Cocoa production in Cameroon: From cash-crop plantations to agroforests

Proceedings of the Workshop:

Cultivating (in) Tropical Forests? The evolution and sustainability of systems of management between extractivism and plantations 28. June -1. July 2000 Kraemmervika, Lofoten, Norway Cocoa plantations of Central Africa have always been understood simply as cash-crop monocultures by agricultural services. This perception has served policies aimed at maximizing cocoa yields, via (i) rehabilitation of old plantations (complete substitution of old trees by juveniles, following precise agronomic standards) and (ii) intensification of plantation (adoption of selected high-yield cultivars, and elimination of any other tree susceptible to cause overshading). Agricultural policies are driven by international market rules, which assert that intensification is the only way of saving cocoa production in this region.

Nevertheless, farmers are reluctant to conform to such initiatives. Cocoa plantations of the old centre are in fact true agroforestry systems in which cocoa and dominant trees are closely intermingled. Some of these trees are remnants of the pristine forest and were maintained during the creation of the plantation, some are light-demanding species that established during the fallow period, and others were voluntarily transplanted or even introduced into parcels by farmers. Most of these trees -a mixture of native and exotic species -provide non-timber forest products.

Because the association of cocoa with trees is substantially reduced in areas where cocoa production began more recently (such as the western province of Cameroon), the ecological need for shade of old cocoa cultivars was stressed to justify such associations. Because this ecological "alibi" no longer applies for newly selected cocoa cultivars that are self-shading, the specter of fungal attacks favoured by over-shading is put forward to persuade farmers to eliminate the dominant trees.

The historical context of cocoa introduction in Central Africa shows clearly that the incorporation of trees was justified not only by ecological needs of cocoa. The presence of trees was mainly motivated by drastic changes concerning (i) the socio-cultural organisation (fragmentation of production units, from extended families to nuclear household) and (ii) land tenure (progressively, the temporary usage right on land has turned into permanent land ownership). These changes were directly induced by sedentarization, pacification, taxes and hard labour imposed by the colonial administration. Today, plantations do not exceed 2 hectares and are managed independently by households. The decrease in land availability, combined with a growing monetary economy, have encouraged more individualist attitudes and have increased competition for land access.

Since their origins, cocoa plantations of southern Cameroon have combined three economic perspectives: that of cash crop (standing reserve in case of need for cash, payment of taxes and consumable goods), that of producing NTFPs (local and selfsufficient economy), and that of land tenure perspective (permanent land ownership). By maintaining such an integrated system, farmers reject the dictates of the market, which does not consider any of these perspectives. They express massively a preference for an optimized intermediate system, rather than a monospecific cash-crop plantation corresponding to productivist models.

Box 9. Cocoa plantations of southern Cameroon (by: Edmond Dounias)

Cocoa plantations of southern Cameroon are very old and their yield is low. Most of the plantations were created during the 1920's and 1930's. They are permanently rejuvenated by planting juveniles to replace old and unproductive plants or in gaps caused by dead trees. Average size of plantations is around two hectares and the basic production unit is the household. Cocoa plantations are generally established in mature swidden fields. Seedlings are transplanted into swidden parcels and grow among the post-agricultural regrowth. Cocoa trees have always been and still are associated with dominant trees that have diverse origins. Some are remnants of the pristine forest and were maintained during the creation of the swidden, some are light-demanding species that established during the fallow period, and some others were voluntarily transplanted or even introduced into the parcel. Most of these trees -a mixture of native and exotic species -provide NTFPs. The original shading function of the dominant trees is no longer justified, now that improved cocoa cultivars are "selfshading". Agricultural policies that aim at encouraging the intensification of cocoa production push for a systematic elimination of shade trees, because excessive shade encourages fungal attacks that drastically affect production of improved cultivars. Farmers are reluctant to eliminate these trees and prefer to maintain older cocoa varieties which are more resistant to fungal pathogens. Farmers express their preference for an optimized intermediate system (justified by socio-economic choices and land tenure strategies) rather than a maximized yield of a monospecific and cash-crop plantation.

  9056 CNRS, 1919,Route de Mende 34293 Montpellier Cedex 5, FRANCE Tel: (+33) 467613234, FAX: (+33) 467412138, Email: dounias@cefe.cnrs-mop.fr Central and Southern Cameroon are known to form an old centre of cocoa cultivation, which was massively introduced in the beginning of the century by the colonial administration. Most of the plantations that are still in production today were created during 1920's and 1930's. Cocoa trees are old and yields are globally low.


Shun Kashiwaya

Céline Olivier

Jérôme Majimel

Andreas Klein

Wolfram Jaegermann

Thierry Toupance
email: thierry.toupance@u-bordeaux.fr

Nickel oxide selectively deposited on the {101} facet of anatase TiO 2 nanocrystal bipyramids for enhanced photocatalysis

Keywords: Facet-engineered NiO-anatase TiO 2 materials, Interface experiments, Band alignment diagrams, Photocatalysis, Organic dyes

Facet-engineered anatase TiO 2 with NiO nanoparticles heterocontacts were successfully prepared by selective photodeposition of NiO nanoparticles onto the {101} facet of the top-truncated bipyramidal TiO 2 anatase nanocrystals coexposed with {001} and {101} facets. The morphology and electronic properties of the resulting 0.1-10 wt % NiO-decorated TiO 2 were investigated by X-ray diffraction, high-resolution electron microscopy, N 2 sorption analysis, and UV-vis spectroscopy. Furthermore, a careful determination of the energy band alignment diagram was conducted by a model experiment using XPS and UPS to verify charge separation at the interface of the NiO-TiO 2 heterostructure. The model experiment was performed by stepwise deposition of NiO onto oriented TiO 2 substrates and in-situ photoelectron spectroscopy measurements without breaking vacuum. Core levels showed shifts of 0.58 eV toward lower binding energies, meaning an upward band bending in TiO 2 at the NiO-TiO 2 interface. Furthermore, 0.1 wt % NiO-TiO 2 exhibited 50% higher activities than the pure TiO 2 for methylene blue (MB) photodecomposition under UV irradiation. This enhanced photocatalytic activity of NiO-TiO 2 nanocomposites was related to the internal electric field developed at the p-n NiO-TiO 2 heterojunction, leading to vectorial charge separation. Finally, mechanistic studies conducted in the presence of carrier or radical scavengers revealed that holes dominantly contributed to the photocatalytic reactions in the case of NiO-TiO 2 photocatalysts while electrons played the main role in photocatalysis for the pure TiO 2 materials.

Introduction

Chemical reactions on titanium dioxide (TiO 2 ) surfaces have attracted worldwide attention in the fields of energy conversion,(1-3) energy storage, [START_REF] Wang | Self-Assembled TiO 2 -Graphene Hybrid Nanostructures for Enhanced Li-Ion Insertion[END_REF][START_REF] Yang | In Situ Fabrication of Hierarchically Branched TiO 2 Nanostructures: Enhanced Performance in Photocatalytic H 2 Evolution and Li-Ion Batteries[END_REF] and photocatalysis [START_REF] Fujishima | Electrochemical Photolysis of Water at a Semiconductor Electrode[END_REF][START_REF] Linsebigler | Photocatalysis on TiO 2 Surfaces: Principles, Mechanisms, and Selected Results[END_REF][START_REF] Walter | Solar Water Splitting Cells[END_REF] because of the relatively high abundance, low cost, environmental friendliness, high chemical stability, and both outstanding optical and electronic properties of TiO 2 -based systems. Thus, TiO 2 materials, mainly as anatase polymorph, showed a great ability for decomposition of various pollutants in air or water, [START_REF] Linsebigler | Photocatalysis on TiO 2 Surfaces: Principles, Mechanisms, and Selected Results[END_REF] carbon dioxide reduction, [START_REF] Dhakshinamoorthy | Photocatalytic CO 2 reduction by TiO 2 and related titanium containing solids[END_REF] and water splitting [START_REF] Fujishima | Electrochemical Photolysis of Water at a Semiconductor Electrode[END_REF] under light illumination. Nonetheless, pure TiO 2 materials still suffer from some limitations for practical use in photocatalysis stemming from too fast recombination of photogenerated electron-hole pairs and too slow redox reactions. [START_REF] Kamat | Manipulation of Charge Transfer across Semiconductor Interface. A Criterion That Cannot Be Ignored in Photocatalyst Design[END_REF] A first efficient way to improve the photocatalytic activity consists in building heterojunction nanocomposites combining TiO 2 nanostructures with other metals [START_REF] Linsebigler | Photocatalysis on TiO 2 Surfaces: Principles, Mechanisms, and Selected Results[END_REF][START_REF] Takai | Shuttling Photogenerated Electrons across TiO 2 -Silver Interface[END_REF][START_REF] Yang | Roles of Cocatalysts in Photocatalysis and Photoelectrocatalysis[END_REF] or semiconductors. [START_REF] Li | TiO 2 Nanobelts Decorated with In 2 S 3 Nanoparticles as Photocatalysts with Enhanced Full-Solar-Spectrum (UV-vis-NIR) Photocatalytic Activity toward the Degradation of Tetracycline[END_REF][START_REF] Hu | Synthesis of Few-layer MoS 2 Nanosheets-coated TiO 2 Nanosheets on Graphite Fibers for Enhanced Photocatalytic Properties[END_REF][START_REF] Li | Ti 3 C 2 MXene-derived Ti 3 C 2 /TiO 2 Nanoflowers for Noble-metal-free Photocatalytic Overall Water Splitting[END_REF][START_REF] Wei | Construction of Noble-metal-free TiO 2 Nanobelt/ZnIn 2 S 4 Nanosheet Heterojunction Nanocomposite for Highly Efficient Photocatalytic Hydrogen Evolution[END_REF] For instance, coupling TiO 2 to noble metal cocatalysts with large work function, e.g., platinum and rhodium, enables to enhance the photogeneration quantum yield in minority carriers by separating electron-hole pairs with a Schottky barrier at the interface and subsequently transferring them to acceptor molecules. [START_REF] Linsebigler | Photocatalysis on TiO 2 Surfaces: Principles, Mechanisms, and Selected Results[END_REF][START_REF] Takai | Shuttling Photogenerated Electrons across TiO 2 -Silver Interface[END_REF][START_REF] Yang | Roles of Cocatalysts in Photocatalysis and Photoelectrocatalysis[END_REF] Furthermore, along with providing reaction sites, these noble metals have beneficial effects on the kinetics of the photocatalytic reactions, for instance, by decreasing the overpotential and the activation energy for hydrogen evolution from water according to the volcano plot for metal-hydrogen bonding energy. [START_REF] Trasatti | Work Function, Electronegativity, and Electrochemical Behaviour of Metals[END_REF] However, the use of noble metals led to further drawbacks such as their scarcity and cost and their ability to catalyze the back-reaction of the formation of water from hydrogen and oxygen, limiting the photocatalytic activity for water splitting. Thus, transition metal oxides that have a negligible activity for water formation have been chosen as cocatalysts to avoid back-reactions. [START_REF] Maeda | Photocatalytic Water Splitting: Recent Progress and Future Challenges[END_REF] On the other hand, the selective deposition of Pt and PbO 2 on specific orientations of rutile and anatase particles was first reported by Ohno et al., [START_REF] Ohno | Crystal Faces of Rutile and Anatase TiO 2 Particles and Their Roles in Photocatalytic Reactions[END_REF] revealing that the different facets of a crystal exposed to the surface promote the separation of photoinduced electrons and holes. [START_REF] Tachikawa | Evidence for Crystal-Face-Dependent TiO 2 Photocatalysis from Single-Molecule Imaging and Kinetic Analysis[END_REF][START_REF] Liu | Crystal Facet Engineering of Semiconductor Photocatalysts: Motivations, Advances and Unique Properties[END_REF][START_REF] D'arienzo | Photogenerated Defects in Shape-Controlled TiO 2 Anatase Nanocrystals: A Probe to Evaluate the Role of Crystal Facets in Photocatalytic Processes[END_REF][START_REF] Zhang | Selective Photoredox Activity on Specific Facet-Dominated TiO 2 Mesocrystal Superstructures Incubated with Directed Nanocrystals[END_REF] More recently, this approach was also exploited to obtain dual heterojunction photocatalysts [START_REF] Wang | Highly Efficient TiO 2 Single-crystal photocatalyst with Spatially Separated Ag and F bi-Cocatalysts: Orientation Transfer of Photogenerated Charges and Their Rapid Interfacial Reaction[END_REF] or spatially separated bicocatalysts onto TiO 2 .(25) These findings have therefore triggered crystal facet engineering of TiO 2 particles. Crystallographic surface orientation and structures indeed play a crucial role in photocatalysis with metal oxide semiconductors since the surface atomic structures rule the physisorption/chemisorption of reactive species, surface transfer and trapping of photoinduced charge carriers, and, as a consequence, redox processes. [START_REF] Sun | Titania-Water Interactions: A Review of Theoretical Studies[END_REF] Over the past decade, many efforts have been devoted to maximizing the surface area of the highest energy {001} facet of anatase crystals, which was believed to be the most reactive, to reach high photocatalytic efficiencies. [START_REF] Gong | Reactivity of Anatase TiO(2) Nanoparticles: The Role of the Minority (001) Surface[END_REF][START_REF] Yang | Anatase TiO 2 Single Crystals with a Large Percentage of Reactive Facets[END_REF][START_REF] Chen | Engineering a High Energy Surface of Anatase TiO 2 Crystals Towards Enhanced Performance for Energy Conversion and Environmental Applications[END_REF] By contrast, improved photocatalytic efficiencies were reported for particles exposing both low-energy facets {101} as reduction reaction sites and high-energy facets {001} as oxidation reaction sites. [START_REF] Murakami | Shape-Controlled Anatase Titanium(IV) Oxide Particles Prepared by Hydrothermal Treatment of Peroxo Titanic Acid in the Presence of Polyvinyl Alcohol[END_REF][START_REF] Amano | Decahedral Single-Crystalline Particles of Anatase Titanium(IV) Oxide with High Photocatalytic Activity[END_REF][START_REF] Roy | Synergy of Low-Energy {101} and High-Energy {001} TiO(2) Crystal Facets for Enhanced Photocatalysis[END_REF][START_REF] Yu | Enhanced Photocatalytic Co(2)-Reduction Activity of Anatase TiO(2) by Coexposed {001} and {101} Facets[END_REF] These findings were recently related to the lower Fermi level of the {001}facets compared to that of the {101}facets which induces a potential difference at the interface between both facets, favoring the spatial separation of the photogenerated electrons and holes. [START_REF] Kashiwaya | Fermil Level Positions and Induced Band Bending at Single Crystalline Anatase (101) and (001) Surfaces: Origin of the Enhanced Photocatalytic Activity of Facet Engineered Crystals[END_REF] As a result, the presence of both {101} and {001} facets in anatase nanocrystals is relevant to obtain high photocatalytic activities. Moreover, enhanced photocatalytic activities could be reached with TiO 2 -based materials via the formation of a p-n heterojunction by combining appropriate p-type metal oxide cocatalysts together with facet engineering of TiO 2 nanocrystals for their synergetic effect. [START_REF] Yang | Roles of Cocatalysts in Photocatalysis and Photoelectrocatalysis[END_REF] In this context, nickel oxide (NiO), which is rather abundant and reactive, is one of the most investigated cocatalysts in combination with TiO 2 and can act as both reduction and oxidation cocatalysts. Thus, various NiO-TiO 2 nanostructures with a random distribution of NiO onto TiO 2 have been reported which led to improved photocatalytic properties for pollutant degradation [START_REF] Ku | Characterization of Coupled NiO/TiO 2 Photocatalyst for the Photocatalytic Reduction of Cr(VI) in Aqueous Solution[END_REF][START_REF] Iwaszuk | Origin of the Visible-Light Response of Nickel(II) Oxide Cluster Surface Modified Titanium(IV) Dioxide[END_REF][START_REF] Wang | 2 /NiO Hybrid Shells: p-n Junction Photocatalysts with Enhanced Activity under Visible-light[END_REF][START_REF] Pandi | Synthesis and Characterization of TiO 2 -NiO and TiO 2 -WO 3 Nanocomposites[END_REF][START_REF] Wahyuningsih | The Influence of NiO Addition in TiO 2 Structure and Its Photoactivity[END_REF][START_REF] Kashiwaya | Supercritical CO 2 -assisted Deposition of NiO on (101)-anatase-TiO 2 for Efficient Facet Engineered Photocatalysts[END_REF][START_REF] Liu | Black NiO-TiO 2 nanorods for solar photocatalysis: Recognition of electronic structure and reaction mechanism[END_REF] or hydrogen production. [START_REF] Wang | 2 /NiO Hybrid Shells: p-n Junction Photocatalysts with Enhanced Activity under Visible-light[END_REF][START_REF] Fujita | Photocatalytic Hydrogen Production from Aqueous Glycerol Solution Using NiO/TiO 2 Catalysts: Effects of Preparation and Reaction Conditions[END_REF][START_REF] Uddin | Band Alignment Investigations of Heterostructure NiO/TiO 2 Nanomaterials Used as Efficient Heterojunction Earth-Abundant Metal Oxide Photocatalysts for Hydrogen Production[END_REF] However, the selective deposition of NiO onto oriented TiO 2 nanoparticles has never been achieved, and both electronic and photocatalytic properties of the resulting heterostructures have not been investigated yet. Furthermore, even though different energy band alignments at the NiO-TiO 2 interface have been proposed for NiO species randomly distributed onto nanoparticles TiO 2 , [START_REF] Kashiwaya | Supercritical CO 2 -assisted Deposition of NiO on (101)-anatase-TiO 2 for Efficient Facet Engineered Photocatalysts[END_REF][START_REF] Uddin | Band Alignment Investigations of Heterostructure NiO/TiO 2 Nanomaterials Used as Efficient Heterojunction Earth-Abundant Metal Oxide Photocatalysts for Hydrogen Production[END_REF][START_REF] Ibupoto | The Synthesis of NiO-TiO 2 Heterostructures and Their Valence Band Offset Determination[END_REF] the energy band alignment of well-defined NiO-anatase (101) and NiO-anastase (001) interfaces still remains unknown.

In the following, we described the selective deposition of NiO onto the {101} facet of anatase TiO 2 nanoparticles coexposed with {001} and {101} facets by employing an original photodeposition method. The formation of cubic NiO particles on the {101} facet was clearly evidenced by high-resolution transmission electron microscopy (HRTEM), high-resolution scanning transmission electron microscopy (HRSTEM), and energy dispersive X-ray spectroscopy (EDX) mapping. Furthermore, a so-called interface experiment [START_REF] Ensling | In-Situ Preparation and Analysis of Functional Oxides[END_REF] was performed by in-situ X-ray (XPS) and ultraviolet (UPS) photoelectron spectroscopy measurements during stepwise NiO deposition via dc sputtering onto a stoichiometric single crystal anatase (101) surface. An energy band alignment of the NiO-anatase (101) interface was experimentally derived from the obtained XP and UP spectra of the interface experiment. Finally, photocatalytic activities of the obtained nanocomposites as a function of NiO loading were examined by degradation of methylene blue (MB) under UV irradiation, and mechanistic aspects were investigated by comparing the degradation rate determined with those obtained in the presence of hole and radical scavengers. The main species ruling the photocatalytic properties of the NiO-TiO 2 nanomaterials appeared to be different from that governing those of pure TiO 2 nanocrystals.

Experimental section

Photocatalysts preparation

All the chemicals were purchased in analytical grade and used without any further purification. Crystalline TiO 2 anatase particles coexposed with {001} and {101} facets obtained by gas-phase reaction were provided by Prof. Ohtani's group. [START_REF] Amano | Decahedral Single-Crystalline Particles of Anatase Titanium(IV) Oxide with High Photocatalytic Activity[END_REF][START_REF] Janczarek | Decahedral-shaped Anatase Titania Photocatalyst Particles: Synthesis in a Newly Developed Coaxial-flow Gas-phase Reactor[END_REF] According to SEM images recorded at different magnifications, the shape of these nanoparticles is rather uniform (Figure S1). NiO-TiO 2 anatase heterostructured photocatalysts were synthesized in two steps. First, Ni-B was selectively deposited onto the {101} facet of the anatase particles by photodeposition.(47) 1.27 g of boric acid (Sigma-Aldrich) was added to 70 mL of distilled water under stirring. The solution was sonicated for 5 min. Potassium hydroxide was then progressively added to the solution until reaching pH = 9.2 under stirring, and 30 mL of distilled water was added to the solution. The resulting solution is referred as a KBi buffer. The KBi buffer, deionized water, and nickel(II) nitrate hexahydrate solution (34 mM, Sigma-Aldrich) were stirred and subsequently centrifuged at 4000 rpm for 10 min to remove a slight amount of precipitated nickel hydroxide, which has a lightgreen color. 500 mg of the single crystalline anatase particles was dispersed into the solution, treated with sonication for 10 min, and stirred for 1 h. To realize the photodeposition of Ni-B onto the anatase particles, the solution was irradiated by UV light under stirring for 3 h. After washing with distilled water three times, the resulting particles were dried at 70 °C and dried under vacuum for 1 h to obtain Ni-B deposited onto the {101} facet of the anatase particles. The {101} facet of anatase crystals was found to accumulate photogenerated electrons, [START_REF] Tachikawa | Evidence for Crystal-Face-Dependent TiO 2 Photocatalysis from Single-Molecule Imaging and Kinetic Analysis[END_REF] and thus nickel nitrate (Ni 2+ ) is supposed to be reduced to Ni-B (Ni 0 ) selectively on the {101} facet of anatase crystals under UV irradiation. The resulting Ni-B/TiO 2 heterostructured particles were then annealed at 400 °C for 4 h to oxidize deposited Ni-B and achieve the target NiO-TiO 2 photocatalysts where NiO is selectively deposited onto the {101} facet of the anatase particles coexposed with {001} and {101} facets. Photocatalysts containing 0.1, 0,2, 0.5, 1, and 10 wt % of NiO were obtained by adding 0.2, 0.4, 1, 2, and 20 mL of nickel nitrate solution (34 mM), respectively, as a precursor added to 500 mg of anatase particles. The KBi buffer/nickel nitrate solution ratio was fixed at 2.5, and distilled water was also added to reach 100 mL of the total volume of the precursor solution containing KBi buffer and nickel nitrate. The resulting photocatalysts are hereafter named 0.1 wt % NiO-TiO 2 , 0.2 wt % NiO-TiO 2 , 0.5 wt % NiO-TiO 2 , 1 wt % NiO-TiO 2 , and 10 wt % NiO-TiO 2 .

Photocatalyst characterization

XRD analyses were performed using a Bruker AXS diffractometer (D2 PHASER A26-X1-A2B0D3A) equipped with a Cu anode (Kα radiation).( 48) DRS spectra were recorded with an UV-vis-NIR Cary 5000E spectrophotometer endowed with an integrating sphere and by using a Suprasil cell bearing a quartz window. Halon standard (6 mm deep and 1 g cm -3 density) was employed as a reference. Specific surface areas (S BET ) were determined by applying the BET (Brunauer-Emmett-Teller) equation( 49) between 0.1 and 0.3 relative pressures of N 2 adsorption-desorption isotherms recorded at 77 K with an ASAP2010 Micromeritics apparatus. HRTEM images were taken with a JEOL JEM 2200F (JEOL, Tokyo, Japan) operating at an acceleration voltage of 200 kV (wavelength λ = 2.51 pm) equipped with a Schottky type FEG and an EDX system (Oxford, Wiesbaden, Germany). For the sample preparation for HRTEM, HRSTEM, and EDX, the photocatalysts nanoparticles were dispersed in ethanol under ultrasound sonication, and a few droplets of the suspension were placed on holey carbon grids (Cu-300HD, Pacific Grid-Tech).

Electronic properties of the synthesized photocatalysts were investigated with DAISY-MAT(50) consisting of a multitechnique surface analysis system (Physical Electronics PHI 5700). XP spectra were measured by using monochromatic Al Kα (1486.6 eV) radiation with an energy resolution of 0.4 eV as determined from the broadening of the Fermi edge of a sputter-cleaned Ag substrate. Binding energies of core levels and the valence band maximum E VB can be obtained with an accuracy of 50 meV and 100 meV, respectively. Binding energies determined by XPS were calibrated by the Fermi level position of the sputter-cleaned Ag sample. Thus, all binding energies are provided with respect to the calibrated Fermi level set to 0 eV. Photocatalyst samples were pressed onto indium foils as a conductive substrate to avoid a charging problem and transferred into the XPS chamber without any further treatments.

Photocatalyst experiments

Photocatalytic properties of the different NiO-TiO 2 nanocomposites were determined by decomposition of MB (Alfa Aesar) under UV-light illumination using a previously established procedure.( 51) Typically, a suspension of photocatalyst (50 mg) in a MB aqueous solution (10 mg/L, 50 mL) was introduced into a Pyrex beaker and was then illuminated with a 125 W high-pressure mercury lamp (Philips, HPL-N 125 W/542 E27) positioned 60 mm above the solution. After stirring in the dark for 30 min to reach adsorption/desorption equilibrium and switching on the light, 2 mL of the suspensions was collected at given irradiation time intervals and then centrifuged (4000 rpm, 10 min) to remove the photocatalyst particles from the MB solution. The remaining MB amount was monitored by UV-vis spectroscopy following the change in absorption at λ max = 664 nm by using a Shimadzu UV-1650 pc spectrophotometer. Blank experiments were also conducted with catalysts in the dark and without catalysts under light irradiation.

Furthermore, the same experiments were reproduced in the presence of different scavengers for charge carriers and radicals to investigate the role of radicals in the photocatalytic reactions. Potassium iodide (KI), isopropanol (IPA), and 1,4-benzoquinone (BQ) were selected as holes, hydroxyl radicals (OH), and superoxide radicals ( , and 1 × 10 -3 M, respectively.

Interface model experiment

Natural anatase crystals were employed for both (101) and (001) surfaces (SurfaceNet GmbH, Germany). The size of the substrates is 5 × 5 × 1 mm 3 . A surface roughness less than a lattice constant was achieved after epi-polishing. As-is surfaces are contaminated from the air and the polishing step. Cleaning cycles of Ar + ion sputtering with an energy of 1 keV for 15 min and annealing under vacuum at 873 K for 30 min were repeated until no remaining emissions from contaminations were observed by XPS. This cleaning procedure has been often employed to remove the contamination and achieve clean TiO 2 surfaces.(56) However, Ar sputtering not only leads to a highly reduced surface, and thus forms O vacancies and Ti 3+ surface states, but also partially ruins the crystallinity of the surface and introduces an amorphous phase. [START_REF] Setvín | Surface Preparation of TiO 2 Anatase (101): Pitfalls and How to Avoid Them[END_REF] The O vacancy concentration for the (101) and (001) surfaces prepared by the procedure is expected to be 7 ± 2% and 1 ± 2%, respectively.( 58) Therefore, the sputtered surfaces were oxidized by an O plasma at room temperature in an atmosphere of 7.5 × 10 -5 mbar O 2 for 15 min to reoxidize the surface and subsequently annealed at 873 K in UHV to recrystallize the surface. Here the O plasma was selected to replenish the lattice O instead of annealing in O 2 atmosphere, which has been generally used for oxidation, as this annealing procedure can form additional incomplete reconstructions for rutile surfaces,(59) which could be also formed for anatase surfaces. It may also result in iron oxide surface contaminations as at elevated temperatures Fe surface segregation, which is typical contamination in natural anatase crystals, may take place.(57) Furthermore, atomic O in the O plasma is more oxidative than a molecular O 2 during annealing in an O atmosphere. [START_REF] Wu | Surface Modification of Indium Tin Oxide by Plasma Treatment: An Effective Method to Improve the Efficiency, Brightness, and Reliability of Organic Light Emitting Devices[END_REF] The reoxidized and recrystallized surfaces were treated with an O plasma at the same conditions mentioned above to oxidized them as they were slightly reduced due to annealing under vacuum at the elevated temperature. This further oxidation procedure is expected to introduce adsorbed O atoms such as peroxo species adsorbed on the surfaces. Finally, the stoichiometric surfaces were obtained by removing adsorbed O atoms by annealing the oxidized surfaces in UHV condition at 473 K for 20 min. [START_REF] Klissurski | Study of Peroxide-Modified Titanium Dioxide (Anatase)[END_REF] To experimentally determine energy band alignments of NiO and oriented anatase surfaces, the interface model experiments were performed in DAISY-MAT capable of XPS and UPS and different deposition chambers in UHV without breaking the vacuum. This system allowed the in-situ characterization of the prepared surfaces and interfaces. NiO was grown on the stoichiometric anatase (001) and (101) substrates by using a direct current (dc) reactive magnetron sputter deposition from a metallic Ni target with 40 W of dc power under a 20% O 2 /80% Ar mixture atmosphere at 0.5 Pa pressure and room temperature. The gas flow of oxygen and argon was kept at 4 and 16 sccm, respectively, during the deposition. The target-to-substrate distance was set to 8.4 cm. The deposition rate was about 2.0 nm min -1

. We are aware of possible additional defect states induced by sputter deposition of oxides. However, a number of systematic studies on different oxides have proven that for our experimental conditions this approach is applicable. After each stepwise deposition of NiO by dc sputtering onto the substrates, XPS spectra were recorded without breaking the vacuum to follow shifts of the binding energies of core level emission lines and the evolution of peak shapes. To determine work function and ionization potentials of the stoichiometric anatase substrates and NiO films at the end of all the stepwise deposition, UPS spectra were recorded in normal emission with He I radiation (hv = 21.2 eV) from a He discharge lamp and a negative sample bias of 4.0 V. The total energy resolution of UPS was 0.2 eV. No charging problems were observed during the XPS and UPS measurements. Binding energies obtained by XPS and UPS were calibrated by the Fermi level energy of the sputter-cleaned Ag sample. Hence, all the binding energies are given with respect to the calibrated Fermi level position set to 0 eV. More details of the DAISY-MAT system and the experimental approach in performing the experiments may be found elsewhere. [START_REF] Ensling | In-Situ Preparation and Analysis of Functional Oxides[END_REF]

Results and discussion

Characterization of photocatalysts

To prepare well-defined heterostructure NiO-TiO 2 particles, the electroless nickel plating strategy was adapted to deposit amorphous Ni-B catalyst on specific facets of TiO 2 nanocrystals.(47) Instead of using silver-based materials as a reducing agent, UV-light combined with the charge separation offered by the different redox properties of {101} and {001} facets were exploited for the selective deposition of amorphous Ni-B on the {101} facets which led to NiO particles after further calcination (Scheme 1).

Scheme 1. Preparation Strategy for Selective Deposition of NiO onto {101} Anatase TiO 2 Facets

Regardless of the NiO loading, the main diffraction peaks observed in the XRD patterns can be indexed as the (101), (004), ( 200), (105), ( 211), ( 204), ( 116), (220), and (215) diffraction lines of the anatase phase of TiO 2 (Figure 1). The weak feature at 27.5° (2θ) can be attributed to the presence of small amounts of the rutile polymorph (110 diffraction line) due to the high temperature (1573 K) used to synthesize the oriented TiO 2 particles. [START_REF] Amano | Decahedral Single-Crystalline Particles of Anatase Titanium(IV) Oxide with High Photocatalytic Activity[END_REF][START_REF] Janczarek | Decahedral-shaped Anatase Titania Photocatalyst Particles: Synthesis in a Newly Developed Coaxial-flow Gas-phase Reactor[END_REF] Peaks assigned to NiO were not observed clearly probably due to the small crystalline domain size (nanometers order) of the NiO species. It is indeed well-known that the crystallite size is inversely proportional to the full width at half-maximum (fwhm) of each peak, which yields broadened and attenuated signals in the XRD patterns.( 62) The morphology of the different materials was then studied by TEM, HR-TEM, and EDX analyses (Figures 2 and3). A square shape of the particle indicates that the {001} facet of the anatase single crystal is parallel to the sample carbon grid as anatase single crystals coexposed with {001} and {101} facets have a bipyramidal shape with truncation of their top and bottom edges (Figure 2A). A gradation of the contrast observed at the edge of the particle can be caused by thinner parts of the sample and might represent a slope of the {101} facet. N 2 sorption analyses were performed to determine the textural properties of the nanocomposites prepared. Regardless of the nickel oxide content, the sample showed a type II adsorption-desorption isotherm typical of nonporous or macroporous solids with BET specific areas of about (13.4-14.0) ± 0.4 m 2 g -1 and total pore volumes of (0.036-0.044) ± 0.001 cm 3 g -1 (Table 1 and Figure S3). These data are consistent with TEM data showing rather well-dispersed nanoparticles with few aggregates. Moreover, depositing up to 10 wt % NiO does not change significantly specific surface areas and total pore volumes.

Optical properties of photocatalysts were investigated by DRS (Figure 4). Heterostructured NiO-TiO 2 samples exhibit visible light absorption of which edges shift toward lower energies with increasing the NiO loading even though the band gap of NiO has been reported to be 3.4-4.3 eV, which corresponds to an absorption edge of 288-365 nm. [START_REF] Patil | Preparation and Characterization of Spray Pyrolyzed Nickel Oxide (NiO) Thin Films[END_REF][START_REF] Sawatzky | Magnitude and Origin of the Band Gap in NiO[END_REF] The origin of the visible light absorption exhibited by NiO-TiO 2 nanocomposites can be ascribed to Ti-Ni bondings at the NiO-TiO 2 interface or interfacial defect states.( 65) DFT calculations revealed that after modification of the TiO 2 surface with NiO clusters new states appear above the valence band and just below the conduction band of TiO 2 .(30,66) Then, this might result in the narrowed band gap found for NiO-TiO 2 samples and thus the visible light absorption. Nonetheless, it was not possible to determine accurately the optical band gap of the various materials prepared by using the conventional Tauc plot analysis postulating that TiO 2 is an indirect semiconductor(67,68) due to the complex composite nature of these materials. [START_REF] Makula | How to Correctly Determine the Band Gap Energy of Modified Semiconductor Photocatalysts Based on UV-Vis Spectra[END_REF] To obtain a deeper understanding of the electronic properties of pure TiO 2 and heterostructured NiO-TiO 2 , XPS measurements were carried out. In addition to carbon, nitrogen, and calcium species which are typical contaminations of ex-situ samples synthesized chemically, emissions from titanium, oxygen, nickel, and indium were detected in the survey spectra (Figure S4). High-resolution scanning for Ni 2p, O 1s, and Ti 2p core level emissions and valence band region emissions are shown in Figure 5. The Fermi level shifts toward a lower energy level from 3.38 eV for pure TiO 2 to 3.07, 3.12, 3.15, and 2.20 eV for 0.1, 0.2, 1, and 10 wt % NiO-TiO 2 , respectively (Figure 5D). The 0.5 wt % NiO-TiO 2 sample does not show any bending, which is evidently related to a smaller amount of Ni as deposited onto the analyzed area (comparing the Ni 2p vs Ti 2p substrate energies). Core levels of prepared samples follow the shift of the Fermi level position except for the 10 wt % NiO-TiO 2 sample, the valence band edge of which is affected not only by the Fermi level shift of the TiO 2 but also directly by the presence of O 2p orbitals of NiO. The exact analysis of the binding energy values and the possible influence of band bending and possible charging effects are hard to discriminate for the samples with different and inhomogeneous distributions of NiO deposits onto nanostructured supports pressed into indium. Therefore, the shifts can only be qualitatively related to the buildup of space charge layers, and details of the contact formation are given from independent layer-by-layer heterojunction formation experiments (see section 3.2). The emission from the Ti 2p 3/2 doublet contains only the Ti 4+ line at 459.47 eV without Ti 3+ for the pure TiO 2 and the peak shifts to the lower binding energy by 140 meV with the presence of NiO (Figure 5C). O 1s spectra show an intense peak assigned to Ti-O bonds accompanying a shoulder at 1.6 eV lower binding energy from the main emission line (Figure 5B). The main line of O 1s emission also shows an upward bending with NiO as well as the Ti 2p 3/2 peaks. The presence of Ni was evidenced by the Ni 2p 3/2 emission line at 856.64 eV with a satellite line at a binding energy higher than that of the main peak by 7.7 eV for the NiO-TiO 2 samples (Figure 5A). Although typical NiO shows a main peak of Ni 2p 3/2 accompanying a shoulder peak at 1.5 eV higher binding energy position which is known as induced by a nonscreening effect, [START_REF] Van Veenendaal | Nonlocal Screening Effects in 2p X-Ray Photoemission Spectroscopy Core-Level Line Shapes of Transition Metal Compounds[END_REF][START_REF] Soriano | Surface Effects in the Ni2p X-Ray Photoemission Spectra of NiO[END_REF] the emission of our samples shows broadened shapes, typical also for small thicknesses, from which the main and satellite peaks cannot be distinguished. This broadening might be attributed to the ultrathin layer of NiO on the anatase particles as the typical intensity pattern of the satellite of NiO is pronounced when the thickness is less than a few nanometers, resulting in the overlapping of the main and satellite peaks which cannot be clearly identified.

Interface analysis

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To obtain further insight into the interface properties of the NiO-anatase heterojunction, in-situ interface experiments were performed by XPS and UPS measurements after each deposition step of NiO with increasing thickness onto the stoichiometric anatase ( 101) and ( 001) substrates. Figure 6 presents the core level photoelectron spectra recorded during the stepwise deposition for the stoichiometric anatase (101) substrate (for anatase (001) see Figure S5). The shoulder appearing at a binding energy 1.5 eV higher than the one of the main emission is assigned to a satellite emission of the Ni 2+ rather than Ni 3+ species. This satellite is a so-called nonlocal screening satellite which appears by a screening process due to oxygen atoms belonging to the octahedral NiO 6 coordination.(70) A broad emission at the low coverage of NiO is evident and might be associated with an imperfect octahedral structure due to surface effects. [START_REF] Soriano | Surface Effects in the Ni2p X-Ray Photoemission Spectra of NiO[END_REF] With increasing the thickness of the NiO layer, the typical NiO emission signature is detected. The contribution of both the nonlocal and surface effects prevents a determination of the exact binding energy peak position. Thus, the main satellite peak of Ni 2p 3/2 at about 7 eV higher binding energy from the main line of Ni 2p 3/2 , which is characteristic for Ni 2+ , is adopted instead of the main peak to follow the peak shift in the course of NiO deposition.

The Ti 2p 3/2 peak at the beginning of the NiO deposition is rather sharp and symmetric, indicating a mostly stoichiometric surface with dominant Ti 4+ oxidation state (Figure 6). However, the asymmetry of the Ti 2p line to lower binding energy evidently increases, suggesting a slight increase of Ti 3+ states during NiO deposition. In the O 1s region, a shoulder peak is observed at higher binding energy than the main peak by 2.3 eV which might be related to the initial formation of the O poor surface species and possible contributions of adsorbed OH -.

The evolution of the Fermi level (E F -E VBM ) of NiO and anatase substrates obtained by following the core level of Ni 2p satellite and Ti 2p 3/2 , respectively, is shown in Figure 7.

The values E F -E VBM of pristine stoichiometric anatase substrates and the NiO top layer correspond to the values of E VB as determined from the valence band spectra. The Fermi level of the uncovered anatase (101) substrate is higher than that of the (001) substrate by 90 meV. This might be due to a higher density of oxygen vacancies on the surface and subsurface of the (101) substrate than that of the (001). The Fermi level of both NiO and anatase shows a parallel shift to lower binding energy, indicating that the band bending occurs only in the anatase substrates and the emission line of Ni 2p just follows the band bending in anatase. The Fermi level shift is attributed to the contact formation between NiO and anatase, forming the p-n junction leading to band bending in contact. It is worth mentioning that the Fermi level of anatase (101) shows a smaller band bending than that of the anatase (001) by 210 meV. This is attributed to stable oxygen subsurface oxygen vacancies of the anatase (101) which would pin the Fermi level at 2.16 eV. As is evident from these diagrams, the band bending at the interface of p-doped NiO and n-doped TiO 2 for the anatase (001) is larger than that for the anatase (101) by 210 meV. In p-NiO the Fermi level is pinned by a high concentration of defect states (Ni 3+ related states) in the bulk of the material as there is no additional band bending observed in the NiO layers. The pinning level is found at a value of E F -E VBM between 0.68 and 0.75 eV. Strong band bending within the TiO 2 substrates is achieved by the deposition of thin NiO layers of thicknesses larger than 1 nm, which would contribute to charge carriers separation in nanoparticle photocatalyst systems. The fact that the space charge layer observed for the NiO particles on the {101} facets of TiO 2 is considerably smaller can be understood based on the different surface properties of the two facets which have already been discussed in details in a previous paper. For small particles the space charge layer is inhomogeneous, and only a small fraction is detected for the rare areas. Second, for small dimensions of the NiO overlayer onto TiO 2 substrates the complete space charge layer may not be found due to exhaustion of charge carriers. To examine the photocatalytic activity of the pure TiO 2 and NiO-TiO 2 nanomaterials, the photodecomposition of methylene blue (MB) was performed under UV illumination.

Photocatalytic properties

Figure 9A shows stepwise attenuation of the characteristic absorption peaks of MB during its photocatalytic decomposition in the presence of 0.1 wt % NiO-TiO 2 . A slight hypsochromic shift of the maximum absorption wavelength, from 664 to 650 nm after 40 min of irradiation, was observed, which suggests the demethylation of methylene blue yielding Azur B and Azur A chromophore intermediates. [START_REF] Zhang | Photocatalyzed N-Demethylation and Degradation of Methylene Blue in Titania Dispersions Exposed to Concentrated Sunlight[END_REF][START_REF] Zhang | Photooxidative N-demethylation of methylene blue in aqueous TiO 2 dispersions under UV-irradiation[END_REF] Finally, MB was completely decolorized within 60 min under UV-light irradiation, evidencing the high photocatalytic efficiency of the 0.1 wt % NiO-TiO 2 heterostructure. The photocatalytic activities of pure TiO 2 and NiO-TiO 2 photocatalysts with various NiO contents are shown in Figure 9B. For quantitative evaluation of the reaction kinetics of the MB degradation, a pseudo-first-order model expressed as ln(C/C 0 ) = -K app t was applied. Here, C 0 and C correspond to the concentration of MB in solution at time 0 and t, respectively, and K app stands for the apparent first-order rate constant for MB degradation. K app can be obtained from the slope of the curve ln(C/C 0 ) vs t. Regardless of the photocatalyst used, the MB degradation follows the pseudo-first-order reaction kinetics at the beginning of the process. Furthermore, to avoid the influence of the surface area of the photocatalysts on the rate constant, the normalized rate constant (K norm ) defined as K norm = K app /S BET was calculated as summarized in Table 1. The 0.1 wt % NiO-TiO 2 photocatalyst shows the highest activity, and its K norm is higher than that of the pure TiO 2 by 50%. This result confirms the key role of the combination of NiO with TiO 2 for enhancing the photocatalytic activities which can be attributed to the enhanced charge separation due to the electric field at the interface of the p-n junction between NiO and TiO 2 as described below.

Mechanistic consideration

According to literature data, the following mechanism can be proposed. • . Thus, the charge carriers, the lifetime of which is prolonged by spatial charge separation at the p-n junction, would acquire a higher probability to encounter reactants producing radicals in the solution and then account for the high photocatalytic activities.

To obtain a deeper understanding of the photodecomposition mechanism in the presence of TiO 2 and NiO-TiO 2 nanomaterials, potassium iodide (KI), isopropyl alcohol (IPA), and 1,4-benzoquinone (BQ) were employed as hole (h + ), hydroxyl radical (OH • ), and superoxide radical ( • O 2-) scavengers, respectively, for MB degradation. The corresponding kinetic rate constants for MB decomposition normalized by that without scavengers are reported in Figure 10. First of all, it can be clearly seen that the species involved in the photocatalytic decomposition of MB depend on the photocatalyst used. Thus, for pure TiO 2 photocatalyst, the degradation efficiency of MB decreased obviously in the presence of BQ while it was almost not affected by the addition of KI and IPA. This means that • O 2-radicals, which are created via photogenerated electrons, are the dominant active species in the photocatalytic reaction whereas OH • radicals play a minor role. The TiO 2 crystals used in this study have an averaged aspect ratio (B/A), [START_REF] Selloni | Anatase Shows Its Reactive Site[END_REF] which is defined by the ratio of the short B and long A sides of the top-truncated bipyramidal crystal, of around 0.7. [START_REF] Amano | Decahedral Single-Crystalline Particles of Anatase Titanium(IV) Oxide with High Photocatalytic Activity[END_REF] The value corresponds to 72% exposure of the {101} facet, and thus it is expected that electrons, which are supposed to be accumulated onto the majority (101) facet, are relevant in photocatalysis for the pure TiO 2 . By contrast, photocatalytic activities of 0.1 wt % NiO-TiO 2 nanomaterials were decreased by addition of KI and IPA, whereas BQ has almost no effect. Although both holes and electrons would contribute to create OH • and oxidize the MB, BQ as a scavenger of • O 2-created by only electrons did not affect much the photocatalytic activities. This reveals that holes mainly dictate the photocatalytic MB decomposition on the 0.1 wt % NiO-TiO 2 photocatalysts. Despite the TiO 2 crystals used mainly expose {101} facets acting as electron sinks, NiO deposited onto the {101} facets would not only effectively collect holes at the p-n NiO-TiO 2 {101} junction but also catalyze the reaction by lowering the activation energy, and therefore holes can be competitive rather than electrons. However, simultaneous consumption of both electrons and holes is required to achieve optimal photocatalytic performances since the accumulation of electrons or holes might result in a higher recombination rate. The deposition of more than 0.2 wt % NiO onto TiO 2 might cover the naked (101) anatase surface more than enough and generate unreacted electrons. These charge carriers will accumulate and trigger a higher possibility of the recombination. Thus, for this NiO-TiO 2 (101) system, the 0.1 wt % NiO-TiO 2 photocatalyst showed the best photocatalytic activity.

Conclusion

The selective photodeposition of NiO nanoparticles onto the {101} facets of oriented TiO 2 anatase nanocrystals coexposed with {001} and {101} facets was achieved for the first time to yield efficient NiO-anatase TiO 2 photocatalysts. The NiO-TiO 2 heterostructure nanoparticles showed light absorption properties shifted toward visible light region compared to those of pure TiO 2 . Furthermore, interface model experiments of NiO deposited onto TiO 2 substrates were conducted and compared to NiO-TiO 2 faceted nanostructures to experimentally determine the energy band alignment diagrams of the NiO-oriented TiO 2 heterostructure. The 0.58 eV shifts of core levels toward lower binding energy were assigned to an upward band bending in TiO 2 anatase (101) substrates which is expected to favor electron-hole charge separation at the interface. The difference of band bending of only smaller than 0.3 eV is related to size effects. The 0.1 wt % NiO-TiO 2 actually showed 50% higher photocatalytic activity than the pure TiO 2 for MB photodegradation due to effective charge separation by the internal electric field formed at the p-n NiO-TiO 2 heterojunction. Finally, further mechanistic studies performed by using carrier and radical scavengers revealed that holes dominantly dictate the photocatalytic reactions for NiO-TiO 2 nanocomposites whereas electrons mainly participate in the photocatalytic reactions rather than holes for the pure TiO 2 . These results underline the key role of selective deposition of NiO onto anatase TiO 2 nanocrystals on photocatalytic processes. This work will inspire optimal designs of heterostructure systems for the efficient photocatalytic process by combining an appropriate choice of cocatalysts and selective deposition onto the specific facet of supported photocatalyst crystals.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.9b00729.

Authors

Figure 1 .

 1 Figure 1. XRD patterns of the pure TiO 2 (a, black), 0.1 (b, orange), 0.2 (c, magenta), 0.5 (d, blue), 1.0 (e, red), and 10 wt % (f, olive) NiO-TiO 2 composites.

  Figure2Bshows TiO 2 particles with other angles showing both {001} and {101} facets. STEM images, EDX elemental mapping, and HR-TEM images of 10 wt % NiO-TiO 2 depicted in Figure3clearly indicate that Ni elements are selectively distributed at the {101} facets, which constitute the edge of the particle (Figures3E and 3F). HR-TEM images confirm the selective deposition of 2-3 nm of NiO particles onto the {101} facet (Figures3G and 3H). The interplanar spacings of 0.18 and 0.20 nm match with the {200} and {111} planes of the face-centered cubic (fcc) crystalline phase of NiO. STEM images before and after electron beam irradiation also indicate the selective deposition of NiO onto the {101} facet of anatase (Figures3A and 3B). Moreover, selected area diffraction patterns clearly confirm that NiO particles were selectively deposited on the {101} anatase facet, with no diffraction feature typical of NiO being detected on the {001} anatase facet (FigureS2).

Figure 2 .

 2 Figure 2. Bright-field TEM images of the pure TiO 2 recorded along (A) the [001] zone axis and (B) the tilted along the [010] direction.

Figure 3 .

 3 Figure 3. Bright-field STEM images of 10 wt % NiO-TiO 2 (A) before and (B) after electron beam irradiation. EDX elemental mapping of (C) Ti, (D) O, (E) Ni, and (F) overlay. HRTEM images (G, H).

Figure 4 .

 4 Figure 4. UV-vis diffuse absorption of pure TiO 2 (a, black) and NiO-TiO 2 nanocomposites containing 0.1 (b, orange), 0.2 (c, magenta), 0.5 (d, blue), 1.0 (e, red), and 10 wt % (f, olive) NiO.

Figure 5 .

 5 Figure 5. XPS spectra of (A) Ni 2p, (B) O 1s, (C) Ti 2p and (D) valence band emissions for the pure TiO 2 (a, black), NiO-TiO 2 containing 0.1 (b, orange), 0.2 (c, magenta), 0.5 (d, blue), 1.0 (e, red), and 10 wt % (f, olive).

Figure 6 .

 6 Figure 6. (A) XPS and (B) UPS spectra of the interface experiment performed by stepwise NiO deposition onto the stoichiometric anatase (101) substrate.The emission of Ti 2p shows the typical doublet peak, but only the Ti 2p 3/2 line is employed to follow the peak shift during the deposition. Complex main line splitting caused by multiplet contributions and satellite structures at higher binding energies cause difficulty in identifying and interpreting NiO spectra. A binding energy of 853.8 eV is assigned to Ni 2p 3/2 spectra for Ni2+ of NiO x which is clearly observed after completing NiO deposition of thicker than 2.5 nm which shows the expected two satellite peaks (compared to the 50 nm thick NiO layer, which is representative of pure NiO undisturbed by the substrate). The spectral feature within the main satellite at a binding energy of around 861 eV indicates that mostly NiO starts to form already from the very beginning of the deposition sequence. The shoulder appearing at a binding energy 1.5 eV higher than the one of the main emission is assigned to a satellite emission of the Ni 2+ rather than Ni 3+ species. This satellite is a so-called nonlocal screening

Figure 7 .

 7 Figure 7. Fermi level (E F -E VBM ) measured during stepwise deposition of NiO onto the stoichiometric anatase (101) substrate. The work function of the uncovered anatase substrates and top layers of deposited NiO was derived from the binding energies of the secondary electron edge E SE according to Φ = hv -E SE . The work function of the stoichiometric anatase (101) and (001) substrates is 5.58 and 5.63 eV, respectively. The work function of NiO on the anatase (101) and (001) substrates is 5.15 and 5.17 eV, respectively. No noticeable difference in work function of NiO indicates that NiO on both orientations of anatase would possess similar stoichiometry without being influenced by different surface coordination of these orientations. Finally, the band alignments at the interface of NiO-anatase (101) and NiO-anatase (001) as experimentally established by using the above data are shown in Figure 8 and Figure S6.

Figure 8 .

 8 Figure 8. Band energy diagrams of p-NiO-n-TiO 2 anatase (101) before and after contact as determined from the experimental data. The important values are given in the figures.

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 10

Figure 9 .

 9 Figure 9. (A) Absorbance change in the visible range of methylene blue solution after various illumination times for 0.1 wt % NiO-TiO 2 : before irradiation (black), 10 min (red), 20 min (blue), 30 min (magenta), 40 min (olive), 50 min (cyan), and 60 min (purple). (B) Pseudo-first-order kinetics expressed by ln(C/C 0 ) vs irradiation time t of the degradation of MB for the pure TiO 2 (a, black), NiO-TiO 2 containing 0.1 (b, orange), 0.2 (c, magenta), 0.5 (d, blue), 1.0 (e, red), and 10 wt % (f, olive) NiO.

( 37 )

 37 After light absorption by the photocatalyst, the photoexcited electron and holes are transferred to TiO 2 and NiO, respectively, at the p-n junction. The electrons transferred to TiO 2 take part in the reduction of dissolved oxygen, forming a superoxide radical O 2 •-. The O 2 •-radicals react with H 2 O to produce hydroxyl radicals OH • which are strong oxidizing agents leading to dye decomposition. At the same time, the holes transferred to NiO directly react with surface adsorbed dyes leading to the dye degradation and also oxidize the surface hydroxyl groups or physisorbed H 2 O forming OH

Figure 10 .

 10 Figure 10. Kinetic rate constants for MB decomposition normalized by that without scavengers for pure TiO 2 (black) and 0.1 wt % NiO-TiO 2 (orange) without scavengers or in the presence of potassium iodide (KI), isopropanol (IPA), and 1,4-benzoquinone (BQ).

:

  Shun Kashiwaya -http://orcid.org/0000-0002-3164-8403 Jérôme Majimel -ICMCB UMR 5026, CNRS, Univ. Bordeaux INP, F-33600 Talence, France Andreas Klein -http://orcid.org/0000-0001-7463-1495 Wolfram Jaegermann -Fachbereich Material-und Geowissenshaften, Technische Universität Darmstadt, Petersenstr. 23, 64287 Darmstadt, Germany Notes : The authors declare no competing financial interest.

Acknowlegments : Prof. Ohtani is warmly acknowledged for providing the single crystalline TiO 2 anatase particles coexposed with {001} and {101} facets. Dr. Marie-Anne Dourges (ISM) and Mrs Muriel Alrivie (LCTS) are acknowledged for their help in collecting the N 2 sorption and SEM data. This work was supported by the European Commission ((S. K. Fellowship, Erasmus Mundus Doctoral School IDS-FunMat, Grant Agreement EMJD FPA 2010/0004 SGA 2014-0966) and the "Université Franco-allemande" (German-French Doctoral College in Functional Materials: Contract CDFA 02-15 (2018-2021)). We thank the PLACAMAT structure for its electron microscopy facilities and EMMI (European Multifunctional Material Institute).