Fully Solar-Powered Photoelectrochemical Conversion for

May 13, 2014 - We report the development of a multifunctional, solar-powered photoelectrochemical (PEC)–pseudocapacitive–sensing material system f...
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Fully Solar-Powered Photoelectrochemical Conversion for Simultaneous Energy Storage and Chemical Sensing Yongcheng Wang,† Jing Tang,† Zheng Peng,† Yuhang Wang,† Dingsi Jia,† Biao Kong,† Ahmed A. Elzatahry,‡,⊥ Dongyuan Zhao,† and Gengfeng Zheng*,† †

Laboratory of Advanced Materials, Department of Chemistry, Fudan University, Shanghai, 200433, People’s Republic of China Department of Chemistry, King Saud University, Riyadh, 11451, Riyadh, Saudi Arabia ⊥ Polymer Materials Research Department, Advanced Technology and New Materials Research Institute, New Borg El-Arab City, Alexandria, Egypt ‡

S Supporting Information *

ABSTRACT: We report the development of a multifunctional, solar-powered photoelectrochemical (PEC)−pseudocapacitive−sensing material system for simultaneous solar energy conversion, electrochemical energy storage, and chemical detection. The TiO2 nanowire/NiO nanoflakes and the Si nanowire/Pt nanoparticle composites are used as photoanodes and photocathodes, respectively. A stable open-circuit voltage of ∼0.45 V and a high pseudocapacitance of up to ∼455 F g−1 are obtained, which also exhibit a repeating charging− discharging capability. The PEC−pseudocapacitive device is fully solar powered, without the need of any external power supply. Moreover, this TiO2 nanowire/NiO nanoflake composite photoanode exhibits excellent glucose sensitivity and selectivity. Under the sun light illumination, the PEC photocurrent shows a sensitive increase upon different glucose additions. Meanwhile in the dark, the open-circuit voltage of the charged pseudocapacitor also exhibits a corresponding signal over glucose analyte, thus serving as a full solar-powered energy conversion−storage−utilization system. KEYWORDS: Photoelectrochemical, solar energy conversion, pseudocapacitor, nanowire, sensor

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water oxidation, as well as the electric energy loss when driving charge carriers through multiple material interfaces and external circuits.16 Very recently, it has been developed for the direct coupling of solar conversion junctions with electrical energy storage materials/approaches, which may allow for efficient storage of the solar generated electrical energy. For instance, a solution-grown TiO2/Ni(OH)2 nanocomposite was synthesized to exhibit integrated solar hydrogen production and pseudocapacitive energy storage, in which the oxidative energy is stored by chemical conversion of Ni2+ into Ni3+.17 Nonetheless, due to the relatively low conduction band minimum of TiO2, an external electrical field is necessary to drive the electron flows for the water reduction, which has not warranted the goal of direct energy conversion and storage. More importantly, a material system which can mimic plants to simultaneously address solar energy conversion, storage, and utilization has not been realized yet. Herein, we report a multifunctional, solar-powered PEC− pseudocapacitor−sensor system for simultaneous photocurrent generation, electrochemical energy storage, and chemical

lants represent a highly hierarchical, multifunctional architecture that allows for efficient solar energy conversion, storage, and utilization, with limited types of required input sources such as sun light, water, and carbon dioxide. The semiconductor material-based photoelectrochemical (PEC) conversion, inspired by mimicking the energy conversion scheme of plants, is an attractive approach for solar energy utilization with minimum carbon footprints.1−6 Due to the stringent requirements including the semiconductor band gap size and alignment, charge transport kinetics, material stability, and cost, the energy conversion efficiency of PEC and other solar energy harvesting technologies are still limited.7−13 The recent substantial development of artificial photosynthesis approaches suggests that using two semiconductor light absorbers, with band diagrams configured as the “Z-scheme”,14 provides an effective approach to cover a larger part of solar spectrum for enhanced photoabsorption, as well as allows for efficient reduction and oxidation at each photoelectrode. For instance, Liu et al. reported the growth of a branched TiO2 nanowire (NW) photoanode and an Si NW photocathode, with an unbiased, spontaneous PEC water splitting efficiency of 0.12%.15 Nonetheless, the storage and utilization of the oxidative energy carried by the photogenerated holes at the photoanodes are generally low, due to large overpotential for © 2014 American Chemical Society

Received: April 19, 2014 Revised: May 10, 2014 Published: May 13, 2014 3668

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Figure 1. Schematic of the TiO2/NiO photoanode and the Si/Pt photocathode for the solar-powered PEC−pseudocapacitive system. (a) Structures of the materials and devices. (b) Solar-powered PEC−pseudocapacitive mechanism. (c) Energy diagram of the system.

Figure 2. (a) SEM, (b) TEM, and (c) HRTEM images of the TiO2/NiO photoanode. (d) SEM, (e) TEM, and (f) HRTEM images of the Si/Pt photocathode.

to H2 at the Si NW photocathode with the assistance of Pt nanoparticle catalyst. Distinct from the previous report of an electric field-biased device,17 this TiO2/Si system fully relies on solar energy for charge carrier separation and transport. More importantly, as the oxidation potential ordering of NiOOH/ NiO (∼1.45 V vs reversible hydrogen electrode, RHE)17 is located between that of water and glucose, the solar-energy

sensing. TiO2 and Si NWs, prepared by hydrothermal growth and chemical etching, are used as a photoanode and photocathode, respectively (Figure 1a, b). A thin layer of nickel oxide (NiO) nanoflakes is grown on the surface of the TiO2 NWs as the hole receptor, which can be converted into nickel oxyhydroxide (NiOOH) by the photogenerated holes. In the meantime, the photogenerated electrons can reduce water 3669

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Figure 3. Electrochemical measurement the solar-powered TiO2/NiO PEC-pseudocapacitor. (a) Photocurrent density measured at the shortcircuited connection of the photoanode and photocathode. (b) Open circuit potential versus the illumination (charging) time for the TiO2/NiO composite (red curve), pure NiO nanoflakes (black curve), and pure TiO2 NWs (blue curve). (c) Reflectance spectra of the TiO2 NW (black curve), as-made TiO2/NiO (blue curve), TiO2/NiO after 300 s light illumination (red curve), and TiO2/NiO after light illumination and then discharging (purple curve). Inset: photographs corresponding to the same order of the aforementioned four samples. (d) Discharge curves at different charge− discharge cycles.

∼150−200 nm and length of ∼1.5 μm (Figure 2a). Transmission electron microscopy (TEM) images reveal that each NW is uniformly covered by a layer of thin nanoflakes (Figure 2b). Lattice spacings of 0.209 and 0.241 nm are observed, corresponding to the (012) and (101) planes of NiO (Figure 2c). The uniform coating of NiO of the entire TiO2 NWs is further confirmed by the elemental mapping (Figure S1). The Si NW photocathode is synthesized by the metalcatalyzed electroless etching (Methods in the Supporting Information).18 After etching, the whole Si substrate is uniformly covered by a highly ordered array of thin wires, with lengths and average diameters of 7 μm and 200 nm, respectively (Figure 2d). Pt nanoparticles (NPs) are then deposited onto the Si NW surface by an electroless metal deposition method to increase the photocatalytic activity (Methods in the Supporting Information). The surface of each Si NW is decorated by small, ∼2−5 nm Pt NPs (Figure 2e). High-resolution TEM (HRTEM) images exhibit lattice spacing of 0.227 nm, consistent with the (111) planes of Pt NPs (Figure 2f). The electrochemical performance of the solar-powered PEC pseudocapacitor is first characterized by the photocurrent measurement. The time-dependent photocurrent of both the Pt NP-coated Si NW (designated as Si/Pt) photocathode and the NiO nanoflake-coated TiO2 NW (designated as TiO2/NiO) photoanode are first measured under simulated sunlight, and the pristine TiO2 NW photoanode is also measured for comparison (Figure S2). The photocurrent of TiO2/NiO photoanode shows a slightly higher photocurrent density than that of the pristine TiO2 NW photoanode. Then the TiO2/NiO

charged NiO nanomaterials may represent an efficient nonenzymatic glucose sensor in aqueous solution, with the capability of simultaneous/sequential electrochemical energy storage and glucose sensing. This work is the first time that a photoelectrochemical conversion is coupled with the pseudocapacitive energy storage, without the assistance of additional electrical bias voltage. In addition, it is the first time that an electrochemical sensor is fully driven by the solar energy, without any signal amplification methods. As proofs-of-concept, under AM 1.5G sun light illumination and no external electric field, the photogenerated electrons of TiO2 NW photoanode flows through the external circuit to the Si NW photocathode (Figure 1c). A stable open-circuit voltage of ∼0.45 V, a photocurrent of ∼0.1 mA cm−2, and a high pseudocapacitance of up to ∼455 F g−1 are obtained, which also exhibit repeated charging−discharging capability. Moreover, this TiO2 NW@ NiO nanoflake composite photoanode exhibits excellent glucose sensitivity and selectivity. Under the sun light illumination, the PEC photocurrent shows a sensitive increase upon the glucose addition, with a detection limit of 0.1 μM. While in the dark, the charged PEC pseudocapacitor also exhibits corresponding drops of the open-circuit voltage over glucose, thus allowing for a fully solar-powered PEC− pseudocapacitive sensor. The TiO2 NW@NiO nanoflake composite photoanode is fabricated by a hydrothermal growth method of TiO2 NWs on a fluorine-doped tin oxide (FTO) substrate,7 followed by a chemical bath deposition method to deposit NiO nanoflakes on the TiO 2 NW surface (Methods in the Supporting Information). Scanning electron microscopy (SEM) images show that a layer of NW arrays is formed, with the diameter of 3670

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Figure 4. (a) Schematic and (b) photocurrent measurement of the TiO2/NiO PEC−pseudocapacitive glucose sensor under the solar energy driving mode. The Si/Pt is used for the photocathode. The injection times of different glucose concentrations (final concentrations) are indicated by the arrows. (c) Schematic and (d) open circuit voltage measurement of the TiO2/NiO PEC−pseudocapacitive glucose sensor under the pseudocapacitive energy driving mode. A Pt wire is used for the counter electrode. The injection times of 100 μM glucose (final concentration) are indicated by the arrows.

photoanode (Figure 3c, inset), suggesting the conversion of TiO2/NiO to TiO2/NiOOH, consistent with a previous report.17 The reflectance spectra confirm that the TiO2/ NiOOH samples have the lowest reflectance over the entire wavelength range from 350 to 800 nm (Figure 3c). The X-ray photoelectron spectroscopy (XPS) shows a small doublet peak at 855.4 and 854.4 eV, corresponding to the Ni 2p3/2 peaks of Ni−OOH and Ni−O, respectively (Figure S4).17 Furthermore, this open circuit voltage can be stabilized for several days, with a gradual decrease to ∼0.3 V by the third day (Figure S5). To quantify the solar charged energy of the PEC pseudocapacitor, galvanostatic discharge tests are carried out at a current density of 0.5 A g−1. After discharging, the color of the TiO2/NiOOH photoanode changes from black to the original yellow−white, suggesting the conversion of NiOOH back to NiO (Figure 3c and inset). A discharge capacity of ∼455 F g−1 is obtained from the first discharge cycle, comparable to the reported values for the NiO−base electrochemical supercapacitors.21 The solar energy-driven charging and electrochemical discharging cycles can be repeated several times, while the open circuit voltage and the discharge capacity gradually decrease (Figure 3d). The discharge capacity drops to 60 F g−1 after 8 cycles. This degradation of PEC− pseudocapacitive energy storage can be attributed to the slow dissolution of Ni2+ in the electrolyte solution during the repeated charging and discharging cycles, which is confirmed by the gradual color reduction of the TiO2/NiO photoanode. Further coating of a thin carbon layer on the NiO surface can

photoanode and the Si/Pt photocathode are directly connected through a current meter (the short-circuited connection) and tested in a Na2SO4 electrolyte solution under an AM 1.5G simulated sun light (Methods in the Supporting Information). A fast photocurrent increase to ∼0.4 mA cm−2 is observed in the first several seconds, which gradually decreases to ∼0.1 mA cm−2 (Figure 3a). This photocurrent can be stabilized for a long time (Figure S3) and is well-correlated with the on/off cycles of the sun light. The photoconversion efficiency is calculated to be ∼0.12%, based on the equation:19 η = I (1.23 − V)/Jlight, where I is the photocurrent density at the measured potential, V is the applied voltage vs reversible hydrogen electrode (RHE), and Jlight is the irradiance intensity of 100 mW/cm2. In comparison, a similar photocurrent behavior is observed for the pristine TiO2 NW photoanode. Nonetheless, for pure NiO nanoflakes, negligible photocurrent is obtained, indicating the photoactivity is attributed to the TiO2 NWs. The PEC−pseudocapacitive energy storage behavior is then demonstrated by illuminating the PEC−pseudocapacitive device for different periods of time, then measuring the open circuit voltage in the dark to eliminate any interference of any photogenerated photovoltages.20 For pristine TiO2 NWs or NiO nanoflakes, no open circuit voltage is obtained when the sun light is switched off (Figure 3b). In contrast, the TiO2/NiO photoanode exhibits a well-defined voltage increase with the illumination time and reaches a saturation of ∼0.45 V after >100 s illumination. A corresponding color change from yellow−white to black is observed for the TiO2/NiO 3671

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energy for sensitive glucose detection. Further development of the PEC−pseudocapacitive material systems will allow for new opportunities in direct solar energy storage and sensor utilization, with enhanced efficiency, sensitivity, and capability.

help to improve the stability of the system. Nonetheless, the stability of the open circuit voltage is over several days, and the discharging time of each cycle is still maintained over hundreds of seconds, which are sufficient for the subsequent solarpowered sensing measurement. As NiOOH can selectively oxidize glucose,22 this TiO2/NiO photoanode is demonstrated for the solar-driven PECpseudocapacitive sensor for the glucose detection. Two different sensing modes are developed, with and without the sun light. In the first sensing mode (designated as the solar energy driving mode), a TiO2/NiO photoanode and a Si/Pt photocathode are directly connected and illuminated by a simulated sun light. The PEC−pseudocapacitive sensor is charged by converting NiO into NiOOH, and a photocurrent is obtained through the external circuit, as described previously (Figure 4a). When solutions containing different concentrations of glucose are delivered onto the sensor surface, the photogenerated holes can oxidize glucose into glucolactone,22,23 leading to a discrete increase of photocurrent wellcorrelated with each glucose addition (Figure 4b). The concentration dependence of the photocurrent increase is summarized, with the lowest detectable glucose concentration of 0.1 μM (Figure S6a), comparable or better than most of the previously reported nonenzymatic glucose sensors.22 The glucose selectivity of our TiO2/NiO PEC-pseudocapacitive sensor is demonstrated by introducing several interference molecules into the solution, including D-lactose, L-ascorbic acid, and sucrose at concentrations of 100 μM (Figure S6b). The sensor shows much lower responses to all of these interferences, compared to that from glucose at the same condition. In the second sensing mode when the TiO2/NiO photoanode is fully charged, the stored pseudocapacitive energy can be utilized for the electrochemical detection of glucose, designated as the pseudocapacitive energy driving mode (Figure 4c). A Pt electrode is used as the counter electrode, and the open circuit voltage is measured to indicate the discharge process. Upon repeated injections of glucose solutions (e.g., 100 μM), the open circuit voltage shows welldefined drops (Figure 4d), suggesting the increase of discharge process for glucose oxidation. In contrast to the sensing mode under sun light illumination that can be repeatedly tested for a long time, this discharge process has limited stored pseudocapacitive energy and thus can only be used for hundreds of seconds with a small glucose concentration range. Nonetheless, these results demonstrate that the TiO2/ NiO photoanode can successfully be utilized for the fully solarpowered glucose sensor. Further optimization of the NiO loading and stability will allow for higher sensitivity and longer utilization of the photoconverted pseudocapacitive energy. In summary, we have developed a novel solar-powered PEC−pseudocapacitive material system. The photoanode is composed of hydrothermally grown TiO2 nanowires coated with a layer of NiO nanoflakes, and the photocathode is Si NWs fabricated by the electroless etching method, followed by deposition of Pt nanoparticles over the Si surface. This material combination allows for spontaneous PEC reaction under sun light illumination, in which the oxidative energy is stored in the pseudocapacitive form by converting NiO into NiOOH. An open circuit voltage of ∼0.45 V and an initial discharge capacity of ∼455 F g−1 are obtained. Moreover, a solar-powered glucose sensor is demonstrated, which can function in either the solar energy driving mode or using the stored pseudocapacitive

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ASSOCIATED CONTENT

* Supporting Information S

Methods and supporting figures, with additional TEM mapping, photocurrent measurement, XPS spectra, open circuit voltage stability, and sensor results. This material is available free of charge via the Internet at http://pubs.acs.org.

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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 We thank the following funding agencies for supporting this work: the National Key Basic Research Program of China (2013CB934104), the Natural Science Foundation of China (21322311, 21071033), the Program for New Century Excellent Talents in University (NCET-10-0357), the Doctoral Fund of Ministry of Education of China, the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, and Deanship of Scientific Research of King Saud University (IHCRG no. 14102). The authors declare no competing financial interest.

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