Progress in Characterization Techniques in Spintronic Enhanced Photocatalytic Hydrogen Evolution

ZHANG Wen-yan, GAO Wei , ZHANG Xu-qiang, LI Zhen, LYU Gong-xuan

(1. StateKeyLaboratoryforOxoSynthesisandSelectiveOxidation, LanzhouInstituteofChemicalPhysics, ChineseAcademyofScience, Lanzhou 730000, China； 2. UniversityofChineseAcademyofScience, Beijing 10080, China； 3. CollegeofMaterialEngineering, JinlingInstituteoftechnology, Nanjing 211169, China)

ProgressinCharacterizationTechniquesinSpintronicEnhancedPhotocatalyticHydrogenEvolution

ZHANG Wen-yan1,2,3, GAO Wei1,2, ZHANG Xu-qiang1, LI Zhen1,2, LYU Gong-xuan1

(1.StateKeyLaboratoryforOxoSynthesisandSelectiveOxidation,LanzhouInstituteofChemicalPhysics,ChineseAcademyofScience,Lanzhou730000,China； 2.UniversityofChineseAcademyofScience,Beijing10080,China； 3.CollegeofMaterialEngineering,JinlingInstituteoftechnology,Nanjing211169,China)

In recent years, the fossil fuel crisis has triggered the worldwide demand for new clean energy. Hydrogen is a desirable candidate due to its high combustive enthalpy and zero-pollution characteristics. A promising approach for the hydrogen production is splitting H2from water by solar driven photo-catalytic hydrogen evolution reaction (HER). The development of photo-catalytic HER is inhibited by many factors: especiallythe the large energy loss and the electron-hole recombination during electron transportation, and high over-potential of proton reduction and water oxidation.Spintronic science sheds new lights on solving these obstacles by triggering the high efficient spin transfer and electron tunneling, as well aselectrons spin filtering to decrease the over-potential of the reaction and suppress the yield of by-products.The progresses in characterization techniques have contributed greatly to the unveiling of the scientific "secrets" in spintronic enhanced HER research. Yet,few work hasbeen carried out to sum up the setechniques and to analyze the potential challenges that inhibit their future development. Given that, this review focuses on these topics and provides an expectation for its development trends.

photocatalytic;hydrogen evolution;characterization;spin detection;spintronics enhanced photocatalytic HER

In recent years,the worldwide fossil fuel crisis makes it necessary to exploreclean and sustainable energy carriers[1-44]. Due to its high combustion enthalpy and zero-pollution characteristics, hydrogen is recognized as an ideal sustainable energy carrier, in the future, to be used in cars, in houses, forportable power, and in many other applications[44-77]. Since the report of hydrogen evolution reaction (HER) over TiO2electrode in 1972[78], great progress have been realized in obtaining hydrogen from water by solar driven photocatalytic reaction[75-82]. A series of model shave been constructed for photocatalytic HER[80-88],including (1) light harvest by photocatalysts, (2) charge excitation (electrons and holes), (3) charge migrating to photocatalysts surface to reduce protons or to oxidize water, (4) bulk charge recombination, and (5) surface charge recombination[80-84,89-96].

Though solar driven photocatalytic HER is a potential and sustainable approach for renewable hydrogen generation, some problems still exist, such as: limited charge transporting efficiency[40,87-88,97-99,100-102], high over-potential of HER and oxygen evolution reaction (OER) half reactions in water splitting[55,103-105], and low photocatalytic stability of some photocatalysts[106-108]. Recent progress of spintronics sheds new lights on solving these obstacles. On the one hand, triggering spin transfer and electron tunneling in photo-catalysts can effectively enhance the efficiency of electron transporting and turnover frequency (TOF) value, due to lossless electron transportation and low electron-lattice interaction in photo-catalysts matrix[100,108-109]. It is amazing that the spin electron mobility could reach up to 18 000 cm2/Vs in some topological transfer media[100,108, 110-113]. On the other hand,spin filtering photoelectrons can lower the overpotential of solar driven photo-catalytic HER[105,109]and inhibit the yield of by-products (e.g., singlet H2O2)[105,114-115], resulting inhigher catalytic activity and robust catalyticstability.

Bridging spintronic science and photo-catalytic research demands a series of sophisticated characterization techniques, including sensitive spin detection devices, advanced optical measurements, energy analyze equipment, electrochemistry characterization devices, sensitive gas detection equipment, size and morphology detection, composition and elements characterization, as well as work function detection. These characterizations provide abundant information for the unveiling of theeffects of spintronic on enhancing solar driven photo-catalytic HER. Nevertheless, only few work on the progress of these important techniques in the detection of electron spin and transfer has been systematically reviewed.In this review, the classification, designs for devices and detection mechanisms of present characterization techniques for related research fields are systematically summarized, and the significant role of detecting techniques in the future development of spintronic enhanced HER research is given. The challenges and developing trends are discussed in this review to lead to more exploration in this research field.

1 ClassificationsoftechniquesfordetectingspintronicenhancedHER

A series of characterization techniques have been developed for the detection of the relationship among HER efficiency, photo-electronic performance, spin transport and spin filtering[53,116-122],as shown in Table 1. These techniques include the common devices which have already been commercialized, and some self-designed novel devices[53,118-132].

Table1 Characterizationtechniquesforspintronicenhancedphoto-catalyticresearch

2 Commercialized techniques

2.1 Spin detection

2.1.1 Electron spin resonance

ESR characterization provides essential and direct spin information of unpaired electrons. The mechanism of ESR detection based on the interaction between the external magnetic field and electron spin. An electron has two spin quantum number,ms=+1/2,ms=-1/2. When placing an electron in the magnetic field (B0), the electron magnetic moment aligns itself either parallel (ms=-1/2) or anti-parallel (ms=+1/2) to the field, each alignment having a specific energy due to the Zeeman effect:

E=msgeμBBo

geis theg-factor of the electron,ge=2.002 3 for the free electron,μBis the Bohr magneton.

As illustrated in Fig.1, the Zeeman effect results in the energy level splitting, and the energy gap between the lower and the upper state is ΔE=geμBBo.

Fig.1 (a)Energy level scheme of spin state (b) Diagram of ESR spectra

Evidently, the increaseof ΔEis directly in proportion to the magnetic field strengthBo. The unpaired electron can absorb energyhνand move from the lower to upper state, or emit a photon and move from upper to lower state. Both the two processes obey the fundamental equation of ESR spectroscopy:

hν=ΔE=geμBBo

ESR devices are designed and constructed based on the above equation. Usually, ESR measurements are conducted by varying the magnetic field (Bo) while holding the frequency of incident photon (ν) constant. When an unpaired electron is placed in the magnetic field (Bo), the increase ofBowould enlarge the value of ΔE(the energy gap betweenms=+1/2 andms=-1/2 states, Fig.1). When ΔEincreases to match the energy of incident microwaves (hν), the electron would absorb energy and jump betweenms=+1/2 andms=-1/2 spin states. The Maxwell-Boltzmann distribution of electrons yieldnet absorption of energy. Such net absorption can be detected and converted into a spectrum. Fig.1 (a) and (b) illustrate,respectively, the simulated absorption ESR spectrum and the first derivative of absorption ESR spectrum.

ESR is an effective technique to detect unpaired electron both in solid and liquid environments. It has been applied to detect the OH· radicals generated in water splitting to reveal the OER mechanism. Also, Lu et al. used to apply the technique to investigate heavy atom induced spin polarization in the photo-catalytic water splitting reaction. These investigations show a high potential of the technique for bridging the spintronic researches and photo-catalytic reactions.

2.1.2 Physical property measurement system

Physical property measurement system is a common and commercialized complex system designed for the detection of magnetic, electronic and thermal properties of materials. It can provide strong and tunable magnetic field, as well as tunable temperature control system for the measurement. More interestingly, PPMS system provides a high convenience for researches to design novel experiments. Fig.2 shows the sample chamber and sample mount of PPMS. The mount and chamber design are both very convenient for researchers to plug in or remove samples, and detect physical properties under magnetic/electronic/thermal fields and under microwave/laser irradiation.

Considering that spin polarization could result in various magnetic, electronic and thermal properties of photo-catalysts, accurate detection of these properties can offer a large number of information to reveal their spin state variation and build reasonable relationship between their spin state and photo-catalytic properties. Equipped with advanced software and standard hardware, PPMS can provide fully automaticcharacterization for a series of physical properties including the resistivity, magneto resistance,magneto-electric coupling, differential resistance, ferroelectric property, dielectric property, Hall coefficient, volt ampere characteristic, critical current, AC susceptibility, hysteresis loop, specific heat, thermal magnetic curve, thermoelectric effect, Sebek coefficient, and thermal conductivity. It is expected that owing to its high accuracy and convenience, PPMS plays an important role in constructing interdisciplinary research of spintronics and photo-catalytic research.

Fig.2 (a) Sample chamber and (b) Sample mount of PPMS system

2.1.3 Vibrating sample magnetometer

VSM is an effective instrument to measures magnetic properties of photo-catalysts as a function of magnetic field, temperature, and time. As illustrated in Fig.3 (a) and (b), modern VSM devices are mainly composed of an electromagnet, a detection coil, a sample holder, a lock in amplifier and a Gauss meter. When placing a sample inside a uniform magnetic field, a dipole moment proportional to the applied field is induced in the sample. If the sample is made to undergo a sinusoidal motion, an electrical signal will be induced in the suitable located stationary pick-coils. This signal is proportional to the magnetic moment, vibration amplitude and vibration frequency, thus provids magnetic information of the samples.

Fig.3 (a) Schematic model and (b) Device image of VSM equipment

VSM test has some special advantages. Firstly, it has few requirement on the morphology and crystallization of samples, so it is widely applicable to detect powders, solids, liquids, single crystals, and thin film. Secondly, it has enough sensitivity to detect all kinds of magnetic materials, including diamagnetic materials, paramagnetic materials, ferromagnetic materials, ferrimagnetic materials, anti-ferromagnetic materials, anisotropic materials and magnetic-optical materials. More importantly, as the magnetic property originates from the spin polarization of electrons, VSM is effective to detect the magnetic property of spintronic material such as giant magneto resistance, colossal magneto resistance, and exchange biased and spin-valve.

2.2 Gas detection and electrochemical detection

Gas detection and electrochemical detection are two important measurement techniques for the photo-catalytic water splitting. Gas chromatography is applied to characterize the generation of H2and O2. Electrochemical workstation is widely applied to characterize the electronic and chemical properties of photocatalysts. A precise and versatile instrument for the electrochemical detection is the electrochemical workstation, which not only can record the photo-current, HER overpotential and other related photo-catalytic data, but can also reveal the spin polarization information of spintronics enhanced HRE by assembling the anodes with chiral molecules to induce chiral-induced spin selectivity effect on water splitting.

2.3 Morphology and size detection

Size and morphology have important effects on energy level, crystallization, lattice stress, surface energy, and formation of defects of photo-catalysts, thus the size and morphology of photo-catalysts are closed related to their spintronic and photocatalytic properties. TEM, STEM, SEM and AFM are inevitably the four keycommon techniques to display the morphology and size of photo-catalysts.

MRFM is a newly developed imaging technique especially suitable for the spin detection. As shown in Fig.4(a),MRFM devices are composed of a microwave coil, a magnetic tip, a resonant slice, a cantilever and an interferometer. The mechanism of MRFM is to detect magnetic force between a ferromagnetic tip and spins in a sample. The problem of single spin detection is that the force from a single spin is too small to be detected.

Fig.4 Scheme of (a) MRFM devices 38 and (b) MRFM device with one single spin detection sensitivity [120-121]

With the development of ultrasensitive cantilever-based force sensors, the problem is solved and now MRFM can detect magnetic force of one single spin[120-121]. The magnetic resonance force microscopy has a very high detection sensitivity, i.e., up to 10 billion times better than a medical magnetic resonance imaging(MRI) used in hospitals.Yacoby et al. developed the MRFM device, and the device is mainly composed of an excitation laser, a scanning diamond platform, a MW coil and a sensor NV, as illustrated in Fig. 4 (b)[120-121].Using the device accurate, real-space, and quantitative magnetic-field images of single molecule imaging at room temperature can be obtained. Considering its single-spin detection sensitivity, it is reasonable to prospect that the MRFM device will provide more useful spin information on the unveiling of interdisciplinary mechanism between spintronic science and photocatalytic reaction.

2.4 Work function detection

Work function (Φ) of a material is the energy difference between Fermi energy and vacuum level. As shown in Fig.5(a), the work function of metals is the energy difference between Fermi energy and vacuum level. The Fermi energy of a semiconductor is theenergy within the band gap (between the conductive band and valance band), so the work function of a semiconductor is somewhat more complex than that of metal, as illustrated in Fig.5(b). In that case, Fermi distribution should be considered since there are no allowed electronic states within the band gap. Fermi distribution is a statistical function, which gives the probability to find an electron in a given electronic state.

Fig.5 Scheme of work function in (a) metal and (b) n-type semiconductor(VBM: valence bands maximum, CBM: conductive bands minimum)

The work function detection is important both for spintronic research and photo-catalytic water splitting. For one thing, as illustrated in Fig. 6,the electron tunneling spin injection and transfer orientation in materials depend highly on the energy level matching of each component[133]. In order to promote the spin injection and spin transfer in photo-catalystic materials, it is necessary to realize suitable energy level matching according to their work function parameter. For another thing, the work function of materials is very sensitive to their surface conditions, so the work function detection can reveal many surface properties, including catalytic activity, reconstruction of surfaces, doping and band-bending of semiconductors, charge trapping in dielectrics and corrosion. These parameters provide abundance information for scientists to analyze photo-catalytic mechanism of water splitting reaction.

Fig.6 (a) Scheme of spintronic material Co/Al2O3/Si multi-layers (b) Dependence of Spin conductance on component variation of Co/Al2O3/Si multi-layers[133]

Work function can be measured by ultraviolet photoelectron spectroscopy [UPS,also known as photoemission spectroscopy (PES)], Kelvin probe force microscopy, and photoelectron microscopy. In this section, we briefly summarize the characteristics and mechanism of UPS and Kelvin probe method.

2.4.1 Ultraviolet photoelectron spectroscopy

UPS has a similar measure mechanism with another detection technique, the XPS. The only difference between UPS and XPS detection is the wavelength of ionizing radiation. UPS applies ultraviolet photons as the irradiation source to induce photoelectric effect, while XPS uses photons higher than 1 keV to excite the photoelectric effect. Ultraviolet irradiations of UPS are produced using a gas discharge lamp which typically filled with helium. He photons emitted by helium gas are of 21.2 eV (He I) and 40.8 eV (He II). UPS not only can detect work function but also can measure the valence band of photo-catalysts.

2.4.2 Kelvin probe force microscopy

KPFM is very sensitive to detect the work function of materials at atomic or molecular scales. KPFM has been considered to be a unique and ideal method to characterize the electronic/electrical properties of metal/semiconductor surfaces, considering that the work function relates to many surface properties, including catalytic activity, reconstruction of surfaces, doping and band-bending of semiconductors, charge trapping in dielectrics and corrosion. Besides, KPFM can provide detection information with high accuracy, as it can be applied in ultra high vacuum environment to avoid contaminates. In addition, with KPFM, researchers are able to recognized surface potential distribution of nano-scaled materials via the two-dimensional or three-dimensional work function images (Fig. 7)[123]. These two-dimensional or three-dimensional images give abundant information on the composition and electronic state of material surface.Therefore,KPFM is an effective characterization technique for exploring intrinsic mechanisms and optimizing properties for photo-catalysts and spintronic devices design.

Fig.7 (a) Cross-sectional KPFM image of Cu In Ga S Se solar cell (b) Three-dimensional KPFM image of Co Ga Se solar cell[123]

3 Self-designed novel devices

3.1 Chiral-inducedspinselectivitydeviceforspindetection

In order to detectCISSeffect, a new type of device has been designed to realize the effective measurement for charge transfer andCISSinduced spin polarization. Fig. 8 illustrates a typicalCISSdevice constructed for the spin detection.The assembly consists of dye decorated deoxyribonucleic acid(DNA) oligomers, a Ag film, an AlOx film and a ferromagnetic Ni layer[53,118-119]. Under light irradiation, the electrons are excited from dye molecules. These photo-generated electrons then migrate through the DNA molecules and arrive at the Ag substrates (Metal 2). The photo electrons, which spin parallel to the net spin of ferromagnetic Ni layer, could tunnel through the AlOx layer and migrate from the Ag substrates (Metal 2) tothe ferromagnetic Ni layer (Metal 1).The spin detection sensitivity ofCISSdevice can be enhanced by reducing the thickness of silver films and replacing the Ni layer with more spin specific substrate[119].

Fig.8 (a1, b1) Scheme of CISS device (red circles are dye molecules, metal 1 is Ni, AlOx is a dielectric layer, and metal 2 is Ag), (a2, b2) energy level diagram for measuring spin detection; (c) SEM image of the device and its electrical connection scheme[118]

3.2 Nano-scaledspin-mechanicaldeviceforspindetection

Since spin is a quantum property characterized by the angular momentum, the spin flip of electrons results in a tiny variation of angular momentum, which can be converted to the tiny variation of mechanical torque. This is the basic mechanism of the spin-mechanical device for spin detection. Besides, the sizes of spin-mechanical devices are usually at the nano scale to enhance the detection sensitivity for tracing spin flip. Fig.9 (a1, a2) and Fig.9 (b) illustrate two typical nano-scaled torque-shaped spin-mechanical device for the spin detection[124-125]. Both of them are composed of nano-scaled materials, such as nano-sheets and nano-wires, to form the torsion oscillator[124-125]. The nano-scaled torsion oscillators can flip sensitively in respond to the flip of spin electrons. Moreover, the direction of spin flip (up or down) can be determined by observing the flip direction of the torsion oscillator.

Fig.9 (a1) SEM image and (a2) Scheme of a single-crystal silicon torsion oscillator[124-125]

3.3 Nano-scaledspin-mechanicaldeviceforspindetection

Taking advantages of the tunnel barrier penetration [Fig. 10 (a)], the constructed tunnel contacts are recognized as the viable and robust method to detect spin polarizations. Fig. 10(b-c) illustrates several tunnel contacts devices composed of ferromagnetic material (FM), tunnel barrier (TB) and non-magnetic material (NM), with a nano-voltmeter to measure voltage and a galvanometer to record currents[126-132]. Tunnel contacts mainly include 2-terminal (2T) [Fig. 10 (c)], nonlocal (NL) measurement [Fig. 10 (d)] and 3-terminal (3T) [Fig. 10 (e)], and measure the spin signal based on the mechanism of local magneto-resistance effect, Hanle effect, and non-local spin transport/diffusion[126-132]respectively. Tunnel contact devices are useful tools for the analysis of spintronic phenomenon in photo-catalysts. By choosing suitable tunnel contact devices, researchers can measure the spin transport, spin lifetime and diffusion length, as well as spin state of spin-polarized surface and Quantum spin Hall (QSH) edge[101,105,126-132]. For example, Parkin et al. utilized the 3T measurement [Fig. 10 (e)] to study the spin transportation in SrTiO3, an efficient photo-catalyst for solar driven HER,and found that the short spin lifetime in SrTiO3originated from the Ti3+defects of the formed SrTiO3[86]. Based on their investigation, the Ti3+concentration in SrTiO3lattice can be controlled if one makes effort on SrTiO3-based spin photocatalysts for water splitting.

Fig.10 Schemes of (a) tunnel barrier penetration (b)Spin current through a FM/TB/NM junction (c) Local magneto-resistance detection device (2T) (d) Scheme of non-local spin transport detection device (NL), (e) Three-terminal spin transport detection device (3T), (f) 3T spin detection device for detecting spin transfer in SrTiO3[86, 126-133]

4 ChallengesandbrightfutureofdevelopmentofcharacterizationforspintronicenhancedHER

Through the above discussions we know that the progresses of characterization techniques have greatly promoted the development of spintronic enhanced photo-catalytic HER research, from the aspects of mechanism discovery, catalysts design, and HER efficiency improvement. Consequently, development of characte-rization displays bright future in the spintronic enhanced HER research and other interesting catalytic problems[134-147].

Nevertheless, the contributions to spintronic enhanced photo-catalytic HER are highly limited due to the lack of real-timedetection, low convenience, limited detection sensitivity, as well as high detection cost. As the spin flipping takes place in a very short time, in order to obtain precise data of spintronics effect on HER, establishing areal-time detection system with high sensitivity is a must done work, otherwise it is difficult to pursue some ultra-fastreaction phenomena which are meaningful in the unveiling of the secrets in spintronic enhanced HER.Another Achilles′ Heel is the complexity and high cost of some common and commercialized devices (such as PPMS, ESR and PEEM). Considering that, it is necessary to accelerate the development of self-designed novel devices, for their complexity and cost could be manipulated more conveniently than those common commercialized devices.

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自旋电子学-光催化产氢交叉学科研究中的测试表征技术进展

张文妍1,2,3, 高 薇1,2, 张旭强1, 李 振1,2, 吕功煊1

(1. 中国科学院 兰州化学物理研究所 羰基合成与选择氧化国家重点实验室,甘肃 兰州 730000; 2. 中国科学院大学,北京 10080; 3. 金陵科技学院, 江苏 南京 211169)

清洁能源的研究和开发为解决化石燃料的日益枯竭问题带来了希望. 氢能燃烧热值高，产物零污染，是理想的清洁能源. 利用太阳能，通过光催化反应从水中制取氢气，是一条极有发展前景的制氢途径. 然而，太阳能光催化制氢的发展受到许多因素的限制，特别是光电子传输过程中的电子-空穴复合及能量损失导致的电子输运效率低以及高的产氢产氧过电位导致水分解过程的势垒增大. 自旋电子学的发展，为太阳能光催化制氢中的这些问题提供了解决之道. 通过将自旋电子学的思路及原理应用于太阳能光催化制氢，借助自旋输运及电子隧穿可有效提高电子的输运效率，光电子的自旋极化还可降低产氢产氧过电位并抑制副产物的生成. 测试表征技术的发展为揭示自旋电子学-太阳能光催化制氢交叉科学的内秉机理做出了重要贡献. 然而，目前尚无相关文籍对此类测试表征技术的发展进行总结和评述. 考虑到这些测试表征技术在自旋电子学-太阳能光催化制氢交叉科学研究中的重要作用，对它们进行归纳和总结，评述其发展面临的问题与挑战，探索并合理预测其未来的发展方向.

光催化；产氢；表征；自旋检测；自旋增益的光催化产氢

综述(219～236)

date: 2017-11-20;

date: 2017-12-04.

Foundation: The National Natural Science Foundation of China (Grant Nos. 21433007 and 21673262) and the 973 Program of Department of Sciencesand Technology China (Grant No. 2013CB632404)

Zhang Wen-yan(1985-), female, PhD, photocatalysis and new materials, E-mail: zhangwenyan8531@163.com

LYU Gong-xuan(1964-), PhD, Researcher, E-mail: gxlu@lzb.ac.cn, Tel: +86-931-4968178.

O643.32Documentcode: AArticleID:1006-3757(2017)04-0219-18

10.16495/j.1006-3757.2017.04.004