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Keywords: software engineering, requirements, Hardwarein-the-Loop, testing, automotive

The required knowledge and skills that should be provided to the novice developer, designing and testing the safety critical device in automotive industry using Hardware-in-the-Loop (HiL), are presented in the paper. They should be available to the student finishing the MSc level of Electrical Engineering or Mechatronics that aims to seek the employment in the automotive industry. The development process is presented in short, together with the brief explanations of phases, which include some typical examples of written text in the documentation (requirements, test cases, etc.). The process follows the Automotive SPICE and focuses on the tip of the V-model. The presented text can serve as a useful information in the process of adapting the existing curriculum to the new occurring needs introduced by the new development and testing processes in industry.

I. INTRODUCTION

N the modern vehicles there is a growing number of electric, electronic and programmed electronic devices. This is not only the case for the main drive (drivetrain), but also for a big number of auxiliary devices. In many cases previously mechanic or hydraulic devices are being replaced with the electric ones. For many such devices (so-called safety critical devices), failures could result in serious, even fatal, injuries of the passenger in the vehicle. These failures must be avoided as far as possible at reasonable cost and in a way sustainable for the producer of the vehicle.

Using extremely reliable devices would in many cases result in a very high cost of the final product, making it unaffordable for the customer. The solution is therefore to develop in such a way, that the devices remain safe even in the case of their failure, i.e., to achieve functional safety of the device and as a result also vehicle. Failures shall be avoided, however if this is not possible, their effects have to be mitigated. This goal can be achieved with the use of fault-tolerant devices developed according to the proper development cycle.

The development cycle in the automotive industry is based on the classical V-model, where the development from the user requirements to the validation of the product is described. In automotive industry processes are specified in compliance with the Automotive SPICE standard [START_REF]VDA QMC Working Group 13 / Automotive SIG, Automotive SPICE Process Assessment / Reference Model (PAM)[END_REF]. Both the system and software development process is upgraded with the processes and methods prescribed in IEC/ISO 26262 [START_REF]Road vehicles -Functional safety[END_REF], the functional safety standard for the electrical, electronic and programmable electronic devices in road vehicles. Automotive SPICE and IEC/ISO 26262 thus represent the most important documents in automotive industry to comply with the Quality Management and Functional Safety assessment. The risk classification scheme defined in the IEC/ISO 26262 is called Automotive Safety Integrity Level (ASIL) and is used to rate hazards of the system based on the consideration of malfunctions in particular driving situations.

To verify the applied system, architecture, software and hardware, testing is an essential tool. It is used in order to determine whether the objectives set are met. Testing is performed based on the requirements, where the most important upgrade introduced by the IEC/ISO 26262 is the introduction of the testing of functional safety objectives. For this purpose, the ISO/IEC does not only specify the requirements regarding the processes, but also prescribes the methods which shall be applied.

Among the methods required by IEC/ISO 26262, the Hardware-in-the-Loop (HiL) testing has become indispensable in the automotive domain [START_REF] Sivashankar | Design and Implementation of HIL Simulators for Powertrain Control System Software Development[END_REF]. It represents the method where the device under test is placed in an emulated environment. The basic principle of the method is presented in the Figure 1.

Tested component HIL simulator

Figure 1:HIL testing basic principle

With the use of HiL, systems testing can be performed under close-to-real conditions. It is also important to note, that the repeatability of tests is assured, and the same testing hardware Knowledge and skills requirements for the software design and testing of automotive applications J. Zabavnik, A. Riel, M. Marguč, M. Rodič I can be re-used for tests of various devices with only limited modifications. Since the device under test is available in real during testing, and no understanding of its internal design is therefore required, HiL testing is a form of black-box testing. Black-box testing can be performed for testing of units, integration testing, system testing and up to the certain level, even system validation. The testing processes as described above have been widely accepted in the automotive industry. They represent a must for any company wishing to be a part of the automotive development process or supply chain. It has been observed, however, that there is a serious lack of engineers familiar with these processes and methods, and consequently many companies, especially smaller ones, experience serious difficulties in adopting them. Definitely the knowledge in this field will be required from the future employees and therefore it is important that teaching it becomes a part of the education process, especially at higher education levels. This is especially important for the students of Electrical Engineering and Mechatronics.

The requirements for new topics and the extension of the existing ones to the education process will be presented in the following text. There have already been extensive activities in the field of adapting university curricula accordingly, in particular the Automotive Universities project [START_REF]Best Practices for Testing Safety Compliant Systems[END_REF], however those have to be extended by the testing methods, especially HiL-related ones.

This paper is organized as follows. After the introduction first the design process in the automotive industry is presented in more details. The V-model is described together with its activities. The testing in the automotive industry is described next. The main approaches, methods and processes are presented. Based on this explanation some knowledge and skills required for the HiL testing are proposed. To visualize the problem and propose a methodology of teaching, an example of testing is proposed. In the final section, the conclusion is given to summarize the most important topics required in the future education.

II. DESIGN PROCESSES IN AUTOMOTIVE INDUSTRY

Automotive SPICE and the IEC/ISO 26262 provide the frameworks for development processes of embedded automotive systems. In the following we will briefly describe the essentials of both these standards.

A. Automotive SPICE

Automotive SPICE is based on the SPICE -ISO/IEC15504 standard. It was developed for process quality management in the development of products suitable for automotive industry. In short, Automotive SPICE defines how a development process must be organized. The standard is basically a set of rules obtained from the good practices in automotive industry which were put together. Based on these practices, automotive suppliers are assessed per development project and receive the rating which determines the supplier's eligibility as a member of the supply chain for all the major global automotive OEMs, and which is usually also the basis for targeted improvement.

Figure 2 shows the V-development process that forms the backbone of both Automotive SPICE and the ISO 26262. An essential property is the bidirectional traceability between process steps on the left-hand side of the V, as well as between related process steps on the left and right-hand side of the V. While the left-hand side covers requirements engineering and design from system level to software (SW) unit level, the righthand side is dedicated to verification and validation in the form of tests from SW unit level up to system level. In the following we will briefly describe every process step on the left-hand side of the V. System Requirements -In the system design phase, engineers technically analyze the stakeholder requirements documents and derive company-internal system requirements from them. Each stakeholder requirement shall lead to at least one atomic (i.e., indivisible) system requirement.

The System Architectural Design maps the system requirements to system components (mechanics, electronic hardware or software) and their interfaces. At this stage, the architectural design is still a high-level black-box design specifying the main system components and their interconnections via clearly defined interfaces.

Software Requirements Analysis leads to requirements to software related parts derived from system requirements. The System Architectural design shall be used to better identify what is needed from the software.

The Software Architecture Design maps software requirements to software components and their interfaces. As in the system architectural design this is a high-level design focusing on software components and their interconnections via clearly defined interfaces.

The Software Detailed Design and Unit Construction comprises the phase for the specification and development of software units, the smallest indivisible design element. Units are the building blocks of components in the software architecture.

B. Functional Safety (ISO/IEC 26262)

Functional safety is becoming important in all major fields of industry. Its application together with the requirement to formalize and standardize the processes, methods and documentation resulted in creation of several standards. In the development of electric and electronic automation devices the basic standard covering functional safety is IEC/ISO 61508 [START_REF] Iec/Iso | Functional safety of electrical/electronic/programmable electronic safetyrelated systems[END_REF], intended to be applicable to all fields of industry. For the specific requirements of the different industries additional functional safety standards were developed. In automotive industry, the ISO 26262 is applied (standard for road vehicles), whereas in aerospace the DO-178/ED-12 (Software Considerations in Airborne Systems and Equipment Certification) [START_REF]12C: Software considerations in airborne systems and equipment certification[END_REF] and the DO-254/ED-80 (Design Assurance Guidance for Airborne Electronic Hardware) [START_REF]Design Assurance Guidance for Airborne Electronic Hardware[END_REF] are relevant. DO-178/ED-12 and DO-254/ED-80 are guidelines rather than standards, which are however treated as de-facto standards by the relevant authorities.

Functional safety does not mean that there are no faults in the final products and its operation. These will always exist, since it is a common knowledge that every system will fail at some point. The concept of functional safety means that the operation of the system will remain safe (transfer into safe state, not cause injuries or at least mitigate them) also in the presence of faults.

C. ASPICE and Functional Safety (ISO/IEC 26262)

ASPICE and ISO 26262 are complementary, the ASPICE process reference model can be seen in some parts of ISO 26262 chapters 4, 6, 7 and 8. Therefore, the ISO 26262 already incorporates the ASPICE process reference model (Figure 3). The ISO 26262 extends the Life Cycle Management of designing a product with safety relevant issues and adds methods to the processes.

III. TESTING IN AUTOMOTIVE INDUSTRY

Testing is crucial in automotive industry, as ISO 26262 and ASPICE require that every requirement from stakeholder requirements to unit construction must be verified and validated. The V-model described before contains multiple testing levels and indicates the relationships between development and testing.

Problems fulfilling standardized process definitions and strictly following the V-cycle are manifold. Effort estimations for each V-cycle activity are difficult to do, especially in new developments. In most cases the number of available experienced personnel is far less than needed. Some companies use the same resources for both development and testing, which compromises the availability of specialized competences as well as the independence of developers from testers. HiL systems provide a key element for facilitating the V-cycle approach. However, using them properly requires undergoing a substantial learning curve unless engineers have prior experience from education or related professional activities. The device under test is the system or component that needs to be tested. It can be a complete system or its part.

The HiL simulator consists of several components [START_REF]Best Practices for Testing Safety Compliant Systems[END_REF]. It is built around a system model, which represents the system's physical and/or logical behavior. Usually this is a software unit executed either on microprocessor hardware or FPGA and can be programmed with as few limitations as possible. The software for this unit can be model-based (using software like Matlab/Simulink as for HiL systems provided by dSPACE) or programmed in classical programming languages [START_REF] Misra C | Guidelines for the use of the C language in critical systems[END_REF] [START_REF] Barr | Embedded C Coding Standard[END_REF]. The model is connected to the DUT through the input and output software (usually present in the form of drivers), which control the input and output hardware. The output hardware can consist out of PWM outputs, digital to analog converters, digital outputs and busses. The input hardware contains of analog to digital converters, digital inputs, busses, comparators, capture units etc.

The HiL simulator can be applied using the dedicated selfdeveloped software and hardware, but because also the formal qualification is required for industrial use, commercial products from companies like dSPACE, National Instruments and Typhoon HiL are more common.

In our case, the testing was done with the help of a dSPACE HIL system that uses the model-based Simulink programming software. The device under test was a motor controller that was communicating with outside world only with bus communication. Testing is needed to verify if the DUT performs and behaves according to the stakeholder and system requirements specifications.

The test engineers shall be able to formalize every scenario of the DUT's normal operation. However, they also have to cover a wider range of scenarios than normal operation and limits of the system parameters in order to verify the system's correct and safe behavior outside the specification ranges.

IV. EXAMPLE OF DEVELOPMENT AND TESTING

Example of development and testing is represented as Application that a customer (stakeholder) is requesting a developer to develop and reproduce in the manner of ASPICE process management and part of ISO26262.

A. Application (Stakeholder requirements document)

Basic description of functionality and what shall be done is presented to developer in as few words as possible. If there are too many demands inside one requirement, it should be broken down to more requirements. An example of stakeholder requirement (ShReqxxx, where xxx represents the requirement ID number) is given as: ShReq001: Electronic device shall measure voltage and transmit information over communication link.

In this example it is obvious that a device shall be used that measures the voltage and transmits the information regarding its value through the communication link. The method of voltage measurement is not given, also the communication network or protocol are not defined, as this would be in a too early stage and would enforce the decision upon the developers in later stages. Namely the idea is to let them have as much freedom as possible.

B. System Requirement

SW related requirements are derived from Stakeholders requirements according to ISO 26262 and ASPICE. In application requirement we only have one requirement, but it already indicates the need for HW and SW requirement(s) but for the purpose of presentation only one SW requirement is extracted.

SysReq001: System shall monitor and transmit input voltage value over communication link with accuracy of 3% of input signal.

Requirement is minimalistic. It shall contain system limits, but it shall not dictate final design if it is not given by the stakeholder document.

At this point it is important to define an acceptance/validation criteria. It shall contain the crucial information regarding the functionality of the system. An example can be given as:

Measured voltage shall be communicated in the range of Umin to Umax with dynamic response of at least dTime.

The values of Umin and Umax represent the minimal and maximal operating voltage, dTime represents maximum refresh time of measured value. It is important to note that symbols can be used instead of actual (numerical) values. When using symbols to give requirement flexibility, variables can be grouped in a System parameters document.

C. Software Requirement

As first prerequisite for the frame of HW design the physical communication system or HW is chosen. Based on that decision the SW engineer can start developing SW requirements. For the purpose of presentation only one SW requirement is written here and in order to enable writing of SW requirements following HW decisions are needed. 1.) Vehicle bus standard Controller Area Network (CAN) shall be used for communication link. 2.) Translation between ADC pin and external connection shall be documented in Hardware Software Interface (HSI) document.

SwReq001: Voltage shall be represented over CAN bus communication in SI units and accuracy shall be within ±Uacc over full measured voltage operating range.

Requirement is again minimalistic with strict limitations and demands. As it is the case in system requirements, symbols can be used, and variables can be grouped in Software parameters document.

Software requirement Acceptance/Validation criteria has the form similar as it is the case for System requirements: Accuracy of voltage representation over CAN bus communication shall be met. Dynamical accuracy shall be considered according CAN to bus communication delay.

D. Software Architecture

Software architecture is defined on a higher level than Software Detailed Design and Unit Construction (later stage) and this shall be respected. It shall roughly define Software Requirement functionalities, interfaces and timing constrains. Software architecture shall group Software functionality that covers all SW requirements. SW Architecture is already described in Software requirements, but there it is not represented yet in the graphical form. Architecture is developed in a graphic way and represents a starting point of Software Detailed Design and Unit Construction stage. Simplified Software Architecture document (Figure 5) is used as a basis for SW integration strategy, but it can also be used for overview of the SW Qualification Test plan.

E. Software Detailed Design and Unit Construction

Software Detailed Design shall be chosen by analyzing Software requirements and Software Architectural design document. Great care shall be taken in data flow and what functions are time critical. Several Software Units can be derived from single Software requirement. Dashed line frame in (Figure 6) indicates which Software Units belongs to which SW requirement. It shows that at least one Software Unit can be extracted from one Software requirement.

SwUnit001: Voltage on the pin named VDC shall be acquired using the driver for the voltage measurement with ADC every 100 µs. Output value named U_DC shall represent voltage value in SI units.

SwUnit002: U_DC value shall be sent over CAN bus communication according CAN communication document.

F. Test Case

Tests are required in order to verify or validate the correct operation or design of the device. For that purpose, the Test Cases (TC) need to be developed. Format of Test Case shall be in a form that can be easily reproduced, repeated or copied. Test Case should be easy and simple to understand.

An example of test case is given below:

TC001: Verify voltage accuracy represented over CAN bus communication.

Using HiL system to communicate and acquire voltage information over CAN bus communication.

With changing DUT DC-link voltage over full-scale operating range.

To

verify if supply voltage on DUT DC-link and represented voltage over CAN bus communication meet voltage accuracy of at least ±0.5V. Dynamical accuracy shall be considered according to CAN bus communication delay.

Verify, Using, With, To form[13] is used in above example for description section to make it easy to understand. As process develop the consistent usage of this form makes it easier for test engineer to understand what test case developer was thinking. "Verify" is used as a first word in test statement, explaining what will be tested/verified. "Using" defines the applied tools and devices. Sentence that starts with "With" gives the procedure/method of test and sentence that starts with "To" gives information about the expected results of the success of the verification.

G. Test (steps)

To perform the test as required by Test Case, the following steps must be performed in the presented sequence:

1. Initialize DUTpower up DUT with normal operating voltage,

Pool power supply voltage and voltage represented over CAN bus communication, 3. Change power supply voltage to minimal operating voltage value and start incrementing voltage in steps of 0.1V every 1s and finish at maximum operating value, 4. Voltage from DC-link power supply and voltage represented over CAN bus communication shall not deviate for more than ±0.5V, 5. No error massages shall be triggered during full operating voltage change

This text also must be a part of the Test Case document.

V. KNOWLEDGE AND SKILLS REQUIRED FOR HIL TESTING More and more educational institutions teach how teamwork is important in everyday life but practice independent work. In fact, teamwork is something that research & development (R&D) industries are seeking to achieve. It is known that most experts are prone to bad communication and poor choice of words at crucial times (public appearances). Learning to communicate in the team and exchange the data clearly is essential. Ability to function in the team is highly appreciated for an engineer.

Basic knowledge about the operation of the system is required. Likewise, the use of semi-formal modelling methods, like UML (Universal Modelling Language) or similar, is very valuable. In the future it is expected that formal methods will be required.

System engineering is an important skill. The device to be designed needs to be understood as a system and details should be avoided in early stages, because they can lead to limiting the design possibilities. The design needs to remain open to different solutions and at the earlier stages the details don't have to be solved. The urge for and engineer to solve the problems too early in many cases leads to the problems in performing the processes required by ISO 26262 and ASPICE. Actually, those two guidelines/standards incorporate hierarchy as the main methodology in the design of the system. The roles of the involved personnel must be clearly divided and clarified. This gives all the their tasks and enables them to act individually, interacting based on clear instructions. Thus, each participant can check the predecessor in the process and give a clear output to the developers of the following stages, not limiting them in their work [START_REF] Kreiner | Automotive Knowledge Alliance AQUA -Integrating Automotive SPICE, Six Sigma, and Functional Safety[END_REF].

Knowledge about hardware design and its limitations is required for the development of hardware requirements and testing. Software developers, more precisely the software engineers developing hardware related software (drivers, etc.), also require knowledge about the processor and its peripherals. Especially it is important to note that a hardware software interface (HSI) is required and represents an essential part of design.

A. Application

Application information is typically provided by customer especially if the device to be designed is a component of the system. In other case (if complete system is designed) the general knowledge about system operation is required.

B. System Requirements

For the purpose of system requirements definition, the knowledge regarding system operation needs to be more technical. Knowledge from the fields of system engineering and quality management need to be added to the general technical knowledge but detailed knowledge regarding applied hardware and software is not required.

C. System Architecture

To map the system requirements to the system components knowledge from the fields of system, hardware, software and mechanical engineering need to be used to develop a good system platform that is partitioned into many sub components that can be developed separately. Basic engineering skills are required, but the main issue is to be able to understand interaction among the subcomponents. This could be a typical position for a mechatronics engineer.

D. Software Requirements

When writing software requirements expert knowledge is needed about software tools to be applied. Basic knowledge about hardware items is needed in order to understand how the hardware is constructed and interfaced with outside world. Functionality of hardware shall be well known to the software engineer that writes Software Requirements.

E. Software Architecture

A higher level of approach is needed in Software Architecture development. To form architecture from Software requirements, knowledge of system and software engineering need to be known. This would be typically a task performed by a software engineer.

F. Software Detailed Design and Unit Construction

To develop a detailed design from Software architecture and Software requirements expert knowledge is needed about software tools. To develop drivers higher understanding of the hardware setup is needed. In case of the more hardwareoriented units this task could be performed by an embedded software engineer, for more general units a knowledge of programming should be sufficient.

G. Test Case development

To write test cases knowledge about quality management and consistency is a key for success. Before starting to write the Test Case, System, Software, Hardware and Architecture related documents for whole System under test are needed. The possibilities of the testing equipment and software must be well known to the test case developer. Hardware and Software Interfaces shall be well defined. Schematic diagram of DUTs Hardware shall be available to tester and all DUT interfaces to the outside world need to be described in high detail. In order to be able to perform these tasks, test case developer should understand the operation of the system or unit for which the test case shall be written. The internal structure and software should not be known to the test case developer, because the test cases need to be developed from requirements and not from the design. Therefore, the test case developer should possess the knowledge from the field of system engineering and be able to utilize the available testing tools to recreate the input/output behavior. The test engineer should be able to use the testing tools, which requires some programming skills (in the programming tool used in testing devices) and ability to connect the DUT to the testing device. The role of test case developer should be given to an experienced engineer whereas the role of test engineer can be assumed by less experienced personnel, giving them a good possibility to learn about the processes and devices.

H. HiL Test programing

Expert knowledge is needed about software tools used by the testing equipment especially if advanced HiL system is used for testing software and system requirements. Expert knowledge is needed about limitations of testing hardware equipment. Test programmer must be able to predict what test cases can be executed and what test cannot be tested with available equipment. Model-based approaches and tools are essential for the effective use of the modern HiL equipment.

To be able to develop more complex HiL tests the knowledge about the system modelling and simulation is required. The HiL test developer needs to be able to develop the simulated environment for the DUT, which typically consists of the physical and logical model of the DUT environment, together with the interfaces. For that purpose, at least basic knowledge in the field of electrical engineering is required. Control engineering is also essential, because the models in HiL need to be controlled in order to emulate the behavior of an actual environment. Modelling and identification are highly useful tools.

CONCLUSION

Based on the findings described in the previous text and experiences gathered in the projects dealing with the development of the safety critical applications in the automotive industry it can be determined that besides the required knowledge in the specific field of study, which the student should acquire in the frame of education process, should also contain the knowledge of software engineering, especially methods and processes of design and testing. More specific, this knowledge includes some essential contents from the requirements engineering, design of experiments, software engineering and system engineering. The typical skills required include the use of Application Lifecycle Management (ALM) tools (like Polarion, Doors, etc.) and use of version-control systems (Git, SVN). It is not very important which tools are used, the main goal is to make students familiar with the fact that they exist and understand their basic meaning and function. Model-based approaches (like Universal Modelling Language, UML) are also important, as they are gaining popularity in the automotive industry, significantly reducing the time-to-product and reducing the possibilities of mistakes in the development process, not only due to faults in programming, but also in failures to understand the specifications and requirements. Tools like Matlab/Simulink are becoming classical choice in the development process. In future even advanced approaches like formal methods will become important.

The knowledge required for writing this paper was obtained in the frame of projects Automotive Quality Universities (Erasmus+ Strategic Partnership project 2015-1-CZ01-KA203-

Figure 2 :

 2 Figure 2:V-model design (ASPICE) Stakeholder Requirements are (internal or external) customer requirements describing what the system shall do and how the system shall perform. The stakeholder requirements documents are analyzed with respect to the feasibility, risk and priority of requirements.System Requirements -In the system design phase, engineers technically analyze the stakeholder requirements documents and derive company-internal system requirements from them. Each stakeholder requirement shall lead to at least one atomic (i.e., indivisible) system requirement.The System Architectural Design maps the system requirements to system components (mechanics, electronic hardware or software) and their interfaces. At this stage, the architectural design is still a high-level black-box design specifying the main system components and their interconnections via clearly defined interfaces.Software Requirements Analysis leads to requirements to software related parts derived from system requirements. The System Architectural design shall be used to better identify what is needed from the software.The Software Architecture Design maps software requirements to software components and their interfaces. As in the system architectural design this is a high-level design focusing on software components and their interconnections via clearly defined interfaces.The Software Detailed Design and Unit Construction comprises the phase for the specification and development of software units, the smallest indivisible design element. Units are the building blocks of components in the software architecture.

Figure 3 :

 3 Figure 3:ISO26262 and ASPICE implementation

Figure 4 :

 4 Figure 4:HiL system

Figure 5 :

 5 Figure 5: Simplified Software Architecture document

Figure 6 :

 6 Figure 6: Simplified Software Architecture with SW Units development

  Such exhaustive behavior-based black-box testing is efficient, however time consuming. Test engineer and Test Case developer shall not be the same person. Test Case developer has a wider view of the system and knows system limitations. Test engineer only knows what Test Case developer wants to test and how it should be tested. Test engineer writes code on testing equipment according to Test Case developer requirements and verifies Software/Unit requirements or validates System requirements in development. If verification/validation fails (test gives a negative result) the requirement and Test Case requirement shall be analyzed, incorrect item shall be corrected or modified to meet the desired criteria. Test Case developer and test engineer are in constant communication that involve test cases and writing tests on testing equipment -HiL system.

013986 AutoUniverse), HYPSTAIR (Development and Validation of Hybrid Propulsion System Components and Subsystems for Electrical Aircraft, 605305 FP7-AAT-2013-RTD-1) and EVA4green (Ecological Safe Vehicle for green mobility, Republic of Slovenia, Ministrstry for Education, Science and Sport, European Regional Development Fund), and Motzart (Efficient development of electric motor by an expert system and new technologies, Republic of Slovenia, Ministrstry for Education, Science and Sport, European Regional Development Fund).


J Bessousa

R Benasseur

Chtourou

L Samet

K March

O Stephan

N Brun

F Hosni

F Bessousa

J Benasseur

R Chtourou

Radiocatalytic Cu-incorporated TiO 2 nano-particles for the degradation of organic species under gamma irradiation

Keywords: Copper-doped titania, Structure, Optical and radiocatalytic relationship, ELNES

Radiocatalytic Cu-incorporated TiO 2 nano-particles for

the degradation of organic species under gamma irradiation

Introduction

Water is a scarce resource and wastewater is a major environmental problem. Advanced oxidation processes, like photocatalysis and radiocatalysis, are widely studied for the treatment of persistent organic compounds. Among several semiconductors used as heterogeneous photocatalysts and radiocatalysts, titanium dioxide (TiO 2 ) is the most widely used and has been intensively studied thanks to its many promising properties . It presents a high chemical and mechanical stability and is amongst the synthetic nanoparticles that present especially interesting properties in the fields of nanotechnology, sustainable development and ecology. Titanium dioxide is a semiconductor that can only be activated with light of energy higher than about 3.2 eV or 3.0 eV for the anatase and rutile structures respectively. When TiO 2 is subjected to illumination, the electron-hole pairs formed can be trapped by surface hydroxyl groups. In the presence of oxygen and water, the trapped carriers form free radicals that are strong oxidizing species and are likely to mineralize organic pollutants. In this respect, the efficiency of TiO 2 catalysis needs to be further improved. Many strategies, such as doping [START_REF] Pan | Synthesis of nitrogen doped faceted titanium dioxide in pure brookite phase with enhanced visible light photoactivity[END_REF][START_REF] Samet | Heat treatment effect on the physical properties of cobalt doped TiO 2 sol-gel materials[END_REF][START_REF] Xia | Origin of significant visible-light absorption properties of Mn-doped TiO 2 thin films[END_REF][START_REF] Hudson | TiO 2 -Cu photocatalysts: a study on the long-and short-range chemical environment of the dopant[END_REF][START_REF] Aguilar | A route for the synthesis of Cu-doped TiO 2 nanoparticles with a very low band gap[END_REF][START_REF] Ganesh | Preparation and characterization of Cu-doped TiO 2 materials for electrochemical, and photocatalytic applications[END_REF][START_REF] Choudhury | Defect generation, d-d transition, and band gap reduction in Cu-doped TiO 2 nanoparticles[END_REF][START_REF] Karunakaran | Cu-doped TiO 2 nanoparticles for photocatalytic disinfection of bacteria under visible light[END_REF][START_REF] Zuas | Synthesis of nanostructured Copper-doped titania and its properties[END_REF][START_REF] Hu | Origin of the improved photocatalytic activity of Cu incorporated TiO 2 for hydrogen generation from water[END_REF][START_REF] Navas | Experimental and theoretical study of the electronic properties of Cu-doped anatase TiO 2[END_REF][START_REF] Wu | Photocatalytic performance of Cu-doped TiO 2 nanofibers treated by the hydrothermal synthesis and air-thermal treatment[END_REF][START_REF] Yin | Enhanced photocatalytic reduction of chromium (VI) by Cu-doped TiO 2 under UV-A irradiation[END_REF][START_REF] Vargas | Effects of metal doping (Cu, Ag, Eu) on the electronic and optical behavior of nanostructured TiO 2[END_REF], co-doping [START_REF] Huang | Enhanced photocatalytic performance of anatase TiO 2 substitutionally co-doped with La and N[END_REF][START_REF] Dashora | Formation of an intermediateband in the energy gap of TiO 2 by Cu-N-codoping: first principles study and experimental evidence[END_REF], surface treatment [START_REF] Fang | Modifications on reduced titanium dioxide photocatalysts: A review[END_REF], coupling with noble metal nanoparticles [START_REF] Méndez-Medrano | Surface modification of TiO 2 with Ag nanoparticles and CuO nanoclusters for Application in Photocatalysis[END_REF] or other semiconductors [START_REF] Wang | Ultrasound-assisted synthesis and solar-light-driven photoelectrocatalytic activity of CdS sensitized TiO 2 nanotube array photocatalysts[END_REF][START_REF] Yang | Enhanced photoelectrochemical properties and water splitting activity of self-ordered MoO 3 -TiO 2 nanotubes[END_REF], synthesis of nanomaterials with different morphologies [START_REF] Wang | Engineering the Surface/Interface Structures of Titanium Dioxide Micro and Nano Architectures towards Environmental and Electrochemical Applications[END_REF], have been tried over the last few decades, in order to reduce the electron-hole recombination rate and improve the photocatalytic efficiency of TiO 2 .

Titanium dioxide has a high oxidation power so is easily doped with active ions. It has been shown that doping with transition metal cations is an efficient way of promoting the photocatalytic activity [START_REF] Wu | Photocatalytic performance of Cu-doped TiO 2 nanofibers treated by the hydrothermal synthesis and air-thermal treatment[END_REF][START_REF] Yin | Enhanced photocatalytic reduction of chromium (VI) by Cu-doped TiO 2 under UV-A irradiation[END_REF][START_REF] Vargas | Effects of metal doping (Cu, Ag, Eu) on the electronic and optical behavior of nanostructured TiO 2[END_REF]. Such doping can extend the light absorption range from the UV down to the visible by inducing additional energy levels within the band gap, leading to the improvement of the TiO 2 photo-response. Several investigations on doped TiO 2 have succeeded in providing some indication about the substitutional or interstitial dopant location and its impact on the physical and electronic structure of TiO 2 [START_REF] Long | New Insights into the band gap narrowing of (N, P)codoped TiO 2 from hybrid density functional theory calculations[END_REF][START_REF] Long | Electronic structure of cation-codoped TiO 2 for visible-light photocatalysts applications from hybrid density functional theory calculations[END_REF]. Doping with copper appears to be a particularly promising approach for photocatalytic applications [START_REF] Vargas | Effects of metal doping (Cu, Ag, Eu) on the electronic and optical behavior of nanostructured TiO 2[END_REF]. For example, it has been found that the incorporation of Cu 2+ ions into a TiO 2 matrix can substitute for Ti 4+ and/or segregate to the surface of TiO 2 crystallites in the form of CuO, Cu 2 O or metallic Cu [START_REF] Hu | Origin of the improved photocatalytic activity of Cu incorporated TiO 2 for hydrogen generation from water[END_REF][START_REF] Logar | Cu and CuO/Titanate nanobelt based network assemblies for enhanced visible light photocatalysis[END_REF][START_REF] Li | Enhanced visiblelight activities for PEC water reduction of CuO nanoplates by coupling with anatase TiO 2 and mechanism[END_REF][START_REF] Huang | Highly aligned Cu 2 O/CuO/TiO 2 core/shell nanowire arrays as photocathodes for water photoelectrolysis[END_REF][START_REF] Hu | Effective water splitting using CuO x /TiO 2 composite films: Role of Cu species and content in hydrogen generation[END_REF] and improve significantly the photocatalytic activity.

Theoretical investigations show that Cu doping can effectively narrow the band gap by creating impurity bands [START_REF] Navas | Experimental and theoretical study of the electronic properties of Cu-doped anatase TiO 2[END_REF][START_REF] Dashora | Formation of an intermediateband in the energy gap of TiO 2 by Cu-N-codoping: first principles study and experimental evidence[END_REF], and thus improve the absorption of visible light in photocatalytic reactions. Alternatively, Wu and al. [START_REF] Wu | Enhanced TiO 2 photocatalysis by Cu in hydrogen production from aqueous methanol solution[END_REF] reported that the dissolution of Cu ions in the TiO 2 lattice leads to a reduction in photocatalytic activity. It is now admitted that a compromise must be achieved between the band gap value and charge transfer at the surface of TiO 2 catalysts. However, the nature of the active copper species (CuO, Cu 2 O, metallic Cu, or isolated Cu 2+ ions incorporated into the TiO 2 lattice) and the factors that govern the Cudoped TiO 2 photo-reactivity are still under debate. Also, there is a need to develop treatment methods that are more effective in eliminating dyes from waste streams. Ionizing radiation is known to degrade numerous pollutants. As in the case of photocatalysis, some publications reported that the addition of nanoparticles such as TiO 2 and AlO 2 in solution enhances the degradation of pollutants and accelerates the reaction induced by ionizing radiations [START_REF] Hoertz | Photocatalytic and radiocatalytic nanomaterials for the degradation of organic species[END_REF][START_REF] Cecal | Use of some oxides in radiolytical decomposition of water[END_REF][START_REF] Follut | Radiolysis of aquous 4-nitrophenol solution with Al 2 O 3 or TiO 2 nanoprticules[END_REF][START_REF] Gonzalez-Juarez | Gamma radiation induced catalytic degradation of 4-chlorophenol using SiO 2 , TiO 2 and Al 2 O 3[END_REF].

However, the effect of doping on radiocatalysis has not been reported. This work explores the local chemical environment of Cu dopants in the core and the near-surface regions of TiO 2 nanoparticles prepared by a simple sol-gel route and annealed at 400°C or 600°C. The characterization studies were conducted using various investigation techniques, including ultraviolet-visible spectroscopy (UV-vis), X-ray diffraction (XRD), transmission electron microscopy (TEM), electron energy-loss spectroscopy (EELS) and electron paramagnetic resonance (EPR) spectroscopy. Methylene blue (MB) was selected as a model compound for evaluating the potential of doped and undoped TiO 2 to remove dyes from wastewater. By investigating the effect of the copper doping level and heat treatment on the structural and optical properties of a series of TiO 2 catalysts and subsequently on their MB degradation efficiency under gamma illumination, we show that the performance of such systems strikingly depends on the nature and distribution of the Cu species incorporated.

Experimental details

Pure and Cu 2+ -doped TiO 2 powders (Cu-TiO 2 ) were prepared via a sol-gel technique.

Details are presented in our previous work [START_REF] Samet | Heat treatment effect on the physical properties of cobalt doped TiO 2 sol-gel materials[END_REF]. This method produces powders of high chemical purity. In this case the measured atomic ratio of Cu to Ti is between 0 and 12 at.%.

Undoped TiO 2 samples were prepared as a controlin the same way. The obtained millimetric solids were ground in an agate mortar to get fine nanopowders. The nanopowders were systematically characterized after heat treatment at 400°C or 600°C in order to optimize their synthesis. In this regard, XRD, TEM and EELS are suitable characterization techniques as they allow one to identify the structures of crystallized phases and to observe possible structural changes after doping and heat treatment.

The X-ray diffractometer used was a Siemens D5005 in Bragg-Brentano geometry, furnishing CuKα radiation (λ = 1.5406 Å). The diffractograms were recorded at room temperature and the X-ray data were collected from 20° to 80° (2Ɵ), with a measurement time of 10 seconds and a step size of 0.02° (2Ɵ). TEM (TOPCON 002B at 100 keV) was used to study the morphology and the crystallographic structure of the powders, and their possible changes after heat treatment and doping. Nanopowders were dispersed in ethanol before drop casting a very dilute suspension on a carbon-film copper grid.

The electronic structure of Cu-TiO 2 powders was investigated by means of EELS performed in two dedicated scanning transmission electron microscopes (STEM), a probe C scorrected Nion Ultra-STEM and a STEM-VG HB501, both equipped with a field emission source operated at 100 keV and coupled to a Gatan EELS spectrometer optically coupled to a high-sensitivity (either nitrogen-cooled or electron amplifying) CCD camera. EELS does not only perform elemental analysis of Cu inserted into the TiO 2 structure after doping, it also provides information related to the local electronic structure of the materials (valence state of each element for example) with a high spatial resolution (down to the atomic scale).

For EPR analysis, the sample was inserted into a standard rectangular cavity in an EMX 71 X-band EPR spectrometer with 100 kHz magnetic field modulator operating at room temperature.

The optical measurements were performed by diffuse reflection over the 250 to 800 nm wavelength range. The reflection spectrum was measured using a UV-vis-near infrared spectrometer (Lambda-950) equipped with an integrating sphere.

Finally, the radiocatalytic activity of the synthesized samples was evaluated through the degradation of methylene blue (MB) in aqueous solution under gamma-ray irradiation at doses varying from 10 to 1500 Gy using the Tunisian Cobalt-60 radiation facility at a 80Gy/min dose rate [START_REF] Hosni | Effect of gamma-irradiation on the Colorimetric properties of epoxy-resin films: potential use in dosimetric application[END_REF][START_REF] Hosni | A study of the fractionation dose on the radiation response of Harwell Red-Perspex PMMA dosimeter[END_REF]. This dose rate was determined by alanine dosimeters irradiated by the facility and returned to Aerial for dose rate assessment.

In a typical experiment, a 10 mg/l aqueous solution of MB was mixed with the catalyst in a 100 ml glass dish to obtain a solution suspension of 2 g/l. The mixture solution was initially stirred for about 2h in the dark, to reach the adsorption equilibrium between MB and the powder catalyst surface. The MB concentration in the solution after the radiocatalysis reaction was estimated by using a UV-vis spectrophotometer (IC6400) and measuring the intensity maximum of the MB absorption peak at about 620 nm.

Results and discussion

Microstructural analysis

XRD was used as a first characterization step to identify the structures of the crystalline phases present in the powders and to monitor any possible structural changes upon heat treatment and doping. The presence of a possible second phase was also looked for. Fig. 1 shows XRD patterns of the Cu-TiO 2 samples annealed at 600°C. Importantly, we note that powders with 0-3 at.% Cu concentration are a mixture of anatase and rutile phases without any detected additional phase. The fraction of the rutile phase increases with doping levels, from 20% for undoped TiO 2 to 44% for 3at.% Cu-TiO 2 . Therefore, it is clear that within this doping level range, the Cu doping promotes the anatase-to-rutile phase transition. This may be caused by the substitution of Cu 2+ ions for Ti 4+ ions within the anatase structural framework. Indeed, it is a well-known fact that the TiO 2 anatase structure is metastable with respect to rutile. In both rutile and anatase, Ti is six-fold coordinated, but the number of shared octahedral edges increases from two in rutile to four in anatase [START_REF] Arbiol | Effects of Nb doping on the TiO 2 anatase-to-rutile phase transition[END_REF]. The tolerance of the TiO 2 structure to Cu doping can be qualitatively related to the local environment of Ti and the introduced strain. When Cu 2+ ions enter the TiO 2 lattice, substituting for Ti 4+ ions, the ion charges should be compensated by an increase in oxygen vacancies, that act as nucleation sites for the anatase-to-rutile transformation. At the annealing temperature of 600°C, these defects promote the transition from a metastable anatase phase to a more stable phase, more tolerant of defects, i.e. rutile. At a higher Cu doping level (6 at.%) we note the almost complete disappearance of the rutile phase and the appearance of an additional phase identified as CuO. The amount of this phase increases with Cu concentration. Once part of the Cu 2+ ions is segregated outside the TiO 2 structure, the number of oxygen vacancies decreases, leading to the inhibition of the anatase-to-rutile transformation. Other notable changes were observed: the estimated "a (= b)" and "c" lattice parameters for undoped TiO 2 powder are respectively 3.781 Å and 9.478 Å in the anatase structure and 4.592 Å and 2.957 Å in rutile.

With the increase in Cu 2+ concentration from 0 to 3 at.%, the value of these lattice parameters hardly changes. Upon further increase of the Cu 2+ concentration, only anatase crystallites are observed (as already mentioned), accompanied by a shift of the (101) and (200) reflections toward larger angles. According to [START_REF] Aguilar | A route for the synthesis of Cu-doped TiO 2 nanoparticles with a very low band gap[END_REF], this shift may reflect the interstitial incorporation of Cu 2+ ions within the TiO 2 lattice. Such incorporation is promoted by an excess of copper.

Interstitial doping generates stress in the anatase structure. Moreover, it does not provide additional oxygen vacancies and therefore inhibits the transition to the rutile phase. XRD measurements on pure TiO 2 and Cu-TiO 2 sol-gel powders annealed at 400°C (not shown here) exhibit only diffraction peaks corresponding to the anatase phase without any secondary phase nor any notable change in lattice parameter, within the detection limits of XRD measurements.

The average crystallite size as a function of the Cu levels for samples annealed at 400°C and 600°C (Fig. 2) has been evaluated from the full width at half maximum (FWHM) of the most intense peaks (the (101) and (110) peaks for anatase and rutile respectively) using the Scherrer method [START_REF] Nuffield | X-ray diffraction methods[END_REF]. As previously reported [START_REF] Samet | Heat treatment effect on the physical properties of cobalt doped TiO 2 sol-gel materials[END_REF], the crystallite size increases with annealing temperature. Moreover, it appears that Cu incorporation has a different effect on crystal growth for each temperature. For samples annealed at 400°C the crystallites are smaller than those in the undoped material, by 25% for the 3at.% Cu-TiO 2 sample, with no detectable change of lattice parameters. The crystallite size evolution according to the doping level must be related to the insertion of doping atoms in the TiO 2 structure. The substitution of metal ions of lower valence values for Ti 4+ increases the oxygen deficiency to maintain charge neutrality.

The reduction in crystallite size could be due to the formation of oxygen vacancies. In fact, defects can influence considerably the nucleation, growth, and structure of crystals [START_REF] De | Colloidal Hard-Sphere Crystal Growth Frustrated by Large Spherical Impurities[END_REF][START_REF] Kubota | Effect of Impurities on the Growth Kinetics of Crystals[END_REF].

For samples annealed at 600°C, the opposite is observed: for a doping level not exceeding 3 at.%, doping leads to an increase in the average crystallite size (in both rutile and anatase phases). This result will be further discussed following a nano-structural study (see section 3.2). For both annealing temperatures and at a high Cu doping level (> 6 at.%) a slight decrease in the size of the crystallites (present only in the anatase phase) is observed. Such a relative reduction in crystallite size in the presence of segregation of copper cations and/or a copper oxide additional phase at the grain boundaries has already been observed for Co-doped TiO 2 [START_REF] Samet | Heat treatment effect on the physical properties of cobalt doped TiO 2 sol-gel materials[END_REF]. This effect seems to increase with temperature as the relative difference between the crystallite size of doped and undoped samples increases with temperature. Above 6 at.% the Cu 2+ doping level does not show any significant effect on grain growth. Thus, we assume that at these high doping levels, the solubility limit of copper in the TiO 2 structure is exceeded and Cu 2+ ions are segregated to the grain surfaces. We will come back to this statement in the nanoscale analysis section.

Nanoscale analysis

As the energy-loss near-edge fine structures (ELNES) are sensitive to the oxidation state and to the bonding and local environment, it is not only possible to determine the chemical composition of the doped TiO 2 samples, but also to sort out spectral signatures of the copper oxidation state and to locally identify the TiO 2 crystalline phase (anatase vs rutile) in the studied samples. Chemical analyses were performed by EELS in the spectrum-imaging mode, in which the focused beam is scanned over a region of interest and a whole spectrum is acquired at each position of the scan [START_REF] Jeanguillaume | Spectrum-image: the next step in EELS digital acquisition and processing[END_REF][START_REF] Gloter | Atomically resolved mapping of EELS fine structures[END_REF]. Such a spectrum-image contains typically 10000 spectra. These spectra can be processed individually, usually by removing the background and summing the intensity corresponding to a characteristic edge and thus building elemental maps. However, as this data set contains a lot of redundant information, it is often very useful to process it as a whole using multivariate statistical techniques [START_REF] De La Peña | Mapping titanium and tin oxide phases using EELS: An application of independent component analysis[END_REF]. We used principal component analysis (PCA) as a filtering method for separating meaningful signal components from noise. In a few favorable cases, we used spectral unmixing methods. These methods assume that the whole spectrum-image can be described by a few reference spectra, each individual spectrum of the original data cube being a linear combination of these basic components [START_REF] Dobigeon | Blind Linear unmixing of EELS spectrum-images[END_REF][START_REF] Dobigeon | Linear and non linearunmixing in hyperspectral imaging, in Resolving spectral mixtures -With application from ultrafast time-resolved spectroscopy to super resolution imaging[END_REF]. We used Vertex Component Analysis [START_REF] Nascimento | Vertex component analysis: A fast algorithm to unmix hyperspectral data[END_REF] to perform this unmixing step.

TEM and high angle annular dark field (HAADF) STEM images of the 3at.% Cu-TiO 2 sample annealed at 400°C are displayed in Fig. 3(a The inspection of the diffraction pattern confirms the anatase structure of the prepared samples with no detectable trace of the rutile phase. The Ti, O and Cu elemental maps extracted from the EELS spectra acquired in the spectrum-imaging mode are shown in Fig. 3(c,d,e) respectively. Fig. 3 also shows the presence of metallic copper nanoparticles of size less than 2 nm (indicated by an arrow in Fig. 3(b)) which are not detected by XRD. The corresponding EELS Cu L 2,3 spectrum is shown and labeled (1) in Fig. 3(f). The Cu-L 2,3 spectrum from the square region, labeled (2) in Fig. 3(e), displays a lower signal-to-noise ratio and characteristic features of Cu 2+ (Fig. 3(f)). In fact, the Cu-L 2,3 edge shape, which reflects electronic transitions from 2p to 3d levels, is very sensitive to the oxidation state of the copper ion. The absorption edge of metallic copper Cu 0 (d10s1) exhibits a typical step like shape, while for Cu 2+ (d9s0) most of the intensity in the near-edge region is concentrated in two narrow peaks, called L 3 and L 2 white lines. The Cu 2+ L edge threshold is also shifted to lower energy by around 2 eV in comparison to the metallic state. It should be noted that the Cu 1+ (d10s0) ELNES (not shown here) exhibits an increase in L 3 white line intensity in comparison to metallic copper, but no chemical shift [START_REF] Backahaus-Ricoult | Interfacial chemistry in internally oxidized (Cu, Mg)-alloys[END_REF].

Fig. 4(a) shows a higher magnification TEM image together with its corresponding selected area electron diffraction (SAED) pattern of 6at.% Cu-TiO 2 annealed at 400°C. The SAED pattern confirms the anatase structure of these polycrystalline powders [START_REF] Samet | Heat treatment effect on the physical properties of cobalt doped TiO 2 sol-gel materials[END_REF]. We also note the presence of grain boundaries. Elemental maps as extracted from EELS spectrum-images of the region of interest associated with the HAADF image displayed in Fig. 4(b), are shown in Fig. 4(c,d) for the energy regions including the Ti-L 2,3 and Cu-L 2,3 edges respectively. These elemental maps show the presence of copper-rich nanoparticles (1-2 nm in diameter) at grain surfaces and a copper segregation at the grain boundaries. These secondary phases were not detected by XRD. In Fig. 4(e) we have manually applied two thresholds to the Cu map in order to separate the particles (visible in red) from the grain boundaries (in green). The spectra summed over the corresponding pixels are shown in Fig. 4(f). It appears that the grain boundaries contain mostly oxidized Cu 2+ as indicated by the strong intensity in the white lines. The EELS fine structures corresponding to the particles exhibit a muchreduced white line intensity due to a strong contribution of a metallic Cu signal. From doping levels of about 3 at.%, the TiO 2 structure begins to be saturated with Cu. The electrons trapped in oxygen vacancies (whose presence assures electron neutrality of the sample upon Ti 4+ substitution) may be transferred to the Cu 2+ ions segregated on the crystallite surface, resulting in metallic Cu formation.

Results from a nanoscale analysis of Cu-TiO 2 samples annealed at 600°C are summarized in Fig. 5. For samples with a Cu concentration of 3 at.%, the typical crystallite morphology is shown in the HAADF images of Fig. 5(a) and Fig. 5(d). Large TiO 2 particles with faceted shapes are observed. The mean particle size is found to be around 55 nm (in agreement with XRD analysis). The Cu map (Cu-L 2,3 edge intensity map) of Fig. 5(b) clearly shows a copper segregation at the TiO 2 crystallite surface and grain boundaries. As in the case of the 400°C annealed sample, we have applied a thresholding to the Cu map in order to separate the signal coming from the larger metallic copper particles (in red) from that of a weaker segregation at surfaces and grain boundaries (in green). Cu-L 2.3 edges thus obtained are presented in Fig. 5(c) and show slightly different oxidation states. In a few cases a strong copper segregation is observed at grain boundaries allowing for the signal extraction by statistical analysis of this secondary phase. Maps obtained after spectral unmixing of sets of spectra encompassing the Ti-L 2,3 to O-K energy region are presented in Fig. 5(e). These maps show the presence of both anatase and rutile phases (blue and green pixels in the image of Fig. 5(e)). A third component is extracted, which is characteristic of the grain boundaries (red pixels). The EELS components corresponding to these maps are presented in Fig. 5(f ). An elemental quantification of this VCA EELS component gives an estimated doping level of about 3 at.% Cu. This is much more than the measured Cu content within the grains, which is well below 1 at.%. The remaining copper is probably present in an undetected additional phase.

The Ti-L 2.3 ELNES corresponding to 3at.% Cu-TiO 2 samples annealed at 400°C and 600°C are presented in Fig. 6(a). All are composed of two main contributions, namely the L 3 and L 2 edges, separated by the 2p core-hole spin-orbit coupling. The L 3 and L 2 edges are then both subdivided into two peaks by the strong crystal field splitting created by the surrounding oxygen atoms [START_REF] Brydson | Electron energy loss and x-ray absorption spectroscopy of rutile and anatase: a test of structural sensitivity[END_REF][START_REF] Corombette | Ti2p X-ray absorption in tianium dioxides (TiO 2 ): the influence of the cation site environment[END_REF][START_REF] Henderson | A Ti-L edge X Ray absorption study of Tisilicate glasses[END_REF][START_REF] Bertoni | Quantification of crystallineand amorphous content in porous TiO 2 samples from electron energy loss spectroscopy[END_REF]. All the spectra have a similar form and are consistent with the TiO 2 anatase structure for samples annealed at 400°C and with both anatase and rutile structures for samples annealed at 600°C. A splitting reduction of the Ti-L 3 -e g edge induced by Cu 2+ incorporation is observed for both anatase and rutile structures. Fig. 6(b) shows EELS O-K edges for undoped and 3at.% Cu-TiO 2 annealed at 400°C and 600°C. The spectra match well with the anatase and rutile TiO 2 structures. They can be decomposed in two main spectroscopic regions labeled as (1) and ( 2). The pre-edge (1) results from hybridization of the oxygen 2p with the titanium 3d orbitals. A stronger crystal field splitting is observed for rutile than anatase (associated with a 2.75 eV and 2.5 eV t 2g -e g splitting respectively) [START_REF] Gloter | Electronic structure of titaniabased nanotubes investigated by EELS spectroscopy[END_REF]. [START_REF] Pichat | Fundamentals of TiO 2 Photocatalysis. Consequences for Some Environmental Applications[END_REF] corresponds to electron transitions from 1s to 2p oxygen states hybridized with titanium 4s and 4p states that are sensitive to long-range connectivity changes between oxygen octahedra in the TiO 2 structure. For all doped samples, the spectra of Fig. 6 show that the Ti-L 2.3 and O-K ELNES modifications due to the structural distortion induced by Cu 2+ substitution are more marked at the grain boundaries, but are also present within the bulk of the TiO 2 crystallites.

Importantly, according to the recorded ELNES, we can rule out the presence of significant quantities of Ti 3+ in all studied samples. We now come back to the previous discussion on the particle size. For samples annealed at 400°C, the average crystallite size decreases by 25% when the Cu content increases from 0 to 3 at.%. The opposite is seen for samples annealed at 600°C: a strong increase of about 50% and 30% of the crystallite size, respectively in anatase and rutile phases is observed. Nanoscale investigation reveals that the TiO 2 structure annealed at 600°C has a lower Cu saturation limit. Therefore, the true Cu doping concentration in the TiO 2 lattice annealed at 600 °C is lower than that in samples annealed at 400°C. Any size reduction due to growth inhibition in the presence of oxygen vacancies created by Cu 2+ substitution is thus expected to be much more limited. However, oxygen vacancies and their effect on crystal growth cannot explain the crystallite size evolution discrepancy between both annealing temperatures.

In fact, the growth of such small nanoparticles is probably more dominated by surface processes than bulk ones. We speculate that the growth mechanism during annealing is affected by the presence of copper at the surface. For example, it has been suggested by Wu et al. for Fe-doped TiO 2 samples that the presence of Fe 3+ ions at the surface decreases the free energy and thereby reduces the thermodynamic driving energy of particle growth [START_REF] Wu | Creating Oxygen Vacancies as a Novel Strategy To Form Tetrahedrally Coordinated Ti 4+ in Fe/TiO 2 Nanoparticles[END_REF].

Alternatively, other mechanisms such as sintering [START_REF] Tuichai | Effects of sintering temperature on microstructure and giant dielectric properties of (V + Ta) co-doped TiO 2 ceramics[END_REF] may be at the origin of an increase of particle size in the presence of copper.

Electron paramagnetic resonance (EPR) study

EPR spectroscopy analyses were performed in order to monitor the involved paramagnetic centers and to follow the valence state and the coordination environment of the doping ions as a function of Cu doping concentration in the TiO 2 lattice. The evolution of the EPR signal of undoped and copper-doped TiO 2 samples, annealed at 400°C, are shown in Fig. 7. The sharp peak observed at g = 2.003 closely approaches the free electron g value (g = 2.002). This value is associated with single electrons trapped in oxygen vacancies from the TiO 2 lattice [START_REF] Sun | Novel method to directly prepare high-surfacearea anatase titania nanoparticles with trapped electrons on oxygen vacancies[END_REF]. Undoped samples have only this paramagnetic signal. For a doping level not exceeding 0.3 at.% of Cu 2+ the intensity of this EPR peak increases and an asymmetric EPR signal shape appears whose intensity also increases. This EPR signal corresponds to the presence of Cu 2+ (3d9) ions in octahedral coordination in the TiO 2 lattice. This result is in agreement with that obtained by B. Choudhry et al. [START_REF] Choudhury | Defect generation, d-d transition, and band gap reduction in Cu-doped TiO 2 nanoparticles[END_REF], confirming the substitution role of Cu 2+ ions in the TiO 2 network. The substitution of Cu 2+ for Ti 4+ ions in the TiO 2 lattice leads to the formation of oxygen vacancies. It has been disclosed that for anatase the doping with a cation, such as Cu 2+ , Co 2+ , Fe 3+ , with a lower valence than that of the Ti 4+ ion, introduces oxygen vacancies inhibiting the formation of Ti 3+ ions [START_REF] Choudhury | Defect generation, d-d transition, and band gap reduction in Cu-doped TiO 2 nanoparticles[END_REF][START_REF] Navas | Experimental and theoretical study of the electronic properties of Cu-doped anatase TiO 2[END_REF][START_REF] Samet | Heat treatment effect on the physical properties of cobalt doped TiO 2 sol-gel materials[END_REF][START_REF] Wu | Creating Oxygen Vacancies as a Novel Strategy To Form Tetrahedrally Coordinated Ti 4+ in Fe/TiO 2 Nanoparticles[END_REF]. We also note that these oxygen vacancy sites are electron traps [START_REF] Li | Calculation of TiO 2 Surface and Subsurface Oxygen Vacancy by the Screened Exchange Functional[END_REF][START_REF] Papageorgiou Anthoula | Electron traps and their effect on the surface chemistry of TiO[END_REF]. Following the Cu 2+ doping, the place occupied by the O 2-anion in the regular TiO 2 lattice is taken by one or two free electrons, thus forming a donor level below the conduction band. It is demonstrated that these donor states in both anatase and rutile increase with increasing oxygen vacancies and can overlap the conduction band [START_REF] Pan | Defective TiO 2 with oxygen vacancies: synthesis, properties and photocatalytic applications[END_REF]. This may explain the great decrease in the band gap of TiO 2 produced by Cu 2+ doping.

At a higher doping level (0.6at.% Cu-TiO 2 ) a decrease in oxygen vacancy sites is noted, which is accompanied by an increase in the peak-to-peak height (PPH) of the Cu 2+ ion spectral signature (Fig. 7(b)). This may be due to the insertion of Cu 2+ ions in the interstitial sites in addition to substitution. At doping levels up to 3 at.%, the decrease in the oxygen vacancies PPH is more pronounced and is accompanied by a decrease in the Cu 2+ ion signal PPH. The Cu 2+ ion EPR signal also broadens, suggesting that a dipolar interaction among neighboring Cu 2+ ions is taking place. The nanoscale investigation in section 3.2 reveals the presence of metallic copper nanoparticles in the grain boundaries at and above a 3 at.% copper doping level. The electrons trapped inTiO 2 oxygen vacancies can be transferred to the Cu 2+ ions and metallic Cu is thus formed. This charge transfer can explain the decrease in the Cu 2+ EPR signal PPH.

Optical properties

The optical properties of Cu-TiO 2 powders annealed at 400°C and 600°C were investigated by UV-vis spectroscopy in diffuse reflectance mode (DRS). The reflectance data R% was converted to the Kubelka-Munk function F(R) by the equation: which is proportional to the absorption coefficient . The optical gap value is estimated using the Kubelka-Munk method combined with the Tauc relation [START_REF] Tandon | Measurement of forbidden energy gap of semiconductor by diffuse reflectance technique[END_REF]:

, where is the photon energy, A is an energy dependent constant and m an integer depending on the nature of the electronic transitions. For an indirect transition m = 2 and for a direct one m = 1/2. By plotting as a function of energy excitation, we estimate the indirect energy band gap and likewise to estimate the direct band gap energy .

According to theoretical investigations, the substitution of Cu 2+ for Ti 4+ in anatase (2at.% Cudoped TiO 2 ) should lead to a decrease in the band gap, down to 1.7 eV, and an increase in the visible light absorption [START_REF] Navas | Experimental and theoretical study of the electronic properties of Cu-doped anatase TiO 2[END_REF][START_REF] Dashora | Formation of an intermediateband in the energy gap of TiO 2 by Cu-N-codoping: first principles study and experimental evidence[END_REF]. Indeed, as already mentioned, Cu 2+ substitution in TiO 2 is accompanied by the formation of oxygen vacancies to preserve electric neutrality. In a simplistic ionic model, along with Cu 2+ acceptor defect states, oxygen donor band states are formed in the band gap [START_REF] Dashora | Formation of an intermediateband in the energy gap of TiO 2 by Cu-N-codoping: first principles study and experimental evidence[END_REF]. Using more sophisticated models, the appearance of new states at the top of the valence band and the bottom of the conduction band after Cu strong broad absorption band in the visible range. Two typical effects are observed: (i) the appearance of an absorption band between 400 and 500 nm that could be caused by the hybridization between O2p from TiO 2 and Cu 2+ 3d states [START_REF] Colon | Cu-doped TiO 2 systems with improved photocatalytic activity[END_REF][START_REF] Choudhury | Defect generation, d-d transition, and band gap reduction in Cu-doped TiO 2 nanoparticles[END_REF][START_REF] Navas | Experimental and theoretical study of the electronic properties of Cu-doped anatase TiO 2[END_REF][START_REF] Dashora | Formation of an intermediateband in the energy gap of TiO 2 by Cu-N-codoping: first principles study and experimental evidence[END_REF]. This band has been shown also to appear due to charge transfer between TiO 2 and CuO clusters [START_REF] Li | Role of surface/interfacial Cu 2+ sites in the photocatalytic activity of coupled CuO-TiO 2 nanocomposites[END_REF] at the TiO 2 /CuO interface. The possible presence of metallic copper can also affect the absorbance of this material in the visible range, via plasmonic excitations. (ii) the appearance of abroad absorption band between 600 and 900 nm, which is not observed in pure TiO 2 , and which is attributed to d-d transitions between the newly created energy levels at the top and bottom of the valence and conduction bands when Cu 2+ is inserted into the TiO 2 crystalline environment [START_REF] Choudhury | Defect generation, d-d transition, and band gap reduction in Cu-doped TiO 2 nanoparticles[END_REF][START_REF] Navas | Experimental and theoretical study of the electronic properties of Cu-doped anatase TiO 2[END_REF]. This typical behavior and these types of electronic transitions are consistent with those observed in the literature [START_REF] Choudhury | Defect generation, d-d transition, and band gap reduction in Cu-doped TiO 2 nanoparticles[END_REF][START_REF] Karunakaran | Cu-doped TiO 2 nanoparticles for photocatalytic disinfection of bacteria under visible light[END_REF] and indicate once more the substitution of Ti 4+ by Cu 2+ . Resulting indirect ( and direct ( ) energy band gaps were estimated graphically in a first approximation via the Tauc relation [START_REF] Tandon | Measurement of forbidden energy gap of semiconductor by diffuse reflectance technique[END_REF] by extrapolation of the Tauc plot to the axis with a linear fit. The obtained values are shown in Fig. 8(c). In the TiO 2 energy band structure, the top of the valence band originates mainly from O 2p states and the conduction band from Ti 3d states. The presence of the extra element in the TiO 2 lattice leads to a decrease of both direct and indirect band gaps. However, the most significant decrease is recorded for the indirect gap. For undoped samples, the band gap decreased slightly with increasing annealing temperature. This is in agreement with an increase in the rutile fraction with temperature. The presence of Cu 2+ ions in the TiO 2 structure has an even stronger effect on the band gap value reduction. In fact, we note that, at doping levels greater than 0.6 at.%, samples annealed at 400 °C present the largest band gap reductions, peaking for the sample with 3 at.% of copper (1.5eV). For Cu-TiO 2 samples annealed at 400°C, when passing from a copper incorporation level of 3 at.% to 6 at.% the intensity of the 400-500 nm absorption band increases while the 600-900 nm absorption band decreases (Fig. 8(a)). This suggests that the effective copper doping level in the 6at.% Cu-TiO 2 samples is lower than in 3at.% Cu-TiO 2 .

Indeed, at higher copper incorporation levels, microanalysis investigations confirm the segregation of copper oxide species at the surface of the crystallites.

For Cu-TiO 2 samples annealed at 600°C the incorporation of Cu 2+ into the TiO 2 structure also enhances the absorption by the catalyst in the visible range and induces a shift in the band gap to lower energies, though less pronounced than at 400°C. According to the structural investigations, this finding can be attributed to the absorption caused by: (i) metallic copper clusters formed in grain boundaries, and (ii) the disordered, highly Cu-doped TiO 2 structure formed in the grain boundaries. These interpretations concern samples with doping level no greater than 3 at.% of copper. The band gap decreases from 3.04 to 2.06 eV and 1.98

eV for a doping level of 6 at.% and 12 at.% respectively. These variations are probably due to the presence of the CuO additional phase, which presents a gap of around 1.5eV. As shown in Fig. 8(b), the contribution of this extra phase becomes rather large, which makes difficult the estimation of the gap through the methods used here on the diffuse reflectivity data.

Radiocatalytic activity evolution

Fig. 9 shows the effects of radiolytic and radiocatalytic degradation of methylene blue (MB) as a function of gamma-ray doses. Fig. 9(a) shows the radiocatalytic degradation curves of MB as a function of dose for different Cu dopant content in Cu-TiO 2 catalysts calcined at 400°C for 6 h. It is found that the degradation ratio of MB increases gradually with gamma dose. Under gamma irradiation and without any TiO 2 catalyst, MB presents a similar degradation to that reported previously in the literature [START_REF] Zheng | Degradation of Methylene Blue in aqueous solution by gamma irradiation[END_REF]. We then find that the insertion of copper into the TiO 2 structure is beneficial to the radiocatalytic efficiency. In the presence of Cu-TiO 2 catalyst the MB degradation is about 25% higher than for undoped TiO 2 . At low gamma radiation doses (≤ 100 Gy) the best degradation was obtained with 3at.% Cu-TiO 2 .

The presence of more copper in the TiO 2 structure becomes slightly beneficial to the radiocatalytic activity only for high gamma radiation doses (≥ 500 Gy). The best radiocatalytic performance obtained for a 3at.% Cu-TiO 2 catalyst annealed at 400°C is probably due to the large reactive surface area and the low band gap energy values. As revealed by XRD measurements, at this doping level the produced samples have the smallest particle size and possibly have the highest specific surface area. In fact, as reported by many authors, e.g., [START_REF] Aguilar | A route for the synthesis of Cu-doped TiO 2 nanoparticles with a very low band gap[END_REF], the specific surface area increases in inverse proportion to the sol-gel TiO 2 particle size. Our structural and optical results revealed that Cu 2+ internal doping of the TiO 2 structure occurs and leads to a very low band gap. During the radiocatalytic process, gamma irradiation can also lead to the desorption of oxygen from the crystallite surface. The oxygen vacancies so created can form donor levels in the TiO 2 forbidden band gap and can also act as traps for photogenerated electrons, thereby limiting the electron-hole recombination. With further copper doping level increases, CuO partially covers the surface of the TiO 2 catalyst.

CuO/TiO 2 heterostructures thus formed can facilitate the carrier separation and lead to improved radiocatalytic properties.

Regarding the second set of Cu-TiO 2 samples annealed at 600°C, the degradation rate as a function of gamma-ray dose is presented in Fig. 9(b). Pure TiO 2 synthesized at 600℃ presents better radiocatalytic performance than at 400℃, even though for TiO 2 catalysts it is generally accepted that anatase exhibits a higher photocatalytic activity than rutile [START_REF] Zhang | New understanding of the difference of photocatalytic activity among anatase, rutile and brookite TiO 2[END_REF].

Indeed, anatase is an indirect band gap semiconductor in contrast to rutile. This leads to a longer lifetime for photogenerated electrons and holes in anatase than in rutile. However, an optimal amount (not exceeding 20 %) of the rutile phase can result in enhanced catalytic properties. This synergistic effect (like that observed in commercial TiO 2 powders, Degussa P25) is controlled by the band alignment between rutile and anatase, which is favorable to charge separation (see [START_REF] Mi | Band alignement and contrillable elecron migration between rutile and anatase TiO 2[END_REF] for more details). Higher rutile percentages will generate trap sites for carrier recombination.

We observe at this temperature that Cu doping leads to a decrease in the radiocatalytic activity of the TiO 2 catalyst. At this annealing temperature samples present a much lower specific surface than undoped ones (crystallites are about 45% larger after doping) and are densely covered with copper species, inhibiting the catalytic properties of TiO 2 . For samples doped with 3 at.% of copper, structural investigations show an increase in the proportion of rutile (about 44%). For higher doping levels, the specific surfaces are even smaller, but the return to a pure anatase phase can explain the slight increase in the MB catalytic degradation efficiency as compared to that obtained with 3at.% Cu-TiO 2 catalysts.

In summary, the best catalytic performance under gamma radiation was observed for copper-doped TiO 2 samples annealed at 400°C, copper being inserted in substitutional sites of the anatase structure. These samples have a high specific surface area and a small band gap.

We note that during radiocatalysis, high-energy radiation is known to generate oxygen vacancies at the catalyst surface [START_REF] Wang | The adsorption of liquid and vapor water on TiO 2 (110) surfaces: the role of defects[END_REF][START_REF] Eriksen | Electronic excitations at oxygen deficient TiO 2 (110) surfaces: A study by EELS[END_REF][START_REF] Knotek | Ion Desorption by Core-Hole Auger Decay[END_REF] that act as electron traps [START_REF] Pan | Defective TiO 2 with oxygen vacancies: synthesis, properties and photocatalytic applications[END_REF]. Such defects have been reported to affect surface adsorption of H 2 O and O 2 on TiO 2 and to promote H 2 O dissociation [START_REF] Schaub | Oxygen Vacancies as Active Sites for Water Dissociation on Rutile TiO 2 (110)[END_REF] and formation of hydroxyl groups, therefore accelerating the organic pollutant degradation process.

Conclusions

This work investigates a novel approach for destroying organic pollutants involving transition-metal-doped titania. Amongst the advantages of using ionizing radiation is the fact that it overcomes optical transparency limitations associated with visible and near-UV illumination. The performance of such systems depends strongly on the insertion of impurities in the TiO 2 structure and their resulting distribution. We have shown that 3at.% Cu-TiO 2 catalysts annealed at 400°C present very small crystallites and therefore the highest specific surface, the most reduced band gap, due to Cu 2+ insertion in the TiO 2 lattice and so the best radiocatalytic performance. We suggest that oxygen vacancies generated during the radiocatalytic process form donor levels in the forbidden band gap of TiO 2 and can also act as traps for the photogenerated electrons thereby limiting the electron-hole recombination. We also suggest that at higher annealing temperature, the much lower specific surface and the copper species present on the surfaces and at grain boundaries inhibit the radioacatalytic performances. (g)

  ) and Fig. 3(b) respectively, for two different areas in the sample. Typical samples are well crystallized and crystallites exhibit polyhedral shapes with a mean size around 10-15 nm, in agreement with XRD investigations.

  Fig. 5(g).The associated Cu-L 2,3 edge after background subtraction is shown in Fig.5(h). The

8 .

 8 2+ doping has been shown to result from the strong iono-covalency of the Cu-O bonding which has a strong O2p-Cu3d hybridized character [15]. The absorbance spectra corresponding to the Kubelka-Munk function versus wavelength and the band gap energy estimations for Cu-TiO 2 powders annealed at 400°C and 600°C are represented in Fig. As expected, the insets in Fig. 8(a) and Fig. 8(b) show a general red shift in the UV-visible absorption spectra as a function of the Cu concentration. Undoped TiO 2 exhibits strong absorption in the UV region and high transparency in the visible. Copper incorporation into the TiO 2 lattice leads to a general shift in the UV-visible absorption spectra and to the creation of an increasingly (with Cu content)

Figures captions:Fig. 1 :Fig. 2 :

 12 Figures captions: Fig. 1: (a) XRD patterns of undoped and Cu-TiO 2 powders annealed at 600°C. (b) Enlarged view of the XRD patterns around the anatase (101) reflection (c) same for the (200) reflection. Fig. 2: Crystallite size (a) and full-width at half-maximum (FWHM) estimation of the anatase (101) and rutile (110) peaks (b) for pure TiO 2 and Cu-TiO 2 powders annealed at 400°C and 600°C versus Cu doping levels.

Fig. 3 :

 3 Fig. 3: Copper distribution characteristics in 3at.% Cu-TiO 2 samples annealed at 400°C. (a) TEM micrograph and corresponding fast Fourier transform (FFT) pattern from a typical area. (b) HAADF image showing a nanometer size copper particle (arrow). (c) Ti, (d) O and (e) Cu elemental maps from the area displayed in (b). (f) associated EELS Cu-L 2.3 to the areas labelled (1) and (2) in (e).

Fig. 4 :

 4 Fig. 4: Copper distribution characteristics in 6at.% Cu-TiO 2 samples annealed at 400°C. (a) TEM micrograph and the corresponding fast Fourier transform (FFT) pattern from a typical area. (b) HAADF image showing a large number of Cu particles decorating at grain boundaries or decorating the grain surface. (c) Ti and (d) Cu elemental maps from the area displayed in (b). (e) thresholding of the Cu map in order to separate the nanoparticles (visible in red) from the grain boundaries (in green). (f) the EELS Cu-L 2.3 spectra summed over the corresponding nanoparticle pixels (1) and that at grain boundaries (2).

Fig. 5 :

 5 Fig. 5: Copper distribution characteristics in 3at.% Cu-TiO 2 samples annealed at 600°C. (a) HAADF image and (b) Cu-L 2,3 intensity map with thresholding from a protruding grain to isolate the spectroscopic contribution from the nanoparticles (in red) to that from the grain surface and boundaries (in green). (c) associated Cu-L 2,3 spectra. (d) HAADF image of several adjacent grains. (e) Composite image after VCA processing of a spectrum image acquired from a reduced area and in the energy range of Ti-L 2,3 and OK edges. The blue, red and green pixels correspond to different TiO 2 phases. (f) VCA component EELS spectra associated to (e). (g) VCA component map associated to Cu, extracted from a spectrum image from the same reduced spatial area but encompassing the whole energy range of Ti-L 2,3 , OK and Cu-L 2,3 edges. (h) Cu-L 2,3 edge associated to the VCA map displayed in (g).

Fig. 6 :

 6 Fig. 6: EELS for Cu-TiO 2 powders annealed at 400 (Cu 3 at.%, Cu 6 at.%) and 600°C (Anatase: Cu 3 at.% (A), Rutile: Cu 3 at.% (R), grain boundaries Cu 3 at.% (GB) and Cu 6 at.%). (a) Ti-L 2,3 ELNES. (b) OK ELNES.

Fig. 7 :

 7 Fig. 7: EPR spectroscopy of Cu-TiO 2 catalysts annealed at 400°C. EPR signal for (a) divalent copper ions (Cu 2+ ) and (b) oxygen vacancies (Vo). (c) Evolution of the peak-to-peak height of both signatures as a function of Cu doping levels.

Fig. 8 :

 8 Fig. 8: Plots of versus for estimating the band gap energies for sol gel undoped and Cu-TiO 2 powders annealed at 400°C (a) and 600°C (b), insets display the absorption spectra. (c) Evolution of the direct and indirect band gap energy estimation versus Cu incorporated levels.

Fig. 9 :

 9 Fig. 9: MB degradation by radiocatalysis using pure and Cu-TiO 2 catalyst annealed at 400° C (a) and 600°C (b).

Figure

  Figure

Fig. 1

 1 Fig.1

Fig. 2

 2 Fig.2

Fig. 3 Fig. 4 Fig. 5

 345 Fig.3

  Fig.6

  Fig.7

Highlights :

 Copper doped and undoped TiO 2 powders were prepared by sol-gel route.

 Nanoscale analysis show evidence of internal doping by Cu 2+ in the TiO 2 sublattice.

 Substitution reduce the optical band gap down to 1.5 eV.

 Metallic copper segregation is formed at the crystallite surfaces.

 Anatase Cu doped TiO 2 improves significantly the radiocatalytic efficiency.

*Highlights (for review)