocurrent value of TH CdS is really higher than that of SH CdS, but it still has ample room for improvement. For future research, the nanostructure of applied semiconductors could be optimized by means of more advanced fabrication methods, so that more efficiency nanoheterojunction could be constructed for charge separation and transfer.

(CB) of CdS and are picked up by TiO2 and then by the CB of FTO substrate.26 Meanwhile, the holes transfer may occur from the valence band (VB) of TiO2 toward the VB of CdS. There is competition with the reaction of recombination hole−electrons in the respective semiconductors.41 Finally, the excess of holes on CdS oxidizes the adsorbed compounds, and may also anodically autocorrode the CdS particles.42 If the photogenerated holes do not react quickly with Cd−OH groups, or chemisorbed water and/or the adsorbed compounds, photoanodic corrosion occurs.41,43 It induces a release of ion cadmium in solution and the formation of an elemental sulfur layer on the surface of the CdS particles (eq 2). This may not only affect significantly the photocatalysis reaction but also cause additional environmental damage. It is in accordance with the results of photocurrent responses (Figure 5) and spectra changes (Figure 6). In TH CdS, charge transfer is also directional. However, for the TH CdS, the situation is completely different (Figure 7b). It is generally acknowledged that the energetic differences between the corresponding bands of the semiconductors bring about a driving force. The greater the energetic difference between the bands is, the greater the driving force of photogenerated charge becomes. In Figure 7b, there is an n− n (CdS/TiO2) heterojunction and two p−n heterojunctions; that is, p-BDD/n-CdS and p-BDD/n-TiO2 exist in the TH. Before the formation of heterojunction, the Fermi level of ptype semiconductor is lower than that of n-type. When these three semiconductors come into contact with each other, the energy bands of BDD slope upward while the energy bands of CdS and TiO2 slope downward, until their Fermi levels are equal. In this situation, the CB position of TiO2 is lower than the corresponding band positions of CdS and BDD, so the former can act as an acceptor for the photogenerated electrons; the VB position of BDD is higher than those of CdS and TiO2, so the former can act as a sink for the photogenerated holes in the TH. When the hybrid is activated by the simulated solar irradiation, the electrons transfer from the CB of n-CdS toward the CB of n-TiO2 and are trapped by chemisorbed oxygen molecules to produce superoxide radical anions. On the other hand, the photogenerated holes on both CdS and TiO2 flow from their VB to the lowest VB of BDD, rather than to CdS like SH CdS. In TH CdS, the potential barrier of the p−n junction is much higher than that in SH CdS, so the separation effect of TH CdS might be stronger than that of SH CdS. The process mentioned above could be expressed through heterojunction theory as well. The photogenerated electrons could be forced into CB of TiO2 by the energy difference in the junction interface. To the holes, once they arrive at the space charge region of the p−n heterojunction, they would be swept into BDD by force of the internal electrostatic potential. Thus the separation of photogenerated charge carriers is enhanced, and there are not enough holes on CdS to induce the anodic autocorrosion. So the antiphotocorrosion performance can be found and presents durable, efficient photoresponses for the TH CdS hybrid.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21077077, 20877058), 863 Program (2008AA06Z329) from the Ministry of Science, P.R. China.

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REFERENCES

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4. CONCLUSION In conclusion, we present here the first use of triple heterojunction as a candidate for electron−hole pair separation and holes transfer. p-type BDD was introduced into coupled n−n type CdS/TiO2, forming a CdS TH. The structure of the as-prepared hybrid was optimized for charge separation and transfer. The TH CdS hybrid has 36.5% improvement on the 12834

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp300374s | J. Phys. Chem. C 2012, 116, 12829−12835