PbS Nanoparticle Sensitized ZnO Nanowire Arrays to Enhance

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PbS Nanoparticles Sensitized ZnO Nanowire Arrys to Enhance Photocurrent for Water Splitting Xianchang Li, Jianxin Li, Chaojun Cui, Zhendong Liu, and YongSheng Niu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10003 • Publication Date (Web): 04 Feb 2016 Downloaded from http://pubs.acs.org on February 5, 2016

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The Journal of Physical Chemistry

PbS Nanoparticles Sensitized ZnO Nanowire Arrys to Enhance Photocurrent for Water Splitting Xianchang Li,* Jianxin Li, Chaojun Cui, Zhendong Liu,Yongsheng Niu Anyang Institute of Technology, Anyang, 455000, China *

Corresponding Author: Xianchang Li, e-mail, [email protected]

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PbS Nanoparticles Sensitized ZnO Nanowire Arrys to Enhance Photocurrent for Water Splitting Xianchang Li,* Jianxin Li, Chaojun Cui, Zhendong Liu,Yongsheng Niu Anyang Institute of Technology, Anyang 455000, China Abstract: Improving the visible light absorption is one of the key ways to optimize the photo electrochemical performance of Zinc oxide （ZnO） nanowire arrays (NWs). In this study, as-synthesized PbS NPs, which are adsorbed onto ZnO NWs through a dip coating method, are used to enhance the photocurrent of the ZnO NWs photo electrochemical anode for water splitting. The morphology crystalline nature and optical properties of the ZnO NWs and PbS nanoparticles (NPs) were characterized by TEM, HRTEM, XRD and UV-NIR absorption spectra. The hybrid anode exhibits a significant photocurrent density enhancement which is about ten times larger than that of pristine ZnO NWs. Moreover, we believe through some effective modifications, there is ample room for improvement of the photo electrochemical performance of PbS NPs sensitized ZnO NWs photo anode that can be achieved. Keywords: PbS nanoparticles, ZnO nanowire arrys, photoelectrochemical anode

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Introduction: Metal oxide nanowire arrays NWs refer to the photo electrode materials with great potential for solar energy conversion devices. These devices include: solar cell, photo electrochemical (PEC) cell, light emitting diodes, photodetectors, gas sensors, field emitters, (due to their quantum size effect) large surface-to-volume ratio, short exciton diffusion length, better cost-effectiveness, free electrons and holes or photons propagation1-3. In this area, ZnO and ZnO-based materials have been largely investigated for various technological applications in the fields of energetics, optics, electronics, catalysis and photo catalysis. Anodes which were made from n-type ZnO NWs are useful for light emitting diodes, photodetectors, gas sensors, field emitters, photonic crystals and solar or photo electrochemical cells because of their low electrical resistance and good corrosion resistance in aqueous solution. They are also an appropriate bandgap and have flat band potential for the splitting of water using solar energy4-8. Nevertheless, ZnO NWs have large bandgap energy (3.1～3.3 eV9), leading to limited light absorption in the visible region of interest which is the fundamental obstacle in containing high photo to hydrogen efficiency. Considerable efforts have been made to improve the visible light absorption of ZnO NWs in order to improving the PEC performance for water splitting. This includes nitrogen doping11-13 or hydrogen treatment14, constructing 3D11,15 or hybrid16,17 structures, piezotronic18 or plasmon19 inducing effect and nanoparticles NPs sensitization20-26. Semiconductor NPs are favored as superior sensitizers because their absorption spectra are readily matched to the power spectrum of the sun with tunable

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particle size. In this field, cadmium containing NPs are focalized on a photovoltaic device and have successfully sensitized ZnO NWs for improving the PEC performance21-24. Furthermore, PbS NPs as both infrared and visible absorber had obtained the highest solar AM 1.5G power-conversion efficiency for NPs photovoltaic device27. However, few studies focused on the PbS NPs sensitized PEC anodes for water splitting, including ZnO NWs photoelectrode. Recently, PbS nanoparticles decorated Al-doped ZnO (AZO) nanorod arrays via successive ionic layer adsorption and reaction (SILAR) route had been used as a photo anode for solar water splitting26. Although the hydrogen treated AZO nanorod arrays which were sensitized by the PbS NPs obtained higher photocurrent density than that of pristine AZO nanorod arrays, it is not recognized as an optimal approach for the reason that NPs in situ grew on metal oxide electrode28. Because the in situ NPs growth methods such as SILAR and chemical bath deposition (CBD) processes exhibit some loss of control over certain properties (particle size distribution, surface passivation, etc…) of the nanocrystallites29. To our knowledge, the majority of the PbS NPs solar cells with higher solar power conversion efficiency did not adopt the in situ NPs growth methods27,30. As we know, the PbS/ZnO NWs hybrid photoanodes which, made of ex situ growth method for hydrogen generation have not been investigated. Figure 1, shows a schematic of the fabrication steps for PbS NPs/ZnO NWs photo anode. ZnO NWs were synthesized over the entire surface of a fluorine-doped tin oxide (FTO) glass substrate using the hydrothermal method. PbS NPs were synthesized through the hot

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injection method and adsorbed on ZnO NWs through the dip coating method. Scanning electron microscopy (SEM), X-ray diffraction (XRD), UV-NIR absorption spectroscopy, transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) were utilized to further characterize specific nanostructures of a series of samples. The photo electrochemical property of pristine ZnO NWs photo anode and PbS NPs as well as ZnO NWs were also examined. Experimental section: Materials: Zinc nitrate (99.99%), hexamethylenetetramine (HMTA, >99%), polyethyleneimine (PEI, 30% in water), ammonia solution (20% in water), Oleylamine (OLA, >80%), ammonium hydroxide, lead chloride (PbCl2, 99.5%), elemental sulfur (S, 99.99%), 1,2-ethanedithiol (EDT, >98%), acetonitrile (anhydrous, >99%), hexane (anhydrous, >97%), ethanol (anhydrous, >99%), tetrachloroethylene (C2Cl4, >99%). ZnO NWs Synthesis: ZnO NWs were synthetized using the hydrothermal method, as adopted by Fan31. Firstly, ZnO nanoparticles were prepared according to the Pacholski method32. Zinc nitrate (0.01 M) was dissolved in methanol (125 mL) while vigorously stirred at approximately 60 °C. Subsequently, a 0.03 M solution of KOH (65 mL) in methanol was added dropwise at 60 °C. The mixture was then stirred for two hours at 60 °C to obtain the seeds of ZnO nanoparticle. Next, the seeds of ZnO nanoparticle were dropped onto the FTO substrate and the substrate was annealed at 120 °C to ensure particle adhesion to the surface and affixed at the kettle at a certain

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angle. The ZnO NWs growth solution consisted of 0.02 M zinc nitrate, 0.015 M HMTA, 0.004 M PEI, and 0.024 M ammonia solution. Next, the reaction kettle was placed in a vacuum drying oven and heated at 90 °C for 3 hours. Finally, the substrate was rinsed by the deionized water, and grew three fold in a fresh growth solution. PbS NPs Synthesis: The PbS NPs synthesis was based on the procedure of Cademartiri et al.33. Briefly, a stock solution of 0.032 g of S dissolved in 4 mL of OLA was prepared at 80°C. In a three neck flask, 0.56 g of PbCl2 was mixed with 12 mL of OLA and heated at 150 °C for 30 minutes. The sulfur OLA solution was then quickly injected into the flask while the temperature was allowed to decrease to 120 °C where it was then kept three hours for the growth of the nanocrystals. After centrifugation, the PbS NPs were re-dispersed in hexane for device fabrication. Device Fabrication: The PbS NPs were deposited onto ZnO NWs through the dip coating method. In this case, ZnO NWs substrates were dipped sequentially into the solutions of PbS NPs in hexane, EDT in acetonitrile and then actonitrile in a fume hood. After immersion in acetonitrile solution, the substrate was dried by nitrogen and then the next dip coating cycle was repeated. Characterization: JEM2010FEF (UHR) microscope with an acceleration voltage of 200 kV was used for transmission electron microscopy (TEM) imaging and high resolution TEM (HRTEM) measurement. PbS NPs in hexane were dripped onto the copper grids coated with amorphous carbon film. The hybrid PbS NPs and ZnO NWs sample was scraped from a FTO substrate and dispersed in ethanol. After five minutes of ultrasonic vibration, the sample was dripped onto the copper grids coated with

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amorphous carbon film for TEM and HRTEM measurement. The morphology and Elemental analysis of the ZnO NWs and hybrid anode were investigated with a Nova NanoSEM 450 field-emissions canning electron microscope (FE-SEM). The x-ray powder diffraction (XRD) patterns were obtained with the Bruka D8 Advanced X-Ray Diffractometer (Bruker axs) using Cu K-alpha radiation of wavelength 1.5406 Ǻ and the scan rate of 0.5 degree/min. The optical properties of the PbS NPs suspended in C2Cl4 and the reflectance spectroscopy of the hybrid photo anode were obtained using a Shimadzu UV-3600 spectrophotometer. Photoresponse Measurements: The photo electrochemical property was investigated with an AM 1.5 solar simulator (Newport, 100 mW) and an Autolab (model AUT84315) electrochemical workstation in a conventional three-electrode quartz cell. The reference electrode and auxiliary electrode were an Ag/AgCl electrode and platinum plate, respectively. A 0.1 M Na2SO4 aqueous solution was used as the electrolyte. IPCE data was obtained through an IPCE testing system (Newport). The photo response of PbS NPs sensitized ZnO NWs photoanodes for water splitting can be quantitatively studied by the incident photon to electron conversion efficiency (IPCE) curves. IPCE can be expressed as: IPCE = (1240 × I ) /(λ × Plight ) where I is the photocurrent density (mA/cm2), λ is the incident light wavelength (nm), and Plight (mW/cm2) is the power density of monochromatic light at a specific wavelength. The photo conversion efficiency (η) was evaluated using the equation:

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η=

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I (1.23 − Vapp ) Plight

where Vapp is the applied voltage versus RHE, I is the externally measured current density, and Plight is the power density of the illumination.

Results and discussion Vertically aligned ZnO NWs grown on FTO are characterized by SEM. Figure 2 displays top-down and plane-view SEM micrographs of ZnO NWs with the average lengths of 15 µm and the uniform diameter of 100 nm. The TEM and HRTEM image (Fig. 3 and Fig 5(c),(d)) reveals that the ZnO NW is single-crystalline in wurtzite crystal structure and the lattice fringe of d = 0.26 nm, corresponding to the (002) plane of the hexagonal ZnO, implying the growth along c-axis for the ZnO plates. Notably, the HRTEM measurement points out to a local structure, while the XRD pattern is representing the global nature of the sample. Figure 3(b) shows the XRD pattern of the ZnO NWs. All of the diffraction peaks can be indexed to the wurtzite hexagonal phase of ZnO (JPCDS #36-1451). It also can be seen that these diffraction peaks are sharp and intense, indicating its highly crystalline nature. The morphology and crystallinity of the as-synthesized PbS NPs is investigated using HRTEM. As shown in Figure S1, the PbS particles show spherical shape and exhibit well crystalline nature. The strong particle contrast allows easy detection of the border in such a way that the single-nanocrystal diameter can be measured with a good precision. The typical TEM image of the PbS nanostructures clearly shows that NPs are of nearly identical size with an average diameter of 6 nm and HRTEM image confirms the crystallinity showing well-resolved lattice planes with an inter-planar 8


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distance of 0.29 nm corresponding to (200) planes of PbS bulk rock salt structure. The XRD spectrum obtained from as-synthesized PbS NPs powder is shown in Figure S2(a). All of the diffraction peaks match well with Bragg reflection conditions of the standard face-centred cubic (fcc) structure of bulk PbS (JCPDS #05-0592). The strongest intensity of the 200 reflection is supportive to the previous HRTEM observations. For the 6 nm size of PbS quantum dots, its first exciton absorption maximum peak always observed at around 1440 nm28. In Figure S2(b), a strong intensity peak at 1490 nm which shows the optical absorption spectrum of the OLA capped PbS NPs in C2Cl4 is supportive of this viewpoint. In order to compare the photochemical performance of the bare ZnO NWs with PbS decorated ZnO NWs, the FTO substrates are cut to the same size and the ZnO NWs are grown under the same conditions. The lengths of 15 µm ZnO NWs are used because they have a large surface area for PbS NPs adsorption, contrasted to that of shorter ZnO NWs. It is a simple method for the direct absorption of as-synthesized PbS NPs onto ZnO NWs surface. We employ EDT to link PbS NPs to ZnO NWs surface by using the dip coating method. After three dip coating process, there are a few oily patches on the ZnO NWs surface(see Fig. 4), which are not completely substituted by EDT legands. As can be seen from the Figure 5, PbS NPs are firmly bound onto ZnO NWs surface, although the hybrid PbS NPs and ZnO NWs film had been ultrasonically shaken for a few minutes for TEM sample preparation. To ensure the efficient charge transfer the interspace between PbS NPs and ZnO NWs is no more than one nanometer.

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The photocurrent was measured under chopped AM 1.5 illumination (100 mW/cm2), so the dark and light current could be monitored simultaneously. The representative current-voltage (I-V) and current-time (I-t) plots of the PbS NPs/ZnO NWs electrode (6 dip-coating cycles) as well as the pristine ZnO NWs electrode are presented in Figure 6. As we can see, both PbS NPs/ZnO NWs and pristine ZnO NWs electrode show small dark current density and pronounced photoresponse under light illumination. Due to the low efficiency of pristine ZnO NWs, the PEC performances of this study was not superior. In ref.[10, 26], the authors increased the prinstine ZnO NWs carrier concentration by doping N or Al, and thereby increasing the concentration of photo-generated carriers. In present study, we increased the prinstine ZnO NWs photo-generated carriers by PbS NPs sensitizing like ref.[23]. The energy gaps of the as-synthesized PbS NPs are approximately 0.86 eV and the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) is -4.1 eV and -5.0 eV, respectively, and both LUMO and HOMO of PbS NPs lie above those of ZnO (-4.4 & -7.6,). In such a configuration, the photo generated electrons can be easily injected from the PbS NPs into the ZnO NWs, driven by the band alignment, and then transported to the FTO substrate along with the single-crystal ZnO, which provides a direct path for electron transport. ZnO NWs decorated with PbS NPs show a significant photocurrent density enhancement with 0.6 mA/cm2 at +0.8 V (vs. Ag/AgCl electrode) which is about 10 times larger than that of pristine ZnO NWs. Figure 6(b) plots the I-t curve, at +0.8 V, of the ZnO nanowires that were decorated using PbS NPs. Significantly, upon illumination, a

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spike in the photoresponse was obtained, which indicates efficient charge separation in ZnO NWs upon illumination. The photoconversion efficiency of light energy to chemical energy in the presence of applied potential could be calculated from the I-V curve(which shown in Fig S3). The potential was measured against an Ag/AgCl reference and converted to RHE potential by using the equation E(RHE) = E(Ag/AgCl) + 0.1976 V + 0.059 pH. The photo conversion efficiency result is displayed as well in Figure 7. The pristine ZnO NWs in our study were obtained with a maximum efficiency of 0.008% at applied voltage of +0.9 V versus RHE. The PbS NPs sensitized ZnO NWs yielded a maximum efficiency of 0.09% at applied voltage of +0.85 V versus RHE,, which is approximately ten times higher than that of the pristine ZnO NWs. Measured IPCE spectra of PbS NPs-sensitized ZnO NWs (square) and pristine ZnO NWs (circle) in the region of 350 to 600 nm at a potential of +0.6 V are also apparent (Fig. 8). It is clear that IPCE data collected from the PbS NPs-sensitized ZnO NWs illustrated a significant enhancement compared to the pristine ZnO NWs in both the visible and UV regions. This is caused, primarily, to the increase in the light absorption by the PbS NPs. As a full spectrum adsorber, PbS NPs can adsorb light from the UV to the infrared region. However, IPCE response of hybrid anode shows low enhanced efficiency in the 500~600 nm region in this study. It maybe because ZnO NWs strongly absorb only in the UV region, and then the PbS NPs improved the absorption efficiency mainly in the visible and UV regions. To explore the effect of dip coating cycles on the photochemical performance of

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electrodes, every five pieces of identical ZnO NWs substrates were divided into a group to fabricate the electrodes. PbS NPs were deposited on each group of 3, 6, 9 and 12 iterations of dip coating process, respectively. The representative I-V plots of the PbS NPs/ZnO NWs electrodes through different dip coating cycles are also visible (Fig. 9). The results clearly confirm that the electrode through six dip coating cycles has the highest photocurrent density under the same condition. Following the six dip coating cycles, the photocurrent density decreases with the increase of dip coating cycles. It indicates that the thickness of PbS NPs film will affect the chance of electron hole recombination. This limits the electron collection efficiency and thereby reaches the maximum photocurrent. Thus, there is a proper thickness of PbS NPs film to obtain the highest photocurrent density.

Conclusion In summary, in our study, PbS NPs were used to enhance the ZnO NWs photo electrochemical anode for water splitting. ZnO NWs that were synthesized over the entire surface of a FTO glass substrate using the hydrothermal method. PbS NPs were synthesized through the hot injection method and adsorbed onto ZnO NWs through the dip coating method. It was demonstrated that the morphology and crystalline nature and optical properties of the ZnO NWs and PbS NPs were characterized by TEM, HRTEM, XRD and UV NIR absorption spectra. PbS NPs were sensitized ZnO NWs through the dip coating method and HRTEM showed that PbS NPs were firmly bound onto ZnO NWs surface. Although the PEC performances of this study was not superior than the state of the art literature values on homologous systems. The typical

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hybrid photo anode showed a significant photocurrent density enhancement with 0.6 mA/cm2 at +0.8 V (vs. Ag/AgCl electrode) which is about ten times larger than that of pristine ZnO NWs. IPCE data approved that the PbS NPs sensitized ZnO NWs possessed an enhanced light absorption compared with the pristine ZnO NWs in both the visible and UV regions. In addition, the study of dip coating cycle’s number effect demonstrates that there is a proper thickness of PbS NPs film to obtain the highest photocurrent density. The result proves that PbS NPs can improve the visible light absorption of ZnO NWs photo anode and we believe much higher efficiencies can be obtained through certain effective modifications. We speculate that much higher efficiencies can be obtained through the following modifications: (i) Use longer ZnO NWs or high surface area branched nanostructures to increase PbS NPs uptake and enhance photocurrent; (ii) Optimize the size of PbS NPs to match the bandgap of ZnO NW to improve the photo generated electron injection efficiency from PbS NPs to ZnO NWs; (iii) Nitrogen doping or hydrogen treatment as well as a severe annealing process of ZnO NWs to reduce crystal defects; (iv) Select a better bifunctional molecule to reduce the PbS NPs surface defects and lower the rate of electron hole recombination across the QD-NW interface. Acknowledgements. This work was financially supported by the National Natural Science Foundation of China (No. 11447150).

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[22] Chouhan, N.; Yeh, C.; Hu, S.; Liu, R.; Chang, W. and Chen, K. Photocatalytic CdSe NPs-decorated ZnO nanotubes: An Effective Photoelectrode for Splitting Water. Chem. Commun. 2011, 47, 3493-3495. [23] Wang, G.; Yang, X.; Qian, F.; Zhang, J. and Li, Y. Double-Sided CdS and CdSe Quantum Dot Co-Sensitized ZnO Nanowire Arrays for Photoelectrochemical Hydrogen Generation. Nano Lett. 2010, 10, 1088-1092. [24] Chen, H.; Chen, C.; Chang, Y.; Tsai, C.; Liu, R.; Hu, S.; Chang, W. and Chen, K. Quantum Dot Monolayer Sensitized ZnO Nanowire-Array Photoelectrodes: True Efficiency for Water Splitting. Angew. Chem. 2010, 122, 6102-6105. [25] Chen, H.; Chen, C.; Lin, C.; Liu, R.; Yang, H.; Chang, W.; Chen, K.; Chan, T.; Lee, J. and Tsai, D. Multi-Bandgap-Sensitized ZnO Nanorod Photoelectrode Arrays for Water Splitting: An X-ray Absorption Spectroscopy Approach for the Electronic Evolution under Solar Illumination. J. Phys. Chem. C 2011, 115, 21971-21980. [26] Hsu, C.; Chen, C.; Chen, D. Decoration of PbS nanoparticles on Al-doped ZnO Nanorod Array Thin Film with Hydrogen Treatment as A Photoelectrode for Solar Water Splitting. J. Alloy. Compd. 2013, 554, 45-50. [27] Chuang, C-H. M.; Brown, P. R.; Bulovic, V.; Bawendi, M. G. Improved Performance and Stability in Quantum dot Solar Cells through Band Alignment Engineering. Nat. Mater. 2014, 13, 822-828. [28] Hyun, B-R.; Zhong, Y.; Bartnik, A. C.; Sun, L.; Abrun, H. D.; Wise, F. W.; Goodreau, J. D.; Matthews, J. R.; Leslie, T. M. and Borrelli, N. F. Electron

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Figures captions:

Figure 1. The schematic of the fabrication steps for PbS NPs/ZnO NWs photoanode.

FTO

ZnO nanowire

PbS nanoparticle

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Figure 2. The top-down(a) and plane-view (b) SEM micrographs of ZnO NWs.

(a)

(b)

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Figure 3. The TEM image (a) and XRD pattern (b) of the ZnO NWs.

(a)

100 nm

(b)

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Figure 4. SEM image and EDX spectrum(insert image) of PbS/ZnO film.

Figure 5. The TEM and HRTEM of PbS/ZnO heterojunction.

(a)

(b)

(c)

(d) ZnO (002) d = 0.26 nm

ZnO (002) d =0.26 nm PbS (200) d =0.29 nm

PbS (200) d = 0.29 nm 5 nm

5 nm

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Figure 6. The I-V (a) and I-t curve (b) of pristine ZnO NWs and PbS/ZnO heterojunction.

(a)

(b)

Figure 7. The PCE of pristine ZnO and PbS/ZnO heterojunction.

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Figure 8. The IPCE of pristine ZnO and PbS/ZnO heterojunction.

40 PbS/ZnO ZnO

30 IPCE(%)

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20 10 0 350

400 450 500 Wavelength(nm)

550

600

Figure 9. The representative current-voltage (I-V) plots of the PbS NPs/ZnO NWs electrode via different dip-coating cycles.

3

6

9

12

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Table of Contents Graphic

Sun light

A eO2

e-

H2

h+

eH+

H 2O

PbS NPs/ZnO NWs/FTO anode

Pt cathode

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Supporting Information

PbS Nanoparticles Sensitized ZnO Nanowire Arrys to Enhance Photocurrent for Water Splitting Xianchang Li,* Jianxin Li, Chaojun Cui, Zhendong Liu,Yongsheng Niu Anyang Institute of Technology, Anyang, 455000, China *

Corresponding Author: Xianchang Li, e-mail, [email protected]

Figure S1. The morphology and crystallinity of the as-synthesized PbS NPs.

(a)

(b)

PbS (200) d = 0.29 nm

2 nm

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Figure S2. The XRD (a) and optical absorption spectrum(b) of as-synthesized PbS NPs.

Figure S3. The I-V curve of pristine ZnO and PbS/ZnO heterojunction.

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PbS Nanoparticle Sensitized ZnO Nanowire Arrays to Enhance

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