Unbiased Sunlight-Driven Artificial Photosynthesis of Carbon

Jun 30, 2016 - Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology (KIST), Seoul, 02792 South Korea. § KU-KIST Gradua...
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Unbiased Sunlight-Driven Artificial Photosynthesis of Carbon Monoxide from CO2 Using a ZnTe-Based Photocathode and a Perovskite Solar Cell in Tandem Youn Jeong Jang,†,⊥ Inyoung Jeong,†,‡,⊥ Jaehyuk Lee,†,# Jinwoo Lee,† Min Jae Ko,*,‡,§ and Jae Sung Lee*,∥ †

Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang, 37673 South Korea Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology (KIST), Seoul, 02792 South Korea § KU-KIST Graduate School of Converging Science and Technology, Korea University, Seoul, 02841 South Korea ∥ Department of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulsan, 44919 South Korea ‡

S Supporting Information *

ABSTRACT: Solar fuel production, mimicking natural photosynthesis of converting CO2 into useful fuels and storing solar energy as chemical energy, has received great attention in recent years. Practical large-scale fuel production needs a unique device capable of CO2 reduction using only solar energy and water as an electron source. Here we report such a system composed of a gold-decorated triple-layered ZnO@ZnTe@CdTe core− shell nanorod array photocathode and a CH3NH3PbI3 perovskite solar cell in tandem. The assembly allows effective light harvesting of higher energy photons (>2.14 eV) from the front-side photocathode and lower energy photons (>1.5 eV) from the back-side-positioned perovskite solar cell in a single-photon excitation. This system represents an example of a photocathode−photovoltaic tandem device operating under sunlight without external bias for selective CO2 conversion. It exhibited a steady solar-to-CO conversion efficiency over 0.35% and a solar-to-fuel conversion efficiency exceeding 0.43% including H2 as a minor product. KEYWORDS: artificial photosynthesis, unbiased photoelectrochemical cell, ZnTe photocathode, perovskite solar cell, CO2 reduction

A

rtificial photosynthesis (AP) refers to conversion of CO2, the most troublesome greenhouse gas, to storable fuels using sunlight and has received great attention recently.1−4 Although the combustion of the generated fuels reproduces CO2, the AP process is useful to recycle CO2 without adding a new carbon source to the environment. Photoelectrochemical (PEC) CO2 reduction suggests an ideal pathway to produce carbon-based fuels from CO2 using abundant solar energy and water in a sustainable manner. However, researches on solar fuel production have been directed heavily toward hydrogen as the target product by PEC water splitting. Relative to water reduction, efficient and costeffective CO2 reduction poses a greater challenge due to higher overpotential of the reaction and low selectivity of products.5,6 A photoelectrode−photovoltaic (PV) dual absorber tandem system has been suggested for a stand-alone PEC device especially for solar water splitting to produce H2 with the advantages of smart management of incident photons from each absorber and supplying sufficient photovoltage for the spontaneous PEC water splitting.7−11 For effective CO2 © 2016 American Chemical Society

conversion in the tandem system, the photocathode needs to meet several criteria: a good transparency, a wider band gap relative to the PV cell yet small enough to absorb ample visible light, a low overpotential for the CO2 reduction, and selective product formation. ZnTe is an attractive photocatalyst for CO2 reduction because it has a suitable band gap (2.26 eV) for effective light harvesting and its most negative conduction band edge position (−1.63 VRHE) among known p-type semiconductors offers a strong driving force to reduce CO2.12−14 In an attempt to obtain a highly effective and transparent photocathode, here we fabricated a ZnO@ZnTe nanorod array film on F-doped SnO2 (FTO) with modification by ntype CdTe to form a p−n junction of the p-ZnTe@n-CdTe core−shell nanorod (NR) structure.15−17 Furthermore, coupling with a Au electrocatalyst on the photocathode facilitated Received: May 4, 2016 Accepted: June 30, 2016 Published: June 30, 2016 6980

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Figure 1. (a) XRD patterns of ZnO (ZO), ZnO@ZnTe (ZT), and ZnO@ZnTe@CdTe (ZCT) films. The peaks of SnO2 from the FTO substrate are marked as *. (b) Profiles of element composition along the depth of ZCT determined by XPS and schematic structure of a ZCT core−shell nanorod in the inset. (c) Absorbance and transmittance of ZO, ZT, and ZCT.

Figure 2. SEM images of ZnO (a), ZnO@ZnTe (b), and ZnO@ZnTe@CdTe (c) films (the cross-view in insets). HRTEM of a ZCT film in low magnification (d) and in high magnification of the marked area of d (e).

conversion device by Schreier et al.1 is based on a conventional configuration, i.e., a triple-junction perovskite solar cell connected with electrodes for electrochemical CO2 reduction.

charge separation by forming an ohmic junction and produced CO selectively by providing active catalytic sites. As mentioned, a PV system is needed in a tandem system to provide bias potentials to drive the spontaneous CO2 reduction on the electrode. Recently, organic−inorganic halide perovskite compounds have emerged as powerful PV materials with their suitable band gaps, an excellent carrier diffusion length, a small exciton binding energy, and a rapidly growing power conversion efficiency with a record of 22.1%.18−20 Taking advantage of its high open-circuit photovoltage (Voc) and simple and low cost fabrication, the perovskite solar cell can be utilized as an efficient bias potential supplier for unassisted CO2 reduction. As a stand-alone solar CO2 reduction device working under sunlight without any external bias, here we fabricate an AP device composed of the modified ZnTe photocathode− perovskite solar cell in tandem. The tandem device is capable of utilizing incident light effectively, i.e., higher energy photons (>2.14 eV) by the front ZnTe electrode and relatively lower energy photons (>1.5 eV) by the solar cell. The device has demonstrated a steady solar-to-CO efficiency of over 0.35% and a solar-to-fuel efficiency exceeding 0.43% including H2 as a minor product. To our best knowledge, this system represents an effective photocathode−PV tandem device operating under sunlight without external bias, which exhibits selective CO production from CO2. The recently reported CO2-to-CO

RESULTS AND DISCUSSION Fabrication of a Transparent Triple-Layered ZnO@ ZnTe@CdTe Core−Shell Nanorod Array Film. A highly active, transparent ZnTe-based photocathode was fabricated by three steps of solution-phase reactions as depicted in Scheme S1 of the Supporting Information (SI). The hydrothermally grown ZnO nanorod array on FTO glass was superficially converted into ZnTe by an anion exchange reaction with a Te2− source, and then the surface of ZnO@ZnTe NR was again partly transformed into CdTe, a well-known light absorber having a narrow band gap, by a cation exchange reaction with a Cd2+ source to complete the formation of a triple-layered ZnO@ZnTe@CdTe core−shell NR array film on FTO. The ion exchange reactions were based on the difference of solubility product constants (Ksp) for ZnO (6.8 × 10−17), ZnTe (5.0 × 10−34), and CdTe (3.2 × 10−42).21 The optimization processes of ZnO, ZnTe, and CdTe synthesis reactions are presented in Figures S1−S3 in the SI to find the optimal thickness of each layer (controlled by reaction durations) that gives the highest PEC activity. The corresponding scanning electron microscopy (SEM) images and optical properties are also presented. Coincidentally, the ZnO@ 6981

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ACS Nano ZnTe@CdTe photocathode fabricated by the three synthesis reactions of 2 h each showed the best activity with a good transparency. The ZnO nanorods that were shorter than optimum length had a low active surface area with a minimal benefit of one-dimensional (1D) nanorods. The longer ZnO nanorods reduced the transparency of the film. Also, the thinner middle shell of ZnTe presented insufficient light harvesting, and the thicker ZnTe layer showed mechanical instability due to crystal expansion. Additionally, too thick a CdTe layer lowered the photocathodic performance with a cathodic-shifted onset potential. The transformation of ZnO (ZO) to ZnO@ZnTe (ZT) and to ZnO@ZnTe@CdTe (ZCT) was confirmed by X-ray diffraction (Figure 1a). In the X-ray diffraction (XRD) pattern of ZT, the peak intensity of hexagonal wurtzite ZnO (JCPDS no. 01-089-0511) was reduced with the appearance of the zinc blende ZnTe (JCPDS no. 01-065-0385) peak. In the case of ZCT, the pattern of CdTe (JCPDS no. 00-015-0770) was observed together with those of ZnTe and ZnO. The profiles of elemental composition (Zn, O, Te, and Cd) were obtained by depth-profiled X-ray photoelectron spectroscopy (XPS) in Figure 1b. In the core within a radius of ca. 30 nm, ZnO was positioned and ZnTe densely covered the ZnO core in the range ca. 30−50 nm. Then a thin CdTe layer of ca. 7 nm formed the outer surface of ZCT, confirming a triple-layered ZnO@ZnTe@CdTe core−shell NR structure (see inset). The finite oxygen content in the ZT and CT regions is due to superficial oxidation of telluride by exposure to air and solution during the fabrication steps.22 The extension of the light absorption range to visible light was observed by ZnTe and CdTe layer formation on ZnO, as shown in Figure 1c. Thus, the white-colored ZnO absorbed only UV light, but the absorption edge drastically shifted to the visible light region to ca. 550 nm for reddish ZT and to ca. 580 nm for dark reddishbrown ZTC. The optimized ZCT films exhibited a good transparency required for assembling a stacked tandem cell system. The detailed morphology of the films was investigated by SEM in Figure 2 and Figures S1−S3 of the SI. The sharp ZnO nanorods 10−40 nm in diameter and 500 nm in length became a ZnO@ZnTe composite with enlarged diameters and rough surface texture after the anion exchange reaction as shown in Figure 2a and b. After CdTe modification, the change in diameter and length in ZnO@ZnTe@CdTe nanorods was not significant because the CdTe layer was so thin in Figure 2c. The high-resolution transmission electron microscopy (HRTEM) revealed the presence of CdTe and ZnTe in Figure 2d and e. A lattice spacing of 0.351 nm for zinc blende ZnTe (111) was observed in the inner layer, whereas that of 0.370 nm CdTe (111) was observed in the thin outer CdTe layer. Photoelectrochemical CO2 Reduction to CO under Simulated Sunlight. The fabricated electrodes were applied to photoelectrochecmical CO2 reduction in the three-electrode configuration with Ag/AgCl reference and Pt wire counter electrodes. The performance was monitored under chopped 1 sun illumination to estimate the activities under dark and light conditions simultaneously in CO2-saturated KHCO3 electrolyte as shown in Figure 3. To demonstrate beneficial effects of the NR morphology, a ZnO@ZnTe NR electrode was compared with a planar-type electrode (ZT PL). The ZT PL was fabricated from the sputtered ZnO on FTO followed by the ion exchange reaction, and its morphological and compositional properties were investigated in Figure S4. The ZT PL

Figure 3. J−V curves for photocathodes synthesized with and without a CdTe outer layer on a ZnTe nanorod (NR) electrode. The inset is J−V curves for the planar photocathode (PL) with and without a CdTe upper layer. All experiments were carried out in CO2-purged 0.5 M KHCO3 electrolyte under 1 sun illumination.

photocathode showed the onset potential of ca. 0.35 VRHE and photocurrent of −0.17 mA cm−2 at −0.11 VRHE (theoretical CO2/CO redox potential). The ZT NR exhibited a positively shifted onset potential of ca. 0.42 VRHE with an improved photocurrent of −0.90 mA cm−2 at −0.11 VRHE. Then the photocathode was additionally modified with a thin n-type CdTe layer by a cation exchange reaction. The ZnO@ ZnTe@CdTe nanorod (ZCT NR) electrode exhibited a much more positively shifted onset potential of ca. 0.6 VRHE with an enhanced photocurrent of −2.15 mA cm−2 at −0.11 VRHE. The effect of CdTe was also observed in the planar type electrode. The onset potential and photocurrent at −0.11 VRHE of ZCT PL were recorded ca. 0.55 VRHE and −0.38 mA cm−2, but still its performance was much inferior to that of the NR electrode. Thus, the ZCT NR is an optimized photocathode with excellent light harvesting, charge transport, and charge collection, which originate from its 1D nanostructure and p− n heterojunction formation between ZnTe and CdTe. In order to further improve the charge separation and provide catalytic reaction sites for CO2 reduction, the ZnO@ ZnTe@CdTe NR photocathode was decorated by Au nanoparticles as a cocatalyst. Thus, Au nanoparticles were deposited on a ZCT NR (ZCT NR-Au) photocathode by e-beam evaporation. The presence of Au was confirmed by SEM-EDS and XPS analysis in Figure S5. The 0.7 wt % of Au was loaded on the ZCT electrode as determined by inductively coupled plasma (ICP) measurements. The performance of the ZCT-Au photocathode was evaluated by linear sweep voltammetry in the CO2-purged KHCO3 electrolyte under chopped 1 sun illumination in Figure 4a. The onset potential of ZCT NR-Au was the same as the value of ZCT NR. However, its photocurrent density was enhanced significantly relative to ZCT NR. At −0.11 VRHE, the photocurrent of ZCT NR-Au (−3.88 mA cm−2) was at least 1.5 times higher than that of ZCT NR (−2.15 mA cm−2). Also, at 0.3 VRHE, ZCT NR-Au (−0.56 mA cm−2) exhibited a ca. 4-fold improved photocurrent relative to that of ZCT NR (−0.13 mA cm−2). To investigate the distribution of CO2 reduction products, chronoamperometry was carried out at −0.11 and 0.3 VRHE for 90 min under simulated 1 sun irradiation as shown in Figure S6 6982

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Figure 4. Effects of Au nanoparticles as an electrocatalyst. (a) J−V curves for photocathodes with and without Au nanoparticles on ZnO@ ZnTe@CdTe nanorod electrodes. The inset photograph shows the transparent ZCT NR-Au electrode. (b, c) Gas evolutions at −0.11 and 0.3 VRHE for ZCT NR and ZCT NR-Au photocathodes, respectively. (d) Faradaic efficiencies of CO and H2 for ZCT NR and ZCT NR-Au photocathodes at different applied potentials (−0.11 and 0.3 VRHE) in the CO2-saturated KHCO3 electrolyte under simulated 1 sun illumination. The numbers on the top of the columns are the selectivity of CO (%) from CO2 reduction relative to H2 production from H2O reduction.

Figure 5. Unbiased photoelectrochemical CO2 reduction in stacked tandem cells. (a) Schematic diagram of tandem cell comprising a ZnO@ ZnTe@CdTe-Au NR photocathode (ZCT-Au PC)-perovskite solar cell (Perov SC)-cobalt bicarbonate (Co-Ci) anode.7 (b) Plots of utilized incident photons of AM 1.5G spectrum by two light absorbers. (c) IPCE responses of SC, PC, and the tandem device. The IPCE of ZCT-Au was measured at −0.11 VRHE. (d) J−V curves of ZCT-Au photocathode and Co-Ci anode measured in three-electrode configuration with overlaid response of the solar cell in the stacked tandem device. The actual size of the PC and solar cell was 1.5 × 1.5 and 0.3 × 0.43 cm2, respectively. (e) Chronoamperometry and time-profiled production of CO and H2 for 3 h in the tandem device. (f) Determined faradaic efficiencies of CO and H2 during 3 h in the tandem cell with a CO2-saturated KHCO3 electrolyte under simulated 1 sun illumination. The values on top of the columns represent CO selectivity.

in the SI. According to the analyses of high-performance liquid chromatography and gas chromatography, the products from photoelectrochemical CO2 reduction with our ZCT NR-Au photocathode were only H2 and CO. Other possible CO2 products such as formic acid, alcohols, and hydrocarbons were

not detected at all. The time-profiled gas products were monitored for ZCT NR and ZCT NR-Au photocathodes as shown in Figure 4b and c, respectively. The faradaic efficiency (actual gas evolution/gas evolution expected from current generation) and CO selectivity are summarized in Figure 4d 6983

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Au photocathode provides the sites for CO2 reduction as the main light absorber, while the solar cell provides the bias voltage needed for the spontaneous reaction. The Co-Ci/Ni foam anode balances the overall reaction by oxygen evolution under dark conditions. The intersection of individual J−V curves of the solar cell and the photocathode determines the predicted operating point of the tandem system. Since the three components are connected in series, the tandem system operates at the same current. Thus, it is expected that the tandem system operates at the current of 0.85 mA when the solar cell supplies a voltage of 1.04 V. The actual capability of the tandem system was monitored by measuring the evolution of gaseous products without external bias under 1 sun illumination for 3 h in Figure 5e. The steady evolution of CO and H2 was observed with faradaic efficiencies above 81.9% and a CO selectivity above 74.9%, as summarized in Figure 5f and Table S2 of the SI. The imperfect faradaic efficiency was originated from the photoelectrochemical instability and corrosion of Te-based materials in aqueous solution and Au detachment (0.5 wt % Au on the electrode) as shown in Figure S9. The solar-to-CO efficiency (ηSTC) was determined by eq 1:1

and Table S1 in the SI. The amounts of generated gases increased with increasing external bias, reflecting stronger electrical field formation provided by the bias. Interestingly, Au deposition brought a dramatic shift in product distribution. The bare ZCT NR photocathode produced mainly H2 (>80%) by H2O reduction, whereas Au-decorated ZCT NR produced mainly CO (∼80%) by CO2 reduction. Additionally, the ZCT NR-Au electrode produced larger amounts of overall gas products than the bare ZCT electrode, reflecting its enhanced photocurrent in Figure 4a. The faradaic efficiencies were higher than 80% in all cases (Table S1). Unbiased CO2 Reduction in a Photocathode−Photovoltaic Tandem Device. For practical artificial photosynthesis, we need to develop a stand-alone device that can produce CO from CO2 reduction under only solar light without any external energy supply. Here we propose such a device as shown in Figure 5a, which assembles a ZnO@ZnTe@CdTe-Au photocathode (ZCT NR-Au PC) and a CH3NH3PbI3 perovskite solar cell (Perov SC) in tandem.7 The solar cell in the back supplies the bias voltage needed for spontaneous CO2 reduction by using low-energy photons that are transmitted through the front photocathode unused. Here, we prepared a highly efficient CH3NH3PbI3 perovskite solar cell by a modified adduct-induced growth method originally reported elsewhere.23 The recently developed solar cell is known to have as high an efficiency yet much lower cost than conventional silicon solar cells. The photovoltaic characteristics are presented in Figure S7 of the SI. A pinhole-free and uniform perovskite film with a grain size of 200−300 nm was fabricated that exhibited a shortcircuit current density (Jsc) of 20.86 mA cm−2, an open-circuit voltage (Voc) of 1.04 V, and a fill factor (FF) of 0.75, realizing a power conversion efficiency (PCE) of 16.31% with negligible hysteresis. The incident photon-to-current conversion efficiency (IPCE) curve depicted in Figure S7C shows a broad and strong response over the range from 350 to 800 nm, and the calculated current density by integrating the IPCE curve with AM 1.5G solar photon flux was 20.4 mA cm−2, which is well consistent with the measured Jsc in the J−V curve. In the proposed tandem device, the ZCT-Au photocathode is positioned as the front light absorber for light harvesting of the majority of UV and high-energy photons in visible light region (< ca. 580 nm). The long-wavelength photons are transmitted through the photocathodes and are utilized for power generation in the back solar cell. Its altered light absorption property is shown in Figure S7 and Figure 5b,c. Most of light below 550 nm is utilized in the front absorber, i.e., the ZCT NR-Au photocathode. Thus, the power density available for the solar cell is reduced to ca. 55%, but the open-circuit voltage remains almost unaltered. Another critical component of the photocathode−PV tandem system is the counter electrode for electrochemical water oxidation in the bicarbonate electrolyte. Here, we used a cost-effective cobalt-bicarbonate (Co-Ci) electrocatalyst deposited on Ni foam instead of expensive platinum. The physical and electrochemical properties of Co-Ci are presented in Figure S8, which showed a high performance for water oxidation when it was loaded on the Ni foam electrode. The Co-Ci anode was placed out of the incident light pathway to avoid light blocking. Figure 5d illustrates the J−V performances of each component in the assembled tandem system, where the currents are based on actual areas of the photocathode and solar cell without normalization. In unbiased photoelectrochemical CO2 reduction in the tandem system, the ZCT NR-

ηSTC =

o ECO2/CO × J × FECO

Isolar

(1)

where EoCO2/CO, J, FECO, and Isolar are the thermodynamic stored energy of the CO2/CO redox couple (1.34 V), photocurrent density, faradaic efficiency of CO, and incident solar power density. Thus, the STC efficiency of our tandem device is over 0.35% and the solar-to-fuel efficiency (ηSTF) exceeds 0.43% including H2 as a minor product. Although the efficiency values are still lower than the one reported recently for the PV− electrode combination (6.5−7.0%), it is more efficient than most natural photosyntheses by plants (0.1−2.0%).1 In this work, we have demonstrated an unassisted solar-lightdriven AP of CO from CO2 reduction using a transparent ZnO@ZnTe@CdTe-Au photocathode and an organic−inorganic halide perovskite (CH3NH3PbI3) solar cell in tandem. The system has several unique features: (i) an AP device by combination of the photocathode and a single-junction CH3NH3PbI3 perovskite solar cell in tandem using only sunlight without any external bias, (ii) a transparent photocathode based on a triple-layered ZnO@ZnTe@CdTe core− shell nanorod array accommodating modification strategies of a p−n heterojunction formation between p-ZnTe and n-CdTe for facile charge separation and an ohmic junction formation with a Au cocatalyst for charge separation and formation of selective reaction sites, (iii) a cost-effective cobalt−bicarbonate (Co-Ci) electrocatalyst deposited on Ni foam instead of expensive platinum as an oxygen evolving anode, and (iv) carbon-monoxide-selective (∼80%) CO2 reduction with H2 as the only byproduct with a ηSTF of 0.43% under simulated 1 sun illumination. It is well established that the 1D nanorod-type photocatalysts have benefits of charge separation, photogenerated carrier collection, increased surface area, and effective light harvesting due to its geometric effect relative to its planar geometry.24−26 We have demonstrated the superior PEC CO2 reduction performance of the nanorod photocathode to that of a planar electrode. In addition, the formation of a heterojunction is essential for effective charge separation in PV or PEC devices.27−30 The optimized ZCT NR photocathode showed 6984

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CONCLUSION We have demonstrated unbiased sunlight-driven artificial photosynthesis of carbon monoxide from CO2 for 3 h under 1 sun illumination using a Au coupled ZnO@ZnTe@CdTe nanorod photocathode and perovskite solar cell in tandem. The tandem cell exhibited four features highly desirable for a standalone CO2 reduction system using light energy: (i) the trial of artificial photosynthesis by combination of a ZnTe-based photocathode and a single-junction organic−inorganic halide perovskite solar cell, (ii) triple-layered ZnO@ZnTe@CdTe core−shell accommodating a p−n heterojunction with Au coupling, forming an ohmic junction and providing selective reaction sites, (iii) a cost-effective cobalt−bicarbonate anode instead of platinum for oxygen evolution, and (iv) selective CO production (∼80%) from CO2 while suppressing competing H2 evolution from water. The device shows a solar-to-CO efficiency (ηSTC) of more than 0.35% and a solar-to-fuel efficiency (including H2) (ηSTF) exceeding 0.43%. With further improvement in the efficiency, we believe that our proposed tandem device has a high potential to become an important milestone en route to practical artificial photosynthesis.

dramatically improved PEC performance of photocurrent generation as well as onset potential, as demonstrated in Figure 3. The straddling band alignment of n-type CdTe and ptype ZnTe shown in Figure S10 would cause band bending, resulting in facilitated electron transfer from light absorber to electron acceptor in the electrolyte.31 In addition, the narrower band gap of CdTe extends the range of the light absorption by the photocathode further into the visible light region. In spite of the great attention worldwide on PEC CO2 reduction, the progress has been slow because of technical challenges including production of diverse products of the CO2 reduction and required high overpotentials. Overlapped redox potentials and multiple proton coupled electron transfer in an aqueous solution make a variety of products such as H2, CO, HCHO, HCOOH, and hydrocarbons.5,6 Incorporation of Au nanoparticles as a cocatalyst addressed these issues. The work function of Au metal (5.1−5.3 eV) is lower than that of n-type semiconductor CdTe (5.72), resulting in the formation of an ohmic junction.32,33 It enhances the band bending of the semiconductor into electrolytes, and photogenerated electrons are driven to electron acceptors (H+ or CO2) easily via Au.34 Also the ohmic junction promotes migration of photogenerated holes into the inside of the electrode. This effective charge separation results in a great improvement in PEC activity. Another critical role of Au is to provide catalytic sites for selective CO2 conversion into CO. Au metal can easily adsorb CO2 in the electrolyte to form CO2-ads, which is converted into HOCO-ads by bonding protons in the electrolyte; then H2O and CO-ads are generated by the reduction of HOCO-ads with transferred electrons and protons, and finally CO desorbs as a product.35,36 Since Au favors CO2 adsorption over H+, CO production is dominant over H2 production in Au-cocatalyzed CO2/H2O reduction. Our ZnO@ZnTe@CdTe-Au photocathode was applied to an unbiased CO2 conversion cell with a single perovskite solar cell. The scheme in Figure 5a and Figure S10 depicts the wellmanaged light harvesting by the photocathode-PV tandem in a single-pass photon excitation. The schematic energy diagram with accompanying charge flow represents how solar-driven CO2 reduction operates in the tandem cell. As mentioned, previous solar light-driven CO production systems from CO2 are made of a PV−electrolyzer combination. However, the triple-junction a-Si or triple-series connected perovskite solar cell were required to supply sufficient voltage to CO2 reduction due to the high overpotential required for the reaction.1,2,37 In our design, the photocathode reduces the overpotential, and thus, the voltage generated from the single perovskite solar cell is enough to drive CO2 reduction without external bias. In addition, the high absorption coefficient of perovskite in a range up to 800 nm facilitates the effective light harvesting to the near-IR region of the solar spectrum, which is transmitted through the photocathodes in the tandem system. The performance could be further improved from that of our current proof-of-concept device by several ways including the enhanced open-circuit voltage (Voc) of the solar cell,38−40 improved cocatalysts for CO2 reduction and water oxidation for higher currents and onset potentials, and better charge separation in the photocathode by modifying the bulk of the semiconductors. The stability could also be developed by a surface protective layer, preventing non-negligible electrochemical corrosion and Au detachment.

METHODS AND EXPERIMENTAL Preparation of the Transparent ZnO@ZnTe@CdTe Nanorod Array Electrode. All chemicals used in this work were of analytical grade and used as received. The ZnO@ZnTe@CdTe NR array was synthesized in three steps. First, ZnO nanorods were grown on a ZnO (50 nm)-sputtered fluorine-doped tin oxide (FTO) (PECTM 8, 6−9 Ω, Pilkington) in the 10 mM Zn(NO3)2·6H2O (99%, Aldrich) with ammonia−water (28−30%, Aldrich) by a solvothermal method at 95 °C for 1.5 h. The duration time determined the length of the ZnO nanorods. For the fabrication of the ZnTe layer outside the ZnO NR, a modified anion exchange method was applied. Thus, 0.1 g of Na2TeO3 (99%, Aldrich) and 1 g of NaBH4 (98%, Aldrich) were mixed in 100 mL of water, and then the ZnO NR film was introduced in the solution for the reaction at 95 °C for 2 h. To form the CdTe layer on the surface, the films were immersed in a 0.1 M Cd(CH3COO)2·2H2O (98%, Aldich) aqueous solution to exchange Zn2+ cations with Cd2+ at 120 °C for 2 h. Finally, the ZnO@ZnTe@CdTe NR array was rinsed with deionized water and ethanol several times and then annealed at 350 °C for 2 h under vacuum. Deposition of Au Nanoparticles on ZnO@ZnTe@CdTe Nanorod Arrays. Au nanoparticles (NPs) were deposited onto the ZnO@ZnTe@CdTe NR substrate using an e-beam evaporator (Temescal, USA). The substrate temperature was kept at 250 °C with a deposition rate of Au at 0.02 nm s−1 in a vacuum chamber of 1.6 × 10−4 Pa. The size and loading amount of Au NPs were determined by the deposition time. For the Au deposition, a calibrated quartz crystal reference was used. The Au-deposited film was annealed at 350 °C for 1 h under N2 flow (100 sccm). Preparation of Co-Ci/Ni Foam Anode. As a water oxidation electrode, cobalt−bicarbonate (Co-Ci) on Ni foam was fabricated by electrodeposition in three-electrode configuration with a Ag/AgCl reference and a Pt counter electrode. The cyclic voltammetry was repeated 15 times within a potential range of 0−1.6 VRHE in 4.0 mM cobalt nitrate, Co(NO3)2·6H2O (98%, Aldrich), and 0.2 M potassium bicarbonate, KHCO3 (99.7%, Junsei Chemical), solution with CO2 purging. Preparation of Perovskite Photovoltaics. Patterned FTO substrates (TEC8, Pilkington) were cleaned by washing with ethanol, acetone, and 2-propanol, followed by UV-ozone treatment for 15 min. The 50-nm-thick compact TiO2 layer was coated on the FTO glass by spin-coating a titanium diisopropoxide bis(acetylacetonate) solution (75% in 2-propanol, Aldrich) diluted in 1-butanol at 2000 rpm for 40 s. After heating at 500 °C for 30 min, lab-made TiO2 (∼30 nm sized particles) paste diluted with ethanol (1:3.5 weight ratio) was spincoated on the compact TiO2 layer and dried at 125 °C for 10 min, 6985

DOI: 10.1021/acsnano.6b02965 ACS Nano 2016, 10, 6980−6987

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ACS Nano followed by annealing at 500 °C for 40 min. A perovskite solution composed of 1 mmol of PbI2 (99.9985%, Alfar Aesar), 1 mmol of CH3NH3I (Dyesol), and 1 mmol of dimethyl sulfoxide (99.5%, Aldrich) dissolved in N,N-dimethylformamide (600 mg, 99.8%, SigmaAldrich) was spin-coated on top of the mesoporous TiO2 layer with dripping of diethyl ether during spinning and then heated to 100 °C for 3 min, resulting in the formation of a dark perovskite film. The hole transport material solution composed of spiro-OMeTAD (56 mg, Merck), 4-tert-butylpyridine (30 mg, Aldrich, 96%), and bis(trifluoromethane)sulfonimide lithium salt (5.8 mg, LiClO4, Aldrich, 99.95%) dissolved in chlorobenzene (1 mL, Aldrich, 99.8%) was deposited with a spin rate of 2500 rpm for 20 s. Finally, the Au electrode was deposited by thermal evaporation with a thickness of 80 nm. The metal electrodes were deposited using shadow masks to define the active area. The active area of each cell was measured using an optical microscope. Characterization. Analyses were done by field-emission scanning electron microscopy (FESEM, JEOL JMS-7401F and Philips Electron Optics B. V. XL30S FEG, operated at 10 keV), high-resolution transmission electron microscopy (Cs-corrected, JEOL JEM-2200FS), X-ray diffraction (Mac Science, M18XHF using Cu Ka radiation, λ = 0.15406 nm), UV−vis diffuse reflectance spectroscopy (UV−vis DRS, Shimadzu, UV2501PC), and X-ray photoelectron spectroscopy (ESCALAB 250Xi). For the XPS depth profiling, the ZnO/ZnTe/ CdTe electrode was soaked in 1 mM H2SO4 solution shortly to loosen the strength between ZnO/ZnTe/CdTe and the substrate and sonicated in an 2-propanol/water (5:1) solution. Then the solution was drop-casted on cleaned FTO. The photoelectrochemical CO2 reduction was carried out in a threeelectrode configuration using a Ag/AgCl reference and a Pt counter electrode. A potentiostat (Gamry Reference 600TM) provided an external bias to the circuit. The light source was solar simulator (91160, Oriel) with an air mass 1.5G filter, and the light intensity of 100 mW cm−2 was calibrated using the guaranteed reference by National Renewable Energy Laboratories, U.S. The electrolyte was CO2-purged 0.5 M potassium bicarbonate, KHCO3 (99.7%, Junsei Chemical). The IPCE was measured under light from a Xe lamp (300W, Oriel) passed through a monochromator with a bandwidth of 5 nm. For the tandem assembly, the Co-Ci anode was placed at the side of the reactor to avoid light blocking. The ZCT NR-Au photocathode was positioned at the front side, facing the light source in the reactor, and the perovskite solar cell was placed on the outside of the reactor behind the photocathode. The FTO (electron collector) and Au (hole collector) of the perovskite solar cell were connected with the ZnO@ ZnTe@CdTe NR-Au photocathode and Co-Ci/Ni foam anode, respectively. To monitor the circuit current, the potentiostat was also connected in two-electrode configuration with zero external bias. The reaction in the tandem cell proceeded for 3 h, and the gas products were analyzed by a gas chromatograph with TCD (HP 7890, molecular sieve 5 Å column, and Ar carrier gas).

Author Contributions ⊥

Y. J. Jang and I. Jeong contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by Brain Korea Plus Program of Ministry of Education, Climate Change Response Project (2015M1A2A2074663, 2015M1A2A2056824), Korean Centre for Artificial Photosynthesis (NRF-2011-C1AAA0001-20110030278), the Basic Science Grant (NRF2015R1A2A1A10054346) funded by MISIP, and Project No. 10050509 funded by MOTIE of Republic of Korea. This work was also supported by the KIST institutional programs. REFERENCES (1) Schreier, M.; Curvat, L.; Giordano, F.; Steier, L.; Abate, A.; Zakeeruddin, S. M.; Luo, J.; Mayer, M. T.; Grätzel, M. Efficient Photosynthesis of Carbon Monoxide from CO2 Using Perovskite Photovoltaics. Nat. Commun. 2015, 6, 7326. (2) Sugano, Y.; Ono, A.; Kitagawa, R.; Tamura, J.; Yamagiwa, M.; Kudo, T.; Tsutsumi, E.; Mikoshiba, S. Crucial Role of Sustainable Liquid Junction Potential for Solar-to-Carbon Monoxide Conversion by a Photovoltaic Photoelectrochemical System. RSC Adv. 2015, 5, 54246−54252. (3) Schreier, M.; Gao, P.; Mayer, M. T.; Luo, J.; Moehl, T.; Nazeeruddin, M. K.; Tilley, S. D.; Grätzel, M. Efficient and Selective Carbon Dioxide Reduction on Low Cost Protected Cu2O Photocathodes Using a Molecular Catalyst. Energy Environ. Sci. 2015, 8, 855−861. (4) Arai, T.; Sato, S.; Kajino, T.; Morikawa, T. Solar CO2 Reduction Using H2O by a Semiconductor/Metal-Complex Hybrid Photocatalyst: Enhanced Efficiency and Demonstration of a Wireless System Using SrTiO 3 Photoanodes. Energy Environ. Sci. 2013, 6, 1274−1282. (5) Benson, E. E.; Kubiak, C. P.; Sathrum, A. J.; Smieja, J. M. Electrocatalytic and Homogeneous Approaches to Conversion of CO 2 to Liquid Fuels. Chem. Soc. Rev. 2009, 38, 89−99. (6) Gattrell, N.; Gupta, N.; Co, A. A Review of the Aqueous Electrochemical Reduction of CO 2 to Hydrocarbons at Copper. J. Electroanal. Chem. 2006, 594, 1−19. (7) Chen, Y.-S.; Manser, J. S.; Kamat, P. V. All Solution-Processed Lead Halide Perovskite-BiVO4 Tandem Assembly for Photolytic Solar Fuels Production. J. Am. Chem. Soc. 2015, 137, 974−981. (8) Dias, P.; Schreier, M.; Tilley, S. D.; Luo, J.; Azevedo, J.; Andrade, L.; Bi, D.; Hagfeldt, A.; Mendes, A.; Gträtzel, M.; Mayer, M. T. Transparent Cuprous Oxide Photocathode Enabling a Stacked Tandem Cell for Unbiased Water Splitting. Adv. Energy Mater. 2015, 1501537, 1−9. (9) Luo, J.; Im, J.-H.; Mayer, M. T.; Schreier, M.; Nazeeruddin, M. K.; Park, N.-G.; Tilley, S. D.; Fan, H. J.; Gträtzel, M. Water Photolysis at 12.3% Efficiency via Perovskite Photovoltaics and Earth-Abundant Catalysts. Science 2014, 345, 1593−1596. (10) Kuang, Y.; Jia, Q.; Nishiyama, H.; Yamada, T.; Kudo, A.; Domen, K. A Front-Illuminated Nanostructured Transparent BiVO4 Photoanode for> 2% Efficient Water Splitting. Adv. Energy Mater. 2016, 6, 1−7. (11) Shi, X.; Zhang, K.; Shin, K.; Ma, M.; Kwon, K.; Choi, I. T.; Kim, J. K.; Kim, H. G.; Wang, D. H.; Park, J. H. Unassisted Photoelectrochemical Water Splitting beyond 5.7% Solar-to-Hydrogen Conversion Efficiency by a Wireless Monolithic Photoanode/DyeSensitised Solar Cell Tandem Device. Nano Energy 2015, 13, 182− 191. (12) Jang, J.-W.; Cho, S.; Magesh, G.; Jang, Y. J.; Kim, J. Y.; Kim, W. Y.; Seo, J. K.; Kim, S.; Lee, K.-H.; Lee, J. S. Aqueous-Solution Route to Zinc Telluride Films for Application to CO2 Reduction. Angew. Chem. 2014, 126, 5962−5967.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b02965. Fabrication scheme, SEM, UV−vis DRS, J−V, XPS, choronoampere results, IPCE, and energy diagram in figures and time-profiled product distribution in tables (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail (M. J. Ko): [email protected]. *E-mail (J. S. Lee): [email protected]. Present Address #

School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York 14853, United States. 6986

DOI: 10.1021/acsnano.6b02965 ACS Nano 2016, 10, 6980−6987

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DOI: 10.1021/acsnano.6b02965 ACS Nano 2016, 10, 6980−6987